Rotors that have minor imperfections or are slightly unparallel can be turned true and smooth with a brake lathe. A brake lathe cuts metal away to achieve the desired surface finish.
There are basically two types of brake disc lathes used by the industry. The first is one that has the capability of resurfacing brake drums and brake discs after they have been removed from the vehicle.
A typical brake lathe. Courtesy of Accu Industries.
The lathe rotates the disc as cutting tools work their way across the braking surface of the disc. The second type is an on-vehicle brake lathe.
An on-the-vehicle brake lathe. Courtesy of Hunter Engineering Company.
This type of brake lathe is a time saver because the rotor does not need to be removed from the vehicle. Special fixtures are used to straddle the rotor so the cutting tools can precisely cut both sides of the rotor. An electric motor is used to rotate the disc and hub assembly during cutting.