Question about GD&T. 2

[woodwood]

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.

 

[pmarc]

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.

 

[powerhound]

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

 

[woodwood]

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?

 

[powerhound]

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

 

[pmarc]

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.

 

[CheckerHater]

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!

 

[pmarc]

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:

[Ø]33.984-34.030
|pos.|[Ø]0(M) [Ø]0.023 MAX|A|
|pos.|[Ø]0(L) [Ø]0.023 MAX|A|

Would that be different to what you showed?

 

[SeasonLee]

CH

Glad to learn this concept.

SeasonLee

 

[CheckerHater]

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.

 

[pmarc]

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?

 

[CheckerHater]

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.

 

[axym]

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.

http://www.axymetrix.ca

 

[woodwood]

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.

 

[MechNorth]

"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

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[fsincox]

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!

 

[pmarc]

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?

 

[fsincox]

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.

 

[pmarc]

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?

 

[fsincox]

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.