28 ELEMENTARY WOODWORKING 



the wood, and so a smooth hole could be bored ; but, 

 considerable pressure was necessary to force the tool 

 through the wood. 



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 lifis, C C, can touch 

 the wood. The point A allows us 

 to accurately place the center of 



FIG. 38. Details 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. 



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 y^, or a 

 {-inch bit. If the number is 16, we have a }-f, or a 

 one-inch bit, etc., always referring to the diameter of the 

 hole which the tool will bore. 



