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Imagine that fig. 2-7 from Y14.5-2009 shows a pin instead of the rectangular block, and the flatness callout has been replaced by straightness FCF.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.

 

[CheckerHater]

I also want to see the lathe that will produce this error

And just in case, there is such thing like default circularity in ISO.

It equals to the diameter tolerance (if there is no runout involved) if you invoke 2768-2

 

[pmarc]

First I have to invoke ISO 2768-2. Someone would probably have to put a gun to my head to force me to do this

 

[CheckerHater]

Pmarc,

Well, you are not happy with 8762. You have to admit ASME doesn’t have any general tolerancing standard.
You may not like independence, or the way it is presented in ISO, but I don’t think ASME definition of local size gives you any comfort.
So, what is left to mere mortals like us other than doing the best with the tools available?

Either way it was a great discussion. Much better than the regular stuff:

OP:
1. We don’t follow any standard in our company
2. Recently I’ve had an argument with my co-workers
3. Please tell me that I was right

Poster 1: Is it ISO or ASME you do not follow?

Poster 2: You don’t have to do anything; the Simultaneous Requirement will take care of it

Poster 3: Use Profile

The End

 

[bxbzq]

CH,
You are a genius. It would take me 4 hours to create those graphs.

J-P,
Back to your perfect form at LMC discussion, can I interpret your statement to this: there is a boundary at LMC size for each of the cross section of a shaft, and no circumferential point of cross section may violate the boundary? If this is the point you are trying to make, I say no. Picture a cross section of the shaft, a nipple at 12 o'clock, and a pit of same height and same diameter at 6 o'clock. It's possible that all 2-point measurements give you LMC reading. Obviously this shape violates the LMC boundary.

 

[Belanger]

I guess I have to toss up my hands on this one. It sounds like something that really needs a mathematical treatise, with formulas etc.

CH, your summary of the typical thread is hilarious! And quite true. But all of these discussions are fun, from the banal questions to the more academic stuff.

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

http://www.gdtseminars.com

 

[Belanger]

Bxbzq -- I don't know if all 2-point measurements will be the same LMC reading. Think about the rising side of that nipple: what is directly opposed to it?

This is a good point, though, and I'll have to think some more about it. Maybe the standard should clarify if MMC and LMC are taken purely on individual cross-sections or not. Notice that paragraph 1.3.38 and 39 speak of them in regards to features of size, which in our case is a "circular element."

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

http://www.gdtseminars.com

 

[pmarc]

J-P,
No special mathematical treatise needed (assuming your comment was to my question).

 

[Belanger]

So then what is the answer to your OP? What is the minimum perfect cylinder which will always contain the 5-star flower part? (It was probably up here somewhere but I missed it.)

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

http://www.gdtseminars.com

 

[bxbzq]

J-P,

I thought it a bit more and figured it does not have to be a nipple at one side and a pit the other, and it does not need to have all 2-point readings at LMC. The point is the LMC boundary.
In attached, suppose the LMC dia is 50. The blue curve is as-produced profile. Any 2-point measurement (cross the center) is no less than 50. But the boundary is at 48.4.

This brings similar question to OP, how wild the shape can go while still meeting size tolerance?

 

[pmarc]

The answer to my OP is 22.1 in theory of course. This is 21.6 (maximum possible circle in each cross section) plus 0.5 of maximum straightness tolerance.

 

[pmarc]

But there is much more important message coming out of this discussion:
When one invokes independency principle for a cylindrical FOS, form of the FOS should be still limited in both directions (longitudinal and cross-sectional) in order to keep the feature form somehow controlled. Focusing solely on longitudinal aspect is not enough.

 

[Belanger]

Thanks pmarc -- yes, the "I" modifier really opens the door to some crazy geometries.

Bxbzq -- the part you've drawn isn't really at LMC, though. See attached sketch; I added a pink line across the blue profile and rotated it to 3 places. The blue profile doesn't have an actual local size of 50 at every possible pair of opposing points. (You're correct that it doesn't have anything less than 50, but to be truly LMC they would all have to be 50 exactly.)



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

http://www.gdtseminars.com

 

[CheckerHater]

Belanger, pmarc,
In your opinion, which definition of “local size” will result in crazier geometry – ISO or ASME?

 

[bxbzq]

J-P,
I think it is possible that by modifying the upper arc in my sketch all 2-point measurements are 50 exactly, don't you think?
If you agree, I see this would apply to MMC as well...

 

[Belanger]

Bxbzq -- let me try a different tack: We all agree that perfect form is required at MMC. But why?
If something is truly at MMC, what type of deformation would cause it to violate that rule? (There are two choices: longitudinal deformation, or circular deformation. Which one or both?)

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

http://www.gdtseminars.com

 

[Belanger]

In your opinion, which definition of “local size” will result in crazier geometry – ISO or ASME?

I don't know if there's really much of a difference.

ASME defines it this way: "The measured value of any individual distance at any cross section of a feature of size."

I don't have ISO's definition at hand (I think pmarc said it's in ISO 14660-2?). Maybe someone can post their definition of actual local size and then we can discuss your question.

Come to think of it, ASME's definition doesn't say anything about passing through a center point. Read it literally, and you could say that a chord of a circle meets their definition for actual local size!

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

 

[CheckerHater]

That's what gives me uneasy feeling - the lid looks the same, but what size can of worms is underneath?
I guess explicitly specifying the roundness, or having a note like “unless specified roundness is so-and-so” will be good in both cases.

 

[fsincox]

Good discussion guys,
I wish I did not feel so empty now.
I also think it shows, I constantly point out about ASME, that actually both systems over simplify the real world just to handle things conceptually and it always leaves gaps to fill in the real world situations.
Frank

 

[pmarc]

This is how ISO 14660-2:1999 defines actual local size of extracted cylinder:

http://files.engineering.com/getfil...8a6-ab3a-3453ae9efa01&file=local_size_ISO.png

 

[Belanger]

That leaves me pretty confused. Is there more than just that page, pmarc? It simply says that those two conditions apply, but that's not a definition, just conditions. It also mentions the "local diameter," but is that really the same as the "local size"?
IOW, is a local diameter one consistent number or can a local diameter vary as we rotate around the circle?

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