Fig. 120. Gimlet bit and centre bit
"The centre bit, an old-fashioned form, had all the necessary features of a good boring tool but one. It had a sharp centre for accurately locating the hole, a knife edge for cutting the fibres, and a chisel for removing the wood, but it lacked the spiral screw thread of the modern tool, and had to be forced through the wood by main strength. On a modern auger bit, this spiral screw relieves the worker of a large part of the labour; all he has to do is to turn the brace and keep it straight, supposing of course that the bit is sharp. ([Fig. 121].)
"The auger bit is most commonly used by woodworkers. It has two knife edges and two chisels besides the spiral spur in the centre. A short form of this tool, called the dowel bit, has the advantage of bending less readily than the ordinary auger bit. The size in sixteenths of an inch is stamped into the metal shank, but if this number is not distinct or for any reason is missing, the diameter may be measured by holding the rule across the knife edges."
Fig. 121. The auger bit
"What's the Yankee invention you were going to tell me about?" interrupted Harry.
"Well, suppose you wanted to bore a large hole, say 21⁄2 inches in diameter, the probabilities are that you wouldn't have a bit that size. In fact, to have a full set of bits from 3⁄16 inch up to 3 inches would mean a very expensive lot of tools. This difficulty has been overcome by a very clever invention called the extension or expansive bit. ([Fig. 122]). On this tool the knife edge and chisel are part of a moving lip, which may be fastened at any desired point by means of a set screw.
"Besides being adjustable in diameter, the lip of the bit has a scale, and the body a single line engraved on it. By bringing this line to the various measurements on the scale, you can set it to a definite size without the trouble of measuring it.