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Cm and Cmk are measurements of machine capability. Machine capability is the ability of a machine to produce parts within tolerance limits. Machine is a common source of variation in a process. Machine variation describes short-term, part-to-part variation from the same machine, with the same operator, under the same conditions.
Use Cm to evaluate the potential capability of a machine based on the spread. Use Cmk to evaluate the potential capability of a machine based on both the location and spread. Potential capability indicates the capability that could be achieved if shifts and drifts were eliminated.
The formulas for Cm and Cmk are the same as the formulas for Cp and Cpk. However, Cm and Cmk are different from capability Cp and Cpk because the data collection is different. To calculate Cm and Cmk, the data vary only because of variation from the measurement process and the variation from the standard operation of a single machine. To calculate Cp and Cpk, the data include sources of variation from the entire process, such as materials, operator, and environment.
The procedure for a machine capability study depends on your application. Usually, the data collection includes 20 to 50 measurements on consecutive parts from a single machine. Larger sample sizes usually produce more precise estimates of machine capability. Consult industry experts to determine best practices for the data collection in your application.
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