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Precision micro-milling process: state of the art

Precision micro-milling process: state of the art Adv. Manuf. (2021) 9:173–205 https://doi.org/10.1007/s40436-020-00323-0 1 1 1,2 • • Lorcan O’Toole Cheng-Wei Kang Feng-Zhou Fang Received: 8 March 2020 / Revised: 11 June 2020 / Accepted: 30 August 2020 / Published online: 27 October 2020 The Author(s) 2020 Abstract Micro-milling is a precision manufacturing combine conventional micro-milling with other technolo- process with broad applications across the biomedical, gies, which have great prospects in reducing the issues electronics, aerospace, and aeronautical industries owing to related to the physical process phenomena, are also intro- its versatility, capability, economy, and efficiency in a wide duced. Finally, the major applications of this versatile range of materials. In particular, the micro-milling process precision machining process are discussed with important is highly suitable for very precise and accurate machining insights into how the application range may be further of mold prototypes with high aspect ratios in the micro- broadened. domain, as well as for rapid micro-texturing and micro- patterning, which will have great importance in the near Keywords Precision machining  Micro-milling  Size future in bio-implant manufacturing. This is particularly effect  Deflection  Runout  Tool wear true for machining of typical difficult-to-machine materials commonly found in both the mold and orthopedic implant industries. However, inherent physical process constraints 1 Introduction of machining arise as macro-milling is scaled down to the microdomain. This leads to some physical phenomena The trend toward miniaturization of precision micro-com- during micro-milling such as chip formation, size effect, ponents, such as for microelectromechanical, nanoelec- and process instabilities. These dynamic physical process tromechanical, and micro-medical systems, has led to phenomena are introduced and discussed in detail. It is advances in microfabrication techniques in recent years. important to remember that these phenomena have multi- This demand for micro-sized parts with high aspect ratios factor effects during micro-milling, which must be taken has necessitated the biomedical, electronics, automotive, into consideration to maximize the performance of the and aerospace industries to adopt and apply both new and process. The most recent research on the micro-milling old manufacturing processes at the microscale. Although process inputs is discussed in detail from a process output microfabrication techniques have existed for many years, perspective to determine how the process as a whole can be the stringent requirements of extremely tight tolerances on improved. Additionally, newly developed processes that form, dimension, and surface characteristics [1], high machining efficiency, and machine positioning accuracy have led to further developments in precision machining processes [2]. Micro-milling is a precision micromechani- & Feng-Zhou Fang fengzhou.fang@ucd.ie cal cutting process, which has been developed to facilitate the increasing requirements [3]. Center of Micro/Nano Manufacturing Technology (MNMT- Micro-milling is an effective and efficient precision Dublin), University College Dublin, Dublin 4, Ireland machining process for manufacturing components with State Key Laboratory of Precision Measuring Technology microstructures such as complex three-dimensional (3D) and Instruments, Center of Micro/Nano Manufacturing surfaces at the microscale. Typically, micro-milling can be Technology (MNMT), Tianjin University, Tianjin 300072, characterized by the size of the cutting edge diameter of the People’s Republic of China 123 174 L. O’Toole et al. micro-milling tool, which lies between the range of 1 lm chip formation, size effect, and process stability, may not and 1 000 lm[4], whereas the diameter of the cutting edge be overcome by micro-milling alone in the future. in the conventional milling process is greater than 1 000 The application of micro-milling across the biomedical, lm. However, this definition focuses only on the tool and electronics, automotive, and aerospace industries is also does not incorporate the important aspects of the machin- discussed in relation to the versatile nature of the precision ing process, namely precision, accuracy, and underlying machining process. Such application ranges from machin- material removal mechanism. Therefore, a more technical ing of microstructures and textures to precision machining approach to characterize the micro-milling process can be of very hard and wear resistant materials for utilization in as follows: a precision mechanical cutting process with the mold manufacturing industry. Therefore, this review geometrically defined cutting edge tool diameters of less introduces the current state-of-the-art micro-milling pro- than 1 000 lm for precise material chip removal to within cess, beginning with the issues of physical process phe- less than 1 lm tolerance on form and dimensional accuracy nomena associated with machining at the microscale and [5]. including an in-depth examination of how to minimize However, the micro-milling process is limited by these negative issues. The process inputs are examined in inherent physical process issues when machining at the relation to the process outputs, offering insights into areas microscale, which are not present when milling at the for improvement in the future, which will further advance macroscale. Such constraints relate to material removal the development of the micro-milling technology. Finally, mechanisms at the microdomain, which include chip for- applications of the micro-milling process are described in mation, size effect, and process stability. Therefore, the detail, followed by insights into the future perspective of major physical processes that limit the process efficiency micro-milling. The objective of this work is to present the and precision in micro-milling are thoroughly discussed. development, benefits, applications, limitations, and future The effects of these negative phenomena on the machining insights of micro-milling and to discuss the recent publi- process outputs are considered, while insights into how cations regarding this precision machining process. these effects can be minimized, if not eliminated, are presented. Recently, there has been a strong research interest in the 2 Fundamentals of the process micro-milling process, with much work focusing on the process inputs, such as workpiece material and Because the precision micro-milling process has been microstructure, geometry and materials of tools, efficient developed from conventional milling, the two milling toolpath generation, and cutting fluid. Examining the latest processes share many similar characteristics such as works concerning the influence of these process inputs on machine components and configuration, tool geometry, and the cutting force, surface roughness, and tool wear during cutting fluid. However, the material removal mechanisms machining provides a clear depiction of the current micro- between these two mechanical cutting processes cannot be milling process. This review therefore investigates the mutually correlated. Conventional milling primarily con- theoretical, analytical, and experimental works most siders shearing forces acting on the rake face and far lesser recently published to identify areas of the process for future ploughing forces on the flank face [7], which are mainly development. caused by machine chatter stability [8]. The shearing- In terms of process advancement, one of the key areas dominant regime is the desired material removal mecha- will be in supplementing the micro-milling process with nism during any cutting process, where material is removed other successful technologies. Section 4 of this review will as distinctive chips along the rake face. The ploughing- examine the recent successful studies that implemented dominant regime is the unwanted material removal mech- secondary systems to produce advanced processes, such as anism, where material deforms plastically under the flank ultrasonic-assisted, laser-induced oxidation-assisted, and face and no chips are formed. The ploughing-dominant plasma jet-assisted micro-milling processes. The impor- regime results in extremely poor surface finish and very tance of such assisted processes will become even more high tool wear due to high cutting and friction forces, high apparent with future developments of higher hardness and temperature, etc., during machining. In contrast to preci- wear resistant materials. Such materials are classified as sion milling, micro-milling is subject to both considerable ‘‘difficult-to-machine’’ materials, which include hard and ploughing and shearing regimes. The contribution of each wear resistant superalloys, refractory metals, structural mechanism depends heavily on numerous factors, such as ceramics, composites, polymers, and magnesium alloys [6]. chip formation, undeformed chip thickness (UCT), size The consideration is that the limitations of the micro-mil- effect, tool deflections, and process stability. Each of these ling process in terms of physical process constraints, i.e., factors can also significantly influence each other, leading to a more dynamic and complicated effect when 123 Precision micro-milling process: state of the art 175 determining which material removal mechanism will workpiece [15]. However, the minimum UCT will be dominate during micro-milling. mainly affected by the tool geometry and material [16], workpiece material, and microstructure [17]. Generally, the 2.1 Chip formation UCT is developed as a prediction model because it is not necessarily a physical parameter and cannot be identified The minimum chip thickness is the critical limit deter- directly during machining. The results of measurements mining whether the material flows along the rake face, such as cutting force and surface integrity can be examined forming chips as the shearing mode of material removal, or to determine which mechanism of material removal is along the flank face causing elastic or plastic deformation dominant during machining. Up until recently, the depending on the material, as the ploughing mode [9–11]. mechanical models for micro-milling were based on scal- Therefore, it can be defined simply as the minimum UCT, ing conventional milling models with adaptations [18, 19] below which a defined chip cannot be formed stably. This and investigation of single factor influences. As discussed critical value will depend on the process parameters, however, simply reducing the scale from macro to micro material properties, and microstructure [12]. When the does not present a constitutive model. More importantly, UCT is less than the minimum value, chips due to the micro-milling is far more complex owing to numerous ploughing-dominant mode of material removal will not be factors having significant influences on the process. The generated. In contrast, when the UCT is larger than the models for UCT in micro-milling are established to predict minimum value, a defined chip will be generated and the process outputs, such as cutting forces, surface quality, and process can be compared to that of conventional milling [9] temperature, as well as to predict fundamental physical (see Fig. 1). Consequently, there will be no chip removal at processes, including the process stability and material very small depths of cut during micro-milling. Instead, the removal mechanisms. Therefore, to select optimal workpiece will undergo pure elastic deformation when the machining parameters, the material removal behaviour cutting tool passes through the workpiece material, which during micro-milling operations must be fully understood then recovers to the original height. However, with an and implemented by accurate models. The establishment of increase in the depth of cut, the material instead begins to such UCT models in micro-milling is clearly an important plastically deform. With the continuous increase in the research topic to obtain a much higher precision and more depth of cut, the material removal mechanism then begins efficient micro-milling process. to shift from plastic deformation to shear chip formation, if The early work by Son et al. [20] on ultra-precision the minimum UCT approaches a certain threshold. There- diamond cutting found that the minimum UCT was deter- fore, chips can be formed and removed only when the mined by the tool edge radius and the friction coefficient of the workpiece-tool interface. Their work then led to depth of cut exceeds the minimum UCT [13]. The UCT is one of the most important aspects that important research by Malekian et al. [21], who confirmed determine which material removal process will dominate in that the minimum UCT was a function of both the edge micro-milling, and it can be influenced by many factors radius and friction coefficient and was dependent on the [14]. The combined effects of factors such as tool setting tool geometry and properties of the workpiece material. errors and toolholder and spindle errors will result in sig- Through their proposed analytical model based on the nificant tool runout of the cutting edge with respect to the minimum energy principle and infinite shear strain method, Fig. 1 Chip formation mechanism of a macro-milling and b micro-milling in terms of minimum undeformed chip thickness h and cutting min edge radius r (Adapted and reprinted from ‘‘Machining scale: workpiece grain size and surface integrity in micro end milling’’ by Rodrigues and Jasinevicius [28], with permission from Elsevier) 123 176 L. O’Toole et al. the normalized minimum UCT of Al6061 was approxi- Fig. 1. The ductile mode of material removal dominates mated as 0.23 of the edge radius. However, it was noted by when the UCT decreases to sufficiently small values during the authors that the minimum UCT was a range of values, micro-milling, particularly at the submicron level as rather than a single point. This may be attributed to the described above [27]. However, the transition between the stagnation region, instead of a stagnation point, as observed shearing-ploughing modes of material removal remains a by others. Ramos et al. [22] also developed a model for large issue during the micro-milling process, while such estimating the minimum UCT of AISI 1045 based on their factors as the minimum UCT, size effect, effective rake experimental results. The minimum UCT was found to angle, and tool edge radius, all influence the process of chip substantially decrease with higher cutting velocities and to formation, leading to one mode of material removal into moderately increase with higher cutting edge radii. Such another. Therefore, a quantitative identification of the chip prediction models that estimate the minimum UCT are formation process and its influence on other micro-milling important because they can help to minimize the amount of phenomena, such as built-up edge (BUE) and burr forma- ploughing-dominant material removal and offer the opti- tion, is a crucial aspect for research to move forward, for all mum cutting conditions. One example of the importance of types of materials [13]. The scientific and systematic prediction models is when working with materials such as understanding of the multifactor effect will become even magnesium, where the risk of fire is a major concern during more significant in the future, particularly when dealing high-speed cutting, because magnesium in the molten state with the increasingly stringent requirements for industrial is flammable when exposed to oxygen. Therefore, accurate scale applications of the micro-milling process. models to predict cutting temperature at the flank face in relation to the UCT are very important, as determined by 2.2 BUE Fang et al. [23]. Chen at al. [11] developed a model of chip formation, When ductile materials, such as aluminum, steel, and even which was capable of connecting the minimum UCT, UCT, some titanium alloys, are machined using the micro-mil- and periodicity of cutting force together. Their model can ling process, BUEs can be observed on the rake face of the predict the normalized value of minimum UCT (k ), which tool, as illustrated in Fig. 2. This is due to the adhesion of represents the ratio of the minimum UCT to the cutting chips or material onto the cutting tool face, which greatly edge radius r . They estimated this value to be 0.43 B k B affects the process outputs and has a significant negative e e 0.48 for cutting edge radii between 2 lm and 3 lm for effect on surface roughness, also causing such problems as potassium dihydrogen phosphate crystal, which was higher cutting forces and shorter tool life [29]. Because the another difficult-to-machine material owing to its proper- BUE periodically develops and breaks off the tool rake ties of being soft, brittle and deliquescent. Their systematic face, the UCT is affected, which further leads to poor work could serve as a reference for similar works on other surface quality as well as deposits and smeared regions on difficult-to-machine materials, and obtained results could the machined surface. The BUE is typically more promi- potentially guide the selection of cutting parameters and nent when using lower cutting speeds, such as in conven- cutting edge radii for improving the integrity and quality of tional milling. However, it remains an important issue in machined surfaces in the micro-milling of other brittle micro-milling, where even small deposits of adhered materials. material on the cutting face will have considerable negative Recently, Lu et al. [24] investigated the tool trajectory in effects. micro-milling, with the aim of building a more accurate The BUE formation in metal machining has been a well- UCT prediction model while taking into consideration known phenomenon, with research into its unwanted radial tool runout on the cutting edge as well as deter- effects beginning even before the 1970s [30]. Similarly, mining the effect of tool setting errors on the UCT. much work has been conducted on the BUE in conven- Comparisons of cutting forces under this UCT model with tional milling more recently, such as Children’ work experimental data indicated that their model could be used toward simulating this phenomenon [31] or Ozcatalbas’ to accurately predict cutting forces during the micro-mil- study of orthogonal cutting, which indicated that the BUE ling process, offering theoretical insights into micro-mil- affected the chip formation and cutting ratio for different ling force models for further study. The construction of cutting conditions [32]. However, the influence of BUE on such accurate, instantaneous undeformed cutting thickness the micro-milling process has not yet been characterized in models is important to further establish cutting force detail. Thepsonthi and Ozel [33] carried out investigations models. on 3D finite element (FE) modeling and simulation of the The transition of material removal mechanism from micro end milling process for Ti-6Al-4V to determine the shearing to ploughing is an important phenomenon when influence of increasing tool edge radius due to wear on the machining at the microscale [25, 26], as can be seen from process performance. They found that the BUE might be 123 Precision micro-milling process: state of the art 177 Fig. 2 Scanning electron microscopy (SEM) images of both flutes of a micro-milling tool rake face exhibiting uneven BUE on each flute a Edge 1 and b Edge 2 (Adapted and reprinted from ‘‘Microstructure effects on process outputs in microscale milling of heat treated Ti-6Al-4V titanium alloys’’ by Ahmadi et al. [39], with permission from Elsevier) formed after the tool was severely worn. More recently, diamond-like carbon (DLC) coating could be used in Wang et al. [34] presented one of the first experimental micro-milling of Inconel 718 to substantially reduce the investigations on the effects of BUE on surface quality and BUE and burr formation, which improved surface rough- its prediction in micro-milling. They studied the influence ness. On the other hand, Aslantas et al. [37] showed that of BUE while machining 316L stainless steel and reported DLC, titanium aluminum nitride (TiAlN), and tungsten that the BUE was the main cause of surface finish deteri- carbide carbon layer-coated tools showed better perfor- oration in micro-milling besides the chip load. They also mance against BUE formation than nanocrystalline dia- showed that when the BUE was not present, theoretical mond-coated and uncoated tools. surface roughness models yielded acceptable predictions. The BUE affects the friction conditions at the tool-chip Davoudinejad et al. [35] confirmed that the presence of and tool-workpiece interfaces by acting like a cutting edge BUE generated unequal chip load and chip formation so that the cutting tool material is no longer in contact with among different tooth engagements. Their results also the chip and the machined surface. This suggests that a proved that burr height was negatively affected by the stable BUE formation may protect the tool from rapid presence of BUE. Finally, analysis of their results con- wear, leading to a higher machining efficiency. Oliaei and firmed the importance of the developed 3D FE modeling Karpat [38] investigated the relationship between approach for future work. stable BUE formation and process outputs in the micro- Ucun et al. [36] and Aslantas et al. [37] both investi- milling of Ti-6Al-4V using an experimental approach, gated how coated tools could minimize the BUE to taking into consideration tool geometry, surface roughness, improve surface integrity. Ucun et al. [36] confirmed that a and process forces. Their results determined that it was 123 178 L. O’Toole et al. possible to customize a micro-milling tool to have stable BUE formation and design it to machine titanium alloys with long tool life and acceptable surface quality. They concluded that a micro end mill with a low clearance angle yielded the most stable condition for BUE formation, while a large unstable BUE would result in surface quality deterioration. Therefore, the ability to predict and control the BUE size, together with a customized tool design, may be beneficial in the micro-milling of other difficult-to- machine materials. Clearly, simulation models and pre- diction models that will quantify the dynamic mechanisms of BUE formation, such as tool coating, tool wear, process parameters, and workpiece material properties, are impor- tant aspects for future research in micro-milling. Addi- tionally, understanding the chip morphology as well as stable and uniform BUE formation will have significant effects in prolonging tool life, increasing machining effi- Fig. 3 SEM 5009 magnification of a machined slot for burr width ciency and improving surface quality. measurement (Adapted and reprinted from ‘‘Novel method for burrs quantitative evaluation in micro-milling’’ by Medeossi et al. [52], 2.3 Burr formation with permission from Elsevier) A major issue during micro-milling pertains to the forma- tion of burrs, which is an accumulation of material forming a raised edge or volume on the workpiece surface, as can be seen from Figs. 3 and 4. Burr formation is a complicated mechanism involving plastic and elastic deformation, which can be influenced by material properties, tool geometry, and even process instabilities, such as tool run- out [40, 41]. It affects the quality of the machined surface significantly, reducing the capability of the part to meet the desired performance and thus the required functionality. The effect is even more significant at the microscale for precise and freeform components; however, burr reduction, characterization, and evaluation remain to be challenging tasks facing the micro-milling process. In addition, burr formation not only decreases the machined part surface and assembly quality, but also increases the production cost by Fig. 4 Types of milling burrs (Reprinted from ‘‘The effect of spindle up to 9% of the total machining cost [42]. This is due to a speed, feed rate, and machining time to the surface roughness and second machining operation, so-called deburring, which burr formation of aluminum alloy 1100 in micro-milling operation’’ may be necessary to remove such materials from machined by Kiswanto et al. [51], with permission from Elsevier) edges and holes. While the complexity and degree of deburring will depend on a number of factors including Jin et al. [46] determined early on that the feed per tooth burr size, location, and material [43], the focus of research had a major impact on the surface topography in micro- should instead be on the reduction and altogether elimi- milling and therefore proposed to use higher feed rates. At nation of burr formation during the micro-milling process low ratios of feed per tooth to cutting edge radius, high through tool geometry development, suitable machine amounts of burrs are obtained in micro-milling. Saptaji parameters [44], and toolpath optimization. As verified by et al. [47] revealed that top burrs could be reduced by either Fang and Liu [45], although burrs may not be eliminated strengthening the side edge of the workpiece or introducing completely through optimization of the cutting parameters a taper angle in the micro-milling tool. Their results sug- in micro-milling, they may be minimized to less than 25 gest that a combination of a large tool taper and large side nm in height. Among the most important factors are UCT edge angle produces the minimum burrs. Although a and tool sharpness, further showing that an optimal tool tapered wall angle, also known as draft angle, is essential in geometry is necessary to reduce burr formation [45]. 123 Precision micro-milling process: state of the art 179 mold machining, it may not always be a desired feature. vibration assistance in the feed direction during micro- Chern [48] classified burr formation into five types based milling of Ti-6Al-4V alloy. By inducing alternating chan- on in-plane exit angle: knife-type burr, wave-type burr, ges in the relative direction of movement between the curl-type burr, edge breakout burr, and secondary burr. workpiece and the tool on both sides of the slot through Hashimura et al. [49] classified burrs by location, shape, small amplitude, high frequency vibrations, chip formation and formation mechanisms. Litwinski et al. [50] on both sides of the slot then became similar, leading to acknowledged bottom burrs in their toolpath planning less burr formation on both sides of the slot. The results concept; however, they provided no insights into bottom from their FE model simulation and experimental work burr formation or prevention. Kiswanto et al. [51] then confirmed the benefit of vibration assistance, which performed a significant study concerning top, bottom, reduced the average top burr height on the down-milling entrance, and exit burr formation, as well as the effect of side by 87%. However, this proposed method in burr tool wear on burr formation mechanisms, as shown in reduction only utilized vibration assistance in the feed Fig. 4. Furthermore, the team analyzed the average sizes of direction and had only been applied for slot micro-milling. top burr for each cutting parameter to determine the rela- Further work is necessary to optimize even this basic tionship between the cutting parameters and burr forma- unidirectional vibration-assisted micro-milling process. Li tion. Their results showed that bottom burr occurred during et al. [54] provided some of this developmental work also longer machining times, in comparison to top, entrance, in the feed direction. They determined that larger vibration and exit burrs, due to the deterioration of the tool. There- amplitudes actually increased the exit burr size. Hence, fore, tool wear due to machining time was found to be the larger vibration frequencies and smaller vibration ampli- most influential factor affecting burr formation. The team tudes are recommended. Clearly, much more work is also determined that in order to produce a burr-free com- necessary to apply vibration in two or three directions for ponent, it was recommended to perform up milling during burr removal during freeform surface machining and end the micro-milling process. Finally, it was shown that milling operations, including both theoretical and experi- appropriate selection of cutting parameters could minimize mental works. burr formation. Their important work provided adequate Any burr left on the machined surface deteriorates the knowledge of appropriate cutting parameter selection dur- component quality, precision, function, and performance. ing the micro-milling operation of aluminum alloy 1100 to This is particularly true for microparts and features. produce a product with minimum burr. Therefore, burr minimization, and where possible elimi- More recently, Medeossi et al. [52] proposed a novel nation, is essential for high-quality micro-milling opera- method for quantitatively evaluating burrs based on optical tions. This is achievable through extensive research on control techniques and further investigations into under- microscopy using an innovative approach to take advan- tage of the a priori information on the manufacturing standing the phenomena. Such key areas of interest for operation and an unconventional use of void pixels for future work therefore lie in cutting parameter optimization, rapid and non-destructive evaluation of multiple geomet- toolpath generation, tool geometry and material, and tool rical quantities. They applied their proposed methodology coatings and lubrication investigations. to slotting micro-milling operations on pure titanium grade II. The results showed that their method had the potential 2.4 Size effect for on-machine monitoring of burr evaluation during micro-milling operations, which had further potential in Micro-milling raises significant issues when removing reducing and eliminating burr formation through process material at the microscale owing to the effect of scaling, optimization. However, it was noted by the authors that otherwise known as size effect. It has been shown that the appropriate modeling of the specific machining operation size effect modifies the mechanism of material removal in was necessary. Moreover, there are inherent limitations of conventional milling [55, 56]. However, the characteriza- online vision-based measurement techniques, such as dif- tion and exact cause of this effect remain a point of con- ficulties in measuring burr height or burr features over tention among researchers, indicating that many factors freeform surfaces without the additional cost of extra rotary influence chip formation and material removal mechanisms axis or right-angle optics for the online measurement at the microscale. In simple terms, size effect is a phe- system. nomenon that modifies the material removal and chip for- In the micro-milling of slots, the relative size of burrs mation mechanisms at the microscale [57]. In conventional formed on the up-milling side is smaller than that on the milling, the shearing mode of material removal dominates, down-milling side, as can be seen from Fig. 3. To