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]

I did not realize ... the level of complexity of my question and I 've fallen in Evan's "swamp"

Since the standard is quite explicit as to how the VC boundary is defined and established, and the values needed for calculation, I think that dragging you into this swamp was unnecessary.

I have. Perhaps you should point out what you think is the problem with them.

The problem is not with "them" (the fundamental rules) but with the assertion that "If it was not critical it would not have a tolerance." If you truly read them, it wouldn't take long for you to realize where you are wrong (unless you detect another "committee screw up" which seems to be one of your favorite excuses).

You have said that the VC represents a clearance for a mating part. Are there any negative value mating parts in your designs or will you back fill this as having yet again meant something else?

I also said that "A negative value has the same practical meaning as zero."

May be useful? At no point did you provide an example; at best you postulated without proof that one might exist. If you claim something exists you need to provide an example in order for others to believe you. Right now, I cannot believe you.

On my part, explaining the general reasoning, which I did more than once already, is more than enough. greenimi introduced the question originally and a bit later stated that he was involved in a discussion invoked by a real case. If you don't imagine what could possibly justify Independency + MMC position, the least you could do is ask him for the application details. That is if you actually cared about anything other than being "right".

"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" describes a part subject to free state variation, not independency.

Not necessarily. Both the free state variation and Independency modifiers/specifications override rule #1.
Since by default all tolerances and dimensions apply in the free state condition, specifying the free state explicitly is mainly required when there are controls that apply in the restrained condition. The scenario I described is not necessarily a case of this type.

However, per your interpretation there is no difference if it is used or not. If the Independency allowed straightness is magically limited by the position tolerance (no direct proof in the standard,) then that is exactly the same as not using Independency at all; which means that using it alone violates those fundamental rules you should know about and it should not appear on a drawing in that capacity.

This is already proven not to be true, as I addressed it in my response to greenimi at 6 Mar 20 14:30. In greenimi's example, the difference between accepting a hole of RAME 4.75 and rejecting any hole that violates a VC if 4.8, results from the specification of independency as opposed to the default requirements.

 

[3DDave]

Another deflection.
You said it was not critical - nothing in the fundamental rules discusses "critical." You have a personal interpretation that I cannot guess at. I know what they say and what they mean. I cannot know how you interpret them. It's up to you to make that interpretation explicit for the rest of the class.

The standard is explicit for the non-Independency condition; claiming that it applies here is unsupported.

Another deflection.
You said the only reason for VC was to provide clearance and now back track and cover the error. The standard does not discuss what the negative value means. So, your interpretation again.

Another deflection.
You said you provided an example of Independency. Where is it? Don't point at greenimi and blame him.

Another deflection.
You claimed your example was of Independency. It is not. Changing the subject to describe what we all know about free state variation is a particularly clumsy argument to avoid admitting it was not an example.

Another deflection.
Your position is that straightness of the feature, even if it is explicitly not to be controlled, cannot violate the available position tolerance. This is the default without Independency and Independency does not change. It is therefore a redundant control and should be removed from the drawing.

You still haven't answered about the material condition that applies to the implied control. Actually there is a large number of cases that have been deflected from. I know you know them because you have specifically skipped them.

 

[3DDave]

pylfrm and chez,

The standard is explicit that the specified straightness tolerance shall be less than the position tolerance.

It does not say that making that straightness tolerance infinite by using Independency will force it to be limited by the position tolerance. In particular, because there is no way to know if the implied control is on an MMC, LMC, or RFS basis.

As long as the explicit straightness is less than the position tolerance then there is a clear interpretation.

Leaving it off results in the benefits that have been claimed as if it applied so, as a stand-alone callout, it is redundant.

 

[3DDave]

pylfrm - to specifically answer your question - the allowed straightness is smaller than the position tolerance so it cannot cause the virtual condition to be any smaller and would not be subtracted in that case.

 

[axym]

All,

I stand by my assertion that this topic is a swamp ;^).

