GD&T

Related terms

Datum

A datum is a theoretically exact point, axis, or plane, established by a real feature on a part, from which other features are located and measured. Drawings list datums in order of precedence (primary, secondary, tertiary), which tells inspection how to fixture the part.

True position

True position is a GD&T location control that defines a tolerance zone, usually cylindrical, around the theoretically exact location of a feature such as a hole, relative to datums. A diameter-zone position callout gives about 57% more tolerance area than the equivalent plus-minus square zone.

Flatness

Flatness is a GD&T form control that requires every point of a surface to lie between two parallel planes a specified distance apart. It does not reference a datum. Flatness is common on sealing faces, mounting pads, and machined plates, and very tight callouts often need grinding or lapping.

Maximum material condition

Maximum material condition is the state where a feature of size contains the most material allowed by its tolerance: the smallest hole or the largest pin. When a position tolerance is applied at MMC, the tolerance grows as the feature departs from MMC, which is called bonus tolerance.