Concentricity Callout. What does it mean?

[cmmguy75]

Hi All,
I see this callout on a part I'm checking. I've not seen the way it called out for concentricity like this. Does it mean center of 3 .250"Holes (Boldcircle) concentric to |B|?
(Normally, I see concentricity of 2 circles at the same locations.)
Thanks!

 

[CheckerHater]

I would ask if you work to ISO or ASME (because definition of concentricity is different in two), but you are right either way: the call-out you are showing is wrong in both of them.

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

 

[cmmguy75]

We are working to ASME.
If it means the boldcircle (of 3x .250" holes) concentric to |B| then I just added a yellow boldcircle to the sketch.
But I still don't understand why concentricity while True Position is already called out? It may tight up the tolerances and could add composite tolerance for those 3 holes....
Any idea to this callout?

 

[drawoh]

cmmguy75,

Are the holes centre-bored? I am trying to make sense of datum[ ]C. It is logical that a centre-bore should be concentric with the through hole, although a location tolerance would work just as well.

--
JHG

 

[CheckerHater]

I guess OP's major concern is holes being "concentric" with

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

 

[cmmguy75]

Drawoh: Datum C is a Cbore.
CheckerHater: Yes, you're correct. The concern is the 3 holes, .250" to |B|.

 

[drawoh]

cmmguy75,

Your pitch circle is imaginary. You cannot apply a tolerance to it, particularly concentricity. You can apply positional tolerances to the individual holes.

--
JHG

 

[powerhound]

Just echoing everyone else...this is a bad callout. There's no reconciling it either drawing. The way to correctly tolerance the hole pattern is pretty straight forward. Do you have a copy of the ASME GD&T standard?

John Acosta, GDTP Senior Level
Manufacturing Engineering Tech
SSG, U.S. Army
Taji, Iraq OIF II

 

[cmmguy75]

powerhound: Yes, I do have that standard.
I will find out what the mating part will do to this part.

 

[powerhound]

page 137 in the 94 standard, page 121 in the 2009.

John Acosta, GDTP Senior Level
Manufacturing Engineering Tech
SSG, U.S. Army
Taji, Iraq OIF II

 

[cmmguy75]

Thanks John (powerhound).
To my understanding, the design would go with fig 7-26 (on p122 in the 2009).
The main thing is they want each .250"hole concentric to the cbore, |C|.
Anyway, the callout is NOT right.
Thanks for all of your guys' help.
Have a great day!

 

[ewh]

I don't think the really want concentricity as the holes would all be in the same location, effectively being one hole in the center of the part.
You are right that fig. 7-26 is most likely what they need.

"Know the rules well, so you can break them effectively."
-Dalai Lama XIV