Understanding The Basics of ISO 10360-2

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When certifying CMM accuracy manufacturers had employed the VDI-VDE 2617 specification which provides for a 95% confidence interval and evaluates linear accuracy specified as U1 and volumetric accuracy specified as U3. In each case the specification provided a 95% confidence level, which means 5% of the observations, could be excluded from the testing data. This means "flyers" are tossed and effectively allowed machine accuracy statements to report better accuracy than could actually be achieved.

The ISO 10360-2 specifications changed all of this recognizing that in practical every day measurement the CMM user does not have the luxury of excluding "flyers" and generally users do not even know when flyers are observed. As such the ISO specification requires 100% of all observations to be included in the evaluation of the CMM performance.

Under ISO 10360-2, the machine is evaluated in at least 3 areas of its measuring volume.

MPEE is Volumetric Length Uncertainty and utilizes calibrated gage blocks. Ball bars are not used as their length is arbitrary and only the spheres can be calibrated. (B89 Standard) Moreover, the measurement of a sphere employs many points to resolve the center which does not represent practical measurement. 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) one point as that to be used in the part feature calculation. Under ISO 10360-2 discrete single points are measured bi-directionally to evaluate length and the MPEE value reports the range of measurements from 7 different positions with 5 different gage lengths, repeated 3 times. All 105 measurements with their deviation from certified lengths are considered and are provided as the Uncertainty of Measured Length (MPEE).

The other ISO 10360-2 evaluation for Touch probe CMMs is MPEP which represents Probing Uncertainty.

The machine measures 25 discrete points that are evaluated as 25 individual radii. The range of radii variation, min to max, is the MPEP value.

The other important ISO 10360-2 metric applicable to Scanning CMMs is MPETHP and is similar to MPEP however it is achieved by full contact scanning of 4 lines, of which, only one can be a full 360 degree equatorial scan. This evaluation comes from the ISO 10360-4 specification. As above the points within each scan are treated as radii and the range of radii deviation, min to max, is reported as MPETHP.

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