Composite callout

[aniiben]

Does the addition of B secondary and C tertiary in the lower segment of the composite (FRTZF) brings any value to the callout? If yes, what is the value added?

What would be the differences between only A primary shown in the lower segment (FRTZF) on the composite versus what is shown on the embedded picture and circled in red?

 

[greenimi]

https://m.qualitydigest.com/?url=https://

http://www.qualitydigest.com/inside...t-s-new-y145-2009-gdt-standard.html&width=412

If article does not work google is your friend

 

[3DDave]

Good article.

Neither of those worked for me. Firefox is not a fan of %2F as a substitute for "/" but Google did come through.

https://www.qualitydigest.com/inside/metrology-article/what-s-new-y145-2009-gdt-standard.html

They suggest the PDF version:

https://www.qualitydigest.com/pdfs/Review of the new Y14 5 2009 Standard.pdf

Also -

https://www.qualitydigest.com/read/content_by_author/12188

for all articles by Bill on that site.

 

[greenimi]

I guess page 9 in Bill's T. article should summarize why 3DDave said "the worms are loose"

Copy-paste
"However, if the Datum Features in the second and all lower tiers may only constrain rotational degrees of freedom, the 2009 Standard (see Fig. 7-42 p.138) has failed to correct an error in the 1994 Standard (see Fig..
5-25 p.122) where datum features B and C are left in individual compartments in the second tier, but must in fact be referenced as composite, namely [B-C], in order to perform their function. This is true, because B, as a stand-alone Datum Feature, can only constrain the first two degrees of rotational freedom which A has already constrained, and therefore serves no purpose. C on the other hand, as a stand-alone, can, may and must constrain roll, but must do so on its own, instead of working in conjunction with B as it does in the first tier and must therefore continue to do."

I think it is the current definition in the standard that Bill T. is criticizing.

 

[chez311]

greenimi,

I have given this some consideration, and I feel personally that there is no real concrete support either way. HOWEVER that being said, I would agree that the most unambiguous and clear way to communicate that it is desired for B and C to work together to constrain the final rotational DOF (w) would be indeed to utilize a composite datum reference [B-C]. I read through the applicable section in Bill Tander’s article, as well as the Y14.5 sections on composite tolerance, and while my gut tends to disagree with Bill I can at least concede that I don’t think it is well enough defined in the standard to definitively say one is correct (besides the [B-C] solution).

A few notes, take them for what you will (I apologize if I’m beating a dead horse or being overly stubborn) – as I said I already know what the solution is to prevent being misinterpreted (a composite datum [B-C] or similar) its just some of the logic in the article just doesn’t quite sit well with me:

1a) I don’t understand where Bill comes off stating absolutes about “stand-alone datums” and treating datums individually in the lower segments (FRTZF) of a composite callout. I have re-read the portion on composite controls in Y14.5 and I just don’t see this anywhere. I know the standard mentions several times that datums in the FRTZF can only constrain rotational DOF – however I think it is a bit of a leap to take this lack of clarity and create a new concept which (to my knowledge?) has no real basis in the standard with “stand-alone datums”. I may well be missing something, and if there is such an example or verbiage in the standard someone please point that out – I just don’t think that lack of clarity or ambiguous wording constitutes support for this new concept. I think it also raises more questions than it answers – how is datum precedence taken into account with stand-alone datums? Are these datums that do not constrain additional DOF wholly ignored forming the three mutually orthogonal planes of a DRF? If not, why can further rotational DOF not be constrained as in the OP’s case? There is no roadmap for how to treat datums individually as far as I can see.
1b) A few examples from Bill’s article that I just don’t see as black and white as he does.

a) B and C "must in fact be referenced as composite, namely [B-C], in order to perform their function" – perhaps it is more clear but “must”? where is that in the standard?
​

b) “This is true, because B, as a stand-alone Datum Feature, can only constrain the first two degrees of rotational freedom” – where is the concept of stand-alone datum features?
​

c) "C on the other hand, as a stand-alone, can, may and must constrain roll, but must do so on its own, instead of working in conjunction with B as it does in the first tier" – where again is the concept of stand-alone datum features? Where in the standard is there support for the viewpoint that interaction between datums does not occur in the FRTZF?
​

