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?

 

[chez311]

the more generic answer is that they wanted to provide means to deal with the cases where the default swept-spheres method for evaluation of size limits simply didn't work.

pmarc - please don't take this as confrontational, I'm genuinely curious about your reasoning on this matter. What situations do you believe the swept-spheres method "simply doesn't work" ? It seems to me it works just fine, the main disagreement seems to be the effect of that single sentence in the note in Y14.5 that "the feature form is entirely uncontrolled."

There is no guidance from the Y14.5.1 draft to use either method of evaluating actual local size in place of swept-spheres, and no mention of how it might release all areas of form control vs swept-spheres. In fact one of the methods presented is the circular elements method which I don't think would be too dissimilar from the swept-spheres as far as form control.

In fact, as I am sure you are already aware, there is actually now explicit instruction on how use the swept spheres interpretation in cases where rule #1 does not apply (ie: perfect form at MMC not required), as opposed to implicit in Y14.5.1-1994 (ie: case where the spine S_m is not required to have perfect form). Additionally there is specific instruction that actual local size is NOT to be used in place of the swept-spheres interpretation for determining conformance and is only an estimate of size.

I myself have wondered about the inclusion of the definition of actual local size with these caveats, the best answer I have been able to come up with is that if someone wanted to either through a drawing note or accompanying internal company standard specifically require "SIZE CONFORMANCE TO BE DETERMINED PER Y14.5.1-20XX ACTUAL LOCAL SIZE OPPOSING POINTS METHOD" or similar. I also considered that it allows for methods to "estimate" conformance, however it seems any method capable of deriving a local size spine could also determine conformance to the swept spheres - utilization of hand tools such as calipers/micrometers would provide an "estimate of an estimate" in that case.

 

[chez311]

Evan,

I'd be interested in any insight you could provide on the above (16 Mar 20 13:56) as well. I understand you're the Vice Chair of this subcommittee so if you believe its a conflict of interest or what you're allowed to say/disclose I'm not being sarcastic when I say I understand, so no worries either way.

 

[pmarc]

pmarc - please don't take this as confrontational, I'm genuinely curious about your reasoning on this matter.

Don't worry. I am not taking this as confrontational. These are all good and constructive questions.

What situations do you believe the swept-spheres method "simply doesn't work" ? It seems to me it works just fine, the main disagreement seems to be the effect of that single sentence in the note in Y14.5 that "the feature form is entirely uncontrolled."

I can think of at least two:
1. Already mentioned, Independency modifier applied to the size specification. I trully believe that per the cautionary note, form of the feature is entirely uncontrolled in such case. "Entirely" means no aspect of form shall be controlled, therefore the swept-spheres method cannot be applicable.
2. Size of the circular elements used to determine the actual increase in size of the DML straightness tolerance zone when applied at MMC/LMC basis and with the resolved geometry interpretation used. For the swept-spheres method, the sizes of the two spheres don't change lengthwise, thererfore the method cannot be used to determine individual sizes in the cross-sections.

Having said that, I still have some comments to how Y14.5.1 committee defined these two additional methods of evaluation of local size in the new version of the standard, but as a new member of the committee I better stop here before I say too much.

 

[greenimi]

pmarc,
Are Y14.5.1. meetings open to the general public or only the Y14.5 ones?

 

[pmarc]

greenimi,
Both are open to the general public.

 

[chez311]

Already mentioned, Independency modifier applied to the size specification. I trully believe that per the cautionary note, form of the feature is entirely uncontrolled in such case. "Entirely" means no aspect of form shall be controlled, therefore the swept-spheres method cannot be applicable.

As a follow-up question, do you believe that the swept-spheres interpretation of size when rule #1 is not in effect (per Y14.5.1-1994 para 2.3.1 or Y14.5.1-20xx para 2.3.2.2) is ever valid?

I think the biggest sticking point for me is changing the core definition of size depending on whether rule #1 (envelope principle) is in effect or not instead of being consistent in interpretation. Is there a similar practice in ISO or do they keep the same 2-point definition with or without the envelope principle?

Size of the circular elements used to determine the actual increase in size of the DML straightness tolerance zone when applied at MMC/LMC basis and with the resolved geometry interpretation used. For the swept-spheres method, the sizes of the two spheres don't change lengthwise, thererfore the method cannot be used to determine individual sizes in the cross-sections.

I concede you are correct, some modification to the definition would be necessary for a resolved geometry interpretation of MMC/LMC DML straightness/DMP flatness. Per the Y14.5.1-20xx draft there is only a surface interpretation provided. As you likely recall (and as you mentioned with you now on the committee you are certainly aware - genuine congratulations are in order!!), I mentioned as much in the thread (

https://www.eng-tips.com/viewthread.cfm?qid=464506)

I don't mean to hold your feet to the fire as we are all certainly allowed to change our minds (I know I do) and I don't want to put words in your mouth - suffice to say you seemed pretty ambivalent about the importance of a resolved geometry interpretation in your response (12 Feb 20 15:11). Has this changed notably since then? Or is this purely an example of where the current definition of swept-spheres doesn't work?

 

[Burunduk]

As a follow-up question, do you believe that the swept-spheres interpretation of size when rule #1 is not in effect (per Y14.5.1-1994 para 2.3.1 or Y14.5.1-20xx para 2.3.2.2) is ever valid?

I am aware the question was directed to pmarc. If it's OK that I intervene, judging by this...

