Geometric Tolerancing

Geometric Tolerancing

A size tolerance controls how big a feature is. It does not control its shape or position — and those can be wrong while every size is within tolerance.

The gap this fills. A shaft can measure Ø25 ± 0.02 at every point around its circumference and still be bent, or oval, or not concentric with the part it sits in. Size tolerance alone cannot catch any of that.

Geometric tolerance controls form, orientation, location and runout. Common controls include flatness, straightness, roundness, perpendicularity, parallelism, position and concentricity.

Datums. Most geometric tolerances are measured from a datum — a reference feature identified by a letter in a box. "Perpendicular to A within 0.05" means nothing without knowing which face A is.

The feature control frame is a rectangular box reading: the characteristic symbol, the tolerance value, then the datum reference.

Worked example. A flange face specified perpendicular to the bore axis within 0.05 mm: without that control, a face machined at a slight angle passes every size check and still leaks when bolted up.

Core checkpoint: Size tolerance controls how big; geometric tolerance controls what shape and where. A part needs both.