Concentricity with common axis A-B?

[Jay8833]

Hey there guys, I need some advice here. Setting aside for a moment whether or not concentricity is the correct callout.. (this drawing is to the 2009 standard)
Can Concentricity be used this way in the 2009?
I was expecting to see A-B in the Concentricity datum A FCF, as well as the Ø12 FCF.. but then again, I'm not even sure if it can be used this way.
Thanks!

 

[greenimi]

Setting aside for a moment whether or not concentricity is the correct callout
Setting aside for a moment whether or not what is shown are functional requirements

I'm not even sure if it can be used this way.

Looks legit to me (legit meaning correct GD&T grammar)

I was expecting to see A-B in the Concentricity datum A FCF, as well as the Ø12 FCF.

Expectation versus reality..... Since I do not know the application (nothing has been described and detailed) I guess I have to stop here.

If more information is provided, we can then discuss it further.

 

[axym]

Jay8833,

I agree with greenimi that the GD&T grammar looks correct. I don't see a reason why concentricity cannot be used in this way - it doesn't violate any rules as far as I can tell.

Having said that, I also would have expected that the concentricity tolerance on the 12 mm feature would reference A-B. This would follow the implied functional story that features A and B act together to constrain the part's degrees of freedom, and so the other features are toleranced to the A-B DRF.

I would also have expected that feature A and feature B would both be toleranced to A-B as well, if they act together.

As greenimi says, we don't know the fit and function requirements. So we can't say whether they have been achieved, or if the tolerances were applied with these in mind.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[Jay8833]

Thanks for the input!

 

[Wuzhee]

Am I seeing a cross reference in Datum A calling out Concentricity Ø0.05 to Datum B?

 

[Belanger]

Datum feature A is toleranced with concentricity to within Ø0.05 relative to datum B. But what do you mean by cross-reference? Datum feature B is not toleranced to anything but itself.

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

 

[Wuzhee]

You're right John. I'm just feeling something is wrong when the datums aren't in alphabetical order.

 

[Steven_D_P]

When I look at this drawing the only thing that seems strange to me is to have a 0.1 circularity tolerance on a feature that has a size tolerance of +/- 0.1. This size tolerance should automatically make the circularity within 0.1. It's not wrong but unless I'm mistaken it's unnecessary.

Personally, I tend to use (total) run out rather than circularity and concentricity as it usually fits the design intent better and it's also far easier to measure.

 

[Belanger]

Steven, the size tolerance of ±0.1 is a tolerance of 0.2. So all is OK because 0.2 < 0.1.
Yes, the runout tolerances are much easier to measure, but they differ from circularity because they also include the elements of location and orientation, which circularity does not. Like anything else, it comes down to function, though.

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

 

[3DDave]

Circularity is radial, diameter is diametral. The radial zone is half of the diametral zone.

 

[Belanger]

3DDave, are you thinking that a size tolerance of ±0.1 (diameter) equates to a circularity tolerance of 0.1 (radial)?

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

 

[3DDave]

At least 0.2 < 0.1 isn't true.

There are certainly pathological cases otherwise. I always like those. Like grinding a flat on something nominally round or cutting a relatively deep groove. Then they aren't the same, but the part would be rejected for workmanship - just like a customer at the hardware store would toss that part back in the bin for the defective feature.

Anyway, nothing equates to anything in Y14.5 as there are always exceptions. Even when 0.2 < 0.1.

 

[Belanger]

0.2 > 0.1 (of course...that was a typo on my part.)
That aside, I just wanted to clarify that although circularity is a radial tolerance zone, it still must be less than the total size tolerance, not half of the total size tolerance.
Your post from 20:44 might be inferred by some as saying that it must be less than half of the total size tolerance.

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

 

[Steven_D_P]

Yeah, I was thinking it only is useful if it's less than half of the size tolerance since it's radial and not diametrical. But you're correct that there are certain specific exceptions that would fit the size tolerance without fitting the circularity tolerance so it rules out those.

 

[Wuzhee]

Nitpicking a bit more, but "A callout for circularity shall be specified on a surface and not to a size dimension".

 

[3DDave]

The admonishment about "not to a size dimension" is new in the 2018 version; at least it was not explicitly mentioned in the 2009 version.

I presume this was a dogmatic rather than a practical addition to avoid software somehow interpreting the circularity form tolerance as applying to the axis (how can an axis be circular?) the way a flatness tolerance associated with size applies to the, let me accurately parrot this, "derived median plane (which is not a plane at all but a lumpy surface of median points)?

Applying a feature surface-element-only control to an axis is not a thing that would happen.

Still, there's a quote about a foolish consistency; this fits.

 

[Garland23]

The admonishment about "not to a size dimension" is new in the 2018 version

Yet they still managed to violate that rule in Figure 8-13.