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

 

[greenimi]

Does the name virtual condition is correct when the hole is RFS? I am questioning the nomenclature.
I would say, as far as I can understand GD&T, the VC name does not exist when the feature is RFS.
Inner and other condition boundary names should be used---see fig 2-14/ Y14.5-2009

 

[AndrewTT]

If you have access to ASME Y14.5-2009 then look at appendix B (formulas for positional tolerancing).

MMC modifier is used quite a bit for mating features of parts. Do you need to use RFS? Also, how are you measuring the RFS position of a threaded hole?

 

[3DDave]

The surface of the feature will certainly have a virtual condition established with an RFS modifier, but the standard is concerned only with the axis/centerplane of features at RFS, which don't. The other consideration for this specific case is that the limit for determining if there could be interference requires knowing the MMC of the features involved for the cylindrical elements and is indeterminate for the conical ones. So it ends up using RFS as the modifier, but MMC as the condition, without using any bonus tolerance as would be available to the MMC modifier.

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. It's also the case that there are those who have made careers in training and consulting by insisting that adding or subtracting two numbers is the only way to determine the virtual condition and I cannot change their minds or those who took their classes.

 

[Belanger]

Really, the ASME standard wouldn't want you to use the term virtual condition in an RFS context. VC is proper only when the MMC or LMC modifiers are imposed.
I think the technical term for RFS would be worst-case boundary -- which is then subdivided into inner boundary or outer boundary.

If you have access to the Y14.5 standard, check out the figures on page 32-34.

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

 

[3DDave]

Boundary as a superset includes Virtual Condition. There was little need to create this overlapping definition, but here we are.

 

[3DDave]

As a separate consideration - what are called "Virtual Condition" and "Resultant Condition" are the limits to the volumetric intersection and volumetric union, respectively, of all allowable combinations of size and tolerance for a feature relative to a given datum reference frame, which necessarily form a worst case boundary. Since there is only one associated worst case the boundary is necessarily also constant.

 

[Belanger]

Boundary as a superset includes Virtual Condition.

Of course. But you also wrote that "the surface of the feature will certainly have a virtual condition established with an RFS modifier," which is not true.

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

 

[3DDave]

If I can calculate it and it isn't a real boundary then it is, by default, a virtual one. Just because the standard plays word games with English to label each branch except the RFS one is an oversight on their part. The distinction is important within datum references, but not for the case of fits between two parts.

 

[greenimi]

Bryan R. Fischer, Advanced Dimensional Management LLC said in one of his articles about VC:

" ASME Y14.5-2009 and prior versions defined the virtual condition of a feature as the collective effect of the nominal feature definition (specified size and shape) plus or minus the allowable form, size, orientation, and location variation as applicable. Whether the allowable variation is added or subtracted is a function of whether the toleranced feature is an external or internal feature. According to ASME Y14.5-2009, a virtual condition only exists if a feature is controlled by a geometric tolerance applied with an MMC or LMC modifier. This is an unfortunate technicality, as the concept could be and should be expanded to other cases. But, the definition is what it is.

The idea is that whichever material condition modifier is specified for the geometric tolerance, MMC or LMC, there is a virtual condition in the associated boundary, calculable by the appropriate addition or subtraction."

https://www.advanceddimensionalmanagement.com/advdm-news/resultant-condition-explained/

 

[Belanger]

Just because the standard plays word games with English to label each branch except the RFS one is an oversight on their part.

Yeah, how silly of me to answer Greenimi's question by appealing to the actual standard.

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

 

[3DDave]

I worked with Bryan before he set up his company. Of the two contract engineers brought in after Robert Nickolaisen of Dimensional Engineering Services gave a Y14.5 course to the newbies** and a refresher to the rest, Bryan was the one who was competent and I enjoyed working with him. I recall Bryan asked if I could explain why the projected tolerance calc required multiplying, by a factor of two, the projection divided by the engagement. He seemed surprised that it took about 30 seconds to do as the other contract "expert" had hemmed and hawed and did not get it on his own. I was working a large VSA effort at the time and had been looking at this sort of problem.

However, I think Bryan is incorrect that the definition of Virtual Condition requires an MMC or LMC modifier. The definition mentions that LMC or MMC is used to determine the virtual condition, not that the modifier is required. LMC and MMC are defined independently of any modifier. This is separate from virtual condition applied at LMB and MMB when used as a datum feature.

I do enjoy the part of his article that form errors could influence the calculation.

** The company had gone and hired a bunch of new-grads and then got a contract to review all the dimensioning and tolerancing on product from a sister division for stupid reasons. It was work that needed to be done, but the manufacturing arm was very far from being on board and most of the work was flushed down the toilet. Worse, the key to doing a good job of this is matching the schemes and tolerance allocation to the manufacturing process and manufacturing refused to release their inspection results. Which explained the use of angle grinders on machined parts at final assembly. Sigh.

 

[pmarc]

dthom0425,
It is hard to answer your question without really knowing how the assembly look like. Does the position callout for the true hole use mating surface as primary datum feature reference? Does the position callout for the thread use mating surface as primary datum feature reference? Are you able to provide a rough sketch that could help to understand the assembly?