2) An excerpt of 7.5.1.6:

7.5.1.6 Where Radial Location is Important.
The control shown in Figs. 7-42 and 7-43 may be specified
where rotational constraint is important. The design,
however, permits a feature-relating tolerance zone to
be displaced within the bounds governed by a pattern locating
tolerance zone, while held parallel and perpendicular
to the three mutually perpendicular planes of
the datum reference frame. See also Fig. 7-42, illustrations
(a) and (b).

This explicit language supports what is, I think, an obvious conclusion based on all other examples of setting up a DRF – that the secondary datum, while not constraining any further rotational DOF, is still utilized in setting up three mutually orthogonal planes which constitute a DRF to which the FRTZF is constrained in rotation (if it were not, and it was wholly ignored because it didn't constrain any additional DOF, there would only be 2x orthogonal planes in example 7-42 with which to constrain rotation). If this is the case, how would one rationalize that a DRF like the OP’s case which creates 3 mutually orthogonal planes not be able to constrain all available rotational DOF?
​

 

[chez311]

After having taken yet another look, I think its of note to point out that Bill also provides an example of a composite tolerance that violates the datum precedence of the PLTZF [A|B(M)|C(M)] in the FRTZF [A|B(M)-C(M)]. This may well just be an error, theres no way to tell, and it would be rectified by either utilizing the same composite datum B(M)-C(M) as a secondary datum in the PLTZF or changing to multiple single segment with a customized DRF as suggested.

I also read a little further down in the credits for the article and it looks like Evan Janeshewski from this forum (axym) contributed to editing Bill's article! If you see this Evan I would be very interested to hear your thoughts on this matter - I fully look forward to eating my crow if someone can point out a truly solid and clear support for this "stand alone" datum concept.

 

[greenimi]

Chez311,
I really really (2x intentionally) appreciate your willingness to make sense of all this issue.
Here is a thread where the “tertiary datum problem” has been discussed.

https://www.eng-tips.com/viewthread.cfm?qid=428771

This thread has other links for other threads, so, please, if you like study them / understand them all. And probably, will NOT give you the answer you’re looking for just because it is no such or straight answer (there is no black and white –straight answer).
Disclaimer: I do not like to talk about other people on the forum specially if they do not participate in the conversation, but since you started……….. (I have to blame someone)

Speaking about other people, especially about Evan (the co-author for Bill’s article) his conclusion (see his post from February 3rd, 2018) has been: ”I had only thought that the MMB simulators made the DOF constraint ambiguous. But now I am thinking that the MMB simulators actually violate datum feature precedence. Some of you have already suspected this, but now I think I see why. “

I DO NOT want to take the above quote out of context, but might be related with what we are talking here too.

Also, in one of the previous threads, one of the best GD&T persons on this planet (and I am not sarcastic or ironic in any way shape or form, I am a truly believer of what I am saying) I named here pmarc said that this issue of the “tertiary datum” / datum precedence/ over constraining/ overriding, is one of the most complex GD&T problems. Also, I do not want to take his words out of the context either.

I wish I have that knowledge and abilities to keep this discussion alive (I do have it to a certain point until I reach my level of incompetence), but looks like you do have that knowledge and willingness to move it forward and hopefully together with other people/ experts that might chime in (Evan, pmarc, just to name a few) to arrive to a satisfactory conclusion.

Anyway, enjoy.

