Stackup calculations (X min / X max.) 3

[gabimo]

An “adjusted” (positional tolerance-wise Ø3mm instead of 0 at LMC) picture from ASME standard. (7-17/2009)
Question: What would be X min. and X MAX. for the case shown?

 

[axym]

pmarc,

I checked the public review draft, and the figure hasn't been removed - it's still the same as before. I agree with you that adding a straightness control of zero at LMC would make the LMB 19 instead of 18.5. Here's a question - would this override the Rule #1 boundary, so that perfect form at MMC would not be required?

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[pmarc]

Ah yes, you are right, Evan. The figure is in the draft indeed - it is now fig. 10-18. For some reason (most likely because it is in chapter 7 in 2009 standard) I also searched in chapter 7 in the draft. My mistake.

As for your question, perfect form at MMC would not be required in this case. Para. 2.7.1(d) in 2009 standard does not explicitly say that, but fortunately in the draft it was clarified. Para. 5.7.1(d) says: "Where a geometric tolerance is specified to apply at LMC, perfect form at LMC is required and there is no requirement for perfect form at MMC."

 

[3DDave]

pmarc,

What parts are a mystery? It must be easy enough to show where it's unclear. It was clear enough to correct it.

 

[pylfrm]

I agree with you that adding a straightness control of zero at LMC would make the LMB 19 instead of 18.5. Here's a question - would this override the Rule #1 boundary, so that perfect form at MMC would not be required?

As for your question, perfect form at MMC would not be required in this case. Para. 2.7.1(d) in 2009 standard does not explicitly say that, but fortunately in the draft it was clarified.

ASME Y14.5-2009 para. 2.7.1(d) may not say it, but doesn't para. 2.7.1(a) cover this?

No variation in form is permitted if the regular feature of size is produced at its MMC limit of size unless a straightness or flatness tolerance is associated with the size dimension or the Independency symbol is applied per para. 2.7.3.

pylfrm

 

[pmarc]

subtract the shift from the hole -> (8 -.1)/2 = 3.95

This is unclear but luckily I did not have to touch it to get to the correct number.

pylfrm,
Personally I do not like the statement from 2.7.1(a) because to me it implies that the straightness or flatness of zero at MMC would not be considered by the committee as legal callout.

But if we take that out of equation then I guess 2.7.1(a) could be used to prove that when produced at MMC size the feature controlled with straightness tolerance of zero at LMC does not have to have perfect form.

 

[3DDave]

pmarc - that's because you didn't read the line above it. (11 - 0.05)/2 - (3 + 0.05)/2 = (8 - 0.1)/2

I call 2.7.1 half-baked. It has interlocking requirements that conflict. I bet they have escaped notice due to the rarity of the LMC callout and the lack of good examples in the standard.

 

[pmarc]

pmarc - that's because you didn't read the line above it. (11 - 0.05)/2 - (3 + 0.05)/2 = (8 - 0.1)/2

Yes, no other reasonable explanation comes to my mind

 

[mkcski]

To all who posted:

I have not studied and LMC datum concepts like it appears everyone who is posting has. So I am trying to follow the posts as a expanding-my-understating exercise. The "dialoig" is amazing! But I have one observation I would like confirmed.

The min / max calculations have issues because Y14.5-2009 and the Nov 2105 DRAFT "avoid" clarifying text that would define that "there is perfect from at LMC for LMC datums" (or not).



Certified Sr. GD&T Professional

 

[Belanger]

mkcski -- No version of the standard has said that an LMB reference automatically means perfect form at LMC. I haven't read the new draft completely, but from the snippets given by pmarc and Evan, it says that if you wish for LMB to equal LMC, then you need to put a geo tol of zero at LMC on that datum feature.

I don't think any of that makes for an issue on this calculation, because the designer apparently didn't want that perfect-form requirement. (Maybe perfect form at LMB was desired, thus it can be confusing, witnessed by my flub on the initial answer!)

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

 

[3DDave]

They mostly have problems since LMC/LMB references can not typically (ever?) be verified by go/no-go gauge as they lie on the surface only at LMC or are embedded in the part. Lack of general applicability doesn't help either - most interest in tolerances that affect fit and LMC/LMB can not typically (ever?) affect fit in a way that is any but a side effect. Since there is no gauge there's no physical datum simulator.

The main thing that's missing is a diagram of what an LMC datum reference means in the context of an unmodified feature of size - that the LMB is based on a banana (or similar) shape that forms the limit for the unrelated actual minimum material
envelope, which is not a shape that is often seen in machined parts.

Since the example was from Y14.5 and all it was used for was a cartoon without explanation I'd say that's a good place for improvement.

 

[mkcski]

3DDave:

I totally agree with your thinking. The modern world runs runs on interchangeable parts, which supports application of MMC assembly concepts. I have only seen one justified use of LMC for features (not datums) - controlling the ligament "thickness" between holes in a heat-exchange tube sheet.

Certified Sr. GD&T Professional

 

[greenimi]

This (the OP excercise) could be a good question / problem for the GDTP-2009 certification exam and it is from the standard ( from the "suppose to know material").

 

[3DDave]

Nah - it's what I call a sucker question. If you know the trick it's easy; if you don't it's disproportionately difficult. If there was a 'means this' for that diagram then the trick is obvious. There are a lot of MMC examples. I suspect if this went through there would be a 90-95% fail rate.

Edit - Except for those who add covering this example to their GDTP test prep courses.

 

[pylfrm]

I have only seen one justified use of LMC for features (not datums) - controlling the ligament "thickness" between holes in a heat-exchange tube sheet.

Why not directly tolerance the ligament thickness? Seems like that would be a much more accurate encoding of the functional requirement.


pylfrm

 

[Lee Saxon]

I'm confused about how we're getting 18.5 from "19 +0.5/-0"?

 

[3DDave]

The maximum diameter is 19.5 so that limits the form. At 19 the outer part can bend to wedge into that 19.5. In the bent condition the largest perfectly formed cylinder that can fit is 18.5.

If you draw a picture of these steps - 19.5 diameter, bent 19.0 local size minimum, and then fit a cylinder that lies fully within the bent 19. local size bent version you'll see that you loose .5 from one side of the 19.5 and an additional .5 from the other side.

 

[pyromech]

Lee

I am with you about using 18.5 Here is an exact example from Alex book where he uses the given limits of size.

 

[greenimi]

A.K book does not use the form error for the datum feature inside diameter A (RFS in the positional callout)
Not sure why the book does not use it................., but it is a difference between thin wall and min wall.
If 14.2-13.8 = 0.4 form error is taken into the calculation for the minimum wall calculation then you find find the "correct" answer to be 0.45.
Again: thin wall (probabbly constant accross the section) 0.65, but minimum wall = minimum possible distance x min. that can ever happen in a single cross section is 0.45.

 

[Belanger]

That's not the same example really, because that doesn't have LMB on the datum reference. Having LMB means that there is no Rule #1 at the least material size, which is the reason the other example used 18.5.

edit: sorry greenimi -- I didn't notice your comment is saying the same thing.

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

 

[pyromech]

That's not the same example really, because that doesn't have LMB on the datum reference.

Right. Unfortunately, there is no similar example using LMB. If you want to use Alex approach, the trick is to use 18.5 as LMC. Where the VC=MMC size (19.5) or min datum shift=0 (|MMC rad-VC rad) and the max datum shift is 1.0 (|LMC rad - VC rad|)