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MMC and LMC on a Datum Reference Do not Affect the Feature’s Tolerance
(In accordance with Y14.5M-1994 standard)

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Referencing a datum feature at MMC or LMC does not change the tolerance on the feature(s) being toleranced. These modifiers, however, may allow the part to float or shift relative to the datum reference frame. Notice that the 15 mm diameter hole references both B and C at their MMC. Therefore, if both B and C are produced at their MMC, there is no shift of the datum feature relative to the datum reference frame established by A, B and C. If, however, datum feature B happened to be produced at 8.1 (its LMC size) and was perfectly perpendicular to the primary datum, the part could shift as much as 0.2 mm total, relative to the datum reference frame. In other words, the part could be slid left or right 0.2 mm total while maintaining contact with datum A in order to bring the 15 mm hole into its allowable position tolerance zone. Some inspectors treat this 0.2 mm shift as an additional bonus tolerance on the 15 mm hole which would allow additional deviation in all directions. This would be incorrect since no movement of the part is allowed perpendicular to the primary datum.

Back to Tips Tip added Feb 2003