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Aviation History
1957
1957 - 0075.PDF
18 January 1957 75 Thanking the speaker, SIR ROY FEDDEN said he thought that thetroubles instanced should even more bedevil today's more complex -air- craft. He noted what an improvement there was in the U.S. scene as aresult of the rationalization following the Findlater Report, which had been called for by President Truman in 1947. He hoped we couldlikewise take a few stitches in time. Seconding, MR. T. GlLBERTSON, director and general manager ofFolland Aircraft, Ltd., said he "wished to goodness someone would be enough of a dictator to standardize a drawing system in this country."The Henschel film would send people away thinking of the wicked waste of mild steel thrown away in jigs. Ninety per cent of such exten-sive tooling effort, e.g., drilling heads, is not wasted in this type of manu- facture. Production engineers for the future must be drawn from themost intelligent of the coming generation, whereas at present the tendency is to channel the best brains toward research and development. Session II, under the chairmanship of PROFESSOR E. J. RICHARDS, consisted of one paper, The Fatigue of Aircraft by MAJOR P. LITHERLAND TEED of Vickers-Armstrongs (Aircraft),Ltd. Instead of reading through his printed paper, Maj. Teed gave an "off-the-cuff* discourse, from a typewritten text, in his ownpuckish style. He started by making a series of appeals. To the designer hesuggested two precepts: design to fail safe and where this is im- possible aim for a long fatigue life. To the operator: avoidrepeated loads by flying high and using smooth runways. To the pilots: adopt a policy of slow descents—a rapid let-down is likedriving fast over a rough road. To production engineers: con- centrate upon surface finish, residual stresses, built-in stresses, andmetallic structures, "both micro and macro." Since the surface was always the most highly-stressed area this was the mostimportant and fortunately it could be well controlled. Built-in stresses could be avoided by the use of shims and other methodsof avoiding tight fits—in which connection it was worth noting that the ball in a bearing was the only truly axially-loaded partin the world. Every rivet or bolt in the million in a modern aeroplane was asource of several stresses, which was one good reason for using larger integral components. Discussing the effects of heat treatment and machining, Maj.Teed made the point that metals reacted to the algebraic sum of the stresses imposed upon them. The influence of residualstresses induced by plastic deformation on the fatigue resistance of a notched wrought aluminium-alloy test-piece was shown by aslide (Fig. 1). Having been pre-loaded in tension, the test- piece had a residual compressiye stress at the base of the notch,thereby increasing fatigue resistance. Pre-loading in compres- sion, producing a residual tension on test, the fatigue life is con-siderably reduced. An R.A.E. test had similarly shown that the application of a load well beyond the elastic limit every 20,000cycles had extended the fatigue life of a wrought aluminium-alloy part tenfold.In response to a question by MR. J. GREGSON (Fairey) the speaker said that machining after heat treatment could be very beneficial byengendering residual tensile stresses in a component. MR. J. C KING (English Electric) quoted the case of a forked fittingwhich fitted over a plate and which developed stress-erosion cracks between adjacent attachment bolt-holes. These cracks started on theinner faces, and by inserting a thin spacing foil between lug and web it had been possible to apply a strain-gauge between a pair of holes,from which it was learned that the mean, not peak, stress was as high as 20,000 Ib/sq in when the bolts were tightened. This was due to Fig. 1. Effects of internal strain on the rotating-beam strength of 75S-T6 (Templin). DIA ^/ OOO2"RAD• SPECIMENS NOT S PRELOADED \ SPECIMENS PRELOADED TO PERCENTAGE OF TENSILE STRENGTH D - 9O% ] O - 7O°A> [ INTENSION O - so% j • - so% Oo, IN • - /u /o COMPRESSION _ 9O% INGLIS STRESS CONCENTRATION- 39 8 local bending stresses caused by unevenness under the bolt heads.Offering this experience for the general benefit, the questioner asked how the speaker dealt with such incidents. Major Teed replied that thetechnical office advised on methods—probably after trouble had occurred! In response to other questions, the speaker said that the B.S.I, wasslowly improving the definitions for surface finish. Electrolytic finish gave the best polish, but was bad for fatigue, since it did not resiststresses well. Metal near its melting point usually failed along the inter- crystalline boundary, whereas when it is several hundred degrees belowits melting point it fractured across the crystal. A corrosive atmosphere bleached out the boundary, so causing the crack to start (unnaturally) inthis zone—this phenomenon was prevalent in the aluminium-zinc alloys. Session III consisted of a lengthy paper entitled NumericalControl of Machine Tools in Aircraft Manufacture, by MR. O. S. PUCKLE of E.M.I. Electronics Ltd. This speaker chose to readhis paper out of its printed order, and he showed few of the slides which amplified the text, so that it was less informative than itcould have been. "Numerical control" is a term which originated at the Massa-chusetts Institute of Technology and means that a machine is controlled by the injection of numerical position information,which acts through suitable servo-motors and valves upon the more or less normal machine parts. There are two basic methods:the digital and the analogue. The former can be said to control by incremental dimensions and the latter by positional sensi-tivity. The drawing information is converted into programme sheets, from which it is then converted into tape or card informa- Fig. 2. Numerical control system tor Cincinnati gantry-type miller A—Part drawing. B—Programme sheets. /C1—Paper-tape punch. Method D1—General-purpose 1 I computor. I E1—Card-punch operated I by computor. I C2—Desk calculator. Method 2 ! D2—Manually operated I card punch. F—Punched cards. G—Control console. H—Machine control- cabinet. J—-Skin miller. Di 1 1 E1 lop o s ^*> 1*/ I 1 [r H • 1 tion (Fig. 2). In effect, numerical control replaces the elaboratemachining fixtures and/or the progressive measurement of parts. E.M.I, is currently developing control systems for integral wingstructures, wing spars and ribs, turbine blades and ship pro- pellers. Turbine blades, with their development system of mathe-matical expressions for the aerofoil ordinates, are well-suited to programming. As an example of accuracy, E.M.I, expects thatits system used with a Cincinnati skin miller should be of the order of O.OlOin in the x axis for the full length of the table and0.002in in the z (vertical) axis. There is an agreement whereby E.M.I, is to provide numerical controls to Cincinnati for itsHydrotel and other appropriate machines. The E.M.I, analogue control gives the discrete dimensionaldrawing information to the machine in the form of successive tool positions with an interpolation process to ensure a smoothcurvilinear motion. The control cabinet signals have output voltages which give continuous analogue representation of therequired work. Feedback loops provide a constant check upon the relative positions of tool and work. By a device in the cabinetwhich continually computes the normal to the curve being cut, automatic compensation for cutter wear can be provided. Mr.Puckle said that such compensation could not be achieved with digital control. For his replies Mr. Puckle was joined by MR. R. H. BOOTH, technicalmanager of the industrial applications division of E.M.I. Engineering Development, Ltd. MR. H. H. CHAMBERS (Rockwell Machine'Tool Cq., Ltd.) saw anapplication to the making of forging dies and thereby an increase in the use of forging presses. What happened with a blunt cutter and whatoccurred when a cutter met a sharply rising curve? Mr. Booth said that although cutter wear could be compensated, a deterioration in finish /
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