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MMC and LMC on a Datum Reference do not Affect the Feature’s Tolerance - 2009 & 2018 (#63a)

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Referencing a datum feature at MMB or LMB 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 15mm diameter hole references both B and C at their MMB. Therefore, if both B and C are produced at their MMB, 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 0.2mm total, relative to the datum reference frame. In other words, the part could be slid left or right 0.2mm total while maintaining contact with datum A in order to bring the 15mm hole into its allowable position tolerance zone. Some inspectors treat this 0.2mm shift as an additional bonus tolerance on the 15mm 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.

This tip is in accordance with ASME Y14.5-2009 and Y14.5-2018
This tip was originally released in February 2003.

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Back to Tips Tip added May 2023