Geometric tolerances

Size tolerances control the permitted variation of a dimension. Geometric tolerances add controls for a feature’s form and its relationship to other features. They help communicate function without requiring every surface to be manufactured to unnecessary precision.

Main tolerance groups

Form: straightness, flatness, circularity and cylindricity. Orientation: parallelism, perpendicularity and angularity. Location: position and profile. Runout controls variation as a feature rotates about a datum axis.

Feature control frames and datums

A feature control frame identifies the geometric characteristic, tolerance value and any datum references. Datums establish the reference system used to inspect and manufacture the part. Select functional datum surfaces and a sequence that reflects assembly and inspection.