Page 47 - MetalForming February 2014
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and to maintain accurate punch-to- die alignment. This can greatly reduce punch chipping (a common problem) and premature wear or shearing.
Consider a chip-resistant tool steel for shave punches and wear-resistant tool steel or solid carbide for burnish- ing punches. Ensure a smooth ball- nose radius on the burnishing punch, blended perfectly with the point body. The ball radius and point body must be highly polished to minimize wear.
Other Considerations
The amount of material to be removed by shaving usually is 10 to 12 percent of stock thickness per side for most materials. This provides a good surface finish and
a back angle of 10 to 30 deg.; and some aluminum alloys may need as much as a 45-deg. back angle.
When shaving a free edge (i.e. trim edges), the same rules apply. In addi- tion, the back side of the shave punch must be heeled and properly support- ed to prevent deflection due to unbal- anced cutting forces. Higher holddown
forces also will be required to hold the workpiece in place during shaving.
Finally, carefully select the lubricant used for shaving. Depending on the type of material and its thickness, a heavy-duty EP lubricant used for fineblanking, broaching or in-die tap- ping may be necessary. MF
leaves enough material in the scrap ring for slug retention. This is important—slug retention is one of the most common problems associ- ated with shaving operations.
“Carefully select the lubricant used for shaving.”
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Shaving very thick materials with pronounced die-break or taper may require more than one operation. When employing two shave steps, the amount of material shaved in the second step should be one-half that of the first step. So, if a total of 0.027 in. of material must be shaved, remove 0.018 in. with the first shave punch and 0.009 in. with the second punch.
Shaving is much closer to a machin- ing operation (cutting) than a stamping operation (punching). Therefore, con- sider adding a rake angle to the shave punch face similar to that found on a cutting tool. A conical shape on the punch face (Fig. 2) can accomplish this.
Establishing the proper back-rake angle for each shave application is somewhat of a black art that requires trial-and-error development. Some brass and bronze alloys do not require any back angle at all; mild steels, depending on hardness, may require
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MetalForming/February 2014 45
