BIZOMAT MAKEOS · Manufacturing
Lathe Chuck Grip Force and Speed Limit
How much grip does the chuck need to hold this cut, how much is lost at speed, and how fast can it safely run?
Workholding check for a jaw chuck: cutting and feed forces from the chip area and specific cutting force, the total jaw grip needed to resist the cutting torque and feed force by friction with a safety factor, the grip lost (or gained) to centrifugal force on the jaws at the spindle speed, the static grip to set, and the highest speed at which the chuck’s rated grip still holds the work.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Cut | ||
| Cutting diameter | mm | Diameter being cut. |
| Specific cutting force | N/mm² | Specific cutting force of the material. |
| Depth of cut | mm | Depth of cut. |
| Feed | mm/rev | Feed per revolution. |
| Feed force ÷ cutting force | Axial feed force as a share of the main cutting force. | |
| Chuck | ||
| Gripping diameter | mm | Diameter on which the jaws grip. |
| Gripping | choice | Outside gripping loses grip with speed; inside gripping gains it. |
| Friction at the jaws | Friction coefficient between jaws and work (about 0.1 for smooth hard jaws, 0.2–0.3 for serrated jaws). | |
| Safety factor | Safety factor on holding. | |
| Number of jaws | Number of jaws. | |
| Mass of each jaw set (base + top jaw) | kg | Mass of one base jaw plus its top jaw. |
| Radius of the jaw centre of gravity | mm | Distance from the spindle axis to the jaws’ centre of gravity. |
| Spindle speed | rpm | Spindle speed. |
| Maximum static grip of the chuck | kN | Largest static grip the chuck is rated for (from the chuck maker). |
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.
Cut
What this means: Cutting diameter
- What it is
- Diameter of the work at the point being cut. The cutting force acts at this radius and tries to turn the part in the jaws.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 5000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 60 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- The grip needed to resist cutting torque rises with cutting diameter ÷ gripping diameter.
What this means: Specific cutting force
- What it is
- Cutting force per square millimetre of chip for the work material, from the tool maker’s data or a machining reference.
- Units and format
- Enter a number in N/mm²: newtons per square millimetre (equal to MPa). The form accepts 0.000001 N/mm² to 10000 N/mm². Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 2000 N/mm² as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Cutting force, and so the grip needed, is proportional to kc.
What this means: Depth of cut
- What it is
- Depth of cut on the radius: how much the tool removes from the surface in one pass.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 50 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 3 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Cutting force is proportional to depth of cut.
What this means: Feed
- What it is
- How far the turning tool advances along the part for each spindle revolution.
- Units and format
- Enter a number in mm/rev: millimetres per revolution. The form accepts 0.000001 mm/rev to 5 mm/rev. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.3 mm/rev as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Cutting force is proportional to feed.
- Get it right
- Per revolution, not per minute.
What this means: Feed force ÷ cutting force
- What it is
- Axial feed force as a multiple of the main (tangential) cutting force. It tries to push the part along its axis in the jaws.
- Units and format
- Enter a plain number with no unit. The form accepts 0 to 2. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.5 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Enter a ratio, not a percentage: 0.5 means half the cutting force.
Chuck
What this means: Gripping diameter
- What it is
- Diameter of the surface the jaws clamp on: an outside diameter, or a bore when gripping from the inside.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 5000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 60 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Gripping on a larger diameter lowers the grip needed to resist cutting torque.
What this means: Gripping
- What it is
- Whether the jaws close onto an outside diameter or open against a bore. As speed rises, centrifugal force on the jaws reduces outside grip and adds to inside grip.
- Units and format
- Choose one of 2 options from the list: On the outside (jaws close); On the inside (jaws open).
- Example
- The form opens with “On the outside (jaws close)” selected as a worked example. Choose the option that matches your case.
What this means: Friction at the jaws
- What it is
- Friction coefficient between the jaw faces and the work, which turns clamping force into holding force. It depends on jaw type, surface finish and lubrication.
- 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 0.1 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- The grip needed is inversely proportional to friction.
- Get it right
- Take it from the chuck maker or a test; coolant or oil on the jaws lowers it.
What this means: Safety factor
- What it is
- Multiplier on the grip needed to resist cutting, covering uncertainty in cutting forces, friction and interrupted cuts.
- Units and format
- Enter a plain number with no unit. The form accepts 1 to 5. 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
- The grip needed while cutting is proportional to this factor.
What this means: Number of jaws
- What it is
- Number of jaws on the chuck; each jaw set adds its own centrifugal force at speed.
- Units and format
- Enter a plain number with no unit. The form accepts 2 to 8. Use a whole number. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 3 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Centrifugal force rises in proportion to the number of jaws.
What this means: Mass of each jaw set (base + top jaw)
- What it is
- Mass of one complete jaw set: the base jaw plus the top jaw bolted to it, from the chuck maker or weighed.
- Units and format
- Enter a number in kg: kilograms. The form accepts 0.000001 kg to 200 kg. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1.5 kg as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Centrifugal force rises in proportion.
- Get it right
- Per jaw set, not all jaws together; heavy soft jaws can add a lot.
What this means: Radius of the jaw centre of gravity
- What it is
- Distance from the spindle axis to the centre of gravity of one jaw set at the gripping position.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 2000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 80 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Centrifugal force rises in proportion.
- Get it right
- A radius from the axis, not a diameter.
What this means: Spindle speed
- What it is
- Spindle speed for the cut being checked, at which the centrifugal force on the jaws is worked out.
- Units and format
- Enter a number in rpm: revolutions per minute. The form accepts 0.000001 rpm to 50000 rpm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1500 rpm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Centrifugal force on the jaws rises with the square of speed.
What this means: Maximum static grip of the chuck
- What it is
- Largest static grip the chuck can apply, from the chuck maker’s rating at maximum actuating force.
- Units and format
- Enter a number in kN: kilonewtons (1 kN = 1000 N). The form accepts 0.000001 kN to 100000 kN. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 60 kN as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Enter kilonewtons, as a total for all jaws in the way chuck makers usually state it.
Method and formulas
- Cutting force F_c = k_c a_p f; feed force = ratio × F_c.
- Grip needed while turning = n √[(F_c d_c ÷ d_g)² + F_f²] ÷ μ (the friction at the grip must carry the cutting torque and the feed force).
- Centrifugal force on the jaws = jaws × m r ω²; static grip to set = grip needed + centrifugal force (outside gripping) or − (inside gripping).
- Speed limit (outside gripping) where the rated grip minus centrifugal force equals the grip needed.
- Example: the default cut needs 40.249 kN of grip; at 1500 rpm the jaws lose 8.8826 kN.
Limitations
- Rigid part; thin walls deform under grip and need lower pressure with soft jaws.
- Use the chuck maker’s speed and grip curves; this does not replace them.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Check that a part will not slip in the chuck on a heavy cut
- Find a safe spindle speed for heavy top jaws
- Set chuck pressure for thin-walled parts
Who it is for
- Machinists
- Manufacturing engineers
- Tooling engineers
What you get
- Cutting and feed forces
- Grip needed while cutting and centrifugal change
- Static grip to set and the speed limit
Interpreting the result
- Heavy top jaws can lose most of the grip at high speed: lighter jaws or a lower speed may be needed.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 1084–1101, 3131–3134. Topic reference; no source tables or recommended settings are reproduced.
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