I would say that what we are discussing is an extension of Y14.5's virtual condition concept, to address situations that were not envisioned. The VC concept was originally based on simple features of size, whose form error (and orientation error) is relatively small compared to the tolerance requirement for location. There is also the implicit assumption that the tolerances for form, orientation and location are relatively small compared to the absolute size and extent of the feature. This allows the combination of UAME size and UAME location to be a reasonable approximation of what happens at the surface.

The virtual condition formulas in Y14.5 are a first approximation, that work well for specifications and feature variation that fit the criteria described above. If we try to extend the scope to include cases that are not so "well behaved", then the limitations of the concept start to show. The idea of what is "correct" becomes open for debate, because these cases were not accounted for. Some examples:
-form error that is large relative to the position tolerance (causes is a significant difference between resolved geometry and surface interpretations)
-form error that is large relative to the absolute size of the feature (so that the axis of a hole can disappear entirely)
-orientation error that is large relative to the size and extent of the feature (so that cosine error from tilting is significant)
-Independency (so that there is not a simple size-and-form boundary on which to add orientation and location error)
-location error that is large relative to the absolute size of the feature (so that the surface of the feature crosses over the true position, and makes the VC zero or negative)

In addition to the geometry itself, there are descriptions and terms in the standard that were written with the well-behaved situations in mind. So we have places where it says "MMC" and it probably should have said "MMB". It might not make any difference if we have Rule #1 and the perfect-form boundary in place. If we don't have that in place, then there can be a much bigger difference between MMC and MMB and the distinction matters.

This is part of what I understood "unrealizable geometry" to mean. I don't think that we can derive a meaningful interpretation for what would happen in certain non-well-behaved cases, based on what is currently written in the standard.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[Burunduk]

It's up to you to make that interpretation explicit for the rest of the class.

Didn't imagine a matter that simple may get to this but:

"This means that the location of the group of holes as a pattern is non-critical" (post from 10 Mar 20 03:30)

If it was not critical it would not have a tolerance.

(a) Each dimension shall have a tolerance , except for those dimensions specifically identified as reference, maximum, minimum, or stock (commercial stock size). The tolerance may be applied directly to the dimension (or indirectly in the case of basic dimensions), indicated by
a general note, or located in a supplementary block of the drawing format.

I have no idea how to make it any more explicit. The quoted rule from the standard is right at the beginning of the paragraph you were pointed to and didn't bother to read. Why do you say you read the rules when you didn't?
I don't think you truly need an explanation of what "critical" is but, in essence, it doesn't even matter, as obviously both "critical" and "non-critical" get the same treatment.

The standard is explicit for the non-Independency condition; claiming that it applies here is unsupported.

The standard is explicit in both the dependency and independency conditions. All you need to do is read from it what it says and not what you want it to say.

You said the only reason for VC was to provide clearance and now back track and cover the error. The standard does not discuss what the negative value means. So, your interpretation again.

Everything is an interpretation. The purpose of the standard is to enable to interpret things based on it. There is no contradiction between my statement that the VC boundary provides the means to conserve space for a mating part and my explanation that negative or zero values indicate that no such space is available. In fact, one explanation completes what the other started. As it should be clear from what I already said about negative values - the standard provides the tools, the user can do either useful or useless things applying them.

You said you provided an example of Independency. Where is it? Don't point at greenimi and blame him

I said that I provided a description and I did. You even quoted it and made a pointless attempt to dismiss it by saying that it doesn't describe what it does. I addressed it but you didn't respond with anything other than repeating "It is not" to support this attempt at dismissal. So why should I bother to elaborate on it further?

Blaming greenimi is certainly one of the things I didn't do and I sure hope he realizes that. You thinking otherwise would be a surprising misunderstanding but I have the feeling that it is you "deflecting" again. In case it is a genuine misunderstanding it should be clarified: You said you don't see what function could possibly drive that tolerancing scheme, so I blame you for not asking greenimi for more details after he said there is a real case behind his question. You could do that either to see if there is anything you might be missing or to check if you can suggest an alternative based on the specific problem.

Your position is that straightness of the feature, even if it is explicitly not to be controlled, cannot violate the available position tolerance. This is the default without Independency and Independency does not change. It is therefore a redundant control and should be removed from the drawing.

