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Question about Independency Principle

Question about Independency Principle

Question about Independency Principle

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Imagine that fig. 2-7 from Y14.5-2009 shows a pin instead of the rectangular block, and the flatness callout has been replaced by straightness FCF.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

RE: Question about Independency Principle

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

RE: Question about Independency Principle

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

RE: Question about Independency Principle

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

RE: Question about Independency Principle

(OP)
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?

RE: Question about Independency Principle

Cross-section boundary or envelope boundary?

RE: Question about Independency Principle

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

RE: Question about Independency Principle

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?

RE: Question about Independency Principle

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

RE: Question about Independency Principle

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

RE: Question about Independency Principle

(OP)
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?

RE: Question about Independency Principle

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?

RE: Question about Independency Principle

(OP)
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?

RE: Question about Independency Principle

Any local 2-point measurement cannot exceed 10.8

RE: Question about Independency Principle

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.

RE: Question about Independency Principle

(OP)
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?

RE: Question about Independency Principle

I guess we posted at the same time smile

RE: Question about Independency Principle

Quote (pmarc)

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.

RE: Question about Independency Principle

(OP)

Quote:

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?

RE: Question about Independency Principle

Yes, better
No, they don’t control circularity

RE: Question about Independency Principle

Good stuff! I step out for one day and y'all run with some good comments. After the initial question, I then suspected that pmarc was referring to circularity rather than the longitudinal aspect.

Does CH's drawing of the 3-lobed part mean that the question has been answered? Or are we saying that ISO is a little ambiguous about this point?

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
http://www.gdtseminars.com

RE: Question about Independency Principle

(OP)
What do you exactly mean by saying that "ISO is a little ambiguous about this point"?

RE: Question about Independency Principle

Well, I guess I don't really mean ambiguous. But here's what confused me: It sounded like the theory presented here was that ISO (or use of the I symbol in ASME) means that there is absolutely no control on circularity. I don't think that's true; if each diametrically opposed pair of points is within the size tolerance, there is still only so much deviation from one pair to the next.

Think of it this way: If we measure only 3 pairs of opposing points, then the circularity error could be awful. If we measure 10 pairs then the circularity is a little better. Now, keep increasing the number of pairs you measure ... independency still means that there are an infinite number of opposing-point pairs that must maintain size. If they are all right next to each other, there can't be too much of a jump in terms of their combined roundness. I hope that makes sense.

So is this the answer? -- The independency rule means that circularity is no longer limited within the size tolerance, but there still is a maximum limit to the circularity error.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
http://www.gdtseminars.com

RE: Question about Independency Principle

The reason they like to put lobed shapes in textbooks so much is that said shapes can be measured infinite number of times and return the same number.
I understand the frustration coming from ASME upbringing – the idea that diameter symbol demands perfect circle/cylinder, at least under some condition(s) is very comforting.
Entering the world where diameter is applied to part that may never be round is unsettling.
But guess what?
Just think of all shapes your “round” part may take when approaching LMC and you will realize that even under default envelope principle applied, roundness/circularity is not really controlled.
Imagine part DIA 10.000/.500 (E)
As perfect form is not really required at LMC, square .500 will perfectly fit both the tolerance (no less that .500 cross-section)and envelope (not exceeding 1.000 boundary)
So it’s all in your head, gentlemen smile

RE: Question about Independency Principle

I get that CH -- the lobe shape has often been used to illustrate that opposing points can give a false sense of security with respect to roundness. But do you really think that removing the envelope idea means there is absolutely no limit to the circularity error?


Also... a square of .500 x .500 is not really at LMC... that's a different story.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
http://www.gdtseminars.com

RE: Question about Independency Principle

Yes, square .500 x .500 is not LMC.
It is one of infinite number of intermediate shapes going from MMC to LMC that satisfy both tolerance an envelope requirement in my example.
It’s just it cannot have local measurement lesser than LMC size of .500.

No, I don’t think circularity error will be infinite. It’s just difficult to derive it mathematically.
But I am also curious about your opinion: if perfect round shape is only required at MMC, do you see any limits to how “ugly” shape may become while staying within the tolerance, and what is preventing it from distortion in absence of explicit circularity control?

RE: Question about Independency Principle

Quote:

...if perfect round shape is only required at MMC, do you see any limits to how “ugly” shape may become while staying within the tolerance...
In the sense that we are discussing, perfect form is required not only at MMC, but at LMC. That is, only if we consider the cross-sectional aspect of a part -- forget the longitudinal aspect. In that case, perfect form is required at MMC and LMC. Think about it: if a diametrical shaft is truly at least material condition, it would have to be a perfect circle. If it has a ding going inward, then it violates size. If it has a spike going outward, then it's not at LMC!

