Double datum letters
Double datum letters
(OP)
The ASME standard certainly allows double letters to be used as datum feature identifiers, once the single-alpha identifiers have been exhausted (ref. para. 3.3.2). But here's an interesting question:
Since the single-letter designation doesn't have to follow alphabetical order, would you say that the double-letter designation also doesn't have to follow an order? The standard seems to imply that the double letters must, but that seems like a break in logic from the single letters.
In other words, if I have a huge drawing where I need 60 datum features (hey, it's possible), would you say it's illegal to use BX, for example, without having an AA, AB, AC, etc.?
Since the single-letter designation doesn't have to follow alphabetical order, would you say that the double-letter designation also doesn't have to follow an order? The standard seems to imply that the double letters must, but that seems like a break in logic from the single letters.
In other words, if I have a huge drawing where I need 60 datum features (hey, it's possible), would you say it's illegal to use BX, for example, without having an AA, AB, AC, etc.?
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems





RE: Double datum letters
Nowhere the standard says that “you shall start with AA”.
I personally met people who believed that AZ should be followed by AAA, so actually explaining that “double alpha” means that AZ is followed by BA was absolutely necessary IMHO.
RE: Double datum letters
Frank
RE: Double datum letters
RE: Double datum letters
Frank
RE: Double datum letters
I'm thinking we all can agree with that (but I'm curious what your last post was alluding to, Frank).
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
RE: Double datum letters
Just that the order given as an example was, literally, required. There is a section in the standard that says something about "datum order is not required", that is what I go by.
Frank
RE: Double datum letters
On the side note, some companies (and possibly countries thru national standards) require using unique letters on datums, views and sections.
This means two things:
A. “single alpha” will be exhausted even faster.
B. skipping letters and burning alphabet from both ends should be tolerated.
RE: Double datum letters
However, I would say that not only is the letter irrelevant to the precedence of primary, secondary, tertiary, but that the letter itself could be plucked from anywhere in the alphabet.
I heard of one guy who always used his initials for the datums. Thus, no A, B, C, on the drawing anywhere. But the lone datum feature was labeled as D because his name was Daniel.
"Hey, Dan," people would say, "you can't do that."
"Yes I can," countered Dan, "see, it says here in the standard...!"
Needless to say, although he was right, he was kind of annoying :)
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
RE: Double datum letters
Frank
RE: Double datum letters
“Know the rules well, so you can break them effectively.”
-Dalai Lama XIV
RE: Double datum letters
Is the "note" on page 48 of the 2009 standard (at the end of paragraph 4.2) intended to be worded that way? I guess I understand the gist of it, but what is the deal with the first few words, "In the means this portion..."
I'm not an English major, but that seems like a typo or something.
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
RE: Double datum letters
“Know the rules well, so you can break them effectively.”
-Dalai Lama XIV
RE: Double datum letters