Page 39 - MetalForming November 2014

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To reduce these occurrences, met- alformers can stagger punch length to minimize impact and snapthrough shock. Splitting punch groupings into two or three different lengths can reduce impact and snapthrough shock by 30 to 50 percent. A common practice is to stagger different groups of punch- es by an amount equal to the stock thickness of the workpiece. Although this will reduce initial shock, it will not reduce total shock. Each group of punches is still exposed to the entire impact and snapthrough shock.
A better solution: stagger the punch lengths equivalent to the shear-band width in the hole being punched, typ- ically less than one-third of stock thick- ness. This reduces impact and snapthrough shock. Staggering punch- es in this manner allows the next group of punches to contact the workpiece material prior to the first group snap- ping through. The snapthrough energy
BELOW THE HOOK
from the first group of punches is absorbed and used to drive the next set of punches through the material.
Punching holes in high-speed appli- cations also requires special attention to tool-steel selection. High-speed punching operations generate a sig- nificant amount of frictional heat, while also minimizing (by the same factor) the amount of contact time the tooling has with the workpiece. This reduces the opportunity for the punch to dissi- pate heat into the work material, which affords some additional cooling.
Because small punches have much less ability to dissipate heat, they are more prone to overheat. This can result in reduced hardness, wear resistance and dimensional stability. High-speed or powder-metal tool steels such as M4 and PM-10V are tempered at tem- peratures above 1000 F, giving them temper resistance superior to A2 and D2 in high-speed applications. MF
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