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Workshop and Production Section AIRCRAFT ENGINEERING July, 1943 HE original experiments with tungsten car I t is useful at this stage to classify the whole bide demonstrated very clearly that this range of the Firth-Brown Carbide steel-cutting material was to occupy an important place in grades in order tha t the value of each can be properly the field of machining metals. Early development assessed, showing in what way the grades cover the however, showed tha t the first carbides were of little whole field of the machining of steel. It should be use in the machining of any types of steel, for noted that the grades are given in the order of reasons which, at that time, were not apparent. toughness and hardness; the first grade is the This limiting feature of the early hard metals repre toughest, while the last grade is the hardest: sented a serious handicap in the development of the (1) GRADE TA5—a heavy-duty grade for the rough use of carbide-tipped tools. As a result of world-- machining of steel under bad conditions. To wide research the cause of failure of the original replace high speed steel on machines where tungsten carbide to perform satisfactorily on steel conditions cannot be materially improved. For was established, but a complete and satisfactory machining a t relatively low speeds, heavy feeds. explanation of the phenomenon has never been For intermittent cutting. forthcoming. (2) GRADE TA—a general purpose grade for machin I t was observed that these early tools failed by ing steel under normal conditions on reasonably what is now known as " cratering". This pheno good machines. It will satisfactorily rough menon is due to pressure of the chip on the upper machine forgings, etc., but has not the shock surface of the tool. Where the chip impinges the resistance of Grade TA5, and will not withstand surface is eroded. This form of cavitation is not the abuse for which the latter was designed. unfamiliar to those who have studied the break Grade TA is particularly suitable for the machin down of high speed steel tools used for heavy cuts. ing of alloy steel bar and forgings. The cratering effect only occurs when turning steel (3) GRADE TE10—a grade harder and more wear- and the problem, therefore, does not arise when resistant than Grade TA, which may be used for turning cast iron. rough machining under very good conditions or This characteristic was eventually overcome by for finish machining where conditions, particu the addition to the early hard metals of one or more larly of the machine, are not as good as they of the carbides of titanium, tantalum, niobium and might be. Particularly useful on mild steels molybdenum. The addition of these carbides cer and low alloy steels. This grade has proved to tainly prevented the formation of the chip cavity be particularly effective for the machining of and as a result such complex carbides would, under shells on modern shell turning lathes. certain conditions, work for prolonged periods when (4) GRADE TE—for finish turning all classes of steel. cutting steel. They were found, however, to be Roughing operations: Conditions should be good and fine feeds should relatively brittle compared with the normal tung Speed 80 ft./min. be used. It is possible to use extremely high sten carbides previously used. For the successful Feed 0·040 in. per revolution. speeds with this grade owing to its extreme machining of steel with these alloys conditions had Depth of cut varying from 0 to 1⅛ in. hardness and wear resistance. to be good. Fine feeds and high speeds were the Tool cut twice along bomb between regrindings, a The series is completed by Grades A, B, C and order of the day and only machines in good condi total distance of 12 ft. Dl , designed for machining cast iron, non-ferrous tion were suitable for the use of such carbide tools. Diameter : approximately 15 in. metals, glass, pottery, synthetic substances, etc. Semi-finishing operations: The Brown-Firth Research Laboratories have Speed 107 ft./min. carried out exhaustive investigation into this com- Feed 0·050 in. per revolution. plex system of carbides and as a result of their work Control of Electrical Equipment Depth of cut steady, between £ in. and improvements have been incorporated into their ¾ in. Mitia hard metal which now offers a grade suitable H E Minister of Supply has made an Order The tool used for th e above operation is one speci for almost any set of conditions. The latest addi under which the disposal and acquisition of ally designed for the job, and in all cases where tools tion to the list is Grade TA5, which may be said to Tindustrial electrical equipment is, except in are used for such arduous work a specially developed complete the series. circumstances defined in the Order, subject to design should be used. The shank section is 2 in. by In developing this grade attention has been paid licensing by the Directorate of Industrial Electrical 1½ in. The tool works with a 15 deg. top rake which to producing a material which, while showing the Equipment, Machine Tool Control. is, of course, a departure from normal carbide greatest resistance to cratering, has the strength and The main purpose of