 

[chez311]

greenimi,

First I want to say that I hope that others on this forum are not displeased with my mentioning of credits in the article to a member on this forum, I actually debated quite a bit in posting that however I ultimately decided to go for it since its all public domain and the names have been shared freely and I hoped it was in the spirit of sparking more conversation/probing into the topic since the opportunity presented itself to get close to the "original source" of a published article. I would like to refrain from taking quotes out of context or musing on statements from people not involved directly in this discussion, however as you noted I too hold the mentioned people in high regard as authorities on all topics GDnT related and any quotes would be utilized out of respect for their knowledge. That being said if anything I have stated is out of line, I will happily flag/delete my post(s) if desired.

Additionally, I do appreciate the discussion on this topic - it had actually independently of your post caused me to stumble upon the very same thread you posted, as well as another one linked in that thread where some articles were posted from Herb Voelecker on the tertiary datum problem and proposing alternate methods for defining coordinate systems/DRFs. While this is truly fascinating and complex stuff, I think its actually sort of tangentially related to my inquiry, or at least a step or two beyond what I'm asking about. I think the "tertiary datum problem" has more to do with HOW exactly secondary/tertiary datums interact to set up a DRF and constrain translation/rotation - what I am asking is, specifically in the context of composite tolerance, CAN datums in the FRTZF interact to constrain rotation at all (put another way are they ALLOWED to)? Alternately, if the answer is no - how does one rationalize treating each datum individually (or "stand-alone") in the FRTZF when I don't really see a basis for it in the rest of the standard?

If the answer to these questions is "its not well enough defined so you just have to specify it in the most clear way possible by composite datum [B-C] and/or multiple single segment with CDRF" then I can accept that, though I still believe that there is precedence that the datums would be allowed to interact. If the answer is "no, datums in the FRTZF are absolutely stand-alone and cannot interact to constrain rotational DOF" then I have a hard time swallowing that pill without concrete evidence - apologies again for being hard-headed in that regard.

 

[chez311]

greenimi,

I want to add that I hope it doesn't seem like I'm brushing off the thread/information you presented about the tertiary datum problem - I appreciate your providing it and I will certainly delve into it further, I just didn't want to derail and open up yet another proverbial can of worms. It just seems to me that the tertiary datum problem only comes into play if you are considering interaction between datums - per Bill's article this is not allowed in the FRTZF of a composite control.

 

[greenimi]

chez311,
No problem. I understand your point. Not sure I agree with it, however.

So far I am leaning toward Bill's explanation (a stand-alone datum feature and “can-may-must” rule which rule has been developed by him and also agreed by other experts) ……
Maybe I am biased due to his credentials (copy-paste:”Tandler is currently a member of the ASME Y14.5.1 Mathematization Committee; a subject matter expert for Working Group 4 of the ASME Y14.5 standard focused on datums)".

I agree (if you are saying)...…when I am learning I am very subjective.

 

[chez311]

greenimi,

I don't have an issue with the "can-may-must" rule - I don't really see how it supports his point of view though. If anything I would say that for example in the OP that tertiary datum C working together with secondary datum B (through the process of setting up 3 mutually orthogonal planes that make up the DRF) can constrain the final rotational DOF (w) so then it must. Its the "may" portion where we diverge on whether or not it is allowed. He says it "may" not because they are not allowed to work together to which I ask simply - why..? The standard clearly states in 7.5.1.6 that the FRTZF is oriented to the 3 mutually orthogonal planes that make up the entire DRF, not any individual datum, and the datums must be considered together to create this DRF.

I saw his credentials and I certainly respect them and I have no doubt that his knowledge and grasp of the concepts is much, much more extensive than mine - which is why I am all the more mystified by this whole "stand-alone" datum concept as I just don't see where it is supported.

I realize that theres a good chance I won't get a solid answer on this, however as you noted I would love to get some other members input on it.

 

[greenimi]

If anything I would say that for example in the OP that tertiary datum C working together with secondary datum B (through the process of setting up 3 mutually orthogonal planes that make up the DRF) can constrain the final rotational DOF (w) so then it must. Its the "may" portion where we diverge on whether or not it is allowed. He says it "may" not because they are not allowed to work together to which I ask simply - why..?

chez311,
I would say that datum precedence shall not be violated even on FRTZF (not only on PLTZF).
4.11.6.1 section c.) has it there "where datum feature D is referenced as tertiary to ensure that datum precedence is not violated," and that is why a lot of people (experts) have issues with 4-16 figure too. Violation of datum precedence.

