Auger-Bits. The auger-bit most in use is shown by Fig. 114. It is sold in sets of thirteen bits each, varying in size by sixteenths, from 1/4" to 1". Each bit is marked by a small figure on the shank, which indicates its size in the scale of sixteenths. Thus the figure 9 is to be interpreted as 9/16".

90. Augers and auger-bits are sharpened by filing. The scoring nib B, Figs. 111 and 112, which is usually the first part to become dull, should be filed wholly from the inside. If filed on the outside, the diameter of the cut it makes will be smaller than that of the body of the bit. The cutting lip C should be sharpened from the lower side, the file being inclined to preserve the original angle. With the spur in good order, whenever the tool refuses to "feed," it is clear that the bit needs sharpening somewhere.

91. Center-Bits are convenient for boring holes of large diameter in delicate material, such as would be likely to split under the action of an auger-bit. By reference to Fig. 115, it will be seen that the spur A, which centers the bit in the work, is triangular in section. This form allows the bit to feed rapidly, or very slowly, in accordance with the degree of pressure applied to it. The point, or "nib," B cuts the fibers about the proposed hole, and the cutting lip C removes the material. The center-bit does not work well in end grain. When dull it may easily be sharpened by whetting. g2. Expansive Bits are so constructed as to be adjustable for holes of any size, within certain limits. There are several forms in use, one of which is shown by Fig. 116. This, without the movable cutter C, will bore a hole 3/4" in diameter, the screw A centering and feeding it into the work, B scoring, and a cutting lip in advance of B (not shown) removing the shavings. When C is inserted as shown in the figure, in addition to the action just described, there is a supplementary action on the part of C, its nib, B', scoring, and its cutting edge removing the chips. The cutter C is held in place by the screw D. By loosening D, C may be moved from or towards the center of the bit, or taken out altogether, and replaced by a cutter of different length. By using a short cutter in the place of C, a hole of any diameter from 3/4" to 2" may be bored, and with the cutter shown, any hole from 2" to 3" may be bored. The range of the bit, therefore, is from 3/4" to 3".

Fig. 114

Fig. 115