426 APPLIED SCIENCE 



Wood-working tools should be ground on a grindstone of 

 medium fine grit to which water is freely supplied. The 

 water is used for two purposes: (1) to prevent the heating 

 of the tool (heating destroys the temper and causes the tool 

 to be "soft"); (2) to wash away the waste material which 

 collects between the fine pieces of grit of which the surface 

 of the grindstone is made up. 



The grindstone should be frequently "trued up" by some 

 suitable device so as to keep its face perfectly round and 

 straight, and to break away the rounded projections and 

 present fresh, sharp projections for action. 



491. The Brace and Bit. The brace is a tool used to hold 

 and turn the various kinds of bits used in boring, drilling, 

 and countersinking, or in driving screws. Braces are of two 

 kinds the ordinary brace and the ratchet brace. The 

 latter type is fitted with a ratchet in the grip, so that the bit 

 can be turned only in one direction. This mechanical fea- 

 ture is necessary where lack of space prevents a complete 

 revolution of the brace, and also when boring in hardwood, 

 or turning large screws. 



The most common forms of bits used in the brace are the 

 following: 



(a) The auger bit. This bit has a spur to draw the bit 

 into the wood, two nibs for cutting the fiber, and two lips to 

 remove the waste which is brought by the twist to the surface. 



(b) The drill bit. Bits of this kind are sharpened only 

 at the end of the twist. They are made of tempered steel, 

 and are used in boring either hardwood or iron. 



(c) The countersink bit. This bit has a V-shaped cutting 

 end for enlarging screw holes, so that the screw head is 

 drawn down even with or below the surface. 



