Primary datum A or B-C (primary)

[greenimi]

What a day……. I had a big argument with some of “local” GD and T “experts” and I would like to run this issue past you guys to see what you have to say.
I wouldn’t expect to have arguments about such a simple part, but anyway….
The part is “sandwich between” other two components called mating parts (mating part 1 and mating part 2). The requirement is to have the hole in the middle on the .500 width.
Now, we had position Ø.005 wrt A (no bonus and no datum shift allowed).
Some guys suggest having the position callout changed to position Ø.005 B-C (as primary), where B is the datum feature plane (.750x 1.000) on one side and C is the datum feature plane (.750x1.000) on the other side.
My opinion was that those callouts are equivalent and produce similar results/have the same effect. They are just two different syntaxes to say exactly the same thing.
Others did not agree and said that it’s a difference in the way the datum feature simulators act and because of that it makes the callouts different. On the position only to A the datum feature simulators is allowed to rotate until get the highest points on the flats, but on B-C (primary) the simulators must remain parallel to each other (they invoked some rule of the datum feature simulators such as must have perfect form, perfect orientation to one another and be perfect positioned) ---
4.5.2 -2009 page 53
I am not saying the rule does not exist, but I don’t know if it’s applicable in our case or not, and here I need your help a little bit.
Which position would you consider to be the correct one to be used in this case?

Thank you for your help

 

[KENAT]

What drawing standards are you working to? Your first option of referencing A makes sense to me.

The B-C reference isn't intuitive to me.

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[greenimi]

Kenat,
ASME Y14.5-1994

 

[KENAT]

OK, I took a look at section 4.5.7 and I'm not convinced that the proposed B-C scheme really holds water. I suppose it you make it as analogous to the examples using 2 nominally coaxial diameters to derive and axis but I'm not sure. One of the more expert members may give a better response.

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[Belanger]

I say neither option. If the requirement is to have the hole be in the middle of the width, simply call that width (top-to-bottom dim) the datum feature. It's called a "feature-of-size" datum, and I guess the closest thing in the 1994 standard would be Figure 4-13.

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

http://www.gdtseminars.com

 

[KENAT]

Belanger, did I miss something?

If you look at the attachment greenimi put then the first option where datum A designator is aligned to the arrows of the .500 dimension is the approach you're suggesting isn't it?

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[Belanger]

Sorry to you both -- I was in a hurry I guess! The first option is indeed correct, Kenat. Those who suggested the other alternative are only muddying the waters.

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

http://www.gdtseminars.com

 

[bxbzq]

I would say the second option could achieve same result as the first one if:
the distance between surface B and C is basic dimensioned
both surfaces are Profile controlled and profile tolerance is half of the the default tolerance of .500
use translation modifier on datum B and C

 

[greenimi]

I understood that if we want to use B-C as a primary datum we need to have some basic dimensions added along with some tolerance adjustments (to get the equivalent results and same effect as primary datum feature of size A). I agree with that statement.
Now, the main question/disagreement still remains: does the datum feature simulator argument have any merit? In other words, does the datum feature simulator act/perform differently in the depicted circumstances (datum feature of size A as primary versus B-C as primary)?

 

[Belanger]

I still can't see why the two options are even being weighed. Go with the first one, rather of making life more difficult by entertaining the second option.

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

http://www.gdtseminars.com

 

[greenimi]

We understand that the first option is the one to go and we will go for it, but just to the sake of the discussion what about the datum feature simulators? Can you shed some light over that issue too?
Thank you for your help

 

[Belanger]

The B-C option is quite confusing, first of all because the standard doesn't show such a method. That's not in itself a bad thing (not everything can be shown in the standard). But I don't even think that a basic dimension is required as suggested above because the B-C combination is a feature of size. (The basic dim idea makes sense if you're doing something like Fig. 4-22 in the standard, where the surfaces face the same direction.)
But since you asked, I think the simulator would be two parallel plates that close down, regardless of material boundary, until they hit the high points. The real datum would then be the center plane.

Which brings us right back to what the first option would yield. Thus my confusion about why the question persists: I say to go with the easier method.

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

http://www.gdtseminars.com

 

[CheckerHater]

In second case (B-C) planes cannot “close” on the part, because they always have to stay away from each other for the amount on basic .500.
They will be fixed in space, for better or for worse.

 

[Belanger]

But why is there a basic dimension?

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

http://www.gdtseminars.com

 

[CheckerHater]

It was agreed few posts before that it would be better this way

 

[Belanger]

But read my last post and you'll see why I question that tactic.

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

http://www.gdtseminars.com

 

[CheckerHater]

I agree. I just was hoping you may see something in this setup that I missed.
Would that be correct to say, that with or without basic dimension it still questionable?

 

[Belanger]

To me, the entire B-C option is confusing. Without a basic dim, I see it as being the same as the first option (so do the first option!). And with a basic dim, I see it as being -- well -- more confusing

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

http://www.gdtseminars.com

 

[dlloydks]

I am a little rusty, but I used to read B-C as "common datum B-C". What is common with both B & C? Nothing! If A is not sufficient I lean toward the feature of size suggested by Belanger.

 

[greenimi]

In second case (B-C) planes cannot “close” on the part, because they always have to stay away from each other for the amount on basic .500. They will be fixed in space, for better or for worse.

CH,

You are absolutely right and that was the thought process behind the idea proposed by some engineers.
Because the planes B and C have always to stay away from each other for the amount on basic .500, the inspector can move the part up and down to bring the axis of the hole inside the tolerance zone and accept the part as a functional one.
Again, as I stated at the beginning of the thread, we don’t want any bonus tolerance added (MMC) or any datum shift allowed (MMB on the datum feature of size) and the main reason for not allowing this, as I was told, is those potential extra tolerances, will have negative effect on some stack-ups (if MMC and MMB is there, we have to take it in consideration, but on the B-C scenario MMB is not allowed in 1994 standard). Therefore, the inspector can take advantage of part mobility –moving the part up and down until brings it into the tolerance zone---and qualifying it as a good part—but this “mobility” won’t be considered in a subsequent stackup analysis (since does not have the symbol M).
Yes, I know, does not make too much sense, right?? But that was the method of thinking to circumvent all the negative effects of the MMC/MMB’s in the stackups.
Any validity?