GD&T Dual Tolerance

[BrysonRD]

I've been on this site a couple times before and have decided to make a profile, so this is my first post. I'm also a fairly new engineer, so please excuse my ignorance. I tried searching the forums for this topic and didn't find anything, so hopefully I'm not duplicating any threads.

The process and the outcome for the physical part that I mention below are well established and won't be changing, I'm just trying to figure out the best way to document it.

I'm working on a drawing for a plastic (match metal mold) part. During cooling, the walls can deflect quite a bit. One wall in particular is allowed to deflect inward by a certain amount, but it is not allowed to deflect outward. I was originally thinking of using a straightness callout, but now I'm thinking that a flatness callout for that wall would be the best way to go. In Solidworks, the Geometric Tolerance Properties window has a tolerance field with an optional second tolerance. Normally, a geometric tolerance is a ± condition, but I was thinking I could use the dual tolerances for a (+) / (-) condition. Please see the attached image.

If this is even allowable, I wasn't sure which should be the (+) condition and which should be the (-) condition. As I'm writing this, I'm thinking that the (+) and (-) directions would be relative to the dimension for that wall, so maybe the second image would be correct.

The questions:
[ul]
[li]Is this an allowable way to control the amount of deflection in that wall?[/li]
[li]If so, which tolerance field should be the (+) condition and which should be the (-) condition?[/li]
[li]If not, what would be a better way to control the amount of deflection?[/li]
[/ul]

Thank you,
Bryson

 

[3DDave]

See

https://www.eng-tips.com/viewthread.cfm?qid=348583

which hasn't got a great answer.

You may have to use "dun dun DUN," a note.

Or a very complicated Profile tolerance that only applies over a limited area and has a NONUNIFORM zone.

I'm sure there will also be an "extension of principle" because the ASME doesn't have the foresight to deal with the very common problem of shrinkage in plastic parts.

You should get a copy of the standard - relying on Solidworks to create correct callouts is a bad idea.

 

[BrysonRD]

Thanks for the link and the input Dave. The original drawing of the part I'm revising actually uses a note. I was just trying to see if there was a more concise way to go about it, but I will probably stick with that.

I actually did a bit of GD&T research (outside of SW). So far I haven't been able to find anything about dual tolerances. I'm sure there's a place for it, but this probably isn't it.

 

[3DDave]

Per

https://help.solidworks.com/2021/English/SolidWorks/sldworks/hidd_gtol.htm

Solidworks developers apparently intended that "tolerance 2" function for the special case where a size-related tolerance zone should be restricted.

 

[Burunduk]

Sometimes notations such as "NOT CONVEX" are used adjacent to the flatness feature control frame.

In ASME it is not standardized, and I think it never was, but it is mentioned in some places such as this instructional video (from a trustworthy source), at minute 17:00
Link


In the ISO GPS standards there is a former practice to state "NC" under the callout standing for non-convex, but it is no longer supported because it is claimed to be ambiguous; since real features are never purely convex or purely concave, I guess some as produced shapes may be unclassifiable. However, ISO 1101 (at least the 2012 version, which is not the latest) permits the requirement to be specified as a note, although it considers it a "qualitative concept".
Example of a note:Link

It comes down to whether it is possible to meaningfully inspect it and always determine whether the surface is convex or concave. What is the specific measurement technique being used for this requirement in your case?

 

[drawoh]

BrysonRD,

ASME Y14.5 allows you to specify asymmetric profiles, but I do not recommend it. Your plastic moulder probably is using your CAD model as the primary reference, as opposed to carefully reading your dimensions. Your CAD geometry ought to be in the middle of your acceptable feature size range.

Notes sound like a good idea.

--
JHG