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Position of a basic feature?

Position of a basic feature?

(OP)
Is it acceptable to apply a position tolerance to a basic feature?

The form (and orientation) is already controlled by the profile tolerance, however I need to hold tighter control on certain parts of a the profile as they approach LMC.
Something like the image below, using position of the diametric features in addition to the profile of the whole.

Thanks!



Bear with me an my hand-mouse markup.... the red boundary represents MMB of the profile, the red-dashed circle represents smallest diameter that fits within the profile boundary at worst-case position, the red cross represents the center of that red-dashed circle. Now, what I'd like, is to further restrict the center of that circle, so that it must fall within the blue-dashed circle, while simultaneously the form must fall within the profile min/max boundaries.

Hopefully that makes sense! :)

RE: Position of a basic feature?

cbrf23,

First, I'm assuming ASME Y14.5-2009 is the relevant drafting standard here. If that's not the case, please specify.

I think most would consider it invalid to apply a position tolerance at LMC to a feature with a basic size dimension. For the case you've described here though, I think you can dodge the issue entirely.

In the usual situation of a cylindrical feature with a directly toleranced diameter dimension, a position tolerance applied at LMC creates a cylindrical boundary that must be contained completely within the material of the part. But your profile tolerance already creates a boundary of the same sort, so only the more restrictive of the two actually matters. Why not just adjust the profile tolerance to provide the boundary you want, and forget about the position tolerance?

pylfrm

RE: Position of a basic feature?

(OP)
Hi pylfrm, yes Y14.5-2009 is the applicable standard - I should have stated that. Thanks!

I may be just overthinking this. I'll take a look at it with a fresh set of eyes this morning.
Thanks!

RE: Position of a basic feature?

How about a non-uniform profile tolerance(see 2009 - section 8.3.2 and figure 8-9, 8-10, 8-11)? This will allow you to describe tight tolerances where you need them and let you have looser tolerances in the non-critical areas. This approach should give you the most freedom to describe the exact tolerances you are looking for.

RE: Position of a basic feature?

Also see Fig. 8-24 of the standard. That's somewhat close to what you're trying to do (but using MMC instead of LMC).

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

RE: Position of a basic feature?

I had forgotten about the combination of profile and position at MMC shown in Fig. 8-24. If MMC is kosher, then I can't think of any reason LMC wouldn't be, so I'll retract my earlier statement. However, this only makes sense when the profile and position tolerances have different datum feature references. In OP's case, any part that meets the profile tolerance is guaranteed to meet a position tolerance of zero at MMC.

I agree with AndrewTT's suggestion to use a non-uniform profile tolerance if appropriate. Alternately, just use different tolerance values on different segments of the profile as shown in Fig. 8-6.

pylfrm

RE: Position of a basic feature?

If LMC is used on the internal feature, the tolerance zone will be in the material and require calculation and not hard gauging for acceptance.

Certified Sr. GD&T Professional

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