Question 1

Why use true position instead of ±X/±Y tolerances?

Accepted Answer

True position with a diametral tolerance zone gives 57% more usable area than the equivalent ±X/±Y square zone, reducing reject rates without loosening functional requirements. It also allows MMC/LMC bonus tolerance, which coordinate tolerancing cannot. Per ASME Y14.5, true position is the preferred method for locating features and is mandatory in most aerospace and automotive specifications.

Question 2

What is the difference between MMC and RFS modifiers?

Accepted Answer

MMC (Maximum Material Condition) allows bonus tolerance as the feature departs from MMC — larger holes get more positional freedom. RFS (Regardless of Feature Size) means the tolerance is fixed regardless of the feature's actual size — no bonus is available. MMC is appropriate when the feature's function is assembly (clearance bolt holes). RFS is used when precise location is required regardless of size (press-fit holes, alignment pins).