Average diameter callout for circularity without free state modifier

[bradlw]

Hello All,

Is it valid to call out an average diameter (AVG) for circularity without a free state modifier or restraint condition?

Per paragraph 5.5.3 of the 2009 standard, it says that where circularity is specified in a free state condition, the pertinent diameter is qualified with AVG. In paragraph 1.4 (m) it states that all dimensions and tolerances apply in the free state unless otherwise specified. So, it seems that I could apply AVG to circularity with a free state modifier, since by default the part should be measured in the free state.

What I am trying to do is allow more circularity for a thin walled aluminum cylinder in the free state. The diameter tolerance is ±.001" but I want to allow a circularity of .005" based on an average diameter. I am comfortable that this free state allowance will allow the part to conform to the mating part. The AVG callout would allow me to have the circularity tolerance (.005"), larger than the tolerance of size (±.001"). Without AVG, if you read 5.4.3, you aren't allowed to have a circularity tolerance larger than the size tolerance.

Thanks

 

[chez311]

Is it valid to call out an average diameter (AVG) for circularity without a free state modifier or restraint condition?
[...]
What I am trying to do is allow more circularity for a thin walled aluminum cylinder in the free state.

If you want to control circularity in the free state with average diameter, why are you asking if its valid to call out avg diameter without circularity in the free state?

Anyway, if its purely theoretical - Y14.5-2009 does not define what would be meant by such a callout as it is shown in all the examples along with a circularity tolerance and free state symbol. Use along side the (F) modifier constitutes an exception to rule #1, however without this how are we supposed to interpret the AVG diameter callout - we know it can violate the limits of size* at its extremes as long as the average is within the specified limits, but do we simply assume that it can violate rule #1 as well? I don't know that we can per the letter of the 2009 standard, however I'm not sure how rule #1 would be interpreted in relation to an average diameter.

So to answer your question, it would be valid if we assume AVG diameter is an exception to rule #1 in 2009. If not, I would say no because I don't think rule #1 could be interpreted in the context of average diameter. I would lean towards the latter, that it is not valid and I think its easy enough to include the Independency or Free State modifier instead of trying to make such assumptions.

With Y14.5-2018 this is changed to be explicit - AVG diameter now constitutes an exception to rule #1 and could be applied without the (F) modifier (and it is in Y14.5-2018 fig 8-13).

An interesting side note - they have deleted any ambiguous reference to "free state variation" in 2018 and now explicitly define it in reference to the the (F) modifier. Ie; 2.7.2(b) "The control of geometric form prescribed by limits of size does not apply to the following: [...] (b) parts subject to free-state variation in the unrestrained condition." has been changed to 5.8.2 "The control of geometric form prescribed by limits of size does not apply to the following:[...](b) tolerances applied with the free state modifier." Additionally the free state modifier may only now be applied in conjunction with a restraint note per Y14.5-2018 section 7.20.4

*Actually now that I read through it again, this is sort of implied in 2009. In 2018 this is now explicitly added to the section for average diameter stating "The individual measurements may violate the limits of size, but the average value shall be within the limits of size."

 

[pylfrm]

bradlw,

Unfortunately ASME Y14.5-2009 is rather unclear in this area.

I think the most reasonable approach is to assume that "AVG" just changes the meaning of the toleranced diameter dimension, and doesn't actually have anything to do with circularity tolerances or restraint conditions. What exactly it changes the meaning to is not precisely defined, but presumably you feel it's clear enough for your application. If so, I wouldn't object to the usage you propose.

I'd say "AVG" counts as "otherwise specified" for para. 2.7, so the envelope principle (and everything else about the usual meaning of toleranced size dimensions) would not apply. This would leave one aspect of your feature's form entirely uncontrolled. To control that, you might consider replacing or supplementing your circularity tolerance with a cylindricity tolerance.

A simpler approach might be to apply the size tolerance (without "AVG") in a restrained condition, and specify an upper limit on the restraint force. This could allow elimination of the circularity tolerance.

