The Tolerance Wasn’t The Problem Instead The Fixture Was

We recently machined a batch of Swiss-type precision sleeves for a blind-mate connector system where the CMM report was essentially "perfect." Runout was within spec, and all critical OD/ID dimensions were approved.

Yet, once the parts hit integration, the assembly behavior started drifting. It wasn’t a hard failure, but a subtle, degrading inconsistency: insertion force variance and repeatability issues that only appeared after repeated cycles.

Nothing was out of tolerance.

Most teams would start by reviewing the print and re-checking the tolerance stack-up. But in precision manufacturing, the real culprit is often hidden in the boundary conditions.

This was a multi-operation Swiss machining project. The second operation required re-clamping, and our fixture referenced a surface that was convenient for machining - but it wasn't the surface that functionally constrained the part in the actual assembly. This mismatch introduced a subtle angular bias during re-clamping. CMM probing didn’t flag it because the geometry remained "within spec," but the assembly system amplified that tiny angular deviation into a noticeable force drift.

What fixed it wasn’t a tighter tolerance; it was a different reference.

We redesigned the soft jaws to lock onto the functional seating surface—the one that defines the actual load path in the customer’s assembly. We didn't change the drawing or the tolerances. We simply aligned the fixture constraint with the functional intent. The drifting behavior stabilized.

One takeaway from this case:
Before you tighten tolerances (and add cost), look at your datums. Is your inspection datum the same surface that constrains the part in the field? Does your fixture force the intended axis, or merely suggest it?

“Within spec” only describes geometry at a single moment in time. Assembly is where constraint turns that geometry into behavior.