【转帖】double-lettered datums

yang686526

double-lettered datums
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?

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quote:
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."fff"> - robert hunter

quote:
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.
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.
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
solidworks/pdmworks 08 3.1
autocad 08; catia v5
i beliebe that after single letters are all used you go to aa, ab, ac, ad and so on.  then ba, bb......
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
i can think of a few reasons to avoid i and o, but why q?
looks too much like an o.
good to know you got shoes to wear when you find the floor."fff"> - robert hunter

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
cad engineer/ecn analyst
silicon valley, ca
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 re