orientation or form error considered in min wall thickness stack? 1

[bxbzq]

Hello,

I think this a simple and classic tolerance stack example. Please see the sketch to the left of the attached pdf. In the textbook I have, the answer of the min wall thickness x is 2.9mm. However, to my understanding, the calculation does not take perpendicularity or form errors into account when the two holes are at LMC. So I added the perpendicularity and form errors and did worst case calculations as shown to the right. Any comments on my understanding and calculations?

Thanks.

 

[CheckerHater]

What "zero" tolerance means.

bxbzq, it's from the same book where you found your problem

 

[CheckerHater]

Sorry, lost the picture:

 

[CheckerHater]

bxbzq,

Read carefully what you just wrote:

Where the size of the unrelated actual mating envelope of the considered feature has departed from MMC, a tolerance equal to the amount of such departure is allowed

Actual mating envelope includes both: size and position/orientation TOGETHER.

It is not diameter of your hole that is departing from MMC, it's the space where you fit tilted / crooked hole, BOTH size and position/orientation.

 

[Belanger]

CH -- you have your numbers backwards. If the hole (datum feature B) is at 10.4, it is at LMC, not MMC. So the zero tolerance grows to 0.4 at 10.4 (LMC)!

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

http://www.gdtseminars.com

 

[CheckerHater]

JP,

Feature control frame clearly states 0 at MMC

That means 10.0 hole is perfect.

We can use available 0.4 to “fatten” it 0.4 and keep perfect perpendicularity
We can use available 0.4 to tilt the hole 0.1 and “fatten” it 0.3
We can use available 0.4 to tilt the hole 0.2 and “fatten” it 0.2
We can use available 0.4 to tilt the hole 0.3 and “fatten” it 0.1
We can use available 0.4 to tilt the hole 0.4 and keep the hole at 10.0

This is what actual mating envelope is, and it’s envelope that’s allowed to increase to 0.4 (like bxbzq already mentioned)

 

[bxbzq]

CH,

It states unrelated actual mating envelope. Unrelated AME is not constrained by location or orientation relative to any datum. Here is definition in '09 standard:


1.3.25.1 Unrelated Actual Mating Envelope. unrel-
ated actual mating envelope: a similar perfect feature(s)
counterpart expanded within an internal feature(s) or
contracted about an external feature(s), and not con-
strained to any datum(s).

1.3.25.2 Related Actual Mating Envelope. related
actual mating envelope: a similar perfect feature coun-
terpart expanded within an internal feature(s) or con-
tracted about an external feature(s) while constrained
either in orientation or location or both to the applicable
datum(s).

 

[pmarc]

The funny thing is that regardless of how your dispute about size of outer boundary = resultant condition of datum feature B hole will end, I know where the example was taken from and it indeed shows 2.9 as a correct answer. It is weird to disagree with such authority, but in my opinion the answer is 2.7, no matter what.

 

[CheckerHater]

I am starting to lean towards your point of view; at least I was in the good company

 

[CheckerHater]

I think I found it:

ANSI Y14.5-1994, paragraph 2.11.3:
“A virtual condition exists for a datum feature of size where its axis or center plane is controlled by a geometric tolerance. In such cases, the datum feature applies at its virtual condition even though it is referenced in a feature control frame at MMC or LMC.”

And also:
“Where a virtual condition equal to the maximum material condition is the design requirement, a zero tolerance at MMC or LMC is specified.”

That’s it: “virtual condition” rather than “resultant condition”. Minus instead of plus. Any opinions?

 

[pmarc]

CH,
I am afraid I do not really understand what you are trying to prove by your last post. Any clarification?

I would recommend looking at fig. 2-12 in Y14.5-2009. All what is needed in terms of virtual and resultant condition for internal features positionally toleranced at MMC is there.

 

[Belanger]

CH -- regarding your last post: those statements you quote are all about when the datum is referenced in a feature control frame with the "M". That is irrelevant to this situation because datum B is refernced without the "M" (or RMB to use the 2009 term).

So it goes back to this: The datum is the axis of the Related AME taken from the hole identified as B. But the actual rim of the hole at the top face of the block could be further away from that datum axis and that's where the distance "X" could be a little less than the given answer of 2.9.

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

http://www.gdtseminars.com

 

[bxbzq]

The para 2.8.3 in '94 and '09 is different.

In '94:
2.8.3 Effect of Zero Tolerance at MMC. Where a tolerance of position or orientation is applied on a zero tolerance at MMC basis, the tolerance is totally dependent on the actual mating size of the considered feature. No tolerance of position or orientation is allowed if the feature is produced at its MMC limit of size; and in this case, it must be located at true position or be perfect in orientation, as applicable. Where the actual mating size of the considered feature has departed from MMC, a tolerance is allowed equal to the amount of such departure. The total permissible variation in position or orientation is maximum when the feature is at LMC, unless a maximum is specified. See Figs. 6-41 and 6-42.

