Virtual condition 1

[3DDave]

Suppose there is a basic brick shape - Datum feature A uses the largest face, datum feature B uses the next largest face, and datum feature C uses one of the smallest faces.

In the same face identified as datum feature A is a hole through the part of diameter E, positioned at MMB with [A|B|C] as the DRF with a tolerance of dia. X. Assume the basic dimension to B is b1, and the basic dimension to C is c1.

Now the face opposite of C is identified as datum feature D and is toleranced with a profile tolerance to [A|B|C] with a zone width of Y. Assume the basic dimension from C to D is d1.

The desire is to make a bracket sharply bent of a rectangular piece that mates to datum feature A, will have one edge coincident to datum feature B plane, and hook around the end of the part to mate with datum feature D. A bolt will pass through both parts.

What is the virtual size of that hole in the [A|B|D] DRF?

 

[Burunduk]

It's been a while since I really thought about this issue, but it seems that there are deep issues with regards to surface interpretation versus resolved geometry interpretation when a straightness tolerance is present. I think that 3DDave is right that this is an unrealizable geometry question.

I'm not sure I follow regarding the "deep issues". From functional standpoint, applying position at MMC for a hole in conjunction with overriding rule #1 is a way for a designer to say "I do realize this part has a tendency to deform and I don't require perfect form at MMC for the hole, but I do require that an empty space boundary of a size that allows a fastener to pass through is maintained at the basic location and orientation of the hole". Not sure what is unrealizable.

 

[greenimi]

I always thought of MMC with Independency (or similar overriding of rule #1) as similar to the default behavior of MMC controls in ISO - the feature cannot violate the calculated MMVC and limits of size, otherwise form is uncontrolled.

Per pmarc:
"But this also means that addition of a DML straightness at MMC requirement to a combination size and position at MMC callout on an ASME-based drawing, as suggested by chez311, doesn't produce the same set of requirements as a combination of size (with default lack of envelope requirement) and position at MMC in ISO. It's because in ISO default interpretation of a linear size callout is two-point local size only, therefore there is no reason why a cylinder with all actual two-point sizes equal to MMC couldn't still have a certain amount of circularity/roundness error, like a tri-lobe shape."

https://www.eng-tips.com/viewthread.cfm?qid=454109

 

[chez311]

greenimi,

Note I said "similar" - I was of course involved in that discussion and I realize the implications of 2 point measurement of size vs. swept spheres, the most obvious being that 2 point measurement will accept a tri-lobular shape that swept spheres will reject.

I thought pmarc had responded in the affirmative that for MMC under both ISO and ASME establish the same MMVC or VC that cannot be violated regardless if the envelope principle/rule #1 is invoked. I realized after re-reading it that the question was left open.

*Edit: If I'm going to read between the lines though, it would seem that the response assumes that regardless of whether rule#1/envelope principle is in effect both establish a VC/MMVC which cannot be violated. Otherwise the most obvious comparison between the two would be that ASME MMC with a straightness tolerance does not produce a virtual condition. I don't want to read too far into it and put words in anyones mouth though, apologies to pmarc if I have interpreted this incorrectly.

 

[3DDave]

chez,

As far as I am concerned, virtual condition is what the feature appears to be like within the allowed limits such that the intersection, for internal volumes, and union, for external volumes, of all possible variations is considered as a single volume. In other words, the virtual condition acts as if the feature was all the variations combined and it does so within the defined datum reference frame, if existent, and it is likely unique within that drf.

If it's the case that that combined feature is non-existent then there is no virtual condition to be had.

If the hole feature ends up in any manner of curlicue shapes such that the intersection of them is not a hole, then that not-a-hole cannot have a virtual condition. Being free of Rule #1 allows such curlicues in the absence of any other form controls.

 

[Burunduk]

When derived median line straightness at MMC and position at MMC are applied on a hole, two virtual conditions exist, as each control has its own virtual condition. The straightness virtual condition is the minimum size limit diameter minus the straightness tolerance value. This virtual condition boundary is not defined in orientation and location. The other virtual condition is the minimum size limit diameter minus the position tolerance value. This boundary is basically constrained in location and orientation to the datum reference frame per the datums referenced in the position control. Since the standard instructs us that the derived median line straightness tolerance must be smaller than the position tolerance, the virtual condition boundary for the straightness is larger than the position VC. Otherwise, some of the straightness tolerance would not be usable, as it could allow enough form error for the surface of the hole to violate the position VC.

When an independency symbol is applied to the size dimension of the hole in conjunction with position at MMC, only the position tolerance has a virtual condition. The hole can be verified for not violating this virtual condition even though there is no built-in form control within the size dimension and tolerance specification. Effectively this virtual condition will also limit the form error of the surface of the hole (in a DRF-constrained manner), with the absence of other, more direct limitations on the form of the surface.

 

[greenimi]

When an independency symbol is applied to the size dimension of the hole in conjunction with position at MMC, only the position tolerance has a virtual condition.

That's what I am inquiring about.

