I have a true position tolerance: (position) ø.003 B A D. This is a datum diameter. I also have a basic 5.500 that represents the linear dimension from the plane and the center of the dia.

I put the basic dim +/-.0015 as a tolerance. Engineering requires +/-.003.

Please solve this dilemma.

Adrian

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Adrian,

Your question is both important and common.

Every designer, machinist, and inspector needs to understand the amount of tolerance applied to a part feature. Explaining this is easier if I reference an example, so I have added a figure as a simple illustration of the problem and solution.  I realize that your part is likely much more complex that the figure.

blog_q_true-pos_tol

The basic dimensions, such as 5.500, locate the tolerance zone of the position tolerance. The basic dimensions also communicate the ideal location of the hole’s axis. The interpretation is that the hole is to be located at 5.500 from datum plane A +/- 0.0015. The tolerance zone in the figure is perpendicular to datum plane A and any measurements must be made as a distance traveled parallel over the plane of datum A.

Michael Adcock
ASME GDTP Senior Level
Dimensional Engineering Mentor