CHAPTER VII 



COMMUTATOR CONSTRUCTION 



IN the present chapter it is .proposed to trace the building of the 

 commutator, beginning with the raw material and ending with the 

 finished commutator. 



While tracing the progress of building, general remarks on the 

 materials used and on their properties are inserted ; in the latter 

 parts of the chapter some examples are given illustrating typical 

 modern constructions for commutators to fill various requirements, 

 together with notes on their construction and design. 



Only the softest and most flexible mica should be used between 

 segments ; green shades of mica, and amber-coloured mica, are 

 generally most satisfactory. Amber-coloured flexible Canadian 

 mica has been found specially suitable ; other grades of Canadian' 

 mica are unsatisfactory. In some quarters, green shades of 

 mica are preferred, and it is claimed that they are softer and 

 more flexible than the amber mica. The colour alone cannot be 

 depended upon as a guide to the softness. The white mica from 

 North Carolina, and the mica from India, are often too hard for 

 the purposes of segment insulation. Softness is far more important 

 than high insulation quality; thus while white mica has the 

 highest insulation resistance, it is altogether unsuitable for insula- 

 tion between segments. In no case should a mica be adopted for 

 this purpose without careful tests of its mechanical quality. The 

 mica end rings, however, may be made of almost any good quality 

 of mica, as for this purpose softness is, of course, of no importance. 

 India mica is often employed for the end rings. The shellac or 

 other cementing medium employed in building up the mica seg- 

 ments from the flakes of mica, must be thinned and sparingly 

 applied, and the plate built up in this fashion must be pressed and 

 heated (preferably at one operation) to expel from the plate all but 

 an extremely thin film, barely enough to hold the components 



107 



