Circularity in the 2018 standard

[Belanger]

I suppose we could start an "errata" thread for the new standard, but here's one that I just noticed. In paragraph 8.4.3, it now says that the circularity callout "shall be specified on a surface and not to a size dimension." Yet in Figure 8-13 it has the FCF under the size dimension.
I realize that that figure is a carryover from the previous edition so I suspect it's an oversight. Or do you think there's something about the AVG idea that warrants the FCF being right there? (I doubt it, because "shall" is a pretty strong word.)

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

 

[greenimi]

Or do you think there's something about the AVG idea that warrants the FCF being right there?

If that is true then why in fig 8-14, the circularity FCF is not under the size dimension?
Just because the circularity FCF has F modifier? Is this reason good enough?

 

[mkcski]

JP:

A I read it: The rest of that sentence in 8.4.3 offers the exceptions which includes AVG.

Certified Sr. GD&T Professional

 

[axym]

J-P,

I'm not really sure what they're getting at here. I think it's something to do with the two different ways that straightness can be specified, and for some reason applying a similar clarification to circularity specifications. I'm not sure what the difference would be if circularity was specified to a size dimension as opposed to a surface (I don't think there would actually be any difference). With straightness, if the FCF is applied to the size dimension then it means DML straightness, and if it's applied to the surface it's line element straightness. But there is no such distinction for circularity, as far as I am aware.

Perhaps if circularity was applied to a torus-shaped feature, it could control the form of the nominally circular derived median line. How about that? It would control both the roundness and flatness of the circle, in a torus-shaped zone. Sort of a 3-dimensional circularity instead of 2-dimensional. Maybe I'll make the suggestion at the meetings next month, and bring donuts as demonstration models. That might sweeten the deal ;^).

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[mkcski]

From a "where is the tolerance zone - surface or axis" approach. I prefer it be a leader to the surface and not "attached" to the size - which generally indicates axis control.

Certified Sr. GD&T Professional

 

[Belanger]

Evan -- like you, I think the intent for circularity is that it always applies to a surface. So to me it doesn't matter where the FCF is placed because there's only one way to interpret it (unlike straightness).
Not sure about your torus idea. I prefer to keep it simple and say that they new standard just has a goof!

Mkcski -- That's what I was wondering too. But in reading it again, I don't think the AVG thing in any way nullifies the previous statement that circularity's FCF shall be applied to a surface, not size.

All -- might the same thing happen for the runout tolerances? I know of examples where a circular runout FCF was placed beneath the size dimension.



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

 

[3DDave]

Runout should have required using a dial indicator symbol, the way that "Save" still uses a floppy disk icon and the "Copy" icon uses the folded corner icon from Xerox machines to indicate the original needs to go face-down.

 

[greenimi]

I'm not sure what the difference would be if circularity was specified to a size dimension as opposed to a surface (I don't think there would actually be any difference).

Evan,

The same “additional requirement” has been added in 2018 for cylindricity.
“A callout for cylindricity shall be specified on a surface and not to a size dimension.”

I think that the “new requirement” has been added for circularity and cylindricity to help transition to MBD.

I’ve never been in any committee meetings, but you might know more on why they decided to add this requirement if it is no for MBD

 

[mkcski]

greenimi: What is MBD?

Certified Sr. GD&T Professional

 

[greenimi]

MBD = Model Based Definition.

 

[mkcski]

greenimi:

Thanks. I am not in the model "world".

Certified Sr. GD&T Professional

 

[3DDave]

MBD = all the same work to make a drawing but with the added requirement that everyone shares the exact same tool chain for a huge amount of money and extra confusion about what the user of that data did and did not see (because there's no equivalent of a highlighter to mark off the parts that have been checked.)

MBD reason for existence = CAD companies have run out of the cheap things to develop and the remaining ones are really expensive -> Looking at you 3-letter company, that's promised sweeping 3D Solids to create solid geometry for a half-decade or more, but still can't be bothered to give users CPU/Memory profiling tools to identify optimal model building techniques.

Still, in the CAD database, the dimension should be tied to the surface and a link from the tolerance to the dimension can be followed to the surface.

 

[pmarc]

All -- might the same thing happen for the runout tolerances?

I think it's already happening in 2018 but in a bit inconsistent way. Oh well... Who would even care about that? [sarcasm]