National Physical Laboratory

Diameter of CMM Reference Spheres

Diameter of CMM reference

NPL provides a service for measuring the diameter of coordinate measuring machine (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. Spheres are compared with a gauge block in a machine having flat, parallel measuring faces whose separation is determined interferometrically. The measured size of the sphere is then corrected to zero measuring force using knowledge of the properties of the material. The diameter is measured in a single identified plane and this result is stated on the certificate. The departure from roundness (MZC, 1-150 upr) at the equator is also measured and stated on the certificate.

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 x diameter in mm) μm

For roundness the uncertainty is typically 0.01 micrometres

Additional measurements

Additional roundness and diameter measurements can be made at additional cost. Please contact us for details.


Customer Service tel: 020 8943 8631

Last Updated: 18 Jun 2013
Created: 30 Aug 2012


Please note that the information will not be divulged to third parties, or used without your permission