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How to establish datum plane for a width feature of size.

Question asked by Tran Van Quang on May 5, 2018
Latest reply on Jun 4, 2018 by Steve Calvert

Hello everyone,

I read about establishing datum for a width feature of size in "ASME Y14.5-2009" and "GD&T mechanical design".

But there are seem two different way to establish datum, for example in this picture below:


According to "GD&T mechanical design", datum B is the 7 inch size feature between the left and right edges.

But according to "ASME Y14.5-2009", it will be the  center plane between left and right edges.


So what is the right datum here? And how to make a mating gage for such center plane