Tolerance of Position and Bonus Tolerance / MMC-LMC Concepts
The heart of the senior exam
Position tolerance with material condition modifiers is the most heavily tested area. At MMC, the geometric tolerance is a fixed boundary (virtual condition); as the actual feature departs from MMC toward LMC, the allowable geometric tolerance increases by the amount of departure. That increase is bonus tolerance. At LMC, the same logic applies in reverse, and LMC is used when minimum wall thickness or clearance must be protected.
Concepts to command
Know how to compute virtual condition and resultant condition. Know the difference between bonus tolerance (from size departure) and the fixed geometric tolerance. Know RFS (no bonus), MMC (bonus as size departs from MMC), and LMC (bonus as size departs from LMC). Know how datum features at MMC shift the DRF and create datum shift. Know the zero position at MMC concept (a position tolerance of zero at MMC with a size tolerance that provides the bonus).
Traps that appear in questions
- Adding bonus tolerance when RFS is specified.
- Forgetting datum shift when a datum feature is referenced at MMC.
- Computing bonus from the wrong direction (MMC vs LMC).
- Confusing virtual condition (constant) with resultant condition (actual).
Rule of thumb
Bonus = |actual size − MMC size| for MMC, or |actual size − LMC size| for LMC. Add it to the stated geometric tolerance.
Worked example
Hole Ø10.0 ±0.2, position Ø0.3 at MMC to A|B(M)|C. At actual size Ø10.1, bonus = 0.1, so allowed position = 0.4. If datum B is also at MMC and departs, datum shift adds further allowable movement of the DRF.