Essentially, brake lathes are metal lathes designed to turn automobile brake drums and rotors, as well as a number of other items. While traditional metal lathes suspend the piece to be worked horizontally between the rotating spindle and the tailstock, brake lathes are somewhat different. Brake lathes have only the spindle and the item to be machined is suspended vertically.
As with traditional metal lathes, brake lathes use cutting bits mounted on toolposts to cut into a rotating piece of metal stock. Bench-type lathes do not, however, have a traditional lathe bed simply because they are machining vertically rather than horizontally. Typically, bench-type lathes consist of the motor, which powers a rotating spindle, and the movable toolpost and bit. Various tapered arbors, designed to fit into the several sizes of brake rotors, are used in place of the chuck common to many traditional metal lathes.
In the operation of brake lathes, the arbor is inserted into the center hole of the brake rotor and the arbor-mounted rotor is fitted onto the shaft of the spindle and secured with a locking ring. The motor is turned on and the spindle rotates the rotor assembly at a pre-determined speed. The toolpost and bit are maneuvered into place, either by a hand crank or mechanically, to begin scoring the rotor, thereby removing rust, dirt, and debris. As well, the bits will cut minute grooves into the rotor. These grooves are essential to providing the friction between rotor and brake pads necessary to good stopping power.
In addition to bench-type brake lathes, many repair facilities will have on-car lathes which do exactly what a bench lathe does. On-car brake lathes are much more compact than bench lathes and are designed, as the name implies, to be mounted directly to the vehicle, ordinarily on the wheel hub or brake calipers.
Though somewhat cumbersome and time-consuming to mount and set up, on-car brake lathes have two advantages over bench lathes. First, on-car lathes reduce rotor runout where the cut may not be deep or shallow enough by providing a more even application of cutting bit to rotor. Runout is simply an uneven rotor surface caused by imperfect machining or rotor wear and will result in a pulsation in the brake pedal when braking. Secondly, on-car lathes eliminate the expense and time needed to take off stuck or “captured” rotors, a common problem with abused or inferior brake systems.