Constructing specialized tooling and workholders

Modern Machine Shop, March, 1996 by Edward G. Hoffman

One very simple way to reduce the amount of labor necessary in building any workholder is by incorporating general purpose, off-the-shelf, components into your specialized workholder designs. This is a common practice with items such as locators, supports, and common hardware. However, these are not the only areas where standard components may be applied. Though used less frequently, standard clamps are another commercially available component that can very easily be designed into specialty workholders.

Although many designers are quite familiar with the wide range of standard, general purpose clamps, less well known is the variety of specialized clamp designs found in many of today's modular workholding systems. This is definitely a category of workholding components that should always be considered anytime specialized clamping devices are required.

The Up-Thrust clamp [ILLUSTRATION FOR FIGURE 1 OMITTED] is a modular style clamping device that can be applied in a variety of very special clamping situations. The novel design of the U-Thrust clamp incorporates both a locating element and a clamping device within the same unit. This clamp design applies the clamping action from below and holds the workpiece between the locating jaw and the clamping jaw with an upward clamping force.

The unique design of the Up-Thrust clamp is well suited for special applications or clamping situations where the workpiece must be located and machined on the same surface. As shown in Figure 2, the Up-Thrust clamp design also elevates the workpiece off the workholder base, or the machine table. In this example the workpiece must be drilled and milled on the top surface. The opposite side of the workpiece is curved and would be impossible to locate without an elaborate locating system. Here the Up-Thrust clamping action permits the workpiece to be accurately located and securely held in the same clamping device. In addition, since the surface being machined is also used as the locating surface, the negative effects of the irregular opposite side are of little concern.

The Up-Thrust clamp uses a cam-action design to apply the holding force. The clamp is operated by pushing against the cam handle, located at the top of the clamp. This clamping action raises the clamping jaw element upward against the workpiece. As shown in Figure 3, the clamping jaw on the clamp also has two clamping positions. The first position is suitable for workpieces between 0 inch and 1.38 inches. The second position may be used for workpieces up to 2.75-inches thick. The underside of the top jaw element has a precision ground locating surface. When the clamp is mounted, the locating surface of the fixed (top) jaw is exactly 4 inches off the mounting surface. Precise clamping adjustments are made by simply rotating the cam screw. This moves the clamping (bottom) jaw either up or down, to suit the workpiece thickness.