Choosing workholding for machining
Compare mechanical, vacuum and magnetic restraint while keeping locating, support and clamping distinct.
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Separate location, support and restraint
Choose how to locate the part, support it against bending and resist the cut. In the vacuum example below, 0.01 m² of sealed area at 60 kPa gives 600 N normal force and 150 N ideal lateral resistance at μ = 0.25. The vacuum-chuck detail shows how losing sealed area changes restraint.
For transfer, use the zero-point seating example to separate fixture repeatability from workpiece location and distortion. For indexed access, compare tombstone pallet payload with trunnion gravity torque and clearance.
Compare the workholding candidates
| Candidate | Useful starting point | Main qualification |
|---|---|---|
| Machine vise | Accessible, opposing gripping surfaces | Seating, jaw engagement, lift and tool clearance |
| Soft jaws | Contact geometry adapted to a particular part | Loaded machining, actual contact and distortion |
| Vacuum chuck | Suitable support and sealed area with open top access | Effective area, leakage, friction and lateral restraint |
| Magnetic chuck | Magnetically responsive work with an appropriate flux path | Material, thickness, pole engagement and cutting-load resistance |
| Zero-point interface | Repeatable exchange of a fixture or pallet | Interface repeatability plus the separate workpiece fixture and load path |
SCHUNK's soft-jaw range and Kurt's soft-jaw guidance describe machinable contact surfaces. Schmalz's vacuum explanation distinguishes normal holding force from lateral resistance.
Worked example: vacuum force has a direction
Assume an illustrative effective sealed area of 0.01 m², a pressure difference of 60,000 Pa and a contact friction coefficient of 0.25. Using Schmalz's relationships:
- Normal force: 0.01 × 60,000 = 600 N.
- Idealized friction resistance parallel to the face: 600 × 0.25 = 150 N.
The lateral estimate assumes unchanged pressure and friction. Evaluate leakage at holes and through-cuts, overturning moments, peak loads and safety factors separately. The vacuum-chuck detail shows what happens if the effective sealed area halves.
Example: machining a thin plate
Compare local jaw contact with distributed face support. Check actual pocket contact, sealing for vacuum or pole engagement for magnetic holding. Verify the workpiece datums in the fixture after transfer and check whether the plate springs back after release. Inspect flatness in the specified free or restrained state. The zero-point example shows how a seating error can shift a feature even when the interface repeats.
Plan the next setup
List the faces machined in each setup and the datums used after transfer. Fixture plates locate and fasten hardware. For tombstones and trunnions, check the loaded envelope, center of gravity and the applicable pallet or rotary limits.
Connect the fixture to the manufacturing route, material condition and setup-cost estimate. Account for loading, alignment and inspection even when the fixture is reusable.
Reviewed November 2, 2026 PST. The examples explain selection and force direction; actual machining requires the applicable fixture and equipment limits.