How to accept a new fixture: what to measure before you pay

The short answer: accept a fixture on four measurements, not on how it looks — fixture geometry against the approved drawing, clamp force at each station, repeatability after removing and re-mounting the fixture, and first-article dimensions on a real part machined in it. If a supplier cannot produce all four numbers, the fixture is not finished.

1. Fixture geometry, measured against the drawing you approved

A CMM report on the fixture itself: datum faces, pin positions, pad heights, parallelism between faces. The reference is the approved drawing, not a verbal specification, which is why the drawing has to be signed before machining starts.

Pass criterion: every dimension inside its stated tolerance, with the actual value recorded, not just a tick. A report that only says “OK” tells you nothing about margin.

2. Clamp force per station

A hydraulic fixture has a pressure gauge, but pressure is not force: force depends on the cylinder area and the lever ratio of the clamp, and on multi-station fixtures the stations may differ. Measure with a clamp force gauge at each station.

Pass criterion: measured force within ±10 % of the design value at every station, at the working pressure stated in the schematic. If one station is 30 % low, a seal is leaking or a sequence valve is set wrong, and both will get worse.

3. Repeatability after re-mounting

This is the test most suppliers skip and the one that decides whether the fixture is useful in production. Machine a test feature, remove the fixture from the table, put it back, machine again, and measure the difference between the two results.

Pass criterion: 0.01 mm or better with a fixed zero-point interface; 0.02 mm with T-slot keys, which is the practical limit of key location. If the fixture is designed for 0.005 mm repeatability, this test is where it shows.

4. First article on a real part

Fixture geometry is not part geometry. A fixture can be perfect and still produce a bad part, because the clamp load distorts the workpiece. Measure the part, not only the fixture, with the part clamped at working pressure.

Pass criterion: every drawing dimension inside tolerance, and the distortion pattern documented — where the part moves when clamped, and by how much. That record is what lets you adjust the process six months later without starting from zero.

What to do when a measurement fails

Separate the three possible causes before accepting a fix: the fixture is wrong, the machine is wrong, or the process is wrong. If the fixture passes geometry and force but the part fails, the machine or the programme is the suspect, and no amount of fixture rework will fix it. A supplier who immediately agrees to rework without this separation is guessing.

Acceptance checklist for a custom fixture

Check	Method	Pass criterion
Fixture geometry	CMM against approved drawing	All dimensions in tolerance, actual values recorded
Clamp force	Force gauge at each station	Within ±10 % of design value
Repeatability	Machine, remove, re-mount, machine	≤ 0.01 mm zero-point, ≤ 0.02 mm T-slot
Hydraulic tightness	1.5× working pressure, 10 min hold	No pressure drop, no seepage
First article	Real part, clamped at working pressure	All drawing dimensions in tolerance
Documentation	Report package	Geometry report, force log, trial-cut video, schematic

Questions

Should I insist on a CMM report for a simple manual fixture?

A full CMM report is overkill for a 600-dollar manual fixture; a runout and parallelism check on a granite plate is enough. The four checks scale with the price — what must not scale is the idea of measuring before paying.

Who pays for third-party inspection?

The buyer, normally, and it is money well spent on a fixture above about 10,000 USD. Ask for the report to be sent to you directly, not to the supplier.

What if the fixture passes but the part is out at one dimension?

Check whether that dimension is affected by clamping. If it moves when the clamp is released, the location scheme is wrong, not the machining. That is a fixture issue and should be fixed by the supplier.

How long should acceptance take?

Half a day for a single-station fixture, one to two days for a tombstone with several faces. If a supplier wants a week, they are commissioning, not accepting — which is fine, but call it what it is.

Published by Liuzhou Dongben Machinery Equipment Co., Ltd. · More notes