In worst case, the flatness is... 7

[SeasonLee]

In worst case, the flatness on this surface is...
Please see the attached for details.
Thanks

Season

 

[3DDave]

My initial answer is looking better all the time.

 

[CheckerHater]

Where we draw the line between first CH picture and the second one - again in the real world - I don't know.

You already started answering your own question.

Could you drive the line between part in OP, and the part with the simple (small) chamfer? Which size chamfer has to be to start worrying?

Standard does not provide exact numbers, ratios, percentages necessary for feature to be considered. Nevertheless, occasionally it provides a clue.

For example in Para. 4.8 abot datum features it says: ..."However, a datum feature should be accessible on the part and of sufficient size to permit its use"

They are steering away from using word "fixture" but they are clearly stating that feature must be big enough to grab on it.

In this line of thought, I believe 30+/-0.3 dimension is a regular feature of size because it is "sufficient".

And little bit of historical perspective.

in 1982 FOS was "set of two plane parallel surfaces"

in 1994 it was "set of two opposed elements or opposed parallel surfaces"

and in 2009 "set of two opposed parallel elements or opposed parallel surfaces"

Nowhere standard says that said "elements" must be exact copies of each other, so I safely assume they may be "sufficient"

You can devise "go-no go" gauge to check size 30 and it will work. When assembled with mating parts it will act as feature of size (imagine it being a spacer of certain size), etc., etc.

The truth is, you cannot always hide behind the standard and avoid making decisions all together.

It is your responsibility to draw the line. If you believe the drawing may be misinterpreted, it is your job to add control (flatness?) to clarify.

In this sense, OP has purely academic interest.

Still, if I remember correctly, when answering "multiple questions" you are supposed to pick the answer as close to what you believe is correct as possible.

Given that part in question is very close to be regular feature of size, I believe the answer will be very close to 0.6, so I chose answer "C"


"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future

 

[greenimi]

CH,

If you read my post from September 11, 2015 at 11:39 see what I said:
Copy-paste:
"Okay I understand.
I agree that for all practical purpose that flatness would be 0.6. Let’s get that out of the way.
Now from a theoretical point of view................""

And from here the discussion starts on what standard requires and what standard does not require.......

Quote:"In this sense, OP has purely academic interest."
I agree. what's is why I have driven the discussion from the theoretical point of view. Therefore, from the theoretical point of view ....... the correct answer is.................. fill the blanks

 

[CheckerHater]

From theoretical point of view straightness does not control flatness (also something that I said long ago)

"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future

 

[greenimi]

Quote:"From theoretical point of view straightness does not control flatness (also something that I said long ago) "

Nobody said it does.

The OP asked a question based on existing skecth / print requirements. The OP's question was what is the worst case flatness of a certain surface. AND that certain surface happened to be controlled by 2 straightness callouts. Nothing to do --directly-- with the straightness requirements.

 

[CheckerHater]

The OP drawing contained straightness requirements and directly toleranced dimension.

We have agreed that straightness does not control flatness.

We have agreed that for practical purpose we can assume the flatness will be controlled by the amount of dimensional tolerance, that is 0.6

The corresponding answer is "C"

What it it, that we did not agree about?


"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future

 

[greenimi]

We agreed that from 10000 miles high, the answer is 0.6.

Someone who is really detailed oriented might say:
Wait, not so fast!

Now , when we really get into the details ( and an engineer should) of this particular part or question, part with the configuration as shown, we found that we disagree where rule#1 is applicable (what area/ portion of this surface, what is covered and what is not covered).

As I stated before, we have to understand the theory, what the standard requires and does not require, before we have some hope to apply such of said theory in any sort of practical way.

 

[dtmbiz]

greenimi

For the sake of discussion, let’s say the surface is not an FOS.

That surface must lie within the size limits.

What would you expect the "Flatness" to be ?

PS
ASME is applied to achieve requirements for real parts.

What does theoretical have to do with your argument?

Curious...

 

[greenimi]

If the feature is not FOS you SHOULD define it with profile. Period.

Direct toleranced dimensions (aka ±) work very well (robust product definition, unambiguous requirements, only one legal interpretation of the functional requirements, whatever you want to say) for FOS and not much else.

 

[CheckerHater]

There is no theory, because GD&T is not science. It's a rule-book written by people barely understanding math.

Re-read the statement from 1.3.32.1: "set of two opposed parallel elements OR opposed parallel surfaces"

Can you visualize parallel elements that are NOT parallel surfaces?

"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future

 

[axym]

Hi All,

Interesting debate.

