datum setup
 

datum setup
it seems there are a lot of problems on the attached print, but i will focus on the datum callout. here is my understanding on the designer's intent:
the bottom surface is primary datum a
datum b is a pilot location hole
datum c is another pilot location hole which is established base on datum b
b-c create the axis orientation
datum b will be the original point for cmm measuring.
my questions are:
1. can we allow two datum symbols (datum a and datum d) on a coplanar surface?
2. is it a right way on datum c callout?
3. which one is correct on the datum reference frame callout
primary datum a -> b-c -> datum b
primary datum a -> datum b -> b-c
primary datum a -> b-c (without tertiary datum)
thanks for all comments
seasonlee

i reviewed you drawing and:
1 each surface could be the datum but having a datum a on one surface and a datum d on the other surface does not make sense. one needs a phantom line between both surfaces and then both are datum a. your drawing in not quite correct here.
2 datum c should be reflected in a feature control frame referencing datum a (bottom surfaces) and datum b (hole) at mmc. the .000 tolerance is fine. it just states that the size of the mmc and the virtual condition size are the same.
3 if you change the bottom surfaces to datum a (not a & d), the reference datums should be a| b mmc| c mmc. when one has a hole as the secondary datum, it is the intersecting point of 2 perpendicular planes. all dimensions come from datum b while datum c is only to orient the part (square it up).
hope this helps.
dave d.
dingy,
i believe that 2 planes intersect in a line, not a point.
"dual datums" like "b-c" are only appropriate for runout. not appropriate for position or profile. use |a|b|c| for primary|secondary|tertiary. the implication is that b positions the part and c clocks the part into position.
|a| and |d| can be individual datums because hey are separately identifiable features. inspectors will need to block part to ensure part does not rest on wrong portion. better to use datum targets to define datums with more clarity.
why is there a |d| datum? it's not used for anything.
thanks for all comments.
when should we need to use dual datum like "b-c"? where can i find it in the standard y14.5m ?
i believe the designer intent to specify the profile tolerance .020 is to control the flatness after forming, there is a note "two surfaces" below the profile control, so it must be a coplanar surface,i agree datum d should be deleted.
seasonlee
ringster:
i agree with that statement about line rather than point. actually, the standard states "axis".
thetick:
one can have 2 planes as a primary datum. one surface could be datum a while the other surface is datum b. when referring to the primary datum in a positional tolerance, one would state "a-b".
it all depends on how the part mounts on the mating part. above, it the part contacts on 2 surfaces at the same time - thus, datum a & datum b. if the part only contacts on 1 of the surfaces, then we only have a datum a.

dave d.
seasonlee,
multiple datum features is defined per 4.5.7 - 4.5.7.1 - 4.5.7.2
fig 4-19 shows a mdf with tolerance of position.
fig 6-21 illustrates a mdf with surface profile.
fig 6-49 & 6-51 shows a mdf with runout and total runout.
i found fig 4-8 in the standard y14.5m is quite similar as the one i posted earlier, the datum reference frame should be i a i b mmc i c mmc i for sure.
thetick :
1. would you please advise where i can find out the rule of "dual datums like b-c are only appropriate for runout" in y14.5m.
2. dual datums b-c not appropriate for position or profile. this makes me recall one part i inspected two years ago, all profile tolerance with dual datum b-c in the drf as shown on the attached
all comments will be appreciated
seasonlee

the example you just posted showing datums b & c as a feature of size taken from theoretical intersections makes complience with 4.3 quite the stretch of the imagination.
as to the illustration posted 07feb09, the b(m)-c(m) is legal, but does not provide 'predictable' repeatability.
see
if dual datum b(m)-c(m) is legal in the post dated at 2-7-09, may i ask what is the difference with this drf a i b (m) i c (m) ?
thanks again
seasonlee