i whittled out a few different mandrels from chunks of 5" aluminum bar stock for late style, hubless rotors. for these, i duplicated the hub register outside diameter to locate the rotor, then use a piece of 5/8" [it might be 3/4"] round plate that just covers the bolt pattern of the rotor, retained with a 3/4" bolt to clamp the rotor to the mandrel. this setup somewhat duplicates the clamping forces the rotor sees on the vehicle.
for the rotors with the cast hubs [like our classics], i use a couple of the cones from an old snap-on wheel balancer i picked up at carlisle many moons ago. this balancer uses a shaft resting on four sealed bearings to find the heavy spot on the tire/wheel combo. [it works pretty good !] anyway, i pick the cones that fit the bearing races, and use a 3/4" bolt to retain the assembly to a small face plate on the lathe.
for cutting, i use 1" diameter boring bars [one right hand, one left hand] with replaceable carbide cutters [diamond shaped] on either the tool post or the tail stock post, depending on whether i'm cutting the front or rear of the rotor.
this setup is very stout, and there is no deflection during the cutting process.
for drums, i have similar mandrels to fit different drum register holes, and i use the same cutting tools as used with the rotors.
the secret to my success is a very rigid clamping setup and short [just long enough to get the job done] large tooling.
the lathe i use is not a crapsman "wobblechuck", so it's a tad more heavy duty than your best brake lathe.