partially opposed FOS 1

[bxbzq]

There has been a lot of discussion on FOS in this forum. I don't intend to stir the pot here but I don't find answer to this question. See attached sketch. For two partially opposed planar surfaces being directly toleranced by 80±1, is there an agreement that the full planar surfaces are part of FOS? I'm asking because I also heard in this case the FOS ends where the opposed elements end, meaning the areas outside of the 30mm length (non-opposed portions) are not part of FOS.

 

[bxbzq]

Sorry forgot to attach the sketch.

 

[MechNorth]

Only the 30mm zone is directly opposed, so that's the extent of the FOS. Consider what used to be called the "caliper rule" in Y14.5. If you can measure directly opposed points using the jaws of the calipers, then it can be a FOS. Note, not the net effect of jaw-to-jaw, but point-to-point.

Jim Sykes, P.Eng, GDTP-S
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[CheckerHater]

Y14-2009 introduced “irregular feature of size” where requirements are loosed a bit.
As long as your directly toleranced feature (or group of features) can be contained by actual mating envelope (in your case pair of parallel planes) it is a feature of size.

 

[ALK35]

I have added a note saying to treat the as a feature of size for the entire length.

 

[bxbzq]

hmm...not agreed.

 

[MechNorth]

CH, it's an "irregular" feature of size; there is still a distinction.
ALK35, I'd have to see it on the drawing, but it's not clear here what the intent would be.

Jim Sykes, P.Eng, GDTP-S
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[bxbzq]

Can we agree if a part measured as shown on the right is good or bad?

 

[CheckerHater]

Bxbzq, it’s bad.

MechNorth, I would really like to see you elaborate on “distinction”.
In Para. 1.3.32 definition section of Y14.5-2009 it is made very clear:

Feature of size
Feature of size: encompasses two types: regular and irregular. See paras. 1.3.32.1 and 1.3.32.2

Nowhere in the standard “distinction” is made to consider one type “not real” feature of size or one type to be somehow inferior to the other.

Also about "caliper rule" in Y14.5.
I would greatly appreciate if someone shows me where exactly words "caliper rule" appear in Y14.5 – paragraph, illustration or better yet scan from the page.

I am design engineer with several years of experience, but I am having trouble to visualize the caliper one might use to measure features of size shown on Fig. 4.33, 4.34, and 4.35 in Y14.5-2009.

All that said I see “caliper rule” being happily thrown out of the window years ago.

Nevertheless I am really interested to see more arguments pro- and contra-.

 

[CheckerHater]

Bxbzq, I am ashamed to ask this question:

From the appearance of your drawing, it looks like you work to ISO standards.
Is that the case?

 

[dingy2]

I agree that there is no "caliper rule" but it is a good "rule of thumb" in cases where we may have 1/2 a diameter or more, as an example. The definition of a Regular Feature of Size 1.2.32.1 states "two opposed parallel elements or opposed parallel surfaces" which a vernier caliper is capable of measuring. One could also use a micrometer but there lies the basis for the caliper rule as Jim stated.

I agree that you are correct that this feature should falls under the category of irregular feature of size 1.3.32.2 (a) which states a directly toleranced feature or collection of features that may contain or be contained by an actual mating envelope that is a sphere, cylinder, or pair of parallel plains.

Dave D.

http://www.qmsi.ca

 

[bxbzq]

CheckerHater,

From your interpretation of FOS, I know you would consider the part bad. I'm actually interested to see Jim's opinion because if the non-opposed portions are not part of FOS, what would contol the distance and form of them.

I work to both ASME and ISO, but I don't know too much about ISO. The proe I used to create the sketch is configured to ISO drafting system.

 

[CheckerHater]

Thank you Dave,
The problem I see with the rules of thumb is that sometimes they may lead to slippery slope.
Look at the enclosed picture: this feature passes “caliper rule”, but it is not feature of size (at least not the regular one), because it isn’t either cylinder or pair of opposite flats.
New irregular FOS idea is meant to fix it. Now the feature is clearly a FOS.
And look at this from other side – any regular FOS perfectly falls under irregular FOS definition, because say, a cylinder has actual mating envelope in shape of, well, cylinder!
So we only need “irregular” definition to define ANY FOS.

Bxbzq,
That makes all the difference in the world.
In ISO size is always two-point measurement. If you don’t have two opposing points, there is nothing to talk about.
In ASME size creates envelope requirement. The question is how far to extend the envelope. New definition of FOS says that anything you can fit into envelope is a FOS.
And about your CAD settings. According to new ISO “invocation principle”, as soon as you put “non-ASME looking” dimensions on your drawing, you automatically invoke the entire ISO GPS system.

