【转帖】question regarding datum and tolerances on it as well as fea

yang686526

question regarding datum and tolerances on it as well as fea
i have a heat sink with 3 plateaus.  i used a flattness call out of 0.30mm and named that my datum a.
now i have additional features which i want to have a tighter tolerance of 0.02mm wrt datum a.
does the feature tolerance become insignificant because datum a has a larger tolerance?  or does the feature tolerance become "with respect to" whatever plane datam a lies on?
if you want it parallel within .02 to datum a (which is flat within .30), that means it may be flat to within .32;  is this really what you want?
believe it if you need it or leave it if you dare.fff"> - robert hunter
ewh,
are you sure of that or might you want to rethink it?
if the standard applied it y14.5, i do believe that the referenced surface will have to be flat within .02 to satisfy the parallel requirement of .02.
i thought a datum plane was a perfectly flat surface upon which your potato chip rests while it's being measured, not an imperfectly planar feature of the article itself.
mike halloran
pembroke pines, fl, usa
ryandis if i understand correctly your 3 'plateau' can't really use flatness if asme y14.5-1994 is in effect, i believe they need to be surface profile see paragraph 6.5.6.1 & figures 6-20 & 6-21.
we had a similar situation recently and i put together the attatched.  it's a compiled extract of asme y14.5, i hope i'm not massively breaking any copyright issues, if so administrator please delete the link or whatever works.
kenat, probably the least qualified checker you'll ever meet...
the flatness of -a- refers to the 3 'datum targets?' designated -a-. datum -a- may still require additional definition, as flatness does not establish orientation; unless the 3 'datum targets' are clearly co-planar.
the refinement of 0.02mm would be confirmed while setting on the 'high points' of datum -a-.
correct me if i'm wrong, but i don't believe you mentioned targets at all.  the three plateaus as you refer to them, should be called out using a surface profile of 0.3 without a referenced to any datum and a note under the fcf indicating application to the three planes.  this will control flatness and co-planarity of the three datum features.  the fos dim/tol defining the additional features you want to control, probably allows for more than 0.02 in orientation and form.  the refinement is the orientation  callout with respect to datum a, not to mention that it also controls form.  it is not accumulative at this point with the geo tol on the datum and should be acceptable, since the measurement originates from the datum not the datum features.  the only thing to be concerned with is validating the actual local size of the features first and i would also make sure to lock the part unambiguously in the six dof  with a tertiary for r&r.
hope this clears things up for you all!  additionally, i really have to question the .3mm form on a datum (foundation)when other features w.r.t. are specified to less than 10% of that.  seems like you could be asking for trouble.  upon performing a stack analysis on the assembly this may be also show up as a contributor.  
i would advise evening them out a bit and if any thing make the datum form tolerance tighter.
xplicator, that's more or less what i was thinking with regard to defining -a-.  not sure where the 'datum targets' came into it.
however weavedreamers last paragraph matches my understanding.
kenat, probably the least qualified checker you'll ever meet...
just picturing 3 plateaus. while datum targets were not mentioned, it is that imagary which prompted the suggestion with the initial question mark ie: 'datum targets?'.
flatness cannot apply to 3 plateaus simultaneously but only on each one separately. as kenat stated, the use of a profile of a surface is most appropriate here. once the surfaces meet the profile tolerance. they become datum a and are assumed perfect.
now that we have established datum a, is it practical to have parallelism (assumed) of another surface 0.02 to datum a? it depends on how large of an area that the other surface covers. it would not be practical to have the relatively tight tolerance if the other surface is the same size as datum a. if the other surface is rather small in comparison to datum a, then maybe 0.02 would be appropriate.
i might suggest that datum targets are placed on datum a to at least give some sort of consistency of measurement.
dave d.
ok, i was shooting from the hip...
i re-thought my post just after i submitted it, but was called away to fight other fires, and wasn't able to re-address it until now.
i agree that the actual datum a is a perfect plane established by the three highest points on the surface, and any feature referring to a is actually referring to that plane.
believe it if you need it or leave it if you dare.fff"> - robert hunter