【转帖】can one feature be used for both secondary and tertiary datu

yang686526

can one feature be used for both secondary and tertiary datu
i have a simple bracket. it only has a slot hole on it. can i use the slot as both secondary and tertiary datum? thanks in advance.
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technically, you could use the long sides as a secondary and one end or middle of both ends as tertiary.  seems quite impractical if you are going to touch off on them for measurement.
sort of...yes/no. depends on the design and how it mates to mating part. normally i would not. is there a reason you would use secondary and tertiary datums?
chris
systems analyst, i.s.
solidworks/pdmworks 05
autocad 06
it is just a simple bracket. the reason using secondary and teriary datums is that i want to set up datum scheme. is it true that every part needs datums (a, b, c)? in other words, do we always need to set datums before puting dimension on the drawing even if it is a simple part? thanks   
not necessarily.  some gdt, like straightness and flatness, rquires no datum reference.  some cases do not require three datums.
sounds like you need to control the slot w.r.t. the rest of the bracket, not the other way around.
quoted from the asme y14.5m - "in some cases, a single datum reference frame will suffice.  in others, additional datum reference frames may be necessary where physical separation or the functional relationship of features require that datum reference frames be applied at specific locations on the part."
one other thing to think about is how this slot performs when it's assembled in the next highest subassembly.
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heckler
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i have seen it a lot where people use a, b & c datums ... no matter what the design does and they base all features off those threee datums. very common.
i agree with thetick & heckler.
we368, curious. do you have a copy of asme y14.5?? if you don't, you should.
chris
systems analyst, i.s.
solidworks/pdmworks 05
autocad 06
how does one establish 3 datum features when dimensioning either a ball bearing or a simple washer. (just something to think about?)
i seem to recall a case of a person showing 3 views of the ball bearing, years ago.
there is a thread posing the question "who needs to have an understanding of gd and t?"  on the surface, it might appear that with a welded structure, you are misdirected in using gd and t in the first place.  have you had any (formal) training on the applications?
the 3 views of a ball bearing is a joke.
chris
systems analyst, i.s.
solidworks/pdmworks 05
autocad 06
ye368,
   you need enough datums to immobilize your part.  does applying a second datum to your slot contribute to this?
   i can give an excellent example of a part with two datums.  i have a round, flat plate with a bolt circle.  datum_a is the bottom face.  datum_b is the outside diameter.  the bolt circle has a positional tolerance using datums_a and_b.  a third datum has no meaning, unless i add another feature that must line up with the bolt circle.
   i keep trying to type "pitch circle".  
   the first hole i drill in ringman's ball bearing can only be located wrt datum_a.
                      jhg