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's not a negative zone, it's a negative offset to the away-from-material side of the tolerance zone. It's no different than having the offset be larger than the zone.

I think the Committee flubbed by not changing to a +/- control of the unequal boundary rather than trying to keep the zone width and introduce an offset. Then one could explicitly have +/+. +/-, and -/- offsets from the ideal boundary.

 

[pylfrm]

Mbovens,

Take a look at thread1103-401396.

pylfrm

 

[BiPolarMoment]

I think the Committee flubbed by not changing to a +/- control of the unequal boundary rather than trying to keep the zone width and introduce an offset. Then one could explicitly have +/+. +/-, and -/- offsets from the ideal boundary.

Hear hear! Even before the unequally disposed profile symbol was introduced, profile was/is the biggest source of argument in GD&T at my company.

 

[mkcski]

One of the major focus' of the evolution from the 1994 to 2009 release has been better controls for defining material boundaries. The "U" symbol is fine with me. My interpretation is how much material is "added" to the BASIC profile to establish the "MMC" boundary and how much is left "inside" to establish the "LMC" boundary. I prefer to avoid + and - as they have X-Y-Z direction which can confuse especially in 3D space.

Certified Sr. GD&T Professional

 

[3DDave]

+ adds material, - removes material. There's no xyz direction to it.

 

[BiPolarMoment]

One of the major focus' of the evolution from the 1994 to 2009 release has been better controls for defining material boundaries. The "U" symbol is fine with me. My interpretation is how much material is "added" to the BASIC profile to establish the "MMC" boundary and how much is left "inside" to establish the "LMC" boundary. I prefer to avoid + and - as they have X-Y-Z direction which can confuse especially in 3D space.

Except this isn't what it means? If you have a Profile .004 U .001 it's still a .004 variance between MMC and LMC (i.e. +.003/-.001 if linear NOT .005 total)... or did I sleep through that part of my training?

 

[BiPolarMoment]

And this conversation just proves the point.

 

[3DDave]

BiPolarMoment,

The point you made was about profile before unequally disposed tolerance. What point about the prior technique was proved in a discussion about the later technique?

 

[Belanger]

Dave -- imagine your suggestion of an unequal profile tolerance given with the + designation on a hole. Your logic means that this makes the hole smaller. Good luck with that being understood by everyone!

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

 

[3DDave]

It's already part of MMC where a "Maximum" means a smaller hole vs "Maximum" means a larger shaft. People can cope with the difference there as well as can be expected. "U" is just as tough but has no corollary anywhere.

 

[Mbovens]

Hey all,
Thx for the responses so far; it does help but now I need to get the callouts correct. Am I on the right track with the callouts? Can I leave a negative number after the U symbol? Does this mean a tolerance zone can lie on the other side of the datum? The intent is to try to encourage the tool shop to bias their tooling to get to the end profile size after paint and make sure the parts can fit together.


Cheers,
Mike

 

[greenimi]

Sorry to bother, but what means “negative” number after U?

If this is a product drawing, then let the end user (manufacturer, inspection) know the profile limits you will be happy with. (after the paint operation/ sequence)

The tool shop will bias their tooling to the best of their knowledge and abilities to make sure the final product will meet the product specification you defined above. Do not try to do their job. Do yours the best you can and more important do not mix the product specification with the process requirements/ specifications.

I think the standard (2009) offers good tools (callouts) for this job (your specific scenario) without trying to find “better ways” (non-standardized) to define the product requirements.

As far as how the standard should have been written I prefer to have no comment about it….no committee is perfect, no standard is perfect and won’t ever be perfect (same as our parts we/ the companies make for living/ selling)…..

 

[mkcski]

Sometimes we try too hard to "cram" all product definition "into" GDT symbols. Don't forget, you can use notes to "assist" with communication.

Certified Sr. GD&T Professional

 

[axym]

Mbovens,

There isn't an official definition of "correct" for these callouts. Y14.5-2009 doesn't define or mention the "plus-plus" and "minus-minus" callouts that you've specified, in which the tolerance zone doesn't include the true profile. They didn't envision these applications when the standard was written, and the (U) format makes it cumbersome to specify them. Here is how I would interpret the callouts, and you can check whether this agrees with the intent:

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

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[CheckerHater]

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.

That is,

1. no "minus" sign, as it always extends in positive direction.

2. "portion" cannot be larger than "whole", so the number to the right from (U) should be smaller or equal than the one to the left.

Very simple.

"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future

 

[gabimo]

Evan,

I understand your approach, but how do you know where the tolerance zone starts? If TZ is 0.1 wide and is “extended” 0.189 my question is from where? I am relatively new here.
Same question for the negative one -0.013. TZ is within the material and is 0.25 wide, where starts?

I am a little confused.

Thanks
Gabi

 

[mkcski]

I agree with CheckerHater. The symbology is non-standard, which defeats the whole purpose of standardization - universal interpretation - like a dictionary for words. If you have improvements to the Standard, send them to the Y14.5 Committee for consideration in the next release (after the one that is due out "soon").

Certified Sr. GD&T Professional

 

[greenimi]

mkcski,

I think Evan is part of the Y14.5.1 (Math Standard) commitee....so he already has some inside info....

Evan,

Am I correct?

 

[mkcski]

That would be a BIG plus: to have these "discussions" directly exposed to Committee members who might advocate of improvements in the content in the standard.

Certified Sr. GD&T Professional