A coordinate measuring machine (CMM) measures points on a part in a controlled coordinate system and uses those points to estimate geometric features such as planes, circles, cylinders and positions.

The machine does not directly measure an abstract 'perfect plane'. It samples physical points from the real surface. Software then fits an ideal geometric feature according to a defined evaluation rule.

For example, several points measured on a bore can be used to estimate a circle centre and diameter in one section. Repeated sections can contribute to a fitted cylinder axis.

A CMM inspection begins by establishing the part's reference frame from physical datum features. That alignment is essential: a hole-position tolerance relative to datums cannot be evaluated meaningfully in an arbitrary machine coordinate system.

Measurement strategy affects the result. Too few points may miss local shape error; probe size and access limit what regions can be touched; temperature and fixturing can alter dimensions.

CMMs are powerful because one programmable system can evaluate many dimensions and geometric relationships, but they do not eliminate measurement uncertainty or specification interpretation.

The chain is

physical datum features
        ↓
part coordinate frame
        ↓
sampled surface points
        ↓
fitted geometric features
        ↓
comparison with drawing requirements

Coordinate metrology therefore connects GD&T to actual measured data.