The reason we say that perfect form is not required at LMC is that in the longitudinal direction the shaft could bend. It's still at LMC, and it's still within the envelope rule.

Anyway, I concur that with the independency concept invoked, circularity is not controlled by size. But there has to be some limit to the circularity error, and I suppose that it is more difficult to derive. So we're right back to pmarc's original question!

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
http://www.gdtseminars.com

RE: Question about Independency Principle

That’s very interesting
Could you quote where the standard says that perfect circularity is required at LMC?
Also, what about infinite number of shapes BETWEEN MMC and LMC?
Are all of them perfectly round as well?
Simple question: Part on my drawing. Will it pass the inspection? Simple Yes or No, please.

RE: Question about Independency Principle

I can't quote that from the standard. But I ask you to re-read my explanation: I was clearly limiting my statement to only the 2-D concept of a circle. If that still doesn't make sense, I will try to post a sketch later today.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
http://www.gdtseminars.com

RE: Question about Independency Principle

size, actual local: the measured value of any individ-
ual distance at any cross section of a feature of size.

what does this mean to you? Which two points would you pick to measure odd shape?

RE: Question about Independency Principle

Bxbzq, If you are asking me, I already made this picture:
http://files.engineering.com/getfile.aspx?folder=9...

The rule seems to be pretty clear:

From Para 27.1 (b):
“Where the actual local size of a regular feature of size has departed from MMC towards LMC, a local variation in form is allowed equal to the amount of such departure.”

So my part is departing from MMC and it still far away from perfect LMC cilcle.

Belanger, you put me in awkward position:
On one hand it’s impolite to insist that you answer my question.
On the other hand I only asked for simple “yes” or “no”.

RE: Question about Independency Principle

(OP)
This is how I see the answer to my initial question and to the question asked by J-P about maximum possible circularity error when the independency principle is in charge. Feel free to comment and disagree.

http://files.engineering.com/getfile.aspx?folder=6...

RE: Question about Independency Principle

Sorry CH -- yes, that part will pass inspection.

Now, my turn :)
Do you see how a part strictly at LMC (diametrically only; not axial-wise) has to be at perfect form?

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
http://www.gdtseminars.com

RE: Question about Independency Principle

Pmarc,
Where does it say that I am forbidden from measuring across the star?
Does “point-to-point” measurement mean “only between the points pmarc has chosen?
And as soon as I measure anything above 10.8, the part fails inspection.
I cannot believe you were serious.

Belanger,
If the drawing explicitly states that certain requirement applies at LMC, than perfect form is required.
That’s the letter of the standard.
In any other situation real part always “departs” from one to another.


RE: Question about Independency Principle

(OP)
CH,
As for your measurement accross the star - this is one of the most serious shortcomings of Y14.5 standard. The definition of actual local size is really unclear. In ISO this is clearly standardized in a way that each line connecting two points must pass through the center of an associated circle.

So if it makes you feel any better, we can say that my picture follows ISO definition of actual 2-point local size.

RE: Question about Independency Principle

Pmarc,
You are going to have to explain it one more time, now with more words. Reference to the standard(s) may help as well

RE: Question about Independency Principle

(OP)
ISO 14660-2:1999 is what you need - Figures 1 and 4 and associated paragraphs.

If you have no access to the standard, I can try to explain it, but to be honest nothing will be better than the two figures and the standard text.

RE: Question about Independency Principle

Quote:

If the drawing explicitly states that certain requirement applies at LMC, than perfect form is required.
That’s the letter of the standard.
Arrrgh! You're not reading my posts, CH :)
Yes of course, if the shaft is at LMC then the LONGITUDINAL aspect is not required to have perfect form (straightness). But if you just consider one cross-section (which I clearly stated as the caveat), and that cross-section happens to be at LMC, tell me exactly how it could be out of round and still be called LMC?

Try it this way, using the MMC perfect-form rule: If an external circle is at MMC, you agree that it must have perfect form. Would you say that the circle could have a lobe extend outward? (No -- because it would exceed the size limit.)

Now, if that MMC circle had a ding inward, is that allowed? Yes or no?

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
http://www.gdtseminars.com

RE: Question about Independency Principle

Belanger,
If perfect form required at MMC, ding is allowed.
“Flat” cross-section has no “material”. Do you believe part with perfect round cross-section that is also bent is at LMC?

