Page 27 - MetalForming July 2010

Punch
Stock
Shaving
Die
Fig. 2—Hollow-Ground Concept
10 to 20 percent per side, depending on the material thickness and type. These “engineered” cutting clearances opti- mize tool life and minimize work hard- ening inside the hole.
Proper shave clearance, on the other hand, will be very tight, usually 1 to 1.5 percent per side. Too much clearance results in a shear and “rebreaking” of the hole. In the example part, the proper shave punch appears to be 0.375-in. dia. However, when specifying shave punch diameters, remember that the resulting hole size will be slightly smaller than the actual punch size, usually by 0.0002 to 0.0004 in., due to elastic recovery after shaving. This means the shave punch in our example part should be ordered at 0.3753 in. dia. The corre- sponding die matrix should be 2 to 3 percent larger and have a 1⁄4 in. land before transitioning into 1⁄2-deg. taper.
For very close tolerances, or when shaving very thick materials with pro- nounced die-break or taper, more than one shaving operation may be needed. When two shave steps are employed, the amount of material shaved in the sec- ond step should be one-half that of the first step. So if a total of 0.009 in. of material is to be shaved, 0.006 in. should be removed by the first shave punch and 0.003 in. removed by the second.
Since shaving is more of a machining operation (cutting) than a stamping operation (punching), a rake angle machined on the shave-punch face usu- ally is necessary. This would be similar to the rake angle found on a cutting tool.
The “hollow-ground” shave punch concept is shown in Fig. 2 (courtesy of
some aluminum alloys may need as much as 45 deg.
Due to the close cutting clearances and high cutting forces associated with shaving operations, a guided stripper plate may be required to adequately support the punches and to maintain accurate punch-to-die alignment. This can greatly reduce punch chipping (a common problem) and premature wear or shearing.
When shaving a free-edge (i.e. trim line edges), the same rules for punched holes also apply. In addition, the free- edge shave punch must be heeled and properly supported to prevent deflec- tion due to unbalanced cutting forces.
Finally, a good lubricant should be chosen whenever shaving is performed. Depending on the type of material and its thickness, a heavy-duty EP lubri- cant used for fine blanking, broaching or in-die tapping operations may be necessary. Work closely with your lubri- cant supplier for optimum results. MF
Fig. 3—Close-Up View
David Smith and Associates) and pro- vides a cutting-tool action. This modi- fication can improve surface-finish quality and extend punch life. Fig. 3 provides a close-up view of the concept.
Establishing the proper back-rake angle for each 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 20 to 30 deg.;
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