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   When Inspecting Flatness, There Is No Datum.

Flatness is another geometric tolerance that is challenging to inspect. It requires isolating the feature from the rest of the part since there can never be a datum referenced with flatness. In this example, the flatness has been applied to datum feature A. Three possible inspection methods are illustrated. In all cases, the considered feature is isolated from the rest of the part and aligned relative to the indicator. In the first case, the part is leveled on the surface plate. In the second illustration, the surface is leveled by placing it on three equal height gage blocks. The indicator is then moved across the surface. In the third illustration, the CMM will mathematically "level" the points of the surface contacted by the probe. In all cases the FIM (Full Indicator Movement) may not exceed 0.2mm.

Problem: If the surface is convex, the part will rock making it difficult to determine the minimum indicator reading over the entire surface.  Problem: Ideally, the gage blocks should be placed under the high points on the surface. Otherwise, the indicator movement may not be the lowest possible. 
In Examples 1 and 2 the problem may cause an acceptable surface to be rejected.

Of course, a CMM will automatically align the points to evaluate the flatness error.

Problem: Often insufficient points are taken to evaluate the flatness error. As a result, an out of spec surface may be accepted.

Bottom line, inspecting flatness requires time and patience.

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