One problem I always had with the description of size in Y14.5 is the emphasis on the "size also controls form" idea. The first sentence in the Limits of Size section states "Unless otherwise specified, the limits of size of a feature prescribe the extent within which variations of geometric form, as well as size, are allowed". This would naturally lead one to believe that size controls form. But the "size controls form" idea is not a rule, and does not bring in specific form controls. For the width example in the OP, the size tolerance does not impose a flatness tolerance of 0.6 on each surface. The form control from a size tolerance is indirect, and is a consequence of two specific geometric requirements:

1. The feature must conform to the Rule #1 boundary (a boundary of perfect form and MMC size)
2. The feature's actual local sizes (opposed point distances) must all be within the size limits.

When these two requirements are applied to regular features of size whose points are all opposed, then the "indirect form control" concept works. If we look at the possible as-produced surfaces that meet both requirements, we see that the form error of the surface cannot be larger than the size tolerance.

But if the requirements are applied to features whose points are not all opposed, then things change. In other words, if the feature has points that do not have a corresponding opposed point. This may be by design, as in the parts with partially opposed widths that have been discussed (or cylinders with a blind hole on one side). This may also be a symptom of the as-produced part, where a cylinder with a nominally perpendicular end face is produced with a tilted end face. In either case, we get some non-opposed points. What do we do with those?

I don't think that the problem is deciding whether or not Rule #1 applies. The problem is with the definition of actual local size. In Y14.5-2009, actual local size is defined as "the measured value of any individual distance at any cross section of a feature of size". There are several problems with this definition (this discussion could fill another thread), but the main one for this thread is the idea of a "distance". This can't deal with non-opposed points. In the places where the points are not opposed, we can't define the actual local size. What is a distance when there is only one point? So the unopposed sections of the surface can pretty much do anything within the Rule #1 boundary, and we don't get the indirect control of form. To appreciate what type of control the actual local sizes apply, imagine measuring the size of the feature using a micrometer with pointed anvils. All you can measure are sets of opposed points. If the surface has areas that are not opposed, you can't measure those areas. It's as simple as that.

So what do we do with those areas that are not opposed? Strictly speaking, the form in those areas is not controlled. In most cases, this does not cause a practical concern. If the unopposed areas are relatively small as in CH's part with the blind hole, we probably assume that the surface is continuous enough that there won't be significant local deviations in the non-opposed areas. If the unopposed areas are relatively large as in CH's part with the rectangular cavity, we might not assume this. Where do we draw the line? There are no rules - this is where "practical considerations" come in. As CH alluded to, it comes down to risk management. It is up to the designer to decide whether to go with just the size tolerance or to add additional controls (if they feel that the size tolerance would allow undesirable outcomes that have a significant chance of occurring).


Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[greenimi]

Evan, great post!!

And I am glad we pushed this thread so far (we wouldn’t have Evan’s input otherwise)

Definitely worth the 70 replies.

 

[CheckerHater]

Thank you Evan,

It is very true that in real life we never have perfectly opposing features. Does it mean Rule 1 should never apply?

There is no way standard will eventually provide strict rules for every possible part. There always will be space for making personal decision.

OP example didn't ask what control is better to use. The question was, how bad part can we get with controls already there.

Since nothing ever is 100% perfect, "none of the above" is (almost) always technically correct.

0.6 was good, reliable guess. Sometimes creators of exercise books don't really dig as deep as we think. That's another moment to be taken into consideration.

All together it was great discussion, but I think it's getting too long.

Question to Season: HAVE YOU FINALLY FOUND THE ANSWER IN THE END OF THE BOOK?

"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future

 

[dtmbiz]

greenimi

The OP question was regarding the max flatness considering the straightness of planar line elements controls.

The apparent answer is the size limits control the flatness.

Considering the possibilities of surface curvature that is possible within the size limits
I cant possibly see how straightness of planar line elements would be useful for a planar surface. Dont want to make the thread longer... thats just me.


That is your "opinion". Dont know how you can claim "should".
(over 70 comments on a limit dimension. you missed the "points of view"?)

A parallelism control could work just as well IMO. (if there was a datum to reference)

However, back to the question / example. The example is obviously incomplete.
It does not reflect comprehensive functionality with just the example shown.
It was a question to consider "flatness".

You never answered my question on what effect this question regarding flatness would differ if the limit dimension is not relatve to a FOS. No rule #1; then what?

 

[axym]

greenimi,

I'm glad that the thread is getting to some interesting outcomes. Your questions tend to make us dig deep and extract the subtle details. Keep in mind that the writers of GD&T textbooks and exercise books (and even standards) can only get into the intricacies to a certain extent, without losing most of their audience. People want things to be simple and easy, not complicated and difficult. I know from experience that it is much easier to make money glossing over these subtleties than it is addressing them. I'll stop there before I get into a self-righteous, bitter rant ;^).

