Figure 105. Measuring commutator out of round.
the commutator is out of round or if it has rough spots on the surface.
(e) Undercut the mica after turning the commutator. The mica must be undercut to a depth of one thirty-
second to three sixty-fourths of an inch. Use an undercutter if available (fig. 106). If no undercutter is
available, use a three-cornered file and a piece of hacksaw blade. Start the groove in the mica with the
file and undercut it with the hacksaw blade. The mica must be cut away clean between the commutator
segments. Do not leave a thin edge of mica next to the segments. Samples of good and bad
undercutting are shown in figure 107.
(f) Check the armature for open and grounded circuits with a test lamp. Touch the test probes to each pair
of adjacent commutator bars (fig. 108). If the lamp does not light, an open circuit is indicated and the
armature must be replaced.
(g) Check the armature for grounds by touching one test probe to the shaft and other probe to each
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