take which leads to chip formation. However, the size effect advantage of this cutting phenomenon and chip formation becomes more significant as the machining scale is reduced mechanism, Chen et al. [53] investigated the effect of to the microlevel, where ploughing of the material surface 123 180 L. O’Toole et al. dominates. This phenomenon produces a major challenge [65], as well as tool specifications [1, 66]. The physical of preventing chip formation by a tooth during a cutting mechanisms that govern the size effect will be discussed in pass, which leads to high cutting forces, high friction, high the following section, including the specific energy, temperature, and significant tool wear. However, as men- shearing and ploughing-dominant modes of material tioned earlier, the exact characterization of the size effect removal, as well as the effect of tool edge radius. has not been fully agreed upon. As an example, Qin [58] defined it as the relationship between the specific energy 2.5 Tool edge radius during cutting and the tool rake angle, which were two important physical parameters that affected the chip A small tool edge radius, rather than a sharp point, is an removal process. As the depth of cut decreases, the effec- important feature of micro-milling tools to limit crack tive rake angle increases, influencing the specific energy. initiation and failure points at the cutting edge of the tool. Therefore, the larger the rake angle, the greater the specific However, because of the size effect, downscaling of con- energy, which has been widely accepted as the main con- ventional milling tools makes the cutting edge radius of tributing factor to the size effect phenomenon [59]. microtools comparable to the instantaneous UCT. Micro- Experimental observation by Mian et al. [60] determined milling tool edge radii are usually less than 5 lm; however, that the specific energy, besides the burr root thickness and they can be up to 20 lm[67]. This means that the tool edge surface roughness of machined surfaces, could be used as a radius is in the same order of magnitude as the chip being relevant measure of the size effect in micro-milling. The formed [68], leading to an increase in cutting force [69, 70] team also used wavelet transformation to extract energy and surface roughness [71]. In micro-milling tools, the bands related to the deformation mechanisms involved in edge is deliberately rounded to impart strength, prevent machining, while high frequency bandwidths in the plastic deformation, and avoid early tool breakage [72]. acoustic emission signals could also be exploited to iden- Therefore, chip formation occurs along the rounded edge of tify the size effect phenomenon. The size effect can also be a tool, resulting in a negative value of the effective rake described as the phenomenon whereby the ratio of the UCT angle, even if the nominal rake angle is positive [73]. to the cutting edge radius of the tool, or the grain size of the Vipindas et al. [74] presented an investigation on the workpiece material, will influence chip formation, material effect of cutting edge radius on cutting force, coefficient of removal mechanisms, and material flow, as shown in friction, surface roughness, and chip formation during Fig. 1. This effect can become significant when the thick- micro end milling of Ti-6Al-4V, for a wide range of feed ness of the material to be removed is of the same order of per tooth. It was found that the feed per tooth within 1 lm magnitude as the tool edge radius or grain size of the range was the critical value, which was approximately one- workpiece material [60]. The influence of tool edge radius third of the cutting edge radius. Below this critical value, on the size effect has also been demonstrated through a the size effect is predominant, leading to the ploughing strain gradient plasticity-based FE model of orthogonal mode of material removal, as illustrated in Fig. 5. Moges micro-cutting by Liu and Melkote [61]. et al. [75] developed a comprehensive mathematical model Actually, the above two definitions are correct because that incorporated the edge radius of the micro-cutting tool, many factors will affect the chip formation and material removal mechanisms; thus, it can be simply said that the size effect is characterized by a nonlinear increase in the energy consumed per unit volume of material removed as the UCT decreases to the same order of magnitude as the cutting tool edge radius or grain size [60, 62]. Therefore, it is very clear that conventional milling mechanisms cannot be used to describe the micro-milling process, because simply reducing the scale of the system will not reproduce the same representative model [63]. The size effect becomes even more significant at the nanoscale, particu- larly for nanometric cutting, where ploughing of material dominates, rather than shearing and chip formation [64]. Consequently, this variation from the general behavior of Fig. 5 Cutting model of microtool edge showing ploughing, shear- both the tool and the workpiece microstructure at the ing, and elastic recovery zones as a result of tool edge radius r microscale during machining will depend on many factors, (Reprinted from ‘‘Experimental research on micro-milling force of a such as the material properties and microstructure [39], single-crystal nickel-based superalloy’’ by Gao and Chen [76], with micro-milling tool parameters [59], machining parameters permission from Springer Nature) 123 Precision micro-milling process: state of the art 181 so that a more accurate prediction of cutting force models morphology, and surface integrity of martensitic aged steel. could be obtained. Therefore, even though rounding of the They proposed a new method for calculating the effective cutting edge was necessary in micro-milling tools, an energy and non-effective energy by the criterion of whether extremely large tool edge radius would greatly influence it contributed to chip formation or not, respectively. Their the size effect. This suggests that a stronger cutting edge to results showed that chips became more segmented with prevent crack initiation could reduce the size effect issue, decreasing proportion of the effective energy, whereas as it would lead to a smaller radius requirement, which in increasing the proportion of the non-effective energy turn would result in a more dominant shearing mode of resulted in surface integrity deterioration and contributed to material removal. To fulfil this demand, further investiga- the formation of a plastic deformation layer. Then, by tion on the cutting edge tool geometry is necessary. assessing the trade-off between surface quality and specific cutting energy, optimized machining parameters were 2.6 Specific energy suggested to achieve a precision surface finish with low specific cutting energy and high energy efficiency, which The energy consumption during the machining process had significant application for the realization of sustainable affects both the environmental and manufacturing costs. manufacturing. Gao et al. [59] examined the size effect in Therefore, evaluating and limiting the energy consumed relation to the tool edge radius and cutting parameters on during micro-milling can lead to more efficient manufac- specific energy in micro-milling of heat resistant stainless turing [77]. One such way, according to Fang et al. [78], is steel. They showed that the specific cutting energy could be to compare the experimental cutting force and specific fully controlled by regulating the geometrical characteris- cutting energy. To compare the energy consumption during tics of the cutting tool, i.e., the cutting edge radius, and by machining operations such as micro-milling, the specific the machining parameters recommended by their devel- energy parameter, which was defined by Li and Kara [79] oped minimum chip thickness prediction model. Precise as the energy consumed to remove a unit volume of control of the specific energy during micro-milling can material, may be used [80]. The specific energy is a par- therefore lead to more efficient chip formation, which has ticularly important parameter to consider during micro- great significance on improving machining efficiency, tool milling, as it can be used to evaluate the cutting effec- life, and surface quality. Lauro et al. [82] also analyzed the tiveness of the process. The ratio of specific energy to the influence of the size effect on the specific cutting energy of UCT can be helpful in characterizing the size effect in AISI H13 steel in relation to austenitic grain size, while relation to surface generation, as can be seen from Fig. 6. examining the response from a cutting force perspective. It has been shown that the size effect strongly affects the They observed that the grain size had a significant influ- ence on both cutting force and specific cutting energy in specific energy necessary for material removal through chip formation mechanisms, which can alter the material micro-milling. Their results revealed that larger grain sizes removal mechanisms [16]. An experimental investigation displayed lower specific energy compared with smaller was carried out by Yao et al. [81] to determine the rela- grain sizes. They also showed that increasing the feed rate tionships between the specific cutting energy, chip had a significant effect on reducing specific energy (ap- proximately by 70%) for both small and large grain sizes. Therefore, the recent research suggests that by reducing the specific energy during cutting, the size effect was therefore lessened, resulting in improved machining efficiency, tool life, surface finish, and material removal rates. 2.7 Process stability Relatively large form error and poor component geometric accuracy are still major obstacles toward achieving higher precision in the field of micro-milling. The main cause of these inaccuracies is the inherent process instabilities dur- ing the micro-milling process. Among several factors, the influences of tool deflection, tool runout, and machining chatter are the main sources of surface and dimensional Fig. 6 Variation in specific cutting energy with uncut chip thickness accuracy errors in micro-milled components. These process at 240 m/min (Adapted and reprinted from ‘‘Size effects in instabilities further lead to high cutting forces, excessive manufacturing of metallic components’’ by Vollertsen et al. [83], tool wear, and tool failure, as well as high cutting with permission from Elsevier) 123 182 L. O’Toole et al. temperatures, as a result of frictional forces due to rubbing both ploughing and shearing modes of material removal. and ploughing during unstable machining conditions, as On the other hand, Lu et al. [88] proposed a revised 3D will be discussed in this section. Because of the relatively analytical model of micro-milling forces, which considered low strength and stiffness and very small cutting diameter the effects of cutting temperature and ploughing force of micro-milling tools, micro-milling must be performed at caused by the arc of the cutting edge during shearing- very high speeds between 20 000–100 000? RPM, to dominated cutting. Therefore, considering the seriousness ensure productive machining. Moreover, material removal of the tool deflection issue, it is of great importance to rates can be maintained during the process by increasing study its effect on the mechanics of the chip formation the spindle speed to negate the effect of the small cutting process, while examining cutting forces, surface errors, and diameter of the microtool and relatively slow feed rate. cutting temperature, so that reliable and accurate predic- However, high-quality precision air bearing spindles with tions can be made to limit and prevent excessive deflec- closed loop position and very accurate speed control are tions during machining. necessary for high RPM machining to ensure process sta- Cutting forces directly affect tool deflection in the bility. Furthermore, vibration and instabilities during high- micro-milling process because of the relatively low stiff- speed micro-milling must be minimized, whereas feed rate ness of the tool, particularly at the tool tip, and results in and positioning must be smooth and continuous [84]. imperfections on the machined surface as described above. Therefore, it is necessary to develop accurate and reliable As bases for determining tool deflection, accurate analyti- process stability models to analyze and improve the per- cal cutting force models that consider the tool geometry formance of such processes as tool runout and tool and material, the specific cutting mechanism involved, as deflection, as well as minimize self-excited vibration, also well as the vibration dynamics are key areas for research. known as chatter. Mamedov et al. [87] fully understood the importance of cutting force on tool deflection and became significant 2.8 Tool deflection contributors to micro-milling tool deflection analysis early Tool deflection is one of the most significant factors lim- iting the performance of micro-milling processes, particu- larly limiting form accuracy and precision [85, 86], as can be observed from Fig. 7. A micro-milling cutting tool is severely prone to relatively large deflections owing to a significantly smaller diameter to overhang length ratio. This results in a drastic reduction in tool shank section modulus, which lowers its strength and ability to withstand periodically varying cutting forces, leading to tool bending [85]. The increased flexibility and lower stiffness of smaller diameter tools result in large values of cutter edge deflections, which lead to two serious problems: form and feature geometric errors on the machined component and distortion of cutting forces. This effect is again strength- ened even further as the cutting tool diameter reduces from 1 000 lmto100 lm, where even a small deflection of 5 lm will lead to an error comparable to the cutting edge radius of the tool. As the deflected tool rotates, undesirable cycles of shearing to ploughing modes of material removal mechanism will occur, leading to spikes in high and low cutting forces on each tooth. This will in turn cause larger deflections, while the cycle itself will continue until either failure of the tool occurs or the tool skips. Moges et al. [85] presented a methodology for determining such cutting force-induced tool deflections and developed a cutting force model considering tool deflection on the resulting Fig. 7 Deflection of milling tool at the bottom of the workpiece edge cutting forces. Similarly, Mamedov et al. [87] developed a (Adapted and reprinted from ‘‘Analysis of tool deflection errors in novel mathematical model for estimating cutting force and precision CNC end milling of aerospace aluminum 6061-T6 alloy’’ by tool deflection by calculating the UCT, which considered Nghiep et al. [95], with permission from Elsevier) 123 Precision micro-milling process: state of the art 183 on. Using their mathematical model, the distribution of also considered tool runout, consisting of both axial and tilt forces acting on the tool can be predicted and deflection of offsets, including entry and exit angles of the tool. Their a micro-milling end mill tool can be estimated with good developed model can be used to further optimize the accuracy. High cutting forces lead to higher tool deflection. accuracy of the micro-milling process because of the Mamedov et al. [89] presented an updated analytical cut- inclusion of a more complete tool deflection model. ting force model, which considered both the shearing and Clearly, deflection of a cutting tool is dependent on many ploughing phenomena, based on the material elastic factors and must be modeled as a dynamic phenomenon, recovery properties. The tool deflections corresponding to rather than a static one. More accurate tool deflection the cutting force were calculated by considering the prediction models will provide methods for reducing cut- microtool stiffness. This model accurately predicts instan- ting forces, thereby reducing tool wear and breakage, taneous tool deflections through analysis of the cutting increasing surface and feature quality as well as machining force, which was presented as a function of cutting force efficiency. However, tool deflection is an implicit issue of coefficients, microchip thickness model, and tool geome- machining at the microdomain, which also has a multi- try. Oliaei and Karpat [90] investigated the influence of factor influence on the process stability as a whole, similar increased cutting force due to tool wear on tool deflections to tool runout and self-excited machining chatter. There- and tool breakage. Their model for predicting tool deflec- fore, a thorough review of relevant research is presented tion and tool breakage allows for the development of tool below on tool runout and chatter in micro-milling. condition monitoring systems based on the physics of the micro-milling process. In their model, Rodrı´guez and 2.9 Tool runout Labarga [91] for the first time considered variable deflec- tion rather than just static deflection along the cutting edge. Tool runout is a critical issue that affects the micro-milling Their model also has promising benefits in monitoring process significantly. It is in part responsible for influenc- systems and adaptive control systems for the prevention of ing the cutting force [96], tool condition, tool life [97], and tool failure during micro-milling operations; however, it surface integrity of the machined component [98]. Tool does not take tool wear into consideration. Moges et al. runout can be described as a phenomenon caused by the [85] also presented a method for determining cutting force- sum of the geometrical displacement errors of the spindle, induced tool deflections and developed a flexible force toolholder, and tool axis from the ideal or theoretical axis model considering the effect of tool deflections on the of rotation. The sum of these errors produces a deviation resultant cutting force based on previous rigid models. The between the theoretical and actual cutting edge trajectories team presented a methodology for predicting variation in [62]. Tool runout may take the form of axial and/or radial runout. Radial runout is caused by the tool rotating off machine surface error due to tool deflections. Their pro- posed model accurately predicted cutting forces in the center, instead of being centrally aligned, and it will rotate presence of tool deflections. In addition, it was found that about a secondary axis. Cutting tools will be generally deflection of the tool caused considerable deviations of the more tolerant to this type of runout during face milling tool center location, resulting in change of tooth trajecto- operations. However, during side milling, radial runout will ries and uncut chip geometry. Their model provided great have significant effects on the cutting force, and therefore benefits in selecting optimum cutting parameters to control tool wear, due to uneven loading on the flutes, which will tool deflections, resulting in tighter tolerances and lead to surface errors. In contrast, axial runout is the result improved productivity. However, to further improve the of rotating components not being parallel with the center prediction accuracy, their model must consider the axis of rotation, such as the tool axis and spindle axis not dynamic vibration of the tool tip. Lu et al. [92] understood running concentrically. Therefore, axial displacement of the importance of examining the cutting force and how it the tool causes its tip to rotate off center relative to the might be used to limit tool deflection. They proposed an spindle axis. Cutting tools will generally be less tolerant to indirect method of determining the average micro-milling axial runout, especially for micro-milling operations in cutting force, which was both low cost and high precision, both side and face milling operations, but axial runout has a by examining the power of the main transmission system of considerable influence on the surface topography genera- a micro-milling machine. Lu et al. [93] then developed a tion in face milling [99]. The total tool runout is therefore new method for predicting micro-milling tool breakage the sum of both axial and radial runouts, with the effect based on theoretical models by examining the tool bending becoming even more significant in the micro-milling stress. Finally, Zhang et al. [94] formulated a mechanistic domain, as demonstrated by Fig. 8. Because micro-milling model of cutting forces and instantaneous tool deflection in requires very high spindle speeds due to the relatively the micro end milling process, which took into account the small cutting edge diameters, the dynamic characteristics minimum UCT effect and tooth trajectory. Their model of the spindle-tool system dominate the machining process 123 184 L. O’Toole et al. With regard to measurement of tool runout, Jing et al. [100] presented a method using modeling and simulation of the cutting force in micro-milling. The proposed approach uses a charge-coupled device to determine differences in displacement of tool flutes and tool shank. An accurate tool runout value can then be calculated using their model. This is a simple, easy, and precise method for measuring runout in micro-milling and can be easily adapted to on-machine measurements during operation. Another simple method for measurement of tool runout is by displacement mea- surement using capacitive sensors close to the tool shank, according to Chen et al. [101]. They also determined that tool runout resulted in a considerable increase in surface roughness, particularly when the feed per tooth was less than the runout. Finally, their proposed surface generation Fig. 8 Effect of runout is intensified for smaller tools (Reprinted model considering the minimum UCT, which takes into from ‘‘Protocol for tool wear measurement in micro-milling’’ by account tool runout, provides a more accurate surface Alhadeff et al. [63], with permission from Elsevier) topography simulation and roughness prediction in micro- milling. Zhang et al. [102] also developed a simple and quality. Therefore, tighter stiffness loop machines with effective tool runout identification method, designed to higher precision spindles and tools are essential. However, quickly identify the tool runout parameters through tool even with the correct equipment, the tool-spindle interface displacement measurement using a laser displacement can cause undesirable radial runout, while even small sensor, so that the accuracy of tool runout measurements deviations in the spindle or cutting tool edges may result in could be improved. significant runout due to poor stiffness and strength of Guo et al. [103] established the importance of a more microtools [84]. systematic approach to investigating tool runout in relation The significance of tool runout is that it has a major to tool geometry and surface generation. Toolpath opti- influence on the cutting force. This is due to the dis- mization during 5-axis machining was examined in detail placement being in the same order of magnitude as the feed in relation to minimizing geometric errors formed from per tooth, which therefore has a large influence on the tool runout. In their model, the tool runout is defined by surface roughness generated, as determined recently by four parameters, namely, inclination angle, location angle, Chen et al. [99]. They also found that axial runout in offset value, and length of the cutter axis. Although their particular limited the achievable surface roughness. Simi- work only considered conventional machining, much of larly, uneven engagement of teeth caused by runout leads what was learned could also be applied to micro-milling, to uneven rates of wear on each tooth, resulting in cutting with the effect becoming even more significant at the force features that can be largely different for both teeth microdomain. Guo et al. [104] then presented an instanta- [63]. This further leads to increasing cutting forces and all neous UCT model regarding tool runout and tool geometry problems generated by process instabilities. Attanasio [62] in micro-milling. Using their early work as a foundation, developed an easy and reliable method for determining tool they determined that five parameters were necessary to runout in micro-milling by implementing a geometric characterize tool runout in micro-milling, namely, runout model that deduced and estimated tool runout from the tool offset length, inclination angle, cutter axis length, location diameter, channel width, and cutting edge’s phase. Their angle, and initial rotation angle. The team analyzed and procedure can be integrated into an adaptive model for discussed the influencing principles of each runout controlling cutting force, which has practicality for parameter on the instantaneous UCT values. Some improving production quality and process stability while important viewpoints that provided reasonable explana- reducing tool wear and machining costs. Li et al. [15] tions for each runout parameter were introduced. However, established a cutting force model that further strengthened no method for detecting each of the runout parameters was the understanding of the micro-milling process, through a offered. Their work would be a good research to begin deeper investigation of tool eccentricity. This multifactor more thorough investigations into fully characterizing the model considers the influence of runout on the UCT, the tool runout parameters and how this phenomenon could be equivalent rake angle and cutting force, and how the minimized or eliminated to reduce cutting forces. As combined effects of each factor influence the surface mentioned, tool runout causes unbalanced chip thickness quality. removal between flute teeth, which leads to uneven cutting 123 Precision micro-milling process: state of the art 185 force loading on each cutting edge. This causes not only as a function of the spindle speed, as depicted in Fig. 9. unwanted vibrations that affect process stability, but also This diagram is an essential tool to find the range of high and uneven tool wear. Throughout this section, it is machining parameters that results in a maximum explained why it is important to limit tool runout as much stable (i.e., chatter free) material removal rate [110]. The as possible. However, it may not always be feasible to idea is to seek regions within the lobes for optimal completely eliminate runout owing to a number of reasons machining parameters, depending on such criteria as time, including tool, toolholder, and spindle setup, and tool cost, and accuracy [105]. However, SLD is based on manufacturing tolerances. Therefore, it is essential to limit individual machine setups as indicated in Fig. 10, which tool runout to an acceptable level, i.e., at least below 2 lm. considers the machine tool stiffness loop, tool geometry, This can be considered prior to machining to realize a etc. Therefore, predicting the stability lobe boundaries can precise and accurate process, avoid accelerated tool wear be a very difficult task that will rely on fundamental or tool breakage, and improve surface finish. understanding of the dynamics of the entire micro-milling process. To do so, a combination of theoretical models of 2.10 Chatter Chatter also greatly influences the process stability, resulting in increased tool wear, poor surface finish, and limiting precision and efficiency. Chatter is a form of self- excited, unstable vibration during specific cutting edge machining. It is generally accepted that there are four types of chatter during the cutting process, namely frictional chatter, regenerative chatter, mode coupling chatter, and thermomechanical chatter [105]. Frictional and regenera- tive chatters are generally the most common types and notably most important in micro-milling. Frictional chatter is mainly attributed to nonlinear dry friction force, i.e., rubbing on the clearance face, which leads to excitation vibration of the cutting force, limiting the thrust force [106]. However, regenerative chatter is the most significant issue in cutting processes in general because of the high Fig. 9 Stability lobe diagram plotting axial depth of cut against spindle speed to identify areas of chatter-free operation. Reprinted spindle speeds involved [105]. It occurs owing to varying from ‘‘Chatter in machining processes: A review’’ by Quintana and cutting forces acting on each tooth of the tool, which create Ciurana [105], with permission from Elsevier a relative displacement between the tool and the workpiece at the cutting point [107]. Depending on the characteristics of the system and the phase between the varying cutting forces, the dynamics of the cutting system can be unstable. This in turn leads to large chip sizes and higher cutting forces and vibrations. This process will continue if the system remains in an unstable condition, until the vibration amplitude increases to the point that the tool jumps or skips, damaging either the tool, workpiece, or spindle [108]. Therefore, to prevent chatter and unstable machining conditions, accurate models of the dynamics of the micro- milling system are necessary to predict the relationships between the workpiece material, structural dynamics of the machine tool including toolholders, tool geometry, and cutting conditions. By analyzing these models, a stability lobe diagram (SLD) can be created, which will offer insights into ideal machining parameters that can be chosen to prevent process instabilities such as chatter, greatly Fig. 10 Dynamic model of micro-milling system showing stiffness improving the machining efficiency [109]. loop and how chatter can be modelled (Adapted and reprinted from The distinction between a stable and unstable cut can be ‘‘Chatter modeling in micro-milling by considering process’’ by visualized with the SLD, which plots the axial depth of cut Afazov et al. [98], with permission from Elsevier) 123 186 L. O’Toole et al. the machine tool and toolholder, as well as deflection considering rotational degree-of-freedom (DOF) and tool testing of the tool will be necessary. To begin constructing point FRF of micro-milling. The FRF at the micro-milling an SLD, an analytical model of the frequency response tool point can therefore describe the dynamic behavior of function (FRF) of the cutting tool, toolholder, spindle, and the entire micro-milling machine system. Lu et al. [121] machine tool is required. Next, experimental testing or an further developed this work by considering the centrifugal accurate theoretical model is required to determine the force and gyroscopic effect caused by the high-speed dynamics of the tool tip. Thus, SLDs can be created for a rotation of the micro-milling spindle to better simulate the system setup using the specified cutter, workpiece material, real scenario and increase the accuracy of modal etc. Finally, the operator can select combinations of axial parameters. depth of cut and spindle speed, which ensure chatter-free To obtain more accurate SLDs, higher accuracy models operation. of chatter and process stability are necessary. Typically, In contrast to that of conventional milling tools, per- models designed based on solving the