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Aviation History
1957
1957 - 0076.PDF
7S FLIGHT THE PRODUCTION CONFERENCE . . . might be the criterion. It was possible to code cutter regrinding orreplacement into the control tape. The machine would slow down as it met a gradient. At present the control operated in three dimensions,but four or five were quite feasible. MR. R. S. BROWN (Bristol) asked how, as it was difficult to buyU.S. machine tools, it would be possible to use the E.M.I, control. Could the British machine tool industry make tools suitable for use with it?Three British companies were already doing so, said Mr. Booth: Research Engineers, Wadkin, and the British Cincinnati. The S.B.A.C.was, furthermore, always willing to specify requirements to the British Machine Tool Association. A skin miller for use with the analoguecontrol was under construction. The Second Day The second day consisted of a tight-packed morning programmeof three papers on actual manufacturing methods, taken at a rattling pace by MR. S. P. WOODLEY (Vickers-Supermarine). Thetime available necessitated condensation of two of the three inter- esting papers, which were taken in succession in an hour and ahalf with a composite discussion at the end. Of MR. R. G. WILKINSON, development manager of MagnesiumElektron, Ltd., it might have been said that he took as his text: "If you know how to use it, it is a marvellous material." InBritain wrought magnesium products had always averaged less than 10 per cent of the total for engineering purposes, while in theU.S. it was 30 per cent—with a peak of 50 per cent in 1950 when the Convair B-36 was in full production. This aeroplane usedabout 7,500 1b of magnesium sheet. The crystal structure of mag- nesium limited its capacity for cold deformation and any majorforming must be done between 230 and 350 deg. C. Once the incon- veniences associated with hot handling had been accepted, said thespeaker, numerous advantages were to be found. Generally, greater deformation could be achieved in one pass than with othermaterials; power requirements were less; dimensional accuracy was good, because of low spring-back; dimensional control couldoften be achieved by tool and material temperature adjustment. Cold forming was possible, but was restricted to simple bends oflarge radius; while, on the other hand, operators soon got the "feel" of preheating sheet with a blowtorch. The use ofmagnesium-alloy tools for small batches of press work gave good wear and obviated the need for temperature differential allowances.In stretch forming, essentially a tension process, the working tem- perature range was between 130 and l$0 deg. C. Properly used, magnesium and its alloys were not subject toexcessive corrosion. It was used, unprotected, in the Volkswagen car and in guided weapons, which might be stored for long periodsunder poor conditions. The latter case had led to the replacement of chromate treatment by, in the U.S.A., the H.A.E., CR-22 andDow 17 electrolytic treatments, and in this country by the fluoride anodizing and Higgins's surface sealing processes. Because it wasnotch-sensitive, riveted and bolted joints in magnesium had to be very carefully designed; but it could be successfully Argonarc- orspot-welded. The author showed a curve suggesting that certain magnesiumalloys maintained strength well into the "thermal barrier" range. HK 31, an American magnesium-thorium alloy compared favour-ably with the aluminium alloys up to 315 deg C, while an even better British alloy was on the way which had a Young's Modulusof 5.3 at 300 deg C and 5.5 at 250 deg C. The correct way to use magnesium was to take advantage of the stiffness possibilities. MR. N- K. GARDNER'S paper, Recent Advances in the Applica-tion of Resistance Welding to Airframe Construction was also, unfortunately, read in brief, despite the considerable informative-ness of the full text. Only four years ago, he said, an eminent aircraft designer had said at this conference that "stiffness couldnever be spot-welded into the skin." Today Boeing, Grumman, Handley-Page and Sud-Est Aviation were using resistance weldingfor primary structures (Mr. Gardner is with Handley-Page). Out of 300,000 welds radiographically examined in Victor pro-duction only 2-3 per cent were defective-r-and this was principally due to poor mating of parts. He considered that flash-butt welding,much used for joining light-alloy fork-ends and tubes, provided one of the few methods of joining titanium without inert gases,and it gave a magnesium weld of 100 per cent efficiency. The paper Contour Etching, by MR. A. W. SHEPPAKD ofSaunders-Roe, Ltd., described a relatively new application of an ancient technique. The speaker said that his company turned tothis method because of the trouble experienced with machined-in stiffness spoiling skin profiles; they had a pilot production shopoperating. Mr. Sheppard offered four points in favour of the method: economy in machine tools; weight saving; elimination ofretail tools; much simplified paperwork. There are four stages in the process: (1) Pre-treatment, i.e.,decreasing, pickling and washing. (2) Masking, application of a strippable coating, over which is placed a stencil, cut round witha knife; the "permanent" areas are then stripped of, after which a primer and the "resist" coating are sprayed on and, finally, after Fig. 3. Sections through sheet, showing results of faulty and good coating application. stoving, the strippable coating on the etching areas is stripped.