There is a limit to how far inward metal will flow when drawing from the blank diameter to the first draw diameter and from a drawn shell diameter to a smaller shell diameter。 This is expressed as a percentage of draw reduction。 The maximum percentage of reduction is limited by the flow of material inward that causes the metal to go into compression, which in turn causes a resistance to flow。 Too much resistance will cause fracture near the cap of the shell, which is the weakest area in tension。来*自-优=尔,论:文+网www.chuibin.com

The percentage of reduction varies with the metal thickness。 For example, for a deep drawing steel blank, the percentage of reduction to the first draw shell diameter varies from 32 percent for 0。015-in。 thick material to 48 percent for 0。125-in。 thick material。

There is a minimum and maximum draw radius on the draw ring that will control the flow of material。 For deep drawing steel parts, the correct radius varies from 5/32-in。 minimum to 1/4 in。 maximum for 0。015-in。 stock, and 11/32-in。 minimum to 15/32 in。 maximum for 0。125-in。 stock。

If the radius is too small, the metal will not flow well, which increases the resistance to flow, causing excessive thinning or fractures near the cap of the cup。 If the radius is too large, the metal will wrinkle after it leaves the point of pinch between the draw ring and the pressure pad, and before it is formed into the vertical wall of the cup。

The normal tendency is to make the radius too small because "it's easy to make the radius larger during die tryout and it's difficult to make a smaller radius。" The result is that needless stress is put on the cup, which results in excessive thinning or fracturing。 Many times the problem of an improper percentage of reduction or improper draw radius in the first draw station will not show up in the first draw station, but in a later redraw station, with the result that considerable time is spent trying to fix the wrong station。