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Question about GD&T.
2

Question about GD&T.

Question about GD&T.

(OP)
Hello all.

I've not had much to do with drawings that contain GD&T up until now, just because of the standard of drawings that we get, and I was after a bit of assistance interpreting a feature.
In the sketch you can see two diameters, the larger one being datum A and the smaller one with a concentricity of 0.000 in minimum and maximum metal conditions. I'm having difficulty understanding the min and max metal requirements and was hoping someone could shed some light (until Ican get some training!)

Thanks.
WoodWood.

RE: Question about GD&T.

Concentricity tolerance value cannot equal 0, because that would mean there is actually no tolerance allowed at all.
It is against a fundamental tolerancing concept saying that perfect part does not exist in reality, so there always has to be a tolerance defined.

The note is poor also because it does not say anything about the amount of concentricity tolerance allowed when the smaller diameter is somewhere in between 33.984-34.030.

RE: Question about GD&T.

I agree with pmarc. This is just a bad callout. To locate using the "zero at MMC" concept, you have to use a position callout. So the feature control frame would be attached to the boss dimension and it would symbolically say "position within a diameter of zero at MMC with respect to datum A". So if the boss actually measured 34.030, it would have to be positioned perfectly. If the boss measured 33.984 then it could have positional error up to 0.046.

Powerhound, GDTP S-0731
Engineering Technician
Inventor 2013
Mastercam X6
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

RE: Question about GD&T.

(OP)
Thanks for the replies.
I see what you mean about it being a bad callout and, Powerhound, your description of a boss measuring 34.030 having to be positioned perfectly, makes sense.
As the note on my drawing says 'with a concentricity of 0.000 in minimum and maximum metal conditions' does that mean that they are saying that a diameter of 34.030 and 33.984 would have to be positioned perfectly and you would only have any tolerance if you stayed around mid limit?

RE: Question about GD&T.

That's probably what the intent was but the verbage is ambiguous and per the ASME definition of concentricity, you can't invoke the "zero at MMC" concept. There is a lot about this callout that begs to be defined. What is "metal condition"? Is it a misstatement of "material condition"? If it is, then invoking the "zero at MMC and LMC" concept on the exact same feature simultaneously makes no sense whatsoever. I think you need to have a conversation with the designer/customer and find out what he/she is trying to say.

Powerhound, GDTP S-0731
Engineering Technician
Inventor 2013
Mastercam X6
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

RE: Question about GD&T.

Assuming that Y14.5 is in charge in this case, and that Concentricity was misused here (like in 99,9% of similar cases), and Position could be applied without a risk of violating any functional requirements (to make a presence of material condition modifiers legal), I am trying to imagine a functional requirement for which application of such callout and note would be justified, but I am not able to figure out anything.

I think technically this could be done with Y14.5's tools, but, as Powerhound said, would that make sense? I would really like to hear how "designer/customer" explained this.

RE: Question about GD&T.


It looks like the concept of applying MMC and LMC simultaneously is quite old. I actually expected it to be embraced by “extension of principle” crowd.

It doesn’t seem very attractive to me, and in today’s world should probably be replaced by statistical requirement.
Nevertheless it’s here – enjoy!

RE: Question about GD&T.

CH,
Interesting.
I was thinking of applying two positional callouts - first 0 at MMC, second 0 at LMC with additional information though. The whole thing would look like this:

Ø33.984-34.030
|pos.|Ø0(M) Ø0.023 MAX|A|
|pos.|Ø0(L) Ø0.023 MAX|A|

Would that be different to what you showed?

RE: Question about GD&T.

CH

Glad to learn this concept.

SeasonLee

RE: Question about GD&T.

Pmarc,
Looks like you can drop your .023 MAX requirement, as your MMC and LMC have to meet in the middle anyway, but I have another concern.
According to the standard, requirements in two FCF applies to the same feature have to be met “together” but not “simultaneously”.
So, using one FCF like in my example and 2 like in yours could have some subtle difference, but I cannot put my finger on it.

RE: Question about GD&T.

CH -- you are right - 0.023 MAX can be dropped.
As for simultaneous requirement, hmmm... I thought about it, but couldn't imagine the subtle differences in OP's case. Are you thinking of something in particular?

RE: Question about GD&T.

Not really.
For some reason the “tip” has it in one single frame.
As we have only one feature and not a “pattern”, there is probably no difference.

RE: Question about GD&T.

Hi All,

Here's a thought. The requirement for zero positional variation at both MMC and LMC would be very similar to a Surface Profile tolerance on the feature (with a basic diameter). There would be an annular tolerance zone for the surface of the hole. If one wants to look at the axis offset of the feature's actual mating envelope, the largest possible offset would exist when the feature is perfectly round and mid-sized. If the feature was made at the smallest or largest size, there could be no axis offset without violating one of the profile boundaries.

Evan Janeshewski

Axymetrix Quality Engineering Inc.
www.axymetrix.ca

RE: Question about GD&T.

(OP)
Thanks all for the input. It seems it's not straight forward! I'm trying to get in touch with the designer to see what the initial intent was.

RE: Question about GD&T.

"zero @ MMC with Zero @ LMC" is not a defined concept, and is clearly NOT an extension of principles. Gary Whitmire's example isn't a definition, it's just how he personally feels it would go. He chose that the split would be 50/50, but that's not necessarily intuitive.

