It is found that the image focusing is a major factor of the measurement uncertainty for 3D components in our testing machine. By using the top focus (or surface focus) approach the experimenters obtained more accurate and repeatable results than using the edge focus approach. Experiments show that the illumination intensity is a relatively minor factor in the measurement uncertainty, compared to the focusing and stage errors. It also is observed that systematic errors of oversized and undersized occurred in the outer diameter and inner diameter measurements respectively. By knowing the correlation between the measurement uncertainty and error sources, one can design a procedure to assess the most critical error factors in the performance evaluation. Thus, a VCMM machine performance evaluation procedure can be established for initial purchasing acceptance and periodical performance assurance.
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