The Symptom and the Usual Diagnosis
A vacuum fixture stops holding. The operator checks the gauge, sees it dropping, and the first response is to blame the pump. In most cases the pump is working exactly as specified and the leak is somewhere between the part and the plate, in a place the gauge cannot see.
Diagnose in This Order
- Cap the ports and run the decay test. If the plate holds vacuum with ports capped, the plate and its internal passages are sound and the leak is at the part interface.
- Inspect the sealing surface. A nicked O-ring, compressed gasket tape or a burr at a groove edge will leak at a rate that looks like a pump failure.
- Check face flatness. Leakage scales with the gap, so 0.05 mm of face error matters more than 50 percent more pump capacity.
- Check groove volume against pump capacity. A deep, wide groove network on a small pump takes time to evacuate and looks like a slow leak.
- Check the part, not just the plate. A warped workpiece or one with a rough back face will not seal regardless of plate quality.
Numbers Worth Knowing
| Parameter | Typical value | Failure threshold |
|---|---|---|
| Face flatness (plate up to 300 mm) | 0.01 mm | Above 0.03 mm holding drops noticeably |
| Sealing land width | 3 mm minimum | Narrower land distorts easily |
| Groove width and depth | 2-4 mm, 1.5-3 mm | Oversized grooves outrun small pumps |
| Clamping force at full vacuum | About 1 kg per square centimetre | Depends on coverage, not on pump brand |
Maintenance That Prevents the Call
- Replace O-rings on a schedule, not on failure, and keep a spare set with the fixture.
- Skim the face and re-cut grooves when flatness drifts, keeping the original groove program on file.
- Keep the chip filter clean; a blocked filter starves the plate and mimics a leak.
- Store plates covered, because a dented sealing land cannot be repaired by adjustment.
- Log the decay value at each setup, so a slow trend is visible before the fixture stops holding mid job.
When to Replace Instead of Repair
If the plate has been skimmed twice and the groove depth is now below the design minimum, replacement is cheaper than another repair. A plate that has lost groove depth cannot hold a thin part no matter how well it is sealed, because the vacuum has nowhere to travel.
Our vacuum chuck fixture plate page lists groove patterns, sealing options and the decay test supplied with every plate.