An interesting side note - they have deleted any ambiguous reference to "free state variation" in 2018 and now explicitly define it in reference to the the (F) modifier. Ie; 2.7.2(b) "The control of geometric form prescribed by limits of size does not apply to the following: [...] (b) parts subject to free-state variation in the unrestrained condition." has been changed to 5.8.2 "The control of geometric form prescribed by limits of size does not apply to the following:[...](b) tolerances applied with the free state modifier."

I'm glad ASME Y14.5-2009 para. 2.7.2(b) didn't make it into the next version.

ASME Y14.5-2018 para. 5.8.2(b) seems perfectly clear, but extremely strange. I thought the purpose of the free state modifier was to override a default restrained condition specified in a general note or similar, not to change the geometrical meaning of the tolerance.


pylfrm

 

[bradlw]

Thanks for the replies. I agree that 2009 isn't perfectly clear on the use of AVG. 2009 doesn't say you can use AVG this way, but it doesn't say that you cannot. Since 2018 has changed AVG to be a clear exception to Rule #1, I must believe that the intent of 2009 was to be able to use it in this way.

 

[chez311]

I'd say "AVG" counts as "otherwise specified" for para. 2.7, so the envelope principle (and everything else about the usual meaning of toleranced size dimensions) would not apply.

Good point, I would tend to agree. Rationally I figured it didn't make sense to apply rule #1 to AVG diameter, but this puts it in more concrete terms.

]I think the most reasonable approach is to assume that "AVG" just changes the meaning of the toleranced diameter dimension[/b], and doesn't actually have anything to do with circularity tolerances or restraint conditions. What exactly it changes the meaning to is not precisely defined

I think this is a good way of looking at it. Would you say that the main issue with ambiguity is the lack of definition of how exactly to measure "diametric measurements across a circular or cylindrical feature" ? Presumably this would be the average of all 2-point "actual local size" measurements at each cross section - which would be more clearly defined with the latest release of Y14.5.1-20xx?

ASME Y14.5-2018 para. 5.8.2(b) seems perfectly clear, but extremely strange. I thought the purpose of the free state modifier was to override a default restrained condition specified in a general note or similar, not to change the geometrical meaning of the tolerance.

I hadn't considered this. Based on the linking of 2.7.2(b) with 5.5 in 2009 I assumed that the free state modifier should do double duty - both override a specified restraint as well override rule #1. Unless I'm reading it wrong, it seems that it does this in both 2009 and 2018, however the difference being that 2018 explicitly states it can only be utilized in conjunction with a restraint specification.

If I'm understanding you correctly, you're suggesting that the free state modifier should ONLY override a restraint note, and not be an exception to rule #1 - that if one desires to additionally override rule #1 then another mechanism such as the independency symbol should be utilized, correct? If so, after consideration I am in agreement - this oversight is unfortunate. I guess currently if one does not want to override rule #1 with the free state modifier (in either 2009 or 2018) a note such as "PERFECT FORM AT MMC REQUIRED" would have to be used.

 

[pylfrm]

Would you say that the main issue with ambiguity is the lack of definition of how exactly to measure "diametric measurements across a circular or cylindrical feature" ?

I was being generous when I said "not precisely defined". The meaning of "AVG" is not really defined at all. It's only mentioned once, and that sentence only claims to describe when it's used:

Where form control, such as circularity, is specified in a free state for a circular or cylindrical feature, the pertinent diameter is qualified with the abbreviation AVG.

Free state is the default per paras. 1.4(m) and 4.20 though, so this implies that "AVG" should be used whenever there is a form control which is not specified to apply in a restrained condition. I highly doubt that was the intention.

Presumably this would be the average of all 2-point "actual local size" measurements at each cross section - which would be more clearly defined with the latest release of Y14.5.1-20xx?

That seems like a lot of presuming.

Based on the linking of 2.7.2(b) with 5.5 in 2009 I assumed that the free state modifier should do double duty - both override a specified restraint as well override rule #1.

