Manual toggle clamp selection for fixturing.

Author:Ruffley, Doug

New developments in manufacturing seldom highlight the benefits of proper workholding. However, clamping requirements must be considered carefully in order to reduce setup time and maximize productivity. Full value is realized only if all parts in a fixture are carefully evaluated for useful function. The cost of misusing a clamp often far exceeds the initial purchase price.

The choice of various manual clamp actions is extensive and includes: toggle-action and cam-action clamps, C-clamps, strap clamps, and T-slot clamps.

Manual toggle-action is much faster than tightening nuts and bolts. It is relatively inexpensive and readily available since it is manufactured in large quantities. Manual toggle-action clamps maintain a positive mechanical lock unless they are stressed beyond their ratings. Limitations of toggle clamps include the fact that they need a very consistent clamping point and exerting force is not easily determined.

Cam-action clamps depend on a low wedge angle to exert clamping force and to keep the clamp in a closed condition. Their main advantage is that they can adapt to varying work piece heights as is often found on rough castings and forgings.

C-clamps, strap clamps, and T-slot clamps have adjusting screws to apply clamping force. Advantages of these clamps are high exerting force and wide adjustabil-ity to adapt to different part thicknesses. A limitation is that they are not fast acting nor is the exerting force easily repeatable.

Determining holding capacity, number

When determining the holding capacity necessary for a clamping solution it is essential to understand the difference between holding capacity and exerting force. Exerting force is a measure of how much force (lb) the clamp applies to the work. This force varies and depends on the relative position of the clamp components, the adjustment of the spindle or the hook), part variation, and point of application on the bar. The maximum exerting force generated by a clamp, for any given set of variables, occurs just as the clamp goes over-center into mechanical lock. The magnitude of this force can still vary considerably depending on the variables noted. The exerting force should never exceed the clamp's rated holding capacity.

Holding capacity is the maximum force (lb) that the clamp will sustain when closed and locked without causing permanent yielding of any components sufficient to damage the clamp. Holding capacity applies only when the clamp is in its...

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