Circularity versus self-referencing circular runout

[aniiben]

I read some discussions about self-referencing runout (circular and total) and I'm trying to understand why cylindricity could be considered "equal" to "self- referencing" total runout, but not the same thing could be said about circularity and circular runout.

No practical application. Just purely academic and theoretical discussion.

Also, I agree that "self referencing" should be avoided to eliminate any misinterpretation and inspection problems that must be overcome with non classical inspection techniques.




From different thread copy-paste:
"I agree that in case of total runout it would be equal to cylindricity, however for circular runout I think it is different from circularity and in certain, though very untypical, situations it may make sense. And I also agree that standard approach is to use runout for controlling feature(1)-to-feature(2) relationship, and not "runout being used on a single feature with a single self-referencing datum". " end of copy-paste.

 

[aniiben]

Forgot the thread:

https://www.eng-tips.com/viewthread.cfm?qid=336494

 

[CheckerHater]

To me trying to replace cylindricity with self-referencing runout (whatever that is) is the equivalent of looking for black cat in the dark room.

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

 

[Sem_D220]

Total runout relative to features own axis (derived from UAME) is not the same as cylindricity, for which the "common axis" involved in the definition in para. 5.4.4 in Y14.5-2009 is not necessarily the UAME axis. I suppose it is not different in the case circularity / circular runout.

 

[greenimi]

With that said, I am not suggesting that this kind of callout should ever be used. It is simply because I believe, like semiond, that it does nothing else that cylindricity would not do.

https://www.eng-tips.com/viewthread.cfm?qid=438912

Well, this topic has been discussed above.

Sem,

Is your today's quote "Total runout relative to features own axis (derived from UAME) is not the same as cylindricity, " in conflict with the one I posted above (from pmarc) ?


I suspect pmarc knows something that special things happening for circularity/circular runout (self-ref) relationship, otherwise he wouldn't have posted his quote above.

 

[Sem_D220]

Is your today's quote "Total runout relative to features own axis (derived from UAME) is not the same as cylindricity, " in conflict with the one I posted above (from pmarc) ?

You would probably need to ask both me and pmarc to get a sure answer if there is a conflict or not but my answer is no - there is no conflict. It seems to me that by the quoted statement from pmarc he meant that the so-called "self-referencing runout" can be considered as an attempt to do something similar to what cylindricity does, but not one that makes sense - as cylindricity is the more adequate alternative, exactly because of the differences between the two. Bottom line is - measurements results would not be identical per produced part.

 

[lfw618]

Why would they not be the same theoretically? You would be rotating both around their axis, and essentially TIR would be your tolerance. If total runout is applied to a cylinder, wouldn't the theoretical tolerance zone be two coaxial cylinders? Using the same logic I'd come to the conclusion that circularity and runout on a circle would be the same though so I'm not sure.

They only practical difference to me might be in inspection. Total runout to me kinda implies v blocks and indicator. Where maybe cylindricity might go to CMM? In that case I could see different results because with v-blocks the theoretical axis is could change as you rotate. But I still think in theory they're the same callout.

 

[chez311]

I think the below quote from Evan in the referenced thread

https://www.eng-tips.com/viewthread.cfm?qid=438912

does an excellent job in stating the differences, especially the bolded portion. That being said while theoretically in the way they are defined they are different - I'd be interested if someone could come up with an actual example where the end result is different. It seems to me in almost every case the end result would be almost identical.

https://www.eng-tips.com/viewthread.cfm?qid=438912[/URL]]So at the end of the day cylindricity can be applied and inspected without using or defining an axis. It's two coaxial cylinders - we can imagine an axis down the middle, but we don't have to.

When total runout applied to a cylindrical feature, the tolerance zone amounts to two coaxial cylinders that are also coaxial to a datum axis. I would say that the self-referencing total runout tolerance in CH's figure B would apply a control very similar to, but not exactly the same as, cylindricity. There are cases in which the measured cylidricity is slightly better than the total runout w.r.t the UAME axis (in other words, the UAME axis is not necessarily the optimal axis for getting the 2 coaxial cylinders as close together as possible). If there were no datum reference (and I know that Y14.5 does not allow this) then the total runout control would be exactly the same as cylindricity. Cylindricity is equivalent to total runout with an arbitrary "datum" axis (I suppose that it's not really a datum axis if it's optimized). I would like to see Y14.5 allow total runout tolerances to be specified on groups of features without any datum references - I think that this would make more sense than the partially self-referencing C-D scheme in figures 9-4 and 9-6.

