Examples of Equivalent Dimensioning & Tolerancing Schemes

[pmarc]

Hi,

Throughout the years I have come to a conclusion that in general there are not many examples where changing dimensioning and tolerancing scheme from one to another would keep the geometric requirements for the system unchanged.

One example where this conclusion would not be true is changing from perpendicularity wrt A to total runout wrt A when applied to a flat face normal to datum axis A.

Another one would be a simple bushing where its ID and OD are controlled with the same +/- tolerance, and then it does not really matter which of the features will be datum feature A and which will be controlled with position or runout relative to A.

I have some more, but I would like to see what others can offer. So could anyone share some examples?

 

[Belanger]

In your example of the ID/OD of a bushing, there would indeed be a difference "with position or runout relative to A." That's because whichever diameter is the datum feature will not have its form error enter the picture (for the GD&T). But whichever diameter is tagged with runout will have its form error be part of the GD&T measurement.

My additions to your list:
1) Profile of a surface applied to a nominally flat plane, with no datum references. This would be identical to flatness.

2) Profile of a surface applied to a nominally flat plane, with a single datum reference (datum feature being the opposite surface). If the distance between the two surfaces is not basic, but plus/minus, then this would be identical to parallelism.

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

 

[pmarc]

J-P,

Thank you for your reply.

You are right about runout. I added it to the example at the very last moment and apparently did not give it enough thought.

As a matter of fact, now that I am re-reading my original post, I see that I was not specific enough in explaining the point of the example with bushing. The point was that no matter which diameter of the bushing is datum feature and which is toleranced feature, orientational and locational relationship between axes of these features will remain unchanged. I would say this is true for all combinations of position tolerance (RFS/RMB/MMC/MMB/LMC/LMB) as long as size tolerances for ID and OD are the same. If the size tolerances are different, then only the RFS/RMB case will not be affected.

I agree with your additions. However (and I really would not like to spend too much time on this), in example #1 would you say (or not) that use of profile instead of flatness is kind of similar dilemma to using position wrt A instead of perpendicularity wrt A in case of a feature size nominally perpendicular to datum A?

 

[greenimi]

Pmarc,
One I stole from you (different discussion)
Composite callout versus multi-single segment with customized datum reference frame adjusted with or without simultaneous requirement (SIM REQT or SEP REQT)

 

[Belanger]

Pmarc, I know that you don't want to spend much time on this side item, but since I'm always criticizing the use of position for perpendicular-only situations, allow me to explain this one about profile.

Profile's main job in life is to control form. Thus, in the first addition I gave to your list, profile is fulfilling its main job. It is legal to do that, even though I agree that flatness would be more proper.

However, when position is used on a single feature while referencing a single perpendicular datum, it is not fulfilling its main job in life, which is location. That's why the perpendicularity symbol would not only be more proper, but required (in my view).

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

 

[pmarc]

J-P,
I understand and do not have any objections. Not sure if you see it the same way, but I would say it is kind of a similar story to the use of angularity tolerance to control features that are parallel or perpendicular to a datum.

greenimi,
Thanks for your example. Yes, I can imagine cases where both options would result in the same geometrical requirements. But since you mentioned composite and multiple single segment callouts, I would have a follow up question to you and everyone else: Let's take figure 7-38 from Y14.5-2009 vs. modified version of it where all three composite callouts have been changed to multiple single segment callouts. Would there be any difference between these two scenarios? If yes, what exactly would it be?

 

[chez311]

greenimi,
Can you clarify what you mean by "customized" DRF? Also do you have an example of what you are referencing? By definition multiple single segment is never subject to simultaneous requirement as you can never repeat the identical DRF in subsequent control frames - it would be redundant. I also don't want to derail this thread, but I have to mention it was discussed in a previous thread (

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

) that it can be a bit ambiguous, depending on who you ask, what simultaneous requirements means in regards to composite tolerancing.

pmarc,
I would say those would be identical. Note that the related example for multiple single segment Fig. 7-46 adds datum B, otherwise it would have been a rehashing of the initial example.

