Machining force vs. clamping force & friction

The picture below illustrates machining and clamping forces in the fixture.

Clamping force F created by OK-VISE clamp are indicated with the yellow arrows. In each stopper, a force that is equal the clamping force is created. Red arrows indicate forces that are applied to a workpiece while machining.

When the fixture is working with friction closure principle, friction between the jaw and the workpiece creates a holding force that resists machining forces.
Forces created by the weight of a workpiece are not presented on the diagram.

In the example below, a workpiece with a size of 300x300x900 mm is being clamped using two SRM-152 Multi-Rail sets (based on RM-500 Multi-Rail). In this example, Multi-Rail sets are installed on a sub-plate.

calc force rm1

Estimation of  how much force the fixture can resist in direction X:

Using 4 tons (40 kN) clamping power in two standard clamps with smooth jaws, where the coefficient of friction is low (µ = 0.15), the maximum allowed force X is:

4 * F * µ = 24 kN = 2.4 tons


Using 4 tons (40 kN) clamping power in two clamps and a stopper*, where coefficient of friction is high (µ = 0.8), the maximum allowed force X is:

X = 4* F* µ = 128 kN = 12.5 tons

* Grip jaw against the steel, the gripper leaves marks on the workpiece surface.

Please note that estimates above are rough.

If in the example above a workpiece is made of steel and its weight is approx. 600 kg, a coefficient of friction against the Multi-Rail base and workpiece is even higher.

Additionally, the force, which generates sliding movement of a workpiece in the fixture, and such constraints as bending or vibration of the fixture can be calculated as well.