Double datum letters

[Belanger]

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.?

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

 

[CheckerHater]

The standard (Para. 3.3.2) says: “When datum features… are so numerous as to exhaust the single alpha series, the double alpha series (AA through AZ, BA through BZ, etc.) shall be used…”

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.

 

[fsincox]

I do not believe any order is required. Too many people belive "A" must be the first datum!
Frank

 

[CheckerHater]

But I think you still have to exhaust your single alpha option before you can start using AA (or ZZ for that matter)

 

[fsincox]

Taken literally it can be read to imply more than that!
Frank

 

[Belanger]

Well, here's how I see it, if you want to approach it from a "legalistic" point of view: Double letters can be used, but only if the 23 single letters have all already been used. Then, if you do get into double letters, they don't have to start with AA (although that is implied as the logical way to start) but they can be random double letters, if desired.
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

 

[fsincox]

JP,
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

 

[CheckerHater]

I think standard meant just because the datum is alphabetically first, it doesn’t make it primary.

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.

 

[Belanger]

Good points, CH. I was just using ASME thinking.

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

 

[fsincox]

Mistakes are made, changes happen, we don't want to have to reletter everything each time we change datums frameworks, at least I don't.
Frank

 

[ewh]

I feel that it is still a good idea to initially use leters consecutively, especially if you know that double letters may be required. This makes it much simpler to avoid repeating letters unintentionally.

“Know the rules well, so you can break them effectively.”
-Dalai Lama XIV

 

[Belanger]

One other question for the group -- it's still about datums so I will tack it on here instead of starting a new thread...

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

 

[ewh]

Mean average?

“Know the rules well, so you can break them effectively.”
-Dalai Lama XIV

 

[CheckerHater]

As confused as you are