The shaft is a piece of steel rod, three-sixteenths of an inch in diameter, and four and one-half inches in length.
The portion of the field into which the armature fits is bored out to a diameter of one and five-sixteenth inches. Considerable care is necessary in performing this operation in order not to break the field magnet apart by taking too heavy a cut.
Fig. 257.—Details of the Commutator.
The armature should be turned down to a diameter of one and one-quarter inches or one-sixteenth of an inch smaller than the tunnel in which it revolves between the field magnets. The center of the armature is bored out to fit the shaft.
Figure 257 shows a two-part commutator for fitting to an armature of the "Siemen’s H" type. It consists of a short piece of brass tubing fitted on a fiber core and split length-wise on two opposite sides, so that each part is insulated from the other.
The fiber is drilled with a hole to fit tightly on the shaft. It is then placed in a lathe and turned down until a suitable piece of brass tube can be driven on easily.
Two lines are then marked along the tube diametrically opposite. A short distance away from each of these lines, and on each side of them, bore two small holes to receive very small wood screws. The screws should be counter-sunk. It is very important that none of the screws should go into the fiber core far enough to touch the shaft.
The commutator may then be split along each of the lines between the screws with a hacksaw. The saw-cut should be continued right through the brass and slightly into the insulating core. The space between the sections of the commutator should be fitted with well-fitting slips of fiber, glued in.
The commutator should now be trued up and made perfectly smooth.