Resurfacing is needed when the drum is scored, out-of-round, or worn unevenly. Some shops resurface a drum anytime the brake linings are replaced, others only when needed. Drums are resurfaced using a lathe in the machine shop of an NMCB and at some shore installations. Commercial brake drum lathes can be found in some shops. Make sure you know how to operate the lathe before attempting to resurface a drum. Using the wrong procedures will damage the drum and possible deadline the vehicle.

Before resurfacing the drum, check the specifications that are cast into the drum (fig. 7-34) or are provided in the maintenance manual. These specifications tell you the maximum amount of surface material that can be removed from the drum and still provide adequate braking. Typically, a brake drum should not be more than .060 inch oversize. For esample. a drum that is 9 inches in diameter, when new, must not be over 9.060 after resurfacing. To measure brake drum diameter, use a special brake drum micrometer (fig. 7-35). It will measure drum

Figure 7-34.- Example of specification cast into a brake drum.

Figure 7-35.- Using a drum micrometer to measure a brake drum.

diameter quickly and accurately. Replace the drum if it is worn beyond specifications.

For maximum braking efficiency after the drums have been resurfaced, the arc of the shoes must match the drums. This means that the linings must be ground to match the curvature of the drum when it is resurfaced. There should be a small clearance between the ends of the lining and the drum. The shoes should rock slightly when moved in the drum. If the center of the linings is not touching the drum, the linings should be arced (ground). Shops equipped with a commercial brake lathe have a special attachment to perform this task. If no attachment is available, the shoes can be installed but the brakes will not become fully effective until the linings wear enough to match the braking surface of the drum. Frequent adjustments will be needed until they wear sufficiently.