Cylindrical feature of size straightness modified LMC

[Burunduk]

ASME Y14.5-2009 specifies that flatness associated with a size dimension can be applied RFS, MMC, or LMC. For straightness applied to FOS there is no mentioning of LMC and only RFS and MMC are covered. Anyone has an idea why?

 

[greenimi]

see this

https://www.eng-tips.com/viewthread.cfm?qid=451071

 

[Burunduk]

Thank you, greenimi, I was searching in the forum but didn't see this thread. Perhaps I didn't include the exact words in the search.

One detail caught my attention:

Having said that, I would still proceed with caution. Y14.5 allows LMC for DMP flatness, but does not provide details on how it is applied. There may be differences in what envelopes apply, as there are when LMC is applied to position tolerances.

Evan, if you are reading this, why the mention of envelopes in the context of FOS flatness with material condition modifiers? I know that envelopes are relevant for position/orientation at LMC/MMC, but I don't see what they have to do with form controls applied to features of size - where the actual local size is used for additional tolerance calculation.

Edit: with that said, relevant "envelopes" may be VC and RC (although to be exact these are "boundaries"), that is for form tolerances too. But they always apply for both MMC and LMC regardless of the type of control.

 

[pmarc]

The definitions of DML and DMP mention cross-section (for DML) and linear elements (for DMP) normal to the axis / center plane of the Unrelated Actual Mating Envelope (UAME) of a feature.

I believe what Evan meant is that in case of DML Straightness at LMC and DMP Flatness at LMC the envolope to use should technically be the Unrelated Actual Miminum Material Envelope (UAMME).

 

[Burunduk]

pmarc,
I'm afraid I'm missing the reason for the parallels drawn between these cases regarding the different envelopes that may be involved.
Let's take for example position MMC/LMC on parallel planes FOS versus DMP flatness MMC/LMC.

1. Position MMC:
Element limited within the tolerance
zone: center plane of UAME

Additional tolerance results from:
departure of UAME size from MMC

2. Position LMC:
Element limited within the tolerance
zone: center plane of UAME

Additional tolerance results from:
departure of UAMME size from LMC

3. DMP flatness MMC:
Element limited within the tolerance
zone: DMP comprised of center points of
line segments normal to centerplane of
UAME.

Additional tolerance results from:
departure of ALS from MMC.

4. DMP flatness LMC (extension of a
principle):
Element limited within the tolerance
zone: DMP comprised of center points of
line segments normal to centerplane of
UAME

Additional tolerance results from:
departure of ALS from LMC.

 

[pmarc]

Burunduk,

1. Position MMC:
Element limited within the tolerance
zone: center plane of UAME

Additional tolerance results from:
departure of UAME size from MMC

2. Position LMC:
Element limited within the tolerance
zone: center plane of UAMME

Additional tolerance results from:
departure of UAMME size from LMC

3. DMP flatness MMC:
Element limited within the tolerance
zone: DMP comprised of center points of
line segments normal to centerplane of
UAME.

Additional tolerance results from:
departure of ALS from MMC. This is what the standard is currently saying but this is not what it should be saying. The local increase in the flatness tolerance zone width should depend on the length of the linear element established in the cross-section perpendicular to the center plane of the UAME of the feature.

4. DMP flatness LMC (extension of a
principle):
Element limited within the tolerance
zone: DMP comprised of center points of
line segments normal to centerplane of
UAMME

Additional tolerance results from:
departure of ALS from LMC. The local increase in the flatness tolerance zone width should depend on the length of the linear element established in the cross-section perpendicular to the center plane of the UAMME of the feature.

Do you now see the parallel?

 

[Burunduk]

pmarc,
I would see the parallel if I was convinced.

"2. Position LMC:
Element limited within the tolerance
zone: center plane of UAMME"

Center plane of UAMME based on which specification? If we are talking about Y14.5-2009:
Para. 7.2 provides a definition for all position tolerance zones based on axis/center plane interpretation which is relevant to RFS, MMC and LMC (for MMC and LMC there is also the surface interpretation but there we are talking about a boundary, not a tolerance zone with the distance between planes or a diameter that equals to the tolerance value).

"(a) a zone within which the center, axis, or center plane of a feature of size is permitted to vary from a true
(theoretically exact) position".

Wherever "feature axis/center plane" or the "axis/center plane of a feature of size" is mentioned it is understood to be derived from the UAME and not from any other envelope.

This is what the standard is currently saying but this is not what it should be saying. The local increase in the flatness tolerance zone width should depend on the length of the linear element established in the cross-section perpendicular to the center plane of the UAME of the feature.

Why?
The current definition is based on the actual local size being compared to MMC. Actual local size is not properly defined in Y14.5 and not defined at all in Y14.5.1M but that is a problem of the standards, not of the industry. In the industry "2-point checks" is well established and reliable process.

