Comments on: Dimensions with Datum Targets http://gdtseminars.com/blog/2008/03/06/dimensions-with-datum-targets/ Geometric Dimensioning & Tolerancing tips, questions and issues Sat, 19 May 2012 06:19:40 +0000 http://wordpress.org/?v=2.3.1 By: John-Paul Belanger http://gdtseminars.com/blog/2008/03/06/dimensions-with-datum-targets/#comment-1109 John-Paul Belanger Tue, 13 Mar 2012 18:21:57 +0000 http://gdtseminars.com/blog/2008/03/06/dimensions-with-datum-targets/#comment-1109 Yes, that is legal. Just because a surface is labeled as a datum doesn't actually impose any tolerance on it. So it's very common to have a surface be labeled as a datum and then also apply a flatness tolerance to it. That is why, to be more precise, we say that the actual surface of the part isn't really the datum -- it is called the "datum feature." The true datum would be a theoretically perfect plane that contacts the actual surface at the high points. Yes, that is legal. Just because a surface is labeled as a datum doesn’t actually impose any tolerance on it. So it’s very common to have a surface be labeled as a datum and then also apply a flatness tolerance to it.

That is why, to be more precise, we say that the actual surface of the part isn’t really the datum — it is called the “datum feature.” The true datum would be a theoretically perfect plane that contacts the actual surface at the high points.

]]> By: vladimir http://gdtseminars.com/blog/2008/03/06/dimensions-with-datum-targets/#comment-1103 vladimir Wed, 15 Feb 2012 19:47:48 +0000 http://gdtseminars.com/blog/2008/03/06/dimensions-with-datum-targets/#comment-1103 Please explain the situation, On the part drawing I specified a flat surface as damum A. The actual part has this surface bowed. The toolmaker asked me to put a flatness requirement on this datum. Is it legal? Thank you Please explain the situation,
On the part drawing I specified a flat surface as damum A.
The actual part has this surface bowed.
The toolmaker asked me to put a flatness requirement on this datum.
Is it legal?
Thank you

]]> By: John-Paul Belanger http://gdtseminars.com/blog/2008/03/06/dimensions-with-datum-targets/#comment-208 John-Paul Belanger Tue, 27 May 2008 22:34:38 +0000 http://gdtseminars.com/blog/2008/03/06/dimensions-with-datum-targets/#comment-208 Sorry for the delay in replying, Mario. Basic dimensions themselves have no tolerance, but they point to a feature that usually does have a tolerance. That tolerance is shown in the form of a GD&T "feature control frame." So for quality control tracking, any statistical data are not really looking at the basic dimension, but at the tolerance given in the feature control frame. It's a minor point, but remember that the purpose of the basic dimension is merely to avoid having us mistakenly apply title block tolerances to that number. My blog entry about basic dimensions and datum targets emphasized that there is no direct tolerance, but the datum targets would never be measured or tracked by quality control. Sorry for the delay in replying, Mario. Basic dimensions themselves have no tolerance, but they point to a feature that usually does have a tolerance. That tolerance is shown in the form of a GD&T “feature control frame.” So for quality control tracking, any statistical data are not really looking at the basic dimension, but at the tolerance given in the feature control frame. It’s a minor point, but remember that the purpose of the basic dimension is merely to avoid having us mistakenly apply title block tolerances to that number. My blog entry about basic dimensions and datum targets emphasized that there is no direct tolerance, but the datum targets would never be measured or tracked by quality control.

]]> By: Mario Varela http://gdtseminars.com/blog/2008/03/06/dimensions-with-datum-targets/#comment-202 Mario Varela Thu, 01 May 2008 21:42:18 +0000 http://gdtseminars.com/blog/2008/03/06/dimensions-with-datum-targets/#comment-202 I work in Quality Control and sometimes we are required to prepare dimensional reports that have Basic Dimensions. How can that be done if they have no tolerances? I work in Quality Control and sometimes we are required to prepare dimensional reports that have Basic Dimensions. How can that be done if they have no tolerances?

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