equations of motion forming an impact hammer test at the tool tip of micro- in either the frequency or time domain, where both cutting milling tools is not feasible because of their inherent tool force and modal parameters are implemented, will lead to fragility. Therefore, new methods for analyzing micro- results that are more robust. This is because ‘‘cutting milling tool dynamics are necessary to build the SLD in instability consists of deterioration in both time and fre- micro-milling. Lu et al. [111] developed a vibration dis- quency domains due to the highly nonlinear nature of the placement measurement system that utilizes a laser dis- micro-milling process’’ [122]. When the cutting forces placement sensor to collect vibration signals during micro- exhibit a linear behavior in cutting processes, the frequency milling. The frequency of the micro-milling cutting force domain solution should be used, i.e., when the process is was obtained using the varying cutting parameters method, more or less stable. However, at small UCTs and feed rates, whereas the relationship between the cutting force ampli- the micro-milling process experiences nonlinear behavior tude, frequency, and vibration displacement was ascer- owing to the size effect, chip formation, etc. Since the tained by using a neural network method to realize cutting forces can become nonlinear, the equations of vibration displacement prediction under given cutting motion must therefore be solved in the time domain using parameters. Before this important work, the methods used numerical methods for integrating the ordinary differential for conducting stability analysis mainly included zero- equations of motion [98]. The dynamic system can then be order solving, semi-discrete, and time domain methods reduced to a 2-DOF system through the assumption that the [112]. The zero-order solving method was applied by helix angle of the microtool is negligible, simplifying the Mascardelli et al. [113], Tajalli et al. [114], and Jin and equations of motion (see Fig. 10). Regarding the time domain method, Lu et al. [112] proposed a micro-milling Altintas [115]. However, only stability prediction results considering the shear effect were obtained for the SLDs force prediction model based on chatter stability analyzed drawn. Tyler et al. [116] presented a method for producing in the time domain. However, because the time response is SLDs that included process damping ranges (low cutting bounded, the process can become significantly unstable and speed) and high cutting speeds. This method defined the chaotic in the frequency domain, which can lead to geo- stability boundaries by radial rather than axial depth of cut, metric errors and tool damages due to chatter. Liu et al. because of the approach taken by computer-aided modeling [122] developed a novel simultaneous time-frequency toolpath programs. This work is particularly significant in control theory to regulate and counteract the various non- defining machining parameters for difficult-to-machine linear dynamic instabilities including chatter and tool res- materials. For these materials, high tool wear prohibits high onance. This model controls the dynamic response of the spindle speeds, which therefore leads to smaller system under various axial depths of cut and spindle speeds stable zones in the SLDs. Park and Rahnama [117] to prevent an unstable state of motion. The simultaneous obtained micro-milling tool tip dynamics indirectly time-frequency control model was found to demonstrate through mathematical coupling of the substructures using the capability of reducing chatter and process instabilities the receptance coupling method. Song et al. [118] and during micro-milling operations, leading to improved tool Tajalli et al. [119] drew an SLD using a semi-discretized performance and machined surface quality. numerical approach to predict chatter stability based on Chatter worsens the machining precision and efficiency, cutting force. However, the authors noted that further study as well as tool and surface integrity [123]. By under- was necessary to investigate how the burr formation standing the dynamics of the system through analysis of the mechanism would affect the process stability, which led to cutting forces and developing accurate prediction models, chatter. Lu et al. [120] provided a clear basis for the the effect of chatter on the micro-milling process can be dynamic study of the tool-toolholder-spindle system based minimized, if not entirely eliminated. Using SLDs, suit- on receptance coupling substructure analysis and by able machining parameters can be chosen to avoid chatter 123 Precision micro-milling process: state of the art 187 and unstable machining conditions. To obtain such dia- temperature increases considerably with the increase in grams, it is suggested that both time and frequency spindle speed [88, 125]. Another important parameter to domains should be considered simultaneously to control evaluate the effectiveness of the micro-milling process is the process. surface quality, often examined as surface roughness, burr formation, and remaining artefacts of the micro-milling tool on the machined surface. Lu et al. [126] established a 3 Process inputs comprehensive floor surface model to predict both the surface roughness and such artefacts or grooves under As with any manufacturing process, the input variables, different cutting parameters and tool parameters. It was such as workpiece material, tool parameters, toolpath, and found that surface roughness decreased first and then cutting fluid, are all widely known to affect the process increased with the increase in spindle speed, but it outputs of cutting force, surface quality, tool wear, etc. The increased with the increase in feed per tooth and depth of influence and multifactor effect of the process inputs have cut. However, as noted by the authors, to obtain the indi- even more significance at the microscale, particularly when vidual and combined interaction effects of each cutting the microstructure of the tool and workpiece must be parameter and how they influence the process outputs, considered and they are of major concern to the micro- more experimental data are required. Lu et al. [127] further milling process performance. Therefore, it is essential to developed this study through an analysis on the effects of fully understand the process physics and characterize the spindle speed, radius of a ball end mill, axial cutting depth, effect of each variable in a systematic way, so that the input and feed per tooth on the curved surface roughness. parameters that have considerable influences on the output Moreover, they also built a surface roughness prediction quality can be easily recognized and accounted for. In model to provide an accurate reference for the selection of theory, the optimization of the process for all machining cutting parameters in the micro-milling of Inconel 718. variables appears to be very complex. However, consid- All of this important research work concerning micro- erable achievements focusing on predictive models, milling has been carried out with the goal of advancing the numerical simulations, statistical analysis, and experi- micro-milling process in terms of efficiency and produc- mental investigations have been made recently, such that tivity, so that it may develop new applications across new the multitude of factors influencing the process outputs can industries. Therefore, a method for quantitatively analyzing now be examined accurately. Therefore, a full review of the efficiency during experimental work is essential to existing investigations on input variables and their effect determine the progression of the micro-milling process. on the micro-milling process outputs is presented. The material removal rate, which is a measure of the amount of material removed per unit time when performing 3.1 Process outputs machining operations, is often used to do so. Lu et al. [128, 129] established an optimization approach based on Firstly, a brief introduction and investigation of major genetic algorithm to achieve the maximum material process outputs are necessary to understand how they are removal rate under the constraints of surface roughness and influenced by process inputs by examining the predictive cutter breakage. Peng et al. [125]. determined that when the models, as well as theoretical and experimental works rate of material removal was the same, a higher spindle presented recently, so that the optimal micro-milling pro- speed was better for reducing surface deformation. Simi- cess can be determined for a range of difficult-to-machine larly, when the spindle speed is the same, a higher material materials. Beginning with the cutting force, it has been removal rate is better for reducing deformation. To shown that typically within the range of selected cutting improve the machining efficiency, selecting a high spindle parameters, the spindle speed has a relatively weak influ- speed and feed rate has a great significance in promoting ence on the cutting force, while it tends to increase initially the workpiece quality in micro-milling. An undesirable when the feed per tooth is close to the radius of the cutting process output that can occur in metals with a crystal edge, leading to a dominant ploughing mode of material structure is work hardening, also known as strain harden- removal. It then tends to decrease almost linearly with a ing. The strengthening of the material is due to dislocation further increase in feed per tooth, while the shearing mode movements and dislocation generation within the crystal of material removal dominates [74, 88, 124]. The cutting structure of the material when it is strained beyond its yield force also clearly increases with an increase in depth and point. An increasing stress is then necessary to produce width of cut, as does the cutting temperature. In this regard, additional plastic deformation, leading to significant tool the cutting temperature also tends to increase at first but wear, higher cutting forces, higher cutting temperature, and then decreases with the increase in feed per tooth, where overall lower machining efficiency during micro-milling. the turning point is at the UCT. In contrast, the cutting Lu et al. [130, 131] used 3D FE analysis for simulating the 123 188 L. O’Toole et al. process of micro-milling to predict the surface hardness of team observed lower cutting forces, lower BUE, and both Inconel 718 and a nickel-based superalloy. With reduced tool wear for the fully lamellar microstructures. regard to other process outputs, the team then studied the Understanding the variation in cutting force is essential in influence of cutting parameters, including the spindle developing a more complete model of the excitation and speed, feed rate per tooth, and axial cutting depth on sur- process stability between the tool and the workpiece. This face Vickers hardness, as well as the relationship between will lead to further development of the micro-milling strain and hardness [132, 133]. According to their analysis, process as a whole and offer insights into better the spindle speed has the greatest influence on Vickers microstructure design when micro-milling. With regard to hardness, whereas the axial cutting depth has an interme- surface roughness, Elkaseer et al. [136] presented a model diate influence, while the feed per tooth has the least to simulate the surface generation process in micro-milling influence. Their work can help guide the selection of cut- of multiphase materials. They confirmed that their devel- ting parameters to reduce surface work hardening, and oped model could be used to predict the surface quality thereby improve the quality of the final product. Clearly, after machining under various machining parameters and selection of appropriate cutting parameters prior to micro- could further be used to optimize the process for multi- milling operations is essential for improving machining phase materials. An important feature of the model is that it efficiency and quality, prolonging the tool life and main- considers micro-burrs at the phase boundaries. taining good surface quality. However, the micro-milling Concerning workpiece microstructure characteristics, process can only be optimized to a certain degree through Ahmadi et al. [39] investigated the influence of grain size, selection of machining parameters. Therefore, a detailed grain boundary, and phase fractions in the micro-milling of investigation and discussion of the other major process Ti-6Al-4V on the process outputs. A smaller grain size inputs, namely the workpiece microstructure, the micro- (both a and b) and lower b phase fraction led to a higher tools themselves, the toolpath, and cutting fluid, are nec- cutting force in micro-milling. Although, the hardness of essary to develop better understanding of the process as a the sample containing enlarged equiaxed grains was found whole. to be higher owing to the greater b phase fraction as dis- played in indentation tests (see Fig. 11), it experienced a 3.2 Workpiece microstructure lower cutting force as a result of its lower ductility. Moreover, the team found that the microstructure could The influences of the microstructure of multiphase mate- greatly affect the BUE formation in terms of size and rials and the process outputs in micro-milling require very shape; therefore, lower grain sizes can result in more detailed investigations to accurately develop robust ana- BUEs. Aksin and Karpat [137] also investigated the influence of microstructure on the process outputs as a lytical models, because the workpiece can no longer be described as homogeneous at the microscale. Better models function of grain size and grain morphology on commer- and understanding of the material microstructure and its cially pure titanium using their developed mechanistic machinability will help develop more accurate predictive model. The microstructure was modified using heat treat- models to avoid tool wear, improve surface quality, etc., ment methods, so that a gradual transition from acicular to during the process design and machining phases [39]. equiaxed grain morphology was obtained. They also Clearly, the anisotropic behavior of multiphase material microstructures is an important factor that must be con- sidered throughout the machining process when the size effect and chip formation mechanisms are influential at the microscale. Vogler et al. [134] presented a very early significant mechanistic model for the micro-milling process that explicitly accounted for different phases of heterogeneous materials. This model explicitly considers the multiple phases and the effect of determining the magnitude and variation in cutting force. The team showed that the microstructural effects could account for more than 35% of the energy in the cutting force signal. Attanasio et al. [135] Fig. 11 Indentation marks on a and b phases showing relevant depths of indentation, which are used to identify areas of different also investigated the influence of material microstructures microhardness due to phase change (Adapted and reprinted from on the cutting force, with a detailed examination of four ‘‘Microstructure effects on process outputs in microscale milling of different microstructures, namely bimodal, fully equiaxed, heat treated Ti-6Al-4V alloys’’ by Ahmadi et al. [39], with permission fully lamellar, and mill annealed, of Ti-6Al-4V alloy. The from Elsevier) 123 Precision micro-milling process: state of the art 189 established that as the microstructure becomes more tool appeared blunt and grinding grooves could clearly be equiaxed, the hardness increased. However, unlike those of observed. This shows that the tool diameter cannot be Ahmadi et al., their results showed increased cutting forces reduced by only scaling the tool geometry, which supports in this case. Elkaseer et al. [138] examined the effects of the conclusion that the microgeometry and microstructure material homogeneity of copper (Cu99.9E) on the mini- of the tool must be adapted to accommodate even smaller mum UCT and showed that by refining the material diameter cutting edges. Finally, the team determined that microstructure, the minimum UCT could be reduced. It the machining parameters must also be optimized to ensure was also confirmed that material homogeneity improve- a high-quality cutting edge surface and overcome the size ments led to a reduction in surface roughness and surface effect. Similarly, Cheng et al. [142] presented a thorough defects in micro-milling. study on micro-milling tools, noting how commercially It is evident that the resulting surface integrity after available tooling was generally downscaled from macro- micro-milling is highly dependent on the material milling tools that were not accurately fabricated nor microstructure, especially for multiphase materials. entirely suitable. Therefore, the team proposed a design Therefore, deeper consideration of the cutting conditions criterion for custom micro-milling tools and developed a and material microstructure must be given prior to micro- new micro-hexagonal end mill, fabricated using wire milling operations, and the analytical models introduced electrical discharge machining (EDM) based on their above can help significantly. Good understanding of these considerations. Their developed tool achieved submicron relationships will lead to future development of more surface roughness values for the side and bottom surfaces. accurate microstructure-based predictive models of the Most importantly, Fang et al. [143] determined that the tool micro-milling process based on computational techniques. tip rigidity of a semi-circle-based (D-type) end mill was much higher than that of a two-flute (commercial type) end 3.3 Microtools mill. This shows that tool geometry plays a major role in tool stiffness in micro-milling, which is important in The continuing trend toward smaller feature sizes in micro- reducing tool deflection, cutting force, and therefore tool milling with higher precision and accuracy has led to wear. demands for higher quality microtools, as the cutting tool Adhesion of material on the cutting edge of microtools edge radius defines the minimum UCT [139]. It was shown can quickly lead to surface quality deterioration, as by Kirsch et al. [140] that the material specifications of tool reported by Katahira et al. [144], who performed ultra- blanks highly influenced the quality and application of precision machining of a single crystalline sapphire using a ultra-small microtools in the range of 4–50 lm. Generally, polycrystalline diamond (PCD) micro-milling tool. To restore the milling capabilities of the PCD tool, the team microtools are manufactured via grinding operations fol- lowing the famous procedure by Aurich et al. [141] for the implemented an electrochemical-assisted surface recondi- design and machining of single-edge micro end mill tools tioning process to remove the surface contaminant and with diameters between 10 lm and 50 lm and a variable restore the machining performance of the PCD micro- helix angle. Cemented carbides, such as tungsten carbide, milling tool. Adhesion of material on the cutting edge of are predominantly used as micro-milling tool materials microtools can be prevented by coatings. It was shown by owing to their high stiffness, hardness, and resistance to Swain et al. [145] through a direct comparison between wear. It was shown how sharper and more homogeneous TiAlN-coated and uncoated tungsten carbide micro-milling cutting edges without breakouts might be achieved with tools that TiAlN-coated tools exhibited superior perfor- smaller grain sizes of cemented carbide, while the appli- mance in terms of tool life and micro-burr formation. On cation of these tools generated smaller cutting forces and the other hand, Thepsonthi and Ozel [146] attempted to resulted in a considerably longer tool life. The quality of improve the performance of carbide micro-milling tools by the manufactured tool may depend on the material, overall applying a cubic boron nitride (cBN) coating to the end geometry, cutting edge radius, surface conditions, and mill tools. Their study clearly showed that the cBN-coated coating, while the tool design influences the dimensional carbide tool greatly outperformed the uncoated carbide tool accuracy, surface quality, burr formation, and tool life in terms of tool wear and cutting temperature. [142]. Therefore, it is extremely important to thoroughly As for recent advances in tool materials, Suzuki et al. investigate all possible factors and influences that the [147] developed and manufactured micro-milling tools micro-milling tool may have on the machining process. from binderless ultra-hard nano-PCD (NPCD) to machine In relation to microtool geometry, Kirsch et al. [140] silicon carbide (SiC) molds using laser fabrication tech- discovered that although their 50 lm diameter cutting tool niques. The NPCD consists of very fine grains having a provided defined sharp cutting edges and faces that length of several tens of nanometers and is harder and more appeared smooth, the cutting edges of the 10 lm diameter thermally stable than conventional PCD. It was 123 190 L. O’Toole et al. Fig. 12 Surface topography of the micro-slot middle region at the cutting length a 1 mm, b 166 mm, c 249 mm, and d 332 mm (Adapted and reprinted from ‘‘Effect of the progressive tool wear on surface topography and chip formation in micro-milling of Ti-6Al-4V using Ti(C7N3)- based cermet Micro-mill’’ by Wang et al. [150], with permission from Elsevier) demonstrated that the tool wear was exceedingly small in Fig. 12. On the other hand, surface quality at the up- compared to that of PCD tools, while a microtextured milling side was better than that at the down-milling side. surface with very fine textures was created on the SiC mold Currently, the smallest commercially available micro- in the ductile mode of material removal. Another material milling tools have diameters of 50 lm, with minimum commonly used for micro-milling tools is chemical vapor achievable machined features depending on workpiece deposition (CVD) diamond owing to its exceptional hard- material, machine tool accuracy, and feature geometry ness and wear resistance. However, the fabrication of such [151]. Therefore, further research and more in-depth tools by conventional grinding processes is inefficient in investigations will be necessary to explore the geometry of terms of productivity and edge quality. Yang et al. [148] more efficient tools, examine the limits of tool and feature developed a novel hybrid machining process that combined aspect ratio, and move toward submicron tool diameters in laser-induced diamond graphitization with precision the distant future. In addition, the influence of ultra-thin grinding to attain high-quality CVD diamond micro-mil- coatings and tool reconditioning processes on micro-mil- ling tools. Zou et al. [149] presented a micro-milling ling tools will be interesting areas for further research in investigation of Ti(C N )-based cermet tools developed in- the future. 7 3 house, taking into account the wear forms and wear mechanisms [150]. Their results showed that adhesive wear 3.4 Toolpath and microchipping were the main wear mechanisms of the major and minor cutting edges, respectively. It was also Thepsonthi and Ozel [152] proposed an integrated method demonstrated that metal debris and plastic side flow for selecting both toolpath and optimum process parame- became more severe as the tool wear progressed, as shown ters to meet certain machining requirements and 123 Precision micro-milling process: state of the art 191 constraints. The method outlined considers a mathematical cutting process. Therefore, depending on the toolpath, this model for determining the optimum toolpath strategy by may lead to two material removal regimes, i.e., where the using data from their experiments and FE simulations. cutting edge removes material by the shear mechanism and Optimal toolpath and process parameters can be used to where the tool center extrudes material through the establish more accurate predictive models by considering ploughing mechanism, as investigated by de Souza et al. and maintaining an acceptable tool-workpiece engagement [157]. Again, this effect will be even more substantial at load. However, it was verified that the resultant optimal the micro-milling scale, especially leading toward freeform toolpath strategy could only determine a certain level of micro-milling. Similarly, modeling the cutter envelope process performance, while a more in-depth study would surface is another important aspect as it can be used to be needed to ensure burr free micro-milling. Finally, the predict geometric errors and optimize toolpaths in con- team demonstrated that the toolpath strategy strongly ventional machining, according to Guo et al. [103]. affected tool wear and burr formation, while FE simula- Therefore, compensating tool runout errors during micro- tions provided an effective platform for toolpath selection. milling in toolpath planning may be helpful for maintaining An analysis of micro-milling vibration minimization and process stability in the future for 5-axis micro-milling. It is surface quality was presented by Wojciechowski and very surprising that toolpath planning and optimization of Mrozek [153], who used ball nose end mill tools at various the micro-milling process remain so underdeveloped at this tool axis inclination angles along the toolpath. They time, with conflicting results and conclusions on its effect showed that the tools axis of inclination in the direction as reported above. Toolpath development must become an perpendicular to the feed motion significantly affected both important area for future research to really advance the the dynamics of the process and the surface quality. micro-milling process as a whole, particularly for freeform Decreasing the inclination angle caused nonlinear growth machining, so that it will find further application in of vibration amplitude and surface roughness. The findings industries such as optics and biomedical devices were attributed to the ploughing-dominant regime resulting manufacturing. in growth of the cutting edge forces at low angles of inclination. 3.5 Cutting fluid For thin-walled structures less than 100 lm, it was found by Annoni et al. [154] that the down-milling strategy was Cutting fluids are essential to all cutting processes. They more influential with regard to geometrical errors, such as can fall into categories of coolant, lubricant, or both. They flatness deviation and average thickness error, compared to are used extensively to supply a steady flow of fluid into the up-milling strategy. Their results also showed that the the working area to cool, lubricate, flush away chips, reduce friction forces, etc. This leads to increased tool and toolpath factor did not influence the geometrical response. However, they recommended the application of step sup- machine tool life, improved surface quality, effective chip port, i.e., removing material from either side of the wall in management, and more efficient machining. Other desir- an offset technique and using the up-milling strategy. able properties of a cutting fluid are being nontoxic and Zariatin et al. [155] determined that there was no specific safe to handle, while preventing any chemical corrosion or correlation found among spindle speed, feed rate, and degradation of the tools or components [158]. Cutting machining strategy with the thin-wall accuracy [155]. fluids can be applied to the working area in various ways, Regarding path strategies, Koklu and Basmaci [156] such as by compressed air as minimum quantity mist, in a presented a study on the influence of cutting path on the flooding process, and at high and low pressure. Fang et al. cutting force and surface quality during micro-milling [159] even introduced chlorine mist and cooled air with pocket operations through analysis of the hatch zigzag and success. Koklu and Basmaci [156] implemented flood contour climb toolpath strategies under different cooling coolant during micro-milling and it was shown that the tool conditions. It was revealed that better results of up to 40% marks were homogeneously formed, while the deteriora- reduction in cutting forces and better surface quality were tion of the machined surface was minimized. However, this obtained with the use of the contour climb, also known as process of applying cutting fluid to the working area is down milling, compared to those of the hatch zigzag inefficient, expensive, and can cause negative effects to strategy for AA 5083 H116 aluminum alloy. These results operator’s health as well as the environment through bac- contradicted those of Annoni et al. [154] who recom- teria and fungi growth, which leads to bad odor, dissocia- mended conventional, also known as up milling, on 0.4% tion of emulsion, reduction in lubrication, and spread of carbon steel (C40). diseases [160]. In milling of freeform surfaces using ball nose end mill In terms of reducing the required amount of cutting tools, it may not always be possible to maintain the incli- fluid, Li and Chou [161] analyzed the performance of the nation angle so that the tool center is taking part in the minimum quantity lubrication (MQL) technique, as 123 192 L. O’Toole et al. depicted in Fig. 15b, in near micro-milling with respect to thermal expansion of the workpiece, as well as metallur- dry cutting on process outputs. It was found that the gical damage to superficial layers [167]. All of these application of MQL substantially improved the tool life, methods are heavily dependent on the material being surface roughness, and burr formation compared dry cut- machined, exact machining process, size of the chip to be ting based on slotting tests with micro end mills on a formed, design and geometry of the tool, etc. However, mesoscale machine tool. Huang et al. [162] used a nano- most of the work carried out so far has been based on fluid/ultrasonic atomization MQL technique with ultrasonic conventional machining processes, such as macro-milling dispersion during micro-milling of SKD11 steel. They and turning. Therefore, further investigations of the MQL compared different MQL nanofluids in terms of effects on process, dry machining, chilled air cutting fluid, and micro-milling cutting force, micro-milling temperature, cryogenic cooling must be carried out specifically for micro-milling tool wear, and surface burr. Pham et al. micro-milling processes under dry cutting conditions, [163] revealed that high-viscosity ionic lubricants provided while maintaining efficient chip removal, low coefficient of a slightly better machined surface and exhibited extremely friction, adequate cooling, and achieving close tolerances. low volatility compared with conventional oils or other The environmental impact of cutting fluids is currently lubricants in the micro-milling process. Javaroni et al. an important consideration in micro-milling; however, it is [160] showed that the conventional cutting fluid provided applicable across all machining processes. It will even better results for the output variables analyzed in advanced become more important in the coming