2.3.2. Reference GD&T Values

The aim is to calibrate the measured dummy part and obtain the reference values for each evaluated feature to realise the uncertainty budget according to the ISO 15530-3 technical specification [46]. Thus, the dummy part is measured in a ZEISS UPMC 850 CARAT CMM, in which the ZEISS VCMM™ tool is available for task-specific uncertainty assessment according to the ISO 15530-4 technical specification [47]. The ZEISS VCMM™ tool considers the mathematical model of the UPMC 850 CARAT CMM to perform the task-specific uncertainty assessment process through multiple iterations (×1000 repetitions). To feed the mathematical model running within the ZEISS VCMM™ tool, the error and influence factors, as well as their variability affecting the measurement accuracy, were previously characterised and introduced into the model. Figure 7 shows the dummy calibration process.

**Figure 7.** Dummy part calibration on the ZEISS UPMC 850 CARAT CMM.

In this manner, a selection of the GD&T features to be considered within the experimental implementation is performed. The types of geometric elements and tolerances that were considered are explained next, according to the type of tolerance and the number of features measured.


The positioning tolerances are evaluated considering the coordinate system created by the three planes which define the ABC datums. In total, the task-specific uncertainty of the 52 GD&T features is evaluated. Figure 8 shows the task-specific uncertainty assessment exercise based on the ZEISS UPMC CARAT 850 CMM measurements.


**Figure 8.** Task-specific uncertainty assessment based on the ZEISS CMM measurement and the ZEISS VCMM™ simulation. Example of measurement report including the VCMM outputs.

From these data, the standard uncertainty associated with the uncertainty of the MMC calibration of the dummy part (*ucal*) is obtained.
