Clamping
Clamping holds the part against the locators against cutting forces. The clamp must be strong enough, applied in the right direction, and must not deform or damage the workpiece.
Purpose of Clamping
The clamp keeps the workpiece pressed against the locating points so that cutting forces, inertia and vibration cannot move or lift it during the operation.
- Press the part firmly against locators.
- Resist cutting forces and torques.
- Prevent vibration and chatter.
- Be quick to operate for batch production.
Clamping Rules
- Clamp in the direction of the locators — the clamp force should press the part onto its supporting points.
- Apply the clamp over a solid support, never over a hollow or overhang that will flex.
- Direct the clamp force toward the main cutting force direction when possible.
- Use the minimum clamp force that keeps the part stable to avoid distortion.
- Use pads or soft tips on clamps to avoid marking finished surfaces.
- Keep clamping points as close to the cutting area as practical.
- Do not clamp so that it lifts the part off a locator (clamp against, not away from, the datum).
Clamp Types
Screw clamp
Simple and reliable; high force from a small handle. Slower to operate; use quick-release studs for production.
Strap / clamp plate
A strap with a center stud and adjustable end; versatile for flat workpieces. Follow the 1:3 lever rule to reduce force loss.
Toggle clamp
Fast open/close with locking over-center; very popular for quick loading in batch production. Force drops at end of stroke.
Cam clamp
Very fast action. Use a self-locking cam angle (below friction angle); not suitable for large forces.
Wedge clamp
Converts a small axial movement into a large clamping force; self-locking if wedge angle < friction angle.
Quick-acting (stud & nut)
A stud with a slotted head and quick-release washer lets the strap slide on/off without fully removing the nut.
Power clamps (hydraulic / pneumatic)
Consistent, repeatable force and rapid operation; ideal for automated and high-volume work. Requires supply and controls.
Friction Coefficients (dry, static)
| Contact pair | μ (approx.) |
|---|---|
| Steel – steel | 0.15 – 0.25 |
| Steel – cast iron | 0.16 – 0.30 |
| Steel – aluminium | 0.20 – 0.30 |
| Ground hardened steel – ground hardened steel | 0.10 – 0.16 |
| Rubber / elastomer pad on steel | 0.50 – 0.80 |
Clamp Force Requirement
The required clamp force is the cutting force multiplied by a safety factor and divided by the friction between workpiece and locator:
- Fcl — required clamp force (N)
- K — safety factor (2 to 3 for general machining, higher for interrupted cuts)
- Fc — resultant cutting force (N)
- μ — friction coefficient between workpiece and locator
Clamp mechanism details
Toggle clamp
Fast open/close with locking over-center; very popular for quick loading in batch production. Force drops at end of stroke.
Screw clamp
Simple and reliable; high force from a small handle. Slower to operate; use quick-release studs for production.
Cam clamp
Very fast action. Use a self-locking cam angle (below friction angle); not suitable for large forces.
Wedge clamp
Converts a small axial movement into a large clamping force; self-locking if wedge angle < friction angle.