 

[greenimi]

greenimi,
Thanks for your example. Yes, I can imagine cases where both options would result in the same geometrical requirements. But since you mentioned composite and multiple single segment callouts, I would have a follow up question to you and everyone else: Let's take figure 7-38 from Y14.5-2009 vs. modified version of it where all three composite callouts have been changed to multiple single segment callouts. Would there be any difference between these two scenarios? If yes, what exactly would it be?

Ref: fig 7-38
I would say that if you add SEP REQT only for the lower segment of the multi-single segment (position of Ø0.25(M) to A primary) both schemes (as shown scheme in Fig 7-38 and the adjusted multi-single segment scheme) would be virtual identical. Am I missing something?
But, again case / scenario valid only for this particular, shown, configuration where the holes are nominally perpendicular to the primary datum (A).

By definition multiple single segment is never subject to simultaneous requirement

Not sure I understand your statement. see fig 7-46. are you saying that position within Ø0.8(M) to A, B and C on the three patterns is NOT subject to simultaneous requirements? Yes or no question.

that it can be a bit ambiguous, depending on who you ask, what simultaneous requirements means in regards to composite tolerancing.

Please provide a little more details. I am really not understanding the issue. Not saying that the issue is not there, but for some reasons I am not getting it.

 

[pmarc]

chez311,

Those would not be identical. In the original scheme (unmodified fig. 7-38) by default the lower segments are not subject to simultaneous requirement rule, therefore spacing between groups of holes (group of 6, group of 4 and group of 3 holes) is controlled by the upper segments of the composite callouts. In the modified scenario, due to the fact that the tolerance zones defined by all 3 lower feature control frames become a single pattern (simultaneous requirement applies here), spacing between the 3 groups of holes is controlled by the lower FCFs.

So a follow up question #2:
If in fig. 7-38 there was only one pattern of holes shown, say the pattern of 6 holes, would there be a difference between composite and multiple single segment scenarios?

 

[chez311]

greenimi,

I have retracted my statement about multiple single segment/simultaneous requirements. I got that mixed up - I was thinking of something else. I would also agree about your statement regarding adding "SEP REQT" to make those two schemes identical.

If you read through the last few posts on that thread I linked to myself and Evan discuss why simultaneous requirement with composite tolerancing can be ambiguous or open to interpretation. I'll quote myself below:

Imagine if there were a clocking datum plane in the figures you mentioned 4-40/4-41. Then both keyseats would be constrained both in location and orientation to the center axis (datum A in the figure) and imaginary datum B (clocking datum - not shown because i made it up). The same is not so for a composite tolerance - even in your original example if A|B|C were called out in the FRTZF it only constrains rotation/orientation of the pattern - it is still free to translate. I could see it argued that if it is not constrained in translation/location to the datum reference frame then why should they be constrained in relation to each other.

Basically since composite tolerancing does not inherently constrain translation of the FRTZF I don't believe theres necessarily a requirement to constrain translation relative to other features/FRTZF's with simultaneous requirements - maybe in rotation but not translation.

 

[greenimi]

Ref:"I would say that if you add SEP REQT only for the lower segment of the multi-single segment (position of Ø0.25(M) to A primary) both schemes (as shown scheme in Fig 7-38 and the adjusted multi-single segment scheme) would be virtual identical"

pmarc,

yes, you are correct, SEP REQT will "distroy" not only the relationship between the patterns of 6, 4 and 3 holes, but also the mutual relationship between themselves.

 

[pmarc]

yes, you are correct, SEP REQT will "distroy" not only the relationship between the patterns of 6, 4 and 3 holes, but also the mutual relationship between themselves.

Not sure what you mean by that, but I am glad you agree with me .
But seriously, I would like to be sure that my last comment was understood properly, so could you clarify the quote?

 

[greenimi]

pmarc,

The referenced quote is comming from my previous comment and after your input, I found out that I was incorrect saying that Fig 7-38 and the adjusted fig 7-38 +SEP REQT would have the same meaning.