Note that in the above quote Evan proposes a method to possibly utilize total runout without a datum reference (not currently supported in 2009 or 2018 as far as I know) and in another thread

https://www.eng-tips.com/viewthread.cfm?qid=266348

in Evan's post on (5 Mar 10 18:48) there is an interesting explanation on some of the history and mention that the entire concept of total runout referencing its own axis is perhaps flawed, but has been carried over through the years as an accepted practice and could be eliminated/replaced if additional rigor was applied to the concept of total runout. I would copy it here but it is rather lengthy and I think is best read in entirety. It is notable that the figure of interest in that thread Y14.5-2009 9-7 has been modified in Y14.5-2018 12-12 so datum feature D is just held perpendicular to C instead of with total runout to C-D (which utilizes its own axis). The other similar figures 9-4/9-6 (12-9/12-11 in 2018) have been carried over with very few changes, especially not to the total runout control of datum feature C-D referencing its own axis in both cases. In fact the control of datum feature C in 9-4 has been changed from circular to total runout to C-D in 12-9. Apparently the rigor envisioned for the new standard was not sufficient to supplant this age old practice - I would be interested to hear if Evan (if he doesn't mind me referencing some of his quotes of course - they are enlightening!) or anyone else maybe had some insight as to why. Note that this practice is still enshrined in the preserved section 9.5.5 in Y14.5-2009 / 12.6.5 in Y14.5-2018.

 

[lfw618]

But for a self referencing total runout, wouldn't the UAME be the most optimal axis?

 

[chez311]

Ifw618,

For self referencing total runout the UAME axis is the axis that is utilized. What Evan's quote says is that there could be cases where this axis does not necessarily coincide with "the optimal axis for getting the 2 coaxial cylinders as close together as possible" which would be concentricitycylindricity.

As I said, I understand this difference in theory but I would have to really try hard to come up with a situation which resulted in a measurable difference.

 

[lfw618]

I guess what I was trying to say, is that my thinking, is that from a pure math/theory perspective, by definition the UAME axis for self referencing total runout, would always be the most optimal axis for cylindricity.

 

[Sem_D220]

chez311, you I suppose that in your last post where you said "concentricity" you meant cylindricity.

 

[chez311]

For the most part I would agree that the two would be identical, but since they are defined differently I can see the possibility of a case where the measured result deviates between the two. Not sure why you said from a "pure math/theory perspective" as that is the only way I am looking at it - they literally have different definitions.

Think of it this way - the UAME for an external cylindrical feature is the smallest cylinder of perfect form which can fit around the feature, which defines the axis for runout. For cylindricity you have two concentric cylinders of perfect form within which the surface of the feature must fall - these can be optimized in any way which minimizes the separation (diametrical difference) between these two cylinders, of which neither concentric cylinder is required to coincide with the UAME. This is the way that cylindricity would be optimized and actually no discussion of axis has to take place - as Evan put it "at the end of the day cylindricity can be applied and inspected without using or defining an axis. It's two coaxial cylinders - we can imagine an axis down the middle, but we don't have to."

Sem - thanks for the catch. I absolutely of course meant cylindricity, I fixed it.

 

[Sem_D220]

That being said while theoretically in the way they are defined they are different - I'd be interested if someone could come up with an actual example where the end result is different.

Two examples by Evan in the thread you referenced:

24 May 18 22:18, and the post right below it.

 

[axym]

Sem_D220,

Wow, I have very little recollection of making those figures even though it was less than a year ago ;^).

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[chez311]

Well there you go, I guess it pays to re-read through the entire thread.

Ifw618 in the thread

https://www.eng-tips.com/viewthread.cfm?qid=438912

there are two examples as Sem noted of how total runout and concentricity differ. Hopefully this answers your question.

Perhaps this would also answer my question about the assertion that this is a dated practice and why it was carried on, due to the fact that total runout is closely tied to a features mating envelope whereas cylindricity can deviate from that in certain cases. I'd be interested on others thoughts on that.

 

[chez311]

Evan,

I wasn't going to speculate but since you've joined the thread I am curious - do you still believe that as you said in

https://www.eng-tips.com/viewthread.cfm?qid=266348

on (5 Mar 10 18:48) that this practice with total runout doesn't stand up to scrutiny? That post was 9 years ago and only a year after the 2009 standard came out, so I absolutely won't hold your feet to the fire on that - hopefully you won't feel like I am, as I said I am just curious.

 

[lfw618]

OK, I see, that makes sense. When I was envisioning it, for some reason I had it in my head that the axis for the cylinders needed to be parallel to UAME axis. But that clearly isn't the case. Makes much more sense now.

 

[aniiben]

All,
The quote I copy-pasted in my initial post came also from pmarc

J-P,
I agree that in case of total runout it would be equal to cylindricity, however for circular runout I think it is different from circularity and in certain, though very untypical, situations it may make sense. And I also agree that standard approach is to use runout for controlling feature(1)-to-feature(2) relationship, and not "runout being used on a single feature with a single self-referencing datum".

I have no doubt in my mind that is correct and accurate. The scope of the initiated thread is to extend the discussion on the circularity and circular runout (SF) --read "self-referencing" or "science fiction " as you wish---

I see that the main discussion is going back to cylindricity and total runout (SF). It is helpful, but not the intent. I trust pmarc and other forum members who participated in the inital and subsequent discussions about cylindricity correctly indicated by chez311, greenimi and others.

I would like now to get the subtle details about circularity ( and circular runout SF), if all possible.

 

[aniiben]

Chez311,
Why are you keep saying Y14.5-2019? (Couple of instances on your post from 24 April at 16.27)

There is no such. And probably won't be. Yes, I know I am nit picking your posts.
Thank you for your contribution. I appreciate your posted previous discussions and old threads.