Maximum Material Condition
What is the MMC of a 10.00 ±0.10 mm hole?
9.90 mm, because the hole has the most material to remove at its smallest permitted size.
What is the MMC of a 10.00 ±0.10 mm shaft?
10.10 mm, because the shaft has the most material at its largest permitted size.
Why do designers use MMC on position tolerances?
To allow bonus tolerance and make parts easier to manufacture while preserving assembly limits at the worst-case condition.
Maximum Material Condition (MMC) is the condition of a feature of size when it contains the greatest amount of material within its size tolerance. For an external feature like a shaft, MMC is the largest permissible size; for an internal feature like a hole, MMC is the smallest permissible size. It is used with an M modifier in GD&T to provide bonus tolerance for geometric controls.
On a CNC shop floor, MMC is applied to control positional, perpendicularity, and other geometric tolerances while ensuring part function in mating assemblies. For example, in a bolt hole pattern, if a hole is at its smallest allowed diameter (MMC for internal features), the geometric tolerance zone is tightest. If the hole is made larger, the part gains bonus tolerance, improving manufacturability and reducing scrap risk. Conversely, for a shaft or dowel pin, MMC is the largest allowable diameter because that condition leaves the least clearance in assembly. Inspection often uses functional gaging to verify parts against the worst-case boundary that guarantees assembly when size and geometric error are combined. In millwork or cabinetry hardware drilling, the same logic applies to mating dowel and connector holes: the hole’s minimum size is the most restrictive condition, while extra size may absorb more location error if the drawing permits MMC-based geometric control.
Confusing MMC with 'largest size for everything': Applying the shaft rule to holes causes incorrect inspection, as holes reach MMC at smallest size, not largest.
Assuming bonus tolerance without MMC modifier: Extra geometric tolerance only exists when the M modifier is explicitly used; without it, tolerance does not increase.
Ignoring assembly worst case in gaging: If inspection ignores virtual condition, parts may pass size checks but fail assembly because hole location and size violate mating envelope.