Roundness vs diameter for Tolerance Stack

[ejthoma2]

For tolerance stacking an assembly which includes a cylindrical component with both a diameter tolerance and a roundness tolerance, how do the two tolerances combine within the tolerance stack? I assume it would be best to just use whichever is largest, but opinions would be most welcome!

Thanks.

 

[Belanger]

It depends on the stack. But if the stack goes across the part or includes the diameter tolerance in any way, then the roundness tolerance is ignored. This is because the roundness must operate within the size limits, and never go beyond them.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

http://www.gdtseminars.com

 

[ejthoma2]

Thank you! That helps!

So - on the drawing of the cylindrical component, should the roundness be adjusted so that the tolerance is not greater than the diameter tolerance?

 

[CheckerHater]

on the drawing of the cylindrical component, should the roundness be adjusted so that the tolerance is not greater than the diameter tolerance?...

If the component is rigid-yes.
Flexible component in free state may have roundness tolerance specified larger than diameter tolerance.
(Think of rubber O-ring)

 

[pmarc]

ejthoma2,
Regardless whether you work according to ASME or ISO GD&T (means regardless whether Evelope Principle governs the print or not), roundness (circularity) tolerance value can never be greater than the diameter tolerance. If it is otherwise, then the callout is wrong.

 

[ejthoma2]

Fantastic - thanks guys!

 

[PaulJackson]

Pmarc,

ejthoma2,
Regardless whether you work according to ASME or ISO GD&T (means regardless whether Evelope Principle governs the print or not), roundness (circularity) tolerance value can never be greater than the diameter tolerance. If it is otherwise, then the callout is wrong.

Not true in the "free state" when the diameter size is specified "AVG" as CheckerHater implied. See ASME Y14.5 (1994 pg. 195), (2009 pg. 97)

 

[ejthoma2]

It's okay - these are solid steel components. Definitely rigid!

 

[pmarc]

Paul,
I was absolutely aware of "AVG" specification while writing my response. Please notice that technically the average diameter callout itself, though shown in fig. 5-13 of Y14.5-2009 as a limit tolerance, does not define the tolerance of diameter (feature's size). Limits of size can be established only if average dimension tolerance AND circularity tolerance are simultaneously taken into consideration. Each of them separately cannot do it.

Additional thing, if we are talking about "AVG" - I am not so sure that you need free state modifier applied in order to be able to use "AVG" diameter concept and to have circularity tolerance greater than the tolerance of the average diameter. It is true that the standard says about it when describing application of free-state symbol, but does that mean the concept can't be used for "typical" rigid parts if needed for some reasons?

 

[PaulJackson]

Pmarc,

I agree the they must be used together on semi-rigid designs.

I saw a 1000mm H8 diameter specification with a [O|1] Circularity and recomended that they add the AVG after the size so that the two specifications would not conflict.

The design intent was to control the circumference and limit the free state form so that when heated and it would assemble and when cooled the assembled hoop-stress would be achieved.

Paul

 

[fsincox]

Paul,
What is a [0|1] circularity?
Frank

 

[Belanger]

I think he was just trying to use keyboard symbols to show a circularity feature control frame of 1 mm.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

http://www.gdtseminars.com