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Dimensional measurements

CMM verification artefacts

Accredited verification service

ISO 10360 strongly recommends that a coordinate measuring machine (CMM) be checked regularly during the periods between periodic re-verification. Depending on the measurement tasks for which the CMM is used, the most relevant of the following commonly used artefacts should be chosen. Some examples are given below:

  • A purpose-made test piece that has features representing typical geometrical shapes, is dimensionally stable, mechanically robust, and which has a surface finish that does not significantly affect the uncertainty of measurement
  • A ball plate
  • A hole plate
  • A ball bar
  • A hole bar
  • A bar that can be kinematically located between a fixed reference sphere and the CMM probe-stylus sphere
  • A circular artefact  

NPL can calibrate a number of these artefacts including purpose-made test pieces, ball plates, hole plates, ball bars, hole bars and ring gauges.

Diameter of CMM reference spheres: NPL also provide a service for measuring the diameter of CMM reference spheres. CMM reference spheres are used for stylus system qualification or as test spheres when carrying out an ISO 10360 probing test. It is therefore important that the diameter and form of these spheres are known with an associated uncertainty. The diameters of reference spheres are measured by comparison with calibrated gauge blocks of similar size. Both metal and ceramic spheres can be calibrated.

Calibration and measurement capability: The calibration and measurement capability for diameter expressed as an expanded uncertainty (k=2) is given by the formula: 0.07 + (0.0011 × diameter in mm) μm. For roundness the uncertainty is typically 0.01 micrometres

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