By:Keith Mills


Xspect Solutions, Inc.


When certifying CMM accuracy, manufacturers have used the VDI-VDE 2617 specification, which provides for a 95% confidence level. Linear accuracy is specified as U1 and volumetric accuracy is specified as U3. In each case, the 95% confidence level means 5% of the observations can be excluded from the testing data. This means the “flyers” are disregarded and effectively allow the CMM machine accuracy statement to report better accuracy that can actually be achieved.

The ISO 10360-2 specification made the necessary changes to recognize that in practical every-day measurement the CMM user does not have the luxury of excluding “flyers”. In fact, most users do not even know when “flyers” have been measured. As a result, the ISO specification requires 100% of all measurements to be included in the evaluation of the CMM performance.

Under ISO 10360-2 the CMM machine is evaluated in at least three areas of its measuring volume:

1) MPEE is Volumetric Length Uncertainty, Fig. 1, and utilizes calibrated gage blocks. Ball bars are not used because their length is arbitrary, and only the spheres can be calibrated (B89 Standard). In addition, the measurement of a sphere employs many points to resolve the center, which does not represent practical measurement. For example, in practical CMM measurement, point data is taken as required for the respective part feature with the expectation that each point is accurate. There is no luxury to measure multiple points and resolve (1) point as that to be used in the part feature calculation. In ISO 10360-2, discrete single points are measured bi-directionally to evaluate length and the MPEvalue reports the range of measurements from (7) different positions with (5) different gage lengths…and this is repeated (3) times. All 105 measurements with their deviation from certified lengths are considered, and they are provided as the Uncertainty of Measured Length (MPEE ).

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2) The second ISO 10360-2 evaluation for touch probe CMMs is MPE, Fig. 2, which represents Probing Uncertainty. In this case, the CMM measures (25) discrete points that are evaluated as (25) individual radii. The range of radii variation, min to max, is the MPEP .

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3) The third important ISO 10360-2 metric is MPETHP  , Fig. 3, which is applicable to scanning CMMs and is similar to MPEP . In this case however, it is achieved by full contact scanning of (4) lines, of which, only (1) can be a full 360-degree equatorial scan. This evaluation comes from the ISO 10360-4 specification. Like MPEP , the points within each scan re treated as radii and the range of radii deviation, min to max, is reported as MPETHP .

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Xspect Solutions, Inc. headquartered in Wixom, Michigan, and is a wholly owned subsidiary of Wenzel Gmbh of Germany. Xspect Solutions is the Number 3 supplier of CMMs in the North American metrology market including; new Wenzel CMMs, and is the world’s largest supplier of pre-owned CMM equipment. Xspect Solutions also supplies new Wenzel CMMs with its OpenDMIS™ software to the North American markets.

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