Pmarc,
While I am on the look-out for ISO 14660, could you at least clarify which one of your point-to-point dimension is a diameter?
After all your OP was about DIAMETER 10.7-10.8?

RE: Question about Independency Principle

(OP)
My picture shows theoretical as-produced geometry of nominally round shaft. I know, it does not look realistic, but we are debating on what can be possible from theoretical point of view.

RE: Question about Independency Principle

Based on what theory exactly?

Quote (pmarc)

we can say that my picture follows ISO definition of actual 2-point local size.

In ISO the size can only be defined on features of size.

So, let me rephrase my question: "could you at least clarify which one of your point-to-point dimension is a feature of size?

RE: Question about Independency Principle

(OP)
Oh man, this is getting difficult.
Isn't the perfect pin a feature of size? How can point-to-point dimension be feature of size? What do you exactly mean? Can't you see opposed points in my example?

RE: Question about Independency Principle

pmarc,
You keep avoiding my questions.
You are calling your dimension “sizes”
ISO says the size can only be defined on features of size.
Dimension between two random points on random doodle is not a size.
I am asking to show me features of size and associated with them size dimensions on your drawing.
Am I really that difficult to understand?

RE: Question about Independency Principle

(OP)
Apparently you are, because I do not know what you are asking for. smile
How can I show you features of size (plural) if we are talking about single pin produced as a 9-arm star? What do you want me to show you? Shall I draw infinite number of opposed points and connect each pair with the line passing through the red point? Are 9 pairs and lines not enough? Aren't the 9 point-to-point distances local sizes of the pin in a single cross section?

RE: Question about Independency Principle

Just show me 2 points that you'd consider "opposite"

RE: Question about Independency Principle

(OP)
Here you are:
http://files.engineering.com/getfile.aspx?folder=e...

The two orange points are opposite, that is they lie along a line passing through the center of associated circle (red point).
The same applies for 8 other pairs of points and for infinite number of pairs not directly shown on the picture.

RE: Question about Independency Principle

Quote:

If perfect form required at MMC, ding is allowed.
This illustrates my point...
You say MMC requires perfect form (even at an individual cross-section).
Then you say that the MMC part can have a ding in it.
So we have a discrepancy at hand: Either the dinged part is not at MMC, or it doesn't have perfect form. Which is it?
Based on this answer we can hop over to my LMC analogy.

I don't mean to derail the main discussion (still thinking about pmarc's 9-pointed star!), but this is important in that it ties in with the relationship between size and roundness.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
http://www.gdtseminars.com

RE: Question about Independency Principle

Thank you Belanger,

I think our major disagreement in in terminology than anything else. We have to clearly distinguish between “drawing demands” and “part actually is”
Yes, part at MMC has perfect size and shape.
If perfect form at MMC is demanded on the drawing, part at perfect size and shape with ding on it is still a good part, as long as ding doesn't violate tolerance.

Area where I am less comfortable is to have “local” LMC limited to roundness, but allowing longitudinal variation.
Imagine part made to “LMC” where cross-section has perfect size, is perfectly round, but axis of the part is allowed to bend. Can we bend axis so much that “LMC” boundary will actually violate MMC boundary? Is that a good part? Is it really LMC part?

So I am leaning towards interpretation that in “perfect form required at MMC/LMC” “form” means roundness and straightness combined.

Actually straightness seems to be the only thing required.
When we say “perfect form at MMC/LMC required” we imply that we may have MMC/LMC without perfect form, which is perfect size, but not straight.

I hope I didn't muddy the water even more.

RE: Question about Independency Principle

I do agree that it's a terminology thing, because if a cross-section is truly at MMC or LMC, it can't do a darn thing -- it's walking a very fine line in terms of circularity.

But you don't have to be uncomfortable with an LMC part that is bent; if we go with the envelope rule then the part can bend until the envelope hits MMC. The part still has the "least material."

On the other hand, if an external diameter is at MMC cross-sectionally, the tiniest bend already puts it outside the envelope. This is why MMC requires perfect form (longitudinally) but LMC does not require perfect form (longitudinally).

But I must admit I am still having trouble with this 9-pointed star (see next post)...

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
http://www.gdtseminars.com

RE: Question about Independency Principle

Pmarc -- your latest graphic shows the orange points, and they indeed seem to cross through a center point. And each of the other dots illustrated would also pass through that same center.

But I'm having trouble with all the other infinite pairs of points; those along the sides of the spikes. Suppose we take one orange dot and move it down the side just a little. Is it correct that the opposing point also moves down its angled side, maintaining the same local size, and the line connecting them still passes through the center?

