Double-Lettered Datums

[JLang17]

In an assembly each part should have its own identifying letters for its DRF. i.e. one drawing, or part, has A|B|C, the next drawing has D|E|F, etc... Is there a standard way to deal with double lettered datums after the single letters are all used?

AA|BB|CC?
AA|AB|AC?
AA|BA|CA?

Or is this more of a designer's choice/company policy?

 

[ewh]

In an assembly each part should have its own identifying letters for its DRF

This is a new one on me. I've always treated individual part drawings as a "stand-alone" definition, starting my datum letters over for subsequent drawings.
In answer to your question though, it would proceed from Z to AA to AB to AC etc until you run out of A configurations. It would then go from AZ to BA, BB, etc.
I'm not sure which standard would cover this, but this is the method defined in the Genium Drafting Manual, and they don't usually violate the standards.

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

 

[TheTick]

i.e. one drawing, or part, has A|B|C, the next drawing has D|E|F, etc...

You are very much mistaken. Datum letters on one drawing have no impact on datums in the next drawing.

 

[JLang17]

Ok, my mistake then. I learned that if the parts are in the same assembly they shouldn't share datum letters, but that must not have been taken from any standard. Thanks.

 

[ctopher]

I agree. Each part is unique to itself. The datums have no impact on each other, only how they fit to each other in an assembly.
If I were checking a part drawing and the datums started at 'G' for example, I would mark it up to start with 'A'.

Chris
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[ringster]

I beliebe that after single letters are all used you go to AA, AB, AC, AD and so on. Then BA, BB......

 

[SeasonLee]

ringster

You are right, pls see 3.3.2 page 41 ASME Y14.5M.
"When datum features requiring identification on a drawing are so numerous as to exhaust the single alpha series, the double alpha series AA through AZ, BA through BZ etc shall be used and enclosed in a rectangular frame."

But we should aviod to use these three I,O and Q letters.

SeasonLee

 

[JLang17]

I can think of a few reasons to avoid I and O, but why Q?

 

[ewh]

Looks too much like an O.

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

 

[fcsuper]

As a point of consistancy, I would also avoid letters "Z" and "S", just to make the common prohibition against particular letters as identifiers (throughout the ASME standards).

And to iterate the point, there is no need to role datum letters between separate drawings. You might as well not reuse letters for details and section views too.

Matt Lorono
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Silicon Valley, CA
Lorono's SolidWorks Resources
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and Mechnical.Engineering Yahoo! Group

 

[ewh]

I have worked at places that omitted those letters also, Matt. One place (radar pedestal manufacturer), you had to start your section/view letters from A-A down, and datums from Z up, so as "not to confuse anyone". I don't remember any of their drawings having to go through enough letters though to reach the situation of doubling up. Still wonder how they would have handled that.

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

 

[SDETERS]

Usually we have anything that is molded or formed have a datum letter of XYZ and so on and if it is machined use ABC and so on. I have heard not using View A and Section A-A on the same drawing but including not matching datums with views and sections that is different.

 

[KENAT]

I think ewh & co set the OP straight.

Fundamentally I think this comes out of ASME Y14.5M-1994 1.4 which states that Dimensions & tolerances apply only at the drawing level where they are specified. Assuming you accept datum’s are part of the dimensioning then this backs up the consensus above.

As to letters to leave out, this is inconsistent between various ASME standards. The newer ones such as Y14.3-2003 state that for views I, O, Q, S, X, and Z should be left out.


KENAT,

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[MechNorth]

There are some conventions used in industry that might be of interest;
- in castings, forgings, moldings, etc., the precedence Z/Y/X is commonly used to quickly distinguish between formed and machined datum features
- in machined parts, precedence A/B/C is commonly used

These are popular conventions, but they are not defined as the default. Any acceptable individual letters can be used in any sequence to reflect the design intent.

The question that I get with increasing frequency is how to approach datums on a system basis, essentially as an interface map. I agree that there is no requirement to go systematically thru the alphabet for datum progression, but it can be useful when trying to establish interfaces, and to avoid some confusion when doing TSUs.

Jim Sykes, P.Eng, GDTP-S
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