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Flatness is an individual control. When you want multiple surfaces to lie in the same plane the Y14.5 Standard says to use profile. Many designers and engineers think that if flatness is used in conjunction with an extension line, it means that the surfaces must be contained within two parallel planes with a maximum separation not greater than the specified flatness tolerance. As illustrated, however, this is not the correct interpretation.
The desired condition is called coplanarity [2009 para 18.104.22.168, 2018 para 3.12,22.214.171.124]. The correct method to control coplanarity of multiple features, is by using profile as illustrated below.
This tip is in accordance with ASME Y14.5-2009 and Y14.5-2018
This tip was originally released in July 1998.