I'm not doubting it. I just don't feel like doing some sort of geometric proof. But that would have to be true to make me feel better about the "actual local size" truly being a constant.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
http://www.gdtseminars.com

RE: Question about Independency Principle

(OP)
J-P,
Yesterday when I was posting my sketch I did not have access to my CAD software, so could not really verify if the 9-pointed star was meeting the requirement for having all local 2-point sizes equal to MMC size. Now I have the access, checked it and my answer is NO - the star does not and actually cannot have 10.8 dimension for each pair of opposite points (I am talking about my very initial geometry of the pin).

But my another attempt is the shape similar to shown in attached picture. In my opinion this could meet the requirement (for simplicity I just shown 5-pointed-star-like contour instead of 9-pointed). All 2-point local sizes passing through the center established by minimum circumscribed circle will be equal.

Now, if you imagine that the root circle is approaching dia. 0, the diameter of circumscribed circle will be approaching dia. 21.6 in order to keep 10.8 local sizes everywhere. This will result in 10.8 of maximum possible circularity error. So repeating the answer to your question, in absence of explicit circularity control the maximum possible circularity error will be limited to half of MMC size of shaft's diameter.

Of course all said above may be true only if we assume that all actual local 2-point sizes intersect a common center and not when interpreted in accordance with muddy definition proposed by Y14.5.

http://files.engineering.com/getfile.aspx?folder=f...

RE: Question about Independency Principle

(OP)
I am not ignoring anything.
First of all, I would suggest to stop using my first sketch, because I already admitted it was wrong.
Secondly, if you look at my most recent sketch, you will not find such "shorter measurement" that would pass through the center of the circle. All 2-point measurements passing through the center should return the same value.

Maybe it was you who ignored my last post? smile

RE: Question about Independency Principle

(OP)

Quote (CH)

It is still possible to take “long” and “short” measurement in your new drawing.

This may be surprising for you, but in 2D model I have the lengths of both lines drawn by you are equal to 10.8.


Quote (CH)

As long as you try to keep your measurements within tolerance you will find that circularity error is shrinking together with tolerance.

I would not agree with this. Again, you are using incorrect shape to prove your point. If your most recent bunch of pictures was showing "flower-like" contour proposed by me, there would be a chance to observe that even for root circle diameter very close to 0 there would be a contour for which all 2-point distances would equal MMC size, so actually in theory there would be no size tolerance needed at all.

RE: Question about Independency Principle

(OP)
Of course. But we have been speaking theoretically. The original question was solely about theoretical interpretation of the independency principle. J-P's question about maximum possible circularity error was also of a theoretical nature (I suppose), because in reality no one will ever produce a flower when the cylinder is desired.

RE: Question about Independency Principle

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

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

RE: Question about Independency Principle

(OP)
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 smile

RE: Question about Independency Principle

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

RE: Question about Independency Principle

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.

RE: Question about Independency Principle

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

RE: Question about Independency Principle

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

RE: Question about Independency Principle

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

RE: Question about Independency Principle

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

RE: Question about Independency Principle

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?

RE: Question about Independency Principle

(OP)
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.

RE: Question about Independency Principle

(OP)
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.

RE: Question about Independency Principle

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

RE: Question about Independency Principle

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

RE: Question about Independency Principle

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...

RE: Question about Independency Principle

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

RE: Question about Independency Principle

Quote:

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

RE: Question about Independency Principle

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.

RE: Question about Independency Principle

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

RE: Question about Independency Principle

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

RE: Question about Independency Principle

(OP)
Para. 3.5.
LOCAL SIZE OF AN EXTRACTED CYLINDER, LOCAL DIAMETER OF AN EXTRACTED CYLINDER
distance between two opposite points on the feature, where
— the connection line between the points includes the associated circle centre; and
— the cross-sections are perpendicular to the axis of the associated cylinder obtained from the extracted surface


Quote:

IOW, is a local diameter one consistent number or can a local diameter vary as we rotate around the circle?
Well, the lower picture shows two local diameters, so I would not interpret it as one consistent number in a single cross-section.

RE: Question about Independency Principle

Thanks -- that helps a lot. Unlike ASME's definition, this one states that "the connection line between the points includes the associated circle centre."

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

RE: Question about Independency Principle

(OP)
That is right. Of course there are other issues with this definition, but if we are solely talking about the connection line between two points, I think there is way less ambiguity in ISO in comparison to what has been defined in ASME.

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