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Aviation History
1971
1971 - 0073.PDF
68 CONCORDE NC FLIGHT International, 14 January 1971 M y 5!^^j*5f SNI ^J i|W' ft**-8 *g3P*f, •M** 5 ^^*''i |#A and difficult to meet because of the characteristics of the material, the wide variations in the sectional thicknesses of the machined panels and the severity of the transverse curvatures—which change in form and become smaller from front to rear, and are not symmetrical with the width of the panel. In the forming of these curvatures the toler ance specified allows a departure of only O.OOlin/in from the design curvature. An additional difficulty is created by the fact that, in order to achieve the required close flatness tolerance on the billet as supplied for machining, extra rolling may be needed after the 2 per cent stretching process for the elimination of residual stress in the billet. The additional work reintroduces a degree of internal stress, which has the effect of complicating the forming of the panels after they have been machined. Forming procedure affords one of the few examples of real "know-how" being required and, indeed, essential to success. Precise process planning is not possible, although a reasonably accurate standard procedure can be estab lished after experience has been gained upon a particular part. However, slight variations in the specification and work done in the original billet may enforce departures from such a procedure and a knowledge of what to do, based upon experience, remains a basic requirement in this work. Shot-peening, three-point press-bending, shrinking and rolling are all used in forming the panels. Equipment includes two shot-peening installations, one designed by BAC and built by Tilghman's Ltd and one by Spencer and Halstead, two portable Vacu-Blast shot-peening units, an Eckold* shrinking (or stretching) machine and a rolling *Tilghman's Ltd, Broadheath, Altrincham; Spencer and Halstead Ltd, Wolverhampton; Vacu-Blast Ltd, Slough, Bucks. Eckold machines are supplied in this country by Alfred Hervert Ltd, Coventry. Left, local shrinking of stringers on an Eckold machine, fitted with special jaws, during the forming of a fuselage skin panel. Above, three-point press-bending of a fuselage skin panel on a specially designed BAC press, during the forming of the panel to the trans verse curvature of the fuselage machine which has also been designed and built by BAC. As received by the forming department, the machined fuselage panels have been processed to remove cutter marks. At this stage any distortion caused by the release of residual internal stresses has become apparent; the panel may have become hog-backed or saddle-backed and the first step is to assess what initial treatment is necessary. Forming proper to the transverse fuselage contour begins by shot-peening the panel between the frame-lands to lift the skin in both transverse and longitudinal directions. It has been found necessary to begin forming between the frame-lands; if the lands are formed first, the areas between them cannot be raised sufficiently to match. Shot-peening of the between-lands areas is followed by three-point press-bending of the frame-lands themselves to bring them to the same contour as the between-lands areas. The machine used for this operation is of BAC design and is a scaled-up and improved version of one designed for similar work on Vanguard skin panels. This sequence of shot-peening and three-point bending is then repeated to bring the panel to the required transverse curvature. During the course of this procedure some trans verse stretching occurs and it is necessary to "lose" metal along the edges of the panel. This requirement is met by use of the Eckold shrinking machine. Panel curvature is checked on an inspection fixture comprising templates mounted at stations along the length of the panel. The forming procedure thus briefly outlined does not, of course, really indicate the skill and experience needed to produce an acceptable result. Longitudinal internal stringers, alternating with transverse frame-lands of much heavier section, to say nothing of changes in skin thickness to accommodate the stress pattern, must all be contained, in the sense that a smooth external curvature must be produced, irrespective of the many interior changes of the section. •
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