i aint used one for a few years now as i have a wire eroder and clean up any damaged sprues using that.
But when we used to use them we had a rougher and a finisher for each size.
we would run the rougher in a machine nice and slow with plenty of tapping oil and with it gripped lightly in the chuch so if it did grab it would spin rather than break.
we then just used the finisher by hand held in a tap wrench again with tapping oil and as previously stated polish it afterwards.