There must be a valid case for the swept spheres concept where rule #1 is not in effect.

My understanding of pmarc's assertion is that when perfect form at MMC is not required and no supplementary form control is specified, applying any concept that will contradict the cautionary note ("... entirely uncontrolled") is not valid, as it will be against what the standard says. So the swept spheres application of limits of size is not allowed (and same for a virtual condition...?). In other words, we are not allowed to apply a concept the application of which might lead to the conclusion that the cautionary note is not 100% correct.

 

[axym]

chez311,

I'm not able to provide details or official information as vice chair, since the subcommittee discussions are private and the final version of the Y14.5.1-20XX document has not been published yet. I can make some general comments which can be taken as my own opinion.

Actual local size was not addressed at all in Y14.5.1M-1994, as the swept-sphere definition defines actual values that apply to the entire feature. I would say that the new material on Actual Local Size Limits in Y14.5.1-20XX represents an attempt to add rigor to the actual local size definition in Y14.5-2009. This proved to be a daunting task, as the definition in 1.3.54 leaves a lot of room for multiple interpretations:

"Size, Actual Local. The measured value of any individual distance at any cross section of a feature of size. See Fig 1-1."

The term "individual distance" is not defined, and for cylindrical features has been interpreted as a 2-point distance by some GD&T experts and as a circular element by others. There were conflicting indications in Y14.5-2009, so each camp has been able to point to evidence that supports their opinion. I would say that the text in the Definitions and the Size section is mainly consistent with 2-point distances, but does not explicitly mention or illustrate them. The figures show actual local sizes in side views, without showing what is going on within the cross section. The Form section mentions actual local size involving circular elements, but does not provide an explanation.

There is also the issue of what is meant by "at any cross section of a feature of size". We know that we can't really use any cross section, because it can be easily shown that certain cross sections would yield silly results. There are indications in certain figures that the cross sections are (or at least can be) approximately normal to a median line, but there is nothing in the text actually stating this. So we know that the cross sections need to be normal to the center geometry of the feature in a general way, but the details are lacking.

Actual local size continues to be a difficult issue. The content of Y14.5-2018 shows that consensus has not been reached, as it contains the same conflicting indications as before. It is not Y14.5.1's role to resolve these issues and conflicts. What we can do is propose more rigorous definitions for the main ingredients of the different interpretations - 2-point distances, circular elements, and the orientations of the cross sections.

There is also the issue of reconciling the swept-sphere size definition from Y14.5.1M-1994 with the size definitions in Y14.5-2009. It is obvious that these swept sphere definition is fundamentally different than what is defined in Y14.5. The size of an enveloping volume derived from a sphere swept along a spine has only an indirect connection with the value of any individual distance at any cross section. If we get into a detailed comparison of things like form control within the cross section, there will inevitably be differences. I would say that the swept-sphere concept doesn't address the details of actual local size - it provides a definition that is conservative. My understanding is that if the feature conforms to the swept-sphere size requirement, it would also conform to the actual local size requirement defined by either 2-point distances or circular elements.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[pmarc]

As a follow-up question, do you believe that the swept-spheres interpretation of size when rule #1 is not in effect (per Y14.5.1-1994 para 2.3.1 or Y14.5.1-20xx para 2.3.2.2) is ever valid?

If we are talking about cylindrical features of size, I think it is still valid in case of DML straightness tolerances, even though for the tolerances at MMC basis interpreted in terms of resolved geometry the sizes of the circular elements also need to be known.

I think the biggest sticking point for me is changing the core definition of size depending on whether rule #1 (envelope principle) is in effect or not instead of being consistent in interpretation.

I would say it is more about changing the definition of size depending on whether the independency principle is in effect or not. Rule #1 may be overridden but this doesn't have to mean that the swept-spheres interpretation is nullified.

Is there a similar practice in ISO or do they keep the same 2-point definition with or without the envelope principle?

In ISO the 2-point size is the default with or without the envelope principle.

[...] I don't mean to hold your feet to the fire as we are all certainly allowed to change our minds (I know I do) and I don't want to put words in your mouth - suffice to say you seemed pretty ambivalent about the importance of a resolved geometry interpretation in your response (12 Feb 20 15:11). Has this changed notably since then? Or is this purely an example of where the current definition of swept-spheres doesn't work?

No, I have not changed my mind. I still think that the importance and usefulness of the resolved geometry interpretation for DML straightness and DMP flatness tolerances is low, so I guess the latter is correct.

 

[3DDave]

The correct process would be to define how evaluations are done in the mathematical definitions and only then incorporate them in the standard with examples. I also think no version of the standard would be released without a simultaneous release of the related certification test, much like one develops software by first setting the goals for it and then creating the tests to see if it works. The 144.5 process is more like adding features and hoping they don't crash the system.

The problem with 2-point measurements is they do not represent anything except 2-point measurements. So, if a part interfaces with other parts by 2-point contact, it's representative; otherwise what it is short hand for is saying - the form is so perfect that any two points that seem to the operator that they might, with reasonable skill make some estimation, are probably close enough to the mean/median/average/RSS/whatever other analysis scheme, that any error is likely not enough to worry about; not exactly a rigorous mathematical basis for concluding a part will always be usable.

The addition of local size is to placate those who demand who ad-hoc evaluations; measurements that may incorrectly evaluate the situation. This fits clearly within the inability to describe mathematically what they want and what it means.