Buna N O-ring measurements

[gabimo]

O-ring definition states
OD 1.525 -1.485 AVG

With circularity .050 and F symbol into the FCF box.

Two questions I have:
1.) Is F mandatory or if no F symbol used, the drawing will have the same meaning?
2.) Could the circularity value be smaller than .040 (total size on average measurements)?

 

[CheckerHater]

Using Free state modifier implies that your part is flexible and special considerations are required (typed this twice today )

Without it you have to treat your ring as solid.

How do you measure a gummy bear?

I included the citation from Genium manual regarding O-rings

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

 

[greenimi]

Let me stir the pot on this: F symbol is NOT mandatory in the FCF for the circularity. My opinion.
Part is already flexible due to the AVG callout. F symbol is redundant.
Am I right?

 

[CheckerHater]

From ASME Y14.5-2009:

Para. 5.4.3: “The circularity tolerance must be less than the size tolerance…”
Para. 5.5.3: “Note that the free-state circularity tolerance is greater than the size tolerance on the diameter” (Fig. 5-13)

I guess, there is a difference between "circularity" and "free-state circularity". And how to indicate said difference on the drawing?


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

 

[greenimi]

Re:"And how to indicate said difference on the drawing"

I would say by using AVG note. Thus, F symbol is redundant. That would be my point.

 

[CheckerHater]

Diameter and circularity are two different things. AVG applies to size. F applies to form tolerance.

Y14.5 is not mandatory either.

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

 

[greenimi]

So how the same part could be flexible for one characteristic and rigid for another? Flexible for diameter size make it flexible. Am I missing something? I have no much experience with flexible parts.

 

[CheckerHater]

Part doesn't have to be flexible to have average diameter. This is why I believe specifying AVG is not enough.

Also free state doesn't apply exclusively to roundness. By the way, all the parts are measured in free state by default. Using (F) helps you to distinguish that measurement in free state will not return functional requirement.

Let say, in case of o-ring both are about 2/3 of what necessary - together too much, alone not enough.


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

 

[Nescius]

Some more discussion in this past thread:

http://www.eng-tips.com/viewthread.cfm?qid=324500

 

[3DDave]

Aren't all measurements defaulted to the free state? Seems redundant.

 

[greenimi]

Re:"CheckerHater (Mechanical)3 Mar 16 18:10
Part doesn't have to be flexible to have average diameter."

I do not say that is not true. I say, I haven't seen any rigid part that is defined/dimensioned with AVG and is not considered flexible. Could you, please, provide some examples/ books and/or references.

Thank you for your input

 

[pylfrm]

Referring to ASME Y14.5-2009 here:

Para. 4.20: "Unless otherwise specified, all tolerances apply in a free-state condition."

Para. 5.5.3: "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."

This would mean that "AVG" is always required for diameters with form control unless restraint is specified. I doubt this was the intent though. AVG is not shown in Fig. 5-12 for whatever that's worth.


- pylfrm

 

[CheckerHater]

greenimi,

It's illegal to have roundness larger than diameter tolerance.

(F) symbol makes it legal for flexible parts.

End of story.

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

 

[dgallup]

Quote " I haven't seen any rigid part that is defined/dimensioned with AVG and is not considered flexible. Could you, please, provide some examples/ books and/or references."

We have deep drawn stainless steel tubes that we use average diameter on. They are not exactly flexible, you can not deform one with your hand, but the tolerance is very small and we will press fit thicker wall cylinders inside them which brings them back into near perfect cylindricity.

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The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

 

[greenimi]

dgallup,

So, are the tubes (the ones using with AVG abbreviation) exceptions to rule #1 or you are still enforcing the perfect form at MMC? Just curious.

I guess the most of the discussion went to the "free state" thread, but I would like to know, how would you understand AVG note in relationship to rule#1.

Thank you

 

[dgallup]

No we don't enforce perfect form at MMC when we use AVG on the tube diameter.

----------------------------------------

The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.