EURAMET Project 1105 - Bilateral comparison on micro-CMM artefacts

A growing number of industrial workpieces feature geometrical micro-structures with dimensions of some µm up to 1 mm which need to be measured with suitable measurement uncertainties. Typical measurement uncertainty specifications of commercially available instruments are in the range from a few tenths of a µm to about 1 µm. To test these instruments, measurement standards are required which are to be calibrated with uncertainties below 0.1 µm. The calibration of such standards with uncertainties of less than 100 nm is very demanding and, notably relevant for multidimensional standards, no experience existed regarding the stability of the standards and the comparability of measurement results associated with very small uncertainties. Therefore, the EURAMET contact persons for length of the Swiss Federal Office of Metrology (METAS) and of the German national metrology institute Physikalisch-Technische Bundesanstalt (PTB) decided in May 2009 to carry out a bilateral comparison for the calibration of artefacts by micro-coordinate measuring machines. This comparison was part of the EURAMET research collaboration project #1105 “Bilateral comparison on micro-CMM artefacts” and was the first comparison in the field of micro-coordinate measurement techniques. PTB was the pilot laboratory of the comparison.

Within the bilateral comparison measurements, three micro-CMM standards were circulated: a precision sphere and a ring both 1 mm in diameter as well as a Zerodur plate with hemispheres shown in Figure 1. As an example of the results obtained the distances between the hemispheres are shown in Figure 2. All parameters to be measured and the measurement results are described in detail in [1]. The results obtained at both laboratories agree within the small uncertainties given. A small length dependency of the observed deviations is observed (dashed curve).

This very good agreement highlights the state-of-the-art of the very precise micro-CMMs used and, moreover, the advanced practice of calibration work of micro-CMM artefacts at METAS and PTB. Furthermore, valuable information was obtained about the exchange of suitable artefacts and practical issues for handling and measurement strategies for future comparisons in the new field of micro-CMM calibration.

The analysis of both the differences and the uncertainty contributions pointed out that the major restrictions in the field of micro-CMM measurement are the traceability of the length measurements, the calibration of the diameter and the form deviations of the probing sphere and last but not the least, the influences of the object to be measured itself. Therefore forthcoming tasks are related to the development of practice-qualified solutions for these problems.
 
[1] EURAMET Project 1105 “Bilateral comparison on micro-CMM artefacts between PTB and METAS”, Final report, 22.03.2011


Figure 1: The Zerodur plate, 90 mm x 90 mm x 5 mm, with nine hemispheres with a diameter of 5.6 mm. The hemispheres are arranged in that manner to realize a large number of different distances as shown in figure 2 with the aid of a low number of hemispheres.


Figure 2: Zerodur hemisphere plate: deviations of the distances from the calculated weighted mean reference values, expanded uncertainties U(L_i) for k = 2,  PTB,  METAS, - - - least-square fit line of data points of METAS (red) and PTB (blue), respectively, as function of distance indicates a small dependency on length.