You are still welcome to address my example provided with numerical values that there is no redundancy. Look up the reference to it in my previous post. Rewording the same argument and ignoring the response that was already provided is a waste of time.

You still haven't answered about the material condition that applies to the implied control.

Not sure what you are asking about. If it is the same thing you address here:

It does not say that making that straightness tolerance infinite by using Independency will force it to be limited by the position tolerance. In particular, because there is no way to know if the implied control is on an MMC, LMC, or RFS basis.

Then my answer is that the only thing that needs to be said about the independency principle is that it removes the requirement of perfect form at MMC, and the standard says just that. Regardless, any other controls that may limit the form of the feature, still apply. A virtual condition boundary is capable of limiting all possible variations of the feature simultaneously. The easiest way to grasp it is by analyzing how the surface interpretation of MMC/LMC controls works. Surface is the interpretation that always applies for MMC/LMC controls, even for cases where the axis interpretation is being "sloppy". Usually, form variations are managed by other means. Usually not equals always.

 

[chez311]

The standard is explicit that the specified straightness tolerance shall be less than the position tolerance.

It does not say that making that straightness tolerance infinite by using Independency will force it to be limited by the position tolerance. In particular, because there is no way to know if the implied control is on an MMC, LMC, or RFS basis.

I agree, the standard says nothing explicitly about the interaction between Independency, straightness, and position. However, I didn't mean to suggest that I meant there is some sort of implied control of straightness/form - only that form is limited indirectly as a result of having to satisfy the equations for the resolved geometry interpretation.

Ie: as long as b>=0 , D/2<=b , and the feature satisfies the limits of size then the feature passes and satisfies the MMC position tolerance according to the resolved geometry interpretation. If there were some possible geometry that would allow straightness variation to be greater than t_0 while not causing b to be negative then I would say it would be allowed per the resolved geometry as the equations for position at MMC would be satisfied. I can't imagine every possible geometry but it does seem that the straightness would be limited to the position tolerance t_0, but this isn't an additional implied tolerance - just an indirect result.

 

[chez311]

As an addendum to my above post, although I provided the equations for the resolved geometry I am of the opinion the equations for the surface interpretation are still valid. After reviewing the math standard, it seems that per Y14.5.1-1994 section 2.3.1(a) r_MMC as it is referred to in the standard, especially table 5-2 for the calculation of the position tolerance zone for the surface interpretation, is simply the "maximum-material condition limit (r_MMC)" which for example dictates the radii of the spheres to be swept along a spine and not the MMC boundary/envelope itself.

As a result I see no issue in utilizing the surface interpretation equations for position at MMC as r_MMC for a toleranced feature can certainly be determined and used to calculate the position tolerance zone per the surface interpretation the same as it can be used to derive a solid G(Sm,Br_MMC) per the limits of size where the spine Sm is not required to be of perfect form along which spheres B of size r_MMC are swept - for example when the Independency symbol is used or another mechanism to override rule #1.

I'd be interested to hear anyones view as to why the equations for the surface interpretation of position at MMC which utilize this same r_MMC are not relevant or break down. The same value of r_MMC would be utilized to determine the solid G(Sm,Br_MMC) (per section 2.3.1) for conformance to limits of size as it would to determine the position tolerance for the surface interpretation *b=size_MMC/2-t_0/2 b=r_MMC-t_0/2 (per table 5-2 and section 5.2.1). The value of r_MMC would remain the same regardless of rule #1 and therefore it would follow the calculation for position tolerance at MMC remains the same. Coincidentally r(p)=b is the virtual condition per 5.2.1(b) so I would contend you still have a virtual condition, but no matter what we want to actually call the boundary it seems to me that all signs point to the calculation being unaffected no matter whether rule #1 is in effect or not.

Thoughts?

*Edit - apologies, I included the equation with size_MMC/2 instead of r_MMC. I typically refer to them as diameters so I converted r_MMC=size_MMC/2. I have changed the equation to reflect what is shown in the standard which includes r_MMC.

