Feature of Size definition

[Belanger]

I promise that I won't bring up the thing about directly opposed/calipers.

Last week I had a discussion with a colleague about some of the changes in the 2009 ASME standard. He pointed out something that changed, but I wonder if he's interpreting it as it was intended:

The new standard defines a feature of size (FOS) as "one cylindrical or spherical surface, a circular element, and a set of two opposed parallel elements or opposed parallel surfaces, each of which is associated with a directly toleranced dimension."

His contention is that you can't just look at a drawing of a part with a hole and say that it's a feature of size, because it may lack a "directly toleranced dimension." For instance: a hole might be dimensioned as Ø18, but is has no plus/minus tolerance. We would of course look to the title block tolerance, but GOTCHA -- this is not a "directly toleranced dimension," so the hole isn't a FOS.

I say that's stupid (not to his face) because that leads to the conclusion that we can't put a position tolerance on the hole, because the standard also says that position must be applied to a FOS.

Of course I could be wrong. But any thoughts on why they added the business about "directly toleranced dimension"?

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

 

[pmarc]

John-Paul,
Y14.5M-2009 standards says that dimensions with plus/minus tolerances are not the only method of direct tolerancing that can be used. Please take a look at para. 2.2 (a)-(c). Subparagraph (c) clearly says that Geometric Tolerances Directly Applied to Features should be considered direct as well. So in your case position tolerance on a hole (which is for sure a FOS) is absolutely legal and does not interfere with FOS definition.

 

[Belanger]

Thanks -- and I agree with that part. The dilemma is for title block tolerances -- are they considered "direct tolerances"? I say yes, because they are not "derived" tolerances. He says no: direct tolerances must be "directly" applied to the individual dimension.

Another angle to approach it from: Is there such a thing as a potential FOS that might not have a "directly toleranced dimension"?

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

 

[dgallup]

Another angle to approach it from: Is there such a thing as a potential FOS that might not have a "directly toleranced dimension"?

Sure, you could dimension one side from a first reference and the second side from a different reference.

 

[Belanger]

Yep -- that makes sense.

So on the attached graphic, the rectangular cutout has three possibilities:

The first example would be called a feature of size (FOS).
The second example is not a FOS.
We cannot say if the third example is a FOS or not -- we need to see the dimensioning

My point is that in conversation, we often refer to holes/pins/slots as features of size, but we can't do that without first verifying the dimensioning scheme.

And back to the original question, are we all in agreement that dimensions which appeal to title block tolerances count as "directly toleranced dimensions"?

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

 

[tk369]

IMHO, I agree with John-Paul.

In my titleblock, there is a statement that :"Unless otherwise specified, dimensions are in millimeters, and tolerance on 1-place decimal: +/-___, 2-place decimal: +/-___," and so on.

IF one has tighter/looser tolerances then the titleblock calls for, it's added it to the dimension. Otherwise, the titleblock tolerance determines the FOS yielding a directly toleranced dimension.

teddykaye

 

[pmarc]

John-Paul,

I am starting to worry that my understanding of FOS definition is not good enough . I am very close to agree with you that only knowing dimensioning scheme we can tell whether a feature is a FOS or is not. But as I told it some time ago in a different thread, this FOS definition seems to be so ambiguous in almost every word, that I wouldn't be surprised if somebody had different opinion about the issue.

Regarding title block tolerances: I think they should be considered as direct ones, but this is only my opinion and unfortunately I do not have anything from the standard that could support this statement.

 

[pmarc]

Maybe one more thought from my site:
In most cases general title block tolerances are +/-. So if +/- tolerances are considered as one of direct tolerancing methods then I do not see a reason why general tolerances should not be treated as such. The fact that tolerance is not specified right next to a dimension value does not mean that it is not there.

 

[fsincox]

J-P,
Only the committee knows for sure, but, your example makes sense to me, dimensions from a base line are not directly toleranced, even were it discribes a feature, like you show, that could be a feature of size if dimensioned another way, interesting. I must say the whole obsession with how to determine a feature of size has blindsided me, I am too ISOish I guess.
Frank

 

[tk369]

According to Jim Meadows, a member of ASME Standards Committee Y14, Subcommittee 5.1, I submit to the group:

"Feature of size: One cylindrical or spherical surface, or a set of two opposed parallel surfaces, associated with a size dimension."

No mention of tolerance, either direct or block.

This information follows up on my earlier post.

ted kralovic
liverpool, ny

 

[Belanger]

Hi Ted,

The question has to do with the new 2009 standard, which made a change to the definition of a feature of size from the one you quote.

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

 

[ewh]

You mean that the '94 standard includes "or"? Then why all of the discussions regarding FOS requiring opposing surfaces?
;-)

"Good to know you got shoes to wear when you find the floor." - [small]Robert Hunter[/small]

 

[KENAT]

Dude!

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[fcsuper]

Direct Tolerance is also defined in the standard at paragraph 2.2. It doesn't include annotation methodology, only type:
Limit Dimensioning, Plus and Minus Dimensioning and Geometric Tolerances that are directly applied to the feature.

This says nothing about where such types are located on a drawing or how they are noted. Any of these dim types can be located anywhere on a drawing as long as the link between them is understood as defined by the drawing itself or ASME standards. You can have them in your notes, the title block, directly attached to a dim, one a second sheet or even another document. That doesn't change it from being "direct".

Matt Lorono
Lorono's SolidWorks Resources & SolidWorks Legion

http://groups.yahoo.com/group/solidworks & http://twitter.com/fcsuper

 

[fsincox]

"applied directly to the feature" it seems to fit j-p's example.

 

[Belanger]

Thanks all! Sorry that my question diverged into two different questions. But it seems that the consensus is:

title block tolerances do qualify as "direct tolerancing" even though they don't appear next to the dimension

and

the new definition of a FOS requires that it's not merely something with opposing sides, etc. but that it also must be directly dimensioned and toleranced across those opposing sides (as in my JPG attached above)

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