I guess you can allowed it (violation), but you have to customize, which either the OP or figure 4-16 didn't.

Me too: I would love to hear some other opinions about this subject. Evan, pmarc, Belanger, 3DDAve, pylfrm, powerhound, CH, ...just to name a few....maybe...maybe...they get out of their confort zone.

 

[axym]

Hi All,

Great thread. Wow, there's a lot to cover here.

The reference to Bill Tandler's article is interesting - I had to go back and look up what article that was. Full disclosure - Bill wrote that article and the ideas are his. All I did was provide some editorial suggestions on wording here and there.

I had the privilege of working with Bill years ago and taught his courses as a subcontractor, and consider him my mentor with regards to GD&T. One of the most important things that I learned from Bill was to listen to anyone's opinion regardless of who they are, and to not blindly trust anyone's opinion regardless of who they are. Part of this is questioning the authors of Y14.5 (or Bill himself). In the years since Bill's article on the 2009 standard was written, I had to study the gory details of composite FCF's for some Y14.5.1 development work. In the end, I came to different conclusions than Bill did.

Here's the short version.

Y14.5 states the following with regards to the FRTZF:

"If datums are specified in a lower segment, they govern the rotation of the FRTZF relative to the datums and within the boundaries established and governed by the PLTZF."

The way that this is commonly interpreted, with support from descriptions in the standard, is that each datum feature controls only rotational degrees of freedom. But I found that this interpretation doesn't work. We run into problems in applications such as the OP drawing or Fig. 7-42, with a secondary datum feature that cannot control any rotational degrees of freedom on its own. I found that there is another interpretation (also supported by some descriptions in the standard) that does work:

1. Establish the DRF for the lower segment in the usual way (each datum feature constrains the applicable translational and rotational degrees of freedom). Bill's can/may/must concept applies.
2. The tolerance zone set (FRTZF) is then allowed to translate relative to the DRF.

In other words, the way that the lower segment datums constrain rotation (and not translation) is that the zones are allowed to translate (together) relative to the DRF. There is no special "rotation only" constraint for each individual datum feature.

Y14.5 also states the following:

"In some instances the repeated datum feature references may not constrain any degrees of freedom, however, they are necessaary to maintain the identical datum reference frame, such as datum feature B in the lower segment in Fig. 7-42."

Unfortunately, this statement is not correct (in my opinion). Datum feature B does participate in the constraint, by providing a center axis that is then clocked by C. If the B reference were not there, then the DRF would be directly aligned to the centerplane of slot C.

What do you think?

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[chez311]

The reference to Bill Tandler's article is interesting - I had to go back and look up what article that was. Full disclosure - Bill wrote that article and the ideas are his. All I did was provide some editorial suggestions on wording here and there.

I'm glad you're not offended by my referencing you - I didn't mean to imply that your opinions were reflected in that article, only that perhaps you might have some extra insight having been involved at some level. Regardless I knew you would have additional insight just due to your extensive knowledge, thank you for your input! I think its always good to question even the most respected of sources as I usually come out of it with a better understanding either way, in this case it feels good to have my gut feeling validated. I hope I didn't come off as too irreverent.

1. Establish the DRF for the lower segment in the usual way (each datum feature constrains the applicable translational and rotational degrees of freedom). Bill's can/may/must concept applies.
2. The tolerance zone set (FRTZF) is then allowed to translate relative to the DRF.

I fully agree with you here, this is the exact logic I was using in my earlier posts and I think it follows much more directly from the standard and much more consistent with the rules governing datum precedence/establishment of a DRF than Bill's interpretation.

Y14.5 also states the following:

"In some instances the repeated datum feature references may not constrain any degrees of freedom, however, they are necessaary to maintain the identical datum reference frame, such as datum feature B in the lower segment in Fig. 7-42."

