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orientation or form error considered in min wall thickness stack?

orientation or form error considered in min wall thickness stack?

orientation or form error considered in min wall thickness stack?

(OP)
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.

RE: orientation or form error considered in min wall thickness stack?

(OP)
The sketch shows asme y14.5-2009.

RE: orientation or form error considered in min wall thickness stack?

Position of your smaller hole is specified at RFS.
Is it the same in your textbook?

RE: orientation or form error considered in min wall thickness stack?

(OP)
Yes. The hole is positioned at RFS base. The example is from Alex's fundamental gd&t book which is based on '94.
I think '94 and '09 would not be different for case like this, would it?

RE: orientation or form error considered in min wall thickness stack?

(OP)
envelope principle, rule #1, perfect form at MMC, not perfect form at LMC, right?
In this calculation I'm dealing with holes at LMC. Am I missing something?

RE: orientation or form error considered in min wall thickness stack?

Quote (ASME Y14.5M-1994 2.7.1.2(c))

There is no requirement for a boundary of perfect form at LMC. Thus, a feature produced at it's LMC limit of size is permitted to vary from true form to the maximum variation allowed by the boundary of perfect form at MMC.

Unless I'm missing something the 'variation in form' has to be in the direction of MMC - so adding material not removing additional material.

Posting guidelines FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm? (probably not aimed specifically at you)
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?

RE: orientation or form error considered in min wall thickness stack?

I think the tolerance zone Ø0.4 should be zero on both of your scenario #1 1nd #2, since the bonus tolerance are automatically included in the analysis for the calculation of min distance, there is no step #4 (-0.2) on the list.

SeasonLee

RE: orientation or form error considered in min wall thickness stack?

I get an answer of 2.7 minimum (but see my next paragraph). As the large hole gets bigger it doesn't affect datum B, because that's created from the Related Actual Mating Envelope (to use the 2009 terminology). However, the actual hole that we call datum feature B could have a larger "sweep" around the top because as that hole departs from MMC it could tilt up 0.2 in any direction. That's why our answer is different from the textbook, Bxbzq.

The real issue is whether the person asking the question is worried about the wall thickness "X" as a consistent thickness (which is why the texbook answer didn't include the bonus tolerance on the large hole), or just the worst case at any cross-section of the hole such as at the top face or rim (which Bxbzq and I are thinking of -- Scenario #1 in the OP).

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

RE: orientation or form error considered in min wall thickness stack?

bxbzq,

I get 2.7mm if I treat the Ø10mm hole as having a positional tolerance. It doesn't. It has perpendicularity to datum A. It is datum B. It has zero positional error, by definition.

There is no MMC specification on the datum B called up on the Ø3.4mm hole, we must consider the Ø10mm hole to be centred on the datum. This will be fun to measure. Sloppy holes make poor datums.

The wall is...

10mm - 3.6mm/2 - 0.2mm/2 - 10.4mm/2 = 2.9mm.

It has been pointed out by Dingy that FOS datums can be fixtured if they are called up at MMC. This changes the answer.

10mm - 3.6mm/2 - 0.2mm/2 + 10mm/2 - 10.4mm = 2.7mm.

--
JHG

RE: orientation or form error considered in min wall thickness stack?

JHG -- while datum feature B has no position tolerance, it does gain tilt if the hole is enlarged. This tilt doesn't change datum B itself, but it does bring the rim of the hole somewhat closer to the other hole, thus reducing the distance X at that rim by another 0.2.

IOW, you're partially right that with no MMB modifier on datum B, "we must consider the Ø10mm hole to be centred on the datum." However, the Related AME of the hole is what's centered on the datum, but that doesn't consider the actual possible rim of the hole. See the sketch in the OP for Scenario #1.

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

RE: orientation or form error considered in min wall thickness stack?