 

[pylfrm]

Just like position tolerances, I think MMC and LMC straightness and flatness tolerances would be a lot simpler with a surface interpretation based on a virtual condition boundary.


pylfrm

 

[pmarc]

Burunduk,
As for the first of your questions, just look at fig. 1-2 (compare it with 1-1) in Y14.5-2009 or para. 7.3.5.3 where "minimum material size" is mentioned. This unveils some lacks in terminology in the standard but the idea is there.

As for the second question, sorry but that is just too much for me right now to explain. I don't want to step into another 170+ long thread trying to convince you to something that has not been even mentioned in the standard ;-)

As pylfrm noted (and I should have said that before), life would be a lot simpler if surface interpretation was the only available interpetation for this kind of tolrances.

 

[Burunduk]

pmarc,
Notice paragraph 1.3.28:

Feature Axis
feature axis: the axis of the unrelated actual mating envelope of a feature.

NOTE: In this Standard, when the term “feature axis” is used, it refers to the axis of the unrelated actual mating envelope unless specified otherwise.

Is it "otherwise specified" in the defining paragraphs of chapter 7 in relation to LMC position tolerance? I think it isn't and it is not in the more general para 2.8.4 ("Effect of LMC") either. If it is in any of the paragraphs I must be missing it. I see that Figure 1-1 indeed shows the "axis of unrelated actual minimum material envelope" indicated in brackets as "feature of size axis" but this type of "feature axis" doesn't seem to be supported in the defining paragraphs or used in any of the concepts as defined in the body of the standard. Am I wrong?

As pylfrm noted (and I should have said that before), life would be a lot simpler if surface interpretation was the only available interpetation for this kind of tolrances.

Agree with both of you, unless it may complicate CMM inspection (I don't know if it may or may not).

 

[pmarc]

Is it "otherwise specified" in the defining paragraphs of chapter 7 in relation to LMC position tolerance? I think it isn't and it is not in the more general para 2.8.4 ("Effect of LMC") either. If it is in any of the paragraphs I must be missing it. I see that Figure 1-1 indeed shows the "axis of unrelated actual minimum material envelope" indicated in brackets as "feature of size axis" but this type of "feature axis" doesn't seem to be supported in the defining paragraphs or used in any of the concepts as defined in the body of the standard. Am I wrong?

Haven't you already noticed based on at least 2 or 3 recent threads here that trying to find a crystal clear logic in everything that is in the standard may only get people into weeds of endless and unproductive debates?

I searched through the 2009 standard (Ctrl+F) and found only one place where "axis of unrelated minimum material envelope" is explicitly mentioned. It is at the beginning of para. 7.6 and I think it has been mentioned there for a reason. It (i.e. the axis or center plane of UAMME) should be mentioned in more places (for example in fig. 7-16) but we have what we have.

By the same token, for some unknown reason fig. 7-16 shows that a "slot center line" should fall within the position tolerance zone. Do you think we need a separate conversation on what the "center line" is and why it does not have its own definition in the standard, or can we just simply accept that this was a typo (which by the way has not been corrected in 2018 version)?

 

[chez311]

pylfrm/pmarc/Burunduk,

For what its worth, the Y14.5.1-20xx draft ONLY provides a surface interpretation for MMC/LMC DML straightness/DMP flatness.

Conformance is that no point on the surface violates the VC boundary. Actual value is the same as RFS, the smallest straightness/flatness tolerance to which the feature will conform. None of the concepts of UAME/UAMME axis/centerplane or actual local size are even mentioned.

 

[pmarc]

chez311,

No wonder. It is hard to come up with math definition for something that doesn't have solid ground in Y14.5. Not to mention that usefulness of the definitions compared to the effort needed to create them and then proceed through all committee ballots would be very low.

 

[Burunduk]

Haven't you already noticed based on at least 2 or 3 recent threads here that trying to find a crystal clear logic in everything that is in the standard may only get people into weeds of endless and unproductive debates?

Endless debates - do you mean like the one that might develop if I insist that straightness of a surface is applicable to a cone because the standard says the tolerance zone is two parallel lines on the same plane with the axis of the UAME but doesn't say that those parallel lines shall be parallel to that axis?

Interesting point about the "axis of actual minimum material envelope" at the beginning of para 7.6. I didn't notice this one. Now I am wondering what might be the meaning of this term appearing only there and in fig. 1-2, and not in any other place where it should be relevant to support what you told me about LMC position. Perhaps I shouldn't put that much thought into it as consistency and clarity is too much to ask for from a document that is made with the purpose to establish "uniform practices for stating and interpreting dimensioning, tolerating, and related requirements for use on engineering drawings and related documents"?

I noticed the "slot center line" wording before and somehow repressed it in order not to get frustrated. Thinking of it now, it looks like a committee member caught some random guy passing by and asked him to do him a favor to finish the figure for him and submit it because he needed to go somewhere in a rush.

 

[pmarc]

Endless debates - do you mean like the one that might develop if I insist that straightness of a surface is applicable to a cone because the standard says the tolerance zone is two parallel lines on the same plane with the axis of the UAME but doesn't say that those parallel lines shall be parallel to that axis?

