KEVOS® MAKEOS · Manufacturing
Fixture Clamping Force and Clamp Screw Torque
How much clamping force holds this workpiece against the cutting forces, and how tight must each clamp screw be?
Clamping force needed to hold a workpiece in a fixture against cutting forces, either by friction alone or with the cutting force taken by a positive stop, with a safety factor; the force per clamp, the screw force for a strap clamp from its lever geometry, the screw tightening torque from the nut factor and the screw stress.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Cutting force taken by | choice | Whether a solid stop takes the cutting force, or friction alone must hold it. |
| Cutting force along the locating face | N | Largest cutting force acting along the locating surface. |
| Force tending to lift the work | N | Largest force trying to lift the work off its locators. |
| Safety factor | Safety factor on holding (2–3 for interrupted cuts). | |
| Friction, clamp to work | Friction between clamp and workpiece. | |
| Friction, work to locators | Friction between workpiece and locators or base. | |
| Number of clamps | Number of clamps sharing the load. | |
| Strap clamp and screw | ||
| Screw to workpiece contact distance | mm | Distance from the clamp screw to where the strap presses the work. |
| Screw to heel distance | mm | Distance from the clamp screw to the heel support. |
| Screw diameter | mm | Clamp screw diameter. |
| Screw pitch | mm | Clamp screw pitch. |
| Nut factor | Nut factor for the screw (about 0.2 dry, 0.15 oiled). | |
| Screw proof strength | MPa | Proof strength of the clamp screw or stud. |
Method and formulas
- Friction only: clamping force = n F_cut ÷ (μ₁ + μ₂) + n F_lift (friction acts at both faces).
- With a stop: clamping force = n F_lift (the stop carries the cutting force).
- Strap clamp: screw force = clamp force × (A + B) ÷ B; torque = K × screw force × d.
- Screw stress = screw force ÷ A_t, with A_t = π ÷ 4 (d − 0.9382 P)².
- Example: 2000 N of cutting force held by friction needs 13,933 N of clamping in total and 33.44 N·m on each screw.
Limitations
- Static force balance; vibration and chatter may need extra margin.
- Clamping should press the work onto its locators, not between them.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Size clamps for a milling or drilling fixture
- Decide whether a positive stop is needed
- Set clamp screw torques for operators
Who it is for
- Tooling engineers
- Machinists
- Manufacturing engineers
What you get
- Total and per-clamp clamping force
- Screw force and tightening torque
- Screw stress
Interpreting the result
- A positive stop against the cutting force usually cuts the clamping force needed several-fold and reduces distortion.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 3131–3134, 1522–1532. Topic reference; no source tables or recommended settings are reproduced.
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