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I'm in a bit of a pickle with one of the quality techns that works for us (the company) over a profile of surface tolerance. I initially submitted something resembling figure 1 (top capture) and quality came back to me saying figure 2 (bottom capture) would be better.

I've never seen ''auto-referencing'' of datum and I don't see the point because it doesn't add anything to the GD&T. I've never seen anything like this in any book, reference or drawing I stumbled upon over my (short) carreer. Are there any arguments to which practice would be best? I'm open to any source saying one way should be the preferred one.

Thanks!

What I'm proposing

What quality guy suggests

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    $\begingroup$ Be a heckuva lot easier if you circled the differences. $\endgroup$ – Carl Witthoft Dec 13 '17 at 17:51
  • $\begingroup$ Where are Datums B and C? $\endgroup$ – GisMofx Dec 13 '17 at 18:06
  • $\begingroup$ Differences are now circled in red, datums B and C are the 2 0.2 holes. $\endgroup$ – HotFuzz Dec 14 '17 at 2:44
  • $\begingroup$ I see. You can't reference a feature back to itself. Your first version is more appropriate, but not ideal. How do you plan to inspect the position of the first hole(datum b)? I see what you're trying to accomplish. I think you need to remove the tolerance of position for that hole and give datum C twice the tolerance... $\endgroup$ – GisMofx Dec 14 '17 at 4:39
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Don't do it. It's probably not legal in any GD&T spec.

You can't reference a feature back to itself(think: chicken or egg?).

Your first version is more appropriate, but not ideal. How do you plan to inspect the position of the first hole(datum b)? I see what you're trying to accomplish(intent), but it should be clear from an inspection stand point. I think you need to remove the tolerance of position for that datum B hole and give datum C twice the tolerance...

Btw, do your holes sizes have a tolerance or fall back on a general note?

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  • $\begingroup$ You're right for the tolerance of position on the hole on datum B and giving twice the tolerance for datum C. In the original drawing, datum B had no tolerance of position. The 0.2 is toleranced via the title block of the drawing. For obvious IP reasons, I recreated a quick mock-up of the drawing to illustrate the problem and I oversaw some details. $\endgroup$ – HotFuzz Dec 14 '17 at 12:41
  • $\begingroup$ @HotFuzz I figured as much. I just want to cover all the bases. $\endgroup$ – GisMofx Dec 14 '17 at 14:21

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