 

[Burunduk]

Evan,
I believe that the "limitations of the concept" which you listed, or at least some of them, indicate that even if at some point there was an intention of the Y14.5 committee that the virtual condition would be a generated envelope resulting from all the allowed variations, including the most extreme form variation as unrestricted by every single drawing specification when considered in isolation (such as independency or "PERFECT FORM AT MMC NOT REQD"), then clearly this is no longer the case. Considering that only the MMC/LMC limit of size and the tolerance modified to apply at that material condition are part of the concept definition and boundary calculation is enough for making that conclusion. In this sense "limitations of the concept" actually limit it to being a variation limiting boundary - as can be indicated by the chosen terminology - "boundary" as opposed to "envelope".

So perhaps it is time for the swamp to get dried out? Some swamps can effectively be dried out by planting eucalyptus trees. Others by not expecting tolerancing concepts to be more than they are capable of being per their definition. ;-)

 

[pmarc]

I'd be interested to hear anyones view as to why the equations for the surface interpretation of position at MMC which utilize this same r_MMC are not relevant or break down. The same value of r_MMC would be utilized to determine the solid G(Sm,Br_MMC) (per section 2.3.1) for conformance to limits of size as it would to determine the position tolerance for the surface interpretation *b=size_MMC/2-t_0/2 b=r_MMC-t_0/2 (per table 5-2 and section 5.2.1). The value of r_MMC would remain the same regardless of rule #1 and therefore it would follow the calculation for position tolerance at MMC remains the same. Coincidentally r(p)=b is the virtual condition per 5.2.1(b) so I would contend you still have a virtual condition, but no matter what we want to actually call the boundary it seems to me that all signs point to the calculation being unaffected no matter whether rule #1 is in effect or not.

To continue on one of Evan's thoughts and trying to address your question... Per para. 2.7.3 in Y14.5-2009, application of the independcy modifier leaves "the feature form entirely uncontrolled". To me this puts in question your statement that "the value of r_MMC would remain the same regardless of rule #1 and therefore it would follow the calculation for position tolerance at MMC remains the same". If the form of a cylindrical feature of size, like a hole, is to be entirely uncontrolled, then the swept-sphere-based size concept cannot be applied, because by its nature it controls form of the feature in the cross-sections.

Like I mentioned before in this thread, this is not to claim that according to the current standards (Y14.5 and Y14.5.1) the VC size is different than what you and Burunduk are saying. It is more to support the statement that we are talking about things that were not envisioned originally.

 

[3DDave]

Burundk,

"explicit" does not mean to pick various small bits from one place or another. It means to have an explicit example and explicit language for that exact case. You are looking for implication.

There is an explicit caution to not to this and it states it without reference to position or orientation. I take that to imply never to do this.

 

[Burunduk]

A caution remark is not explicit by nature because it is only a "caution" remark, not a forbiddance. When you are cautioned against something, it means that you need to make a careful judgment when dealing with a given subject.
It is not explicit that applying a tolerance of position at MMC is not a valid way to prevent the uncontrolled form that the remark cautions against:

"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."

Where this control is applied, there is no physical possibility of an entirely uncontrolled form. Look at what the control actually does rather than in what box it is put, what name it is given and at what part of the standard it is specified.

 

[Burunduk]

If the form of a cylindrical feature of size, like a hole, is to be entirely uncontrolled, then the swept-sphere-based size concept cannot be applied, because by its nature it controls form of the feature in the cross-sections

Since the swept spheres concept is the mathematical definition of the control of limits if size, this statement can easily be reversed to mean that wherever limits of size are controlled, the form of the feature cannot be entirely uncontrolled. That is even where there is no requirement of perfect form at MMC, no supplementary form control, or any other means. For me, this seems questionable.

 

[pmarc]

Since the swept spheres concept is the mathematical definition of the control of limits if size, this statement can easily be reversed to mean that wherever limits of size are controlled, the form of the feature cannot be entirely uncontrolled.

By saying this, haven't you just admitted that in your opinion the cautionary note in Y14.5-2009: "Without a supplementary form control, the feature form is entirely uncontrolled" is incorrect?

What makes you think that the swept spheres concept for limits of size has power to override the cautionary note and not vice versa?

