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Aviation History
1932
1932 - 0862.PDF
m SUPPLEMENT TO FLIGHT AUGUST 26. 1932 THE AIRCRAFT ENGINEER DRIVING SPROCKET, DRIVING CHAIN ADJUSTING SCREWS- GRIP CLAW ROLL OR DIE STANDARDS c—. SECTION OF TRACK PLAN AT GRIP. DIAGRAMMATIC SKETCH OF DRAWBENCH. FIG. tf/.S Hol/yJivck. At the end of the travel, the strip is disconnected from the grip and the formed length cut off. The grip is then disengaged from the chain by lifting out the claw and is run back—generally by hand—to the commencing end of the track and is then ready for drawing the next length of material. The speed of working should be approximately 10 feet per minute, though if variation is possible, 20 feet per minute is allowable (except at the start) under favourable conditions; i.e., when the section is a simple one not requiring very severe working of the metal. This increase of speed is not very important, however, since the setting-up and man-handling time—which constitutes something like 50 per cent, of the total— will not be affected. Drawing through Rolls. This process is similar to rolling, in essentials. The rolls are designed in exactly the same way and the same general considerations govern the whole process. One difference, however, is that since the rolls do not drive the work, it is necessary to apply power to them at the commencement until sufficient metal has passed through to enable the grip to be attached. With adequate gearing, man power is sufficient for this pur pose. The gearing, by the way, should be disengaged during the travel so that the rolls can rotate freely at different speeds. This allows the slip to reduce automatically to a minimum and eliminates any tendency, in the case of very thin strip, to crumple between adjacent pairs or rolls. Another difference between this process and ordinary rolling is the fact that the material being worked is constantly in tension. This definitely precludes the adoption of this method when complex sections are to be formed out of very thin strip. The working in such cases being so severe that the tensile load imposed is too great for the thin material. Even if it does not actually tear away or crack, distortion—and consequent weakening—is inevitable at those parts which are most heavily worked. FIG. 12 DIAGRAMMATIC SKETCH OF TYPICAL DIE SECTION OF INTERMEDIATE DIE. Drawing through Dies. In this process the strip is drawn through slits in bar or plate dies which are set up in a vertical plane as shown in Fig. 12. The actual drawing operation is the same as drawing through rolls. The design of the dies does not differ greatly from the design of rolls as regards the shape of the strip and the number of dies required, but the following important considera tions must be kept well in mind. At the commencement of drawing, the strip must be laid into the dies and attached to the grip first. The upper and/or outer portions of the dies must then be fitted and the strip drawn through, a little at a time, the dies being tightened gradually while doing so. It will therefore be realised that the design of the dies needs very careful consideration if this section is at all complex, particularly if it is a closed one. It is impossible to lay down any hard and fast rules on this subject, as every case must be treated on its merits. It is quite obvious, however, that the dies will some times have to be in several pieces, the tightening up being effected partly at the top and partly at the sides instead of from the top only as in the case of rolls. Another point, which is of vital importance, is that the dies must be absolutely rigid when in use. The reasons for this, of course, are obvious. Also, a good lead-in (as shown in Fig. 13) is essential; as, unlike rolling, in all cases and at all times the strip is being rubbed on practically the whole of its surface by the dies. For the same reason it is also necessary to have a really smooth finish on the dies. The number and thickness of the dies and the lead- in are all interconnected. The absolute minimum number will, naturally, be governed by consideration of the material to be worked and the shape of the finished section'; but if the material of the dies is thin, the lead-in will be correspondingly poor and the number of dies required to avoid scoring and stretch ing of the strip will consequently be greater. Generally speaking, the thickness should not be less than f in. Incidentally, a thick die makes for rigidity, but excessive thickness increases the wear on the strip and also, by increasing the frictional losses, increases the power required to operate the drawbench. The most satisfactory material for the dies will, as in rolling, depend upon the class of work to be done and the material to be worked. Since the mass of metal is not great, hardening does not present any great difficulty and therefore case-hardened mild steel may be empioyed in many cases with advantage, on account of its cheapness and easy working qualities. It is not, 802 d
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