Ira M. Cushing

This dynamo is capable of delivering 10 amperes at 15 volts pressure when driven at a speed of 2200 R. P. M. The field frame and base should be cast in one piece. The outline of the base is shown in the lower right hand part of the illustration, which gives a plain view of half the machine. The base should be cored out underneath as much as possible in order to reduce the weight.

The field poles are square with well rounded corners. They are l 1/4 in. vertical dimension and 2 1/4 in. wide in the direction of the shaft. The bearing pedestals are of course, cast separate from the bed plate and can be made of any suitable metal. Cast iron of same grade as field frame is probably the cheapest. They should be - carefully set up in the lathe with the point where the center of the shaft will be as a center, and feet turned off to a radius of 3 5.32 in. This I would suggest as the first operation towards building the dynamos. Do not bore the bearing at this time. Note. that the two bearing pedestals are different. The one at the commutator end has the bearing lengthened on the inside to support the brush yoke.

The field frame should now be centered on the lathe, clamped to the carriage and the fields bored for the armature. The bore is 2 9-16 in. or 2.5625 in., and extreme care should be exercised not to exceed it, as upon this turns as upon no other single operation, the successful working of the completed machine. I would even suggest that skilled workmen may slightly reduce the diameter of the field bore.

After boring the fields, and before taking the machine from the lathe, the place for the bearing pedestal at the side of the dynamo frame nearest the lathe head may be bored. The depth of the cut will depend upon the success attained in turning off the pedestals. If these were accurately turned the cut in the base should have a chord of 2 in. The field frame should now be turned end for end and, carefully centering it again with the aid of a mandrel, the location for the other pedestal can be turned out.

The pedestals should now be clamped in position and the holes for the machine screws that fasten them in place can be drilled and tapped. The field frame with pedestals in place should be centered on the lathe again, and the pedestals bored out to receive the brass bushings for the bearings. The boring should be 5/8 in. diameter. For bushings use a brass tube, with wall thick enough to give a chance to drill out for a running fit for the shaft, which is .375 in. diam-ter. The bushings should be driven into place and bored out before again removing the field frame form the lathe.

A simple way to prevent trouble from the bushings coming out again or turning would be to cut with a cold chisel a slight niche in the edge of the hole in the pedestals, both on the armature side and outside, and then with a prick punch upset the brass close to the niche. This will force the brass into the niche and hold the lining in place.

The pedestal at the commutator end can now be set in the lathe and the place for the brush yoke turned off. This should be 0.875 or 7/8 in. diameter and 1/2 in. long. The pedestals should then be drilled for the oiling device. A plain hole can be drilled or can be drilled and tapped for oil or grease cup. If an oil cup is used a wick feed would be advisable. The grease cup would, however, probably give the best lubrication.

The field frame with its bearings can be set aside and the armature core and shaft started. The shaft consists of a piece of steel at least 11 1/4 in. long. This allows for a pulley at one end. Two inches should be added to this if the builder wishes a pulley at each end. It is not advisable, however, to make the shaft any longer than necessary, as it is a difficult part to turn. The steel, to start with, should be at least 9-16 in. in diameter. Beginning at the commutator end the shaft is 0.375 or 3/8 in. in diameter for a length of 4 1/4 in. The diameter is then increased to .5626 or 9-16 in. for 3 1/2 in. and then reduced to 0.375 in. again for the length.

A No. 14 thread should be cut for about 5-16 in. on the commutator end of the enlarged part of the shaft and to fit this should be made a brass or steel nut 1/4 in. thick. A steel washer 1 1/2 in. in diameter and 1/4 in. thick should be fastened rigidly to the enlarged portion with its outer face 1/4 in. from the shoulder of the shaft at pulley end. The easiest way would be to fit a brass pin through the washer into the shaft.

Between the nut and washer should be fitted enough discs to fill tightly the space of 2 1/2 inches. These discs are 21/2 in. in diameter, with a 9-16 in. shaft hole, and have twelve | in. holes, with their centers on a circle 21/8 in. in diameter, punched around the edge. Botb in making and assembling care should be taken to get the holes lined true. This can be done by putting a | in. rod through them and keeping the rod parallel to the shaft all the time. Clamps can be used to make the bundle of discs as compact at possible and the final tightening of the holding nut should be done with as much strength as can be used on a 6 in. wrench.

If the builder has a milling machine at hand he could assemble the blank discs on the shaft and cut 12 slots 7-16 in. deep by 3-16 in. wide. This would give approximately the same winding space. One objection to slots instead of round holes is that binding wires are necessary to prevent the armature wires from coming out when running at high speeds.

The finishing touch to the armature core is to chuck it in the lathe and take off a fine chip. This to make it as true of balance as possible. The better the armature is balanced the less will be the vibration when running. This is a very important factor in the machine, as the bearings will last the longer the less the vibration, and screws and nuts will not be always working loose.

The armature can now be prepared for its winding. For each end of the armature there should be made 2 fibre washers about 1/8 in. or 1-16 in. thick, 2 1/2 in. in diameter and cut out in the center to fit, one over the nut and the other over the steel washer at the ends of the core. Holes or slots should be cut in the fiber to correspond with the holes or slots in the core punch-ings. These are to prevent the sharp corners from cutting the insulation on the windings. Now cut out two round pieces of fairly heavy cotton cloth about 2 in. in diameter. The center of one should be cut out 3/8 in. in diameter and the other 9-16 in. in diameter. These are to cover the nut and steel washer.