The Relationship between Process Capability and Quality of Measurement System

Abstract

This article deals with the design of appropriate measures that had to be taken for the implemented measurement system. The measurement result was significantly negatively affected by several factors. Forty-five samples of shafts used in the production of surgical drills were measured. Measurements were performed by metrological appraisers with a calibrated digital micrometer. Measurement and subsequent data processing revealed low process capability (CP and PP indices). A large portion of the shafts had an observed size below the lower specific limit (LSL). Therefore, it was necessary to take corrective action. This paper focuses on the corrective measures implemented in the measurement system. The micrometer met the requirements of the standard and was metrologically capable. The shafts were measured by eight metrological appraisers, so attention was focused on the potential impact of the metrological appraiser. The measured data were evaluated by uncertainty analysis, paired t-tests, measurement systems analysis (MSA) and Cohen’s kappa. The number of non-compliant shafts was shown to increase with decreasing measurement capability. The measurement system was evaluated as conditionally capable, even incapable. One possibility was to identify the optimal pair of metrological appraisers. The pair of metrological appraisers E and F appeared to be the most suitable combination for most methods. Due to the relatively high %EV index, the second option was to improve the work with the measuring instrument, that is, improve the training and supervision of metrological appraisers in the measurement process. Repeated measurements by the pair with the highest capability (metrological appraisers E and F) resulted in an increase in the value of the capability indices and a decrease in the number of shafts out of tolerance for the same shafts. As the value of these indices was lower than 1.33 during repeated measurements, corrective measures had to be taken, not only in the measurement system, but also in the production system.