the Order is to ensure that practice and is essential in the use of this grade. toughness to stand up to the arduous conditions electrical equipment, the manufacture of which The tool, when semi-finishing, is employed with a which it is designed to meet. In addition, tools of absorbs scarce materials and labour, is used to best special form of chip-curler. this grade will take a top rake and heavy cuts can advantage to meet essential requirements. The Case 2. Turning large forgings. then be taken at high speed without the absorption control is applied in a manner suited to the normal Size, 20 in. dia. by 3 ft. 6 in. long. of unduly increased power. Used under ideal procedure of the electrical industry and with a Material: Nickel chrome steel 65 tons/sq. in. tensile conditions this grade will prove of great value in the minimum of formality in regard to supplies of strength. rapid machining of heavy forgings, etc. It will equipment for essential purposes. possess, too, the great advantage that it can be Condition: Forged; bad, irregular and scaly sur To avoid duplication of control the Board of successfully used, and with greatly increased tool face. Trade Machinery, Plant and Appliances (Control) life, on machines and under conditions where high Speed: 36 ft./min. (maximum of machine). (No. 3) Order 1942, will no longer apply to indus speed steel normally would have to be used. Feed: 0·070 in. per revolution. trial electrical equipment. Depth of cut : averaging ⅝ in . The Order relates to electric motors, generators Cut for 57 in. (part secondcut) withdrawn for re Speeds and Feeds and other rotary electrical machinery and control conditioning but only slightly worn. The speeds and feeds recommended for Grade gear up to 1,000 kVA, kW or h.p., switchgear over Case 3. Turning large bombs. TA5 vary considerably, since an outstanding feature 60 amperes capacity for use on circuits up to 650 Material and condition as for Case 1 of the grade is its ability to operate over a wide volts, and static condensers for power factor cor Speed (a) 50 ft./min. (6) 65 ft./min. range of conditions. rection. Feed 100 in. per revolution Speed and feed depend upon the quality of the The Order permits the acquisition or disposal of Depth of cut 1 in. material being cut. For low carbon steel the speed certain electrical equipment to Government De Two tools used for test and both capable of machin should be 125/200 ft. per minute, while th e feed may partments, public utility undertakings and firms ing a complete forging before regrinding. vary from0·015 in. up to 0·125 in. per revolution. engaged on essential work, without licence in speci I t should be clearly understood that in introduc For steels having a tensile strength of 40·65 tons/sq. fied circumstances. ing the new grade TA5 it is not suggested that it in in. the feeds used should be from 0·15 in. to The Order makes provision for the acquisition of any way supersedes the other grades of carbides 0·100 in. per revolution. Steels of higher tensile equipment temporarily in an emergency and for the which Thos, Firth & John Brown Ltd. manufac strength have to be treated more individually, use of certain used equipment where only a small ture but is a necessary addition to their already since machineability varies according to the com amount of labour and materials would be required wide range of cutting alloys. Grade TA5 is for use on position, etc., and in these cases advice and assist for reconditioning. machines which are normally considered to be suit ance as to the suitability of Firth-Brown grades and Applications for permission to acquire industrial able only for high speed steel and where the normal cutting conditions should be sought. electrical equipment for any purpose requiring a grades of carbide have proved too hard and too licence must be made to : brittle for successful use. Examples of Performance Directorate of Industrial Electrical Equipment I t is of particular value in dealing with those Machine Tool Control, Grade TA5 has been thoroughly tested in various numerous jobs which call for a better tool material 35 Old Queen Street, machine shops, including Firth-Brown's own works, than high speed steel bu t where th e normal grades of London, S.W.I. and the following examples show the performance carbide cannot, due to the conditions, be applied. Copies of the Order, The Control of Industrial of th e tools: On the other hand, where conditions are good then Electrical Equipment (No. 1) Order, 1943, (S.R. & the old established grades of Firth-Brown Carbide Case 1. Turning large bombs (Fig. 1). O. No. 533, 1943) which came into force on April 19, should be applied since their superior hardness and Material: Alloy steel, machined in 50-60 tons/sq. 1943, may be obtained from H.M. Stationery wear resistance will, in such cases, allow even greater in. tensile strength condition. Office, York House, Kingsway, W.C.2, or through Surface : Extremely bad forged surface. Scale up speeds to be attained, with longer periods between any bookseller. to in. thick. Very irregular in shape. regrinding.
Aircraft Engineering and Aerospace Technology – Emerald Publishing
Published: Jul 1, 1943
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