 

[bxbzq]

CheckerHater,

Good to learn that. When I started this post, I was in ASME world. And I don't want to get into ISO too much until I'm clear in ASME. Just for discussion purpose, let's not be so strict with the looking of my sketch.

 

[dingy2]

CheckerHater:

Again, ASME has tried to clarify a feature of size with the "Irregular Feature of Size" 1.3.32.2 but has it muddied the waters rather than clearing it. Using the ISO method and our regular feature of size using opposing elements (caliper mehtod), there is no question of what features are deemed a feature of size. We even knew that 1/2 radius or more is a feature of size since we can measure the size using a caliper. Now, is 1/8th rad a feature of size? Possibly, a 1/16th rad could be? I have enclosed an extreme situation that might pass the irregular feature of size criteria but from a practical perspective, I would not use it as such. What do you think? Is it a feature of size and would you apply a positional tolerance to such?

Dave D.
www.qmsi.ca

 

[CheckerHater]

Short answer “yes” and “yes”
To elaborate will take some time.

Yes it is feature of size if you dimension it this way.
Now the question is WHY you dimension it this way?
Is it important for function of the part? There is an opinion on this forum that function is the King and the matter of feasibility is absolutely not relevant.

Now, if you are not completely ignorant of manufacturing and quality control you will never dimension the part this way to begin with.
So it is you muddying the water and not the ASME. (I don’t mean you personally, but hypothetical ignorant user).

Every single dimension, tolerance, or geometrical specification can be applied in the way that’s legal, but useless. Every rule can be misused and abused. It is not guns that kill people.
So, shortly, “yes”. Complete answer: “Yes, but I will find better way to dimension it”

 

[dingy2]

Yes, CheckerHater, I have had experience in manufacturing and quality includng CMM. I wanted to reflect that something that may be legal may not be practical and it seems I have done so. You used the word "useless" and I agree.

Dave D.

http://www.qmsi.ca

 

[MechNorth]

CH, you defined the distinction between "regular" and "irregular" FOS quite well, so evidently you do understand there is a distinction between the two, if only in terminology. The distinction is quite important if the OP is working in the '94 standard or earlier, where irregular features of size are not defined and therefore do not exist within the realm of the standard. Perhaps the next release will amalgamate both into a single definition for clarity; time will tell. As the revision level was not noted, you are making a bold statement that they are the same. As for the Caliper Rule, it was removed by the '94 standard, but was in either the '82 or earlier standard; '94 is the earliest version I have at hand. I agree that it is no longer indicated in the standard, but as Dave points out, it's a very useful ROT. My (perhaps foolish) assumption was to use the '94 standard as the baseline for my response, above. Your graphic of the cylinder with a flat would not be a FOS per '94, but would qualify as an irregular FOS per '09, at least at the single opposed point; the entire feature is not defined and therefore it is impossible to tell.

Bxbzq, for a Y14.5M-1994 interpretation, the "wings" are not features of size and therefore are not controlled by the callout; the overlapping portions of the top and bottom surface meet the size tolerance requirement and therefore pass, but there is nothing controlling the tabs. In the '09 standard, it would be considered an irregular FOS, and therefore would fail the spec.


Dave, your figure arguably could arguably be interepeted as a FOS in the '94 standard per 1.3.17, or a regular FOS in '09 per 1.3.32.1 (if one believes that "opposed" doesn't mean "directly opposed" {yes, that's an ongoing argument}. Otherwise, it would be an irregular FOS per '09. However, even here the "caliper rule" has some value if you consider how the part would be encompased by the long jaws of the caliper; it wouldn't be terribly stable, would it; the part would tend to twist within the jaws until it was most stable.




Jim Sykes, P.Eng, GDTP-S
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[fsincox]

I believe the distinction was not that clear in the 1982 standard, either, as we have all discussed here in the past. Remember, there was no "directly" opposed terminology before 1994 and some, obviously, felt it needed to be added. It was clear only to those who had inside knowledge and/or were powerful enough to influence the committee to their way of thinking. IMHO, It doesn’t even really mean they were right, really, it just means they won! “To the victor go the spoils” as they say.

 

[CheckerHater]

Well, it looks like we basically agree on things.

I am not that optimistic, but I like the idea that somewhere in Y14.5-2024 we will see unified definition of FOS.

I find the situation amusing as well though:
ISO stands firm on the idea of size being “two-points”
ASME stays with the idea of size as an envelope.
ASME makes definition of FOS more in-line with the idea of envelope, but for some reason it puts ASME people all up in arms.

Maybe at some point in the future we could all agree that if we invoke envelope principle on our drawings, then our FOS are envelope-based; and if we invoke independence principle, our FOS are “opposite-point”-based. After all it is in line with the standards we already have. Time will tell…?