years as industries ceramics grinding compared to those of the MQL process. strive to become more environmentally and economically However, the MQL can still present satisfactory results sustainable. This has led to interesting areas for future considering the economic, health, and environmental ben- research in micro-milling. For example, Chen et al. [168] efits offered by this technique. MQL can also greatly developed a novel electrochemical micromembrane tech- reduce the consumption of cutting fluid, thereby reducing nology to demulsify oily wastewater and recover oil from environmental pollution and associated costs due to lower oil-in-water cutting fluids. Similarly, Shen et al. [169] volume requirement, subsequent post-processing, disposal, presented an approach to recover cutting fluids and SiC etc. In addition, MQL can also enhance the ability of cut- from slurry waste. Although eco-friendly cutting fluids ting fluid to enter the cutting zone, which can greatly should be the target to move forward, the importance of improve the cooling and lubrication effects. In general, maintaining the process outputs will remain. Burton et al. MQL is a highly efficient and low-cost cutting fluid tech- [170] conducted an investigation into effectively obtaining nique [164]. a vegetable oil-in-water emulsion through ultrasonic Micro-milling processes may not always require cutting atomization. Their experimental results were very positive, showing lower cutting forces, smaller chip thickness, and fluid flooding, such as when light machining some poly- mers, ceramics, and alloys and when the risk of contami- less burr formation for the micro-milling process. Li et al. nation specifically does not allow it, e.g., machining some [164] further developed this vegetable oil-in-water emul- biomaterials for biomedical implants. Therefore, dry cut- sion-based cutting fluid by dispersing graphene and using ting conditions of nonconventional approaches are neces- the MQL technique, meeting the demands of cleaner and sary to lower the cutting temperature while ensuring sustainable manufacturing. The reduction and recovery of efficient chip evacuation. Effective nonconventional cutting fluid can reduce both the cost and environmental methods include MQL as discussed, dry cutting, chilled air, damage caused by machining, which should be viewed as cryogenic cooling, as well as the use of solid lubricants, all an integral component of every micro-milling manufac- of which have been shown to be viable substitutes to cut- turing chain. However, although the environmental aspect ting fluid while maintaining machining performance [165]. is an important consideration, the focus of research on The application of cryogenic cooling and chilled air can micro-milling cutting fluids should first consider the actually lead to lower cutting forces, surface roughness, lubrication properties to minimize BUE, friction, burr and tool wear during some machining processes owing to formation, ploughing, etc., followed by coolant properties the reduction in the coefficient of friction at the interface of to regulate the temperature at the cutting zone. the tool and chip; however, the opposite can occur as well when the mechanical properties of some materials, such as microhardness, increase under the condition of being 4 Advanced processes cryogenically cooled [166]. Dry machining has the benefits of reducing contamination and disposal, and it is the most Currently, the micro-milling process is limited by the environmentally safe option. However, dry machining inherent constraints of cutting material removal mecha- causes problems of high temperature, high friction, oxi- nisms at the microdomain, which include chip formation, dation, the inability to achieve close tolerances due to size effect, and process stability. However, these 123 Precision micro-milling process: state of the art 193 constraints may be overcome by the application and com- direction had a major effect on surface quality, with bination of new technologies with the micro-milling pro- vibration applied in the normal direction improving the cess, such as micro-rotary ultrasonic vibration-assisted machined surface. The vibration assistance also enhanced milling (lRUAM), laser-induced oxidation-assisted micro- the brittle-ductile transition of glass and therefore reduced milling (LOMM), and atmospheric-pressure plasma jet- the surface damage. Finally, they concluded that a higher assisted micro-milling. vibration frequency improved the surface quality by reducing the surface waviness. Bian et al. [174] also con- 4.1 Micro-rotary ultrasonic-assisted machining ducted an experimental investigation on micro-milling of brittle materials, but on ZrO ceramics with diamond- Micro-milling has been shown to be an advantageous coated micro end mills. It was found that the chips formed machining process for manufacturing surfaces, features, in the ductile mode were long and thin curled strips with a and structures in the microdomain with high accuracy and smooth back surface, leading to less edge and surface precision. However, the application of micro-milling in the chipping. However, the compressive forces due to the mold, optics, and biomedical industries requires that this ductile mode of material removal presented an increasing process must be suitable for machining typical difficult-to- trend with random fluctuations of the cutting force, leading machine materials, from very hard and wear resistant to higher tool wear. metallic alloys to very brittle ceramics or deliquescent With respect to very hard and wear resistant metallic crystal materials. One of the major efforts directed toward alloys in particular, Li and Wang [175] recorded lower tool processing these difficult-to-machine materials has been wear and better surface quality when the cutting speed was the application of lRUAM [158]. This process applies an much less than the maximum vibration during lRUAM ultrasonic frequency vibration in the range of 20–100 kHz compared to conventional milling. The material tested was with an amplitude between 5 lm and 50 lm at the tool tip AISI H13, which was suitable for manufacturing molds and in one or more directions, e.g., axially or radially. So far, dies. Xu et al. [176] performed lRUAM research on tita- lRUAM has been shown to reduce cutting forces and nium alloy TC4 and aluminum alloy 6061T6 with ultra- improve tool life during machining, owing to the working sonic vibration in the radial direction. Their experimental mechanisms of material removal, which form smaller chip results also verified that lRUAM could reduce the cutting sizes, reduce contact at the tool-workpiece interface, force, while improving surface quality by reducing reduce frictional forces, and inhibit crack propagation on machining marks. Burr formation was also substantially very brittle materials. However, strict control over the lessened in lRUAM, compared to conventional milling, as machining and vibration parameters, direction of vibration, shown in Fig. 13. Finally, the team determined that the size and the process as a whole is necessary, as tool life can effect appeared at much lower feed rates than in conven- actually be diminished with wrong parameter selection tional micro-milling. They proposed that vibration-assisted [171, 172]. machining at the microdomain triggered a change in the In relation to the abovementioned materials, Jin and Xie material removal mechanism. According to the authors, [173] presented an experimental study on the surface ‘‘impulse impact accelerates the generation and propaga- generation in lRUAM of a BK-7 optical glass using a tion of tiny cracks in the workpiece material, which redu- 2-flute micro end mill tool. They showed that the vibration ces the binding force inside the grains of the material.’’ Fig. 13 Machined surface after ultrasonic vibration micro-milling experiments at amplitudes of a 0 lm, b 2 lm, and c 4 lm, showing that ultrasonic vibration-assisted micro-milling can reduce surface defects and machining marks and thus improve the surface quality (Reprinted from ‘‘Machinablity improvement with ultrasonic vibration–assisted micro-milling’’ by Xu et al. [176], with permission from Sage) 123 194 L. O’Toole et al. Although this may be significant in reducing and elimi- opportunities for theoretical and experimental works yet to nating the size effect in micro-milling, an in-depth analysis be presented. will be necessary for future work. Feng et al. [177] pro- posed a predictive model to estimate flank tool wear to a 4.3 Atmospheric plasma jet-assisted micro-milling high accuracy. They found that a smaller axial depth of cut, larger feed per tooth, or higher cutting speed would result Atmospheric-pressure plasma jet-assisted micro-milling is in higher flank wear rate, while the effects of the vibration another underdeveloped assisted micro-milling process, parameters were less significant. Clearly, lRUAM is an which was proposed by Katahira et al. [183]. The team interesting area in the development of micro-milling, as it performed a feasibility study to investigate the effects of may reduce some inherent limitations of the current con- the application of an atmospheric-pressure plasma jet ventional micro-milling process. Much work needs to be during PCD micro end milling, which compared machined carried out concerning the physical process as well as SiC surfaces for both with and without the application of process inputs, while research will now require focusing on plasma jet. It was revealed that with the plasma jet, the more theoretical work rather than experimental work to formation of a high-quality surface was possible. More- fully characterize the process. over, it was also highly effective in improving the chip formation process by imparting hydrophilicity to the tool 4.2 Laser-induced oxidation-assisted micro-milling and workpiece surfaces, as well as removing surface con- tamination at the tool edge during machining. However, no LOMM is derived from laser-assisted micro-milling additional work had been presented on this process until (LAMM). LAMM combines the mechanical process of Mustafa et al. [184] very recently. They also confirmed that micro-milling with highly localized thermal softening of atmospheric-pressure plasma jet-assisted micro-milling the hard material by continuous wave laser irradiation. was a promising assisted technology with respect to the Subsequently, the softened material is removed by micro- micro-milling process, as it provided the lowest surface milling [178]. Compared to the conventional micro-milling roughness values among various cutting environments: dry, process, the cutting force in LAMM is substantially nitrogen jet, plasma jet, MQL, and plasma jet combined decreased and the tool life is prolonged. However, a high with MQL (see Fig. 15). The material tested was Inconel laser power is required to soften hard materials such as 718. It was determined that the plasma jet could promote ceramics. This would result in the ablation of workpiece fracture of the nickel surfaces and therefore reduce the material, expansion of heat affected zone, and formation of cutting force. However, it was demonstrated that the microcracks [179]. Therefore, Yang et al. [179] proposed residual stresses in micro-milled machined surfaces were compressive, and atmospheric-pressure plasma jet tended the novel process of LOMM, which used a relatively low- power laser to irradiate the surface of a ceramic material. to increase such compressive residual stresses. Again, far An oxidation reaction between the ceramic material and more work needs to be carried out to begin characterizing oxygen occurs, forming a loose and porous oxide layer, this process, which has great potential for reducing the which can be removed easily through the mechanical inherent issues of machining in the microdomain. process of micro-milling with a low cutting force there- after. Compared to the conventional micro-milling, the surface quality by LOMM was better, and the machining 5 Applications efficiency was improved by 104%. Xia et al. [180, 181] also presented a study on Ti-6Al-4V using this novel The development of machine tools and the manufacturing process. They showed that LOMM effectively decreased technology as a whole has led to high-precision micro- the cutting force and tool wear and prolonged the service milling processes in both research and industrial fields. The life of the tool. They verified that the cutting force when increasing demand for micro-structured parts and products removing the oxide layer in LOMM was 50%–65% lower with functional surfaces requires enhancing the process than when removing the material in conventional micro- efficiency to develop new technologies and improve milling under the same cutting parameters. It was also existing ones, so that a faster and more reliable production noted that the top burr width of the machined microgroove can be achieved [185]. The application of the micro-mil- and tool wear were smaller by LOMM. Wu et al. [182] also ling process ranges from fabrication of microstructures and confirmed that far less tool wear occurred for LOMM micro-components to micro-texturing and mold manufac- compared to conventional micro-milling, as shown in turing for industries such as electronics, aerospace, aero- Fig. 14. This is a very new and promising area of research nautics, and biomedicine. for the micro-milling industry, with considerable 123 Precision micro-milling process: state of the art 195 Fig. 14 Tool wear process with different material removal volumes: a with laser-induced oxidation and b without laser-induced oxidation (Reprinted from ‘‘Laser-induced oxidation of cemented carbide during micro-milling’’ by Wu et al. [182], with permission from Elsevier) nanometers [84, 186, 187]. The functionality of compo- nents can be improved by surface modifications such as microstructures using the micro-milling process. These structures can cause changes in the mechanical properties of the components, as reported by Godart et al. [188], who determined that 50 lm wide microstructures with a depth of 10–20 lm could increase tensile strength and decrease the fracture elongation in commercially pure-titanium workpieces. A basic example of such structure is a micro- thin wall, which usually refers to a cantilever structure with a thickness below 100 lm and a height to thickness ratio greater than 10 [189]. Accurate and precise removal of Fig. 15 Processes of machining Inconel 718 alloy a plasma-assisted material to form micro-thin wall structures is very difficult micro-milling process and b MQL process (Reprinted from ‘‘Atmo- spheric pressure plasma jet-assisted micro-milling of Inconel 718’’ by to accomplish in reality, especially for metallic alloys that Mustafa et al. [184], with permission from Springer Nature) tend to deform plastically when the wall thickness is in microns, as exhibited in Fig. 16. As the thickness of the wall is decreased, failure of the microstructure begins to 5.1 Micro-structures occur owing to the wall thickness exceeding the material threshold of rigidity and strength. Other microstructures One of the earliest and most employed application areas for include pipes, blades of an impeller or turbine, walls of a the micro-milling process is in microstructure and micro- microchannel, microcolumns, and fins of a heat exchanger. part fabrication. Microdevices can be defined as having at At present, these microstructures have been widely applied least two critical dimensions in the sub-millimeter range in micro-fuel cells [190], microfluidic chip channels [191], with at least one critical dimension significantly smaller and EDM electrodes [192, 193]. than 0.1 mm and with tolerance ranges of a few microns to 123 196 L. O’Toole et al. Fig. 16 Thin-wall features of 900 um in height (Reprinted from ‘‘Experimental study on micro-milling of thin walls’’ by Wang et al. [201], with permission from IOP) In the energy and electronics industries, the enhance- microstructures remain to be major issues that need to be ment in efficiency of heat transfer devices is crucial as the addressed. trend toward miniaturization of devices requires better understanding of heat transfer in small dimensions [194]. 5.2 Micro-texturing Repeated rib surfaces are known for their effectiveness in enhancing heat transfer and they are widely required in Another major application of the micro-milling process is many scientific and industrial applications. Further in micro-texturing or micro-patterning to reduce frictional improvements to these microstructures were made by forces and reduce wear between parts in industries such as Wang et al. through the introduction of a textured asym- automotive and biomedicine. As summarized by Chen metric arc rib structure on which microstructure arrays of et al. [202], functional microtextured surfaces have high secondary microgrooves were superimposed [195]. It was aspect ratio features, which enable the component to have then verified by Zhao et al. [196] that these hierarchical superior properties such as reduced adhesion friction [203], microstructures, composed of a primary microstructure and improved lubricity [204], increased wear resistance [205], secondary micro V-grooves, could be machined well by an ability to manipulate hydrophilic performance [206], as ultra-precision micro-milling process using a one-step well as influence optical properties [207]. Three possible cutting operation and a diamond tool. mechanisms by which surface micro-texturing improves Another application of microstructures that can be pro- tribological performance, as outlined by Chen et al. [208], duced by micro-milling is for mass sensing in microelec- are described as follows. Firstly, the textured surface can tromechanical system (MEMS) devices [197], which are increase the load-carrying capacity by serving as micro- used in the telecoms market, e.g., mobile phones and hydrodynamic bearings for hydrodynamic lubrication optical modulators [198], using lithium niobate (LiNbO ) [209]. Secondly, surface micro-textures can act as a second material. LiNbO is a crystalline material also often used in lubricant source to permeate the surface and reduce friction surface acoustic wave sensors and optical drives owing to and wear between both surfaces, creating a lubrication its superior electrical, optical, and physical properties boundary [210]. Finally, surface micro-textures can reduce [199]. However, because of its low toughness, it is con- the ploughing induced by abrasive wear and deformation sidered a difficult-to-machine material, which has con- between components by capturing wear debris between the ventionally been used as a substrate with deposited texture features [211]. It was also shown by Kovalchenko microstructures, rather than machined. However, owing to et al. [212] that arrayed dimples on contact surfaces under the ever-increasing demand for higher efficiency, direct lubrication helped to establish hydrodynamic pressure and fabrication of structures on the surface of LiNbO is now decrease the friction force. These three mechanisms of necessary, and this can be accomplished easily by micro- tribological performance improvements have significant milling, according to Huo et al. [200]. Clearly, micro- potential in orthopedic implants for arthroplasty procedures milling is a direct and effective manufacturing method for related to replacement of joints, such as hips, knees, or fabricating microstructures with complex 3D shapes. elbows. It has already been shown that micro-milling is a Nevertheless, the limitations of material deflection, plastic high-precision machining process suitable for difficult-to- deformation, and burr formation during machining of such machine materials, such as titanium alloys, cobalt-chrome 123 Precision micro-milling process: state of the art 197 alloys, and ceramics, all of which are commonly used for the average friction coefficient was reduced by 6.93%, orthopedic implants. while optical micrographs indicated that the microtextured Micro-textures can be fabricated by employing various specimens exhibited the narrowest and shallowest wear techniques, including laser machining [213] and etching track, in comparison to untextured specimens. Micro-mil- [214]. However, the most cost effective and efficient ling using a ball nose end tool is another viable technique method will always be rapid and direct machining owing to for creating such micro-textures, as demonstrated by Gra- the relatively low power consumption, precise and accurate ham et al. [216]. By tilting the tool at an inclined angle, the 5-axis CNC programming, and high surface finish, all of spindle speed and feed rate can be adjusted so that the which are characteristics of the micro-milling process. flutes of the cutter create periodic patterns in a workpiece Although the application of micro-milling to fit this pur- surface. However, the problem of burr formation associated pose is relatively new, there has been some interesting with the machining of microgrooves and micropatterns works presented lately. For example, Syahputra and Ko remains an important issue. In an attempt to solve this [215] developed a rapid process for acquiring complex problem, Fang et al. [217] studied the effects caused by texture data by using an image processing technique for the cutting parameters, work material, and cutting methodol- micro-milling process in which a complex surface texture ogy. However, more detailed work into tool geometry will representing human skin is transferred to a metal surface. be necessary to limit burr formation during micro-milling. Similarly, Chen et al. [208] concluded that the friction Evidently, micro-milling is an efficient and versatile performance of micro-milled Al-Si alloy ZL10 surfaces manufacturing technique, ideal for rapid machining of could be enhanced by malposed rectangle dimple micro- micro-textures and micropatterns across a broad range of textures, as illustrated in Fig. 17. Their results showed that materials and industries. However, the limited research Fig. 17 Microtexture SEM profiles with different distribution angles: a h =15, b h =45, c h =60, and d h =90 (Reprinted from ‘‘Effects of micro-milled malposed dimple structures on tribological behavior of Al-Si alloy under droplet lubricant condition’’ by Chen et al. [208], with permission from Springer Nature) 123 198 L. O’Toole et al. available at this time indicates a significant research gap, manufacturing perspective, the micro-milling process which must be filled so that this precision machining pro- allows for rapid prototyping of microfluidic devices [222]. cess can be applied in the field of orthopedic implant However, several challenges remain in micro-milling of manufacturing. molds and dies, namely burr formation and thin-wall deformation. Thus, it is important to identify and investi- 5.3 Mold making gate these issues in applying this process in the mold making industry. Saptaji determined that micro-milling is The most significant application of the micro-milling pro- capable of creating the features necessary for a thin cess is in the mold making industry, because it permits microfluidic embossing mold, with a thickness and feature precise, rapid, and accurate machining of high aspect ratio height of approximately 160 lm and 100 lm, respectively microfeatures, such as microchannels, microarrays, and [223]. The appropriate selection of the micro-milling thin walls, as already discussed. These molds are essential strategy is also crucial in achieving designed micro-lens to microinjection molding, micro-hot embossing, and array surfaces, according to Gao et al. [224], while dif- nanolithography industries for the polymer micro/nano ferent machining strategies have different machining sur- mass replication of features and surfaces [218]. The most face textures due to the cutting direction. An important obvious contribution is rapid manufacturing and prototyp- work by Ardila et al. [185] aimed at improving the entire ing of molds and mold inserts through finishing processes micro-milling production chain to generate knowledge of roughed out mold cavities, providing a quick and effi- about related process stages, including potential improve- cient processing time. A distinctive application example of ments of productivity and quality. The team concluded that this is for the microfluidics industry, which is important for to increase the application of micro-milling in the mold the employment of disposable medical sensors. This highly industry, the micro-milling process must satisfy the pro- significant area for micro-milling application in the ductivity and quality standards, confirming that the process microfluidics industry is in functionally optimized surfaces needs further research to comply with these requirements. through patterned microstructures on miniaturized biore- actor components, also known as ‘‘lab-on-a-chip’’, as shown in Fig. 18. The microfluidic chip provides a cheap 6 Conclusions and perspective and disposable platform for production and testing of pharmaceutical and biomedical products [219]. The pos- The high-precision micro-milling process was shown to be sibilities for such microfeatures may also offer an impor- a very effective and versatile manufacturing process, cap- tant role in distinguishing biofilm behavior in the future able of machining a broad range of difficult-to-machine [220]. The miniaturized size also allows for lower power materials for applications that require tight tolerance, good consumption and greater portability, while utilizing smaller surface integrity, and efficient machining across all volumes of reagents and samples, which are extremely industries. Although micro-milling is not a particularly new important to the microfluidics industry [221]. From a manufacturing process and has been the focus of consid- erable research work throughout the years, new and novel applications for this process are constantly being estab- lished in the industry. The flexible and versatile nature of micro-milling has guided this precision process from direct manufacturing of MEMS components to 5-axis CNC machining of precision molds for micro/nano-polymer replication. Currently, further development of the process is being driven by the necessity for rapid and highly accurate micro-patterning and micro-texturing of surfaces, for producing large numbers and arrays of micro-sized features such as dimples and rectangular pockets to increase lubricity and reduce friction forces between wear parts. The driving force behind this latest development is the requirement of the bio-implant industry to produce such features in a precise and efficient manner and to improve Fig. 18 Micro-milling of a lab-on-a-chip microfluidic mold: 4 arrays the tribological performance of orthopedic implants, of 28 pins with 0.8 mm diameter and 2 mm height (Reprinted from thereby extending implant life. Therefore, the development ‘‘Impact of deep cores surface topography generated by micro-milling of efficient and precise micro-milling to produce on the demolding force in microinjection molding’’ by Masato et al. microstructures, micro-textures, and high-quality molds [225], with permission from Elsevier) 123 Precision micro-milling process: state of the art 199 has promoted a sustainable future for the micro-milling insights into further development are considered and sig- process, with interesting new areas and applications to nificant research gaps are identified. overcome current limitations of other technologies. Acknowledgments This work was supported by the National Key However, as mentioned throughout the review, many Research and Development Program (Grant No. 2016YFB1102200), problematic and inherent issues of the micro-milling pro- Science Foundation Ireland (Grant No. 15/RP/B3208) and the ‘‘111’’ cess prevent its application in industries until further Project by the State Administration of Foreign Experts Affairs and the research and investigations are carried out. Such issues Ministry of Education of China (Grant No. B07014). include the phenomena of downscaling machining to the Open Access This article is licensed under a Creative Commons microdomain, i.e., the size effect, chip formation mecha- Attribution 4.0 International License, which permits use, sharing, nisms, and fundamental process instabilities involved. adaptation, distribution and reproduction in any medium or format, as Primarily, burr formation during channel or slot milling is long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate the most critical issue limiting the use of micro-milling in if changes were made. The images or other third party material in this microfluidic chip molds, where undesirable projections of article are included in the article’s Creative Commons licence, unless the material form as a result of the plastic flow from cutting indicated otherwise in a credit line to the material. If material is not and shearing operations. Since post processes such as included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted deburring are costly and are non-value added operations, use, you will need to obtain permission directly from the copyright understanding and control of burr formation are research holder. To view a copy of this licence, visit http://creativecommons. topics with high relevance to industrial applications, with org/licenses/by/4.0/. much work yet to be carried out. Similarly, adhered materials on the tool cutting edge, i.e., BUEs, have a large influence on the machining process outputs. 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He has conducted both fundamental studies and appli- Lorcan O’Toole is a prospec- cation development in the areas tive PhD candidate part of the of micro/nano machining, opti- Centre of Micro/Nano Manu- cal freeform design and manufacturing, and ultra-precision machining facturing Technology (MNMT- and measurement benefiting a variety of industries in medical devices, Dublin) situated in the Engi- bio-implants, optics and mold sectors. neering and Materials Science Centre, in University College Dublin. Lorcan received his Bachelor of Mechanical Engi- neering (BE) from the School of Mechanical Engineering in UCD in 2018 and is furthering his studies in the area of preci- sion machining. In particular, the focus of his PhD is on micro-milling of difficult-to-machine materials. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Advances in Manufacturing Springer Journals

Precision micro-milling process: state of the art

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Abstract