(OP)
Looks only Belanger is with me.

drawoh and SeasonLee, the ø0.4 is not position tol. However, there could be orientation and form errors when the hole gets larger than MMC. Think about this, just look at datum feature hole B itself, what is max distance from its axis (datum axis B derived from datum simulator at related actual mating envelope) to the furthest point on an actual hole at LMC size? In scenario #2, it is 5.4mm. In scenario #1, the max distance could be even greater than 5.4mm, depending thickness of the plate to calculate the allowed tilted angle of the hole. On sheet metal parts, it could be a quite big number.

kenat, are you saying only barreled or waisted shape allowed if the hole's max dia. is at LMC? Being curved hole like shown in scenario #2, there is no more material removed. It's just that, curved shape. Actual local size at any cross section equals to LMC size.

RE: orientation or form error considered in min wall thickness stack?

bxbzq,
Not only J-P is with you.
2.7 is what you should get in this case.
2.9 would work if the smaller hole was positioned to B only, and not |A|B|. In |A|B| case you have to take tilt of datum feature B surface into account, exactly as you have shown on Scenario #1 sketch.

I have some comments though.
Both of your scenarios show incorrect inner boundary of smaller hole. That said, if the hole is positioned at RFS and is at its least material condition (3.6), the inner boundary is 3.4, not 3.2.

Now, using your sketch #1 to create a stack-up, we will get (starting at your zero):
-0.2 -- stack-up start point -> right edge of 3.4 inner boundary;
-1.7 -- right edge of 3.4 inner boundary -> center of 3.4 inner boundary;
+10.0 -- center of 3.4 inner boundary -> center of 10.0 virtual condition;
-5.0 -- center of 10.0 virtual condition -> left edge of 10.0 virtual condition;
-0.4 -- left edge of 10.0 virtual condition -> stack-up end point
-----
+2.7

Another thing, part's thickness in scenario #1 has no influence on stack-up results. The thicker the part, the tilt angles will be smaller, but distances in stack-up direction will not change, so the result will always be 2.7

RE: orientation or form error considered in min wall thickness stack?

I don’t think datum feature will ever tilt outside of 10.4 (5.2 RAD).
As your perpendicularity tolerance is zero you cannot really “add” it to calculate virtual condition.
Your hole should always stay within 10.4 envelope. it looks like OP is “double-dipping”, taking 10.4 hole and tilting it another .4, while in fact tolerances are reciprocal:
You have perfect 10.0 hole which may EITHER tip 0.4 while staying 10.0, or stretch to 10.4 while staying perfectly perpendicular. Either way 10.4 envelope is maintained.
Because of that in Scenario_1 0.2 should be dropped, and in Scenario_2 5.4 should be 5.2
Just my 2 cents

RE: orientation or form error considered in min wall thickness stack?

CH,
If the datum feature hole is at 10.4 it can be 0.4 out of squarness. That generates outer boundary of dia. 10.8=10.4+0.4. Which means that extremities of datum feature surface can be 10.8/2=5.4 max radially from datum axis B.

This whole excercise can also be treated as a calculation of distance between outer boundaries of the holes, which centers are spaced basic 10.0 apart, and not their inner boundaries. So:
- outer boundary of datum feature hole is 10.8 (10.4+0.4);
- outer boundary of smaller hole is 3.8 (3.6+0.2).
Which gives: 10-10.8/2-3.8/2=2.7

RE: orientation or form error considered in min wall thickness stack?

(OP)
Another term of the 10.8mm boundary is resultant condition.

pmarc,
I agree if the smaller hole at LMC, the boundary is at ø3.4, but I don't agree you call it inner bounday. Because inner boundary is a constant boundary generated by MMC minus geo tol. In this case it should be 3.4-0.2=3.2mm. Maybe related AME is correct term.

RE: orientation or form error considered in min wall thickness stack?

Quote (pmarc)

If the datum feature hole is at 10.4 it can be 0.4 out of squarness.

No, it cannot. Your "squareness" tolerance is zero.

In order to "tilt" something, say, 0.2, you have to BORROW 0.2 from your "size" requirement, so now your hole is only 10.0-10.2.