Not sure I understand. The problem I see with the application of straightness of line elements to a cone is that it is unclear how exactly the UAME of a conical feature should be established, but this has nothing to do with orientation of the two parallel lines of the tolerance zone relative to the UAME axis, as far as I can tell.

Now I am wondering what might be the meaning of this term appearing only there and in fig. 1-2, and not in any other place where it should be relevant to support what you told me about LMC position. Perhaps I shouldn't put that much thought into it as consistency and clarity is too much to ask for from a document that is made with the purpose to establish "uniform practices for stating and interpreting dimensioning, tolerating, and related requirements for use on engineering drawings and related documents"?

It is definitely not too much to ask for consistency and clarity from Y14.5. But what is too much (forgive me for saying this) is permanent attempt to convince other people - especially different respected members of this forum, sometimes deeply involved in Y14 matters - that consistency and clarity exist when it has been objectively proven that this is not true.

I noticed the "slot center line" wording before and somehow repressed it in order not to get frustrated. Thinking of it now, it looks like a committee member caught some random guy passing by and asked him to do him a favor to finish the figure for him and submit it because he needed to go somewhere in a rush.

Well, in that case the committee apparently hasn't had a chance or time to fix the mistake for at least last 25 years, because Y14.5M-1994 also has the "center line" in the corresponding figure 5-14 [sarcasm]. This is just another proof that the standard was not, is not and probably will never be perfect.

 

[Burunduk]

Not sure I understand. The problem I see with the application of straightness of line elements to a cone is that it is unclear how exactly the UAME of a conical feature should be established, but this has nothing to do with orientation of the two parallel lines of the tolerance zone relative to the UAME axis, as far as I can tell

Not truly intended as a start of another debate, but just to clarify the point: myself and others provided some reference in other threads that some, including committee members and credited experts, consider a cone as an irregular FOS - but that is not the gist here. The gist is that for those who accept that it may be a considered a FOS (and we could probably at least agree it's a grey area) and therefore also that an unrelated actual mating envelope exists, the lack of a requirement for the tolerance zone limiting lines to be parallel to the UAME axis for surface straightness may suggest it can be applicable for a conical feature. The only caveat is the title of the paragraph where this is described "5.4.1.1 Cylindrical Features".

It is definitely not too much to ask for consistency and clarity from Y14.5. But what is too much (forgive me for saying this) is permanent attempt to convince other people - especially different respected members of this forum, sometimes deeply involved in Y14 matters - that consistency and clarity exist when it has been objectively proven that this is not true.

I recognize the parts of the standard where consistency and clarity are lacking. But even a less "respected member of this forum", less "deeply involved in Y14 matters" can tell that where the requirements are simple and explicit, like in the para. 7.2 definition of the axis/center plane interpretation tolerance zone for position and of what it limits in variation, there can be no more than one interpretation of the concepts. If pointing this out is considered "too much" in this forum, then this is not a forum I wish to keep participating in.

 

[pmarc]

Not truly intended as a start of another debate, but just to clarify the point: myself and others provided some reference in other threads that some, including committee members and credited experts, consider a cone as an irregular FOS - but that is not the gist here. The gist is that for those who accept that it may be a considered a FOS (and we could probably at least agree it's a grey area) and therefore also that an unrelated actual mating envelope exists, the lack of a requirement for the tolerance zone limiting lines to be parallel to the UAME axis for surface straightness may suggest it can be applicable for a conical feature. The only caveat is the title of the paragraph where this is described "5.4.1.1 Cylindrical Features".

This all just proves that there is still a lot to clarify in the standard so that discussions like the recent one about cones - regardless if participating people are just regular members of the forum, credited GD&T experts or committee members - don't happen any more

But even a less "respected member of this forum", less "deeply involved in Y14 matters" can tell that where the requirements are simple and explicit, like in the para. 7.2 definition of the axis/center plane interpretation tolerance zone for position and of what it limits in variation, there can be no more than one interpretation of the concepts.

Of course, everyone can tell that. The thing is that if someone else tells that there can be more than one interpretation, this is at least worth thinking about it instead of just claiming that the standard must be right no matter what.

I truly wish Y14.5 was the document that everyone could blindly trust in but the undeniable fact is that the deeper one looks at its details the less trustworthy it becomes.  

 

[Burunduk]

If you followed any of the discussions I was involved in or even paid attention to what I said in this discussion, you would know I am not among those "claiming that the standard must be right no matter what".
But then again, I suppose that some kind of accusation had to be made anyway.

 

[pmarc]

If you followed any of the discussions I was involved in or even paid attention to what I said in this discussion...

Speaking of accusations...

... you would know I am not among those "claiming that the standard must be right no matter what".

This is not what I see in this particular discussion.

Anyway, I believe it is the right moment for me to jump out of the conversation. I just hope you will take a moment to analyze the "UAME vs. UAMME in tolerances at LMC" topic in details. Not by just looking at what is in the standard but by analyzing it from the objective mathematical/geometrical standpoint.

 

[Burunduk]

This is not what I see in this particular discussion.

What do you think was the point of the surface straightness example?

I believe it is the right moment for me to jump out of the conversation

I suppose that for me too.