The principles of sound workholder design

Modern Machine Shop, May, 1995

* One machined hole

* Two machined holes

* Three machined surfaces at right angles

* Two machined and one unmachined surfaces at right angles

* One machined and two unmachined surfaces at right angles

* Three unmachined surfaces at right angles.

Although the last four features listed are not as efficient or accurate as the first two, they are the most commonly used for the six-point or 3-2-1 locational method. In many cases, there is simply no other way to locate the workpiece.

If the workpiece does have one or more holes suitable for location, they should be considered preferred locating surfaces. This is because a single hole can restrict more movement with fewer locators.

As shown in Figure 3, one machined hole can restrict nine degrees of freedom with a single locator. By locating on the center hole shown here, you restrict the workpiece from moving axially, downward along the Z axis, and also from moving axially or radially along or about the X or Y axes. The only way this workpiece is free to move is axially, upward along the Z axis, and radially about the Z axis. As before, the unrestricted degrees of freedom can be held with a clamp. Thus, by using one locator, positioned in a hole, you achieve the same location that formerly required six locators mounted around the outside of a rectangular workpiece.

If the workpiece has an additional hole, one more locator may be positioned there. The added locator restricts the movement about the Z axis, with the result that, now, eleven degrees of freedom are restricted with only two locators. This arrangement allows the workpiece to move in only one direction, upward on the Z axis. Once again, the single remaining degree of freedom is restricted with a clamp.

Rules For Locating

The importance of properly locating and positioning a workpiece cannot be overemphasized. Locating is a critical function of any jig or fixture. Accordingly, the overall position, orientation and relationship of the locators to the workpiece must be carefully thought out when designing and constructing the locating elements. Following are some of the design considerations to remember, and problem areas to avoid, when designing locators.

Duplicate Location

Duplicate or redundant location occurs when a workpiece is located on two parallel surfaces or features. This will create problems in locating the workpiece, and may also cause difficulties in loading and unloading the workpiece. Figure 4 shows an example of duplicate location. Here, the workpiece is a flanged cylinder located by both its bottom surface and the underside of the flange. This type of location is not practical, since an actual workpiece will be located on either the bottom surface or the flanged surface, not on both at the same time.

A better specification is shown in Figure 5. Here, the workpiece location shown is for either the bottom surface or the flanged surface. The actual locating surface will be determined by the workpiece itself. If the overall length of the workpiece is the most important consideration, the workpiece should be located on its bottom surface. On the other hand, if the thickness of the flange is most important, the workpiece should be located on the underside of the flange.