This is fundamental principle known as "Rule 1" or "envelope requirement".

Both size and shape are confined within the same enverope - the more of one means the less of the other.

RE: orientation or form error considered in min wall thickness stack?

Quote (bxbzq)

Another term of the 10.8mm boundary is resultant condition.

Resultant condition = Largest hole + Position/orientation tolerance.

Largest hole = 10.4

Position/orientation tolerance = 0

10.4 + 0 = 10.4

RE: orientation or form error considered in min wall thickness stack?

(OP)
CH,

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 size of the unrelated actual mating
envelope of the considered feature. No tolerance of posi-
tion 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 size of the unrelated actual mating envelope
of the considered feature has departed from MMC, a tol-
erance equal to the amount of such departure is allowed.

The total permissible variation in position or orientation
is maximum when the feature is at LMC, unless a maxi-
mum is specified.

Also see fig. 2-12.

RE: orientation or form error considered in min wall thickness stack?


What "zero" tolerance means.

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

RE: orientation or form error considered in min wall thickness stack?

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.

RE: orientation or form error considered in min wall thickness stack?

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

RE: orientation or form error considered in min wall thickness stack?

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)

RE: orientation or form error considered in min wall thickness stack?

(OP)
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).

RE: orientation or form error considered in min wall thickness stack?

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.

RE: orientation or form error considered in min wall thickness stack?

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

RE: orientation or form error considered in min wall thickness stack?

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?

RE: orientation or form error considered in min wall thickness stack?

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.

RE: orientation or form error considered in min wall thickness stack?

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

RE: orientation or form error considered in min wall thickness stack?

(OP)
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.

RE: orientation or form error considered in min wall thickness stack?

Quote (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. ")


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

RE: orientation or form error considered in min wall thickness stack?

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/jan2...

RE: orientation or form error considered in min wall thickness stack?

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?

RE: orientation or form error considered in min wall thickness stack?

(OP)
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?

RE: orientation or form error considered in min wall thickness stack?

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

RE: orientation or form error considered in min wall thickness stack?

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?

RE: orientation or form error considered in min wall thickness stack?

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 smile Of course if we are talking about boundary perfectly perpendicular to A, which is a cese in our example here.

RE: orientation or form error considered in min wall thickness stack?

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?

RE: orientation or form error considered in min wall thickness stack?

Yes, this is true. But in this case the hole will occupy dia. 10.8 space equal to resultant condition of the hole, not dia. 10.4. That is why I said it would be a magic to meet all 3 conditions.

Is this what your post was refering to?

RE: orientation or form error considered in min wall thickness stack?

And about rules for measuring thickness – makes all the difference in the world.
The entire purpose of GD&T is to ensure fit and interchangeability.
From fit and interchangeability point of view, which “thickness” do you think is more relevant?

RE: orientation or form error considered in min wall thickness stack?

Guys -- remember that the original question was to find the minimum distance indicated as X.

That verbiage doesn't mesh with any particular GD&T terminology, so we need to either make an assumption or ask the questioner to be more specific.

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

RE: orientation or form error considered in min wall thickness stack?

CH,

From fit and interchangeability point of view, that is to assure that both parts will always mate together if all tolerances are met, neither T1 nor T2 is relevant, or at least this is not the most important factor I would care about.

First of all I would focus on assuring that surfaces of holes and pins may never violate their inner (in case of holes) and outer (in case of pins) boundaries (most likely their virtual conditions, if the assembly is the only concern). On your picture it looks like full mate is impossible because diameters of the holes at the bottom are smaller than size of the pins.

RE: orientation or form error considered in min wall thickness stack?

pmarc,

If you REALLY don't understand what I am trying to say, I will provide better picture on Monday.

Belanger,

Yes, this is my assumption: distance indicated as X represents amount of space critical for part to engage mating part / gage, so it's boundary-to-boundary, not worst point to worst point

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