If say, you draw a bearing enclosure in solidworks and then have it printed on a 3D printer... the bearing OD is 30mm... what size hole do you draw on
the solidworks model? a 30mm hole WILL work in theory, but in practice, would you find that the bearing simply wont fit?
What sort of "gap" should I leave to ensure that the bearings etc will actually fit in the printed part?
What you require is an Interference fit
Google came up with a few results for calculating it, such as here
[Edited on 12/2/11 by NigeEss]
You need to know the bearing diameter tolerance. It may be 30mm nominal but what is the tolerance?
After that you should look up a 'limits & fits' chart - there are loads on the internet like this one
HERE
This is a table of standard tolerances that you can use for this kind of stuff. It covers interference/clearance fits for holes and shafts.
Enjoy 
ive just had a turbine housing rapid prototyped, a 36mm hole is a good interfearence fit on a 36.00mm diamater shaft turned on a high speed lathe. that was an ABS prototype machine with a resolution of 0.1mm. not sure if thats any help...
The tolerance on the printed part is going to be far greater than the bearing tolerance, so you need to find this out as well. Your not going to be
able to press a bearing into a printed part because they are not 'mechanical components', well not for standard limits and fits anyway.
They are photocurable resins (for most technoligies) so not particually strong and will creep with load and degrade with UV exposure and time.
Don't mean to sound negative, just trying to save you some potential problems.
You can, to all intents and purposes, ignore the tolerance on the bearing. They are produced to within a few microns.
As previously posted, your problem is the tolerance on the printed part.
If this is +/- 0.1mm and your bearing is 36mm you will need to specify a drawing size of 36.1mm in order to guarantee that the bearing will go in
without interference.
It could, of course, actually end up at 36.2mm but you can use Loctite "Bearing Fit" or similar to fill the small gap.
If you specify 36mm then the part might end up at 35.9mm which will almost certainly be too much in terms of interference.
The part may well split.
It all comes down to the tolerance of the printed part.
Geoff
[Edited on 13/2/11 by SPYDER]
u2u sent 
Is the printed part going to be used as a casting pattern? If it is then I expect it will need to be machined for the bearing.
quote:
Originally posted by interestedparty
Is the printed part going to be used as a casting pattern? If it is then I expect it will need to be machined for the bearing.