GlossaryKind of term: Standard and conventionMaximum material condition (MMC)
What is maximum material condition?
Maximum material condition, shown as Ⓜ in the feature control frame, lets a geometric tolerance grow as the feature's size departs from maximum material.
Strictly, maximum material condition is the state of a feature at its maximum material size; the requirement shown as Ⓜ after the tolerance value is called the maximum material requirement in ISO 2692, while ASME Y14.5 uses MMC for both. It applies to features of size, such as a hole, pin or slot. Maximum material is the limit with the most material: a hole at its smallest size, a pin at its largest. At that size the tolerance holds as written; as the actual size departs from it, the geometric deviation may grow by the difference. A hole ⌀10 +0.1/0 with position ⌀0.1 Ⓜ may therefore lie within a ⌀0.2 zone at ⌀10.1.
What does the standard say about the maximum material condition?
ISO 2692 defines the requirement and works with the maximum material virtual size: for a hole, the smallest size minus the geometric tolerance, ⌀9.9 in the example. The feature may not violate that boundary and must also stay within its size tolerance. Ⓜ usually sits with position, but can also apply to orientation tolerances and to the straightness of an axis, in each case to a requirement on the axis or median plane of a feature of size, never on a surface. After a datum letter, Ⓜ means the datum is simulated at its virtual size, or at its maximum material size if the datum feature carries no geometric requirement of its own. The counterpart is least material condition, shown as Ⓛ.
How does the maximum material condition work in practice?
Ⓜ is meant for features that have to assemble, such as a bolt hole: the virtual size is the boundary that secures assembly, and a part that accepts a gauge pin at that size assembles with a mating part designed to the same boundary. That gives room in the route. If a hole comes out towards the large side of its size tolerance, there is more position allowance, and plain drilling is sometimes enough where you would otherwise bore. In a batch, a functional gauge can inspect faster than measuring every hole on a coordinate measuring machine; the gauge itself has to be made and periodically calibrated. Whether that pays off depends on quantity and repeat orders. Miss the Ⓜ and you price a tighter requirement than the drawing asks for.
What is the maximum material condition often confused with?
Maximum material condition is not the envelope requirement: Ⓔ belongs to a size and keeps form within it, whereas Ⓜ in the feature control frame lets a geometric tolerance increase as the size departs from maximum material. Nor is it least material condition: Ⓛ works from the size with the least material and suits, for example, a minimum wall thickness you must keep.
Frequently asked questions
What is bonus tolerance under maximum material condition?
The extra geometric tolerance gained when a feature with Ⓜ departs from its maximum material size. A hole that comes out larger may sit further from its ideal location, by exactly that difference. The term comes from ASME practice.
May I make the hole larger on purpose to gain bonus tolerance?
Yes, within the size tolerance: the drawing allows it. On a fit the size tolerance is small, so the extra position allowance is too.
What does an M after the datum letter mean?
That the datum is simulated at its virtual size. If the datum feature departs from that size, the part may shift relative to the simulated datum. A functional gauge allows for that shift by itself; a coordinate measurement must include it.