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Inspecting Flatness at MMC
(In accordance with the ASME Y14.5-2009 standard)

PDF of this Tip

In the last Tip I showed that since the 2009 revision of the Standard, it is appropriate to apply flatness to the derived median plane of a width.  This is done by associating the flatness tolerance with the size dimension.  Prior to the 2009 revision of the Standard, this same control was accomplished using straightness.  When flatness is associated with the size dimension, it is permissible to modify the tolerance with MMC or LMC.  Where MMC is applied the tolerance becomes a single boundary control.  In other words, there is a single boundary that may not be violated by the feature.

In this example, the single limit boundary that may not be violated is equal to the largest size, 20.2 mm plus the flatness tolerance of 0.3 mm which equals 20.5.  A hard gage could be used to verify that this boundary isn’t violated or an indicator may be mastered at 20.5 mm.  When the part is placed on a surface plate and the top surface indicated, all readings must be down from the mastered value.  One advantage of this approach is that it provides variables data for process control.

Back to Tips Tip added Apr 2013