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Fully Employing Positional Tolerancing Over Rectangular Tolerancing?

Fully Employing Positional Tolerancing Over Rectangular Tolerancing?

Fully Employing Positional Tolerancing Over Rectangular Tolerancing?

(OP)
I am in the process of updating my activities’ in-house drawing standards  and I would like to hear your opinions on the full use of employing geometric positional tolerancing.

ASME Y 14.5 allows either rectangular coordinate system or geometric positional tolerancing method.
Per ASME Y 14.5, para. 2.1.1.1 it use the term “Preferably”  in regard to the use of positional tolerancing for the location of features. Some of my engineers and draftsman feel that they have a choice (which they do) whether to use rectangular coordinates dimensioning/tolerances or geometric positional tolerancing.  Some feel that since geometric positional tolerancing is more “accurate” ( not me)  one should only employ geometric positional tolerancing to critical features.

The way I understand geometric positional tolerancing in a nut shell is that it opens up the tolerance region ( plus the bonus tolerance) which will result in not rejecting acceptable parts when compared to the rectangular "square" tolerance zone method . With positional tolerancing, one can say that the actual center could be less “accurate” from true center ( since you have a large tolerance zone) but who cares if the part is still acceptable.  

With this reasoning, I do not see why one wouldn’t want to fully employ positional tolerancing thought out all drawings.  

RE: Fully Employing Positional Tolerancing Over Rectangular Tolerancing?

I agree with you.  Geometric tolerancing is an excellent way to free up available tolerances, but should only (in my opinion) be used on an as needed basis.  Overuse can drive up cost considerably.
For some good discussions on this topic, see the Drafting Standards $ GD&T Forum.

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