Position Tolerance at MMC: CNC Hole Patterns That Fit

Position tolerance at MMC is the cleanest way to control CNC hole patterns that must assemble without over-constraining every hole. Maximum material condition (MMC) adds bonus tolerance as a hole departs from its smallest permitted size, so inspection follows function rather than one arbitrary limit.

Start with a stable datum scheme: a primary face, a secondary edge, and a tertiary feature that matches assembly location. On a 6061-T6 bracket, a true position of Ø0.2 mm relative to A|B|C is often reasonable for clearance holes; adding MMC allows larger holes while protecting the minimum boundary. Do not duplicate the control with an unnecessarily tight ±0.05 mm coordinate tolerance—that raises machining and inspection cost without improving fit.

Use MMC mainly for clearance holes and fixed-fastener patterns. For dowel-pin bores, press fits, or parts in POM/Delrin that move after machining, evaluate regardless of feature size (RFS) or a separate size limit; bonus tolerance can misrepresent the fit you actually need. Confirm the mating fastener size and hole spacing before releasing the drawing.

Inspect the pattern with a CMM or functional gage, and check that datum faces are flat and free of burrs. On 304 stainless or 7075 aluminum, specify a fine datum-face finish only where repeatable seating matters; Ra 0.8 may be unnecessary on non-functional surfaces. Keep ISO 2768 general tolerances for non-critical features and reserve position tolerance at MMC for the interfaces that control assembly.

Done well, position tolerance at MMC gives CNC suppliers a clear functional boundary, reduces drawing queries, and avoids over-tightening unrelated dimensions. Review the datum callouts and hole sizes with your machinist before production, especially when prototypes already show assembly interference.

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For more guidance on related work, see: CNC Machining Lead Time: Prototype Versus Production.