 

[Burunduk]

The last sentence was "for me, this seems questionable". Therefore I don't think that the swept spheres concept has the power to override the cautionary note, but I don't see how the vice versa case is valid either.

 

[Burunduk]

In other words, If the swept sphere concept "controls form of the feature in the cross-sections", it is bound to make the cautionary note unnecessary/incorrect. Therefore I question if it really is the case, and think that r_MMC does not depend on the existence of a form control.

 

[chez311]

Per para. 2.7.3 in Y14.5-2009, application of the independcy modifier leaves "the feature form entirely uncontrolled". To me this puts in question your statement that "the value of r_MMC would remain the same regardless of rule #1 and therefore it would follow the calculation for position tolerance at MMC remains the same". If the form of a cylindrical feature of size, like a hole, is to be entirely uncontrolled, then the swept-sphere-based size concept cannot be applied, because by its nature it controls form of the feature in the cross-sections.

I'm not sure I follow why you believe the swept spheres concept of size cannot be applied without rule #1. Y14.5.1-1994 para 2.3.1 describes two solids G_l=G(S_l,Br_LMC) and G_m=G(S_m,Br_MMC) and a set of conditions which they must satisfy. This would be the case used when rule #1 does NOT apply and the spine S_m is not required to be of perfect form. The special case where rule #1 DOES apply would be 2.3.2 where the spine S_m is required to have perfect form.

I realize the text of Y14.5 says "entirely uncontrolled" however I would view that this refers to "macro" form error, not in the cross section. As far as I can tell and as I have shown above the swept spheres can be applied with or without rule #1 therefore cross-sectional form error will always be controlled due to this interpretation of size - I don't think that one sentence nullifies this. Why would the standard provide a definition for size per the swept spheres interpretation in Y14.5.1 which can be utilized where rule #1 does not apply and then nullify its use with a single sentence in Y14.5 ?

I would agree the combination of Independency (or any other mechanism to override rule #1) and position may not have been envisioned originally, however the application of size limits without rule #1 certainly was. If we apply the equations for position (BOTH surface and resolved geometry) consistently and in the same manner as the equations/requirements for the limits of size in Y14.5.1 then one should come to the conclusion I posted previously that position does indeed provide some limit for form. Why would r_MMC be the same value for size conformance with or without rule #1 but not for position?

What makes you think that the swept spheres concept for limits of size has power to override the cautionary note and not vice versa?

Are you suggesting the inverse? That the only mathematically rigorous definition of size provided by ASME per Y14.5.1-1994 which has provisions for interpretation with or without rule#1 should be nullified by this cautionary note?

If the swept sphere concept "controls form of the feature in the cross-sections", it is bound to make the cautionary note unnecessary

I would disagree. See my remark above about "macro" form error. The straightness error of a shaft released from rule #1 could be immense while still limiting cross-sectional (aka "micro") form error due to the swept spheres definition of size. The note could still be necessary in this regard.

 

[pmarc]

The last sentence was "for me, this seems questionable".

OK, I didn't get that.

Therefore I don't think that the swept spheres concept has the power to override the cautionary note, but I don't see how the vice versa case is valid either.

I don't see how the vice versa is valid either, which to me just proves that not all pieces in this whole puzzle match up.

NOTE: Initially, I was going to say that I hope you see my point now, but your most recent post shows that it's probably not true.

 

[Burunduk]

chez311, I simply don't think that size on its own controls form in the same context as in the cautionary note. Regardless of cross-sections. The part I had an issue with was about how the swept spheres concept is not applicable for the "completely uncontrolled" form scenario from that note. That would mean that size controls form in that context. Should have been more precise with the portion of the statement I'm quoting.

 

[pmarc]

Why would the standard provide a definition for size per the swept spheres interpretation in Y14.5.1 which can be utilized where rule #1 does not apply and then nullify its use with a single sentence in Y14.5 ?

Why would the standard be full of different unclear/inconsistent/illogical/conflicting requirements? But seriously speaking... the sooner we accept the fact that not everything in the standard holds water the better for everyone.