I really have no idea what ASME Y14.5-2009 para. 2.7.2(b) is supposed to mean. If you do, I'd be interested to hear it.

For what it's worth, ASME Y14.5-2009 para. 3.3.20 makes no mention of a connection to rule #1.

If I'm understanding you correctly, you're suggesting that the free state modifier should ONLY override a restraint note, and not be an exception to rule #1 - that if one desires to additionally override rule #1 then another mechanism such as the independency symbol should be utilized, correct?

That seems like the most useful meaning to me.

I guess currently if one does not want to override rule #1 with the free state modifier (in either 2009 or 2018) a note such as "PERFECT FORM AT MMC REQUIRED" would have to be used.

I'd prefer to skip the free state modifier and use a flag note such as "INDICATED REQUIREMENT APPLIES IN UNRESTRAINED CONDITION" instead. Considering this simple alternative, the value of the free state modifier seems rather questionable.


pylfrm

 

[chez311]

I agree that AVG per Y14.5-2009 is loosely defined. Y14.5-2018 section 8.5 makes some slight improvements on this, though. For example specifically noting "The individual measurements may violate the limits of size, but the average value shall be within the limits of size" as well as "Invoking average diameter constitutes an exception to Rule #1" (in conjunction with 5.8.2) instead of relying on our logical assumption that this is the case as well as your conclusion that AVG diameter falls under "unless otherwise specified".

Free state is the default per paras. 1.4(m) and 4.20 though, so this implies that "AVG" should be used whenever there is a form control which is not specified to apply in a restrained condition. I highly doubt that was the intention.

I too thought that part of Y14.5-2009 section 5.5.3 was very poorly worded and seemed to imply this, though of course logically we can conclude thats probably not the case. Thankfully this is removed in 2018 section 8.3 stating "Typically, an average diameter is specified for parts that are flexible in a nonrestrained condition; however, its application is not limited to such cases. [...] The pertinent diameter is qualified with the abbreviation 'AVG.' "

I really have no idea what ASME Y14.5-2009 para. 2.7.2(b) is supposed to mean. If you do, I'd be interested to hear it.

For what it's worth, ASME Y14.5-2009 para. 3.3.20 makes no mention of a connection to rule #1.

I've always been wary of Y14.5's attempt to define "free state variation" and am glad to see its removal in 2018 and any previous references to such put in more concrete terms. I didn't really mean to say I fully understood Y14.5-2009 section 2.7.2(b) but to explain that from the reference to 5.5 "Application of the Free State Symbol" that I didn't question the new Y14.5-2018 section 5.8.2(b) but now I see from your statements that perhaps I should have.

Y14.5-2009 section 3.3.20 as you note does note mention rule #1 but it does mention the ever-pervasive and contentious "free-state variation". I'm not defending the text of the standard or any interpretations thereof, simply stating why I didn't question the Y14.5-2018 definition of the free state modifier as something that both overrides a restraint specification as well as rule #1. I'm glad you pointed out that such a control may not always be desirable.

That seems like a lot of presuming.

It is indeed. I concede that because of the lack of definition even in 2018 theres a lot of wiggle room for interpretation for what exactly is meant by "diametric measurements", any references to methods of determining "local size" in the new Y14.5.1-20xx notwithstanding. At least it would provide some standardized basis of measurement even if not required.

Perhaps it doesn't matter much though, on many applications the part might be sufficiently flexible/compliant that precise unambiguous definition of "diametrical measurements" is splitting hairs.

I'd prefer to skip the free state modifier and use a flag note such as "INDICATED REQUIREMENT APPLIES IN UNRESTRAINED CONDITION" instead. Considering this simple alternative, the value of the free state modifier seems rather questionable.

Good point, thats a much better solution than trying to alter the meaning of the modifier with a note as I suggested. The free state modifier could still be used in cases which one desires to override both rule #1 and a restraint specification, though of course this could be accomplished with your note combined with an independency symbol as well.