42 Auto Brakes Copyright by Goodheart-Willcox Co., Inc. shop equipment and are not normally purchased by the technician. Brake Lathes Brake lathes are used to remove the glazed and scored outer layer from rotors and drums. Th is removal process is usu- ally called machining or turning. Th ere are two main divi- sions of disc and drum brake lathes, bench-mounted and vehicle-mounted. Bench-Mounted Brake Lathe Th e most common type of brake lathe is the bench-mounted lathe. Th e modern bench lathe is usually designed to turn both disc and drum brakes. Many modern bench lathes consist of two separate lathes so that both disc and drum brakes can be turned at the same time. Th e bench lathe can be permanently affi xed to the shop fl oor, or can be equipped with wheels to allow positioning closer to the vehicle being repaired. A bench-mounted brake lathe is shown in Figure 3-34. Th ese lathes are equipped with cutting bits, attached to a boring bar. Th e bits are carefully positioned to remove small amounts of metal as the drum or rotor is turned. Drums are turned using a single bit, while rotors are turned using two bits which cut both sides of the rotor in one opera- tion. A special cutter assembly is used to cut rotors. Vehicle-Mounted Brake Lathe On some front wheel drive vehicles, the rotor assembly is pressed to the CV axle shaft , or for other reasons removing the rotor to turn it on a bench lathe would be very time- consuming. For these types of rotors, the vehicle-mounted brake lathe is used. Th is lathe assembly bolts to the caliper mounting and the two bits are adjusted. Th e disc is turned by vehicle power if the rotor is installed on a drive axle, or by an electric motor drive if the rotor is installed on a non-driving axle. One type of vehicle-mounted disc lathe is shown in Figure 3-35. Brake Rotor Burnisher Th e brake rotor burnisher, although not generally used, may still be found in some shops. Burnishing conditions the braking surface fi nish to the manufacturer’s specifi ca- tions. Burnishing tools can be equipped with stones or abra- sive discs. Th e burnishing tool can be attached to the brake lathe, or it may be a handheld device. Some burnishing tools are powered by a hand drill. Today, most technicians prefer to burnish, or bed-in, the brake rotors and pads during the initial road test. Drum Grinder A precision drum grinder is sometimes used to obtain a fi nal fi nish on brake drums. Th e high-speed stone wheel is installed in place of the boring bar and bit. Th e wheel is turned by a high-speed electric motor. Th e grinding wheel passes over the braking surface as the drum rotates, pro- ducing a very even fi nish. Th e grinder is oft en used to Self-aligning spacer Micrometer tool depth control Cutter tool Cross feed Ammco Figure 3-34. This combination disc and drum brake lathe features digitally controlled feed and speed rates. Refinishing is accurate to a thousandth of an inch or tenth of a millimeter. aSuruwataRi/Shutterstock.com Figure 3-35. An on-vehicle disc brake lathe being used to turn a rotor.