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508 HANDBOOK OF PROJECTION FOR
If the segment is high by reason of the fact that, being of harder material than its mates, it has worn down more slowly, then, using a fine file, it may by very careful work be dressed down. If, on the other hand, it is low, then the only remedy is to turn down the rest of the bars to match.
If the fault is slight, this may be done by removing the brushes and holding a piece of grindstone wjiich has been turned out to fit the circumference of the commutator to it while it is revolved rapidly. This process is, however, slow. The best way is to put the armature in a lathe and turn it off. In the case of a motor the grinding may, however, be done with the brushes down and the machine running by its own power, but if this be attempted it must be done with great caution.
When you are through, the face of the brushes should oe thoroughly cleaned by drawing No. Yz sandpaper drawn around the curve of the commutator with the sand side next to the brushes as per Fig. 150, in order to grind off their face and thus remove any particles of sand which may have become imbedded in the brush, since such particles would scratch the commutator and cause undue wear. It is better to do the grinding with the brushes raised and the machine run from some outside source of power, if it is practicable.
(g) A rough or eccentric commutator. This may be caused by improper care, or by the use of defective materials in its construction. A rough commutator may be detected merely by feeling. The mica insulation between the segments will either stand out in ridges or be worn down so that there is a small groove between the segments. An eccentric commutator may most readily be detected by holding some instrument firmly against the frame opposite the commutator so that its end just touches the bars.
If the commutator is true it will touch all the way round as the armature is slowly revolved, but if the commutator is eccentric it will, of course, only touch the high spots. If the eccentricity be bad it will cause the brushes to move in and out of their holders perceptibly when the armature is revolved slowly. The only remedy is to turn the commutator down, and this can only be successfully done in a machine shop where work of this character is understood.
In preparing to turn down a commutator the machinist should note whether the journal or bearing points run true when the armature is revolved on its centers in the lathe. Often the centers themselves are not true with the journals, due to a defective center or a sprung shaft. In either case if