GD&T True Position and MMC Bonus Tolerance (ASME Y14.5)

ASME Y14.5 §7 defines positional error as the diameter of the smallest circle centred on the theoretically exact location (TED) that contains the actual feature centre. Because the tolerance zone is a cylindrical diameter — not a radius — the positional deviation reported on a CMM is always a diameter value: twice the straight-line distance from the TED to the measured centre.

The two-dimensional formula combines the X and Y coordinate deviations and multiplies by two to produce that diameter. If a 3D positional callout is used (common for threaded holes where the depth matters), the formula extends to include the Z-axis deviation in the same way.

The maximum material condition is the feature size that puts the most material in the part: the smallest allowable hole diameter, or the largest allowable shaft diameter. When the MMC modifier ⓜ appears in the feature control frame, the stated positional tolerance applies at that worst-case size. Every unit the actual feature departs from MMC — every increment that a hole grows larger, or a shaft grows smaller — adds exactly one unit of bonus tolerance to the positional zone.

Functionally, this reflects the reality of mating parts: a larger-than-MMC hole needs less positional accuracy to clear a mating shaft, because more clearance is available. The total effective zone is the stated tolerance plus the bonus; the mating boundary that must never be violated — regardless of how large the bonus grows — is the virtual condition.

A positional feature control frame reads left to right: the position symbol (⊕), the tolerance value preceded by the diameter symbol (⌀), and one or more modifiers in a circle (ⓜ for MMC, Ⓛ for LMC). The datum references follow, defining the coordinate system. The tolerance value in the frame is T_stated — the zone diameter that applies at MMC. The MMC size comes from the size tolerance on the same feature: for a hole toleranced as ⌀ 12.00 +0.15/−0.00 the MMC size is ⌀ 12.00 mm (the smallest hole, most material).

If no modifier appears (RFS — regardless of feature size), the zone is fixed and there is no bonus. If Ⓛ (LMC) is used instead of ⓜ, the bonus accrues as the feature approaches its least-material size — the calculation is symmetric but the direction reverses. The worked example below uses MMC on a hole, which is the most common industrial case.

After the total effective tolerance T_total is known, the PASS/FAIL verdict is a single comparison: the feature passes if TP ≤ T_total. The remaining tolerance — T_total minus TP — tells an inspector or process engineer how much margin is left before the part would fail. A small remaining tolerance flags a borderline part that may be acceptable today but is close to the rejection boundary if the process drifts.

When measurement uncertainty is significant relative to the tolerance (a measurement-to-tolerance ratio above 25 % is a common warning threshold), ASME Y14.5 does not itself specify how to handle the uncertainty; that is governed by ISO 14253-1, which applies a guardband inward from each specification limit by the expanded uncertainty U95. The MechanixCalc calculator includes that check and returns an Accept / Reject / Inconclusive decision suitable for attaching to a CMM report or non-conformance notice.