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
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)
The B-C reference isn't intuitive to me.
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: Primary datum A or B-C (primary)
ASME Y14.5-1994
RE: Primary datum A or B-C (primary)
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: Primary datum A or B-C (primary)
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
http://www.gdtseminars.com
RE: Primary datum A or B-C (primary)
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?
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: Primary datum A or B-C (primary)
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
http://www.gdtseminars.com
RE: Primary datum A or B-C (primary)
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)
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)
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
http://www.gdtseminars.com
RE: Primary datum A or B-C (primary)
Thank you for your help
RE: Primary datum A or B-C (primary)
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)
They will be fixed in space, for better or for worse.
RE: Primary datum A or B-C (primary)
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
http://www.gdtseminars.com
RE: Primary datum A or B-C (primary)
RE: Primary datum A or B-C (primary)
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
http://www.gdtseminars.com
RE: Primary datum A or B-C (primary)
Would that be correct to say, that with or without basic dimension it still questionable?
RE: Primary datum A or B-C (primary)
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
http://www.gdtseminars.com
RE: Primary datum A or B-C (primary)
RE: Primary datum A or B-C (primary)
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)
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)
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)
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 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)
RE: Primary datum A or B-C (primary)
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)
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
http://www.gdtseminars.com
RE: Primary datum A or B-C (primary)
RE: Primary datum A or B-C (primary)
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
http://www.gdtseminars.com
RE: Primary datum A or B-C (primary)
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)
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: Primary datum A or B-C (primary)
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)
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.