Virtual Condition Check at RFS 3

[dthom0425]

Hello all-

I am aware of how to perform virtual condition checks for MMC and LMC but I am wondering how does one verify (what is the math?) that the RFS true position applied to a hole will work in a mating condition.

Example:

C'sink with .150" thru hole @ (dia) True position .010" RFS.
mates to
#6 thread (.138) @ (dia) True position .028" RFS

How would you verify through math that your true position tolerancing is OK at RFS?

Thanks

 

[3DDave]

pmarc - they never treated the case, either in the text or in any diagram. The original question goes unanswered.

 

[pmarc]

pmarc - they never treated the case, either in the text or in any diagram.

Not sure what you mean by that. Could you clarify?

 

[3DDave]

After I see an answer to the question.

 

[pmarc]

I thought I answered your question. You said I didn't. So I kindly asked you for clarification. If you don't want to clarify, then so be it. It is your choice.

 

[chez311]

3DDave,

By the case do you mean any case where an RFS feature is referenced at MMB/LMB and then call maximum or minimum material boundary? Because it is shown in Fig 4-20 (c).

 

[pmarc]

By the case do you mean any case where an RFS feature is referenced at MMB/LMB and then call maximum or minimum material boundary? Because it is shown in Fig 4-20 (c).

That is exactly why I wanted to make sure I understood 3DDave's statement correctly.

 

[3DDave]

pmarc - I see where I have missed out. MMB/LMB/RMB do not refer to any feature at all. Only to the identically sized datum feature simulator. The calculations for all these cases, Virtual Condition, Outer Boundary, and MMB are identical boundaries, with identical dimensions, but they have multiple names which depend on context.

Figure 4-20 is, I believe, copied largely from figure 4-18 in the '1994 version, except they eliminated one datum precedence condition and (if I recall correctly) added back the perpendicularity tolerance that was in the '1982 version that should certainly have been in the '1994 version. Someone with a handy copy of '1982 could look that up.

In any case, '2009 Figure 4-20 doesn't refer to MMB in both cases; just "datum feature simulator." It only mentions maximum material boundary in relation to the MMC callout version of the feature. So maybe MMB does not apply to an RFS feature callout.

(edit: I believe I have been mislead by my memory of the original '1982 figure - with B perpendicular to A. The '2009 version reversed this, hence the reason they eliminated the one case that would not make as much sense, to correct their oversight in the '1994 version.)

The '1994 version referred to the True Geometric Counterpart for the RFS reference and the (wait for it) virtual condition for the MMC reference.

(edit: notwithstanding, there are three calculations for exactly the same thing.)

 

[pmarc]

In any case, '2009 Figure 4-20 doesn't refer to MMB in both cases; just "datum feature simulator." It only mentions maximum material boundary in relation to the MMC callout version of the feature. So maybe MMB does not apply to an RFS feature callout.

Figure 4-20(c) mentions maximum material boundary in relation to the MMB version of the callout applied to the pattern of 4 holes. But that version, as well as version (b), still shows RFS version of the callout applied to datum feature A. That was the point of your question, wasn't it?

Figure 4-20(b) could have shown datum feature simulator A at MMB, but that would be very special case and it wouldn't definitely help in grasping the concept of difference between secondary datum feature referenced RMB vs. MMB.

EDIT: You edited your post while I was writing mine, so my apologies if this reply does not apply any more.

 

[pmarc]

3DDave,

Can I still have a question? In your very first reply in this thread you said:

I've been meaning to write up an article about virtual condition, but keep putting it off. Mostly it's a case that anything besides a trivial case is significantly complex to calculate - more than just adding or subtracting two numbers.

This truly isn't to argue with you. I would really like to read such an article, because I see the point in the comment you made. Is there any chance that your intention could turn into reality?

 

[3DDave]

pmarc - We're good. My frustration has more to do with the inconsistency within the standard and between versions of the standard, essentially making it a lose-lose vocabulary test. Between the text and the figures the committee throws in so many faults it's tough to keep each straight.