Adv. Manuf. (2021) 9:173–205 https://doi.org/10.1007/s40436-020-00323-0 1 1 1,2 • • Lorcan O’Toole Cheng-Wei Kang Feng-Zhou Fang Received: 8 March 2020 / Revised: 11 June 2020 / Accepted: 30 August 2020 / Published online: 27 October 2020 The Author(s) 2020 Abstract Micro-milling is a precision manufacturing combine conventional micro-milling with other technolo- process with broad applications across the biomedical, gies, which have great prospects in reducing the issues electronics, aerospace, and aeronautical industries owing to related to the physical process phenomena, are also intro- its versatility, capability, economy, and efficiency in a wide duced. Finally, the major applications of this versatile range of materials. In particular, the micro-milling process precision machining process are discussed with important is highly suitable for very precise and accurate machining insights into how the application range may be further of mold prototypes with high aspect ratios in the micro- broadened. domain, as well as for rapid micro-texturing and micro- patterning, which will have great importance in the near Keywords Precision machining  Micro-milling  Size future in bio-implant manufacturing. This is particularly effect  Deflection  Runout  Tool wear true for machining of typical difficult-to-machine materials commonly found in both the mold and orthopedic implant industries. However, inherent physical process constraints 1 Introduction of machining arise as macro-milling is scaled down to the microdomain. This leads to some physical phenomena The trend toward miniaturization of precision micro-com- during micro-milling such as chip formation, size effect, ponents, such as for microelectromechanical, nanoelec- and process instabilities. These dynamic physical process tromechanical, and micro-medical systems, has led to phenomena are introduced and discussed in detail. It is advances in microfabrication techniques in recent years. important to remember that these phenomena have multi- This demand for micro-sized parts with high aspect ratios factor effects during micro-milling, which must be taken has necessitated the biomedical, electronics, automotive, into consideration to maximize the performance of the and aerospace industries to adopt and apply both new and process. The most recent research on the micro-milling old manufacturing processes at the microscale. Although process inputs is discussed in detail from a process output microfabrication techniques have existed for many years, perspective to determine how the process as a whole can be the stringent requirements of extremely tight tolerances on improved. Additionally, newly developed processes that form, dimension, and surface characteristics [1], high machining efficiency, and machine positioning accuracy have led to further developments in precision machining processes [2]. Micro-milling is a precision micromechani- & Feng-Zhou Fang fengzhou.fang@ucd.ie cal cutting process, which has been developed to facilitate the increasing requirements [3]. Center of Micro/Nano Manufacturing Technology (MNMT- Micro-milling is an effective and efficient precision Dublin), University College Dublin, Dublin 4, Ireland machining process for manufacturing components with State Key Laboratory of Precision Measuring Technology microstructures such as complex three-dimensional (3D) and Instruments, Center of Micro/Nano Manufacturing surfaces at the microscale. Typically, micro-milling can be Technology (MNMT), Tianjin University, Tianjin 300072, characterized by the size of the cutting edge diameter of the People’s Republic of China 123 174 L. O’Toole et al. micro-milling tool, which lies between the range of 1 lm chip formation, size effect, and process stability, may not and 1 000 lm[4], whereas the diameter of the cutting edge be overcome by micro-milling alone in the future. in the conventional milling process is greater than 1 000 The application of micro-milling across the biomedical, lm. However, this definition focuses only on the tool and electronics, automotive, and aerospace industries is also does not incorporate the important aspects of the machin- discussed in relation to the versatile nature of the precision ing process, namely precision, accuracy, and underlying machining process. Such application ranges from machin- material removal mechanism. Therefore, a more technical ing of microstructures and textures to precision machining approach to characterize the micro-milling process can be of very hard and wear resistant materials for utilization in as follows: a precision mechanical cutting process with the mold manufacturing industry. Therefore, this review geometrically defined cutting edge tool diameters of less introduces the current state-of-the-art micro-milling pro- than 1 000 lm for precise material chip removal to within cess, beginning with the issues of physical process phe- less than 1 lm tolerance on form and dimensional accuracy nomena associated with machining at the microscale and [5]. including an in-depth examination of how to minimize However, the micro-milling process is limited by these negative issues. The process inputs are examined in inherent physical process issues when machining at the relation to the process outputs, offering insights into areas microscale, which are not present when milling at the for improvement in the future, which will further advance macroscale. Such constraints relate to material removal the development of the micro-milling technology. Finally, mechanisms at the microdomain, which include chip for- applications of the micro-milling process are described in mation, size effect, and process stability. Therefore, the detail, followed by insights into the future perspective of major physical processes that limit the process efficiency micro-milling. The objective of this work is to present the and precision in micro-milling are thoroughly discussed. development, benefits, applications, limitations, and future The effects of these negative phenomena on the machining insights of micro-milling and to discuss the recent publi- process outputs are considered, while insights into how cations regarding this precision machining process. these effects can be minimized, if not eliminated, are presented. Recently, there has been a strong research interest in the 2 Fundamentals of the process micro-milling process, with much work focusing on the process inputs, such as workpiece material and Because the precision micro-milling process has been microstructure, geometry and materials of tools, efficient developed from conventional milling, the two milling toolpath generation, and cutting fluid. Examining the latest processes share many similar characteristics such as works concerning the influence of these process inputs on machine components and configuration, tool geometry, and the cutting force, surface roughness, and tool wear during cutting fluid. However, the material removal mechanisms machining provides a clear depiction of the current micro- between these two mechanical cutting processes cannot be milling process. This review therefore investigates the mutually correlated. Conventional milling primarily con- theoretical, analytical, and experimental works most siders shearing forces acting on the rake face and far lesser recently published to identify areas of the process for future ploughing forces on the flank face [7], which are mainly development. caused by machine chatter stability [8]. The shearing- In terms of process advancement, one of the key areas dominant regime is the desired material removal mecha- will be in supplementing the micro-milling process with nism during any cutting process, where material is removed other successful technologies. Section 4 of this review will as distinctive chips along the rake face. The ploughing- examine the recent successful studies that implemented dominant regime is the unwanted material removal mech- secondary systems to produce advanced processes, such as anism, where material deforms plastically under the flank ultrasonic-assisted, laser-induced oxidation-assisted, and face and no chips are formed. The ploughing-dominant plasma jet-assisted micro-milling processes. The impor- regime results in extremely poor surface finish and very tance of such assisted processes will become even more high tool wear due to high cutting and friction forces, high apparent with future developments of higher hardness and temperature, etc., during machining. In contrast to preci- wear resistant materials. Such materials are classified as sion milling, micro-milling is subject to both considerable ‘‘difficult-to-machine’’ materials, which include hard and ploughing and shearing regimes. The contribution of each wear resistant superalloys, refractory metals, structural mechanism depends heavily on numerous factors, such as ceramics, composites, polymers, and magnesium alloys [6]. chip formation, undeformed chip thickness (UCT), size The consideration is that the limitations of the micro-mil- effect, tool deflections, and process stability. Each of these ling process in terms of physical process constraints, i.e., factors can also significantly influence each other, leading to a more dynamic and complicated effect when 123 Precision micro-milling process: state of the art 175 determining which material removal mechanism will workpiece [15]. However, the minimum UCT will be dominate during micro-milling. mainly affected by the tool geometry and material [16], workpiece material, and microstructure [17]. Generally, the 2.1 Chip formation UCT is developed as a prediction model because it is not necessarily a physical parameter and cannot be identified The minimum chip thickness is the critical limit deter- directly during machining. The results of measurements mining whether the material flows along the rake face, such as cutting force and surface integrity can be examined forming chips as the shearing mode of material removal, or to determine which mechanism of material removal is along the flank face causing elastic or plastic deformation dominant during machining. Up until recently, the depending on the material, as the ploughing mode [9–11]. mechanical models for micro-milling were based on scal- Therefore, it can be defined simply as the minimum UCT, ing conventional milling models with adaptations [18, 19] below which a defined chip cannot be formed stably. This and investigation of single factor influences. As discussed critical value will depend on the process parameters, however, simply reducing the scale from macro to micro material properties, and microstructure [12]. When the does not present a constitutive model. More importantly, UCT is less than the minimum value, chips due to the micro-milling is far more complex owing to numerous ploughing-dominant mode of material removal will not be factors having significant influences on the process. The generated. In contrast, when the UCT is larger than the models for UCT in micro-milling are established to predict minimum value, a defined chip will be generated and the process outputs, such as cutting forces, surface quality, and process can be compared to that of conventional milling [9] temperature, as well as to predict fundamental physical (see Fig. 1). Consequently, there will be no chip removal at processes, including the process stability and material very small depths of cut during micro-milling. Instead, the removal mechanisms. Therefore, to select optimal workpiece will undergo pure elastic deformation when the machining parameters, the material removal behaviour cutting tool passes through the workpiece material, which during micro-milling operations must be fully understood then recovers to the original height. However, with an and implemented by accurate models. The establishment of increase in the depth of cut, the material instead begins to such UCT models in micro-milling is clearly an important plastically deform. With the continuous increase in the research topic to obtain a much higher precision and more depth of cut, the material removal mechanism then begins efficient micro-milling process. to shift from plastic deformation to shear chip formation, if The early work by Son et al. [20] on ultra-precision the minimum UCT approaches a certain threshold. There- diamond cutting found that the minimum UCT was deter- fore, chips can be formed and removed only when the mined by the tool edge radius and the friction coefficient of the workpiece-tool interface. Their work then led to depth of cut exceeds the minimum UCT [13]. The UCT is one of the most important aspects that important research by Malekian et al. [21], who confirmed determine which material removal process will dominate in that the minimum UCT was a function of both the edge micro-milling, and it can be influenced by many factors radius and friction coefficient and was dependent on the [14]. The combined effects of factors such as tool setting tool geometry and properties of the workpiece material. errors and toolholder and spindle errors will result in sig- Through their proposed analytical model based on the nificant tool runout of the cutting edge with respect to the minimum energy principle and infinite shear strain method, Fig. 1 Chip formation mechanism of a macro-milling and b micro-milling in terms of minimum undeformed chip thickness h and cutting min edge radius r (Adapted and reprinted from ‘‘Machining scale: workpiece grain size and surface integrity in micro end milling’’ by Rodrigues and Jasinevicius [28], with permission from Elsevier) 123 176 L. O’Toole et al. the normalized minimum UCT of Al6061 was approxi- Fig. 1. The ductile mode of material removal dominates mated as 0.23 of the edge radius. However, it was noted by when the UCT decreases to sufficiently small values during the authors that the minimum UCT was a range of values, micro-milling, particularly at the submicron level as rather than a single point. This may be attributed to the described above [27]. However, the transition between the stagnation region, instead of a stagnation point, as observed shearing-ploughing modes of material removal remains a by others. Ramos et al. [22] also developed a model for large issue during the micro-milling process, while such estimating the minimum UCT of AISI 1045 based on their factors as the minimum UCT, size effect, effective rake experimental results. The minimum UCT was found to angle, and tool edge radius, all influence the process of chip substantially decrease with higher cutting velocities and to formation, leading to one mode of material removal into moderately increase with higher cutting edge radii. Such another. Therefore, a quantitative identification of the chip prediction models that estimate the minimum UCT are formation process and its influence on other micro-milling important because they can help to minimize the amount of phenomena, such as built-up edge (BUE) and burr forma- ploughing-dominant material removal and offer the opti- tion, is a crucial aspect for research to move forward, for all mum cutting conditions. One example of the importance of types of materials [13]. The scientific and systematic prediction models is when working with materials such as understanding of the multifactor effect will become even magnesium, where the risk of fire is a major concern during more significant in the future, particularly when dealing high-speed cutting, because magnesium in the molten state with the increasingly stringent requirements for industrial is flammable when exposed to oxygen. Therefore, accurate scale applications of the micro-milling process. models to predict cutting temperature at the flank face in relation to the UCT are very important, as determined by 2.2 BUE Fang et al. [23]. Chen at al. [11] developed a model of chip formation, When ductile materials, such as aluminum, steel, and even which was capable of connecting the minimum UCT, UCT, some titanium alloys, are machined using the micro-mil- and periodicity of cutting force together. Their model can ling process, BUEs can be observed on the rake face of the predict the normalized value of minimum UCT (k ), which tool, as illustrated in Fig. 2. This is due to the adhesion of represents the ratio of the minimum UCT to the cutting chips or material onto the cutting tool face, which greatly edge radius r . They estimated this value to be 0.43 B k B affects the process outputs and has a significant negative e e 0.48 for cutting edge radii between 2 lm and 3 lm for effect on surface roughness, also causing such problems as potassium dihydrogen phosphate crystal, which was higher cutting forces and shorter tool life [29]. Because the another difficult-to-machine material owing to its proper- BUE periodically develops and breaks off the tool rake ties of being soft, brittle and deliquescent. Their systematic face, the UCT is affected, which further leads to poor work could serve as a reference for similar works on other surface quality as well as deposits and smeared regions on difficult-to-machine materials, and obtained results could the machined surface. The BUE is typically more promi- potentially guide the selection of cutting parameters and nent when using lower cutting speeds, such as in conven- cutting edge radii for improving the integrity and quality of tional milling. However, it remains an important issue in machined surfaces in the micro-milling of other brittle micro-milling, where even small deposits of adhered materials. material on the cutting face will have considerable negative Recently, Lu et al. [24] investigated the tool trajectory in effects. micro-milling, with the aim of building a more accurate The BUE formation in metal machining has been a well- UCT prediction model while taking into consideration known phenomenon, with research into its unwanted radial tool runout on the cutting edge as well as deter- effects beginning even before the 1970s [30]. Similarly, mining the effect of tool setting errors on the UCT. much work has been conducted on the BUE in conven- Comparisons of cutting forces under this UCT model with tional milling more recently, such as Children’ work experimental data indicated that their model could be used toward simulating this phenomenon [31] or Ozcatalbas’ to accurately predict cutting forces during the micro-mil- study of orthogonal cutting, which indicated that the BUE ling process, offering theoretical insights into micro-mil- affected the chip formation and cutting ratio for different ling force models for further study. The construction of cutting conditions [32]. However, the influence of BUE on such accurate, instantaneous undeformed cutting thickness the micro-milling process has not yet been characterized in models is important to further establish cutting force detail. Thepsonthi and Ozel [33] carried out investigations models. on 3D finite element (FE) modeling and simulation of the The transition of material removal mechanism from micro end milling process for Ti-6Al-4V to determine the shearing to ploughing is an important phenomenon when influence of increasing tool edge radius due to wear on the machining at the microscale [25, 26], as can be seen from process performance. They found that the BUE might be 123 Precision micro-milling process: state of the art 177 Fig. 2 Scanning electron microscopy (SEM) images of both flutes of a micro-milling tool rake face exhibiting uneven BUE on each flute a Edge 1 and b Edge 2 (Adapted and reprinted from ‘‘Microstructure effects on process outputs in microscale milling of heat treated Ti-6Al-4V titanium alloys’’ by Ahmadi et al. [39], with permission from Elsevier) formed after the tool was severely worn. More recently, diamond-like carbon (DLC) coating could be used in Wang et al. [34] presented one of the first experimental micro-milling of Inconel 718 to substantially reduce the investigations on the effects of BUE on surface quality and BUE and burr formation, which improved surface rough- its prediction in micro-milling. They studied the influence ness. On the other hand, Aslantas et al. [37] showed that of BUE while machining 316L stainless steel and reported DLC, titanium aluminum nitride (TiAlN), and tungsten that the BUE was the main cause of surface finish deteri- carbide carbon layer-coated tools showed better perfor- oration in micro-milling besides the chip load. They also mance against BUE formation than nanocrystalline dia- showed that when the BUE was not present, theoretical mond-coated and uncoated tools. surface roughness models yielded acceptable predictions. The BUE affects the friction conditions at the tool-chip Davoudinejad et al. [35] confirmed that the presence of and tool-workpiece interfaces by acting like a cutting edge BUE generated unequal chip load and chip formation so that the cutting tool material is no longer in contact with among different tooth engagements. Their results also the chip and the machined surface. This suggests that a proved that burr height was negatively affected by the stable BUE formation may protect the tool from rapid presence of BUE. Finally, analysis of their results con- wear, leading to a higher machining efficiency. Oliaei and firmed the importance of the developed 3D FE modeling Karpat [38] investigated the relationship between approach for future work. stable BUE formation and process outputs in the micro- Ucun et al. [36] and Aslantas et al. [37] both investi- milling of Ti-6Al-4V using an experimental approach, gated how coated tools could minimize the BUE to taking into consideration tool geometry, surface roughness, improve surface integrity. Ucun et al. [36] confirmed that a and process forces. Their results determined that it was 123 178 L. O’Toole et al. possible to customize a micro-milling tool to have stable BUE formation and design it to machine titanium alloys with long tool life and acceptable surface quality. They concluded that a micro end mill with a low clearance angle yielded the most stable condition for BUE formation, while a large unstable BUE would result in surface quality deterioration. Therefore, the ability to predict and control the BUE size, together with a customized tool design, may be beneficial in the micro-milling of other difficult-to- machine materials. Clearly, simulation models and pre- diction models that will quantify the dynamic mechanisms of BUE formation, such as tool coating, tool wear, process parameters, and workpiece material properties, are impor- tant aspects for future research in micro-milling. Addi- tionally, understanding the chip morphology as well as stable and uniform BUE formation will have significant effects in prolonging tool life, increasing machining effi- Fig. 3 SEM 5009 magnification of a machined slot for burr width ciency and improving surface quality. measurement (Adapted and reprinted from ‘‘Novel method for burrs quantitative evaluation in micro-milling’’ by Medeossi et al. [52], 2.3 Burr formation with permission from Elsevier) A major issue during micro-milling pertains to the forma- tion of burrs, which is an accumulation of material forming a raised edge or volume on the workpiece surface, as can be seen from Figs. 3 and 4. Burr formation is a complicated mechanism involving plastic and elastic deformation, which can be influenced by material properties, tool geometry, and even process instabilities, such as tool run- out [40, 41]. It affects the quality of the machined surface significantly, reducing the capability of the part to meet the desired performance and thus the required functionality. The effect is even more significant at the microscale for precise and freeform components; however, burr reduction, characterization, and evaluation remain to be challenging tasks facing the micro-milling process. In addition, burr formation not only decreases the machined part surface and assembly quality, but also increases the production cost by Fig. 4 Types of milling burrs (Reprinted from ‘‘The effect of spindle up to 9% of the total machining cost [42]. This is due to a speed, feed rate, and machining time to the surface roughness and second machining operation, so-called deburring, which burr formation of aluminum alloy 1100 in micro-milling operation’’ may be necessary to remove such materials from machined by Kiswanto et al. [51], with permission from Elsevier) edges and holes. While the complexity and degree of deburring will depend on a number of factors including Jin et al. [46] determined early on that the feed per tooth burr size, location, and material [43], the focus of research had a major impact on the surface topography in micro- should instead be on the reduction and altogether elimi- milling and therefore proposed to use higher feed rates. At nation of burr formation during the micro-milling process low ratios of feed per tooth to cutting edge radius, high through tool geometry development, suitable machine amounts of burrs are obtained in micro-milling. Saptaji parameters [44], and toolpath optimization. As verified by et al. [47] revealed that top burrs could be reduced by either Fang and Liu [45], although burrs may not be eliminated strengthening the side edge of the workpiece or introducing completely through optimization of the cutting parameters a taper angle in the micro-milling tool. Their results sug- in micro-milling, they may be minimized to less than 25 gest that a combination of a large tool taper and large side nm in height. Among the most important factors are UCT edge angle produces the minimum burrs. Although a and tool sharpness, further showing that an optimal tool tapered wall angle, also known as draft angle, is essential in geometry is necessary to reduce burr formation [45]. 123 Precision micro-milling process: state of the art 179 mold machining, it may not always be a desired feature. vibration assistance in the feed direction during micro- Chern [48] classified burr formation into five types based milling of Ti-6Al-4V alloy. By inducing alternating chan- on in-plane exit angle: knife-type burr, wave-type burr, ges in the relative direction of movement between the curl-type burr, edge breakout burr, and secondary burr. workpiece and the tool on both sides of the slot through Hashimura et al. [49] classified burrs by location, shape, small amplitude, high frequency vibrations, chip formation and formation mechanisms. Litwinski et al. [50] on both sides of the slot then became similar, leading to acknowledged bottom burrs in their toolpath planning less burr formation on both sides of the slot. The results concept; however, they provided no insights into bottom from their FE model simulation and experimental work burr formation or prevention. Kiswanto et al. [51] then confirmed the benefit of vibration assistance, which performed a significant study concerning top, bottom, reduced the average top burr height on the down-milling entrance, and exit burr formation, as well as the effect of side by 87%. However, this proposed method in burr tool wear on burr formation mechanisms, as shown in reduction only utilized vibration assistance in the feed Fig. 4. Furthermore, the team analyzed the average sizes of direction and had only been applied for slot micro-milling. top burr for each cutting parameter to determine the rela- Further work is necessary to optimize even this basic tionship between the cutting parameters and burr forma- unidirectional vibration-assisted micro-milling process. Li tion. Their results showed that bottom burr occurred during et al. [54] provided some of this developmental work also longer machining times, in comparison to top, entrance, in the feed direction. They determined that larger vibration and exit burrs, due to the deterioration of the tool. There- amplitudes actually increased the exit burr size. Hence, fore, tool wear due to machining time was found to be the larger vibration frequencies and smaller vibration ampli- most influential factor affecting burr formation. The team tudes are recommended. Clearly, much more work is also determined that in order to produce a burr-free com- necessary to apply vibration in two or three directions for ponent, it was recommended to perform up milling during burr removal during freeform surface machining and end the micro-milling process. Finally, it was shown that milling operations, including both theoretical and experi- appropriate selection of cutting parameters could minimize mental works. burr formation. Their important work provided adequate Any burr left on the machined surface deteriorates the knowledge of appropriate cutting parameter selection dur- component quality, precision, function, and performance. ing the micro-milling operation of aluminum alloy 1100 to This is particularly true for microparts and features. produce a product with minimum burr. Therefore, burr minimization, and where possible elimi- More recently, Medeossi et al. [52] proposed a novel nation, is essential for high-quality micro-milling opera- method for quantitatively evaluating burrs based on optical tions. This is achievable through extensive research on control techniques and further investigations into under- microscopy using an innovative approach to take advan- tage of the a priori information on the manufacturing standing the phenomena. Such key areas of interest for operation and an unconventional use of void pixels for future work therefore lie in cutting parameter optimization, rapid and non-destructive evaluation of multiple geomet- toolpath generation, tool geometry and material, and tool rical quantities. They applied their proposed methodology coatings and lubrication investigations. to slotting micro-milling operations on pure titanium grade II. The results showed that their method had the potential 2.4 Size effect for on-machine monitoring of burr evaluation during micro-milling operations, which had further potential in Micro-milling raises significant issues when removing reducing and eliminating burr formation through process material at the microscale owing to the effect of scaling, optimization. However, it was noted by the authors that otherwise known as size effect. It has been shown that the appropriate modeling of the specific machining operation size effect modifies the mechanism of material removal in was necessary. Moreover, there are inherent limitations of conventional milling [55, 56]. However, the characteriza- online vision-based measurement techniques, such as dif- tion and exact cause of this effect remain a point of con- ficulties in measuring burr height or burr features over tention among researchers, indicating that many factors freeform surfaces without the additional cost of extra rotary influence chip formation and material removal mechanisms axis or right-angle optics for the online measurement at the microscale. In simple terms, size effect is a phe- system. nomenon that modifies the material removal and chip for- In the micro-milling of slots, the relative size of burrs mation mechanisms at the microscale [57]. In conventional formed on the up-milling side is smaller than that on the milling, the shearing mode of material removal dominates, down-milling side, as can be seen from Fig. 3. To take which leads to chip formation. However, the size effect advantage of this cutting phenomenon and chip formation becomes more significant as the machining scale is reduced mechanism, Chen et al. [53] investigated the effect of to the microlevel, where ploughing of the material surface 123 180 L. O’Toole et al. dominates. This phenomenon produces a major challenge [65], as well as tool specifications [1, 66]. The physical of preventing chip formation by a tooth during a cutting mechanisms that govern the size effect will be discussed in pass, which leads to high cutting forces, high friction, high the following section, including the specific energy, temperature, and significant tool wear. However, as men- shearing and ploughing-dominant modes of material tioned earlier, the exact characterization of the size effect removal, as