Unfortunately, this statement is not correct (in my opinion). Datum feature B does participate in the constraint, by providing a center axis that is then clocked by C. If the B reference were not there, then the DRF would be directly aligned to the centerplane of slot C.

Once again I am in full agreement - this bothered me as well, and now that I think about it also discredits Bill's interpretation. If datum B no longer participates in the constraint and the DRF is directly aligned to C in the FRTZF - then this is no longer the same DRF as in the PLTZF which I would think violates datum precedence! If [A|B(M)|C(M)] is called out in both the PLTZF and FRTZF then they should result in the same DRF! The lack of translational constraint only governs how the pattern moves in relation to the DRF, not what the DRF actually looks like.

 

[greenimi]

Thank you Evan for your post. If I learn something from this particular thread is what you wrote in the beginning of your post)—(Should I say that sounds like a disclaimer?)

“One of the most important things that I learned from Bill was to listen to anyone's opinion regardless of who they are, and to not blindly trust anyone's opinion regardless of who they are”


Hmmm…… This language (GD&T) becomes harder and harder…… Yes. The life is complicated.

 

[axym]

chez311,

Don't worry about the reference - it was a published document. I'm lucky to have access to Bill and his ideas. He is very well respected, especially in the CMM/metrology community, and his opinions range from insightful to genius to controversial to contrarian. Bill's writing is uniquely Bill.

I'm glad that you think that the "translating zone" concept makes sense. The descriptions in Y14.5 vary a bit from paragraph to paragraph - some suggest the translating DRF and some suggest the translating zone. But I would say most of the descriptions portray composite FCF's in terms of the lower segment DRF shifting relative to the upper segment DRF. But the details of exactly how this happens are not explained, and the figures don't show it either. They tend to zoom in on the tolerance zones without showing the actual datum features, simulators, and DRF's. So there has always been an element of mystery (for me, anyway) as to how the DRF's shift, and how the datum feature simulators would have to work in order to control only rotational degrees of freedom. It never really made sense to me. With the translating zone concept, a lot of things fall into place much better.

After talking to several people that were around during the development of Y14.5M-1994, I believe that the "shifting DRF" idea was a factor in the decision to make FRTZF's exempt from simultaneous requirements. There was the concern that if the FRTZF's were simultaneous, then the lower segment DRF's would have to shift together and effectively force all of the features to translate together (and hence become locked together in mutual location). So the sim reqt default was not applied to FRTZF's, which is quite questionable from a functional point of view (this is currently being discussed in another thread). It has the unfortunate consequence of making the FRTZF's not simultaneous with the PLTZF's. I am told that one of the original purposes of composite FCF's was that the FRTZF was a liberation within the confines of the PLTZF. Having the separate requirements, so that the zones could be oriented differently on rocking datum features, seems to defeat this purpose a bit. If the translating zone concept is applied then I don't see the need for the special rule - the FRTZF's can be simultaneous and it doesn't cause the same problem. Perhaps there were differing opinions when this section was developed, and the result is a bit of a mixture of concepts.

greenimi,

One of Bill's slogans is "the objective is to get it right, not to be right". In other words, listen to everybody and then go with the best solution regardless of where it came from. I have seen him apply this philosophy numerous times, and it has also served me very well.



Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[chez311]

Evan,

I agree, the varying descriptions in the section on composite tolerance is frustrating to say the least - as your example showed from 7.5.1 theres a single sentence which seems to suggest two different things! Personally I think your translating zone interpretation follows much better from the methods established in the standard - if you start dropping datums, treating them individually as "stand-alone" and not allowing to work together (ie: secondary axis datum creates two orthogonal planes clocked by a tertiary axis OR planar datum) like they would in any other FCF then I feel like you've got to formulate a whole new set of rules that aren't found anywhere in the standard.

