April, 1943 AIRCRAFT ENGINEERIN G 93 of defectives", again, has valuable possibilities when the final test of quality depends on visual inspection—of finish or general work Aircraft Engineering manship—as opposed to conforming to dimensional tolerances. Devote d to th e Science an d Practice of Aero Readers may, in these connexions, be directed particularly to Fig. 15 of the article in this issue. nautic s and to Allied and Subsidiary- Branche s of th e Engineerin g Industry "Chance " and "Assignable" Causes Editor: Lieut.-Col. IV. Lockwood Marsh, O.B.E., F.R.Ae.S.,M.S.A.E.,F.I.Ae.S. I t is important to appreciate that the basis of quality control is an inspection routine for the examination of samples at regular in Vol . XV , No . 170 April 1943 tervals during the process of manufacture. It is, therefore, in sharp contradistinction to the normal inspection of finished products; which results merely in the throwing out of defectives. Quality control is designed to avoid the making of these defectives. It N this issue is brought to a conclusion the series of three articles constitutes a scientific method of separating the random variations on " Quality Control in Production Engineering " by MR. H. in'size, or some other quality, of product, due to *' chance " causes, IRISSIK who is one of the pioneers of the use of statistical charts from systematic and recurring variations, due to " assignable " for controlling the quality of products during manufacture. He causes. The moment the latter are eliminated production can be is a member of the B.S.I. Committee on Statistical Methods in allowed to continue with the knowledge that the products will be Standardization and Specifications, and was, we believe, responsible within the " control limits". A serious misconception into which for the first experiment in the application of quality control to many critics of the method fall is to assume that the control limits production engineering in England—at the Croydon works of of the chart are a narrower version of the engineering tolerance limits MESSRS. CREED & Co.—in 1940. In addition to this, he has been set by the customci. This is based on a complete misunderstanding occupied during the last six months or so in giving special lecture of the whole procedure. The two are not in fact directly comparable., courses on the subject at Technical Colleges in various parts of the The " control limits " are designed to produce an average—based country. on experience in the manufacture of the particular product con I t will be remembered that we originally called attention to the cerned—which will ensure the products passing inspection. statistical control of quality in our issue of July last year in con Tolerance limits are individual measurements within which each nexion with an article we were then publishing on inspection of the products turned out must fall. The point of difference will methods and salvage at the Curtiss-Wright factories in America. become clear on an examination of the Figure 15 to which we have already referred. Interpretation for Engineers It was as a result of what we then wrote that MR. RISSIK got in A Possible Future Development touch with us ; the outcome of which was that he agreed to write these articles for the benefit of our readers. They constitute, we There is reason to believe that Quality Control may have even have reason to believe, the most complete reasoned explanation of wider and more fundamental uses than any of the applications to the significance and value of the method yet written for the pro which it has yet been put. It seems not too much to hope that it duction engineer. Hitherto, the only authoritative literature on the may come to the help of aeroplane design by providing a closer subject has been two booklets issued by the BRITISH STANDARDS understanding of the strength properties of materials—and ulti INSTITUTION which have the disadvantage of being somewhat mately of aeroplane structures. With the fuller knowledge and academic and more concerned with the mathematical demonstra control of material strengths that may develop, permissible stresses tion of the soundness of the theory than the practical application for design might be linked more precisely to the actual products of it. Details of these two publications, for those who wish to go and perhaps, in some instances, raised without undue risk. The back to fundamentals, were given by us last July and .are repeated old difficulty of interpreting structural tests on isolated wings and in a footnote on p. 116 of this issue. fuselages, which arises from the possible departure in strength of any one specimen from an average or minimum specimen; might be Practical Examples solved in a more scientific and economical fashion than has hitherto Another advantage of the present series is that the examples been possible. The achievement of any such results would clearly given are taken from experience in works engaged on the production open up important opportunities of saving structural weight. It of general engineering parts, or, in one instance, specifically in an seems evident that Quality Control may have Useful applications aero-engine factory; whereas in the B.S.I, booklets the examples in directions, such as this, outside the field customarily assigned to were taken from the application of the method to the production of it at present and come into the picture in the design stage instead such products as fuzes, glass tubing, etc.—which are somewhat of only later on in the factory. It is too early yet to do more than specialized products. Indeed, the' range of examples quoted by indicate the possibility of these developments. We hope to be MR. RISSIK covers a sufficiently wide field for almost any firm in a position to return to the subject in the future. connected with the aircraft industry to be able to form an opinion as to whether, and How far the method is applicable to its own business. Given the will—which is the crux of the whole matter— THE CASE OF SHORT BROTHERS it is our own conviction that there are extremely few engineering An admirably informative debate in the House of Lords on firms the quality of whose products would not be improved in April 13 served to remove the fears engendered by what had hitherto homogeneity and reliability by the intelligent introduction of appeared to the public as the high-handed action of the MINISTER " quality control". OF AIRCRAFT PRODUCTION in taking over the share capital of Short The "Method of Defectives" Brothers. These fears could have equally well been dissipated by SIR STAFFORD CRIPPS in the House of Commons, but he preferred At first sight, it might not be thought to be likely to be helpful instead to adopt an intransigeant dictatorial attitude which only in a purely repetitive process—such as, lor instance, the large-scale served to intensify mistrust. Why he should have chosen this production of rivets or screws—but in the final instalment in this course is beyond comprehension, and it has not served to increase issue MR. RISSIK shows that by turning to the " method of defec public confidence in his suitability for the position he occupies. tives " valuable improvement can be obtained. This method is the appropriate procedure, also, in all cases of purely manual The position in regard to the shares after the war still remains production processes, such as centre-lathe turning, grinding, etc., obscure. It is obviously going to be a difficult problem, because when adherence to dimensions is entirely under, the operator's there can be no guarantee that the dispossessed shareholders will control. A further advantage of the system in this class of work then be in a position to repurchase, even if they are given the is the opportunity it gives for the introduction of a " quality opportunity to do so. To whom, then, are the shares to be offered bonus " as opposed to the familiar output bonus. The " method when the Government decides to " unload ' ?
Aircraft Engineering and Aerospace Technology – Emerald Publishing
Published: Apr 1, 1943
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