FOS defined with basic dimensions... legal?

[dtmbiz]

If I have a cookie-cutter shape that I want to define with basic dimensions and use a profile tolerance, can two parallel and opposing flat surfaces in that profile be used as a Feature Of Size?

It seems to be that it could be, as the size it the basic dimension with the tolerance applied from the profile FCF.

Is this correct?

 

[MechNorth]

Yes, BUT a hole's (or sphere's) size (i.e. overall, not just at one point) can be controlled with a linear tolerance whereas a cone's size overall cannot be controlled by linear tolerances. Therein lies the difference. And yes, I do like to rap knuckles.

Jim Sykes, P.Eng, GDTP-S
Profile Services

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[axym]

JP,

There are those on the committee who feel that, if a size tolerance were present, a profile tolerance would only control the form of a cylindrical hole.

Here's another reason I don't like these examples that mix profile with a size tolerance. In almost all other cases in Y14.5 (DML straightness overriding Rule #1 notwithstanding), a geometric tolerance must refine what is already provided by other controls. But in 8-17, 8-18, and 8-27 the presence of a size tolerance un-refines (coarsens? overrides?) the control provided by profile. The control provided by profile depends on the presence (or absence) of a directly toleranced dimension. This just does not sit well. It doesn't lend itself to MBD, that's for sure.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[Belanger]

Let me stretch the analogy further. If some on the committee believe that it's OK to profile a hole that has a ± diameter, then I could make the same case for a radius curve. After all, a single radius is merely a portion of a circle, right?

Please tell me the committee would not allow something like R 13 ± 0.5 to then have a profile tolerance! Of course they wouldn't, because it's not a true profile. Ergo, I say same for a hole.

Evan, I don't see the difference between these two statements:

--profile must refine what is already provided by other controls such as size

--a size tolerance un-refines the control provided by profile

To me that's like saying "New York is east of Chicago, but Chicago is west of New York."

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

 

[axym]

JP,

I'm not sure what they would say about the radius suggestion. Probably something like "well, a radius is not a feature of size, so profiling it would be ridiculous" ;^)

It looks like my bitter rant about "un-refining" didn't get the point across. If a smaller tolerance A refines a larger tolerance B, the larger tolerance B does not un-refine the smaller tolerance A. I'll try the explanation again before I've had too much coffee.

Back to Figure 8-27. If the 80 was a basic dimension, the profile tolerance would control the location of the surface within a zone 0.07 wide. In other words, the height relative to datum A would be controlled within 0.035. Because profile controls location.

But if we change the 80 to a directly toleranced dimension, the height is controlled within +/- 0.2. Suddenly profile doesn't control location anymore. The addition of the size tolerance has "un-refined" the control that would have been provided by the profile tolerance.

This is not how combinations of geometric tolerances and size normally work. If I have a cylindricity tolerance of .005 and a size tolerance of +/- .010, the form of the cylinder is controlled within .005. We don't "un-refine" the cylindricity to .020.

Is that a better explanation? I suppose that the opposing argument is that profile only controls location (or relative location) if the true location is defined by basic dimensions. By the same token, profile only controls size if the true size is defined by a basic dimension. Does this mean that profile only controls orientation if the true orientation is defined by a basic angle?

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[Belanger]

Your last paragraph is right on target: Profile only controls location (or relative location) if the true location is defined by basic dimensions. Profile doesn't automatically control location just because a datum reference is invoked. Perhaps the standard doesn't state this outright, but because profile is a hodgepodge of several things we have to be careful to look at the dimensions to see what to extent the profile controls things.

So maybe say it this way ... a datum reference in a profile FCF means that we are tolerancing some sort of relationship with that datum. The "lowest-level" relationship of a surface to a datum is orientation. So that's what Fig. 8-27 is saying. However, if the height were changed to a basic dim, then we'd be controlling orientation and location (and of course size, too).

That said, I still don't follow the analogy about unrefining something. If two things control something, then the tighter one takes precedence. But the other thing can still control a broader characteristic.

Sorry if we deviated far astray from the OP. But I'm having fun anyway...

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

 

[MechNorth]

Exactly right, J-P. It's the hierarchy again. Location first, orientation second, form third. If a feature can be located, it must be located. In 8-27, the top surface is located by a +/- tol from the lower surface (or vice-versa). Now that its location is defined, the lower level functions of orientation and form can be refined ... note, no unrefinement.

Jim Sykes, P.Eng, GDTP-S
Profile Services

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[axym]

You guys are right, the effect of profile depends on the presence of basic dimensions. That's how they did it in the standard. I guess I just wish that it was some other way, because there is all sorts of ambiguity when you bring in implied basic orientation, MBD, etc. It might also be that I'd rather not have the size characteristic being mixed into everything. Size is such a nasty characteristic when you try to put rules on it.

Here's a question. Imagine a garden-variety position tolerance for a single cylindrical hole, with the standard three-plane ABC datum structure and basic X and Y dimensions showing true position. Now change the basic dimensions to directly toleranced dimensions. Does the position tolerance now control orientation only?

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[Belanger]

You guys know my feelings on that one

The very word "position" means that there is some sort of distance involved. So while orientation is one aspect of position, the position symbol must always[/i] have location leading the caravan, as it were.



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

 

[axym]

JP,

That's kind of the way I feel about profile. That it must control size if applicable, and must control location if applicable.

I guess I just want the profile zone to behave in a consistent way. I want the "magnitude" of the profile zone to remain fixed, so that it can't grow and shrink like a cylindricity or total runout zone. This takes care of the size control.

I also want the orientation and location of the profile zone to be constrained to the datums that are referenced. So that the location doesn't float like a parallelism zone. This takes care of the location control.

I believe this is the underlying logic of how profile zones work when basic dimensions are used. It's just subverted when the darned size tolerances are mixed in!

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[Belanger]

Well, let's go to the standard:

Paragraph 7.2 says that the position symbol involves "the location of one or more features of size." That's why I say using position for orientation-only isn't possible; it would be missing the constitutive element of the symbol.

But Paragraph 8.2 says that the profile symbols control "form" or "combinations of size, form, orientation, and location of a feature." That's why I say profile need not have location as part of the package; form might be the only thing profile does in certain cases.

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

 

[MechNorth]

OK, in essence then, we are all agreeing that (1) position SHOULD NOT be used as an orientation-only control <legal or not, it's a bad practice>, (2) profile should be used in as robust a means as possible, controlling as many of the feature's aspects as it can.

I've read several trainers' materials and could never understand why some of them have written things as being "law" when in fact it's just their personalized "best practices". I talked with some and heard it was because trainees miss the subtleties of "must", "should", "should not" and "must not". I was dubious until a year ago when I heard feedback that a trainee had taken what I'd described as "technically" correct but poor practice as the gold seal on the practice.

Jim Sykes, P.Eng, GDTP-S
Profile Services

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com