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318 RICHARDSON'S BLUEBOOK OF PROJECTION
( 141 ) At least once each week the commutator should be brushed vigorously lengthwise, using a brush with bristles as stiff as may be without danger of injuring the surface. This is to remove from the undercut grooves between commutator bars any dust or foreign substance that may have lodged there. Much of the dust is carbon scraped from the brushes and copper worn from the copper bars. This worn copper quickly becomes a conductor.
Sandpapering
(142) In the event it becomes necessary, for any reason, to clean the commutator surface right down to the copper, use No. 00 sandpaper. Do not use anything coarser than No. 00 and under no circumstances use emery paper or emery cloth. Remove all brushes and, having started the motor (we are still considering motorgenerators), hold the sandpaper against the commutator, without exerting too much pressure, until the copper shows clean and bright all over the surface.
(143) Stop the motor. and give the commutator a vigorous brushing endwise. Next whittle a hardwood stick until its point will just fit into the undercut between commutator bars. Draw this through each one of the undercut slots to dislodge any copper bits that may have been drawn either across from one bar to another or down into the groove. Then brush endwise again vigorously.
(144) There are "commutator stones" on the market, which are rectangular bricks of fineness or coarseness to suit any "dressing" job.
The stone is held against the rotating commutator, and its surface wears down readily to fit the commutator curvature, in the meantime cutting freely. Unless the brushes are raised, they too will partake in this rapid ^vear-away-spree. In fact one way of fitting the brush ends to the commutator curvature is to let them rest in their natural position on the commutator and operate with a rather fine grade of commutator stone. The fit thus secured is perfect.