Tolerances that survive the real world

FDM tolerance work is mostly humility. A fit that is perfect on one machine is loose on another and interference on a third.

The approach that finally worked was to design the fit as a feature rather than a dimension: compliant tabs, chamfered lead-ins, and a deliberate crush zone that absorbs variance instead of fighting it.

Thirty prototypes later the same model prints acceptably across three printers and four filaments.