Bill,

A disk probe is a section of a sphere containing the equator. If the sphere is high quality, then the disk will be high quality, with no loss of accuracy.

A high quality steel disk should be hardened, ground, and polished, with a hardness value of at least Rockwell C 62. Even so, a hardened steel disk will wear. Wear can sometimes be detected during calibration by an unusually high standard deviation. However, this is valid only if you touch on a worn spot during calibration! Therefore, we strongly recommend regular visual inspection, under magnification, for signs of abrasion.

There is no cosine error if probing is done with the spherical portion of the disk, just as there is none with a complete sphere. The concentricity of stem to disk is not critical; however, the perpendicularity is, to prevent probing on the non-spherical parts of the disk.

I'm not sure what you mean by "dynamics" coming into play. Remember, a disk is just a section of a ball.

Proper calibration of a disk probe is very important. We recommend a minimum of 24 probings on a disk during calibration. Take at least 8 points near the equator, 8 above it, and 8 below it. Spread the 24 points over as much area as practical. Remember, your CMM software must "create" a complete (and accurate!) sphere through a very limited and constrained point set provided by the disk. Always check your calibration data closely - with particular emphasis on the standard deviation - before using any disk probe.

We manufacture disk probes in diameters from 2 mm and up. We've found that disk probes can be important and productive tools for any CMM, provided certain procedures are followed.

Best regards,
Mark Osterstock
President, Q-Mark Manufacturing Inc.
www.cmms.com