FIG. 61.—Three-pole Armature.
After the patterns are finished they should be given a coat of shellac and carefully rubbed with fine sandpaper so that they are perfectly smooth. Otherwise the sand is liable to stick in moulding and produce an imperfect casting.
Castings may be obtained from any foundry which is equipped to make grey iron castings. They should be as soft as possible. The cost will depend upon the quantity which are ordered. If only one set is required, the charge will probably be based upon the time required for making the moulds but if several sets are ordered the price may be based upon the weight.
After the castings have been received from the foundry, the first operation is to carefully remove all rough spots and burrs with a file.
Those who have a lathe or large drill press can easily finish the tunnel by turning or reaming. In the absence of these facilities, hand filing can be made to suffice, if carefully done.
The holes marked "BBBB" should be drilled with a No. 29 drill and tapped 8-32. These holes must be very carefully located because they serve to fasten the bearings. Each hole should be exactly opposite the other, two and five-sixteenth inches apart and on a line passing exactly through the centre of the tunnel.
The holes, "PP" and "SS", are three-sixteenths of an inch in diameter. The former support the Binding Posts and the latter pass the screws which fasten the motor to the wooden base.
FIG. 62.—Six-pole Armature.
The armature, in the case of either the six or three pole type, has a three-sixteenth inch hole drilled along the axis to accommodate a steel shaft of the same diameter.