Does the hole at MMC have a VC when I (edit: clarification Independency) is applied?

Looks like my question is not too far away in the weeds, but I still ended up in the swamp.

 

[3DDave]

No. Because "only the position tolerance has a virtual condition" makes no sense and it is not allowed to back drive as a form control in any case.

For a hole, position tolerance subtracts from the available MMC; in the case of Independency, that available amount is zero.

 

[Burunduk]

greenimi:
Let's focus on the basics.
This is a quote from the 2009 standard with my own interpretation based clarifications added in brackets:

"1.3.67 Virtual Condition
virtual condition: a constant boundary generated by the collective effects of a considered feature of the size’s specified MMC or LMC (in a feature control frame of a geometric tolerance) and the geometric tolerance (value) for that material condition."

Even when the independency symbol is applied, where it is done in conjunction with an MMC position tolerance:
Is there a geometric tolerance that specifies a tolerance at MMC? check.
Is there a geometric tolerance value that applies to the MMC condition? check.
Is there a virtual condition? You should know the answer.

If you are not convinced yet, look at this another way:

No independency symbol, but a cylindrical feature is specified with derived median line straightness at MMC. We know Rule #1 does not apply. However, the straightness of a derived median line (and size limits) tolerance still produces what is called a virtual condition. If virtual condition would not exist where rule #1 does not apply, we would be forced to say that derived median line tolerance at MMC has no virtual condition. Would that be true?

 

[3DDave]

Setting a straightness limit <> having no straightness limit.

Would that be true?

If that is true then any argument derived from setting a straightness limit does not apply to having no straightness limit.

 

[Burunduk]

With a straightness limit or without it, when rule #1 does not apply a virtual condition can still exist.
All which is needed for the full definition of a virtual condition is having the MMC size limit specified and a geometric tolerance that applies at MMC.
One could always look it up in the math standard if he thinks the Y14.5 definition is not solid enough.
In the math standard it's on page 23: see table 5-2 and look up "virtual condition" in para 5.2.1 (b) Conformance.
Considering table 5-2, none of the values that define the boundary depend in any way on perfect form being required at MMC.

 

[3DDave]

Not surprising that the 1994 version of the math standard does not include any treatment of the Independency principle, whioh was not introduced until the '2009 version of the D&T standard. Neither does the 2018 draft of the math standard.

They seem to agree that there is no virtual condition interpretation for that case.

Perhaps this is familiar:
CAUTION: Without a supplementary form control, the feature form is entirely uncontrolled. See Fig. 2-7.

Now that's not "is potentially controlled by imagining a position control controls it," it is plainly "entirely uncontrolled" without a supplementary form control. Since position is not a form control the form variation can exceed the size of the feature, making the virtual condition vanish.

 

[Burunduk]

The fact that there was no circled "I" symbol at the time the 94' math standard was formed has nothing to do with how the virtual condition definition was managed in that standard. The exact same thing that's stated by the independency symbol used to be specified by a note such as PERFECT FORM AT MMC NOT REQD.

The purpose of the virtual condition boundary for a hole at MMC is to ensure there is enough space at the theoretical location of the hole for assembling whatever passes through that hole. It does it by limiting the surface from violating that boundary, and it does not filter form variations from size, orientation, and location variations of the surface elements. So yes, the rules allow the user of the standards to establish that boundary regardless of direct form controls or rule #1, and effectively it limits the form of the hole where no other direct or indirect form controls are in force.

 

[greenimi]

Burunduk,
I think that what 3DDave is saying is the fact you are missing some subtle but very significant details.
Initially, I was on the same logic / page with you, but after some heated discussions with a friend (initial inquiry is coming from his place of work) and also based on this discussion I am thinking otherwise.

Evan,I think, is alluding to the same conclusion as 3DDave. (Based on his post from March 6, 16:59).

I missed all of these subtleties entirely. I am trying to learn more that's why I am posting and "stir the pot" ......

I am sure the experts of this site (3DDave, Evan, pmarc, just to name a few) will come back and straightened me out. Certainly they are able to explain it to "a six year old one otherwise they wouldn't understand it themselves" (quote per Albert Einstein)

 

[Burunduk]

think that what 3DDave is saying is the fact you are missing some subtle but very significant details.
Initially, I was on the same logic / page with you, but after some heated discussions with a friend (initial inquiry is coming from his place of work) and also based on this discussion I am thinking otherwise.

If whatever was said in these discussions was enough to change your mind surely you can point to at least one subtle detail that I am missing (and you used to miss when you were on the same page but no longer). Please clarify.

 

[Burunduk]

Evan,I think, is alluding to the same conclusion as 3DDave. (Based on his post from March 6, 16:59).

Evan introduced a question:

So we're not allowed to specify a straightness tolerance at MMC that exceeds the position tolerance at MMC. The boundary resulting from the straightness tolerance must somehow get absorbed by the boundary for the position tolerance. Does this mean that specifying Independency, which would allow even more straightness error, would not be allowed?

