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Primary datum A or B-C (primary)

Primary datum A or B-C (primary)

Primary datum A or B-C (primary)

(OP)
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

RE: Primary datum A or B-C (primary)

(OP)
Kenat,
ASME Y14.5-1994

RE: Primary datum A or B-C (primary)

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.

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: Primary datum A or B-C (primary)

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

RE: Primary datum A or B-C (primary)

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

RE: Primary datum A or B-C (primary)

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

RE: Primary datum A or B-C (primary)

(OP)
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)?

RE: Primary datum A or B-C (primary)

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

RE: Primary datum A or B-C (primary)

(OP)
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

RE: Primary datum A or B-C (primary)

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

RE: Primary datum A or B-C (primary)

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.

RE: Primary datum A or B-C (primary)

But why is there a basic dimension?

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

RE: Primary datum A or B-C (primary)

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

RE: Primary datum A or B-C (primary)

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

RE: Primary datum A or B-C (primary)

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?

RE: Primary datum A or B-C (primary)

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

RE: Primary datum A or B-C (primary)

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.

RE: Primary datum A or B-C (primary)

(OP)

Quote (CH)

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?

RE: Primary datum A or B-C (primary)

I am simple person.
To me if your part is allowed to “rattle” between the datum simulators, then yes, indeed you introduce datum shift.
And then you say you do it to eliminate datum shift.
I would stop right there.
It looks like you have created MMD datum without using letter “M” on your drawing. Why?
I was honestly trying to understand what is going on and I think second option (B-C) should not be used just because of the level of confusion it creates.
Just an opinion.

RE: Primary datum A or B-C (primary)

To be honest, I do not understand why some of you keep claiming that in B-C case both datum feature simulators are spaced basic .500 apart (or actually I understand why, but in my opinion this is not how it should be interpreted).

Short exercise:
I assume that current .500 dimension has a tolerance - probably defined somewhere close to a title block. Let it be +/-.010 for the purpose of the exercise. That gives us .490-.510 limits of size. Now, what if I expressed this dimension directly on the print area as .495 +.015/-.005 for instance. Would you still consider the simulators B & C as spaced .500 apart? Basing on what rule?

RE: Primary datum A or B-C (primary)

Pmarc -- they are caught up in this notion of adding a basic dimension along with profile tolerances. But if the first option in the OP were simply embraced, this whole mess can be avoided. Perhaps it's just an academic discussion at this point.

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

RE: Primary datum A or B-C (primary)

I think that referencing a position relative to B-C is unconventional and confusing. If a centralizing datum is the intent, I recommend referencing datum A which should be well-understood.

I'm not sure of the function of the part, but you can probably increase your tolerance without negatively impacting the function of the part by adding a maximum material condition modifier to the datum reference (A) and to the positional tolerance. This will also allow the position of the features to be checked with hard gaging.

Also, you're not currently controlling the perpendicularity or the left-to-right position of the hole... I think you'll want to reference three datums to establish the positional tolerance.

RE: Primary datum A or B-C (primary)

pmarc,

Isn't para 4.5.2 in '09 std quite clear? In your exercise, I don't know how the datum feature simulators would behave. They would not be spaced .500 basic apart, they would not be movable within the range of width tolerance either in my opinion. Not defined to my interpretation.

Question to all, if the .500 was basic, and Profile tolerance was applied, would the height still be considered as FOS?

RE: Primary datum A or B-C (primary)

To the last question -- no it wouldn't, because the standard defines a FOS as something that associated with a "directly toleranced" dimension, and a basic dim doesn't seem to meet that criterion (see paragraph 1.3.32).

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

RE: Primary datum A or B-C (primary)

I'm not sure of the advantage of using a profile control on this part, but even if a dimension is basic, it can still be a size dimension (and thus can define a feature of size). Ref. ASME Y14.5-1994, 1.3.17 (Feature of size definition) and Para 6.5 (Basic size dimension/profile control).

RE: Primary datum A or B-C (primary)

So now the FOS question comes down to which standard you're using; the current standard added the requirement that it be directly toleranced.

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

RE: Primary datum A or B-C (primary)

bxbzq,
So are you saying that if the dimension in my example was specified as .500+/-.010 everything would be nice and clear? Or since the dimension in OP's picture is specified just as a stand-alone .500 everything is nice and clear?

As for profile tolerance application and dim .500 being basic -- this is totally different story. Look at fig. 4-33 in Y14.5-2009. The basic height in OP's case could be assigned as datum feature, just as in 4-33, and the datum would always be a center plane. In addition there would have to be two profile callouts (or one with two leaders) without any datum references applied to both surfaces of the height. And now different scenarios could take place depending on how datum feature A was referenced in positional FCF for the hole in the center:
- A without any modifier - datum center plane simulated at RMB. (So, in fact, this would be more or less equal to case A in OP's question);
- A followed by (M) modifier - datum center plane derived from height's MMB;
- A followed by (L) modifier - datum center plane derived from height's LMB;
- A followed by [BSC] modifier - datum center plane derived from basic .500 height;
- A followed by [VALUE] modifier - datum center plane derived from VALUE height.

RE: Primary datum A or B-C (primary)

pmarc,

No, I'm saying if .500 basic and reference B-C in FCF, it is clear the simulator B and C would be spaced .500 apart. Para 4.5.2 backs me up.

In your exercise, however, it is not clear how the datum feature simulator B and C would behave. Because the distance between surface B and C is not basic dimension. Would you agree?

To go on my question, as J-P pointed out, if the height is not FOS, why does the datum of B-C have to be the center plane?

RE: Primary datum A or B-C (primary)

Yes, I would agree. And this was exactly my point. Without having basic orientational and/or locational relationship between B & C defined on the print one can not say that the simulators are spaced basic .500 apart. Moreover, one can not even say that the simulators are parallel to each other.

As for your question, I was attempting to say that if the HEIGHT was assigned as a single datum feature, like in fig. 4-33, and not surfaces B and C separately, there could be a couple of extra possibilities fully compliant with the standard.
I do not see however any value of having B & C specified as datum features separately, even with basic relationship defined between them.

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