Why Does a Part That Measured Fine on the Fixture Come Back Out of Tolerance After Unclamping?

Because the fixture was holding the part in a shape it doesn't want to be in on its own. Clamping force, not machining error, was doing part of the work of meeting tolerance. Release the clamps and the part springs back toward its natural, unstressed shape, and that shape is out of tolerance.

Clamping can hide distortion instead of preventing it

Every workholding setup applies force somewhere, and thin or unevenly supported parts flex under that force. If a part is clamped flat against a fixture that isn't perfectly flat, or clamped hard enough to pull a slightly warped blank into alignment, the part measures correctly while it's still under load. The moment you unclamp, any elastic energy stored in that deflection releases, and the part moves.

What looked like a good part on the machine was actually a good part plus a clamp.

This is especially common with thin plates, parts with unequal wall thickness, and anything machined from stock that had internal residual stress to begin with. Machining one side of a plate removes material asymmetrically, which shifts the stress balance inside the part. If the fixture was resisting that shift with clamping force, releasing the part lets it move to wherever the new stress balance wants it to sit, and there's no way around that once the stress is already unbalanced short of addressing it before the final cut.

How to tell if this is your problem

The signature is simple: the part checks in-spec on the fixture, but a gauge check after unclamping shows bow, twist, or a shifted dimension. Re-clamp the same part on the same fixture. If the measurement comes back into spec, that's about as close to a confirmed diagnosis as you'll get without instrumenting the clamps themselves.

It's worth ruling out thermal effects first, since a part that was warm on the machine and cooled after unclamping can also shift dimensionally, but thermal drift tends to be uniform and predictable, while clamp-release distortion tends to correlate with where the clamps were and how thin the section is nearby.

What actually fixes it

Reduce clamping force to the minimum that holds the part through the cut. Overclamping a thin part to be extra safe is a common instinct and it's usually the wrong one. It just stores more energy to release later. Support against cutting forces, don't crush the part flat.

Support the part where it wants to move, not just where it's convenient to clamp. A fixture that only contacts the edges of a thin plate leaves the middle free to bow; adding support under the middle, even light contact, reduces how much the plate can flex under clamp pressure in the first place.

Machine in a sequence that balances stress removal. Alternating cuts between faces or working symmetric features together, rather than hogging one whole side before touching the other, keeps the part's internal stress more balanced through the job instead of loading it all onto one side.

Consider a stress-relief pass or rough-then-rest cycle for stock with known residual stress. Letting a roughed part sit, or relieving it before finish passes, gives internal stress somewhere to go before the final cut locks in dimensions.

The fix is never "clamp harder." That just stores more energy for the part to release later.

Figure out where the part wants to move and either support it there or let the stress relieve itself before the final cut.