Answer: Premium quality rotors are machined on very accurate CNC lathes. These lathes cost upwards of $1 million and feature 18" vertical arbors and use ceramic cutters that work at high speeds and machine many rotors in one operation. Basically, if the surface smoothness is below 70 to 80 Ra, then it is smooth enough to use. If you choose to machine new rotors regardless, any removal of metal increases the temperature the rotor will operate at, reduce cooling, and can increase occurances of rotor runout.
