The principles of sound workholder design

Modern Machine Shop, May, 1995

Keeping a workpiece from being loaded upside down can also present a design problem. For those workpieces having either a hollow recess or holes, such as shown in Figure 8, this problem is easily solved. Here, a simple block or pin arrangement can help ensure that the workpiece is mounted only in the correct way.

In cases where the workpiece has no holes or internal recesses, but does have external projections, the projections can be used for foolproofing workpiece location. In such cases, either pins or blocks can be used to ensure that the workpiece will not be loaded upside down.

While turning a workpiece either end-for-end or upside down are frequent locating errors, another problem is incorrectly loading a workpiece on a center axis. This problem often occurs when two hole patterns are machined relative to each other, or when machined details are radially related to a central axis location. As shown in Figure 9 A, the hole pattern in the small hub is to be machined relative to the holes in the large flange. While the primary locator for this arrangement is the center hole, a secondary locator in one of the other flange holes will prevent many locational problems.

In cases where a workpiece must be accurately located relative to a central axis, the easiest method is to use a center hole as a concentric locator and a second hole as a radial locator to prevent rotation about the center axis [ILLUSTRATION FOR FIGURE 9 B OMITTED]. Here, the pin used for radial location of the workpiece also acts as a foolproofing pin to prevent the workpiece from being loaded incorrectly.

Occasionally, the only feature available to initially locate a workpiece is a center hole [ILLUSTRATION FOR FIGURE 10 OMITTED]. Here, the holes to be machined must be positioned relative to the center hole, and also spaced at a specific angular relationship to each other. With this type of workpiece, the first hole can be drilled and a jig pin inserted for location. This pin then serves both as a radial locator and as a foolproofing pin, helping to maintain correct alignment and preventing the holes from being improperly drilled.

Selecting And Positioning Locators

When selecting locators for any workpiece, make sure that they are large enough to provide good contact with the workpiece. The locator must also have enough rigidity to resist all clamping and tooling forces. This is especially important with large or heavy workpieces. A 0.25-inch diameter dowel pin, while quite ample for many small workpieces, could easily break or bend if used for large or heavy workpieces.

Another important consideration in positioning the workpiece is to keep the locators as far apart as practical. This permits more positional stability and helps to prevent rocking or movement of the workpiece. This consideration is especially critical when positioning supports for large workpieces. The locators, or supports, under the workpiece should provide maximum stability. Ideally, they should be positioned to allow operations on the workpiece to be performed within an imaginary circle formed by the supports. Where such positioning is not practical, auxiliary supports can be used to help prevent any workpiece distortion.