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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,

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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

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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 į inch from the edge. Turn the commutator around and mark in the same manner from the other end.

Fig. 22 shows the appearance of the commutator after

FIG. 20.—Dividing Commutator


FIG. 21.-Scribing.

this operation. With a center punch carefully indent these sixteen points. Place a No. 48 twist drill in the chuck of the lathe and

drill a hole at each center punch mark. Fig. 23 shows this operation.

In drilling these holes a V-center is the best thing to employ to hold the com

mutator true. Lacking this, FIG. 22.-Commutator with however, a V-block can be Screw Holes Marked.

used and held firmly against the tail stock while the drilling is being done.

In using the latter appliance it is preferable to hold the commutator with the hole for the shaft in a vertical position, for then the operator can see whether the drill is pointed directly toward the center of the hole in the commutator.

If the commutator is held with this hole in a horizontal position this is not so readily noticed. The greatest danger of failure in the construction of this piece of work


FIG. 23.-Drilling Screw Holes in Commutator.

is in the liability of the lower ends of two screws touching each other and thus causing a short circuit.

An examination of the working drawing for this piece, Fig. 17, will show how easily this could come about by a little carelessness in the drilling.

After the holes are drilled they should be tapped with a No. 2-56 tap. This is quickest done by placing the tap in the lathe chuck and, holding the commutator in the right hand, rotate the lathe by making a downward movement of the belt with the left hand.

If this is carefully done and the movement of the belt properly gauged the result will be much more rapid than by tapping the piece by hand.

When all the holes are tapped insert the sixteen brass screws. In doing this place the round-head screws at one end of the commutator and the flat-head screws at the opposite end as shown in the detail drawing. The holes where the flat-head screws are to be inserted should be first counter-sunk, but not deep enough that the heads of the screws are below the surface of the ring.

When this point is reached the segments are to be marked for cutting apart.

Place the commutator again in the position it occupied in Fig. 22, and with the dividers find the center between any two of the first indicated marks. Now ? reset the dividers to the distance apart of the original: eight spaces and, beginning at the new mark,' again divide the ring into eight other equal spaces. Mark these off across the commutator face, using the square as before.

These last marks, if correctly spaced, will be exactly halfway between the lines on which the screws are located.

As will be readily seen this last operation of dividing could have been done at the same time that the first divisions were made. It is not, however, advisable to do so as the additional lines are liable to lead the constructor into the mistake of center-punching and drilling at the wrong points. These last divisions will indicate where the points of separation of the segments are to be made.

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