At novel hemisphere plate standards, measurement uncertainties of less than 0.1 µm can be achieved by combining a special measurement strategy, stable environmental conditions and a very good reproducibility. This considerably improves the testing of micro coordinate measuring machines in industry.
To be able to verify the dimensions of micro components, micro coordinate measuring machines (micro-CMMs) are used in industry which currently achieve length measurement errors of 0.3 µm and less. Testing these micro-CMMs in compliance with international standards must be carried out with suitable measurement standards, e.g. with special ball plates where hemispheres have been wrung to a Zerodur plate. Due to the very low thermal expansion coefficient of the Zerodur glass ceramics, the precisely defined distances are guaranteed, and the special design of the plate, with drill holes in the substrate, makes it possible for the hemispheres to be probed from both sides of the plate. The novel hemisphere plates are calibrated with high precision at PTB by means of a micro-CMM with a tactile sensor.
The systematic deviations of the measuring instrument used can be largely eliminated by means of an error separation technique. Thereby, the hemisphere plate is measured in various orientations to the measuring instrument axes, in an upright and inverse position. The precision of the error separation technique is significantly limited by the reproducibility of the measurements in the various positions. The requirements placed on the stability of the environmental conditions and on the diligence of the operator are thus accordingly high.
The systematic deviations of the measuring instrument used can be largely eliminated by means of an error separation technique. Thereby, the hemisphere plate is measured in various orientations to the measuring instrument axes, in an upright and inverse position. The precision of the error separation technique is significantly limited by the reproducibility of the measurements in the various positions. The requirements placed on the stability of the environmental conditions and on the diligence of the operator are thus accordingly high.
The calibration uncertainties attained amount to 50 nm (k = 2) for the positions of the hemispheres on the Zerodur plate. Long-term investigations of the procedure showed reproducibilities of better than ± 15 nm for three repeated calibrations on a PTB hemisphere plate.
The measurement results achieved with PTB‘s micro-CMM have – within the scope of a bilateral EURAMET project which has just been completed – been compared to the measurement results of the micro-CMM operated at the Swiss metrology institute METAS. Within the range of the estimated measurement uncertainties, a very good agreement hereby resulted.
Michael Neugebauer
Department 5.3 Coordinate Metrology
Phone: +49 (0) 531 592-5212
E-Mail: 
michael.neugebauer(at)ptb.de
Neugebauer, M: Precision size and form measurement with a micro-CMM F25. Proceedings of the IXth International Scientific Conference Coordinate Measuring Technique, 14.–16. April 2010, Bielsko-Biała, Polen, ISBN 978-83-62292-561, S. 43–49