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Aviation History
1971
1971 - 0072.PDF
FLIGHT International, 14 January 1971 67 Detail of the fin structure showing the mounting of two fin-ribs The outer surface of the panel is first machined. Any access doors and inspection apertures are profiled to a depth within some 0.02in of the inner face of the billet and the rebates round these apertures are machined as well as any rebates along the edges of the panels. Before the inner face of the skin panel is machined, glass-fibre fillers, moulded to fit, are inserted into the rebates to give a vacuum seal over the whole area of the panel. In machining the inner surfaces of the skin panels, the areas corresponding with the stringer tips are machined first. The bulk of the material between the stringers and over the other areas down to the required skin and other section thicknesses is then removed in the profiling and area-clearance sequence. Roughing and finishing cuts are taken in making the stringer and skin thicknesses. Cutter speeds of 9,000 r.p.m. are used with feed rates of up to 30in/min for roughing and up to 40in/min for finishing. Because of the thinness of the finished stringer sections—of the order of 0.09in— the finishing cut is fairly substantial, about 0.07in. This procedure is necessary in order to allow the cutter to bite while maintaining a reasonable thickness of section to support the cut; if only a small allowance were left, the cutter would tend to push the stringer section away rather than to cut to size; also vibration would be set up in the stringer, with detrimental effect on size and finish—and with possibly even fatigue effects. It is an interesting fact that, although one of the benefits of integral machining is a structure less prone to fatigue, fatigue can be induced by the high routing cutter-speeds used in this kind of machining operation. If, for example, a stringer section were first machined to thickness on both sides and then to height, the effect of machining the tip of the stringer, if it were caused by the cutter speed to vibrate at its natural frequency, could be to induce fatigue cracking along the base of the section. Carbide-tipped, straight-flute, two-lip profiling cutters are used to rout the stringer to the thickness and to machine the skin to the thicknesses necessitated by the stress loading. As already mentioned, the teeth on the end- face of the cutter are relieved by a central recess, to minimise end-cutting problems, and do not cut to the centre. In order to avoid the need for incremental grinding of cutters and the multiplication of control tapes to match, full use is made of the tolerance of O.Olin on stringer thicknesses. As supplied new the cutters are some 0.002in to 0.004in above nominal size in diameter. Before the thickness tolerance on the stringer sections is exceeded, the diameter of the cutters can be reduced by regrinding to 0.005in undersize. When they have been reduced to a diameter too small to maintain tolerance, the cutters are used for roughing operations only, until retipping is necessary. Swarf removal A considerable problem can be created by the large amounts of swarf produced by the material removed in high-speed routing. At Weybridge, swarf from the machine is drawn away by pneumatic suction through trunking to cyclones outside the shop. Here, the swarf is separated from the airstream and falls to the floor of what may be termed an outhouse. Several such outhouse dumps are situated adjacent to the shop to serve the different batteries of machines. From these dumps, the swarf is removed, again by pneumatic suction, directly into special box-vans for disposal as scrap. Forming the panels After the skin panels have been machined they must be formed to the cross-sectional curvatures of the rear fuselage. This requirement is both daunting to contemplate Vacuum chucks for machining fin-attachment bracket and fin-rib. Each of the chuck sections is, in effect, a separate chuck holding down a corresponding portion of the part. The vacuum seal between part and chuck is maintained by a Neoprene cord inserted into a groove in the chuck-face. There is a connection from each discrete section of the chuck to the vacuum manifold. A, Neoprene cord; 6, vacuum orifice; C, vacuum manifold
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