Fig. 1758.
The simplest form of wood-piercing tool is the awl or bradawl, shown in [Figs. 1757] and [1758], its cutting end being tapered to a wedge shape whose width is sometimes made parallel with the stem and at others spread, as at c d in figure. It is obvious that when the end is spread the stem affords less assistance as a guide to pierce the hole straight.
It is obvious that the action of an awl is that of wedging and tearing rather than of cutting, especially when it is operating endways of the grain.
Thus in [Fig. 1758] is shown an awl operating, on the right, across the grain, and, on the left, endwise of the same. In the former position it breaks the grain endwise, while in the latter it wedges it apart. Awls are used for holes up to about three-sixteenths of an inch in diameter.
Fig. 1759.
[Fig. 1759] represents the gimlet bit having a spiral flute at f and a spiral projection at s s, which, acting on the principle of a screw, pulls the bit forward and into its cut. These bits are used in sizes from 1⁄16 inch to 1⁄2 inch. The edge of the spiral flute or groove here does the cutting, producing a conical hole and cutting off successive layers of the fibre until the full diameter of hole is produced. The upper part of the fluted end is reduced in diameter so as to avoid its rubbing against the walls of the hole and producing friction, which would make the tool hard to drive.
