Brake rotors (discs) are squeezed by the brake pads, causing friction that reduces the speed of the rotors, which slows or stops the vehicle. Brake rotors also dissipate heat produced during braking.
There are certain circumstances where the rotors must be refinished using an on-vehicle brake lathe:
Rotor thickness variation Brake judder is felt as steering wheel vibration or wiggle when braking and is caused by variations in brake rotor thickness.
Grooves Heavy scoring (grooving) on the rotors because of metal-to-metal contact from worn out pads is cause for refinishing, provided the rotors are not beyond the service limit.
If new brake pads are installed on rotors with deep grooves, it will result in a spongy brake pedal feel and the pedal effort will be higher than normal until the new pads wear to match the deeply grooved rotors.
Tapered or dished rotors This can occur because rotors seldom wear evenly. (The inner surface will wear thinner or thicker than the outer surface.) If new pads are installed on tapered rotors, the brakes will feel spongy until the new pads wear to match the shape of the rotor.