Profile to replace Position/Size

[[Br...]]

It's fully explained in 7.3.3.1. I got to tell ya, over the weekend I talked to some guys who work for Tec Ease who know many of the Y14.5 committee members about this just to know if there was any controversy over this subject. There's not. You would be the lone ranger on this one who believes that RFS can be explained by a surface interpretation. The two guys I quoted earlier are guys who have been on the committee for over 20 years and were pivotal in the implementation of these concepts. If you ever get a chance to get some formal GD&T training, you should ask this question. Those guys can probably explain it to you much better than I can. To me it's pretty evident by the text and I don't know how else to explain it to you.

[[Ja...]]

I found the solution to your arguments. Aaron, read Y14.5.1M Section 5.1.1 carefully.

"A position tolerance may be explained either in terms of the surface of the actual feature or in terms of size and the resolved geometry (center point, axis, or center plane) of the applicable (mating or minimum material) actual envelope."

This is clarifying that we have axis/surface interpretations. Read just a little further in the same paragraph.

"For MMC and LMC callouts, these explanations are not equivalent."

Therefore, technically speaking, Aaron is correct in that we can discuss the surface interpretation of a feature at RFS.
Brett is also correct in that only the axis interpretation at RFS makes any sense.

But I think you both are halfway there in that they are actually the same interpretation in the case of RFS.

[[Aa...]]

Jacob, thanks for your attempts at reconciliation of this disagreement, but my experience tells me they are also not equivalent for RFS.

7.3.3.1 doesn't fully explain SI, it only explains position at MMC in terms of SI. In the same way, 6.4.5 doesn't fully explain it. it merely explains Orientation at MMC interms of SI. They say "may be explained" and "may not be equivalent", but they don't go so far as to say they are unique in that way.

Brett, thanks for your input on this discussion. I know I'm not the lone ranger on my position, and I have heard this from others who are well connected to committee members, as well. At this point I simply wish to concede that it's not as straight-forward as I once thought, and that my post that sparked this controversy was not helpful.

[[Me...]]

I think any in-depth discussion on GD&T is helpful. I love reading positions from people smarter than me.

I know there was a time when Mark Foster would peruse this forum. I'd love to get his take on it.

[[Ja...]]

7.2

"Position is the location of one or more features of size
relative to one another or to one or more datums. A positional
tolerance defines either of the following:
(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
(b) (where specified on an MMC or LMC basis) a
boundary, defined as the virtual condition, located at the
true (theoretically exact) position, that may not be violated
by the surface or surfaces of the considered feature
of size."

[[Aa...]]

Please sign in to view this quote.

Interesting. That's exactly the same as 5.2 in Y14.5M-1994, which was released ahead of Y14.5.1M-1994. Yet, in creating Y14.5.1M-1994, the 5.1 subcommittee went to the trouble of describing how to evaluate surface interpretation for RFS position tolerances.

Like I said, it's not as straight-forward as I once thought.

[[Br...]]

ASME Y14.5-2018 5.9.2 further clears this up.

Please sign in to view this quote.

[[Aa...]]

You're right. That does clear it up. I think that is unfortunate, because the use of SI at RFS, though it apparently doesn't jibe with the thinking of a lot of the current GD&T instructors or last year's committee, can solve a lot of orientation stability issues.

I don't have a copy of 2018. Did they give any justification for that stance?

[[Aa...]]

Also, do they still reference ASME Y14.5.1? 1994 is pretty old, and I would hope they're nearing a re-write of it.

[[Ja...]]

Please sign in to view this quote.

Aaron,

I spoke with Rob Jensen, chair of 14.5.1. He said the next rev has been approved for publication, and he's expecting it to be published before the end of the year. Hopefully we will see some resolution between this next rev and Y14.5.-2018.

Y14.5 - 2018 still references Y14.5.1M "for a mathematical explanation..." but does not specify a unique revision.

[[Br...]]

It's not even listed as one of the principle changes and improvements. I'm telling you this has always been the way it was to be understood.

[[Aa...]]

If that is so, then why would the Y14.5.1 subcomittee have spent the time defining the approach to calculating surface interpretation results for RFS? To me, this seems like a case of amnesia by committee.

[[Br...]]