[/If in fig. 7-38 there was only one pattern of holes shown, say the pattern of 6 holes, would there be a difference between composite and multiple single segment scenarios?

Trying again. No difference if SEP REQT is added on the lower segment pos Ø0.25(M) to A

 

[pmarc]

Sorry but I am still a bit confused, greenimi.

Regarding the first question, I would say that if SEP REQT was added to all 3 lower FCFs in the multiple single segment scenario, this would have the same meaning as the original fig. 7-38. Do you, or anyone else, see it differently?

As for the second question, my answer is that in case of a single pattern there is no difference between the two tolerancing schemes. Again, this is with the assumption that there is no other feature or pattern of features on a drawing that is controlled with position wrt A.

 

[greenimi]

Regarding the first question, I would say that if SEP REQT was added to all 3 lower FCFs in the multiple single segment scenario, this would have the same meaning as the original fig. 7-38

That's exactly what I said initially, but after that I realized that I have made a mistake.

What mistake would you say?

I think because of adding SEP REQT to all 3 lower FCF’s in the multiple single segment the mutual spacing between the holes WITHIN each pattern is now controlled by the upper segment (therefore within Ø0.8) and not by the lower segment (within Ø0.25) as would have been if the original scheme (fig 7-38, composite tolerance would be kept)

Am I wrong saying that SEP REQT will “destroy” the mutual relationship between the holes in the pattern (again WITHIN the pattern, not only BETWEEN the patterns)?

As far as your second question (copy-paste:” If in fig. 7-38 there was only one pattern of holes shown, say the pattern of 6 holes, would there be a difference between composite and multiple single segment scenarios?)
Your answer was: “my answer is that in case of a single pattern there is no difference between the two tolerancing schemes. Again, this is with the assumption that there is no other feature or pattern of features on a drawing that is controlled with position wrt A.”

My comment is: You convinced me. I agree. No need for SEP REQT as I initially thought to make those two schemes identical.

My side comment: the immediate above conclusion valid ONLY for the shown example (not to be generalized for fig 7-42 for example or fig 7-51 of A is to be added in the FRTZF)

 

[chez311]

greenimi,

I don't think that separate requirements severs the connection between the holes WITHIN the pattern, only to other features/patterns on the part. If one wanted to accomplish that (ie: treat EACH hole separately), I think the note "INDIVIDUALLY" would have to be added. At least thats my understanding.

 

[pmarc]

greenimi,

I agree with chez311 that the SEP REQT note does not nullify spacing requirement for the features within the pattern. In ASME the mechanism that groups the features together is simply position symbol. So if one does not want to control spacing between features in the pattern with the lower FCF, I think the most straightforward way is to use perpendicularity symbol instead of position in that FCF.

 

[greenimi]

Thank you pmarc.

I agree with chez311 that the SEP REQT note does not nullify spacing requirement for the features within the pattern. In ASME the mechanism that groups the features together is simply position symbol. So if one does not want to control spacing between features in the pattern with the lower FCF, I think the most straightforward way is to use perpendicularity symbol instead of position in that FCF.

The time spent here has been proven valuable for me as I un-learned something I knew (and was incorrect) and re-learned (and hopefully remeember) the correct thing.
Continuous improvement, the entire essence of this forum.

Keep this thread alive.

I like to learn more about equivalent dimensioning and tolerancing schemes.

 

[Belanger]

pmarc -- if you're willing to include ISO in this discussion then other identical things would be concentricity (ISO) and position applied the same cylindrical FOS, as well as symmetry (ISO) and position on the same planar FOS.
This presumes that ACS is not added to the callout.

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

 

[pmarc]

J-P,
Yes, those would be the same. And I don't mind adding ISO to the discussion although I initially thought it would be easier for all of us to find equivalencies in ASME.

What about <CF> modifier? Would there be a way to define the same requirement without using the <CF> symbol?

Any other ideas?