Originally Posted by
Toolmaker51
Bet this turns out interesting. Two variations that come to mind immediately are direct spindles vs driven spindles, and the bearing packs. A lightweight drill press gets away with just two. A tough mill often has three or four. Other machines are completely isolated.
Don't know the exact term, but there is an issue where stray current erodes contact areas of bearing components and races. IIRC, VFD rated motors have additional frame grounding to bypass the bearings.