Question about Independency Principle 1

[pmarc]

Imagine that fig. 2-7 from Y14.5-2009 shows a pin instead of the rectangular block, and the flatness callout has been replaced by straightness FCF. Everything else stays the same (of course the diameter symbol is added to 10.7-10.8 dimension).

Knowing this, what is the minimum possible diameter of a perfect cylindrical boundary that the pin would never violate?

Thanks.

 

[Belanger]

I would say 11.3, because the actual local size of the pin could be 10.8 and that top surface could bow by 0.5. Of course, the opposing (bottom) surface would also have to bend the same way in order to preserve the actual local size.

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

http://www.gdtseminars.com

 

[pmarc]

And what about impact of independency principle on what happens in each individual cross section of the pin?

 

[Belanger]

Independency means that the actual local size must be met at each cross section, each on its own terms. Then, each longitudinal line element must be straight within the given tolerance, but each on its own terms. The two ideas don't go together, as Rule #1 might normally require.

So I know you're after something But the notion of Independency is that it doesn't have any effect on each individual cross-section, other than the local size. (I haven't considered the circularity aspect; would that be a consideration in your question?)

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

http://www.gdtseminars.com

 

[pmarc]

Yes, I am thinking about circularity aspect.
In the absence of a circularity control, are we able to give exact value of the boundary that the pin will never violate?

 

[CheckerHater]

Cross-section boundary or envelope boundary?

 

[pmarc]

Size of "envelope boundary" is what I am interested in.

 

[CheckerHater]

I was under general impression that the idea of independence principle is “if you need something, you specify it.”
If you want to control boundary, you can specify circularity, as you mentioned, also envelope requirement, MMC, possibly runout or profile (depending on function).
You are not complaining about lack of options, are you?

 

[pmarc]

CH,
No, I am not complaining about lack of options.
The question is solely about understanding of independency principle.

 

[CheckerHater]

If you have an old (before 2011) copy of ISO 8015, the Fig. 2 and Para. 6 pretty much summarize the difference.

 

[pmarc]

Okay, I have this figure in front of me, but still would like to hear your answer to my initial (or follow up) question.

J-P's choice was 11.3. Do you agree with that?

 

[CheckerHater]

Right now I am leaning towards 11.3
Looks like ISO isn’t quite sure how to extract generating line of a cylinder.
Is your question still standing for median line straightness as well?

 

[pmarc]

I do not think straightness of derived median line has anything to do with this. I also do not think this is ISO-specific problem.

The point is that when thinking about independency principle we very often visualize feature's geometry along its axis only (e.g. (I) modifier without any additional geometric control will allow banana shape to happen, etc.). But what does the independency principle mean for feature's form in each cross-section? Is this form controlled at all? Does dia. 10.7-10.8 specification (in conjuction with (I) modifier) mean that the diameter of 10.8 cannot be violated?

 

[CheckerHater]

Any local 2-point measurement cannot exceed 10.8

 

[CheckerHater]

I think I know what you are asking.
If we specify larger tolerance, say DIA 1.414/1.000, will 1.00 square cross-section be acceptable?
I say yes.

 

[pmarc]

Look once again at Figure 2 in ISO 8015:1985 and associated description in clause 5.2. Each local 2-point size is at maximum there (in my case this would be dia. 10.8), but the perfect circular (2D) envelope that cannot be violated is enlarged by circularity tolerance value. If in my case the circularity tolerance was 0.02, the circular envelope would be dia. 10.82, right? So would this still give you dia. 11.3 of cylindrical (3D) envelope?

And going further, in the absence of circularity tolerance, are we able to find the size of the 3D cylindrical envelope at all?

 

[CheckerHater]

OK, here is one possible “bad-case scenario”.
You will need larger envelope, but no physical feature will exceed 10.8 measurement.
So, is “diameter” violated?

 

[CheckerHater]

I guess we posted at the same time

 

[CheckerHater]

And going further, in the absence of circularity tolerance, are we able to find the size of the 3D cylindrical envelope at all?

I think in absence of envelope requirement we shouldn't even try in the first place.

 

[pmarc]

I think in absence of envelope requirement we shouldn't even try in the first place.

Okay, so if my initial question was: "If the independency principle is invoked - either by default (ISO) or by placing I modifier (ASME) - do the limits of size control circularity of a cylindrical feature of size?", would that sound better to you?