Cpk also requires input from the customer for the lower specification limit (LSL) and upper specification limit (USL). The Cpm capability calculation accounts for a nominal value. Cpk will never exceed the Cp Similar to Ppk, the Cpk capability index is only a function of the standard deviation and mean of the data, not a nominal (target) value that may be historical or provided by the customer. and since the maximum value for k is 1.0, then the value for Cpk is always equal to or less than Cp. Both Cpk and Ppk relate the standard deviation and centering of the process about the midpoint to the allowable tolerance specifications. A perfectly centered process will have Cp = Cpk. The minimum value of "k" is 0 and the maximum is 1.0. A perfectly centered process where the mean is the same as the midpoint will have a "k" value of 0. The addition of "k" in Cpk quantifies the amount of which a distribution is centered, in other words it accounts for shifting. The Cp is commonly referred to as the process entitlement because, when centered, it represents the best performance possible. The Cp is the best a process can perform if that process is centered on the midpoint. Think about it this way, if a subgroup size is 5 and all five data points show an average that is vastly different from the averages of the other subgroups (that also have 5 data points) then there is a strong chance that the subgroup was measured under a "unique" or special condition. The greater the subgroups size the higher likelihood the special cause variation is detected and that it exist if it is detected. It is recommended to have at least 25 subgroups and preferably each subgroup having >1 data point. control chart is not in control and free from special cause.data is not a bell-shaped normal distribution.still rising with upward trend), then assessing the current process is unlikely to reflect the long term performance. Use time-series charts and SPC charts to determine process control. This goes for the baseline measurement and the final measurement. This is called the baseline measurement.Īfter the improvements are done, the process is measured again and a new Cpk value is calculated in the CONTROL phase.Īs with all the process capability indices, the process must be in control before assessing capability. In order to measure improvement (or lack of) there must be a starting point. After the MSA is complete, the Six Sigma project has a goal of improving the baseline measurement. NOTE: There are two values calculated and the minimum value (worst case) is used.Ĭpk is an option (along with z-score and PPM or DPMO) when describing process baseline measurement in the MEASURE phase.
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