DO THE SECOND AND THIRD DATUMS NEED TO BE RELATED......

[learningchecker]

Hi, I would appreciate your help. ASME Y14.5-2009, MATL: 302 SS .040 THK, OVERALL SIZE IN INCHES: 4.000 X .075 & .325 X .040. 2 Bends.

The engineer has DATUMS A, B, & C. No FCF showing any relationship with B to A and none with C back to A & B. He does have one bend with profile of a surface with respect to A,B,&C. He also has a straightness callout on the .075.

Is he right that he doesn't need a perpendicularity for B w/respect to A and one with C w/respect to A & B?

Thanks......

 

[fsincox]

I have been in lots of arguments here with people over what was basically just this issue. You have asked about 2009 so this makes it easier. In the past it was never a requirement, In 1994 it was to be "considered". Now, since 2009, it "shall be controlled....".
It always seemed resonable when we were working with important features, the delemma comes when you are working with non-important features only being used for inspection purposes.
We have some "experts" who do not recommend it if the datum feature is too tough for inspection to inspect, etc.
The standard now says: "YES". I am sure many of the drawings you will see for a long time will not.
Frank

 

[learningchecker]

Thank you for answering. Can you tell me where in ASME Y14.5-2009 it writes that? I have to have proof in my hand when I tell him this. I haven't found it yet.

Thanks again....

 

[fsincox]

See ASME Y14.5-2009, section 4.9, DATUM FEATURE CONTROLS.
Frank

 

[learningchecker]

Thank you so much.

 

[CheckerHater]

Am I the only one who thinks Para. 4.9 is seriously half-baked?

First, choice of words. “Shall” shall never be in the same sentence with “appropriate”, as if I shall do something the way I see fit. Who is to decide what’s appropriate?

Second, overly detailed pp. a), b), and c) are implying that there is only one primary, secondary and tertiary feature. I already asked this before: what if I want/need to use the same datums in different order?

Third, “informative” rather than “commanding” style: “This, in turn makes it possible to calculate datum feature simulator boundaries” Makes sense, but not all datum features have boundaries.

Maybe this actually has to be emphasized: “Datum features shall be controlled directly or indirectly if and when it’s appropriate for calculating simulator boundaries or other ways to reduce ambiguity”

You do it when you need/have to do it. Not “shall” to do “appropriate” (and they still use the word “consider”)

Not trying to start a fight. Actually don’t have time for that.

 

[learningchecker]

I agree with you and I'm still trying to decipher it. Still haven't gotten my answer. I'm sure it's my lack of brain power, but I'll keep on trying.

 

[fsincox]

CH,
I have no problem with your still questioning it. They are trying to encourage, for most cases what above in effect I am calling common sense, while at the same time not making every old drawing that does not conform, obsolete, not really an easy task.
We like things down in black and white; they are trying to be “politicians” to get a majority to agree on something.
I also implied above, that I feel not all parts seem to need this kind of discipline. I believe in the older concept that the perfection of the datum system itself should be good enough in some cases. This was a concept I took for granted before pmarc and I discussed my keyway example back and forth.
Frank

 

[Belanger]

CH,
There's nothing wrong with using "shall" and "appropriate" in the same sentence. "Shall just says that something is to be done, but "appropriate" means that you can choose the means to do so.

Suppose I get a notice for jury duty. I "shall" go there on that day, using "appropriate" transportation (car, bike, walking).
No conflict there!

At any rate, I think the benefit of that requirement is for datum features referenced with MMB or LMB, because in those cases a specific number needs to be determined to simulate the datum properly. The requirement might not be so necessary (but still good) when dealing with other datum situations.

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

 

[CheckerHater]

John-Paul,
I was always taught that there is a clear distinction between “shall”, “should”, and “may”, at least in GD&T.
You are yet another person on this thread saying the requirement “might not be so necessary (but still good)”.
From this point, do you think using the word “shall” in Para. 4.9 is appropriate?

 

[pmarc]

Paragraph 4.9 is simply about completeness and unambiguity of drawing specification, regardless of type of datum features and material boundary at which they can be referenced.

Lack of any geometric control means the drawing is incomplete.

Quasi control, that is for example title block angular or linear tolerances, means the drawing is ambiguous.

 

[fsincox]

CH,
It is required to calculate a virtural condition properly. This is not required if we go back to the old concept of the datum reference frame created by "the image of perfection as established by the best tools we have in our shop" sort of thing. The mathmatizaion of the standard really requires it.
Frank

 

[CheckerHater]

I was simply unhappy with the wording, but now will somebody please explain how boundary / virtual condition looks like when datum is NOT feature of size?

 

[learningchecker]

Hello to all,

English really is my first language, but we all have our limitations and I am admitting mine. From reading Para 4.9 I come up with : It's best to control by Geo Tol, DATUM B to DATUM A and DATUM C to A & B, but it is not THE LAW.

If I am interpreting this correctly can I get a simple "YES, you are" or "NO, you are not"? This would help me greatly.

Thanks to all. I have learned much from all of you.

 

[fsincox]

Yes, I read it the same. (of course no one says it has to be A B & C).
Frank

 

[pmarc]

I was simply unhappy with the wording, but now will somebody please explain how boundary / virtual condition looks like when datum is NOT feature of size?

Figs. 4-29 through 4-31, for example, may help you.

But again, the boundaries are just one side of the story. In simple case of 3 mutually perpendicular planar datum features it is indeed really hard to think in terms of boundaries.

It is, however, very easy to picture that the surfaces must be controlled to each other, otherwise the geometry stays not fully defined.

 

[Belanger]

CH -- I don't think you were following my thought there. I was simply stating that it's perfectly fine to use "shall" and "appropriate" in the same sentence.

Did you read my analogy about jury duty? Notice that the concepts of "shall" (you SHALL appear for jury duty) and "may" (you MAY use any method to get there) are speaking to totally different ideas. If I had said, "you shall appear for jury duty, but you may come if you think it's appropriate," then I would agree with you.

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