Comments on: How Literally Should We Take the GD&T Standard? http://gdtseminars.com/blog/2011/08/30/how-literally-should-we-take-the-gdt-standard/ Geometric Dimensioning & Tolerancing tips, questions and issues Sat, 19 May 2012 07:02:54 +0000 http://wordpress.org/?v=2.3.1 By: John-Paul Belanger http://gdtseminars.com/blog/2011/08/30/how-literally-should-we-take-the-gdt-standard/#comment-1050 John-Paul Belanger Thu, 15 Sep 2011 03:57:56 +0000 http://gdtseminars.com/blog/2011/08/30/how-literally-should-we-take-the-gdt-standard/#comment-1050 The situation you describe in the first portion of your question is indeed legal, and in fact it would be the desired way to go if they are sealing surfaces. Reason: Just because a surface is labeled as datum A doesn't mean that it will be flat. The true datum that is taken during measurement will actually be a plane formed from the high points of that surface A. Think of a bumpy surface that sits on a flat gage table ... the gage table actually becomes the datum that we want the opposite face to be parallel to. The flatness of the surface contacting the gage table is not part of the measurement (unless the print adds a flatness tolerance as you mention). Hope that helps! The situation you describe in the first portion of your question is indeed legal, and in fact it would be the desired way to go if they are sealing surfaces. Reason: Just because a surface is labeled as datum A doesn’t mean that it will be flat. The true datum that is taken during measurement will actually be a plane formed from the high points of that surface A. Think of a bumpy surface that sits on a flat gage table … the gage table actually becomes the datum that we want the opposite face to be parallel to. The flatness of the surface contacting the gage table is not part of the measurement (unless the print adds a flatness tolerance as you mention).

Hope that helps!

]]> By: S. Jones http://gdtseminars.com/blog/2011/08/30/how-literally-should-we-take-the-gdt-standard/#comment-1049 S. Jones Wed, 14 Sep 2011 02:17:37 +0000 http://gdtseminars.com/blog/2011/08/30/how-literally-should-we-take-the-gdt-standard/#comment-1049 Hi, Is it okay to call a datum surface (ie DATUM A) and a flatness tolerance( with a leader) on one side of a plate and a parallelism tolerance with reference to the specified datum on the other opposite surface or would you only call out the datum on one surface and the parallelism with respect to the datum on the other side. Both surfaces are sealing surfaces. Thanks Hi,

Is it okay to call a datum surface (ie DATUM A) and a flatness tolerance( with a leader) on one side of a plate and a parallelism tolerance with reference to the specified datum on the other opposite surface or would you only call out the datum on one surface and the parallelism with respect to the datum on the other side. Both surfaces are sealing surfaces.

Thanks

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