BIZOMAT 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. |
What each input means
Open an input to see what it is, its units, the worked example and the common mistakes, with a small diagram.
What this means: Cutting force taken by
- What it is
- Whether a solid stop or locator takes the cutting force directly, or the clamps must hold the work against it by friction alone.
- Units and format
- Choose one of 2 options from the list: Friction only (no stop); A positive stop (clamps hold against lift).
- Example
- The form opens with “Friction only (no stop)” selected as a worked example. Choose the option that matches your case.
- Get it right
- With a positive stop the cutting force and friction values are not used; clamps only resist lift.
What this means: Cutting force along the locating face
- What it is
- Largest machining force acting along the locating face, trying to slide the workpiece sideways.
- Units and format
- Enter a number in N: newtons. The form accepts 0 N to 10000000 N. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 2000 N as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- In friction mode the clamping force rises in proportion.
- Get it right
- Use the peak force for the heaviest cut, from cutting-force calculation or tool supplier data; only used when friction holds the work.
What this means: Force tending to lift the work
- What it is
- Largest force trying to pull the workpiece away from its locators, for example the upward component of a cutting force.
- Units and format
- Enter a number in N: newtons. The form accepts 0 N to 10000000 N. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 300 N as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Adds safety factor × this force to the clamping force in both modes.
- Get it right
- Estimate the lifting component of the cut separately from the sideways force.
What this means: Safety factor
- What it is
- Multiplier on the cutting and lifting forces to give the holding force, covering vibration, interrupted cuts and uncertainty.
- Units and format
- Enter a plain number with no unit. The form accepts 1 to 10. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 2 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Clamping force rises in proportion.
- Get it right
- Take it from your fixture design practice for the type of cut.
What this means: Friction, clamp to work
- What it is
- Friction coefficient between the clamp face and the top of the workpiece.
- Units and format
- Enter a plain number with no unit. The form accepts 0.02 to 1. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.15 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Higher friction lowers the clamping force needed in friction mode.
- Get it right
- Coolant, oil and smooth faces lower it; used only when friction holds the work.
What this means: Friction, work to locators
- What it is
- Friction coefficient between the underside of the workpiece and the locators or fixture base.
- Units and format
- Enter a plain number with no unit. The form accepts 0.02 to 1. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.15 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Higher friction lowers the clamping force needed in friction mode.
- Get it right
- Hardened smooth locators give less friction than serrated ones; used only in friction mode.
What this means: Number of clamps
- What it is
- Number of clamps that share the total clamping force equally.
- Units and format
- Enter a plain number with no unit. The form accepts 1 to 100. Use a whole number. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 2 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Force per clamp, screw force and torque fall in proportion.
- Get it right
- Count only clamps that bear on the work at the same time and share the load.
Strap clamp and screw
What this means: Screw to workpiece contact distance
- What it is
- Distance along the strap from the clamp screw to the point where the strap presses on the workpiece.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0 mm to 1000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 40 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A longer distance raises the screw force needed for the same clamp force.
- Get it right
- Measure from screw centre to the contact point on the work, not to the end of the strap.
What this means: Screw to heel distance
- What it is
- Distance along the strap from the clamp screw to the heel support at the other end of the strap.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.1 mm to 1000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 40 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A longer heel distance lowers the screw force needed.
- Get it right
- Measure from screw centre to the heel contact.
What this means: Screw diameter
- What it is
- Nominal (major) diameter of the clamp screw or stud that pulls the strap down.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 100 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 12 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Tightening torque rises in proportion; a larger screw has less stress.
- Get it right
- Use the nominal size, for example 12 for an M12 stud.
What this means: Screw pitch
- What it is
- Thread pitch of the clamp screw, used to work out its tensile stress area.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 10 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1.75 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A coarser pitch slightly reduces the stress area and raises screw stress.
- Get it right
- Coarse and fine threads of the same diameter have different pitches; check the designation.
What this means: Nut factor
- What it is
- Nut factor relating tightening torque to the axial force in the screw, covering thread and nut-face friction.
- Units and format
- Enter a plain number with no unit. The form accepts 0.05 to 0.5. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.2 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Tightening torque rises in proportion.
- Get it right
- It depends on lubrication and finish; dry and oiled threads differ markedly.
What this means: Screw proof strength
- What it is
- Proof strength of the clamp screw or stud material. The tool checks the screw stress against 75 % of it.
- Units and format
- Enter a number in MPa: megapascals (1 MPa = 1 N/mm²). The form accepts 0.000001 MPa to 2000 MPa. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 600 MPa as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Use proof strength for the property class from the fastener standard, not tensile strength.
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.
Learn the Concepts Behind This Tool
These BIZOMAT Library readings explain the method this tool uses, with worked examples.
Useful alongside · BIZOMAT original book
Holding against cutting forces
In Magnetic Workholding by the Numbers
What it covers: Compare a magnet's sliding resistance with milling forces: cutting force from power and speed, peak factors, friction with and without coolant, stops, and vacuum and clamp alternatives.
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