CMM measurement

CMM measurement (coordinate measurement) is a method in which points on a part's surface are probed in a 3D coordinate system, and software calculates geometric elements from them (planes, cylinders, cones, spheres, circles) and evaluates dimensions and geometric tolerances. It is carried out on a coordinate measuring machine (CMM).

The most widespread type is the bridge CMM with a granite table, air bearings and precision scales in three axes. Large parts such as car bodies are measured on horizontal-arm machines or large gantry machines. Points are captured by a touch probe with a ruby ball: a touch-trigger probe records individual points, while a scanning probe glides along the surface and collects points continuously, so form such as roundness or flatness can be evaluated too. Machines can also be fitted with optical and laser sensors.

Measurement follows a program prepared from the CAD model and the drawing. The part is aligned to its datums, the machine probes the specified features and the software evaluates them against the drawing, including geometric tolerances to ISO 1101. The result is an inspection report with nominal values, measured values and deviations.

Machine accuracy is verified according to ISO 10360 and stated as the maximum permissible error of length measurement, usually a few micrometers plus a term that grows with the measured length. Measurement takes place in a climate-controlled room, because the reference temperature for dimensional specifications is 20 °C (ISO 1). The portable alternative is an articulated measuring arm, which is brought to the part or machine. It is less accurate, typically hundredths of a millimeter depending on arm length, and often carries a laser scanner as well.

CMM measurement is the right choice where hundredths of a millimeter or less decide: bearing and pin fits, hole spacing and positions, flatness of seating faces, coaxiality and other geometric tolerances from the drawing. It serves first article inspection, routine inspection of series production parts with an existing program and independent measurement in quality complaints.

In reverse engineering, a CMM is combined with 3D scanning. The scanner captures freeform surfaces, and the CMM precisely measures the functional dimensions from which the nominal values are derived. It collects fewer points than a scan, but roughly an order of magnitude more accurately.

A measurement is only as good as the drawing. Missing or ambiguous datums force the metrologist to guess the alignment, and two suppliers then measure different values. Clarify the datums in advance, as well as how features are evaluated (least squares, minimum circumscribed or maximum inscribed element) and how many points are probed. A hole measured with three points tells you nothing about its roundness or cylindricity.

Parts must reach room temperature before they are measured. An aluminum part 100 mm long grows by about 0.012 mm when it warms up by 5 °C, which is decisive for tight tolerances. Thin-walled and soft parts can deform under clamping or probing force, and deep narrow holes require long styli.

Measurement uncertainty must be well below the tolerance being checked, in practice at least several times smaller. In your request, specify which dimensions are to be measured and ask for a report with the nominal value, tolerance, measured value and deviation.