In soft stones the hammer blows must be delivered lightly or the cutting edge will produce corrugations in the seat or bottom of the hole, and falling into the same recesses when revolved after each blow the chipping action is impaired and finally ceases. To prevent this the cutting edge is sometimes curved in its length so that the indentations cross each other as the drill is revolved, which greatly increases the capacity of the drill, but is harder to forge and to grind.
Fig. 1777.
The simplest hand-drilling device employed for metal is the fiddle bow drill shown in [Fig. 1777]. It consists of an elastic bow b, having a cord c, which passes around the reel r, at one end of which is the drill d, and at the other a stem having a conical or centre point fitting into a conical recess in a curved breast-plate. The operator presses against this plate to force the drill to cut, and by moving the bow back and forth the cord revolves the drill.
Fig. 1778.
As the direction of drill revolution is reversed at each passage of the bow, its cutting edges must be formed so as to cut when revolved in either direction, the shape necessary to accomplish this being shown in the enlarged side and edge views at the foot of the engraving. It is obvious that a device of this kind is suitable for small holes only, as, say, those having a diameter of one-eighth inch or less. But for these sizes it is an excellent tool, since it is light and very sensitive to the drill pressure, and the operator can regulate the amount of pressure to suit the resistance offered to the drill, and therefore prevent the drill from breakage by reason of excessive feed. In place of the breast-plate the bow drill may be used with a frame, such as in [Fig. 1778]. the frame being gripped in a vice and having a pin or screw a. If a pin be used, its weight may give the feed, or it may be pressed down by the fingers, while if a screw is used it must be revolved occasionally to put on the feed.