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Place a piece of soft steel or brass, about 1 inch in diameter, in the lathe chuck with about one inch projecting from the jaws.

Turn down the outer end of this piece to fit tight in the hole drilled in the metal plate to mark the location of the shaft. The length of this turned-down portion should correspond with the thickness of the plate of metal used for the jig.

The shoulder remaining at the inner end of this piece

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FIG. 16.--Completed Drilling Jig for Bearings.

should be trued up so that it will fit against the surface of the jig squarely when in place.

Next turn down a portion of the rod, say 1 inch long, to a diameter of 3/16 inch, the size of the shaft hole in the bearing. Cut off this portion from the rod remaining in the chuck and insert it in the jig which will now have the appearance shown in Fig. 16.

If now the hub of the bearing be placed down over the projecting pin the two holes in the jig can be used to locate the centers of the holes for the side-bearing rods. To do this see that the casting sets squarely on the jig in such a manner that both bosses of the casting cover the circles marked on the jig, or are divided equally between them.

The casting can now be clamped to the jig by putting a. flat strip of metal across the feet of the casting. Place this flat piece against the tail stock of the lathe after the back center has been removed and, with the No. 42 drill held in the chuck, put a hole through one of the bosses of the casting.

Another drill or a piece of No. 42 steel or brass rod can be put through this hole just drilled to prevent the casting from slipping on the jig while the second hole is being drilled.

These holes are, of course, much too small to admit the ends of the rods and must now be enlarged to į inch. This can readily be done by placing the bearing on a flat piece of metal, which rests against the tail stock of the lathe, and redrilling with a 1-inch drill. This drill will follow the small or “pilot” hole already made.

Another method is to first enlarge the holes in the jig to 1 inch, and then drilling the castings that size at the first operation.

The edge of the jig can now be used as a gauge in filing up the bottom of the feet of the bearings.

Center-punch the outer end of the feet of the bearing and drill small holes for the holding-down screws.

A very small hole should be drilled in the top of the bearing for oiling the shaft, after which the 3/16-inch reamer should be inserted to remove the burr from the hole where it comes through into the shaft hole.

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 4-inch mandrel or arbor

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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

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