COMMUTATOR CONSTRUCTION 



109 



together. As cementing medium, shellac and copal varnishes have 

 been used with good results. In the building up of mica plates, the 

 pressure should be applied gradually, being increased as the baking 

 process proceeds, otherwise the 

 component pieces of mica are 

 apt to slip and slide apart. The 

 plate thus formed is then sawn 

 or sheared slightly larger than 

 the commutator segment; it is 

 afterwards tested and pressed, 

 and is finally put through a 

 milling machine, which reduces 

 it, in the case of large machines, 

 to a uniform thickness of about 

 0*7 mm. The plates must be of 

 extremely uniform thickness a 

 variation greater than "05 mm. 

 not being permissible. The seg- 

 ments should be of hard drawn 

 copper, which are cut up into 

 pieces the length of the commu- 

 tator. To save time in machin- 

 ing the ends of the segments, 

 they are often, in the case of 

 large commutators, sawn out to 

 the shape indicated by the dotted 

 line in Fig. 119; or in smaller 

 commutators the portions 

 marked Ain Fig. 120 are punched 

 out in avertical slotting machine. 

 The use of forged segments has 

 sometimes been found unsatis- 

 factory ; but when drop-forged 

 segments are used they are 

 forged to the required shape, 



and often with the lugs in one FIG. 122. Commutator Building Kings, 

 piece with the segment. All 



the segments for the same commutator must come from the same 

 stock, and in any case must be of uniform quality as regards hard- 

 ness. The segments are first assembled on end, inside massive steel 

 rings (two such rings are often used) studded with stout bolts 

 bearing against clamping segments, as shown in Figs. 121 and 122. 



