## Interpretation of a true position geometric tolerance

## Interpretation of a true position geometric tolerance

(OP)

Hello,

I would like to ask for your help in interpreting the true position geometric tolerance of the attached drawing.

I believe it specifies that the axis of the pocket with diameter DIM A should be contained inside a cylinder with diameter 0.3 and whose axis:

I would like to ask for your help in interpreting the true position geometric tolerance of the attached drawing.

I believe it specifies that the axis of the pocket with diameter DIM A should be contained inside a cylinder with diameter 0.3 and whose axis:

- is perpendicular to the face with datum A;

- is coincident with the plane defined by datum B;

but I don't understand what constraints datum C imposes. Can you help me figure it out?
## RE: Interpretation of a true position geometric tolerance

Datum feature C constrains the final remaining degree of freedom, which is translation in the direction perpendicular to the section view cutting plane.

This strikes me as a rather strange tolerance scheme.

pylfrm

## RE: Interpretation of a true position geometric tolerance

I agree with pylfrm. I would use one of the diameters perpendicular to datum feature A to control location, and use either the slot, or your datum feature C to control rotation.

--

JHG

## RE: Interpretation of a true position geometric tolerance

Any previously controlled degrees of freedom are ignored in later datum references.

There is little reason to think the given set of datums is meaningful, especially without any suggestion as to the relations between the datum features and without the mating parts or their tolerancing. In fact, given the shallow depth of the feature chosen for B, it is suspect all around. Even a slight misalignment in cutting the groove could make determination of C very difficult.

## RE: Interpretation of a true position geometric tolerance

Thank you, that makes sense.

I have one final question: do datum feature A and datum feature C define the top and bottom, respectively, of the theoretical cylinder that contains the axis of the pocket? I.e.: is the attached drawing correct (where the green axis represents the axis of the pocket and the grey square represents the theoretical cylinder)?

## RE: Interpretation of a true position geometric tolerance

The length of the tolerance zone dia. 0.3 cylinder equals the depth of the short cylinder of dia. DIM A. It does not go any deeper, so your drawing is incorrect.

## RE: Interpretation of a true position geometric tolerance

Thank you for your answer, that was what I was expecting. May I ask you if you can tell me which paragraph in ASME Y14.5 explains it?

## RE: Interpretation of a true position geometric tolerance

John-Paul Belanger

Certified Sr. GD&T Professional

Geometric Learning Systems

## RE: Interpretation of a true position geometric tolerance

If you are using Y14.5M-1994, it is 1.4(m).

If you are using ISO or ISO-related standard (your drawing looks a little bit ISO-ish), then it is para. 4.6 in ISO 1101:2012.

## RE: Interpretation of a true position geometric tolerance

John-Paul Belanger

Certified Sr. GD&T Professional

Geometric Learning Systems

## RE: Interpretation of a true position geometric tolerance

## RE: Interpretation of a true position geometric tolerance

Thank you both for your time and great answers. Your answers sparked a few further questions, if you don't mind:

## RE: Interpretation of a true position geometric tolerance

2nd question -- It doesn't matter how it's produced. The position tolerance only applies for the depth of the finished part's DIM A.

John-Paul Belanger

Certified Sr. GD&T Professional

Geometric Learning Systems

## RE: Interpretation of a true position geometric tolerance

## RE: Interpretation of a true position geometric tolerance

Thank you very much for your time and promptly replies.