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Shaving and Burnishing Guidelines

By: Peter Ulintz

Sunday, July 1, 2018
 

While generally associated with burnished openings such as holes, many candidates exist for shaving, including free-edge features, burnished edges and die-cut features requiring close tolerances.


Fig. 1
Consider the 0.188-in.-thick mild-steel plate shown in Fig. 1. The two punched holes require shaving to a finished diameter of 0.5000 in. The following process steps serve as a guide for determining proper punch sizes and required cutting clearances for punching and shaving.

When specifying the diameter of the shave punch, the die component that determines the final size of a hole, remember that the resulting hole size will be slightly smaller than the actual punch size, usually 0.0002 to 0.0004 in. due to elastic recovery after shaving. In this example, the punch, specified at a 0.5005-in. dia., allowed for some wear and elastic recovery.

Next, determine the corresponding die-button opening.

Closer to a machining operation (cutting) than a stamping operation (punching), shaving clearances must be tight, usually 1 to 1.5 percent of the stock thickness per side. Too much clearance in the shave station will result in shearing and fracturing rather than shaving. In the accompanying table, Punch and Button Size, the clearance used for shaving was 1.25 percent.

The die button for the punching station often will be the same size as the shave station. Softer materials such as aluminum may require a button diameter as much as 2-percent-of-stock-thickness-per-side larger, depending on the thickness. A larger die-button diameter in the punching station minimizes burring of the shaved hole.


* A clearance of 10 percent per side is assumed for punching (mild steel) and 1.25 percent per side for shaving.
Punch-to-die cutting clearance is a function of stock thickness. The total clearance in our example is 0.0376 in., subtracted from the die-button diameter to establish the point diameter for the punching station.

To eliminate the metal rollover condition on the punch-entry side of the part, it may be necessary to flip the part over and shave from the opposite side. For progressive-die applications, this may require a partial transfer system or an offline secondary operation.

When high-quality surface finishes and/or tight size tolerances are required, it may be necessary to burnish the hole after shaving. Burnishing does not remove any material; instead, it compresses the remaining surface irregularities to produce a high-quality precision-sized hole.

The size of the burnish punch is equivalent to the shave-punch diameter, plus 0.0002 to 0.0003 in. The corresponding die button will be approximately 0.001-in. larger than the burnishing punch.

Due to the close cutting clearances and high cutting forces associated with shaving operations, a guided stripper usually is required to support the punches and to maintain accurate punch-to-die alignment. This can greatly reduce punch chipping and premature wear or shearing.

Also, consider chip-resistant tool steel grades for shave punches. Chip-resistant tool steels are not as wear resistant as other grades, but since most shave-punch failures result from chipping and breaking, not wear, wear resistance is less of a factor.


Fig. 2

When constructing burnishing punches, always use high-wear-resistant tool steel or consider cemented carbide. The ball-nose radius on the burnishing punch should be highly polished and blend perfectly with the point body, and wear-resistant PVD surface coating is best for tool-steel and carbide burnishing punches.

Other Considerations

In general, material removed by shaving usually amounts to 10 to 12 percent of stock thickness per side for most materials. This provides a good surface finish and leaves enough material in the scrap ring for slug retention, an important consideration because slug retention remains a common problem associated with shaving operations.

Shaving thick materials with pronounced die break or taper may require more than one shaving operation. When two shave steps are required, the amount of material shaved in the second step should be one-half that of the first step. For example, if the requirement calls for shaving a total of 0.027 in., remove 0.018 in. with the first shave punch, then 0.009 in. with the second.

Because a shaving operation acts on the sheetmetal more like a machining operation (cutting) than a punching operation, consider adding a rake angle to the shave-punch face (Fig. 2) to facilitate this cutting action. Establishing the proper back-rake angle for each shave application, somewhat of a black art, 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 10 deg. or more. And some aluminum alloys have required as much as 45 deg.

The same practices apply to free-edge shaving, although this requires much higher holddown forces to secure the work material while shaving. In addition, preventing deflection due to unbalanced cutting forces requires a heeled and properly supported backside of the shave punch.

Finally, shaving requires a good lubricant. Depending on the type of material and its thickness, a heavy-duty EP lubricant used for fine blanking, broaching or in-die tapping operations may be needed. Work closely with your lubricant supplier for best results. MF

 

Related Enterprise Zones: Tool & Die


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