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You Can Calculate a Cpk when MMC Is Used
(In accordance with Y14.5M-1994 standard)

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Many believe that when MMC is applied to a geometric tolerance, hard gaging, must be used. Hard gaging does not provide variable data and therefore is often not desirable for process control. Also, many customers today require that production calculate the Cpk of the process. The application of MMC or LMC may be thought of as specifying a single limit for position.

This drawing, for example, only requires that the size and inner boundary (virtual condition) of the hole be verified. The virtual condition is a single limit requirement. The theoretical design size of the gage pin (1) is 8.0-0.2=7.8. The distance from the axis of the datum simulator to the surface of the gage (2) is 3.9 (7.8/2).

By using an indicator, which is mastered at 3.9 from the datum simulator, variable data may be collected. Recording the distribution of data relative to the inner boundary allows the calculation of Cpk of the process.

Back to Tips Tip added Mar 2002