GD&T question about FOS

[groovemixer]

Hi,

Was wondering if anyone can help me on a question I have about GD&T tolerancing.

What is the proper defininition of feature of size? (FOS) Can a surface itself be a FOS? Is it possible to have a positioning tolerance on a surface itself?? I am having issues with design engineering reguarding a tolerance they put on a drawing. They have a small surface cutout on the side of a cylinder and have a positioning tolerance on the surface itself. I do believe this is not possible but I don't have too much experience with GD&T. What the engineerrequires is that this small surface is perpendicular to the cylinder bottom and parallel to the actual outside diameter of the cylinder. So instead of putting two tolerances on the drawing they put a positioning toleracne to try and cover both. To me this is not proper.

Any help would be great.

Thanks

[cncprofessor]

(1) What is the proper defininition of feature of size?

(2) Can a surface itself be a FOS?

(3) Is it possible to have a positioning tolerance on a surface itself??.

Answers:

1. Feature of Size: One cylindrical or spherical surface, or a set of two opposed elements or opposed parallel surfaces, associated with the size dimension (Ref.1.3.17 ASME Y14.5M-1994).

2. Can not.

3. No

By definition, surface for individual features defines the form of a feature such as straightness, flatness, circularity (roundness) and cylindricity.

For individual or related features, surface can be defined by the profile of a line and profile of a surface. Now, I am not quite sure I fully understand what is meant by “a small surface cutout on the side of a cylinder” (?!).

If the above means a part of that cylindrical surface was to be cut out, and/or perhaps the end of the cylindrical feature was to be cut out then that would call for an orientation tolerance rather than a location tolerance control.

If you could post the attachment (or send me an e-mail) of the drawing part to show exactly what you have, I’ll be more than happy to expand.

[groovemixer]

Thanks cncprofessor, I cannot send the drawing at the moment but you have answered my question.

Just to be more specific:

Picture a cylinder (2" dia X 3" long), on the actual OD (1" from the top of the cylinder) there is a 1/4" wide flat cut out .1" deep from the OD. The OD of the cylinder is set to datum A and the bottom of the cylinder is set to datum B. There is a positioning tolerance given pointing to the surface, referenced to both A and B.

What the engineer is asking for is to control this cut out to be perpendicular to the bottom of the cylinder and parallel to the OD.

I've argued that this is breaking a GD&T rule and that you cannot apply a positioning tolerance to a surface itself. Engineering thought it will simplify the tolerances if positioning was put in instead of parallelism and perendicularity.

Hope this makes my question clearer.

[cncprofessor]

It’s much clearer, but I am still not quite there yet since you’re mentioning Datums-A & B (?). Since you’re talking about a “cylinder” are we talking about where both OD & ID are cylindrical with the common axis (as in tubing), right?

In either case where ever datums A & B are applied (OD/ID, or the ends) a surface can NOT be position (located) to either one of the mentioned features, a surface can only be oriented (parallel, perpendicular, or angular.

In some cases, a profile of a surface for individual or related feature, can be called out when the elements of a profile are straight lines, arcs, and other curved lines, even in the case as you're describing...

[groovemixer]

Yes that's how I figured it, thanks for your input.