The progress that has been made in the manufacture of tools can be easily appreciated by comparing this center bit with the modern auger bit.
Referring to the sketch ([Fig. 38]), B B are two knife points, or nibs, which cut the wood fibers before the chisel edges, or lips, C C, can touch the wood. The point A allows us to accurately place the center of the hole where we wish it, and the screw back of A draws the tool into the wood as it revolves. This part is known as the spur, or worm. On this class of bits no pressure is necessary.
Fig. 38. Details of Auger Bit
The opposite end of the bit, called the shank, fits into the brace. Any tool with such a shank, and designed for use with the brace, is a bit. We have screw-driver bits, gimlet bits, auger bits, etc.
On the shank of an auger bit will be found a number. This is the numerator of a fraction whose denominator is 16. If we find this number to be 4, it is a ⁴⁄₁₆, or a ¼-inch bit. If the number is 16, we have a ¹⁶⁄₁₆, or a one-inch bit, etc., always referring to the diameter of the hole which the tool will bore.
In using the brace and bit care must be taken to see that the bit shank is far enough in the brace to be fastened securely, and that the tool is held at right angles to the wood. It may appear from the front to be perfectly vertical, yet by stepping to one side and looking at it from another position it will frequently be found far from vertical. When starting a hole it is well to do this several times until assured that the tool is working in a true upright position.
Fig. 39. The Gimlet Bit