[Syphonics]

how do i choose ball bearings?
example if i want to get bearings for a 8mm shaft. Should the inner diameter be 8mm or 8.5mm?
If 8mm how do i fit through the shaft?

[Mister H]

normally the inner shaft is fixed with the inner ring of the bearing. You can achieve this with a press-fit or by fixing the inner ring with a nut or something like that.

In the same way the outer ring is fixed in the hole the bearing is mounted in (if appropriate).

gr. Hendrik

[Zumba]

An 8mm shaft gets an 8mm bearing. You either thread the shaft or turn the shaft for press-fit as Hendrik said.

Much easier to thread the shaft. Press fit will require .0001" tolerances.

[PeterCharles]

Or Loctite...

[NC Cams]

Or, better yet, you can read the following threads on how to properly "fit bearings":

http://www.cnczone.com/forums/showthread.php?t=23213

http://www.cnczone.com/forums/showthread.php?t=23883

When the shafts are properly sized, the bearings will "fit" without being either too loose or too tight for the specific application. Moreover, you won't have to resort to "glue" and/or SWAG "oops too tite" fits to hold them in place either.