I almost hesitate to bring it up because I've been in the threads where its discussed and it brings on quite a bit of debate, but I feel like its related - if a custom DRF is specified, no matter what combination of DOF is constrained, would not the actual DRF (your mutually orthogonal planes) be identical regardless of this combination of constraint? I apologize in advance if this is a can of worms you don't want to open - I'll be honest CDRF have always been a bit nebulous to me, but it makes more sense if it follows in the same vein as your "translating zone" interpretation as I think a composite FRTZF is sort of a special case of a CDRF. Perhaps there is the same set of disagreements as there is for composite tolerancing.

In regards to simultaneous requirements, I know you and I have discussed this before (

https://www.eng-tips.com/viewthread.cfm?qid=437957)

and are mostly in agreement. At best the standard is ambiguous but personally, I'm in the camp that if there is no translational constraint between the FRTZF and the datums, there shouldn't be translational/location constraint between different FRTZF's or other features with the same datum structure. The problem arises because I don't think its well defined how simultaneous requirements should be treated when position is acting somewhere between a full position tolerance and an orientation tolerance due to modification of the constrained DOF ie: basic location is preserved between features in a single FRTZF, but only orientation to the datums.

 

[axym]

chez311,

Customized DRF's are another can of worms, but I don't mind opening it. This is another example of Y14.5 introducing a new tolerancing tool without providing sufficient detail on how it works. We are told how to "customize" datum feature references by specifying which degrees of freedom each one is intended to constrain, but not how that would be accomplished when the DRF is established on an actual part. In reality, degree of freedom constraint is accomplished through the interaction of the datum feature simulators with the imperfect datum feature surfaces. These details are completely missing - they're not described or illustrated! The figures only show the final result with datums and DRF, on a perfect part.

I think you're right that there would be a similar set of disagreements as for composite FCF's. The customized DRF is just a much more obscure tool that we don't see on drawings very often, so there has been much less debate over it. I've overheard some informal discussions at Y14 meetings about whether or not composite FCF's are just a special case of customized DRF's, and whether or not composite FCF's could be phased out accordingly. There were opinions on both sides, of course.

Personally, I believe that the customized DRF's are fundamentally different than composite FCF's. If you look at Fig. 4-45 and especially 4-46, the constraint that the position zones apply is not like an orientation tolerance at all - it really is a difference in how the DRF is established. I sat down for a few hours one time and worked out a theory on what would have to happen on a real part to get the "means this" results in Fig. 4-45 and Fig. 4-46. It involved special behavior of the datum feature simulators. I would go as far as to say that the tool should have been called "customized datum feature simulators" - as you say, the resulting DRF is still an identical combination of three mutually orthogonal planes.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[greenimi]

Evan,
Should I understand that the main difference between Bill's interpretation versus yours, lies in the behavior of the tolerance zone versus how the datum features constrain degrees of freedom?

I've been always following Bill's way of thinking (arresting DOF's), but looks like I was wrong all along. Okay at least partially wrong.......

I agree what you said about "who is right" versus " what is right", but that is extremely dificult for an average user (like myslef), when some of these subjects have no consistency and no common denominators with an increased risk of becoming irrelevant for an average Joe (or greenimi).

 

[3DDave]

Evan,

The custom DRFs are from the CMM side of the D&T house. Whenever there is a DRF description that does not have a matching shipping product and no one can describe how to make one or why they would want to, it's a give-away to CMM proponents. I think Voekler took it on as an academic exercise, but I saw not a single application in what he wrote.

As far as I can tell, they are intended to solve problems no one can have. If they did, they would have written notes that told the inspector the steps and analysis required, which is something I did for developing a complex feature evaluation.

Recall how important the Quadrants were and now they are gone? Same line of reasoning.

Shorter view - if it isn't a demonstrated widespread need, there's no need to standardize it.

 

[3DDave]

Further thinking on this - what if instead of B & C as secondary and tertiary datum features they were B1 & B2 targets?
Then would B(M) be usable over B(M)-C(M}?