Bits of different kinds are used in making round holes. Bits for wood are made of a special kind of steel, one end of which forms the cutting portion of the tool, and the other is wedge-shaped, that it may be securely fastened into a handle or brace, by means of which it revolves. In working, the brace is always turned to the right, and the bits are made to cut in the same direction. The edge of the bit is designed to make its way into the wood without great pressure, and without risk of splitting it. The bit must work without hindrance from the shavings; otherwise it will become hot from friction, and boring will be difficult. A good bit cuts like a knife, detaches smooth spiral shavings, and becomes only moderately warm, even when worked quickly.
The brace may vary in construction. Fig. 70 shows a very strong Swedish brace made of iron. The upper end, or tang of the bit, forms a square truncated pyramid, which slots into a hole in the brace socket, and is fastened by a spring. Fig. 71 shows an American brace, also made of iron. It has a screw adjustable socket, into which the bit is secured. The tang of the bit may be of any form, provided it is somewhat rectangular.
How a bit should work.
Fig. 70. Swedish Brace. J.
Fig. 71. American Brace; section of screw adjustable socket, or bit holder, 1/4.
The bits in most general use are shell-bits and centre-bits. Small shell-bits are called pin-bits.
Fig. 72. A, Auger-bit. B, Centre-bit. C, Shell-bit orsPm-bit. D, Hole-rimer drill. E, Screwdriver bit. F and G, Counter-sink drills. 1/3.
1. The shell-bit (Fig. 72, C) is gouge-shaped, with the end curved like the point of a spoon. Unlike the centre-bit, it has no middle point, and it is therefore more difficult to gauge to holes of any given size, especially if the latter are large.
This bit is better than the centre-bit for boring end pieces. Shell-bits are made in various sizes, from those adapted for holes of 1/16 inch in diameter to those suitable for holes of 11/2 inch in diameter. The smallest kind, the pin-bit, is most used in wood slojd. A set of pin-bits includes from 8 to 10, varying in size from 1/8 inch to \ inch.