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Adaptive and Compensation Methods in Free-Form Surface Assessment

The assessment of manufactured surfaces by coordinate metrology techniques has changed in recent years. Speed, accuracy and flexibility have all improved with the introduction of coordinate measuring machines (CMM). However, some technical problems have yet to be solved, in particular those pertaining to free-form surfaces. This paper considers the influence of surface features and probe tip geometry on the assessment of free-form surfaces. Inaccurate measurements due to the problems of probe tip diameter and undulating surface curvature have been investigated, and a simple mathematical compensation technique for different probe diameters and an adaptive measuring point planning strategy have been developed. Experimental data confirm that the measuring errors are significantly reduced when the probe tip diameter compensation approach is used. The problems of machined surface measurement have also been considered and a method, namely least-squares data fitting, is proposed to give a better qualifying technique for the cusped surfaces. This new technique has been implemented and the results from the case study indicate that the least-squares method can give a more precise representation of the quality of a machined surface than standard statistical measurement systems. Experimentation also indicates that the results of the least-squares data fitting method are consistent with the CMM computed values, which verifies the reliability of the method.