Popular Science Monthly
��four small holes should be drilled with a No. 21 drill and threaded 10/32 and the center hole should be the size of the motor shaft. The insulating bushing between the hub and the rotor casting is
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��The cover for the casing is of black sheet fiber and is screwed on like a cylinder head
made of black fiber 5^ in. in diameter and turned as shown. It will be found worth while to corrugate the sides with a round nose tool as shown at D, as this reduces surface leakage and im- proves the appearance of the rotor. The fiber piece should have No. 18 drill holes in it to correspond with those on the 4% in. circle of the casting C. There should also be No. 10 drill holes at the center corresponding to those in the hub. The rotor may then be as- sembled with round head screws and balanced. This may be done by putting a small piece of rod the size of the shaft in the hub and fastening it securely with the set screw. It should then be rested on parallel knife edges and drilled out at various points on the back toward the inner edge of the casting until it will rest at any position. The knife edges must be set absolutely level.
The wooden block shown in Fig. 4 is of quartered oak or maple and is 23^ by 8 by 8 in. This block should be planed absolutely square. If it is turned off center as shown, the casing will be more accurate than if the block were turned and then the three sides cut off. The block should be hollowed out 63^ in. in diameter and i^ in. deep and drilled as shown in the illustrations.
��The holes in the bottom of the block are for mounting it on the base, Fig. 5. C, and a center hole about 3/16 in. in diameter should be drilled for a 3/'i6-in. flat head bolt, and 1/ 16 in. on each side of it to receive a wood screw from the base. The three holes in each side should correspond to those in electrode plates. (Fig. 2, D. E.)
The fiber cover for the casing is shown at A-B, Fig. 5 and is made from 5/16-in. black fiber sheet. The small hole around the periphery should be 5/16 in. in from the edge and drilled with a No. 18 drill to allow the use of about ^^-in. round head brass wood screws to fasten cover to wooden casing. The large holes shown are for small circular windows to observe the spark and for adjustments.
For glasses, the lenses of the cheap grade of automobile goggles ma\- be used by cutting all the metal away except a small rim around the glass. If you have not a drill of the proper size one can be improvised, as shown at Z>, by taking a piece of ^^-in. machine steel with a rectangular hole cut through it and a piece of hardened tool steel about /€ by /^ in. driven in and ground to a cutting edge. When using a drill of this sort, a hole the size of the machine steel — ;^ in. — will have to be drilled through the cover. Drill about 1/16 in.
���Three views of the gap, as it is attached to the motor and the front and back sides
deep on each side of each hole the exact size of the glasses and then grind off the drill about i/i6in. on the radius and con- tinue the hole through. The glasses