A request for information on your mill or lathe

[Tonyg]

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.

SKF have an interesting article on current through bearings. I have come across it on some large plasma and oxy/fuel machines with a potential difference between the gantry and the rail where you can see it sparking and the bearing gets very hot (and damaged). Caused by a poor earthing connection, a star earth to between the two generally solves the problem.

https://evolution.skf.com/en/prevent...n-in-bearings/