well as the effect of tool edge radius. has not been fully agreed upon. As an example, Qin [58] defined it as the relationship between the specific energy 2.5 Tool edge radius during cutting and the tool rake angle, which were two important physical parameters that affected the chip A small tool edge radius, rather than a sharp point, is an removal process. As the depth of cut decreases, the effec- important feature of micro-milling tools to limit crack tive rake angle increases, influencing the specific energy. initiation and failure points at the cutting edge of the tool. Therefore, the larger the rake angle, the greater the specific However, because of the size effect, downscaling of con- energy, which has been widely accepted as the main con- ventional milling tools makes the cutting edge radius of tributing factor to the size effect phenomenon [59]. microtools comparable to the instantaneous UCT. Micro- Experimental observation by Mian et al. [60] determined milling tool edge radii are usually less than 5 lm; however, that the specific energy, besides the burr root thickness and they can be up to 20 lm[67]. This means that the tool edge surface roughness of machined surfaces, could be used as a radius is in the same order of magnitude as the chip being relevant measure of the size effect in micro-milling. The formed [68], leading to an increase in cutting force [69, 70] team also used wavelet transformation to extract energy and surface roughness [71]. In micro-milling tools, the bands related to the deformation mechanisms involved in edge is deliberately rounded to impart strength, prevent machining, while high frequency bandwidths in the plastic deformation, and avoid early tool breakage [72]. acoustic emission signals could also be exploited to iden- Therefore, chip formation occurs along the rounded edge of tify the size effect phenomenon. The size effect can also be a tool, resulting in a negative value of the effective rake described as the phenomenon whereby the ratio of the UCT angle, even if the nominal rake angle is positive [73]. to the cutting edge radius of the tool, or the grain size of the Vipindas et al. [74] presented an investigation on the workpiece material, will influence chip formation, material effect of cutting edge radius on cutting force, coefficient of removal mechanisms, and material flow, as shown in friction, surface roughness, and chip formation during Fig. 1. This effect can become significant when the thick- micro end milling of Ti-6Al-4V, for a wide range of feed ness of the material to be removed is of the same order of per tooth. It was found that the feed per tooth within 1 lm magnitude as the tool edge radius or grain size of the range was the critical value, which was approximately one- workpiece material [60]. The influence of tool edge radius third of the cutting edge radius. Below this critical value, on the size effect has also been demonstrated through a the size effect is predominant, leading to the ploughing strain gradient plasticity-based FE model of orthogonal mode of material removal, as illustrated in Fig. 5. Moges micro-cutting by Liu and Melkote [61]. et al. [75] developed a comprehensive mathematical model Actually, the above two definitions are correct because that incorporated the edge radius of the micro-cutting tool, many factors will affect the chip formation and material removal mechanisms; thus, it can be simply said that the size effect is characterized by a nonlinear increase in the energy consumed per unit volume of material removed as the UCT decreases to the same order of magnitude as the cutting tool edge radius or grain size [60, 62]. Therefore, it is very clear that conventional milling mechanisms cannot be used to describe the micro-milling process, because simply reducing the scale of the system will not reproduce the same representative model [63]. The size effect becomes even more significant at the nanoscale, particu- larly for nanometric cutting, where ploughing of material dominates, rather than shearing and chip formation [64]. Consequently, this variation from the general behavior of Fig. 5 Cutting model of microtool edge showing ploughing, shear- both the tool and the workpiece microstructure at the ing, and elastic recovery zones as a result of tool edge radius r microscale during machining will depend on many factors, (Reprinted from ‘‘Experimental research on micro-milling force of a such as the material properties and microstructure [39], single-crystal nickel-based superalloy’’ by Gao and Chen [76], with micro-milling tool parameters [59], machining parameters permission from Springer Nature) 123 Precision micro-milling process: state of the art 181 so that a more accurate prediction of cutting force models morphology, and surface integrity of martensitic aged steel. could be obtained. Therefore, even though rounding of the They proposed a new method for calculating the effective cutting edge was necessary in micro-milling tools, an energy and non-effective energy by the criterion of whether extremely large tool edge radius would greatly influence it contributed to chip formation or not, respectively. Their the size effect. This suggests that a stronger cutting edge to results showed that chips became more segmented with prevent crack initiation could reduce the size effect issue, decreasing proportion of the effective energy, whereas as it would lead to a smaller radius requirement, which in increasing the proportion of the non-effective energy turn would result in a more dominant shearing mode of resulted in surface integrity deterioration and contributed to material removal. To fulfil this demand, further investiga- the formation of a plastic deformation layer. Then, by tion on the cutting edge tool geometry is necessary. assessing the trade-off between surface quality and specific cutting energy, optimized machining parameters were 2.6 Specific energy suggested to achieve a precision surface finish with low specific cutting energy and high energy efficiency, which The energy consumption during the machining process had significant application for the realization of sustainable affects both the environmental and manufacturing costs. manufacturing. Gao et al. [59] examined the size effect in Therefore, evaluating and limiting the energy consumed relation to the tool edge radius and cutting parameters on during micro-milling can lead to more efficient manufac- specific energy in micro-milling of heat resistant stainless turing [77]. One such way, according to Fang et al. [78], is steel. They showed that the specific cutting energy could be to compare the experimental cutting force and specific fully controlled by regulating the geometrical characteris- cutting energy. To compare the energy consumption during tics of the cutting tool, i.e., the cutting edge radius, and by machining operations such as micro-milling, the specific the machining parameters recommended by their devel- energy parameter, which was defined by Li and Kara [79] oped minimum chip thickness prediction model. Precise as the energy consumed to remove a unit volume of control of the specific energy during micro-milling can material, may be used [80]. The specific energy is a par- therefore lead to more efficient chip formation, which has ticularly important parameter to consider during micro- great significance on improving machining efficiency, tool milling, as it can be used to evaluate the cutting effec- life, and surface quality. Lauro et al. [82] also analyzed the tiveness of the process. The ratio of specific energy to the influence of the size effect on the specific cutting energy of UCT can be helpful in characterizing the size effect in AISI H13 steel in relation to austenitic grain size, while relation to surface generation, as can be seen from Fig. 6. examining the response from a cutting force perspective. It has been shown that the size effect strongly affects the They observed that the grain size had a significant influ- ence on both cutting force and specific cutting energy in specific energy necessary for material removal through chip formation mechanisms, which can alter the material micro-milling. Their results revealed that larger grain sizes removal mechanisms [16]. An experimental investigation displayed lower specific energy compared with smaller was carried out by Yao et al. [81] to determine the rela- grain sizes. They also showed that increasing the feed rate tionships between the specific cutting energy, chip had a significant effect on reducing specific energy (ap- proximately by 70%) for both small and large grain sizes. Therefore, the recent research suggests that by reducing the specific energy during cutting, the size effect was therefore lessened, resulting in improved machining efficiency, tool life, surface finish, and material removal rates. 2.7 Process stability Relatively large form error and poor component geometric accuracy are still major obstacles toward achieving higher precision in the field of micro-milling. The main cause of these inaccuracies is the inherent process instabilities dur- ing the micro-milling process. Among several factors, the influences of tool deflection, tool runout, and machining chatter are the main sources of surface and dimensional Fig. 6 Variation in specific cutting energy with uncut chip thickness accuracy errors in micro-milled components. These process at 240 m/min (Adapted and reprinted from ‘‘Size effects in instabilities further lead to high cutting forces, excessive manufacturing of metallic components’’ by Vollertsen et al. [83], tool wear, and tool failure, as well as high cutting with permission from Elsevier) 123 182 L. O’Toole et al. temperatures, as a result of frictional forces due to rubbing both ploughing and shearing modes of material removal. and ploughing during unstable machining conditions, as On the other hand, Lu et al. [88] proposed a revised 3D will be discussed in this section. Because of the relatively analytical model of micro-milling forces, which considered low strength and stiffness and very small cutting diameter the effects of cutting temperature and ploughing force of micro-milling tools, micro-milling must be performed at caused by the arc of the cutting edge during shearing- very high speeds between 20 000–100 000? RPM, to dominated cutting. Therefore, considering the seriousness ensure productive machining. Moreover, material removal of the tool deflection issue, it is of great importance to rates can be maintained during the process by increasing study its effect on the mechanics of the chip formation the spindle speed to negate the effect of the small cutting process, while examining cutting forces, surface errors, and diameter of the microtool and relatively slow feed rate. cutting temperature, so that reliable and accurate predic- However, high-quality precision air bearing spindles with tions can be made to limit and prevent excessive deflec- closed loop position and very accurate speed control are tions during machining. necessary for high RPM machining to ensure process sta- Cutting forces directly affect tool deflection in the bility. Furthermore, vibration and instabilities during high- micro-milling process because of the relatively low stiff- speed micro-milling must be minimized, whereas feed rate ness of the tool, particularly at the tool tip, and results in and positioning must be smooth and continuous [84]. imperfections on the machined surface as described above. Therefore, it is necessary to develop accurate and reliable As bases for determining tool deflection, accurate analyti- process stability models to analyze and improve the per- cal cutting force models that consider the tool geometry formance of such processes as tool runout and tool and material, the specific cutting mechanism involved, as deflection, as well as minimize self-excited vibration, also well as the vibration dynamics are key areas for research. known as chatter. Mamedov et al. [87] fully understood the importance of cutting force on tool deflection and became significant 2.8 Tool deflection contributors to micro-milling tool deflection analysis early Tool deflection is one of the most significant factors lim- iting the performance of micro-milling processes, particu- larly limiting form accuracy and precision [85, 86], as can be observed from Fig. 7. A micro-milling cutting tool is severely prone to relatively large deflections owing to a significantly smaller diameter to overhang length ratio. This results in a drastic reduction in tool shank section modulus, which lowers its strength and ability to withstand periodically varying cutting forces, leading to tool bending [85]. The increased flexibility and lower stiffness of smaller diameter tools result in large values of cutter edge deflections, which lead to two serious problems: form and feature geometric errors on the machined component and distortion of cutting forces. This effect is again strength- ened even further as the cutting tool diameter reduces from 1 000 lmto100 lm, where even a small deflection of 5 lm will lead to an error comparable to the cutting edge radius of the tool. As the deflected tool rotates, undesirable cycles of shearing to ploughing modes of material removal mechanism will occur, leading to spikes in high and low cutting forces on each tooth. This will in turn cause larger deflections, while the cycle itself will continue until either failure of the tool occurs or the tool skips. Moges et al. [85] presented a methodology for determining such cutting force-induced tool deflections and developed a cutting force model considering tool deflection on the resulting Fig. 7 Deflection of milling tool at the bottom of the workpiece edge cutting forces. Similarly, Mamedov et al. [87] developed a (Adapted and reprinted from ‘‘Analysis of tool deflection errors in novel mathematical model for estimating cutting force and precision CNC end milling of aerospace aluminum 6061-T6 alloy’’ by tool deflection by calculating the UCT, which considered Nghiep et al. [95], with permission from Elsevier) 123 Precision micro-milling process: state of the art 183 on. Using their mathematical model, the distribution of also considered tool runout, consisting of both axial and tilt forces acting on the tool can be predicted and deflection of offsets, including entry and exit angles of the tool. Their a micro-milling end mill tool can be estimated with good developed model can be used to further optimize the accuracy. High cutting forces lead to higher tool deflection. accuracy of the micro-milling process because of the Mamedov et al. [89] presented an updated analytical cut- inclusion of a more complete tool deflection model. ting force model, which considered both the shearing and Clearly, deflection of a cutting tool is dependent on many ploughing phenomena, based on the material elastic factors and must be modeled as a dynamic phenomenon, recovery properties. The tool deflections corresponding to rather than a static one. More accurate tool deflection the cutting force were calculated by considering the prediction models will provide methods for reducing cut- microtool stiffness. This model accurately predicts instan- ting forces, thereby reducing tool wear and breakage, taneous tool deflections through analysis of the cutting increasing surface and feature quality as well as machining force, which was presented as a function of cutting force efficiency. However, tool deflection is an implicit issue of coefficients, microchip thickness model, and tool geome- machining at the microdomain, which also has a multi- try. Oliaei and Karpat [90] investigated the influence of factor influence on the process stability as a whole, similar increased cutting force due to tool wear on tool deflections to tool runout and self-excited machining chatter. There- and tool breakage. Their model for predicting tool deflec- fore, a thorough review of relevant research is presented tion and tool breakage allows for the development of tool below on tool runout and chatter in micro-milling. condition monitoring systems based on the physics of the micro-milling process. In their model, Rodrı´guez and 2.9 Tool runout Labarga [91] for the first time considered variable deflec- tion rather than just static deflection along the cutting edge. Tool runout is a critical issue that affects the micro-milling Their model also has promising benefits in monitoring process significantly. It is in part responsible for influenc- systems and adaptive control systems for the prevention of ing the cutting force [96], tool condition, tool life [97], and tool failure during micro-milling operations; however, it surface integrity of the machined component [98]. Tool does not take tool wear into consideration. Moges et al. runout can be described as a phenomenon caused by the [85] also presented a method for determining cutting force- sum of the geometrical displacement errors of the spindle, induced tool deflections and developed a flexible force toolholder, and tool axis from the ideal or theoretical axis model considering the effect of tool deflections on the of rotation. The sum of these errors produces a deviation resultant cutting force based on previous rigid models. The between the theoretical and actual cutting edge trajectories team presented a methodology for predicting variation in [62]. Tool runout may take the form of axial and/or radial runout. Radial runout is caused by the tool rotating off machine surface error due to tool deflections. Their pro- posed model accurately predicted cutting forces in the center, instead of being centrally aligned, and it will rotate presence of tool deflections. In addition, it was found that about a secondary axis. Cutting tools will be generally deflection of the tool caused considerable deviations of the more tolerant to this type of runout during face milling tool center location, resulting in change of tooth trajecto- operations. However, during side milling, radial runout will ries and uncut chip geometry. Their model provided great have significant effects on the cutting force, and therefore benefits in selecting optimum cutting parameters to control tool wear, due to uneven loading on the flutes, which will tool deflections, resulting in tighter tolerances and lead to surface errors. In contrast, axial runout is the result improved productivity. However, to further improve the of rotating components not being parallel with the center prediction accuracy, their model must consider the axis of rotation, such as the tool axis and spindle axis not dynamic vibration of the tool tip. Lu et al. [92] understood running concentrically. Therefore, axial displacement of the importance of examining the cutting force and how it the tool causes its tip to rotate off center relative to the might be used to limit tool deflection. They proposed an spindle axis. Cutting tools will generally be less tolerant to indirect method of determining the average micro-milling axial runout, especially for micro-milling operations in cutting force, which was both low cost and high precision, both side and face milling operations, but axial runout has a by examining the power of the main transmission system of considerable influence on the surface topography genera- a micro-milling machine. Lu et al. [93] then developed a tion in face milling [99]. The total tool runout is therefore new method for predicting micro-milling tool breakage the sum of both axial and radial runouts, with the effect based on theoretical models by examining the tool bending becoming even more significant in the micro-milling stress. Finally, Zhang et al. [94] formulated a mechanistic domain, as demonstrated by Fig. 8. Because micro-milling model of cutting forces and instantaneous tool deflection in requires very high spindle speeds due to the relatively the micro end milling process, which took into account the small cutting edge diameters, the dynamic characteristics minimum UCT effect and tooth trajectory. Their model of the spindle-tool system dominate the machining process 123 184 L. O’Toole et al. With regard to measurement of tool runout, Jing et al. [100] presented a method using modeling and simulation of the cutting force in micro-milling. The proposed approach uses a charge-coupled device to determine differences in displacement of tool flutes and tool shank. An accurate tool runout value can then be calculated using their model. This is a simple, easy, and precise method for measuring runout in micro-milling and can be easily adapted to on-machine measurements during operation. Another simple method for measurement of tool runout is by displacement mea- surement using capacitive sensors close to the tool shank, according to Chen et al. [101]. They also determined that tool runout resulted in a considerable increase in surface roughness, particularly when the feed per tooth was less than the runout. Finally, their proposed surface generation Fig. 8 Effect of runout is intensified for smaller tools (Reprinted model considering the minimum UCT, which takes into from ‘‘Protocol for tool wear measurement in micro-milling’’ by account tool runout, provides a more accurate surface Alhadeff et al. [63], with permission from Elsevier) topography simulation and roughness prediction in micro- milling. Zhang et al. [102] also developed a simple and quality. Therefore, tighter stiffness loop machines with effective tool runout identification method, designed to higher precision spindles and tools are essential. However, quickly identify the tool runout parameters through tool even with the correct equipment, the tool-spindle interface displacement measurement using a laser displacement can cause undesirable radial runout, while even small sensor, so that the accuracy of tool runout measurements deviations in the spindle or cutting tool edges may result in could be improved. significant runout due to poor stiffness and strength of Guo et al. [103] established the importance of a more microtools [84]. systematic approach to investigating tool runout in relation The significance of tool runout is that it has a major to tool geometry and surface generation. Toolpath opti- influence on the cutting force. This is due to the dis- mization during 5-axis machining was examined in detail placement being in the same order of magnitude as the feed in relation to minimizing geometric errors formed from per tooth, which therefore has a large influence on the tool runout. In their model, the tool runout is defined by surface roughness generated, as determined recently by four parameters, namely, inclination angle, location angle, Chen et al. [99]. They also found that axial runout in offset value, and length of the cutter axis. Although their particular limited the achievable surface roughness. Simi- work only considered conventional machining, much of larly, uneven engagement of teeth caused by runout leads what was learned could also be applied to micro-milling, to uneven rates of wear on each tooth, resulting in cutting with the effect becoming even more significant at the force features that can be largely different for both teeth microdomain. Guo et al. [104] then presented an instanta- [63]. This further leads to increasing cutting forces and all neous UCT model regarding tool runout and tool geometry problems generated by process instabilities. Attanasio [62] in micro-milling. Using their early work as a foundation, developed an easy and reliable method for determining tool they determined that five parameters were necessary to runout in micro-milling by implementing a geometric characterize tool runout in micro-milling, namely, runout model that deduced and estimated tool runout from the tool offset length, inclination angle, cutter axis length, location diameter, channel width, and cutting edge’s phase. Their angle, and initial rotation angle. The team analyzed and procedure can be integrated into an adaptive model for discussed the influencing principles of each runout controlling cutting force, which has practicality for parameter on the instantaneous UCT values. Some improving production quality and process stability while important viewpoints that provided reasonable explana- reducing tool wear and machining costs. Li et al. [15] tions for each runout parameter were introduced. However, established a cutting force model that further strengthened no method for detecting each of the runout parameters was the understanding of the micro-milling process, through a offered. Their work would be a good research to begin deeper investigation of tool eccentricity. This multifactor more thorough investigations into fully characterizing the model considers the influence of runout on the UCT, the tool runout parameters and how this phenomenon could be equivalent rake angle and cutting force, and how the minimized or eliminated to reduce cutting forces. As combined effects of each factor influence the surface mentioned, tool runout causes unbalanced chip thickness quality. removal between flute teeth, which leads to uneven cutting 123 Precision micro-milling process: state of the art 185 force loading on each cutting edge. This causes not only as a function of the spindle speed, as depicted in Fig. 9. unwanted vibrations that affect process stability, but also This diagram is an essential tool to find the range of high and uneven tool wear. Throughout this section, it is machining parameters that results in a maximum explained why it is important to limit tool runout as much stable (i.e., chatter free) material removal rate [110]. The as possible. However, it may not always be feasible to idea is to seek regions within the lobes for optimal completely eliminate runout owing to a number of reasons machining parameters, depending on such criteria as time, including tool, toolholder, and spindle setup, and tool cost, and accuracy [105]. However, SLD is based on manufacturing tolerances. Therefore, it is essential to limit individual machine setups as indicated in Fig. 10, which tool runout to an acceptable level, i.e., at least below 2 lm. considers the machine tool stiffness loop, tool geometry, This can be considered prior to machining to realize a etc. Therefore, predicting the stability lobe boundaries can precise and accurate process, avoid accelerated tool wear be a very difficult task that will rely on fundamental or tool breakage, and improve surface finish. understanding of the dynamics of the entire micro-milling process. To do so, a combination of theoretical models of 2.10 Chatter Chatter also greatly influences the process stability, resulting in increased tool wear, poor surface finish, and limiting precision and efficiency. Chatter is a form of self- excited, unstable vibration during specific cutting edge machining. It is generally accepted that there are four types of chatter during the cutting process, namely frictional chatter, regenerative chatter, mode coupling chatter, and thermomechanical chatter [105]. Frictional and regenera- tive chatters are generally the most common types and notably most important in micro-milling. Frictional chatter is mainly attributed to nonlinear dry friction force, i.e., rubbing on the clearance face, which leads to excitation vibration of the cutting force, limiting the thrust force [106]. However, regenerative chatter is the most significant issue in cutting processes in general because of the high Fig. 9 Stability lobe diagram plotting axial depth of cut against spindle speed to identify areas of chatter-free operation. Reprinted spindle speeds involved [105]. It occurs owing to varying from ‘‘Chatter in machining processes: A review’’ by Quintana and cutting forces acting on each tooth of the tool, which create Ciurana [105], with permission from Elsevier a relative displacement between the tool and the workpiece at the cutting point [107]. Depending on the characteristics of the system and the phase between the varying cutting forces, the dynamics of the cutting system can be unstable. This in turn leads to large chip sizes and higher cutting forces and vibrations. This process will continue if the system remains in an unstable condition, until the vibration amplitude increases to the point that the tool jumps or skips, damaging either the tool, workpiece, or spindle [108]. Therefore, to prevent chatter and unstable machining conditions, accurate models of the dynamics of the micro- milling system are necessary to predict the relationships between the workpiece material, structural dynamics of the machine tool including toolholders, tool geometry, and cutting conditions. By analyzing these models, a stability lobe diagram (SLD) can be created, which will offer insights into ideal machining parameters that can be chosen to prevent process instabilities such as chatter, greatly Fig. 10 Dynamic model of micro-milling system showing stiffness improving the machining efficiency [109]. loop and how chatter can be modelled (Adapted and reprinted from The distinction between a stable and unstable cut can be ‘‘Chatter modeling in micro-milling by considering process’’ by visualized with the SLD, which plots the axial depth of cut Afazov et al. [98], with permission from Elsevier) 123 186 L. O’Toole et al. the machine tool and toolholder, as well as deflection considering rotational degree-of-freedom (DOF) and tool testing of the tool will be necessary. To begin constructing point FRF of micro-milling. The FRF at the micro-milling an SLD, an analytical model of the frequency response tool point can therefore describe the dynamic behavior of function (FRF) of the cutting tool, toolholder, spindle, and the entire micro-milling machine system. Lu et al. [121] machine tool is required. Next, experimental testing or an further developed this work by considering the centrifugal accurate theoretical model is required to determine the force and gyroscopic effect caused by the high-speed dynamics of the tool tip. Thus, SLDs can be created for a rotation of the micro-milling spindle to better simulate the system setup using the specified cutter, workpiece material, real scenario and increase the accuracy of modal etc. Finally, the operator can select combinations of axial parameters. depth of cut and spindle speed, which ensure chatter-free To obtain more accurate SLDs, higher accuracy models operation. of chatter and process stability are necessary. Typically, In contrast to that of conventional milling tools, per- models designed based on solving the equations of motion forming an impact hammer test at the tool tip of micro- in either the frequency or time domain, where both cutting milling tools is not feasible because of their inherent tool force and modal parameters are implemented, will lead to fragility. Therefore, new methods for analyzing micro- results that are more robust. This is because ‘‘cutting milling tool dynamics are necessary to build the SLD in instability consists of deterioration in both time and fre- micro-milling. Lu et al. [111] developed a vibration dis- quency domains due to the highly nonlinear nature of the placement measurement system that utilizes a laser dis- micro-milling process’’ [122]. When the cutting forces placement sensor to collect vibration signals during micro- exhibit a linear behavior in cutting processes, the frequency milling. The