Does anyone know what alloy of tool steel is typically used for CNC machine spindles?

Thanks,
Joe

I doubt they use tool steel at all.

Machinery builders like C8620, which is a carburizing grade of steel. I wouldn't swear on it, but I would guess they might use it. It is particularly popular for gears. However, this grade requires carburizing and hardening after machining, so they expect to have to grind it straight and true after heat treating. This material is quite stable in heat treatment, if properly supported in the furnace. But, for the home shop, it might be easier to stick with the pre heat-treated steels, like C4140 or C4340.

See H3ndrix's thread on this same subject.