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Aviation History
1957
1957 - 0208.PDF
FLIGHT15 February 1957 "HOT ENDS" . . . Jet-pipe work: in the foreground the girls are lagging the jet pipe proper with Alfol; while in the background the light pro- tective hinged outer casings are being fitted. either plain or "gashed" pots, i.e., those from which surplus metalhas been removed (between what will be the flanges) by rough turn- ing while the metal was hot. In order to avoid waste of the expensivealloy the pots are usually welded to mild steel chucking or cramping bases and are then machined to internal and external diameters,faced and parted in the vertical borers. The faces of flanges are trued to remarkably fine limits by grinding, usually as a finaloperation after component-assembly by welding. Each part is measured by the inspection department beforeremoval from the machine tool, since large-diameter hoops fre- quently "spring" after release. However, if it has been machinedtrue, any ovality will disappear upon assembly. Components are also checked on inspection fixtures (i.e., interchangeability gauges)in the W.I.D. inspection cribs which are located strategically round the shops. A Briggs innovation, quite separate from A.I.D. procedure,is the "component control number" which is given to each item when the raw material is issued from the stores. This number,first painted and then etched on as soon as there is a finished surface to take it, enables the whole history of the part, includingits X-ray and metallurgical analysis history, to be known and recorded. The second main source of detail parts is the press shop; herethe principal equipment consists of 150-ton British Clearing hydraulic presses, plus some smaller presses, two 300-ton Miillerpresses and an Erco stretch-forming press. Most of the dies are made from Narite, which is a compara-tively new type of centrifugally cast iron which has a long life (25,000 steel parts) and gives a high finish. Fibreglass and otherplastic dies are used for stretch-forming dies and for the "expanding-block" technique. This last process is a means ofensuring that large-diameter sheet-metal parts are true to size and form before welding to their flanges. The parts are madea known percentage undersize and then put over a segmental core pattern which is expanded momentarily oversize (withoutexceeding the elastic limit of the material) by a tapered hydraulic ram that is forced up its centre. The part is thus stretched tosize by a calculated permanent set and any wrinkles disappear. The presses for this work are located about the shop near eachjob and are not concentrated in the press shop itself. Heat Treatment and De-scaling. Closely associated with thepress shop is the heat treatment department, since the stainless steel (e.g., D.T.D.171) and Nimonic sheet requires annealingbetween the several deep-drawing operations necessary on most A contrasting type of component produced by Briggs Motor Bodies— an Armstrong Siddeley bearing housing. The part, as may be seen, is largely the result of machining from a forged alloy steel billet. To the finished machining the external stifteners are welded. of the combustion-chamber pans. According to the type of treat-ment and the component concerned there are muffle furnaces, a pit furnace and a continuous conveyor-belt electric furnace. Thelast-named is particularly useful for small parts, which pass through it in twenty minutes. Directly associated with heat treatment are the de-scalingfacilities. The system used is an I.C.I, sodium hydride plant— which is comparatively new—in preference to the more usualcaustic-soda bath. De-scaling is of the greatest importance, not only to ensure good finish in press work but also to avoidinclusions during welding. Sodium hydride provides a quicker and more complete removalof scale than caustic soda, but it is a process requiring rigid pre- cautions against explosion. The parts are immersed in a causticsoda bath through which the sodium hydride is passed. The gas is produced in generators containing tablets of metallic-sodiumover which hydrogen from ammonium crackers is passed. Since water reacts violently on contact with sodium hydride the partsare pre-heated in order to dry them thoroughly. The de-scaling plant is impressive in appearance, separatedfrom the shop by a blast screen and with an orange flame from waste sodium flickering on top of the deposit-encrusted tank.The operators wear protective helmets and clothing and handle the parts remotely from outside the screen. After treatment the parts are rinsed in hot and cold water,to neutralize the caustic soda, before being "flash pickled" in a nitric acid bath to remove the grey residue left by the sodiumhydride treatment; after this they are finally spray-washed. The joining of the machined and pressed details, apart froman occasional riveted or bolted joint, is either by fusion or resistance welding. The former includes oxy-acetylene, Argonarc,metallic-arc and submerged-arc welding; while the latter embraces spot, roller, seam, stitch and flash-butt welding. The methodused depends upon the strength requirements of the component, the conditions under which it will operate and the accessibilityand "distortability" during assembly. For all types of welding Briggs adopt the general principle of tacking in a jig or fixture,with subsequent welding "free." The more complex components are, of course, welded on a "north-south-east-west" sequenceto balance-out distortion. With the tolerances of welded airframe components in mind, we nearly wrote "to minimize distortion";but these gas-turbine parts are made to very fine limits—as witness a Dart RDa.7 discharge-nozzle flange with ± 0.0005in ona 4.25in diameter. Deep tubular components are tack-welded in their fixtures byspot-welding "pistols," with long jaws, that are slung from over- head girders. Such a component as a jet-pipe is made in sectionsabout three feet long, then these are joined, first at the centre, then working outward by adding front and rear portions. Themost complex jet-pipe unit made to date is the Avon RA.7R afterburner as fitted on the Swift. The electric welding machines at Briggs are set up in several"batteries" and differ from those commonly used for light metals by being watercooled. The welding pressure is very high (75 lbfor a iin-diameter electrode is representative) and extremely critical. Because of this the central air supply is fed to eachmachine through its own individual accumulator in order to even out any possible fluctuations in delivery pressure from the com-pressor plant. Moreover, above each battery there is a green tell-tale lamp, which goes out, accompanied by the sounding of aklaxon, should the air supply fail—in which event all welding stops immediately. Inspection. Welding inspection is, of course, a very importantsubject indeed. The Briggs laboratory is equipped with several X-ray machines. Certain components call for 100 per centX-ray examination of all the welding, inch by inch; in other cases certain areas of each finished part must be photographed; (Concluded on page 216)
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