Double-Lettered Datums
Double-Lettered Datums
(OP)
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?
AA|BB|CC?
AA|AB|AC?
AA|BA|CA?
Or is this more of a designer's choice/company policy?





RE: Double-Lettered Datums
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." - Robert Hunter
RE: Double-Lettered Datums
You are very much mistaken. Datum letters on one drawing have no impact on datums in the next drawing.
RE: Double-Lettered Datums
RE: Double-Lettered Datums
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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RE: Double-Lettered Datums
RE: Double-Lettered Datums
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
RE: Double-Lettered Datums
RE: Double-Lettered Datums
"Good to know you got shoes to wear when you find the floor." - Robert Hunter
RE: Double-Lettered Datums
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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RE: Double-Lettered Datums
"Good to know you got shoes to wear when you find the floor." - Robert Hunter
RE: Double-Lettered Datums
RE: Double-Lettered Datums
Fundamentally I think this comes out of ASME Y14.5M-1994 1.4 (n) 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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RE: Double-Lettered Datums
- 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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