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Tolerances Modified at LMC and MMC Are a Single Limit
(In accordance with Y14.5M-1994 standard)

A geometric tolerance modified at MMC or LMC becomes a single limit control. This single limit is known as the virtual condition. Features toleranced in this manner need only be inspected for size and to assure that none of the surface of the feature has violated the virtual condition. This is important to realize when calculating Cpk for a manufacturing process. The boundary may be considered a single limit control. By probing the feature and recording the point on the feature that is nearest the boundary being controlled, a Cpk value may be determined.

For this example, once size has been checked, the hole may be probed to determine how close the surface is to the virtual condition of 59.5mm. By recording these values over time, an average and standard deviation of the values may be determined. Calculating the difference between the average of these values and the virtual condition boundary, then dividing that difference by 3 standard deviations will yield a Cpk.

Back to Tips Tip added Dec 2002