Refining a profile tolerance with an inch/inch type spec?

[fsincox]

I believe it has been discussed before here:
Can a profile tolerance be refined with an inch/inch type specification? or a sqin per sqin specification?
Like straightness or flatness does.

 

[powerhound]

Yup. See here:

http://www.tec-ease.com/gdt-tips-view.php?q=112

Powerhound, GDTP S-0731
Engineering Technician
Inventor 2013
Mastercam X6
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

 

[fsincox]

Powerhound,
Thank you and Don!

 

[pmarc]

1. The tips mentions paragraph 8.3.2.1 of Y14.5-2009? Shouldn't it rather be 8.3.2.2?
2. I am curious why flatness per unit area callout was not used instead of lower segment of profile FCF? Does 8.3.2.2 really say anything about profile per unit area application?

 

[Belanger]

Pmarc -- profile would probably still be best on the lower segment because it is a refinement of the upper segment, which must be profile because it references a datum.
The note in 8.3.2.2 does indeed mention "per unit length" but not you've got me thinking that "per unit area" makes more sense because profile of a surface is applied to a 2-D or 3-D surface. So one linear dimension only seems incomplete?

In the OP I think Frank was referring to this thread from a while ago:
[URL unfurl="true"]http://eng-tips.com/viewthread.cfm?qid=325577[/url]

If I recall, the issue was for profile applied to a contoured surface, where it might not always be clear if the given area dimensions conform to the curve or are the flat-only dimensions of the area. Obviously that's not an issue in the Tec-Ease tip.

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

http://www.gdtseminars.com

 

[fsincox]

JP,
Yes, that was it. Don's example does mention non-flat surfaces at the end, there too.

 

[pmarc]

J-P,
Are you saying that flatness per unit area callout as the 2nd FCF would not be equal to what the tip shows? (Of course we are talking about controlling geometry of nominally flat surface here).

 

[Belanger]

No, I don't mean that it wouldn't be the same -- it would be. But it would require two separate FCFs. Having it stay as profile "streamlines" it all into the same symbol, and to me it's more clear because one concept is given a global and then a local tolerance under the same banner called profile. But in the end, what you propose is the exact same meaning.

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

http://www.gdtseminars.com

 

[MechNorth]

J-P, I am an advocate (maybe not so surprisingly) of using composite controls over multiple single segment controls to refine higher-level controls. While multiple single segments may lead the user to understand the orientation & form/size refinements, there is an inherent and legal difference. In a composite profile control, refinement levels must follow the same datum precedence, with the last datum(s) being dropped, but no resequencing permitted. With multiple single-segment FCFs to provide the refinement, each FCF can have a unique DRF, which changes it from a refinement to an entirely independent control. Such a discrepancy jumps out at the reader in a composite control, but not in the multiple FCFs unless the reader is somewhat advanced in knowledge/skill in GD&T.

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

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[pmarc]

Jim,
That is totally true what you are saying.
However in the tip there is only one datum involved, so resequencing is not really an issue.

Let's imagine the tip with slight modification. The upper surface being controlled by 3-segment composite profile FCF:
|prof.|0.6|A|
| |0.3|A|
| |0.1|

And now, let's switch it to 3 separate FCF's:
|prof.|0.6|A|
|paral.|0.3|A|
|flatn.|0.1|

I guess we agree that these are equivalent callouts. So how do you think, which option will be clearer for an average GD&T user?
I wonder how many folks would grasp the difference between 1st and 2nd segment in the composite profile FCF.

 

[fsincox]

Thanks guys,
Out here, we are going to use the three tier composite only when no other control will do, just do to its lack of a strong definition in earlier standards (we still reference 1982). I was a strong advocate of the addition of a definition for the restatement of datums in a composite tolerance and saw no reason to make it the same as 2 separate statements. I am very pleased by the way this issue did turn out in the standard. It just adds more tools to our tool box. It is one of the few things that did turn out like I thought it should!

 

[MechNorth]

I understand perfectly what you are saying, pmarc. Unfortunately we deal with conflicting realities; (1) people are generally undertrained in GD&T, so they don't adequately understand composite profile tolerancing (or profile tolerancing in a general sense), and (2) if we "dumb down" the callouts so that absolutely everyone can understand them, we run a greater risk of the callouts being incorrect due to datum resequencing. In the typical environment of today where we don't have checkers and most designers still aren't catching their own mistakes, I am more afraid of scenario #2 than scenario #1. Personal bias, yes, but based on lots of experience with both situations.

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

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[ptruitt]

I am still thinking about cubical vs. spherical tolerance zones. Noone responded the first time I posted it at

http://www.eng-tips.com/viewthread.cfm?qid=325577.

(I don't blame you guys/gals. I get in over my head all too often.) One of the attached illustrations shows a cube, with a basic surface and its boundaries, similar to Paul’s. The other illustration (same PDF) shows the same basic surface and boundaries, but clipped by a circumscribed sphere. My contention is that the boundaries of each are likely to work equally well, functionally, in the design. (The same argument as square vs. circular tolerance zones.) The table shows how the surface area of the two boundaries, as clipped by the sphere, have a more linear variation from the basic surface than the cube...

Peter Truitt
Minnesota

 

[MechNorth]

Peter,
I reviewed the previous thread again, but I don't see why you would consider these 3-D zones; the zone boundaries would be offset from whatever the basic geometries are. For example, pictue an Eggo Waffle with an all-over profile control. The tolerance zone boundaries would be the expanded & contracted offsets from the nominal surfaces. Thus, I don't see any relevance to the spherical versus cubic tolerance zone, except as an abstract consideration. I seem to be missing the point of it?

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

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[ptruitt]

Hmmmmm. Where are the words??? I suppose you realize that the CAD model used for both illustrations is the same except I circumscribed a sphere around the cube as a last step in the illustration on the right side. Someone might be confused by the fact that I rotated the model after adding the sphere. I should have left it in the same orientation for clarity. It seems as though if I were to make up my own definition of profile to be about a unit of spherical volume, it would allow tolerance that more closely relates to function. (I have never had an Eggo Waffle. Maybe it would provide the nourishment my poor brain needs for comprehension, here.)

Peter Truitt
Minnesota

 

[fsincox]

Peter,
Any waffle would do, it doen't have to be eggo.