Maximum Material Condition (MMC)

The Ⓜ modifier: the tolerance applies at maximum material, and departing from MMC grants bonus tolerance.

Definition

A feature of size is at its maximum material condition (MMC) when it contains the maximum amount of material: a shaft at its largest allowed size, a hole at its smallest. The Ⓜ modifier in a feature control frame means the stated tolerance applies when the feature is produced exactly at MMC — and as the produced size departs from MMC, the tolerance zone grows by exactly that departure. That growth is the bonus tolerance. The same idea applies in both directions: departing from MMC means a shaft is produced smaller or a hole larger — always toward more clearance and less material.

The MMC boundary: virtual condition

At MMC the tolerance zone defines the worst-case boundary for assembly — the virtual condition. For an external feature (a pin) the virtual condition is the MMC size plus the position tolerance; for an internal feature (a hole) it is the MMC size minus the tolerance. Whatever assembles against the part must clear that boundary, and the boundary never moves: as the feature departs from MMC, the zone grows, but the guaranteed clearance boundary stays put. That is why MMC callouts can be checked with a fixed functional gauge — the gauge is built to the virtual condition.

Bonus tolerance in practice

The arithmetic is a straight subtraction. A hole with ⌖ ⌀0.5 Ⓜ toleranced at Ø6.0 minimum is produced at Ø6.4: the departure from MMC is 6.4 − 6.0 = 0.4, so the position zone grows from 0.5 to 0.9 diameter. The part is easier to make than the frame first appears — which is the point of the modifier. The zone only ever grows; it never shrinks below the stated value.

Where Ⓜ applies — and where it can't

MMC exists only for features of size — a cylinder or a width between two parallel planes. The common applications: position (by far the most frequent), axis straightness (the one Form control that accepts it), and profile of a feature of size on later ASME revisions. Never: flatness, circularity, cylindricity, runout, concentricity, symmetry, or any surface-applied orientation control — those reference the surface directly, not a size-derived boundary, so there is nothing for the modifier to act on.

Reading a feature control frame

⌖ ⌀0.5 Ⓜ A B C reads as a position tolerance of 0.5 diameter at MMC, located from datums A, B and C in precedence order. The Ⓜ sits in the tolerance compartment immediately after the value — everything before the first datum letter is part of the tolerance unit. Without Ⓜ (or Ⓛ) the frame means the plain default, regardless of feature size.

Measurement

Functional gauging — a gauge built to the virtual condition. Fast and repeatable, the standard for high-volume production; the gauge passes every part whose boundary clears the virtual condition regardless of how the tolerance was consumed.

CMM coordinate measurement — measure the feature's size and location, compute the departure from MMC, and add it to the base tolerance before judging the measured location error. Slower, but it reports exactly how much of the tolerance was used.

When to use it

• Clearance-hole patterns for fasteners, where the worst case is the smallest hole — the MMC boundary is the assembly guarantee
• Mating shafts and hubs where the fit is decided at maximum material
• Hard-to-hold locations where bonus tolerance can meaningfully cut scrap without relaxing the guaranteed fit
• Any callout a functional gauge must verify — gauges only make sense against the MMC boundary