Finish Cut
To make a finish cut, set the cutters to a small cutting
depth, no more than .002” (.050 mm), and set the feed
speed to a low setting. Engage the feed lever and observe
the rotor as it is cut. Allow the cutting blades to cut the
entire braking surface and exit the outer edge of the rotor.
Then disengage the feed lever and inspect both rotor sur-
faces. After making the finish cut, check the rotor thickness
with a micrometer. If the rotor is now too thin, it must be
Turning Rotors on the Vehicle
On some vehicles, the rotor is pressed into the CV
axle shaft in such a way that removal is very time-con-
suming and difficult. In these cases, it is much easier to
turn the rotor on the vehicle. To turn a rotor on the vehi-
cle, a special on-vehicle lathe, Figure 13-31, must be used.
Both front and rear rotors can be turned using this lathe.
Setup and cutting instructions are similar to the
process for a bench-mounted lathe. The cutters are set to
just touch the rotor surface, then brought to the middle of
the rotor. The depth of cut is set and the feed turned on. As
with any type of machining operation, watch the rotor
carefully as it is being cut.
Some on-vehicle lathes require the rotor be turned by
engine power as the cutters move across the rotor braking
surface. A sequence of installing and using this type of
lathe is shown in Figure 13-32. Other on-vehicle lathes are
equipped with a drive motor which turns the rotor and CV
axle assembly with the transmission in neutral. Some
motor driven on-vehicle lathes have a provision for chang-
ing drive speed to make rough and final cuts.
When using an on-vehicle lathe, it is very important
to set all cutters and drive mechanisms very carefully. This
is because the design of the on-vehicle lathe is less rigid
than the bench lathe, and slight misalignment can cause
the rotor to be cut improperly. If the lathe and rotor appear
to be oscillating (wobbling) excessively when the cut is
started, turn off the lathe immediately and recheck all
Removing Hot Spots
Hot spots, sometimes called hard spots, are rotor sec-
tions that have been overheated by severe brake operation
and become much harder than the surrounding metal.
224 Auto Brakes
Note: If the rotor is warped, observe the
mark that you made when checking maxi-
mum runout. At the mark, one side should
be cutting deeply, while the other side is cutting
lightly or not at all. This indicates that the rotor is
properly installed on the lathe arbor.
Figure 13-31. An ASE certified brake technician using an on-car brake rotor lathe to turn a rotor.This particular vehicle is front-wheel
drive. When using these lathes, carefully follow all the tool manufacturer’s installation and machining procedures.
(Hunter Engineering Company))C (H t E i i
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