If it is desired to file the bearings up smooth it should be done after the machine work is all finished.

The bearings are now ready to assemble on the sidebearing rods, and tested to see that the frame work of the machine stands firm.

CHAPTER V

COMMUTATOR

This commutator is one easily constructed by the amateur. It consists of a fiber core containing a hole for the shaft and surrounded by a ring of thick tubing which, after fitting, is cut into eight parts or segments, and these segments are afterwards insulated from each other by strips of mica clamped into the spaces left by the slitting saw.

Those who have had any experience in working hard fiber know that it is a substance built up like sheets of paper and then subjected to great pressure.

This material is procurable in sheets, rods, and tubing. The tubing cannot be used in this instance as it is not of sufficient thickness compared with its diameter.

Fiber rod will not suffice as this is made by cutting long square strips from the thick sheet fiber and then turning it down to size leaving the grain of the fiber running lengthwise of the rod. A commutator core cut from such a rod is very weak and splits apart when forced onto an armature shaft. The only way to make this core of sufficient strength to stand all the strains to which it is subjected is to take a piece of sheet fiber as thick as the required width of the commutator and saw it into square blocks sufficiently large to finish to the required diameter. This leaves the grain of the fiber at right angles to the shaft. It is such a cube as this which is furnished for the commutator core of this dynamo.

Fig. 17 shows the construction of the parts.

The first operation to be done to this piece of fiber is to locate and drill the hole for the shaft through the center of the block. The center can be readily located by drawing diagonal lines from opposite corners of the block. Center-punch the intersection of the lines and drill a 1-inch hole through the block at that point.

Place the fiber block on a 1-inch mandrel or arbor

and, with a good sharp tool, turn it down to the required diameter that it may be forced into the thick metal ring which is to form the segments of the commutator. Fig. 18 shows the block of fiber mounted in the lathe ready for turning and the metal ring on the slide rest ready to be placed in position.

It will be found that this material, hard fiber, is one

which will wear away the sharp edge of a cutting tool much faster than will tool steel. Every two or three cuts across the commutator core the cutting tool should be reground in order to get good results.

The fiber core should be turned to the proper size,

the ring forced into it, and the two pieces mounted on the mandrel and a light cut started across the outside to true up the ring.

Do not cut away more material than is absolutely necessary in this operation as the commutator must be turned down again when fully assembled. Fig. 19 shows

this operation clearly. The ring is now ready to be marked off into segments.

Set the commutator on end, and with a pair of sharppointed dividers go around the edge of the brass ring and divide it into eight equal spaces as shown in Fig. 20. Next use a small try-square and with a sharp-pointed

scriber mark a line across the face of the commutator at each of the eight spaces previously indicated. This is duly shown in Fig. 21.

Now set the points of the dividers at such a distance apart that when one point is held against the end of the ring the other point will make a mark inch from the edge.