【转帖】datum casting vs machining

yang686526

datum casting vs machining
i am working on the design of a machined casting. tertiary datum used to position some machined features is a cast hole. i think it is appropriate to use tertiary at mmc but i do not want to add the cast hole size and tolerance to the machining drawing unless i can't avoid this.
currently we typically do restate the dimension and tolerance of a cast feature when it is used as a datum but i am not in love with this practice (two controls for same feature on different drawings).
i would tend to think that if the datum is a feature of size on the machining drawing and is modified at mmc and no tolerance or dimension is given one should refer to the casting drawing but i am unsure (and i don't know if this will work in my little world).
i might be stuck sticking with our current method but i though i would ask if anybody has any advise. thanks!
asme y14.5m-1994 states in 1.4(n) the dimension on the casting applies at the casting.
if you want the feature of size to be applicable on the machining, especially since you are referencing it as a datum reference at mmc - it would complete the definition for that drawing.
show it as a reference dimension.  you could even reference the casting drawing in a note.
joebk,
   my problem with machined castings is that i want the same datums to be used by the foundry and by the maching shop.  when the machined features must be more accurate than casting tolerances, the datums must be selected carefully.
   a fixture that locates to the mmc of a casting id is possibly not accurate enough for machining.
   i suggest you select datum targets.  you can use one side of the cast hole.  if you are designing the casting, i suggest you design datum target features into it.  this is described in casting design manuals.  
   this eliminates variation due to mmc.
                        jhg
i concur please see an example of casting and machined casting i design drafted a few yearsback
ray doyle
solidworks 99-07
concur please see an example of casting and machined casting i design drafted a few yearsback
ray doyle
solidworks 99-07