1. The ordinary centre-bit (Fig. 72, B, and 73, B) has a flat blade, the lower portion of which is broader than the upper, as is shown in the illustrations. In the middle of the lower edge is the centre-point a, and at one side is the cutter b. In boring, the cutter makes a circular incision corresponding to the circumference of the hole, and thus determines its diameter, prevents the wood from splitting, and facilitates the removal of shavings and sawdust by the lip c, the edge of which is horizontal to the point and oblique to the blade, and which cuts at right angles to the cutter. The centre point is longer than the cutter, which again cuts deeper than the lip. In sharpening the centre-bit, which may be done with a small half-round file, care must be taken that the edge of the cutter is on the outer side, and the edge of the lip on the under side. A set of bits should contain 8 to 12 centre-bits, from | inch to 1 inch broad.

The ordinary centre-bit.

Fig. 73. A, Portion of Auger bit. B, Portion of Centre bit. aa, Centre point, 666 Cutter, cce Lip.

Fig. 73. A, Portion of Auger-bit. B, Portion of Centre-bit. aa, Centre-point, 666 Cutter, cce Lip.

2. The auger (Fig. 72, A) belongs to the same class. In boring with the bits previously described, it is necessary to exercise a certain degree of pressure, but the auger works its way into the wood by means of the conical screw which forms its centre point, and after the screw has once started all that is needed is to make it revolve. The auger is besides furnished with a cutter and a lip on both sides of the screw. (See Fig. 73, A.)

Way in which the auger works

Above the cutting portion it is spiral in form, and thus we have a double spiral with sharp edges. This gives plenty of room for the sawdust and shavings which are worked out of the hole without the removal of the auger. The American augers are the best. A set includes 6 to 12 pieces, from 3/16 inch to 1 inch.

The best augers.

Fig. 71.   Expansion bit 1/3

Fig. 71. - Expansion bit 1/3.

The expansion bit (Fig. 74) is of American construction. Within certain limits it admits of holes of different sizes being bored with one and the same bit. Its point, lip, and cutter are tolerably like those of the auger, but it is furnished with two loose cutters, which may be screwed in to suit the diameter desired. The adjustable cutter does the work both of lip and cutter. The expansion bit makes holes with remarkably even surfaces, and with two different sizes it is possible to bore holes varying in diameter from 1/4 inch to 3 inches.

Adjustable orexpansion bits.

A screw-driver bit (Fig. 72, E), two or three counter-sink drills (Fig. 72, F and G), and a couple of square or hexagonal hole-rimers (Fig. 72, D) are usually included in a complete set of bits. The counter-sink drill is used to produce a conical hole in wood or metal, suitable for sinking screw-heads. The rimer enlarges holes in thin metal plates, e.g., screw holes in hinge-plates.

3. The Bradawl (Fig. 75). This tool consists of a steel bit 1/16 inch to 1/8 inch thick, and 2 inches long. Its point is like that of an awl, or it may be chisel-pointed. The bit is secured in the handle by a screw-socket. Several bits of different sizes belong to the tool, and the handle, which is hollow, serves as a case for them. The bradawl is used to bore holes for sprigs, nails, etc. When holes are bored with the chisel-pointed bit, the edge is placed across the grain of the wood, and pressure is exerted in this direction to prevent the splitting of the wood.

Screwdriver bit, countersink drill, and hole-rimer.

Fig. 75. Bradawl. 1/3.

Fig. 75. Bradawl. 1/3.