frequency of the micro-milling cutting force domain solution should be used, i.e., when the process is was obtained using the varying cutting parameters method, more or less stable. However, at small UCTs and feed rates, whereas the relationship between the cutting force ampli- the micro-milling process experiences nonlinear behavior tude, frequency, and vibration displacement was ascer- owing to the size effect, chip formation, etc. Since the tained by using a neural network method to realize cutting forces can become nonlinear, the equations of vibration displacement prediction under given cutting motion must therefore be solved in the time domain using parameters. Before this important work, the methods used numerical methods for integrating the ordinary differential for conducting stability analysis mainly included zero- equations of motion [98]. The dynamic system can then be order solving, semi-discrete, and time domain methods reduced to a 2-DOF system through the assumption that the [112]. The zero-order solving method was applied by helix angle of the microtool is negligible, simplifying the Mascardelli et al. [113], Tajalli et al. [114], and Jin and equations of motion (see Fig. 10). Regarding the time domain method, Lu et al. [112] proposed a micro-milling Altintas [115]. However, only stability prediction results considering the shear effect were obtained for the SLDs force prediction model based on chatter stability analyzed drawn. Tyler et al. [116] presented a method for producing in the time domain. However, because the time response is SLDs that included process damping ranges (low cutting bounded, the process can become significantly unstable and speed) and high cutting speeds. This method defined the chaotic in the frequency domain, which can lead to geo- stability boundaries by radial rather than axial depth of cut, metric errors and tool damages due to chatter. Liu et al. because of the approach taken by computer-aided modeling [122] developed a novel simultaneous time-frequency toolpath programs. This work is particularly significant in control theory to regulate and counteract the various non- defining machining parameters for difficult-to-machine linear dynamic instabilities including chatter and tool res- materials. For these materials, high tool wear prohibits high onance. This model controls the dynamic response of the spindle speeds, which therefore leads to smaller system under various axial depths of cut and spindle speeds stable zones in the SLDs. Park and Rahnama [117] to prevent an unstable state of motion. The simultaneous obtained micro-milling tool tip dynamics indirectly time-frequency control model was found to demonstrate through mathematical coupling of the substructures using the capability of reducing chatter and process instabilities the receptance coupling method. Song et al. [118] and during micro-milling operations, leading to improved tool Tajalli et al. [119] drew an SLD using a semi-discretized performance and machined surface quality. numerical approach to predict chatter stability based on Chatter worsens the machining precision and efficiency, cutting force. However, the authors noted that further study as well as tool and surface integrity [123]. By under- was necessary to investigate how the burr formation standing the dynamics of the system through analysis of the mechanism would affect the process stability, which led to cutting forces and developing accurate prediction models, chatter. Lu et al. [120] provided a clear basis for the the effect of chatter on the micro-milling process can be dynamic study of the tool-toolholder-spindle system based minimized, if not entirely eliminated. Using SLDs, suit- on receptance coupling substructure analysis and by able machining parameters can be chosen to avoid chatter 123 Precision micro-milling process: state of the art 187 and unstable machining conditions. To obtain such dia- temperature increases considerably with the increase in grams, it is suggested that both time and frequency spindle speed [88, 125]. Another important parameter to domains should be considered simultaneously to control evaluate the effectiveness of the micro-milling process is the process. surface quality, often examined as surface roughness, burr formation, and remaining artefacts of the micro-milling tool on the machined surface. Lu et al. [126] established a 3 Process inputs comprehensive floor surface model to predict both the surface roughness and such artefacts or grooves under As with any manufacturing process, the input variables, different cutting parameters and tool parameters. It was such as workpiece material, tool parameters, toolpath, and found that surface roughness decreased first and then cutting fluid, are all widely known to affect the process increased with the increase in spindle speed, but it outputs of cutting force, surface quality, tool wear, etc. The increased with the increase in feed per tooth and depth of influence and multifactor effect of the process inputs have cut. However, as noted by the authors, to obtain the indi- even more significance at the microscale, particularly when vidual and combined interaction effects of each cutting the microstructure of the tool and workpiece must be parameter and how they influence the process outputs, considered and they are of major concern to the micro- more experimental data are required. Lu et al. [127] further milling process performance. Therefore, it is essential to developed this study through an analysis on the effects of fully understand the process physics and characterize the spindle speed, radius of a ball end mill, axial cutting depth, effect of each variable in a systematic way, so that the input and feed per tooth on the curved surface roughness. parameters that have considerable influences on the output Moreover, they also built a surface roughness prediction quality can be easily recognized and accounted for. In model to provide an accurate reference for the selection of theory, the optimization of the process for all machining cutting parameters in the micro-milling of Inconel 718. variables appears to be very complex. However, consid- All of this important research work concerning micro- erable achievements focusing on predictive models, milling has been carried out with the goal of advancing the numerical simulations, statistical analysis, and experi- micro-milling process in terms of efficiency and produc- mental investigations have been made recently, such that tivity, so that it may develop new applications across new the multitude of factors influencing the process outputs can industries. Therefore, a method for quantitatively analyzing now be examined accurately. Therefore, a full review of the efficiency during experimental work is essential to existing investigations on input variables and their effect determine the progression of the micro-milling process. on the micro-milling process outputs is presented. The material removal rate, which is a measure of the amount of material removed per unit time when performing 3.1 Process outputs machining operations, is often used to do so. Lu et al. [128, 129] established an optimization approach based on Firstly, a brief introduction and investigation of major genetic algorithm to achieve the maximum material process outputs are necessary to understand how they are removal rate under the constraints of surface roughness and influenced by process inputs by examining the predictive cutter breakage. Peng et al. [125]. determined that when the models, as well as theoretical and experimental works rate of material removal was the same, a higher spindle presented recently, so that the optimal micro-milling pro- speed was better for reducing surface deformation. Simi- cess can be determined for a range of difficult-to-machine larly, when the spindle speed is the same, a higher material materials. Beginning with the cutting force, it has been removal rate is better for reducing deformation. To shown that typically within the range of selected cutting improve the machining efficiency, selecting a high spindle parameters, the spindle speed has a relatively weak influ- speed and feed rate has a great significance in promoting ence on the cutting force, while it tends to increase initially the workpiece quality in micro-milling. An undesirable when the feed per tooth is close to the radius of the cutting process output that can occur in metals with a crystal edge, leading to a dominant ploughing mode of material structure is work hardening, also known as strain harden- removal. It then tends to decrease almost linearly with a ing. The strengthening of the material is due to dislocation further increase in feed per tooth, while the shearing mode movements and dislocation generation within the crystal of material removal dominates [74, 88, 124]. The cutting structure of the material when it is strained beyond its yield force also clearly increases with an increase in depth and point. An increasing stress is then necessary to produce width of cut, as does the cutting temperature. In this regard, additional plastic deformation, leading to significant tool the cutting temperature also tends to increase at first but wear, higher cutting forces, higher cutting temperature, and then decreases with the increase in feed per tooth, where overall lower machining efficiency during micro-milling. the turning point is at the UCT. In contrast, the cutting Lu et al. [130, 131] used 3D FE analysis for simulating the 123 188 L. O’Toole et al. process of micro-milling to predict the surface hardness of team observed lower cutting forces, lower BUE, and both Inconel 718 and a nickel-based superalloy. With reduced tool wear for the fully lamellar microstructures. regard to other process outputs, the team then studied the Understanding the variation in cutting force is essential in influence of cutting parameters, including the spindle developing a more complete model of the excitation and speed, feed rate per tooth, and axial cutting depth on sur- process stability between the tool and the workpiece. This face Vickers hardness, as well as the relationship between will lead to further development of the micro-milling strain and hardness [132, 133]. According to their analysis, process as a whole and offer insights into better the spindle speed has the greatest influence on Vickers microstructure design when micro-milling. With regard to hardness, whereas the axial cutting depth has an interme- surface roughness, Elkaseer et al. [136] presented a model diate influence, while the feed per tooth has the least to simulate the surface generation process in micro-milling influence. Their work can help guide the selection of cut- of multiphase materials. They confirmed that their devel- ting parameters to reduce surface work hardening, and oped model could be used to predict the surface quality thereby improve the quality of the final product. Clearly, after machining under various machining parameters and selection of appropriate cutting parameters prior to micro- could further be used to optimize the process for multi- milling operations is essential for improving machining phase materials. An important feature of the model is that it efficiency and quality, prolonging the tool life and main- considers micro-burrs at the phase boundaries. taining good surface quality. However, the micro-milling Concerning workpiece microstructure characteristics, process can only be optimized to a certain degree through Ahmadi et al. [39] investigated the influence of grain size, selection of machining parameters. Therefore, a detailed grain boundary, and phase fractions in the micro-milling of investigation and discussion of the other major process Ti-6Al-4V on the process outputs. A smaller grain size inputs, namely the workpiece microstructure, the micro- (both a and b) and lower b phase fraction led to a higher tools themselves, the toolpath, and cutting fluid, are nec- cutting force in micro-milling. Although, the hardness of essary to develop better understanding of the process as a the sample containing enlarged equiaxed grains was found whole. to be higher owing to the greater b phase fraction as dis- played in indentation tests (see Fig. 11), it experienced a 3.2 Workpiece microstructure lower cutting force as a result of its lower ductility. Moreover, the team found that the microstructure could The influences of the microstructure of multiphase mate- greatly affect the BUE formation in terms of size and rials and the process outputs in micro-milling require very shape; therefore, lower grain sizes can result in more detailed investigations to accurately develop robust ana- BUEs. Aksin and Karpat [137] also investigated the influence of microstructure on the process outputs as a lytical models, because the workpiece can no longer be described as homogeneous at the microscale. Better models function of grain size and grain morphology on commer- and understanding of the material microstructure and its cially pure titanium using their developed mechanistic machinability will help develop more accurate predictive model. The microstructure was modified using heat treat- models to avoid tool wear, improve surface quality, etc., ment methods, so that a gradual transition from acicular to during the process design and machining phases [39]. equiaxed grain morphology was obtained. They also Clearly, the anisotropic behavior of multiphase material microstructures is an important factor that must be con- sidered throughout the machining process when the size effect and chip formation mechanisms are influential at the microscale. Vogler et al. [134] presented a very early significant mechanistic model for the micro-milling process that explicitly accounted for different phases of heterogeneous materials. This model explicitly considers the multiple phases and the effect of determining the magnitude and variation in cutting force. The team showed that the microstructural effects could account for more than 35% of the energy in the cutting force signal. Attanasio et al. [135] Fig. 11 Indentation marks on a and b phases showing relevant depths of indentation, which are used to identify areas of different also investigated the influence of material microstructures microhardness due to phase change (Adapted and reprinted from on the cutting force, with a detailed examination of four ‘‘Microstructure effects on process outputs in microscale milling of different microstructures, namely bimodal, fully equiaxed, heat treated Ti-6Al-4V alloys’’ by Ahmadi et al. [39], with permission fully lamellar, and mill annealed, of Ti-6Al-4V alloy. The from Elsevier) 123 Precision micro-milling process: state of the art 189 established that as the microstructure becomes more tool appeared blunt and grinding grooves could clearly be equiaxed, the hardness increased. However, unlike those of observed. This shows that the tool diameter cannot be Ahmadi et al., their results showed increased cutting forces reduced by only scaling the tool geometry, which supports in this case. Elkaseer et al. [138] examined the effects of the conclusion that the microgeometry and microstructure material homogeneity of copper (Cu99.9E) on the mini- of the tool must be adapted to accommodate even smaller mum UCT and showed that by refining the material diameter cutting edges. Finally, the team determined that microstructure, the minimum UCT could be reduced. It the machining parameters must also be optimized to ensure was also confirmed that material homogeneity improve- a high-quality cutting edge surface and overcome the size ments led to a reduction in surface roughness and surface effect. Similarly, Cheng et al. [142] presented a thorough defects in micro-milling. study on micro-milling tools, noting how commercially It is evident that the resulting surface integrity after available tooling was generally downscaled from macro- micro-milling is highly dependent on the material milling tools that were not accurately fabricated nor microstructure, especially for multiphase materials. entirely suitable. Therefore, the team proposed a design Therefore, deeper consideration of the cutting conditions criterion for custom micro-milling tools and developed a and material microstructure must be given prior to micro- new micro-hexagonal end mill, fabricated using wire milling operations, and the analytical models introduced electrical discharge machining (EDM) based on their above can help significantly. Good understanding of these considerations. Their developed tool achieved submicron relationships will lead to future development of more surface roughness values for the side and bottom surfaces. accurate microstructure-based predictive models of the Most importantly, Fang et al. [143] determined that the tool micro-milling process based on computational techniques. tip rigidity of a semi-circle-based (D-type) end mill was much higher than that of a two-flute (commercial type) end 3.3 Microtools mill. This shows that tool geometry plays a major role in tool stiffness in micro-milling, which is important in The continuing trend toward smaller feature sizes in micro- reducing tool deflection, cutting force, and therefore tool milling with higher precision and accuracy has led to wear. demands for higher quality microtools, as the cutting tool Adhesion of material on the cutting edge of microtools edge radius defines the minimum UCT [139]. It was shown can quickly lead to surface quality deterioration, as by Kirsch et al. [140] that the material specifications of tool reported by Katahira et al. [144], who performed ultra- blanks highly influenced the quality and application of precision machining of a single crystalline sapphire using a ultra-small microtools in the range of 4–50 lm. Generally, polycrystalline diamond (PCD) micro-milling tool. To restore the milling capabilities of the PCD tool, the team microtools are manufactured via grinding operations fol- lowing the famous procedure by Aurich et al. [141] for the implemented an electrochemical-assisted surface recondi- design and machining of single-edge micro end mill tools tioning process to remove the surface contaminant and with diameters between 10 lm and 50 lm and a variable restore the machining performance of the PCD micro- helix angle. Cemented carbides, such as tungsten carbide, milling tool. Adhesion of material on the cutting edge of are predominantly used as micro-milling tool materials microtools can be prevented by coatings. It was shown by owing to their high stiffness, hardness, and resistance to Swain et al. [145] through a direct comparison between wear. It was shown how sharper and more homogeneous TiAlN-coated and uncoated tungsten carbide micro-milling cutting edges without breakouts might be achieved with tools that TiAlN-coated tools exhibited superior perfor- smaller grain sizes of cemented carbide, while the appli- mance in terms of tool life and micro-burr formation. On cation of these tools generated smaller cutting forces and the other hand, Thepsonthi and Ozel [146] attempted to resulted in a considerably longer tool life. The quality of improve the performance of carbide micro-milling tools by the manufactured tool may depend on the material, overall applying a cubic boron nitride (cBN) coating to the end geometry, cutting edge radius, surface conditions, and mill tools. Their study clearly showed that the cBN-coated coating, while the tool design influences the dimensional carbide tool greatly outperformed the uncoated carbide tool accuracy, surface quality, burr formation, and tool life in terms of tool wear and cutting temperature. [142]. Therefore, it is extremely important to thoroughly As for recent advances in tool materials, Suzuki et al. investigate all possible factors and influences that the [147] developed and manufactured micro-milling tools micro-milling tool may have on the machining process. from binderless ultra-hard nano-PCD (NPCD) to machine In relation to microtool geometry, Kirsch et al. [140] silicon carbide (SiC) molds using laser fabrication tech- discovered that although their 50 lm diameter cutting tool niques. The NPCD consists of very fine grains having a provided defined sharp cutting edges and faces that length of several tens of nanometers and is harder and more appeared smooth, the cutting edges of the 10 lm diameter thermally stable than conventional PCD. It was 123 190 L. O’Toole et al. Fig. 12 Surface topography of the micro-slot middle region at the cutting length a 1 mm, b 166 mm, c 249 mm, and d 332 mm (Adapted and reprinted from ‘‘Effect of the progressive tool wear on surface topography and chip formation in micro-milling of Ti-6Al-4V using Ti(C7N3)- based cermet Micro-mill’’ by Wang et al. [150], with permission from Elsevier) demonstrated that the tool wear was exceedingly small in Fig. 12. On the other hand, surface quality at the up- compared to that of PCD tools, while a microtextured milling side was better than that at the down-milling side. surface with very fine textures was created on the SiC mold Currently, the smallest commercially available micro- in the ductile mode of material removal. Another material milling tools have diameters of 50 lm, with minimum commonly used for micro-milling tools is chemical vapor achievable machined features depending on workpiece deposition (CVD) diamond owing to its exceptional hard- material, machine tool accuracy, and feature geometry ness and wear resistance. However, the fabrication of such [151]. Therefore, further research and more in-depth tools by conventional grinding processes is inefficient in investigations will be necessary to explore the geometry of terms of productivity and edge quality. Yang et al. [148] more efficient tools, examine the limits of tool and feature developed a novel hybrid machining process that combined aspect ratio, and move toward submicron tool diameters in laser-induced diamond graphitization with precision the distant future. In addition, the influence of ultra-thin grinding to attain high-quality CVD diamond micro-mil- coatings and tool reconditioning processes on micro-mil- ling tools. Zou et al. [149] presented a micro-milling ling tools will be interesting areas for further research in investigation of Ti(C N )-based cermet tools developed in- the future. 7 3 house, taking into account the wear forms and wear mechanisms [150]. Their results showed that adhesive wear 3.4 Toolpath and microchipping were the main wear mechanisms of the major and minor cutting edges, respectively. It was also Thepsonthi and Ozel [152] proposed an integrated method demonstrated that metal debris and plastic side flow for selecting both toolpath and optimum process parame- became more severe as the tool wear progressed, as shown ters to meet certain machining requirements and 123 Precision micro-milling process: state of the art 191 constraints. The method outlined considers a mathematical cutting process. Therefore, depending on the toolpath, this model for determining the optimum toolpath strategy by may lead to two material removal regimes, i.e., where the using data from their experiments and FE simulations. cutting edge removes material by the shear mechanism and Optimal toolpath and process parameters can be used to where the tool center extrudes material through the establish more accurate predictive models by considering ploughing mechanism, as investigated by de Souza et al. and maintaining an acceptable tool-workpiece engagement [157]. Again, this effect will be even more substantial at load. However, it was verified that the resultant optimal the micro-milling scale, especially leading toward freeform toolpath strategy could only determine a certain level of micro-milling. Similarly, modeling the cutter envelope process performance, while a more in-depth study would surface is another important aspect as it can be used to be needed to ensure burr free micro-milling. Finally, the predict geometric errors and optimize toolpaths in con- team demonstrated that the toolpath strategy strongly ventional machining, according to Guo et al. [103]. affected tool wear and burr formation, while FE simula- Therefore, compensating tool runout errors during micro- tions provided an effective platform for toolpath selection. milling in toolpath planning may be helpful for maintaining An analysis of micro-milling vibration minimization and process stability in the future for 5-axis micro-milling. It is surface quality was presented by Wojciechowski and very surprising that toolpath planning and optimization of Mrozek [153], who used ball nose end mill tools at various the micro-milling process remain so underdeveloped at this tool axis inclination angles along the toolpath. They time, with conflicting results and conclusions on its effect showed that the tools axis of inclination in the direction as reported above. Toolpath development must become an perpendicular to the feed motion significantly affected both important area for future research to really advance the the dynamics of the process and the surface quality. micro-milling process as a whole, particularly for freeform Decreasing the inclination angle caused nonlinear growth machining, so that it will find further application in of vibration amplitude and surface roughness. The findings industries such as optics and biomedical devices were attributed to the ploughing-dominant regime resulting manufacturing. in growth of the cutting edge forces at low angles of inclination. 3.5 Cutting fluid For thin-walled structures less than 100 lm, it was found by Annoni et al. [154] that the down-milling strategy was Cutting fluids are essential to all cutting processes. They more influential with regard to geometrical errors, such as can fall into categories of coolant, lubricant, or both. They flatness deviation and average thickness error, compared to are used extensively to supply a steady flow of fluid into the up-milling strategy. Their results also showed that the the working area to cool, lubricate, flush away chips, reduce friction forces, etc. This leads to increased tool and toolpath factor did not influence the geometrical response. However, they recommended the application of step sup- machine tool life, improved surface quality, effective chip port, i.e., removing material from either side of the wall in management, and more efficient machining. Other desir- an offset technique and using the up-milling strategy. able properties of a cutting fluid are being nontoxic and Zariatin et al. [155] determined that there was no specific safe to handle, while preventing any chemical corrosion or correlation found among spindle speed, feed rate, and degradation of the tools or components [158]. Cutting machining strategy with the thin-wall accuracy [155]. fluids can be applied to the working area in various ways, Regarding path strategies, Koklu and Basmaci [156] such as by compressed air as minimum quantity mist, in a presented a study on the influence of cutting path on the flooding process, and at high and low pressure. Fang et al. cutting force and surface quality during micro-milling [159] even introduced chlorine mist and cooled air with pocket operations through analysis of the hatch zigzag and success. Koklu and Basmaci [156] implemented flood contour climb toolpath strategies under different cooling coolant during micro-milling and it was shown that the tool conditions. It was revealed that better results of up to 40% marks were homogeneously formed, while the deteriora- reduction in cutting forces and better surface quality were tion of the machined surface was minimized. However, this obtained with the use of the contour climb, also known as process of applying cutting fluid to the working area is down milling, compared to those of the hatch zigzag inefficient, expensive, and can cause negative effects to strategy for AA 5083 H116 aluminum alloy. These results operator’s health as well as the environment through bac- contradicted those of Annoni et al. [154] who recom- teria and fungi growth, which leads to bad odor, dissocia- mended conventional, also known as up milling, on 0.4% tion of emulsion, reduction in lubrication, and spread of carbon steel (C40). diseases [160]. In milling of freeform surfaces using ball nose end mill In terms of reducing the required amount of cutting tools, it may not always be possible to maintain the incli- fluid, Li and Chou [161] analyzed the performance of the nation angle so that the tool center is taking part in the minimum quantity lubrication (MQL) technique, as 123 192 L. O’Toole et al. depicted in Fig. 15b, in near micro-milling with respect to thermal expansion of the workpiece, as well as metallur- dry cutting on process outputs. It was found that the gical damage to superficial layers [167]. All of these application of MQL substantially improved the tool life, methods are heavily dependent on the material being surface roughness, and burr formation compared dry cut- machined, exact machining process, size of the chip to be ting based on slotting tests with micro end mills on a formed, design and geometry of the tool, etc. However, mesoscale machine tool. Huang et al. [162] used a nano- most of the work carried out so far has been based on fluid/ultrasonic atomization MQL technique with ultrasonic conventional machining processes, such as macro-milling dispersion during micro-milling of SKD11 steel. They and turning. Therefore, further investigations of the MQL compared different MQL nanofluids in terms of effects on process, dry machining, chilled air cutting fluid, and micro-milling cutting force, micro-milling temperature, cryogenic cooling must be carried out specifically for micro-milling tool wear, and surface burr. Pham et al. micro-milling processes under dry cutting conditions, [163] revealed that high-viscosity ionic lubricants provided while maintaining efficient chip removal, low coefficient of a slightly better machined surface and exhibited extremely friction, adequate cooling, and achieving close tolerances. low volatility compared with conventional oils or other The environmental impact of cutting fluids is currently lubricants in the micro-milling process. Javaroni et al. an important consideration in micro-milling; however, it is [160] showed that the conventional cutting fluid provided applicable across all machining processes. It will even better results for the output variables analyzed in advanced become more important in the coming years as industries ceramics grinding compared to those of the MQL process. strive to become more environmentally and economically However, the MQL can still present satisfactory results sustainable. This has led to interesting areas for future considering the economic, health, and environmental ben- research in micro-milling. For example, Chen et al. [168] efits offered by this technique. MQL can also greatly developed a novel electrochemical micromembrane tech- reduce the consumption of cutting fluid, thereby reducing nology to demulsify oily wastewater and recover oil from environmental pollution and associated costs due to lower oil-in-water cutting fluids. Similarly, Shen et al. [169] volume requirement, subsequent post-processing, disposal, presented an approach to recover cutting fluids and SiC etc. In addition, MQL can also