CH,

I would say that Rule #1 still applies, even with features that are not perfectly opposed. Assessing the feature's conformance to a boundary is straightforward, even if the feature has unopposed areas.

Again, to me the difficulty lies in the actual local size. It's an old, shop-worn tolerancing tool that serves well for parts that one wants to be able to inspect with a caliper or mic. But for this tool to work, certain conditions have to be in place (such as opposed geometry, and form error that is relatively small). When these conditions are not satisfied, it breaks down and becomes ambiguous. It's just not as robust as zone-based geometric tolerances.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[greenimi]

Dtmbiz,
The fact the straightness requirement is there or not is irrelevant for the question asked by the OP. And yes, the size limits control the flatness and “straightness does not control flatness”

Quote: “That is your "opinion". Dont know how you can claim "should"”
Of course, IT IS my opinion. But looks like I was not entirely far off in the weeds by bringing it up.

Quote:” You never answered my question on what effect this question regarding flatness would differ if the limit dimension is not relatve to a FOS. No rule #1; then what?”

If it is not relative to a FOS (and specially regular FOS), then rule#1 does not apply and the flatness could be controlled indirectly by others controls zone based geometric tolernaces (and not a non-based tolernaces) :
You can use profile to locate the surface, or even composite profile and the flatness would be indirectly controlled there.
You can also use directly flatness, but should be a refinement of the location control and the orientation control. I am sure you know, you locate first and then orient and then refine the form if it is still needed.
I am very sure you all know about this…….


Evan,
Quote: “……actual local size is defined as "the measured value of any individual distance at any cross section of a feature of size". There are several problems with this definition (this discussion could fill another thread), but the main one for this thread is the idea of a "distance"…………

That is why the math standard Y14.5.1 moved toward the LMC sphere concept.
Concept not very developed in Y14.5 (at least not yet) .

Probably in the very near future. By the way do you know if the math standard will get a new revision soon or nothing in sight?

 

[3DDave]

There are a number of articles on math interpretations that preceded the issue of the Y14.5.1, but Google seems to find none since. Voeckler should have generated a new chapter before proposing the out-of-order datum evaluation scheme that got dropped in place. Or the committee should have tabled the new scheme until that math section was generated to match.

 

[dtmbiz]

greenimi,

Thank you for your time to directly respond.
You have explained your interest in FOS and Rule #1.

Agreed regarding other geometric controls are available along with fundamentals
and rules for consideration, however the question was regarding the example as shown.

This thread went to some areas of interest at a deeper level.
(dont they all or at least most? - rhetorical)

Interesting discusions for new threads.

 

[axym]

greenimi,

A new revision of the Y14.5.1 mathematical definition standard is currently in development. I am hoping that it will be released in 2016.

I'm not sure what will happen with the definition of actual local size - to me, this is one of the most difficult problems in GD&T. There is not even agreement on the meaning of the current definition in Y14.5 applied to cylinders, as "any individual distance at any cross section" is interpreted differently by different people. Some interpret it as a distance extracted from 2 opposed points (as one would measure with a mic), and some interpret it as a diameter extracted from a cross-sectional circular element. Unfortunately, the standard contains text supporting both interpretations and no figures that clarify the meaning (only side views are shown, obscuring what is really happening within a given cross section). So we don't even know for sure whether a cross section of a cylindrical feature has one actual local size or many. The committee is in a difficult position, because it is now impossible to choose one or the other without contradicting past practices in some way. The Y14.5.1M-1994 mathematical definitions standard created a novel definition based on an LMC sphere, but this has been largely ignored in industry (partly because it conflicts with the idea of 2-point opposed diameters, and partly because the mathematical definitions standard was itself largely ignored).

The fact that there are camps favoring different interpretations makes it likely that different types of local size are needed for different applications. The ISO GPS standards define several different types of size, but so far Y14.5 has not embraced this approach.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[greenimi]

Regarding the ISO GPS standard’s approach to the actual local value here is what one of the best experts in the ISO standards said:
Pmarc said: (I know he did not give me the permission to copy and paste and I am very sorry about that, but it is a public site)
Sorry pmarc. If I have to remove this post I will do it without any hesitation.


“Yes, in ISO GPS actual local (two-point) size of a cylindrical FOS is a distance between two points measured in a plane perpendicular to the axis of associated LSQ cylinder, and that distance must be measured across the center of associated LSQ circle.

To me this definition is mathematically consistent, although it does not address all issues. Just one example: picture a pin that is perfectly round yet is bowed to a banana shape. Per ISO definition this feature will not have all actual local sizes identical. Only in one cross section (at the middle of pin's length) they will all be equal, because only in that cross section the surface of the pin will be seen as a perfect circle. In other sections the pin's surface will be seen as an oval/ellipse-like shape, thus lead to different local size measurements. “