I don't know without hearing the actual committee meetings. Because they can I guess. It's important to understand that Y14.5.1 is not what defines the tolerances. Y14.5 does all of this. Y14.5.1 is just a mathematical explanation. When Y14.5 the says that position at RFS requires the axis to to fall within a cylindrical tolerance zone, and that MMC/LMC have two options but one takes precedence over the other--that's the definition, no matter how you can mathematically explain it if it were different.

[[Aa...]]

You are casting Y14.5.1 as merely a meaningless mathematical exercise. The Y14 committee created the Y14.5.1 subcommittee in order to create "explicit definitions" that "add mathematical rigor" to the Y14.5 standard.

Y14.5.1 says it "presents a mathematical definition of geometrical dimensioning and tolerancing consistent with the principles and practices of Y14.5M-1994." Y14.5.1 is intended to be not a different approach or a side show, but a rigorous explanation of the concepts in Y14.5.

But set Y14.5.1 aside.

"In certain cases of surface deviation of the feature, the tolerance in terms of the feature axis or feature center plane may not be exactly equivalent to the tolerance in terms of the surface limited by a boundary. In such cases, the surface interpretation shall take precedence. "

I've always thought this wording was a little bit odd. The statement is that if the tolerances are not exactly equivalent, surface interpretation takes precedence. Axis Interpretation takes precedence when it there's no difference? Ignoring the question of RFS, if you're setting up/selecting evaluation algorithms for MMC/LMC you could:

1. Assume it doesn't matter, and simply stick to axis interpretation, because it's simpler and faster to calculate.
2. Calculate both. If they are the same report the result from axis interpretation. If not, report the result from Surface Interpretation.
3. Use Surface Interpretation, knowing you'll end up at the same place as 2, but without having to calculate the result in Axis Interpretation.


But then, that statement does not say this is limited to MMC or LMC. This note precedes the definitions of RFS, MMC, and LMC. (In 1994, this note only appears in a section covering MMC position tolerances. Did that mean it was only applicable for positional tolerances at MMC?)

[[Br...]]

Please sign in to view this quote.

Well that's a strawman. I didn't say that it was meaningless. I said it doesn't define dimensioning and toleranceing-or in this case the Position tolerance zone at RFS. It mathematically defines the principles that have already been established in Y14.5 so you can produce an actual value. It even says Y14.5 should be consulted for practices related to dimensioning and tolerancing.

Please sign in to view this quote.

No need to overcomplicate this. Its a pretty simple statement. One takes precedence over the other. This would only apply to those tolerances that have the option between the two interpretations. So it applies to Position tolerances AND Orientation tolerances at MMC. It doesn't apply to concentricity tolerances, or symmetry tolerances even though it doesn't mention them.

Please sign in to view this quote.

When It explains in the section on how to interpret a positional tolerance at RFS; that it is required for the axis to be within a tolerance zone, then, in the section related to Position at MMC you have 2 options.... I just don't see how you can read this any other way. I find it quite clear. If RFS is required to be one way, then it must not be the other way too.

[[Aa...]]

Strawman...fair enough.

Is this a fairer assessment of our differences?

You read 2.8 Note 2 as sometimes overriding 2.8.1, but 7.3.2 as overriding 2.8 Note 2.

I read 2.8 Note 2 as overriding both 2.8.1 and 7.3.2.

[[Br...]]

Please sign in to view this quote.

Note 2? Note 2 just states what tolerances are always RFS. Maybe you meant note 1 since that was what you just quoted in your last comment. In that regard I don't think it overrides anything. I think its completely consistent with the rest of the standard. It just says that Surface interpretation takes precedence. 2.8.1 just explains what RFS means, namely that the specified tolerance is independent of the feature size. Then 7.3.2 explains the relationship of RFS to the Position tolerance which includes the requirement that the axis fall within the tolerance zone (a.k.a. axis interpretation). Only when we arrive at 7.3.3.1, do we find our friend "Surface Interpretation" under the explanation of Position at MMC. Now when we remember what we read back in 2.8, we know that our surface interpretation takes precedence over the two ways 7.3.3.1 says Position at MMC can be explained.

7.2 effectively states the two interpretations explaining the tolerance zone. (a) is Axis interpretation, and (b) is surface interpretation. Notice the exclusive statement that (b) makes referring to MMC or LMC.

"(b) (where specified on an MMC or LMC basis) a
boundary, defined as the virtual condition, located at the
true (theoretically exact) position, that may not be violated
by the surface or surfaces of the considered feature
of size."

[[Aa...]]

Please sign in to view this quote.

Oops! you're right.

