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lathe chuck in this manner, if the drill does not start true and straight it will be impossible to bring it back into line again and the longer the distance the drill has to travel the farther the hole will be from the true center.
To drill the bearing use a No. 20 twist drill and,
if you have no drill chuck fitted to the tail-stock of the lathe, file a flat spot on the back end of the drill and, holding it in the vise, make a center-punch mark thereon. Clamp the drill in a hand vise or lathe dog, to prevent it from revolving, and place the center-punch mark against the back center of the lathe and the point of the drill in the center located in the work. This is shown in Fig. 12.
Start the lathe and hold the drill firmly against the back center while it is being fed into the work by the tail screw.
The drill should be a new one or one that has been accurately ground, for if the two lips are not ground off
equally or are ground at different angles the hole will not be straight.
When starting the drill, if the point of a lathe tool held in the slide rest is carefully brought up against the side of the drill, just as close to the work as possible, it will tend to hold the drill in position and will also be a great help in starting it straight. This is one of those little “kinks" which will be found very useful in all similar work.
As soon as the hole is drilled through, place a very small boring tool in the tool post of the slide rest and bore out the hole carefully to true it up. Bore out to a size that a 3/ 16-inch reamer will just clean up when it is put through. The reamer is held against the back center in the same manner as the drill in Fig. 12.
Figure 13 shows the boring tool in position to bore out the hole after drilling.
After both bearing castings are drilled and reamed, the long hub, which was held in the chuck during the foregoing operation, should be turned down to a size of 3 inch. Only one is necessary to be made just this size and that is the one on which the brush-holder rocker arm is placed. It is advisable to finish both bearings in this manner because of the improved appearance.
The bearing casting is placed on a 3/16-inch mandrel which is mounted between the lathe centers and turned to size as shown in Fig. 14. A cut should also be taken across the two bosses where the holes for the side-bearing rods are to be drilled. It is advisable to do this with a sharp-pointed tool and feed very slowly, so that there will be no danger of the casting slipping on the mandrel.
After turning and facing both bearings the next operation to be done on them is to locate and drill in each one the two 1-inch holes for the side-bearing rods. To do this accurately it will be necessary to prepare a little “jig” for the purpose.
As only the two bearings are to be drilled with it there will be no necessity for making this of tool steel and hardening it. A piece of soft steel, cast iron, or even brass will answer the purpose. It should be at least š inch thick and the two flat sides must be parallel. One of the edges of this piece must be straight.
If the working drawing of the bearing, Fig. Ir, be carefully studied, it will be noted that the distance from the bottom of the feet of the bearing casting up to the center of the holes for the side-bearing rods is 5/16 inch. If now we let the straight edge of the jig represent the line of the bottom of the feet, a line drawn on the jig parallel to its straight edge and 5/16 inch distant, would represent the center line of these holes. This distance should be very carefully measured off on the metal plate and the line a, Fig. 15, should be drawn by using a straight edge and a sharp-pointed scriber. Near one end of this line mark a point with a sharp center-punch. From this point just located measure along the line a, a distance of I 27/32 inches, at which place a second center-punch mark should be carefully made. This must be accurately done or the next measurement, which represents the position of the shaft, will not be correct.
Around each of these two center-punch marks on the line a, make a circle corresponding to the diameter of the bosses on the bearing castings.
FIG. 15.- Locating Holes in Drilling Jig for Bearings.
Now set a pair of sharp-pointed dividers with the points 1 1/16 inches apart and from each of the two center-punch marks on the line a, describe an arc of a circle. Where these intersect will be the point representing the center of the hole in the bearing.
Before proceeding further, however, all these measurements should be carefully verified according to the figures given on the working drawing, Fig. II. When all is found correct, center-punch the last point located. Now take a small twist drill, say No. 42, and carefully drill a hole at each of the three points centerpunched.