I believe we discussed part of virtual condition before on the subject of figure 4-16(c) where I believe that the MMB of D is not a diameter because of the change in the way the feature is referenced. My last question on that was (approximately) what would the MMB of datum feature D be if it was the secondary and what would the MMB of B be if it was the tertiary datum reference. Obviously the position tolerance on D cannot vanish just because the order is changed, so where does it go? I have a way to answer that - the standard does not.

You had stuck with the 4-16(c) solution, as did Meadows, and the process to step through how I see that as an incorrect evaluation just seemed too long. Even if I did succeed, there's not a chance the committee would agree to make a change and they have people who should know my basis based on the transformation of geometry as it is converted from one frame of reference into another one.

 

[greenimi]

Figure 4-20(b) could have shown datum feature simulator A at MMB, but that would be very special case and it wouldn't definitely help in grasping the concept of difference between secondary datum feature referenced RMB vs. MMB.

Pmarc,
Could you, please, clarify this statement. I am missing on why if datum feature simulator A (secondary) is shown at RMB you say that "could have shown" in the same way as it is depicted in 4-20(c)---with a gap/ not contacting the part. I am sure I am missing something, not sure what.

 

[pmarc]

greenimi,
All I meant by that is that the datum feature A simulator in figure 4-20(b) starts its contraction from MMB size, so technically they could show datum feature A produced at MMB (then there would be no loose between the feature and simulator) and this condition would still match the "smallest circumscribed cylinder ..." description. But that definitely wouldn't help anyone to see the difference between A referenced RMB vs. MMB.

I sometimes think I should refrain from making this kind of comments, as they apparently create nothing but a confusion.


3DDave,
Yes, I remember well our discussion about virtual condition size and shape in fig. 4-16(c). And I admit that it made me thinking. Since then I have had a chance to work on different things where this topic came up a few times. Like I said, I see a point in what you say (although I personally wouldn't really spend too much time on the scenario where D at MMB is secondary and B at MMB is tertiary. First of all, B would not add any value to the position callout for dia. 3.5 hole. Secondly, even if B tertiary made sense, it would look very weird to have D controlled with position to B first, and then D defined more important than B in the position callout for dia. 3.5 hole).

 

[greenimi]

I sometimes think I should refrain ......

No, No, Don’t do just that!

I (and maybe some other people too) do not know how much I don’t know. And you, by making “this kind of comments” (to paraphrase your post above) are helping me tremendously realizing and discovering my weaknesses. So, by coming here, and posting on the forum you help. But, probably, you don’t understand or grasp how much. I can tell you, from my perspective… A LOT!! Better than any Fundamentals/ Basic/ Advanced GDT classes combined.

Sure, people must have some knowledge—general one—to understand the concepts, but the fact this is an open forum and it is not structured as a “class setting”= the mentor has to cover the training material in the book, helps to clarify the concepts otherwise have missed or not covered in the class.

 

[AndrewTT]

Do those worried about their MMB being interpreted differently than their intent use the technique described in 4.11.6.3 (clarifying applicable MMB)?

If I were to make a drawing like figure 4-20(c) I would consider employing this technique to avoid confusion.

 

[chez311]

MMB/LMB/RMB do not refer to any feature at all. Only to the identically sized datum feature simulator. The calculations for all these cases, Virtual Condition, Outer Boundary, and MMB are identical boundaries, with identical dimensions, but they have multiple names which depend on context.

3DDave,

I fully agree, I sort of came to a similar conclusion as I read through this thread. While MMC/LMC/RFS describe and define the actual feature and its limits, MMB/LMB/RMB define how a datum feature simulator should look like and how it interacts with the feature. It seems that in their quest to more clearly define different conditions with a multitude of different terms it has created quite a bit of confusion.

I would also like to say that I would like to echo pmarc - if you ever decide to publish that paper/article on virtual condition I would be extremely interested in reading it and learn about the nuances of the topic.

 

[pmarc]

Do those worried about their MMB being interpreted differently than their intent use the technique described in 4.11.6.3 (clarifying applicable MMB)?