Please sign in to view this quote.

Not even the axis interpretation explanations in 2.8.2 and 2.8.4?

[[Br...]]

Please sign in to view this quote.

Ok I see what you mean there. But that's specifically the point that 2.8 note 1 is making. It says the following paragraphs (which include 2.8.2) describe principles based on axis interpretation. But to be clear, it also explains that surface interpretation takes precedence. So its informing you how it's explaining the effects of each modifier in terms of axis interpretation, with a disclaimer about surface interpretation. They made it very clear to avoid confusion since they would be talking about MMC and LMC only in terms of axis interpretation. Just imagine the discussion we would be having if that note wasn't there. We might be saying, "Why does 2.8.2 describe MMC as applying to the axis but 7.3.3.1 says the Surface interpretation takes precedence!"

[[Aa...]]

Please sign in to view this quote.

(including RFS)

Please sign in to view this quote.

(and RFS)

Please sign in to view this quote.

[[Br...]]

Please sign in to view this quote.

Yes they make the blanket statement that all the following paragraphs describe an axis interpretation

Please sign in to view this quote.

No, since RFS can't be explained in terms of surface interpretation according to this standard. The disclaimer would only apply to tolerances that could be interpreted both ways. You have completely missed the point of the note. It blows my mind a little bit that after all the evidence to the contrary you're still trying to argue this. I already mentioned that those are two separate clauses earlier. now I'm just repeating myself.

[[Ja...]]

2.8 is early on in the standard, no Position has been discussed yet. It is explaining modifiers in general and their application to features/datums. Note (1) states that the following paragraphs (2.8.1 - 2.8.5) are assuming an axis interpretation (really irrelevant as we are just learning what the modifiers mean, and again, no Position is being discussed). Note (1) also gives a reason for why it bothered to mention the interpretation, that is because there can be a difference. It directs you to 7.3.3.1 & Fig 7-6 for more information regarding interpretations. But for now we are still in 2.8. It is simply explaining how the modifiers work. No implication of which modifiers are associated with which interpretation, just general statements. You can read into it all day, but there are no words like "X modifier can/can't be considered under Y and/or Z interpretation".

Now we move over to Position (7.2). 7.2 has two sections (a) and (b). It is apparent to me that (a) describes an axis interpretation, and (b) describes a surface interpretation. Subsection (b) has an exclusive note that it is valid "where specified on an MMC or LMC basis". This appears to say, not even imply, that the surface interpretation is not what the positional tolerance defines at RFS, only at MMC/LMC.

7.3.2 - RFS as related to Positional Tolerancing : Absolutely no mention of surface interpretation is made, but the section states that RFS requires the axis of the feature to be located....

7.3.3.1 - MMC as related to Positional Tolerancing : We see, again, what was mentioned back in 2.8 Note (1), namely that if the interpretations lead to different tolerance values, the value obtained through the surface interpretation should be used. We did not see this under RFS. Further, this section is divided into discussions of each interpretation whereas RFS just mentioned the axis.

It seems quite clear that Brett's mode of thinking more closely adheres to Y14.5M - 2009. Let's not forget what he pulled from the 2018 revision, namely a statement that RFS cannot use surface interpretation.
Also, read 2.11 Boundary Conditions - RFS -> ... guaranteed control of the center point, feature axis, or feature center plane.



Regarding 14.5.1. I think that we can consider a surface interpretation at RFS as was done between Brett and I at the beginning of this entire post. I think that we can even calculate an associated tolerance as is demonstrated in the standard.
That being said, Y14.5, which is the direct standard that governs my drawings, seems to define RFS as applying only through an axis interpretation.

[[Aa...]]

If the argument is that RFS surface interpretation is impossible because it is not explained, I would argue that it is very thoroughly explained in Y14.5.1, and 1.1 of Y14.5-2009 says to go there for explanations.

[[Br...]]

Please sign in to view this quote.

I'm just using the language from the standard. Through Y14.5, it cannot be explained that way. Outside of Y14.5 we can have all sorts of philosophical explanations of all sorts of things, including arguing what the standard ought to include. If you think Y14.5 should allow a surface interpretation, that's perfectly fine, and I think you probably have some good arguments too.

[[Aa...]]

Please sign in to view this quote.

The Y14.5 doen't reference our philosophical explanations...

But I think we have reached the point where we know why we disagree, and we know that we're not going to change each other's minds (or in my case, the 2018 standard).