Question about GD&T. 2

[woodwood]

Hello all.

I've not had much to do with drawings that contain GD&T up until now, just because of the standard of drawings that we get, and I was after a bit of assistance interpreting a feature.
In the sketch you can see two diameters, the larger one being datum A and the smaller one with a concentricity of 0.000 in minimum and maximum metal conditions. I'm having difficulty understanding the min and max metal requirements and was hoping someone could shed some light (until Ican get some training!)

Thanks.
WoodWood.

 

[Belanger]

Frank -- I have to agree with pmarc and MechNorth on this.
Nobody should ever have an issue with zero at MMC (or LMC), unless they don't understand the concept of bonus tolerance. But the one proposed by Gary (zero at MMC & LMC) is inherently ambiguous. There's no ambiguity if you make the feature right at MMC or LMC. But the question is: since we have two points of departure, where do they meet? Suppose someone desires that as you depart from MMC, the bonus tolerance kicks in and keeps adding -- like we're all used to -- right up to the point where you're 1 micron from LMC and then at LMC the tolerance suddenly jumps to zero!
Of course, the opposite could be true. Maybe the bonus tolerance is tied more to the LMC, and suddenly the tolerance jumps to zero only when the MMC threshold is hit.
It's an intriguing idea -- and there may be some practical uses for this -- but as it is right now there is no strict definition of how those two variables interact with each other.

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

http://www.gdtseminars.com

 

[fsincox]

Guys,
Obviously, I must be so rapped in my own thoughts/perspective that I can not see the forest for the trees, here.
I will have to just trust you guys on this then, thanks.

 

[DeanD3W]

Sorry to come into this late. I agree with Evan that using profile would be much better for this case than the proposed (M) & (L) "thing"... I think a good way to explain why this would be more straightforward is to describe Position at (M) and Position at (L) as surface controls, with a virtual condition boundary for each... If you combine (M) & (L) then you get a virtual condition boundary on the MMC side and the LMC side also... These two perfect form boundaries could be more clearly imposed using Profile of a Surface.

As a matter of a fact, my opinion is that a lot of cryptic Position at (M) and Position at (L) tolerances (having two interpretations), especially "zero" tolerances at either condition would be more clearly said with a more capable Profile method that I wish we could move towards... This is a very large ship we're dealing with, with a large committee steering it, though. This opinion that I am stating is mine alone, at this point. Over time maybe profile will become less "evil" in industry :^).

Dean

http://www.d3w-engineering.com

 

[fsincox]

Well here is an example from my book, about 1992, it was intended as an internal company extention of the standard the example was really more as an explanation of the use of "individually".

 

[fsincox]

Sorry, I don't upload enough, I'll try again