If I were to make a drawing like figure 4-20(c) I would consider employing this technique to avoid confusion.

The thing is that it is not just a matter of size of the MMB. It is also about its shape. The technique described in 4.11.6.3 does not provide tools to define MMB shape different than the shape of the datum feature.

 

[3DDave]

pmarc - the interest in re-ordering isn't to deal with the hole; it is to examine how to evaluate the way mating parts will interact. Perhaps "B" is fit over a shaft with a shoulder against "A" and "D" will provide a pivot for a lever that interacts with that shaft. It is useful to know how "B" varies in the reversed context in order to design that lever because "B" is a stand-in for the shaft at the part level.

I figure such a thing could be part of a transmission shift linkage.

Using the more conventional definition of "virtual," such as

virtual. adjective. having the essence or effect but not the appearance or form of

I like to think that, for engineering purposes, it boils down to - if that's what you want, that's what you should accept, but it ends in push back on the "what you should accept" part. Equally, there's no requirement to accept more than the minimum , a minimum I think is defined by a virtual condition.

 

[greenimi]

Pmarc, 3DDave, Chez311 and all,

Somehow loose related with this discussed subject of fig 4-20 (c), I would like to ask you guys: what do you think about fig 4-21 (c) (again not 4-20 (c)) ?

In fig 4-21 (c) doesn’t DOES look like B datum feature is BEING primary? At least in the picture it is shown as acting like primary, doesn’t it?
In other words, primary datum feature A, (which is called out at MMB in the FCF and is simulated at MMB) acts as secondary if the actual size/ actual part is somehow manufactured at LMC. So, A primary (again called at MMB in FCF) has acting as secondary?

I am also thinking about another interesting question posted on linkedin, where it is stated that A-B (multiple datum feature primary) it is allowed in the shown configuration of if 4-21 in the ISO GPS, but it is kind of NO-NO in ASME. In other words, in ISO GPS, A-B primary (as a compound) could be used for fig 4-21, but not in ASME.

I would like to ask you guys, wouldn’t make sense to use A-B primary (multiple) (with no clear distinction between which is primary and which is secondary) if one of the mutual perpendicular datum features is modified at MMB/ LMB?

Do you understand what I am trying to ask? If not, please let me know. I am trying to find analogy/ comparison to ISO in the same time discussing 4-21 (case c) with primary shown at LMC

 

[greenimi]

Here is what ISO GPS allows on shown configuration

 

[chez311]

greenimi,

I've seen it suggested several times here that MMB actually potentially results in a violation of datum precedence, most often in conjunction with the fact that many combinations with MMB can be stated several different ways with identical results (ie: usually where MMB secondary/tertiary can be switched with no change to meaning).

In this case the simulator would make full contact with planar datum feature B (which makes it behave in the way we would think a primary datum would even though its secondary as you noted) all the way until A reaches MMC - past this point [A] then makes full contact with its simulator and B makes partial contact, shown by 4-21 (d).

Truthfully I'm not entirely sure what to make of the fact that MMB can violate datum precedence, how to solve it, or even if it needs solving - what I do know is that it can produce somewhat counter-intuitive behavior.

I would like to ask you guys, wouldn’t make sense to use A-B primary (multiple) (with no clear distinction between which is primary and which is secondary) if one of the mutual perpendicular datum features is modified at MMB/ LMB?

It seems to me the second part of your question (bolded) could be generalized to "if an RMB datum feature is referenced after/in a lower order of precedence in a DRF after an MMB/LMB datum feature?" as it seems to me this behavior would happen in this more general case too.

In regards to making it a multiple datum feature [A(M)-B] the standard doesn't seem to put any restriction on (1) a multiple datum feature primary or (2) mixing of material boundaries in a multiple datum feature. For (1) someone else will have to answer that as I am interested to know the reasons why this is a "no-no" per ASME. For (2) while there may be no restrictions on mixing boundary conditions I don't see how combining them adds any clarity to interpretation over [A(M)|B] as the result I think would be the same (again the issue of violation of datum precedence due to MMB).