The Diseases Of Electrical Machinery 1904

12 THE DISE ASES OF taken from the commutator surface with a sharp pointed tool, after which the surface should be polished with fine glass-paper.* If the process has proceeded still further, it may so happen that the machine will not work, i.e. gives no voltage, since the contact between the brushes and the commutator is broken by the protruding mica insulation. Turning down. of Commutators.—It may be here advisable to say a few words regarding the turn- ing down of commutators. When turning down the commutator, the author recommends that the peri- pheral speed should not be greater than 20 to 25 feet per minute. A sharp pointed tool should be used for the first cut, so that the mica and copper are turned down together, and that no mica segments should remain sticking out. For the finishing cut a similar tool should be used, and it should not traverse at a greater rate than inch per revolution. The cutting speed may, however, be raised to 35 feet per minute. The commutator must then be carefully ex- amined on the lathe, to see whether any of the segments are joined by tags or burrs of copper. These must be carefully removed. The surface must then be smoothed with glass-paper, and as good a polish given as pos- sible, for which purpose the use of a few drops of oil * [A less drastic remedy than taking a cut off the commutator, is to file the surface while it is gently turned round, using a glass-hard file, itself of fine cut. Or instead of a file, a fine carborundum hone may be applied.—Ed.]