![]() The tolerance width is two as big as the data spread. To help understand Cm, if the Cm value is 2.0, that means the spread of the consecutive measurements will fit 2x into the tolerance width. ![]() The value of Cm is the number of times the spread of the machine's performance compared to the width of the tolerance. ![]() The process capability index measurements of Cpk, Cp, Pp, and Ppk will include stoppages and some or all of the above six sources of variation.Ĭm does NOT account for the position of the spread within the tolerance limits. Cm will almost always be greater than Cp and Pp. The machine capability is is very short sampling and the best performance value out Cp, and Pp.Ĭp and Pp will never exceed Cm. It's possible that the machine capability is the same (or equal) to the process capability but very very unlikely. This validates that the measurement system variation is minimal as a % of the total variation. Hence the reason for the name being machine capability and not process capability.Īnd it is the job of the Black Belt to quantify the measurement system variation and verfiy a passing MSA. ![]() Recall as the Fishbone shows, there are six factors that are generally accepted that create variation:įor Cmk and Cm, the ONLY factors of variation that are at play are Measurement and Machine. This is considered a very short term index since this process of collecting these consecutive measurments can NOT include stoppages, operator changes, tool changes, machine changes, environment changes, oil changes, etc. Higher values for Cm and Cmk represent better machine performance.Ĭm and Cmk describes machine capability using 20-50 consecutive measurements. Cmk represents a very short term reflection of machine performance in relation to the tolerance limits (or specification limits). ![]() Cmk (and Cm) are two denotations that represent machine capability. ![]()
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