To my understanding, the actual mating envelope definition in '94 is related AME definition in '09.
So in scenario #1, the related AME is still at MMC (virtual condition in this case). In this case, no position or orientation tolerance allowed...But it leads to no perpendicular tolerance allowed at all as long as the related AME is at MMC regardless of the unrelated AME size. paradoxical...

The '09 std is more logic.

 

[greenimi]

pmarc,
If you see now, the next example from the same book you mentioned fig 9.13-page 264, the min distance "x" calculation should have been also 2.4 instead of 2.6? First case (original post) is position .02 wrt A and B. The one I am enquiring is position .02 at MMC wrt A and B at MMC.
So, again the question is how the bonus tolerance plays it's role in the stckup: .02 (MMC)+.02 (3.6-3.4 size) /2) ?
What about datum shift B in the stackup ?
Are we missing another 0.2 from the total min distance because of (10.4-10)/2 or is that covered by the datum shift?
Thank you

 

[greenimi]

And one more thing: seems to be some confusion about unrelated actual mating envelope (UAME) and related actual mating envelope (RAME), location constrained RAME and orientation constrained RAME etc (at least in my head).
I found this link from another GD and T authority and I hope will help us in these cases /(Subject:Subject: Maximum Material Boundary (MMB)--scroll down somewhere half of the page.
as they said, there are some exceptions to the rule:

http://www.geotolmeadows.com/newsletters/2012/jan2012.htm

 

[pmarc]

Well, first of all I think we should finally tell the others which textbook we are discussing about - just to have more votes in further discussion. It is "Fundamentals of GD&T (2nd Edition)" by Alex Krulikowski.

Knowing that, in my opinion calculations shown in figure 9-13 are OK. In this case, if we want to have maximum datum feature shift, datum feature hole must be at 10.4 and perfectly perpendicular to A. Each other configuration of the hole will give datum feature shift smaller than 0.4, however on the other hand...

...there is another interesting aspect. If, just for the purpose of excercise, we delete (M) modifer standing right after 0.2 in positional feature control frame for the smaller hole in 9-13, so that the bonus tolerance is not available, we should get 2.7 (at least this is what I get). Which leads to a conclusion that according to what I keep claiming about figure 9-12 (so that the correct answer shall be 2.7, and not 2.9), there is no difference whether datum B reference in positional callout is modified by (M) or not. Minimum wall thickness will always be 2.7. Hmmmm...

Does someone see my point?

 

[bxbzq]

Because modifying datum B does not affect Outer Boundary of the smaller hole. In both cases it's ø3.8mm.


Now, back to the original question, if a similar question appears in a test, like certification test for '94, which answer should I give?

 

[Belanger]

It all depends on what the questioner means by "minimum wall thickness." As I stated earlier, it sounds like the answer you were given of 2.9 was assuming the consistent "X" throughout the plate/block, whereas we've shown how the worst case at a cross-section of the hole (i.e., at the top face or rim) could be 2.7.

It's similar to showing you a picture of a Coke bottle and asking you if it's straight. You can answer yes (because you might be thinking of axial straightness) or you could answer no (you might be thinking of surface straightness). Guess what: both answers are correct. The burden is on the questioner to be more specific.

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

http://www.gdtseminars.com

 

[CheckerHater]

I have a few questions.

First, imagine the piece in question is only 0.4 mm thick.
That means we can punch 10.4 mm hole at 45 degrees and satisfy perpendicularity requirement.
(I pick 45, so I don’t have to explain, is it 45 to vertical, or 45 to surface)
Some simple math (or CAD software) will quickly tell us that not only “wall thickness” will become smaller, it will become negative. In fact, there will be little space left for second, smaller hole at all.

Second, is “wall thickness” a dimension? If so, is it subject to “caliper rule”? How do you measure it anyway?

If we can create some magic hole maintaining LMC 10.4 mm, perpendicularity error 0.4 mm and boundary 10.4 all at the same time, and we adopt “caliper” definition of thickness, what will be the minimum wall thickness?

 

[pmarc]

CH,
Like I said in last paragraph of my first post in this thread, part's thickness has nothing to do with these particular calculations. Regardless whether piece is 0.4 or 40.0 thick, the rim of datum feature hole cannot fall outside of dia. 10.8 resultant condition perpendicular to datum plane A. The same applies to the smaller hole - its rim cannot be outside of dia. 3.8 resultant condition. Both resultant conditions are spaced basic 10.0 apart.

As for whether thickness is a dimension and a subject to "caliper rule", in my opinion it completely does not matter from application point of view. If the requirement is that the holes under any cicumstance cannot be closer to each other than 2.9, and the stack-up shows 2.7, there is a risk that something will go wrong in as-produced component. I do not there will be a place then for a debate whether the stack objective was formulated properly and clearly or not.

And the last thing - it would be magic indeed to have the hole at LMC=10.4, with perpendicularity error of 0.4 to A, and not violate 10.4 boundary Of course if we are talking about boundary perfectly perpendicular to A, which is a cese in our example here.

 

[CheckerHater]

Pmarc,
I am not convinced.
In your own words: “If the datum feature hole is at 10.4 it can be 0.4 out of squarness.”
So can, or cannot?