be checked and resoldered if necessary and the commutator examined carefully to make sure that it is tight on the shaft. It is generally preferred to remove the armature from the machine and place it in a lathe, as this offers the best accessibility and more rigid support for turning, although the commutator may be turned with the armature left intact within the machine. The choice between the two methods depends largely on the size of the machine. Before starting to turn or grind a commtator, the windings ofthe armature should be carefully protected from metal chips by wrapping them with canvas or some other closely woven material. This serves to shield the windings from any chips or metal dust which are certain to result in short circuits, When turning or grinding the commutator, extreme caution must be taken to give the cutting tool a rigid support. It is best tc take a light cut, take several if necessary, to remove all bad flat spots on the commutator. A fine grain sandpaper or commutator stone may be used for final surfacing, with care taken to eliminate all scratches, especially those running diagonal on the commutator surface, until the commutator has a highly polished surface. For satisfactory operation of a commutator, it has become standard procedure to undercut the mica segments, as mica wears at a slower rate than the commutator bars, and soon results in an uneven surface, flat bars, and even flashovers. A hack saw blade mounted in a wooden handle serves as a satisfactory undercutting tool where the amount of work to be done does not warrant the purchase of special equipment. Care should be taken not to cut more than 1/32 inch deep, as a deeper slot will accumulate foreign particles, and metal and carbon dust. 3-57