ROBERT GIBSON GRISWOLD

Any amateur who has tried using a twist drill of large diameter in a small lathe has, perhaps, experienced a few of the difficulties attending that operation, especially when the drill begins to break through; the drill is generally drawn off the tail center, canted in the hole, catches on the tool post, and frequently ends the performance by springing the piece out of the chuck or tearing it from the faceplate, provided the drill does not break. Aside from this, a twist drill is apt to feed into the work, if of brass, owing to the shape of the lip. It is true that this maybe obviated by grinding the lip nearly parallel with the drill axis, but that destroys its cutting action in iron or steel, and that much of the drill has to be ground away before a proper edge can again be had. Another very important objection to twist drills of diameters above one half inch is their great cost, especially if a set of any considerable number is purchased.

The object of this article is to explain how to make a set of very satisfactory chucking drills at a very low cost. They are simply flat drills with a center at one end in which to rest the tail center. The opposite end is forged down to the size desired, which should be slightly longer than the largest dimension of the shank, and ground to a fine, true edge after hardening and tempering.

It is surprising what extremely accurate work may be done with these drills if properly sharpened and kept in good condition. Suppose we make a set ranging from 1/4 to 1 in. varying by 1-32 in. up to 1/2 in., and by 1-16 in. from 1/2 to 1 in., which gives us a set of seventeen drills, as follows: i, 9-32, 5-16, 11-32, |, 13-32, 7-16, 15-32, 1/2, 9-16, 5/8, 11-16, 3/4, 13-16, 1/2, 5-16, 1 in. For the drills up to $ in. we will use flat tool steel, 3-16 X 3/8 in., and make them all six inches long, which will require about 4 ft. of this stock. Cut the pieces off for each drill after the end has been drawn out by forging. This drawing increases the length and saves material. Make the 9-16, 5/8 and 11-16 in. drills of 1/2 X 1/4 in., and the remaining four of £ x5-16 in. stock, each finished drill being about seven inches in length.

Forge each drill down to a size a little above the finished size and make it flat, as shown in the sketch. This flat is necessary in order that the chips may clear and have a chance to get out. The cutting edge may be made somewhat thinner, but not so much as to weaken it while cutting. Then cut off the drill, center both ends and place in lathe. Take a cut for the distance indicated in the table to give the drill a true body, and then relieve it slightly, as shown. This makes the drill follow in a straight hole. Unless the amateur has some means of grinding these bodies after turning, a very fine finishing cut had best be taken with a keen finishing tool wet with oil. One cannot file an irregular piece in the lathe to make a good job. The flat dimension of the drill should gradually taper from the dimension D to 2x D, where it runs into the stock.

When finished to the desired shape, file down the point approximately to the angle desired, afterwards drawing the temper to a dark straw. Then grind the final cutting edge, exercising care to see that the same angle is used on both sides, and that both cutting lips are of the same length. Now make three holders, as shown in cut, one for each size stock. In using the drills these holders are fastened in the tool post, the drill stock passed through the slot, which should be slightly larger, and rested on the tail center. The point of the drill is then brought against the revolving work and fed by means of the tail or spindle. The holder permits the drill to move axially without revolving with the work. If the lips have been carefully ground, very straight, true holes can be drilled, and and the drill will not draw in when breaking through.

A very handy wrinkle to prevent the drill dropping off the center when through the hole, is to drill a hole through the stock and drive in a pin, over which latter a heavy rubber band isslipped and passed over some projection of the tail stock. If tightly stretched it will draw the drill out with it. Another scheme is to back the holder up against the pin and move the carriage with the spindle.

This set, while highly efficient and capable of good heavy work, is very much cheaper than a correspond-i»g set of twist drills, and for all practical purposes is every bit as good.

In order that a piece may be so located on a face-plate that a hole may be drilled or bored in any definite position, it is necessary to apply some instrument that will indicate the exact amount that the center of the hole (already laid out on the work) is out of truth and in what direction. Such an instrument is called an indicator, and the one herein described is accurate, durable, cheap and easily constructed.

Make a ring of brass or steel 1 1/8 in. outside diame-eter and 13-16 in. inside diameter by £ in. width. Drill and tap two holes diametrically opposite for a 1/8 in. screw with 40 threads, or any other tap of that diameter. Thread 1/4 in. of one end of a piece of 1/8 in. drill rod 3/4 in. in length, and sharpen both ends down to a 60° point: then harden the ends. Make the other extension in this manner. Cut a piece of the same drill rod six inches in length and thread one end for 3-16 in.; in the other end drill a 3-32 in. hole about 1/2 in-deep. Screw both of these pieces into place firmly. Now make an index out of a piece of straight-grained pine or spruce about 10 in. long, and taper it down as shown, fitting the larger end snugly to the 3-32 in hole. This pointer is very light and will move quickly without vibration. The ratio of movement is as .75 : 14= 18.66, and the end of the pointer will magnify the error in setting 18.66 times, generally a sufficient amount for all practical purposes. An error of .001 in. in the setting will be multiplied to .01866 in-, or nearly 1-50 of an inch. If greater accuracy is required the short piece may be made 1/2in. long, when the multiplication of error will be 28 times.

A holder is made as shown to press the point against the work. This holder is fastened in the tool post, and the vertical adjustment made by moving the spring a up or down, while the horizontal adjustment is secured by moving the tool-post block. Only sufficient pressure to hold it to the work should be applied.

To use the indicator, strap the piece to the face plate with the center of the hole in approximately the correct position. Then apply the indicator, adjusting the holder until the end of the wooden pointer coincides with the tail center. Now revolve the face-plate through half a revolution and move the carriage so that the pointer will approach the center by half the discrepancy. Then lightly tap the block until the pointer again coincides with the center. You have then moved the block the distance that it was off the center in that direction. Then turn the faoe-plate a quarter turn and repeat the operation. The block will then be practically correct, and by light taps in one direction as the other may be brought to the desired position, when there will be no movement at all at the end of the pointer. Then firmly clamp the piece in position and try again to see that the bolts have not drawn the piece out of true or sprung it.