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【正文】 red, or curved sides. The size of the parts may vary from ″() diameter or smaller, to aircraft or automotive parts large enough to require the use of mechanical handling equipment. Die Design Principles Coining Dies. In backward extruding dies the punch is always smaller in diameter than the die cavity in order to give the clearance between punch and die equaling the desired wall thickness of the part to be produced. The punch is loaded as a column. To minimize punch failure it is desirable to coin the slugs to a close fit in diameter to assure concentricity. Figure 866 illustrates a coining die to prepare a slug for backward extrusion. Coining the slug to fit the diepot and coining the upper end to fit and guide the free end of the punch will minimize punch breakage of the extruding die. Backward Extrusion Dies. A typical backward extrusion die is shown in Figure 867. The use of a carbide die cavity will minimize wear due to excessive pressures. The carbide insert is shrunk into a tapered holder. The holder has a 1? side taper that prestresses the carbide insert to minimize expansion and fatigue failure. The inserts are well supported on hardened blocks. The extruding punch is guided by a springloaded guide plate which in turn is positioned by a tapered piloting ring on the lower die. Ejection of the finished part from the die is by cushion or pressure cylinder. Figure 868 illustrates a backward extrusion die with an unusual punch peration ratio of 5：1 made possible with a modified flatend punch profile. Forward Extrusion Dies. Figure 869 is an example of a typical forward extrusion die in which the metal flows in the same direction as the punch, but at a greater rate owing to change in the crosssectional area. The lower carb
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鄂ICP備17016276號-1