(3) Etching is in a 12 per cent sodium hydroxide solution at 80 deg C to give 0.045 in/hour. (4) Post-treatment is a five-minute cold-water wash, five minutes in a 10-30 per cent nitric acid solution, a cold/hot rinse for ten minutes and, finally, removalof the "resist" by a ten-minute immersion in paint, followed by a last rinse. The stiffness of the "resist" coating is critical, asshown by the sketch (Fig. 3); given the correct value the radius at the edges will be equal to the depth. The etching rate is main-tained constant by seeding with aluminium oxide. Parts illustrated by the lecturer showed a wide variety of com-ponents of typical aircraft form which would normally be made by the attachment of stiffeners and doublers, or by machining.Taper thicknesses and curved skins could be produced. An interesting example was a chordwise joint in a curved skin—suchas a leading edge—in which the edges had been recessed to make a flush lap-joint without a butt strap. A perforated grid in material0.006 in. thick was made with a photo-sensitive coating, but was the thickest upon which a vertical edge was obtainable. Points from the Discussion Three points were made from the chair on the three papers. • (1)Magnesium needed a much better sales service than it has : builders of naval aircraft should have been pushed harder with its non-corrosiveproperties—and the magnesium suppliers could start with Super- marine. All in all, it made one wonder if the U.S. were ahead withmaterials. (2) It would be interesting to know a lot more about welding titanium. (3) There must be a tremendous future in contour etching,with its saving on finishing—and why not tape control for the manu- facture of masks? MR. W. E. MILLS (Avro) considered that contour etching was in itsvery initial stages. He thought that stripping by knife was crude and might initiate cracks, so why not a chemical method? Five minutesseemed short for washing. Had any prolonged fatigue tests been made upon etched material? The description of finish was nebulous; couldC.L.A. values be given? He thought that stress corrosion had been dismissed too easily. Mr. Sheppard replied thar knife stripping was themost satisfactory and the girls quickly learned to cut the coating without marking the metal—the polyamide cured epoxy resin "resist" was bothacid- and alkaline-proof. Etched parts had been left lying around for two and a half years without any sign whatsoever of "after-etching.'"MR. W. S. HOLLIS (M.O.S.) said that the N.G.T.E. had prepared a manual of methods for welding turbine components which covered mostof the points raised; this had apparently not reached aircraft firms but would be available to them. He also recalled the successful use ofmagnesium alloy for the tracks of gun turrets during the war, which work-hardened after the chromate wore off and wore well withoutcorrosion. Mr. Wilkinson thanked the speaker for mentioning this and said that it was an excellent material where bearing pressures were low;he instanced de Havilland camshaft bearing inserts, which never failed and never needed replacement. A rather vague question elicited a very informative disse:tation fromMR. WILKINSON. The new American AZ 31 has a good paper per- formance, losing little weight in an immersion test, but it is subject todeep pitting at an inclusion—which can be catastrophic. The new British equivalent loses more weight, but does so all over without anypitting, moreover, it has a higher residual stress after heating to 30C deg. C. The B-52 aircraft carries 4,500 lb of AZ 31B, while the outerskin of the thermally anticed leading edge of the Boeing 707 is also of AZ 3IB. Boeing were investigating, and Chance-Vought had applied tothe Crusader, chem-milled magnesium alloy. The significance of using magnesium is less the actual weight saved than the use of thicker, betterstabilized, stiffer material and the reduction of the number of com- ponents; hence the number of stress-raising fastenings was alsoreduced. To a question from MR. K. E. CHEVERTON (Bristol Aircraft), Mr.Gardner said that spot-weld fatigue tests for the wing case were +6.5 to — 7.5 per cent of design load, and the target of a million reversals wasreached. The pressure cabin test was from zero to design load. Boeing had set a target of 65,000 reversals and found it easy to achieve 125,000Handley-Page aircraft were designed so as not to buckle in the straighi and level flight case. Loading had been stepped up until there wasskin buckling, and even then the welds had taken twenty-five million reversals. Shrinkage cracks at the spot were a welding defect, but evera sandwich specimen with thirty of these did not fail—the breakages all came from rivet holes in the inner skin. Some 150 Victor and Hera'dparts had been tested without a single case of a failure originating from a weld. Corrosion between sheets gave no trouble, but a protective filmcould be used if welding was done while it was wet, as Sud-Est Aviation did with Araldite for the Caravelle. MR. BROADBENT (Sterling Metals) considered that the U.K. was aheadof the U.S. on magnesium castings and said that his company supplied 8,000 a month to one customer—and that from 600 different patterns
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