Effects of part coordinate system appointment on cmm measurement

Accurate measurement of the components before assembly, as well as the spare parts for agricultural machines, is a crucial issue for the correct and failureless machinery work. In the paper, measurement with Coordinate Measuring Machines (CMMs) is discussed from the perspective of accuracy affected by the part coordinate system appointment. In general, the CMM procedure consists of determining the spatial coordinates of measurement points on the surface of a measured object, and then via certain calculations, the best-fitting geometrical elements are ascribed to the probing points either in the CMM coordinate system or in the coordinates of the measured object. From these elements, dimensions of the measured part are derived with certain accuracy. In the experiments, a contact measurement was completed with a stylus equipped with a ball tip maintaining the same distance between its centre and the measured surface in all directions. There are many sources of uncertainty in CMM measurement, but the errors introduced during determination of the Part Coordinate System (PCS) affect all the subsequent measurement results obtained from respective chains of calculations. Thus, the study is devoted to the uncertainty reduction through the analysis of PCS appointment accuracy. Experimental measurements were performed using CMM Mitutoyo Crysta-Apex C7106 and the standard gage blocks. Theoretically, accurate identification of the probing points should provide accurate determination of the PCS coordinates X, Y, and Z. However, when measuring such an accurate object as a gage block, the results appeared to be dependent on the measurement method. In the experiments, four methods were tested. Dependent on the measurement strategy, PCS appointment method and particular settings of the measurement parameters, the Part Coordinate System generated different errors from negligibly small ones up to 10 μm.