datums

[ray99]

Hello all (my first post)
Perhaps an elementary question, but here is is:

In a multi-detail drawing (tooling), can you reuse the same datum letters (A,B,C,..) on the different parts? Or do you have to continue with the next letters in sequence (D,E,F,..) for the other parts. Incidentally each part has line dividers between them.

 

[MikeHalloran]

Re-using datum identifiers within a single document to refer to different datums is just asking for trouble; don't do it.

The only disadvantage to not identifying datums in sequence, e.g. leaving out identifiers, is that you may get some phone calls about the missing identifiers. On the other hand, there are good reasons for not using easily misinterpreted letters like I,J,O, and a few others.



Mike Halloran
Pembroke Pines, FL, USA

 

[pmarc]

Agree with Mike. If more than one part is shown on one drawing I would recommend using different letters for different datum feature identifiers. Letters I, O, Q shall not be used as datum identifying letters. If there are so many datums that single alpha series are exhausted, double alpha series shall be used (AA,...AZ, BA, ..., BZ, etc.)

 

[Belanger]

Or you could be like a designer I knew who always used his intials for the datums: D, A, W (his first name was Daniel, his middle name started with A, etc.)

When questioned, he would always pull out the standard and show that the datum letters don't have to start at the top of the alphabet and don't have to be in sequential. Needless to say, he drove everyone nuts.

So to avoid playing "mind games" with people, most drawings will use A, B, C ... and like the other posts say above, it's best to use fresh letters for different details on the same print.

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

 

[ray99]

Thanks everyone,

As I suspected, good common sense advice. So it seems there is no written standard regarding this.
I happen to have one more two-part question (not related to the datum question above), perhaps better put in a new thread, but here it is:
1)
There is a counter-bored hole with a positional tolerance below it.
Does this positional tolerance apply to the hole or to the counterbore, or to both?
2)
Say I want a loose tolerance on the hole location and tight on the counterbore. Or vise-versa. How is this best shown?

Thanks.

 

[pmarc]

ray99,

Regarding your first post: take a look to paragraph 3.3.2 of Y14.5 (1994 or 2009 edition). It does not specifically state how to proceed with datum lettering when different parts are shown on a print, but it is clearly written that "Each datum feature of a part requiring identification shall be assigned a different letter".

Regarding your recent questions: look at figs. 7-24 through 7-26 in Y14.5-2009 or 5-37 through 5-39 in Y14.5M-1994. These figures are showing 3 different ways of controlling relationship between bore and a hole. I believe this is what you are looking for in this case.

 

[ewh]

I agree strongly with the others regarding not repeating datum letters; however, being the literalist I tend to be, I would like to expound on pmarcs example from the standard... "Each datum feature of a part...". This would require further definition of just what a part is. As I understand it, the assembly in question is composed of several parts, which would allow for repeating datums on the drawing (as poor of a practice as that may be). Perhaps if the standard were worded differently, not allowing repeating datums on a drawing, this issue would go away?
;-)

"Good to know you got shoes to wear when you find the floor." - [small]Robert Hunter[/small]

 

[ray99]

Thanks again all,
re: the c'bored holes, it's all there in y14.5 isn't it? I don't know how I overlooked it. Thanks.