GD&T help - Unequally Disposed Profile tolerance

[Mbovens]

All,

I've used this site for quite some time to find great ,valuable information BUT now I have a question to ask your advice on.

I'm working on detailing up a plastic part that will be painted as a secondary process and I need to get the GD&T right to ensure the profile is held.

Attached is a quick pic of the dwg we're working on. It deals with the Unequally Disposed Profile - (U) modifier

I need some guidance whether or not the GD&T is correctly called out to correctly measure up the part before and after paint. I see some negative values in the FCF and I don't think, IMO, this is correct. (you cant have a negative tol zone). I think the idea here was to indicate that the unique perimeter can be allowed to be slightly bigger than the actual CAD data. Other plastic parts fit into these pockets so it needs to be snug but not too snug.

The part is shot and painted in the same toolshop hence the dual GD&T callout.


Thanks for any help/ advice you can provide.

Mike

 

[3DDave]

It would be better to expose the committee discussions where the alternatives and arguments made would be presented. Whether on purpose or not a notable portion of the standard is written ambiguously.

There was already a method of avoiding the use of the U; simply move the basic outline. There was no need for this function.

The usage is not standard only to the extent that an exact example is not in the standard. Since the standard doesn't contain every valid use, then almost no uses are standard. The wording is clear - the second value is an offset to the added material. Simple arithmetic covers what happens to a negative number under addition.

 

[pylfrm]

I would interpret both as "illegal"

The number to the left of (U) represents width of tolerance field, and the number to the right represents portion of said field extending "outside" of basic profile.

This is pretty much how ASME Y14.5.1M-1994 defines it.

PRF|0.1(U)0.189|A|B|C| is a plus-plus profile with boundaries at +.189 and +.089

PRF|0.25(U)-0.013|A|B|C is a minus-minus with boundaries at -0.013 and -0.263

"Illegal" or not, this is the only reasonable interpretation I can imagine. A small adjustment to a single sentence in ASME Y14.5-2009 paragraph 8.3.1.2 makes it kosher:

A second value is added following the unequally disposed symbol to indicate the tolerance offset from the true profile to the maximum material boundary of the tolerance in the direction that would allow additional material to be added to the true profile.


Mbovens,

What's the purchasing situation here? Will you be buying unpainted parts and then paying someone else to paint them? If so, why not just have two drawings, two part numbers, two sets of basic dimensions, and use equally disposed bilateral profile tolerances for both? Alternately, if you will be buying painted parts complete, then why bother with tolerances before paint?


pylfrm

 

[Mbovens]

pylfrm,

One company shoots the plastic and then the same exact company paints them. Hence, one P/N with 2 sets of GD&T callouts. One to ensure the part is in spec after molding and then the other GD&T callout ensures the exact same part is in spec after painting.

Thanks
Mbovens.

 

[axym]

CH,

I can't disagree with your interpretation. The way Y14.5-2009 describes the number after the (U) is not compatible with the "extended" plus-plus and minus-minus specifications. In other words, the number after the (U) would need to be interpreted as something other than the amount of tolerance in the direction that adds material.

gabimot,

The numbers are all offsets relative to the true profile (which would have an offset of zero).

mkcski,

I agree that the proposed plus-plus and minus-minus specifications are non-standard. They are an attempt to encode something that the Y14.5 standard currently does not support.

greenimi,

Yes, I'm a member of the Y14.5.1 subcommittee and also the Y14.45 subcommittee for Measurement Data Reporting. Part of the reason that I participate in GD&T and CMM forums is to get a better idea of what challenges industry professionals are facing, and what their opinions are on various issues. A new revision of Y14.5.1 is in the committee review stage and Y14.45 is nearing completion as well. There should be public review drafts coming out later this year (I'll keep you posted). I'm hoping for lots of feedback from members of this forum, especially on the profile section.

3DDave,

I agree that there are sections of the standards that are written ambiguously, and that the clarity could definitely be improved.

I'm not sure what you're referring to with a method of moving the basic outline. Before the (U) was introduced, unequally disposed profile zones were annotated with phantom lines showing the offset to one of the boundaries.

The committee discussions are not made public - we're not allowed to publicly discuss the details. But the meetings themselves are open to the public - there are always a lot of visitors at the Y14.5 meeting and we even get a few in Y14.5.1 ;^). The next round of meetings is Oct 23-27 in San Diego.

pylfrm,

I agree that the numbers make sense if the value after the (U) is interpreted as an offset from the true profile to the MMB. I came to exactly the same conclusion.


Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[3DDave]

Evan, what I meant, is that if a profile tolerance zone is going to be offset (unequally-disposed), just change the true profile so that there is no need for the offset and the zone can be symmetrically disposed. This would eliminate the need for (U) and for the former practice, both of which are primarily manufacturing cues and are not required for describing acceptance or performance of the related parts.

I recognize that there are people with a stake in arguing about it, but describing the location of the tolerance zone boundaries doesn't require the additional operators.

 

[pylfrm]

Mbovens,

Why do you care about profile of a surface specifications before paint? Wouldn't some combination of paint thickness tolerances and profile of a surface tolerances after paint get you what you need?

Also, are the datum features being painted? If so, and if you insist on keeping before and after tolerances on a single drawing, matters will be complicated substantially. Has this been considered?

Side note: There is a datum feature symbol attached to what looks like an imaginary vertical line. This is not valid. An actual feature must be identified.


pylfrm

 

[Belanger]

3DDave -- interesting viewpoint. Are you also of the opinion the plus/minus tolerances should never be unequally disposed, but rearranged to always be symmetric about the nominal? The same logic seems to apply with ± tolerances and profile's disposition.

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

 

[3DDave]

Berlanger, nice try.

Directly applied tolerancing already made use of the "+" and "-" signs, and there never was a suggestion that these offsets would be replaced by a directly applied single value that represented a tolerance zone width (or diameter.)

Most geometric controls represent only width (or diameter) with profile being the only one that has an offset exception. It's odd because flatness could also benefit in those cases where controlling convexity/concavity would be desired, but why not many other controls where sidedness might apply? Note, I would not appreciate anyone putting these concepts into any standard.

In either case, asymmetry typically represents a process peculiarity and D&T is supposed to be process neutral. While it is unavoidable to support it for direct tolerances, it was never necessary, even if sometimes convenient, to add it for profile**, but if it's going to be added for convenience, it should have been added in a way that matched prior practice, particularly for +/+ and -/- limit offset cases.

I should be able to design a punch and die and use the exact same true profile on both parts and shift the tolerance zone by the desired amount of clearance on each part. In the strict interpretation of (U) given by some here, that's not an acceptable use.

**Necessary in the same sense that a calculator square root key is unnecessary when there is an X^Y key. Convenient, sure. Necessary, no.