datums
datums
(OP)
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.
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.





RE: datums
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
RE: datums
RE: datums
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
RE: datums
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.
RE: datums
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.
RE: datums
"Good to know you got shoes to wear when you find the floor." - Robert Hunter
RE: datums
re: the c'bored holes, it's all there in y14.5 isn't it? I don't know how I overlooked it. Thanks.