These spots cannot be removed by cutting bits. After turn-
ing is complete, these spots will remain as raised places on
the finished surface.
To remove hot spots, a special motor driven grinder,
Figure 13-33, must be used. This grinder is installed in
place of the cutting bit and rotates a grinding stone, or
wheel, against the braking surface as the rotor turns. To use
this grinder, set clearances in the same manner as when
setting the cutting bits. Then start the grinder and set the
feed to low speed. As the grinding wheel moves over the
hard spot, it will grind it down to match the other areas of
If the grinder cannot remove all the hot spots, the
rotor should be replaced. Ideally, you should replace any
rotor that has hot spots.
Swirl Grinding Rotors
The swirl grinding process is used to make a final
non-directional finish on the rotor. A non-directional finish
eliminates the microscopic tool marks made by the cutting
bits, replacing them with a series of extremely fine random
scratches. This finish helps to eliminate noises and aids in
There are essentially two ways to perform swirl grind-
ing: the lathe-mounted grinder, Figure 13-34, and the
hand-held grinder, Figure 13-35. With either design, the
basic operation is to hold a spinning sanding disc against
the rotor surfaces as the lathe turns. The combination of
lathe and grinder rotation creates a swirl pattern that pre-
vents the development of vibrations that take the form of
squeals or other noise.
To perform swirl grinding, follow the manufacturer’s
instructions to attach the grinder, if necessary. Then rotate
the grinder against the rotor as it turns. It is not necessary
to operate the swirl grinder for a long period; 30-60 sec-
onds on each side is sufficient.
Once the swirl grinding operation has been performed,
remove the rotor from the lathe and clean it thoroughly to
remove all chips. This is especially important if the rotor is
an integral type with bearings installed in the hub. After
cleaning, the rotor can be reinstalled on the vehicle.
Rotor installation is the reverse of removal. Most
rotors simply fit back on over the hub. Once installed, rein-
stall the rotor screw or bolt, caliper, and wheel. To reinstall
an integral rotor, clean the spindle assembly of all old
grease and dirt. Install the inner bearing and seal, place the
rotor over the spindle, and install the outer bearings. Install
226 Auto Brakes
Figure 13-33. Removing hot spots from the braking disc (rotor)
r with a motor driven grinder. The grinder is also handy for
removing rust and lining deposits. Wear your safety glasses.
B disc B raking ki di
Hot o spots
on o the
Figure 13-34. A—Lathe mounted grinder being used to apply
the proper non-directional crosshatch (swirl) pattern to the
brake rotor. B—Finished rotor. Note the grinding pattern.
Crosshatch (non-directional) finish