January , 1942 AIRCRAFT ENGINEERIN G 1 designs which require tolerances closer than these. Refer to Table I giving practical tolerances for the common machining Aircraft Engineering operations. Machining uperations should be planned for straight Devote d to th e Science an d Practice of Aero machine feeds, and designs which involve elaborate "sculp turing " operations, and/or complicated tooling, should be nautic s and to Allied and Subsidiary avoided. Herei n lies th e whole philosophy of th e design of machined parts. Branche s of th e Engineering Industry It s lessons are emphasized over and over again throughout the Editor: Licut.-Col. W. LochwoodMarsh, O.B.E., F.U.Ae.S.,M.S.A .E.,F.I.Ae.S. series an d driven home by numbers of sketches of "awful examples" set alongside reasonable designs. Art for Art's Sake I t is so fatally easy for design staffs to draugh t parts, an d pass them throug h to the works, which fulfil their functions perfectly and are frequently quite fascinating examples of ingenuity; but which E complete in this issue the publication of the scries of immediatel y cause someone else to get out his drawing board and design a tool, of equal ingenuity, capable of doing the job. From articles by MR. JAMES E. THOMPSON of VULTEE AIRCRAFT th e point of view of " art for art's sake " these productions are on " Designing for Machinability " ; for permission to n o doubt worthy of all praise and reflect great credit on their in republish which from the pages of AERO DIGEST we are greatly indebted to the Author and Editor, MR . GEORG E F . MCLAUGHLIN. ventors—bu t they are not business. Let those responsible spare We have never read anything so informative and instructive on an hour for reflection on—and offer up a prayer for—the woes of th e unfortunate toolmaker. Let them think of him finding on his this subject, the importance of which for rapid production it is desk in the morning—along with, perhaps, a Government contract impossible to overestimate, and our admiration for it has increased for many thousands of small tools involving, in some instances, with each rc-pcrusal of it necessitated by the repeated re-rcadings called for in the various stages of proof correcting prior to final " straight runs " of a size running themselves to hundreds—an publication . We should like to sec MR. THOMPSON' S introductory order , with attache d drawing, for a complicated tool which ma y well tak e some hours to produce ; hours which otherwise might see the paragraphs , which appeared on page 289 of our October 1941 issue, appearanc e of hundred s of standar d tools. There is, surely, n o need posted u p in large letters in every detail-design office and we feel t o embroider the tale, or colour a picture which is most truly one tha t the best service we can do is to reprint them verbatim here. of those of which every one " tells a story. " And yet, in practice, it is rarely that any morning does in fact pass without every tool- Mr. Thompson Speaks make r finding one of these drawings in his post. We are not, Throughout the design of every machined part the designer of course, maintaining tha t in all cases the y are avoidable, bu t it is should follow the practice of visualizing the machining in incontrovertibl e tha t in a ver y large percentag e of instances they are. volved, and make a genuine effort to design for " machin ability " by avoiding all difficult operations. This factor cannot be over-emphasized, as it is only too easy to complicate The Part for the Tool th e machining of a part unnecessarily by lack of forethought during the design. I t may be an ideal, but output would be enormously enhanced if Complicated machined surfaces should be avoided, owing everyon e responsible for the design of detail part s kept at his elbow to th e increased production cost and the difficulty of obtaining schedules of standard tools and adopted as his slogan the mott o : and/or replacing the special tools required. Designs should " Design the part for the tool " ; with the emphasis on the last incorporate standard drill and reamer sizes, threads, milling cutte r widths, corner radii, etc., wherever possible, in order thre e words, which arc all too often omitted altogether. to utilize standard shop equipment. The use of standard tools W e quite expect to receive a number of letters and telephone is a more important factor to-day than ever before, because calls from outraged designers pointing out how thoroughly practical of the ever-increasing delay in the delivery of standard ar e the y an d their particular staffs and protesting against our action .machine tools, and in the procurement of special equipment. Machined parts usually represent a higher unit cost than in insulting them. To these we can only reply in advance that if any other type of part, and those which are spoiled usually tha t be so then it is obviously not they to whom we are referring ; have no salvage value. For this reason, it is vitally important with , perhaps, a sotlo voce reference to a certain proverb about the tha t thought be given t o production economy during the design fitting of caps. We wou'd then ask them to go through a batch of of machined parts. thei r detailed drawings and see if they are quite free from any Considerable engineering time can frequently be saved through consulting with the affected shop supervisors regarding bearing a certain resemblance to some of MR. THOMPSON'S the machinability of a given design. When consulting with " warnings." Let none stoop down to pick up the inevitable stone the shop be certain to give complete information about the unti l he is quit e positive tha t he is entirely without guilt. design in question, as too often an erroneous answer is received because of insufficient information regarding all factors A Lesson from Experience involved in a particular part. As it chances, we have awaiting publication an article, for which Limits and Tolerances ther e was no space in this issue, embodying tabulated results The general functions of machining are the removal of of causes of delays in production at the factory.of a well-known firm. metal, and the fitting of one part to another. Each involves I t is interesting to find that, as a result of an entirely independent holding the finished size of the piece to close limits if it is an d unbiased detailed investigation, one of the causes of inter t o be usable after machining. Limits on dimsnsions are ference with the smooth flow of outpu t proves to be this very point the maximum over and tinder variations from the true size, beyond which the piece being machined must be re-worked of inadequate attention to the " machinability " aspect of produc or scrapped. The tolerance is the sum of these two limits. tion in the designing of detail parts in the drawing-office. We arc, In selecting limits for a dimension, the designer should ask a t the time of writing, somewhat hampered by the fact that the himself, " Would I scrap a piece costing as much as this does, typescrip t of the article concerned is not available for reference, if it came from the shop 0.002 in., 0.003 in. or even 0.005 in. being in the hands of the printer for setting up in type, so we are beyond the limit I have specified? " If not, add that much to the limit placed on the drawing. unabl e to give further details. These will, however, be available Always keep tolerances at the most generous practicable t o those interested in our next issue. We are aware tha t it is only value, as nothing increases the cost of a part more than the too easy to dra w erroneous conclusions from statistics of this natur e ; necessity of working to extremely close tolerances. When the bu t the fact remains that there is corroboration of the view so design permits, tolerances should largely fall on the side of strongl y put forward by MR. THOMPSON. possible error : i.e. on a turned shaft, a minus tolerance (thus, +0.000, -0.002) ; on a drilled or bored hole, a plus tolerance (thus, +0.002, —0.000) ; on a close ground diameter or reamed hole, a balanced tolerance (thus, +0.0005). The fact that goods made of raw materials in short supply owing: to war conditions arc advertised in AIRCRAFT KNGINEERING should not be taken as an indication that they arc The designer should have a good knowledge of the practical necessarily available for export. tolerances for high-grade machine work, and should avoid
Aircraft Engineering and Aerospace Technology – Emerald Publishing
Published: Jan 1, 1942
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