Machining Rotors (760) 741-7861

Rotor

It is not always necessary to refinish brake rotors when doing routine brake maintenance such as replacing brake shoe and lining assemblies.

Refinish rotors only under the following circumstances:

  1. There is brake pedal pulsation.
  2. There are heat spots, cracks, or excessive scoring.

Brake rotors have a minimum thickness cast into them. This dimension is called a discard thickness and not a refinish dimension. Do not use a brake rotor that is below the discard thickness after refinishing. Replace it with a new one because a rotor below minimum thickness it will not have the proper heart transfer capabilities. Control of rotor tolerances is necessary for the proper performance of disc brakes.

Always use precision brake lathe for machining rotors. Locate the deepest score and turn the rotor micrometer knobs until the tool bit bottoms out at the deepest point of the score. Zero the scale and back out the tool bits. Advance the cutter hand-wheel until the bits have cleared the inner edge of the rotor face. Adjust the micrometer knobs for approximately .005 " (.0127 mm) more than the first reading. This will ensure clearing the rotor in one cut. It is important that you make the rotor surface non-directional by dressing the rotor surfaces with a sanding block with 150 grit aluminum oxide sandpaper. With the rotor turning approximately 150 RPM, sand each rotor surface using moderate pressure for a minimum of 60 seconds. After sanding the rotor clean with brake rotor with soap and water.

The finished rotor surface should be as close to that of a new rotor as possible.

Rotor Inspection And Repair (Disc Brakes)

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Rotor Tolerance & Surface Finish

Tolerances of the braking surfaces for flatness, parallelism and lateral run out are closely held. The maintenance of close tolerances on the shape of the breaking surfaces is necessary to prevent brake roughness or pulsation. The surface finish must be held to a specific range of 60 Ra roughnesses or less. The control of the surface can improve lining life. Light scoring of the rotor surfaces not exceeding 1.5 mm (0.06 in.) in depth is normal and not detrimental to brake operation.

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Thickness Variation Check

Check thickness variation by measuring the rotor thickness at four or more points around the circumference of the rotor. Use a micrometer calibrated in ten thousands of an inch. Make all measurements at the same distance in from the edge of the rotor. A rotor measuring thicker than .0005" (.013 mm) can cause pedal pulsation and/or front end vibration during brake applications. A rotor like this should be refinished or replaced.

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Lateral Run out Check

The best way to check lateral run out is with the wheels still installed on the vehicle. This gives a much more accurate reading of the total indicated run out under real braking conditions. If equipment is not available to perform the check with the wheels installed, the next best reading can be made with the wheels removed but the caliper still installed.

1. Clean rotor surface. If the wheel must be removed, reinstall the wheel nuts to retain the rotor. Tighten the wheel nuts to the correct torque specification following the wheel nut tightening sequence.
2. Fasten a dial indicator to the steering knuckle so the indicator button contacts the rotor surface about .5 " (13 mm) from the outer edge.
3. Set the dial indicator to zero.
4. Turn the wheel one complete revolution and observe the run out indicated on the dial.
5. The total indicated run out must not exceed .0003" (.08 mm).
6. If lateral run out is not within specifications, refinish or replace the rotor as necessary.

In some cases, excessive lateral run out can be improved by indexing the rotor on the hub one or two bolt positions from the original position. If lateral run out cannot be corrected by indexing the rotor, check the hub and bearing assembly for excessive lateral run out or looseness. If the hub and bearing assembly lateral run out exceeds .0015" (.040 mm), repair or replace the hub and bearing assembly.