Overiding Rule #1 3

[SpaciouS]

Why was the ability to override Rule #1, with straighness on a FOS dimension, created in the first place?

I'm curious to hear other people's opinions.

Thanks,
Sean

 

[fsincox]

Basically, in practice, for too many cases "the envelope principle" is unrealistic.
Frank

 

[Belanger]

Since Rule #1 is a catch-all default that controls form unless otherwise specified, then having a straightness tolerance on a feature of size counts as "otherwise specified."

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

http://www.gdtseminars.com

 

[CheckerHater]

There is a good review of Rule 1 exceptions in this place:

http://www.mechsigma.com/newsletters/2003_02_Newsletter.pdf

Straightness is number 3. I wouldn’t even try to explain it better

 

[SpaciouS]

As I understand it, the only time rule #1 is overridden is by the following:
1) Special note stating such override
2) Stock material is not applicable
3) Straightness control on a FOS dimension

So in terms of geometric characters, there is only the one way. That's why it's curious to me. But after thinking more on it, I tend agree with fsincox and his comment about an unrealistic envelope. In a lot, if not most cases, straightness control of a FOS (axis or median plane) would be for long parts. By virtue of the indirect form control of Rule #1, the envelope for a part that is long can be unrealistic if the size tolerance is relatively small.

I think this logic makes sense.


Thanks,
Sean

 

[CheckerHater]

If you followed my link, you would find 4) When LMS/LMB is applied.

 

[Belanger]

To Sean's list, another would be non-rigid parts: Rule #1 doesn't apply for non-rigid parts subject to free-state variation.

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

http://www.gdtseminars.com

 

[SpaciouS]

Yes that's right, non-rigid parts subject to free-state, as shown in CheckerHater's link, is another case...

I was really focusing on geometric characters, more than anything, and the fact that there is only one that applies and even that has to be specifically relative to a FOS dimension. Just a nice mental note, that no other geometric characters override Rule #1.


Thanks,
Sean

 

[pmarc]

Just a nice mental note, that no other geometric characters override Rule #1.

True as long as you are working to '94 version of Y14.5. In Y14.5-2009 there are two additional ways to override rule #1:
1) Independency modifier (I in a circle) - which is nice and short substitute of text note.
2) Flatness control on a FOS dimension - controlling derived median plane of a non-cylindrical FOS.

 

[SeasonLee]

Rule #1 does not apply to an irregular FOS, it only apply to regular FOS.

Season

 

[SpaciouS]

Ahh okay, thanks for the tip pmarc, I was not aware of this.

I will review the 2009 standard on this topic!

Thanks,
Sean