Mechanical workholding is the oldest and simplest method, and it offers the lowest initial cost. Familiar components used are available in countless variations. The components are easy to manufacture, so they are offered by many, many suppliers and have become essentially a commodity. Mechanical workholding components from many manufacturers are interchangeable and, like Lego© blocks, can be assembled into endless geometries and reused over and over. Typical Components Part positioning is accomplished by use of components such as precision ground pins and spring plungers. Support heads can be used to provide support points required for 3-2-1 fixturing. Probably the most common clamping device is the strap clamp, also called a bar clamp.
In the illustrations below, the clamping is implemented by using a strap or bar together with studs, springs, washers, and a nut or threaded knob or handle to apply force. Clamping force is controlled by torque applied to the nut. The spring lifts the bar away from the part during placement or removal.
Clamping force can be generated by tightening a nut, a knob, or a handle.
|Examples of other mechanical clamping devices are hook and toggle clamps.||A cam can also be used to apply force to the bar clamp.|
In general, mechanical workholding is most appropriate when these two criteria apply:
Modest production run
No critical tolerances
See the "Cost Comparison" on page 8
for details on evaluating the size of a production run.
Mechanical clamping can cause distortion of the part because the operator, using hand tools, cannot tighten the clamp to a specific torque with great accuracy. This can result in significant differences in part height and position on a fixture. However, with a rough casting where the required
finish is not critical, this may be acceptable. Another acceptable example is a large casting, where clamping force is highly unlikely to distort the part.
A modular fixture system can be used
to speed fixture assembly by eliminating special machining tasks. Instead, horizontal and vertical T-slotted or drilled grid plates are used to mount the workholding components.