In my response from 6 Mar 20 17:12, there is a suggested answer that can be read between the lines. I will spell it out: the answer is no, it should not mean that the independency symbol is not allowed in conjunction with MMC position. That is because the standard prevents the user from defining tolerances that can only be partially utilized (as I said if DML straightness MMC tolerance is larger than the position MMC tolerance, the straightness tolerance cannot be fully utilized) - preventing the specification of partially usable tolerances is typical to Y14.5 and can be observed in other places in the standard as well. Since "Independency" on its own is not a tolerance, there is no such restriction in this case. "Independency" only means that you don't use the size dimension and tolerance to limit the form variation.

 

[greenimi]

Burunduk,

but it seems that there are deep issues with regards to surface interpretation versus resolved geometry interpretation when a straightness tolerance is present. I think that 3DDave is right that this is an unrealizable geometry question.

Here is a direct quote from Evan and (in IMO) a direct admission, that made me changing my mind. (Again, it is not reading between the lines).

Basically both gentlemen (Evan and 3DDave) during this discussion told me (directly and and somehow indirectly) that I was wrong.

I wish they would come back and explain their reasoning in more details.

 

[Burunduk]

Fair enough. I just hope that while you are waiting for their clarifications, you find the time to take a look at how the virtual condition is described in the math standard, on the page I referred to in one of my posts above. Ask yourself if perfect form at MMC is involved in that definition. I guess it won't hurt as an addition to getting experts' opinions.

 

[pmarc]

In my opinion this is another "what is in the standard vs. what (according to some) should be in the standard" type of discussion.

As of today Y14.5 clearly says that the virtual condition is a boundary generated by the collective effects of feature's specified MMC or LMC and the geometric tolerance for that material condition. In other words, if only a feature of size has MMC or LMC size limit specified, a virtual condition exists, and its size is calculated using different formulas given throughout the standard, especially in the series of figures in section 2 (2009) or 5 (2018). So why even start a discussion about it? - one camp might ask.

Well, it is because there is another camp saying that the virtual condition definition and all the VC size calculations given in the standard are artificial and don't really reflect the true worst-case sizes of the boundaries that might be created by some special as-produced geometries. If for a hole controlled with a position tolerance t at MMC the virtual condition boundary is to represent the worst-case boundary that the surface of the hole shall never violate, then this another camp will say that even with Rule #1 in charge the formula VC = MMC - t isn't fully correct because for the UAME of the hole produced at MMC size with its axis fully tilted within the position tolerance the size of the worst-case boundary will be different (smaller) than the VC size obtained by the formula. So if addtionally Rule #1 is not in charge, this makes the standard VC calculations even more incorrect.

 

[Burunduk]

pmarc, the points you make are very accurate.

My first observation is that when a person asks a question in order to learn, especially if the type of question is: "Is it possible to calculate X?" Or "is the concept X valid in this and that case?", the answer should first and foremost be based on what's in the standard, not on what one thinks should be in the standard. If an opinion is expressed based on what one thinks should be in the standard, this should be stated. Otherwise, the person that asked the question may be led astray to take the other person's belief of what should be in the standard for a standard based answer, especially if he thinks that the answer is provided by an expert. This is why I hope that whoever is interested in a practically useful answer pays close attention to the part of your post that precedes the second's camp approach (although what follows is also very interesting and valuable).

My second observation is that in order to form an informed opinion on the "issues" the second camp is raising, one has to understand the function of and the intent behind the virtual condition boundary as a concept. The second camp you described sees the virtual condition boundary exclusively as an envelope derived from the allowed variations of the feature. This is somewhat understandable considering how the concept is shown in the figures in chapter 2, the formulas used for the calculation (when not looking too deep into them) and the room that the definition in para. 1.3.67 leaves for interpretation. However, being a resultant boundary is not the essence of this boundary. In fact, the "opposite" boundary which is actually called "the resultant condition boundary" is more relevant to this type of reasoning. This approach does not hold water also because of the issues you described - it does not really work well as a worst-case boundary generated by variations.

The more informed approach is seeing the virtual condition boundary as a controlling / variation limiting boundary imposed by the size and the geometric tolerance requirements which apply with material condition modifiers. This is the more useful approach given the contexts in which the virtual condition boundary is applied throughout the standard and used in the industry. As an example, one can look at para. 7.3.3.1 Explanation of Positional Tolerance at MMC:

"(a) Surface Interpretation. While maintaining the specified size limits of the feature, no element of the surface shall violate a theoretical boundary (virtual condition) located at true position. See Fig. 7-7"

So, for anyone that sees it in the context of its application, it's clear that the VC is actually a tolerance boundary set by the drawing, not a tool to analyze the generated effects of different possible variations of the feature as derived from other controls or lack of them, such as form variations being limited by rule #1 or unlimited by a rule #1 being overridden.

 

[3DDave]

CAUTION: Without a supplementary form control, the feature form is entirely uncontrolled. See Fig. 2-7.

is in the standard.

"The more informed approach " is only your opinion of what you wish the standard said to support yet another argument.