PROCESS AND MACHINE CAPABILITY


Before exploring the procedures for determined capability, it is important to note that there are two recognized forms of capability analysis: process capability and machine capability (short-term and long- term capability).

Process capability generally refers to a long-term (30 “60 days) study intended to reflect the variability of all process elements, including time- related effects such as cycles. Of course, this study is also intended to confirm process control. Many companies, require that long-term capability studies (process) demonstrate that 99.73% of the process output falls within bilateral specifications and that at least 99.87% falls within a unilateral limit. Both of these requirements, of course, correspond to plus and minus three sigma ( ±3 sigma) limits of a normal distribution. This capability is reported as C pk .

A machine capability study is usually a short-term analysis (1 “30 days, but usually less than 1 “2 days) performed (generally) on a single tool or machine. The purpose of this study is to determine individual machine capability. Additionally, short-term machine capability studies are needed to predict the potential for achieving long-term process capability. Typically, short-term machine capability requirements are more stringent than are long-term process capability requirements. This is because we must ensure that there will be leeway for the inevitable variations that will occur across the entire process, originating from the interaction of numerous process elements, time-related factors, and natural variability. Many firms, therefore, will require that machine capability/short-term capability studies predict at least 99.994% of total output to be within specifications on a bilateral specification. For unilateral specification limits, 99.997% is required. These requirements are equivalent to 4 sigma limits from a normal distribution. This capability is reported as P pk .

To restate the most important portion of this discussion, unless short-term capability can be achieved on process elements within ±4 sigma, the potential for achieving process capability within ±3 sigma is very low over the long term.




Six Sigma and Beyond. Statistical Process Control (Vol. 4)
Six Sigma and Beyond: Statistical Process Control, Volume IV
ISBN: 1574443135
EAN: 2147483647
Year: 2003
Pages: 181
Authors: D.H. Stamatis

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