Procure about ten pieces of stiff, hard wrapping paper, and two flat pieces of sheet brass not less than 1/8 inch thick, all of them being the same size as the steel plates. Lay the face-plate on the bench, face up, and pile on it first the paper, second one of the brass plates, third the bundle of steel sheets, and finally the remaining brass. Straighten up the pile as neatly as possible, and have the centers of all the pieces coincide as nearly as may be with the center of the face-plate. The whole must be firmly clamped together by means of four wood or metal clamps, to hold the mass while it is being drilled for the four bolts that are to hold it on the face-plate while it is being bored and turned. To mark off the places for these four bolts, first find the true center of the upper brass plate by measuring from the periphery of the face-plate with a pair of dividers or with a rule and square. From this center strike a circle of 2 5/16 inches radius on the brass. When this circle is divided into four equal parts, the points so found will be at the corners of a square which will measure a trifle over 3 1/4 inches on a side. The bolt holes are drilled through these corners, so that the whole mass may be bolted together with machine bolts not less than 3/8 inch in diameter. (See Fig. 216.) At least two of the bolts may be made to pass through the radial slots in the face-plate, but if the latter is provided with six such slots it will, of course, be necessary to bore right through the plate in making the other holes. As soon as each hole is drilled put in the bolt for which it was made from the front side, and tighten up the nut. When all have been tightly set up, the clamps may be removed and the face-plate will be ready to be screwed on the lathe spindle.

Fig. 216.

Fig. 217.

Fig. 218

Fig. 219.

Figs. 216-219 - Details of face-plate work.

"Make haste slowly" is one of the secrets of success in working a pile of laminae in a lathe. Put in the back gears and run the belt on the largest of the cone pulleys, keeping the speed of the work down to thirty revolutions per minute or even less. An ordinary V-shaped threading tool, as shown in Figs. 216 and 217, is one of the best to use. Feed the tool slowly by hand. As each successive plate becomes nearly cut through the tool will catch in the ragged edge and the entire piece to be removed will be quickly torn out. When the bulk of the metal has been thus cut away, the pieces may be bored and turned to the exact dimensions with ordinary tools and slow power feed. Make the bore 3 3/4 inches in diameter, and the outside 6 5/8 inches.

Before unscrewing the face-plate from the lathe, take a light cut off the face of the brass plate so as to make the part of it lying outside of the bolt heads perfectly true. By placing a sharp pointed tool in the carriage it will then be easy to mark off two circles on the brass, the one being 6 1/8 inches in diameter and the other 5 5/8 inches. These circles will form accurate guides for laying out the permanent bolt holes and the pole pieces, in accordance with the drawing in Fig. 215.

Divide the outer circle into four equal parts, choosing points midway between the bolt heads. If this is done, the removal of the metal between the pole pieces will take away also the old bolt holes, which form no part of the finished core plates. Mark out the outline of the pole pieces on the surface of the brass, and drill all necessary holes before removing the laminae from the face-plate. As shown in Fig. 218 there are to be four 3/16-inch holes, A, for the permanent bolts, four 5/32-inch holes, B, to form the bottoms of the slots in the pole faces, and four circular arcs, C, made by drilling 3/16-inch holes as closely together as can be done without danger of the drill breaking through from one hole to the next. These last holes will have their centers all on the circle 5 5/8 inches previously marked on the brass. It is to be noted that the four holes, A, must pass entirely through the second brass plate, but the others need be only deep enough to pass through the steel plates.

When all the holes are drilled, the laminae will be ready for removal from the face-plate. This can be best done by taking out at first only three of the bolts, after which the bundle of plates may be swung around on the fourth, to permit of the insertion of some of the 3/16-inch bolts. This prevents the springing apart of the plates and avoids the danger of a mix-up. The laminae, now tightly clamped between the brass plates, must next be held in a vise while the eight cuts indicated in Fig. 4 by dotted lines and the four small slots in the pole faces are made with a hacksaw. The pieces containing the original large bolt holes can then be easily removed, thus leaving the stator plates finished except for roughness, which must be carefully removed with a file. Finally the brass plates may be removed and thrown aside. The operations in the lathe have made a very intimate contact between successive laminae, so that as far as being an electrical conductor is concerned the stator might now almost as well have been cut out of a solid block. It is very well worth while, therefore, to take apart the laminae, remove the burrs from each one separately with a fine file, wash them in a pan of benzine to remove oil and loose filings, and finally to give each plate a coat of very thin shellac on one side only before reassembling. It is quite important that the plates be not mixed up during these cleaning operations, as the inevitable irregularities in the form of the different poles and in the location of the bolt holes makes it impossible to reassemble the plates in any other than their proper positions. To avoid this mixing pass a stout string about four feet long through one of the bolt holes and tie a big knot at each end. The plates may then be handled separately, and then be finally put back as they were at first. The finished core must be exactly 1 7/8 inches thick.

The work of making the rotor core plates is much easier than that of the stator, so only a brief description is necessary.

The material required is about 8 1/2 pounds of sheet steel (similar to that used in the stator) cut 4 inches square, and two copper plates of the same size and 1/8 inch thick. These copper plates are not used merely to make the work of clamping and turning easy, as in the case of the stator, but are to be left at each end of the finished core. If suitable copper plates cannot be obtained, some 1/8-inch brass may be substituted. Protect the face-plate with sheets of paper, as before, and bolt on the metal plates with four 3/8-inch bolts. Strike a circle 2 inches in diameter on the upper copper plate, and divide this into four equal parts to find the place for the bolts. On large lathes the hub of the face-plate will be in the way. In this case screw the bolts into tapped holes made for them either in the face-plate itself or in a heavy, flat metal plate bolted on it.

When the material has been fastened, turn off the outside smoothly to a diameter of 3 11/16 inches. Mark off on the top copper plate a circle having a diameter of 3 13/32 inches and divide up this circle accurately into 37 equal parts, and mark the points so found with a center punch. The correct spacing can be found only by repeatedly "stepping off" around the circle with a pair of dividers, trying different distances between their points until it comes out just right.

It may appear at first sight as if 37 were an unnecessarily difficult number of holes to space off, and that 36 might just as well be substituted, but this is not true. It has been found by experiment that the number of slots should be an odd one. One of the reasons for this will be self-evident if one considers what would happen if the rotor were provided with only four such slots, of somewhat larger size, or, to go to an extreme, if an iron cross were to be substituted for the rotor. When the arms of this cross came opposite the four polar projections of the stator they would be very firmly gripped by the magnetic flux, and it would take considerable force to twist the cross out of the magnetic path. After being twisted far enough, however, to become released from the attraction of the poles, the cross would move forward with a jerk to the next favorable position. In a four-pole motor, then, the rotor must not have a number of slots divisible by four, or it will, to a less extent, be found to turn with little jerks that result in vibration and noise when the motor is running, in addition to interfering with its starting.