As for the discussion about terminology... I think I mentioned a few times before in this forum that in my opinion it certainly does not help that the standard uses so many different (weird) terms for the extreme boundaries of the features. And from that point of view I symphatize with 3DDave. But the undeniable fact is that the standard (rightly or not) uses terms Virtual Condition and Resultant Condition only in the context of geometric tolerances applied at MMC or LMC basis. For geometric tolerance applied at RFS basis the standard uses terms Inner Boundary and Outer Boundary.

As mentioned by J-P, one place in the document that somewhat proves that are pages 32-34. The other are figures 5-2 and 5-3 showing application of DML straightness tolerance - in fig. 5-2, that is where the straightness tolerance is applied at RFS, the extreme boundary is named Outer Boundary; in fig. 5-3, that is where the straightness tolerance is applied at MMC, the extreme boundary is named Virtual Condition.

 

[3DDave]

pmarc - this is partly true. The text makes no such distinction; the figures which are only to be an aid, do. It's like there's a disconnect at the committee level as to what they intended to express.

It even trips experts up as the RFS related figures use the terms "Maximum Outer Boundary" and "Minimum Inner Boundary," neither of which is defined or appear in any text. Just ad hoc creations of the figure creator.

 

[pmarc]

pmarc - this is partly true. The text makes no such distinction; the figures which are only to be an aid, do. It's like there's a disconnect at the committee level as to what they intended to express.

In figures 2-12 through 2-17 (pages 32-34), there is a listing of paragraphs in the text that refer to each illustration.

In figures 2-12, 2-13, 2-15 and 2-16 (showing position callouts at MMC or LMC) the referenced paragraphs are: 1.3.2, 1.3.5, 1.3.51, 1.3.67, 2.11.

In figures 2-14 and 2-17 (showing position callouts at RFS) the referenced paragraphs are: 1.3.2, 1.3.5, 2.11. The two paragraphs missing are 1.3.51 and 1.3.67 - these are exactly the definitions of Virtual Condition and Resultant Condition. Do you think it is a coincidence?**

** Of course here you may argue that the note in para 1.1.4 says that: "To assist the users of this Standard, a listing of the paragraph(s) that refer to an illustration appears in the lower right-hand corner of each figure. This listing may not be all-inclusive. The absence of a listing is not a reason to assume inapplicability.".

 

[3DDave]

pmarc - that's not the point. The point is the illustrations are not reflective of the text. They could easily have said something along the lines of "virtual condition applies solely when a feature tolerance FCF uses an MMC or LMC symbol and does not apply to any RFS condition."

Which is weird as there is so very much emphasis on precisely parroting back the exact verbiage in such cases; at least it was on the certification test examples questions.

It doesn't explain why the figures add their own, undefined, terminology when the text would appear to not require it. How is 'worst case' not maximum or minimum?

I don't see it as coincidence as much as a failure to include what had to be significant amounts of intra-committee discussion into the text. It's not a failure to trace back - it's a glaring failure to provide basis to support creating the figures from the text. A small number of people 'know' what it's supposed to mean, fail to write it down, and then try to paper over it with examples that are not exhaustive.

If a feature location/orientation is defined using an RFS tolerance modifier can it be used as a datum feature referenced at MMB? And isn't that MMB the virtual condition of that feature?

 

[greenimi]

I have to apologize by bringing Bryan Fischer’s article in this discussion. Reading this thread and, specially, the parallel one (FRTZF Top Calculating) I realize that the article just increased the animosity between the two most respected and knowledgeable MVP’s members of this forum. We need you both here for your wealthy knowledge and willingness to share.

Again, I am sorry for stirring up the pot.

 

[CheckerHater]

Could we see it as merely matter of terminology?
It's pretty clear that Y14.5 is avoiding using terms "virtual condition" and "resultant condition" in context of RFS.
There are terminology arguments that are going from beginning of time - that bonus tolerance is not really "bonus", that datum shift is not really "shift", etc.
Could we at least agree that having bad rules is better than having no rules at all?

"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future

 

[3DDave]

Hi CH - Bad rules being better is only the case when there is a clear process to manage improvement to the rules. The problem has been the continuing addition of new bad rules which make the parrot test takers happy and forms a stumbling block for everyone else. At least with disagreement the flaws are exposed and the potential exists for correcting them.

I notice a distinct quiet over "If a feature location/orientation is defined using an RFS tolerance modifier can it be used as a datum feature referenced at MMB? And isn't that MMB the virtual condition of that feature?" A clear distinction makes such things impossible.

I fear the answer will be a new level of terminology for describing features used as datum references when the defining modifiers don't match the datum reference modifiers. MMC->LMB, MMC->RFS, RFS->MMB, RFS->LMB, LMC->MMB, LMC->RFS; and both material inside the nominal part and outside, so twelve more cases.

 

[pmarc]

They do not have the problem with naming a boundaries created by a datum feature controlled with a geometric tolerance at RFS or MMC or LMC and then referenced at MMB or LMB because they simply call them Maximum Material Boundary or Least Material Boundary.

That is one of the smartest things they did in this area.