As Evan posted, Profile of a Surface would be the appropriate call to get the net effect and center the max'm tolerance at half the zone.

Jim Sykes, P.Eng, GDTP-S
Profile Services www.profileservices.ca
TecEase, Inc. www.tec-ease.com

RE: Question about GD&T.

I naturally have to disagree with the experts; I see it as exactly an extension of two fairly well understood principles, using position, of course.
Under ISO that might very well be a different story.
I agree profile would do the same thing so why the issue, then?
I actually proposed this concept 20 years ago for location of ball bearing support shoulders being referenced to the bearing bore itself as a datum. That is the way I am forced to interpret the ball bearing manufactures shoulder recommendations.
Remember, the use of profile was expressly banned by the management of the company at the time!

RE: Question about GD&T.

Frank,
Since you keep this thread alive, I agree with Jim on this. It is not 100% obvious that positional tolerances must meet in the middle (though I would probably call it intuitive). What is shown by Mr. Gary Whitmire below feature control frame is only his interpretation and does not stem from any official Y14.5 rule. Don't you think that someone might have thought that tolerance distribution is different? If I tell you that my interpretation is 0.002 for 0.747 going from hole's MMC size and 0.007 for 0.748 going from hole's LMC size, will you be able to prove that I misinterpreted the callout?

RE: Question about GD&T.

pmarc,
Sorry, I just got to it. I am not following your line of thought on this, are you saying we do not have to meet both conditions, simultaneously? My statement was done as 2 separate frames more like conventional refinements.
I had not seen Gary's sheet before. Lowell Foster spends time on zero position tolerancing in his book "Geomtrics" (the books I was taught from). The real problem with “zero tolerancing” is the “zero”, it scares people. There is nothing wrong with the concept. GD&T dances around with perfection a lot (perfect form at MMC) but seems to be afraid to really commit. It sounds to me to be more of a political and lack of courage problem on the committee's part from Gary's description. You are right we can not point and say this is what it means if it is not there.

RE: Question about GD&T.

Frank, I have nothing against "zero at MMC" or "zero at LMC" concepts. My comment was purely about Gary Whitmire's paper and interpretation of callout proposed by him. From what I see you seem to agree that his interpretation (unless clearly standardized) is not the only option possible.

What I do not understand is why you said that you did not agree with the experts? Notice that nobody stated that the approach with 2 different FCFs was illegal or violated any Y14.5 rule. It was only suggested that profile of surface application would be very similar to it. Could you clarify?

RE: Question about GD&T.

It seems like Gary is saying that they (the experts on the committee) prefer to side step this issue but agreed on the basic concept.
I have an application, as I described, where it seems to fit a real world design requirement.
I work with people enough to have seem the shocked reaction to "zero tolerancing". As explained to me it is a valid concept, simply choosing not to reserve some tolerance portion for size and another portion for location. In line with the ASME stated concept of not defining the manufacturing method, simply stating required parameters.
Don (and others) talk about companies customizing the standard to their liking, I am not a big fan of this idea as my former company did that and outlawed profile use completely. I see this as a big opportunity to be driven more by fear of change, I am very suspect of the committee's position on this issue, granted based on limited information, if someone knows better let him speak.

RE: Question about GD&T.

Frank -- I have to agree with pmarc and MechNorth on this.
Nobody should ever have an issue with zero at MMC (or LMC), unless they don't understand the concept of bonus tolerance. But the one proposed by Gary (zero at MMC & LMC) is inherently ambiguous. There's no ambiguity if you make the feature right at MMC or LMC. But the question is: since we have two points of departure, where do they meet? Suppose someone desires that as you depart from MMC, the bonus tolerance kicks in and keeps adding -- like we're all used to -- right up to the point where you're 1 micron from LMC and then at LMC the tolerance suddenly jumps to zero!
Of course, the opposite could be true. Maybe the bonus tolerance is tied more to the LMC, and suddenly the tolerance jumps to zero only when the MMC threshold is hit.
It's an intriguing idea -- and there may be some practical uses for this -- but as it is right now there is no strict definition of how those two variables interact with each other.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
http://www.gdtseminars.com

RE: Question about GD&T.

Guys,
Obviously, I must be so rapped in my own thoughts/perspective that I can not see the forest for the trees, here.
I will have to just trust you guys on this then, thanks.

RE: Question about GD&T.

Sorry to come into this late. I agree with Evan that using profile would be much better for this case than the proposed (M) & (L) "thing"... I think a good way to explain why this would be more straightforward is to describe Position at (M) and Position at (L) as surface controls, with a virtual condition boundary for each... If you combine (M) & (L) then you get a virtual condition boundary on the MMC side and the LMC side also... These two perfect form boundaries could be more clearly imposed using Profile of a Surface.

As a matter of a fact, my opinion is that a lot of cryptic Position at (M) and Position at (L) tolerances (having two interpretations), especially "zero" tolerances at either condition would be more clearly said with a more capable Profile method that I wish we could move towards... This is a very large ship we're dealing with, with a large committee steering it, though. This opinion that I am stating is mine alone, at this point. Over time maybe profile will become less "evil" in industry profile smiley.

Dean
www.d3w-engineering.com

RE: Question about GD&T.

Well here is an example from my book, about 1992, it was intended as an internal company extention of the standard the example was really more as an explanation of the use of "individually".

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