Any fundamental difference here? 2

[DonkeyPhysics]

Hi there,

This may seem like a very basic question, but I just want to make sure I'm not missing anything.

Is there any conceptual difference between these two examples? I believe the answer is no, but I'm willing to be corrected.

In general, I'm in the middle of training my group here how/why GD&T is a better approach vs direct tolerancing, but... in this specific example... I can't think of any way these two statements (shown in the attached figure) are different.

Ultimately, if I wanted to give a looser tolerance to the profile (i.e. 0.3 unilateral), and add in a parallelism refinement of 0.2, I believe I could accomplish that same requirement by using a direct tolerance of +0.3 /-0.0 with a parallelism call-out of 0.2. Is that right?

Thanks!

 

[greenimi]

With my limited GD and T knowledge, I would say that in order to get equivalent results on both schemes, you would need to use the “dimension origin” symbol on the ± dimensioning scheme. In this way, you will have defined direction of measurement with no ambiguity (the inspection measures in a single direction originating from the appropriate feature)

 

[Belanger]

There is a difference in that they allow a different "actual local size." In the first picture, every cross-section (think of measuring with calipers) must be no less than 10.0.
But in the second picture, there could be a cross-section of 9.9. This is because the profile across the top (which allows a minimum height of 10.0) is measured from datum A. And datum A is the plane established by only the extremeties of the bottom surface. There could be a valley of 0.1 lifting up off the inspection plate, and this might make that particular cross-section as small as 9.9.

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

 

[Belanger]

My post is based on the ASME rules, not ISO, by the way.

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

 

[DonkeyPhysics]

OK, I see your point. Thank you!

In this case, then, as I've defined the scenario, it seems like the direct toleranced example more correctly accomplishes the design intent.

 

[DonkeyPhysics]

Is it wrong of me to be enjoying this stuff? I mean, I'm obsessed at the moment out of necessity, but I find the whole process quite entertaining. It's like a giant puzzle.

Sure, there are multiple acceptable ways to control any given set of features, but it's fun finding the "Best" way of doing things, and that means arming one's self with the best possible understanding of the rules. The rules ARE sufficiently exact. So, the game for me is both seeing for myself where the wiggle room is, and also figuring out how I can explain things to my coleagues here (we're not even going to get into the Spanish translation part, which is a whole separate challenge for me).

 

[SeasonLee]

A minor problem on your 2nd picture, phantom line (not dashed line) needed to indicate the unilateral outside of material tolerance zone.

Season

 

[Belanger]

Nothing wrong with enjoying this stuff, DP! And yes, there are multiple ways to say things (with and without GD&T) and half the fun is de-coding all of those.
Always remember, though, that GD&T is a language and like any other language people may say things in different ways. Sometimes they mean exactly the same thing, but at other times a small nuance will make the statements quite different.

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