A coordinate measuring machine (CMM) is a machine that touches a part at many points and measures its size, and software then compares the points with the design to prove that each size is within tolerance. The science of exact measurement is called metrology.
A CMM has a small probe on a movable arm. The probe touches the surface of a part at a chosen set of points. The machine records the position of each point in three directions, and software works out from them the size of a hole, the distance between two surfaces or the roundness of a shaft. It compares them with the design, which it receives from CAD software or from PLM software.
Take, for example, an engine block from a CNC machine. A cylinder bore in it must be 80.000 mm wide, with a tolerance of plus or minus 0.015 mm. Once in every fifty blocks, the block is taken to the CMM laboratory. The probe touches 12 points inside the bore. The software calculates a width of 80.012 mm, which is inside the limit. If the width were 80.020 mm, the block would fail, and the plant would adjust the CNC machine.
Heat changes a size, because metal expands as it warms. A steel part that is 80 mm wide grows by about one thousandth of a millimetre for each degree of temperature. So a CMM is kept in a room held at 20 °C, which is the standard temperature for measuring, and parts are left in the room to reach that temperature before they are measured. A CMM is also checked regularly against a reference piece of known size. A touch probe is accurate but slow. Some CMMs use a laser instead, which scans a surface much faster but less accurately.
The results go to the MES and the quality management system. A customer who buys parts can demand this record.
A CMM measures only the outside and the surfaces that its probe can reach. To see inside a part, plants use industrial CT scanning.