enhance the ability of cut- from slurry waste. Although eco-friendly cutting fluids ting fluid to enter the cutting zone, which can greatly should be the target to move forward, the importance of improve the cooling and lubrication effects. In general, maintaining the process outputs will remain. Burton et al. MQL is a highly efficient and low-cost cutting fluid tech- [170] conducted an investigation into effectively obtaining nique [164]. a vegetable oil-in-water emulsion through ultrasonic Micro-milling processes may not always require cutting atomization. Their experimental results were very positive, showing lower cutting forces, smaller chip thickness, and fluid flooding, such as when light machining some poly- mers, ceramics, and alloys and when the risk of contami- less burr formation for the micro-milling process. Li et al. nation specifically does not allow it, e.g., machining some [164] further developed this vegetable oil-in-water emul- biomaterials for biomedical implants. Therefore, dry cut- sion-based cutting fluid by dispersing graphene and using ting conditions of nonconventional approaches are neces- the MQL technique, meeting the demands of cleaner and sary to lower the cutting temperature while ensuring sustainable manufacturing. The reduction and recovery of efficient chip evacuation. Effective nonconventional cutting fluid can reduce both the cost and environmental methods include MQL as discussed, dry cutting, chilled air, damage caused by machining, which should be viewed as cryogenic cooling, as well as the use of solid lubricants, all an integral component of every micro-milling manufac- of which have been shown to be viable substitutes to cut- turing chain. However, although the environmental aspect ting fluid while maintaining machining performance [165]. is an important consideration, the focus of research on The application of cryogenic cooling and chilled air can micro-milling cutting fluids should first consider the actually lead to lower cutting forces, surface roughness, lubrication properties to minimize BUE, friction, burr and tool wear during some machining processes owing to formation, ploughing, etc., followed by coolant properties the reduction in the coefficient of friction at the interface of to regulate the temperature at the cutting zone. the tool and chip; however, the opposite can occur as well when the mechanical properties of some materials, such as microhardness, increase under the condition of being 4 Advanced processes cryogenically cooled [166]. Dry machining has the benefits of reducing contamination and disposal, and it is the most Currently, the micro-milling process is limited by the environmentally safe option. However, dry machining inherent constraints of cutting material removal mecha- causes problems of high temperature, high friction, oxi- nisms at the microdomain, which include chip formation, dation, the inability to achieve close tolerances due to size effect, and process stability. However, these 123 Precision micro-milling process: state of the art 193 constraints may be overcome by the application and com- direction had a major effect on surface quality, with bination of new technologies with the micro-milling pro- vibration applied in the normal direction improving the cess, such as micro-rotary ultrasonic vibration-assisted machined surface. The vibration assistance also enhanced milling (lRUAM), laser-induced oxidation-assisted micro- the brittle-ductile transition of glass and therefore reduced milling (LOMM), and atmospheric-pressure plasma jet- the surface damage. Finally, they concluded that a higher assisted micro-milling. vibration frequency improved the surface quality by reducing the surface waviness. Bian et al. [174] also con- 4.1 Micro-rotary ultrasonic-assisted machining ducted an experimental investigation on micro-milling of brittle materials, but on ZrO ceramics with diamond- Micro-milling has been shown to be an advantageous coated micro end mills. It was found that the chips formed machining process for manufacturing surfaces, features, in the ductile mode were long and thin curled strips with a and structures in the microdomain with high accuracy and smooth back surface, leading to less edge and surface precision. However, the application of micro-milling in the chipping. However, the compressive forces due to the mold, optics, and biomedical industries requires that this ductile mode of material removal presented an increasing process must be suitable for machining typical difficult-to- trend with random fluctuations of the cutting force, leading machine materials, from very hard and wear resistant to higher tool wear. metallic alloys to very brittle ceramics or deliquescent With respect to very hard and wear resistant metallic crystal materials. One of the major efforts directed toward alloys in particular, Li and Wang [175] recorded lower tool processing these difficult-to-machine materials has been wear and better surface quality when the cutting speed was the application of lRUAM [158]. This process applies an much less than the maximum vibration during lRUAM ultrasonic frequency vibration in the range of 20–100 kHz compared to conventional milling. The material tested was with an amplitude between 5 lm and 50 lm at the tool tip AISI H13, which was suitable for manufacturing molds and in one or more directions, e.g., axially or radially. So far, dies. Xu et al. [176] performed lRUAM research on tita- lRUAM has been shown to reduce cutting forces and nium alloy TC4 and aluminum alloy 6061T6 with ultra- improve tool life during machining, owing to the working sonic vibration in the radial direction. Their experimental mechanisms of material removal, which form smaller chip results also verified that lRUAM could reduce the cutting sizes, reduce contact at the tool-workpiece interface, force, while improving surface quality by reducing reduce frictional forces, and inhibit crack propagation on machining marks. Burr formation was also substantially very brittle materials. However, strict control over the lessened in lRUAM, compared to conventional milling, as machining and vibration parameters, direction of vibration, shown in Fig. 13. Finally, the team determined that the size and the process as a whole is necessary, as tool life can effect appeared at much lower feed rates than in conven- actually be diminished with wrong parameter selection tional micro-milling. They proposed that vibration-assisted [171, 172]. machining at the microdomain triggered a change in the In relation to the abovementioned materials, Jin and Xie material removal mechanism. According to the authors, [173] presented an experimental study on the surface ‘‘impulse impact accelerates the generation and propaga- generation in lRUAM of a BK-7 optical glass using a tion of tiny cracks in the workpiece material, which redu- 2-flute micro end mill tool. They showed that the vibration ces the binding force inside the grains of the material.’’ Fig. 13 Machined surface after ultrasonic vibration micro-milling experiments at amplitudes of a 0 lm, b 2 lm, and c 4 lm, showing that ultrasonic vibration-assisted micro-milling can reduce surface defects and machining marks and thus improve the surface quality (Reprinted from ‘‘Machinablity improvement with ultrasonic vibration–assisted micro-milling’’ by Xu et al. [176], with permission from Sage) 123 194 L. O’Toole et al. Although this may be significant in reducing and elimi- opportunities for theoretical and experimental works yet to nating the size effect in micro-milling, an in-depth analysis be presented. will be necessary for future work. Feng et al. [177] pro- posed a predictive model to estimate flank tool wear to a 4.3 Atmospheric plasma jet-assisted micro-milling high accuracy. They found that a smaller axial depth of cut, larger feed per tooth, or higher cutting speed would result Atmospheric-pressure plasma jet-assisted micro-milling is in higher flank wear rate, while the effects of the vibration another underdeveloped assisted micro-milling process, parameters were less significant. Clearly, lRUAM is an which was proposed by Katahira et al. [183]. The team interesting area in the development of micro-milling, as it performed a feasibility study to investigate the effects of may reduce some inherent limitations of the current con- the application of an atmospheric-pressure plasma jet ventional micro-milling process. Much work needs to be during PCD micro end milling, which compared machined carried out concerning the physical process as well as SiC surfaces for both with and without the application of process inputs, while research will now require focusing on plasma jet. It was revealed that with the plasma jet, the more theoretical work rather than experimental work to formation of a high-quality surface was possible. More- fully characterize the process. over, it was also highly effective in improving the chip formation process by imparting hydrophilicity to the tool 4.2 Laser-induced oxidation-assisted micro-milling and workpiece surfaces, as well as removing surface con- tamination at the tool edge during machining. However, no LOMM is derived from laser-assisted micro-milling additional work had been presented on this process until (LAMM). LAMM combines the mechanical process of Mustafa et al. [184] very recently. They also confirmed that micro-milling with highly localized thermal softening of atmospheric-pressure plasma jet-assisted micro-milling the hard material by continuous wave laser irradiation. was a promising assisted technology with respect to the Subsequently, the softened material is removed by micro- micro-milling process, as it provided the lowest surface milling [178]. Compared to the conventional micro-milling roughness values among various cutting environments: dry, process, the cutting force in LAMM is substantially nitrogen jet, plasma jet, MQL, and plasma jet combined decreased and the tool life is prolonged. However, a high with MQL (see Fig. 15). The material tested was Inconel laser power is required to soften hard materials such as 718. It was determined that the plasma jet could promote ceramics. This would result in the ablation of workpiece fracture of the nickel surfaces and therefore reduce the material, expansion of heat affected zone, and formation of cutting force. However, it was demonstrated that the microcracks [179]. Therefore, Yang et al. [179] proposed residual stresses in micro-milled machined surfaces were compressive, and atmospheric-pressure plasma jet tended the novel process of LOMM, which used a relatively low- power laser to irradiate the surface of a ceramic material. to increase such compressive residual stresses. Again, far An oxidation reaction between the ceramic material and more work needs to be carried out to begin characterizing oxygen occurs, forming a loose and porous oxide layer, this process, which has great potential for reducing the which can be removed easily through the mechanical inherent issues of machining in the microdomain. process of micro-milling with a low cutting force there- after. Compared to the conventional micro-milling, the surface quality by LOMM was better, and the machining 5 Applications efficiency was improved by 104%. Xia et al. [180, 181] also presented a study on Ti-6Al-4V using this novel The development of machine tools and the manufacturing process. They showed that LOMM effectively decreased technology as a whole has led to high-precision micro- the cutting force and tool wear and prolonged the service milling processes in both research and industrial fields. The life of the tool. They verified that the cutting force when increasing demand for micro-structured parts and products removing the oxide layer in LOMM was 50%–65% lower with functional surfaces requires enhancing the process than when removing the material in conventional micro- efficiency to develop new technologies and improve milling under the same cutting parameters. It was also existing ones, so that a faster and more reliable production noted that the top burr width of the machined microgroove can be achieved [185]. The application of the micro-mil- and tool wear were smaller by LOMM. Wu et al. [182] also ling process ranges from fabrication of microstructures and confirmed that far less tool wear occurred for LOMM micro-components to micro-texturing and mold manufac- compared to conventional micro-milling, as shown in turing for industries such as electronics, aerospace, aero- Fig. 14. This is a very new and promising area of research nautics, and biomedicine. for the micro-milling industry, with considerable 123 Precision micro-milling process: state of the art 195 Fig. 14 Tool wear process with different material removal volumes: a with laser-induced oxidation and b without laser-induced oxidation (Reprinted from ‘‘Laser-induced oxidation of cemented carbide during micro-milling’’ by Wu et al. [182], with permission from Elsevier) nanometers [84, 186, 187]. The functionality of compo- nents can be improved by surface modifications such as microstructures using the micro-milling process. These structures can cause changes in the mechanical properties of the components, as reported by Godart et al. [188], who determined that 50 lm wide microstructures with a depth of 10–20 lm could increase tensile strength and decrease the fracture elongation in commercially pure-titanium workpieces. A basic example of such structure is a micro- thin wall, which usually refers to a cantilever structure with a thickness below 100 lm and a height to thickness ratio greater than 10 [189]. Accurate and precise removal of Fig. 15 Processes of machining Inconel 718 alloy a plasma-assisted material to form micro-thin wall structures is very difficult micro-milling process and b MQL process (Reprinted from ‘‘Atmo- spheric pressure plasma jet-assisted micro-milling of Inconel 718’’ by to accomplish in reality, especially for metallic alloys that Mustafa et al. [184], with permission from Springer Nature) tend to deform plastically when the wall thickness is in microns, as exhibited in Fig. 16. As the thickness of the wall is decreased, failure of the microstructure begins to 5.1 Micro-structures occur owing to the wall thickness exceeding the material threshold of rigidity and strength. Other microstructures One of the earliest and most employed application areas for include pipes, blades of an impeller or turbine, walls of a the micro-milling process is in microstructure and micro- microchannel, microcolumns, and fins of a heat exchanger. part fabrication. Microdevices can be defined as having at At present, these microstructures have been widely applied least two critical dimensions in the sub-millimeter range in micro-fuel cells [190], microfluidic chip channels [191], with at least one critical dimension significantly smaller and EDM electrodes [192, 193]. than 0.1 mm and with tolerance ranges of a few microns to 123 196 L. O’Toole et al. Fig. 16 Thin-wall features of 900 um in height (Reprinted from ‘‘Experimental study on micro-milling of thin walls’’ by Wang et al. [201], with permission from IOP) In the energy and electronics industries, the enhance- microstructures remain to be major issues that need to be ment in efficiency of heat transfer devices is crucial as the addressed. trend toward miniaturization of devices requires better understanding of heat transfer in small dimensions [194]. 5.2 Micro-texturing Repeated rib surfaces are known for their effectiveness in enhancing heat transfer and they are widely required in Another major application of the micro-milling process is many scientific and industrial applications. Further in micro-texturing or micro-patterning to reduce frictional improvements to these microstructures were made by forces and reduce wear between parts in industries such as Wang et al. through the introduction of a textured asym- automotive and biomedicine. As summarized by Chen metric arc rib structure on which microstructure arrays of et al. [202], functional microtextured surfaces have high secondary microgrooves were superimposed [195]. It was aspect ratio features, which enable the component to have then verified by Zhao et al. [196] that these hierarchical superior properties such as reduced adhesion friction [203], microstructures, composed of a primary microstructure and improved lubricity [204], increased wear resistance [205], secondary micro V-grooves, could be machined well by an ability to manipulate hydrophilic performance [206], as ultra-precision micro-milling process using a one-step well as influence optical properties [207]. Three possible cutting operation and a diamond tool. mechanisms by which surface micro-texturing improves Another application of microstructures that can be pro- tribological performance, as outlined by Chen et al. [208], duced by micro-milling is for mass sensing in microelec- are described as follows. Firstly, the textured surface can tromechanical system (MEMS) devices [197], which are increase the load-carrying capacity by serving as micro- used in the telecoms market, e.g., mobile phones and hydrodynamic bearings for hydrodynamic lubrication optical modulators [198], using lithium niobate (LiNbO ) [209]. Secondly, surface micro-textures can act as a second material. LiNbO is a crystalline material also often used in lubricant source to permeate the surface and reduce friction surface acoustic wave sensors and optical drives owing to and wear between both surfaces, creating a lubrication its superior electrical, optical, and physical properties boundary [210]. Finally, surface micro-textures can reduce [199]. However, because of its low toughness, it is con- the ploughing induced by abrasive wear and deformation sidered a difficult-to-machine material, which has con- between components by capturing wear debris between the ventionally been used as a substrate with deposited texture features [211]. It was also shown by Kovalchenko microstructures, rather than machined. However, owing to et al. [212] that arrayed dimples on contact surfaces under the ever-increasing demand for higher efficiency, direct lubrication helped to establish hydrodynamic pressure and fabrication of structures on the surface of LiNbO is now decrease the friction force. These three mechanisms of necessary, and this can be accomplished easily by micro- tribological performance improvements have significant milling, according to Huo et al. [200]. Clearly, micro- potential in orthopedic implants for arthroplasty procedures milling is a direct and effective manufacturing method for related to replacement of joints, such as hips, knees, or fabricating microstructures with complex 3D shapes. elbows. It has already been shown that micro-milling is a Nevertheless, the limitations of material deflection, plastic high-precision machining process suitable for difficult-to- deformation, and burr formation during machining of such machine materials, such as titanium alloys, cobalt-chrome 123 Precision micro-milling process: state of the art 197 alloys, and ceramics, all of which are commonly used for the average friction coefficient was reduced by 6.93%, orthopedic implants. while optical micrographs indicated that the microtextured Micro-textures can be fabricated by employing various specimens exhibited the narrowest and shallowest wear techniques, including laser machining [213] and etching track, in comparison to untextured specimens. Micro-mil- [214]. However, the most cost effective and efficient ling using a ball nose end tool is another viable technique method will always be rapid and direct machining owing to for creating such micro-textures, as demonstrated by Gra- the relatively low power consumption, precise and accurate ham et al. [216]. By tilting the tool at an inclined angle, the 5-axis CNC programming, and high surface finish, all of spindle speed and feed rate can be adjusted so that the which are characteristics of the micro-milling process. flutes of the cutter create periodic patterns in a workpiece Although the application of micro-milling to fit this pur- surface. However, the problem of burr formation associated pose is relatively new, there has been some interesting with the machining of microgrooves and micropatterns works presented lately. For example, Syahputra and Ko remains an important issue. In an attempt to solve this [215] developed a rapid process for acquiring complex problem, Fang et al. [217] studied the effects caused by texture data by using an image processing technique for the cutting parameters, work material, and cutting methodol- micro-milling process in which a complex surface texture ogy. However, more detailed work into tool geometry will representing human skin is transferred to a metal surface. be necessary to limit burr formation during micro-milling. Similarly, Chen et al. [208] concluded that the friction Evidently, micro-milling is an efficient and versatile performance of micro-milled Al-Si alloy ZL10 surfaces manufacturing technique, ideal for rapid machining of could be enhanced by malposed rectangle dimple micro- micro-textures and micropatterns across a broad range of textures, as illustrated in Fig. 17. Their results showed that materials and industries. However, the limited research Fig. 17 Microtexture SEM profiles with different distribution angles: a h =15, b h =45, c h =60, and d h =90 (Reprinted from ‘‘Effects of micro-milled malposed dimple structures on tribological behavior of Al-Si alloy under droplet lubricant condition’’ by Chen et al. [208], with permission from Springer Nature) 123 198 L. O’Toole et al. available at this time indicates a significant research gap, manufacturing perspective, the micro-milling process which must be filled so that this precision machining pro- allows for rapid prototyping of microfluidic devices [222]. cess can be applied in the field of orthopedic implant However, several challenges remain in micro-milling of manufacturing. molds and dies, namely burr formation and thin-wall deformation. Thus, it is important to identify and investi- 5.3 Mold making gate these issues in applying this process in the mold making industry. Saptaji determined that micro-milling is The most significant application of the micro-milling pro- capable of creating the features necessary for a thin cess is in the mold making industry, because it permits microfluidic embossing mold, with a thickness and feature precise, rapid, and accurate machining of high aspect ratio height of approximately 160 lm and 100 lm, respectively microfeatures, such as microchannels, microarrays, and [223]. The appropriate selection of the micro-milling thin walls, as already discussed. These molds are essential strategy is also crucial in achieving designed micro-lens to microinjection molding, micro-hot embossing, and array surfaces, according to Gao et al. [224], while dif- nanolithography industries for the polymer micro/nano ferent machining strategies have different machining sur- mass replication of features and surfaces [218]. The most face textures due to the cutting direction. An important obvious contribution is rapid manufacturing and prototyp- work by Ardila et al. [185] aimed at improving the entire ing of molds and mold inserts through finishing processes micro-milling production chain to generate knowledge of roughed out mold cavities, providing a quick and effi- about related process stages, including potential improve- cient processing time. A distinctive application example of ments of productivity and quality. The team concluded that this is for the microfluidics industry, which is important for to increase the application of micro-milling in the mold the employment of disposable medical sensors. This highly industry, the micro-milling process must satisfy the pro- significant area for micro-milling application in the ductivity and quality standards, confirming that the process microfluidics industry is in functionally optimized surfaces needs further research to comply with these requirements. through patterned microstructures on miniaturized biore- actor components, also known as ‘‘lab-on-a-chip’’, as shown in Fig. 18. The microfluidic chip provides a cheap 6 Conclusions and perspective and disposable platform for production and testing of pharmaceutical and biomedical products [219]. The pos- The high-precision micro-milling process was shown to be sibilities for such microfeatures may also offer an impor- a very effective and versatile manufacturing process, cap- tant role in distinguishing biofilm behavior in the future able of machining a broad range of difficult-to-machine [220]. The miniaturized size also allows for lower power materials for applications that require tight tolerance, good consumption and greater portability, while utilizing smaller surface integrity, and efficient machining across all volumes of reagents and samples, which are extremely industries. Although micro-milling is not a particularly new important to the microfluidics industry [221]. From a manufacturing process and has been the focus of consid- erable research work throughout the years, new and novel applications for this process are constantly being estab- lished in the industry. The flexible and versatile nature of micro-milling has guided this precision process from direct manufacturing of MEMS components to 5-axis CNC machining of precision molds for micro/nano-polymer replication. Currently, further development of the process is being driven by the necessity for rapid and highly accurate micro-patterning and micro-texturing of surfaces, for producing large numbers and arrays of micro-sized features such as dimples and rectangular pockets to increase lubricity and reduce friction forces between wear parts. The driving force behind this latest development is the requirement of the bio-implant industry to produce such features in a precise and efficient manner and to improve Fig. 18 Micro-milling of a lab-on-a-chip microfluidic mold: 4 arrays the tribological performance of orthopedic implants, of 28 pins with 0.8 mm diameter and 2 mm height (Reprinted from thereby extending implant life. Therefore, the development ‘‘Impact of deep cores surface topography generated by micro-milling of efficient and precise micro-milling to produce on the demolding force in microinjection molding’’ by Masato et al. microstructures, micro-textures, and high-quality molds [225], with permission from Elsevier) 123 Precision micro-milling process: state of the art 199 has promoted a sustainable future for the micro-milling insights into further development are considered and sig- process, with interesting new areas and applications to nificant research gaps are identified. overcome current limitations of other technologies. Acknowledgments This work was supported by the National Key However, as mentioned throughout the review, many Research and Development Program (Grant No. 2016YFB1102200), problematic and inherent issues of the micro-milling pro- Science Foundation Ireland (Grant No. 15/RP/B3208) and the ‘‘111’’ cess prevent its application in industries until further Project by the State Administration of Foreign Experts Affairs and the research and investigations are carried out. Such issues Ministry of Education of China (Grant No. B07014). include the phenomena of downscaling machining to the Open Access This article is licensed under a Creative Commons microdomain, i.e., the size effect, chip formation mecha- Attribution 4.0 International License, which permits use, sharing, nisms, and fundamental process instabilities involved. adaptation, distribution and reproduction in any medium or format, as Primarily, burr formation during channel or slot milling is long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate the most critical issue limiting the use of micro-milling in if changes were made. The images or other third party material in this microfluidic chip molds, where undesirable projections of article are included in the article’s Creative Commons licence, unless the material form as a result of the plastic flow from cutting indicated otherwise in a credit line to the material. If material is not and shearing operations. Since post processes such as included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted deburring are costly and are non-value added operations, use, you will need to obtain permission directly from the copyright understanding and control of burr formation are research holder. To view a copy of this licence, visit http://creativecommons. topics with high relevance to industrial applications, with org/licenses/by/4.0/. much work yet to be carried out. Similarly, adhered materials on the tool cutting edge, i.e., BUEs, have a large influence on the machining process outputs. 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Schlegel C, Chodorski J, Huster M et al (2017) Analyzing the his Ph.D. degree in Mechanical influence of microstructured surfaces on the lactic acid pro- Engineering from the University duction of Lactobacillus delbrueckii lactis in a flow-through cell of Queensland (UQ) in 2016. His system. Eng Life Sci 17(8):865–873 research interests are micro/nano 221. Weibel D, Whitesides G (2006) Applications of microfluidics in machining of difficult-to-ma- chemical biology. Curr Opin Chem Biol 10(6):584–591 chine materials and surface 222. Guckenberger DJ, de Groot TE, Wan AMD et al (2015) Micro- finishing of next-generation milling: a method for ultra-rapid prototyping of plastic bioimplants. microfluidic devices. Lab Chip 15(11):2364–2378 223. Saptaji K (2016) Micro-milling of thin mould for continuous Feng-Zhou Fang is a joint productions of polymer microfluidic device. ARPN J Eng Appl Professor and the director of Sci 11(24):14225–14230 Centre of Micro/Nano Manu- 224. Gao P, Liang Z, Wang X et al (2018) Fabrication of a micro-lens facturing Technology (MNMT) array mold by micro ball end-milling and its hot embossing. at Tianjin University and Micromachines Basel 9(3):96. https://doi.org/10.3390/ University College Dublin. He mi9030096 received his Ph.D. in Manufac- 225. Masato D, Sorgato M, Parenti P et al (2017) Impact of deep turing Engineering from the cores surface topography generated by micro milling on the Harbin Institute of Technology demolding force in micro injection molding. J Mater Process and has been working in the Technol 246:211–223 field of manufacturing since 1982. He has conducted both fundamental studies and appli- Lorcan O’Toole is a prospec- cation development in the areas tive PhD candidate part of the of micro/nano machining, opti- Centre of Micro/Nano Manu- cal freeform design and manufacturing, and ultra-precision machining facturing Technology (MNMT- and measurement benefiting a variety of industries in medical devices, Dublin) situated in the Engi- bio-implants, optics and mold sectors. neering and Materials Science Centre, in University College Dublin. Lorcan received his Bachelor of Mechanical Engi- neering (BE) from the School of Mechanical Engineering in UCD in 2018 and is furthering his studies in the area of preci- sion machining. In particular, the focus of his PhD is on micro-milling of difficult-to-machine materials.

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Published: Oct 27, 2020

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