If the detail drawing, Fig. 25, be examined it will show the method used to tighten the pulley on the shaft.

This is a No. 4-36 machine screw placed in the bottom of the groove for the round belt and the head of the screw let down below the bottom of the groove.

The location for the hole is first center-punched on the bottom of the groove and the hole corresponding to the head of the screw should be first drilled. When this is sufficiently deep the drill is replaced by a No. 42 twist drill which should continue the hole through to the hole for the shaft.

It might seem, at first thought, preferable to first drill the No. 42 hole and then enlarge the upper end of the hole with the larger drill.

This mode of procedure might spoil the work, as a twist drill in brass has a great tendency to "draw in" unless there is metal ahead of the point of the drill to hold it back.

The only remaining operation to this piece is to tap out the screw hole with a No. 4-36 tap.

CHAPTER VII

BRUSH HOLDERS

The rocker arm is the only casting among the brush holder parts.

This piece requires to be bored out to fit the 3-inch hub of the bearing. To do this the casting is held in the chuck by gripping one side of the hub. Put a tool in the tool post of the slide rest and test the outer sides of the end bosses, the same as the bosses of the bearing were tested, to see that the casting is straight in the chuck.

When the casting is found to be set true, face off the center hub and, with a hand tool or a centering tool held in the slide rest, mark the center where the drill is to start.

Set a 5/16-inch drill against the back center of the lathe as shown in Fig. 29, and put a hole through the hub. In drilling this hole the drill must be held back very hard against the back center to prevent its drawing through too fast and catching in the soft brass as soon as the point of the drill breaks through and there is no metal ahead of the point of the drill.

After the drill has passed entirely through the casting remove it and, setting a boring tool in the slide rest, true up the hole and bore to almost 3 inch, after which a 3inch reamer should be passed through to bring the hole to size. This hole should just fit over the hub of the bear

ing, but should not be so tight thereon that it cannot be readily rotated forward and backward as desired. Fig. 30 shows the operation.

Before the work is removed from the chuck a cut may be taken across the bosses on the ends of the casting, or the casting can first be removed from the chuck and placed on a 3-inch mandrel, between the lathe centers, and the sides of the hubs alternately faced up.

The rocker arm can then be removed from the mandrel

REAM 3

TAP 4-36.

FIG. 28.-Detail Drawings of Rocker Arm.

and a-inch hole drilled in the centers of each end hub, as shown in the detail drawings.

On the upper edge of the center hub will be seen a small boss. In the center of this a hole should be drilled down through to the 3-inch hole, using a No. 42 twist drill. After the hole is drilled it should be tapped out with a 4-36 tap. Into this hole a No. 4-36 screw is inserted which is used as a set screw to tighten the rocker arm in place when the proper position has been found by experiment.

The brush holders are made of pieces of 1-inch round

brass rod and the shape and size of the various parts are shown in the detail drawings, Fig. 31.

To machine these pieces one end is held in the chuck and the projecting end turned down to the proper diameter to be threaded with a No. 4-36 die for a distance of inch. The shoulder should be squared up true so that when

the parts are assembled the brush holders will stand at right angles to the rocker arm.

After the 4-36 die has been run on over the turneddown portion of the holder, the piece can be removed from the chuck and split down from the opposite end as shown. Before slitting a hole should be drilled through the