Which limit is MMC
Maximum material condition is the size at which the feature contains the most material. A smaller hole leaves more metal around it, so for an internal feature MMC is the low limit. A larger pin holds more metal, so for an external feature MMC is the high limit. Every calculation in this guide starts from that single decision, and getting it backwards flips the sign of the result rather than merely changing its size.
The mirror of that rule gives the least material condition, and the two departures always sum to the size tolerance. That identity is a quick check on any bonus calculation you do not trust.
Bonus is departure, bounded at both ends
The bonus is the distance from the produced size to the material condition limit named in the frame. For a hole at MMC it is the actual size minus the MMC size. It starts at zero when the feature is exactly at MMC and reaches its maximum, equal to the whole size tolerance, when the feature reaches LMC.
It also stops there. A feature produced outside its size limits is a size rejection in its own right and earns no bonus at all. An unbounded departure calculation on a hole 0.20 mm past its upper limit reports 0.40 mm of bonus and turns an obvious scrap part into the most generously toleranced feature on the drawing.
Bonus is per feature, and it is not datum shift
Every feature earns its own bonus from its own produced size, so a pattern of four holes made at four sizes has four different allowances. Averaging them rejects good holes and accepts bad ones.
A modifier after a datum letter is a different thing entirely. That one fixes the datum feature simulator at a boundary and grants datum shift, which moves the whole datum reference frame rather than widening any zone. A frame can carry both, and they must be reported separately.
Zero tolerance at MMC
A frame stating a position tolerance of zero at MMC is a deliberate technique rather than a mistake. It fixes the boundary at the MMC size itself and hands the entire size tolerance to position: a feature made exactly at MMC must be exactly located, while one at LMC has the whole size band available.
The result accepts every part that assembles and rejects every part that does not, which is the strongest statement of a clearance requirement available. It also demands that the shop understands the rule, because the frame reads like an impossible requirement to anyone who does not.
A 10.12 mm hole with position diameter 0.20 mm at MMC
The hole is toleranced diameter 10.00 to 10.20 mm. It is produced at 10.12 mm and its axis is 0.13 mm from basic in X, on basic in Y.
- Departure from MMC
- 0.120 mm
- Bonus tolerance earned
- 0.120 mm
- Stated tolerance
- 0.200 mm
- Total allowed
- 0.320 mm
- True position
- 0.260 mm
- Verdict
- Accept
Against the stated 0.20 mm alone this hole fails. The modifier is the difference between scrap and a good part, and it costs the design nothing because a larger hole genuinely can sit further off location and still receive the same fastener.