MMC, LMC and bonus tolerance
Work out the MMC size, the bonus or the total tolerance. Get 10 right to finish.
What is the maximum material condition (MMC) size of this pin?
Material conditions
| Feature | MMC (most material) | LMC (least material) |
|---|---|---|
| Hole | Smallest hole | Largest hole |
| Pin or shaft | Largest pin | Smallest pin |
Bonus tolerance
With a circled M, the stated tolerance applies when the feature is at MMC. As the feature moves away from MMC it gets extra tolerance, the bonus, equal to how far it has moved.
Example: a hole Ø10.0 to Ø10.2 with position Ø0.25 at MMC. MMC is 10.0. The hole measures 10.15, so the bonus is 0.15 and the total position tolerance is 0.25 + 0.15 = Ø0.40. The most bonus a part can get is the size tolerance, here 0.2.
Common mistakes
- Using the wrong limit as MMC for a hole. A hole has the most material when it is smallest.
- Adding bonus when there is no M or L in the frame.
- Thinking bonus lets the size go out of its limits. It only adds position tolerance.
Questions
- What is MMC in GD&T?
- Maximum material condition: the size at which the feature has the most material. For a hole that is its smallest allowed size, for a pin its largest.
- How do you calculate bonus tolerance?
- Bonus = the difference between the measured size and the MMC size (or LMC size, for an LMC callout). Add it to the stated tolerance for the total.
- Is there bonus tolerance without a modifier?
- No. Without a circled M or L the tolerance applies regardless of feature size, so the stated value is all you get.
Explained in our own words and checked against several GD&T references. For the full rules, see ASME Y14.5-2018 and ISO 1101:2017.