COMMON HAND-TOOLS 267 



or teeth extending from a point near the handle to the opposite 

 end. The mechanical principle of the teeth of a file is that 

 of the wedge. The handle acts as a lever. In the course 

 of manufacture, files pass through the successive processes of 

 forging, annealing (gradually heating and cooling), grind- 

 ing, cutting, hardening, and tempering. They are annealed 

 before being ground and cut, and 



,1,111 . IT - *=* FORWARD STROKE 



thus the hardness is reduced. 



File teeth are like a series of 



small chisels cut at an angle ^ WQRK ^ 



to the sides of the file, as shown FIQ 126 _ Action of File 



in Fig. 126. Cutting on the re- Teeth. 



turn stroke dulls the teeth and 



injures the file. It is possible to destroy some of the teeth 



of a brand new file in one minute's careless work. 



Many new kinds of files of all shapes and sizes, have re- 

 cently appeared, so that there are now at least 104 different 

 varieties on the market. All may be divided into three 

 general classes, namely single-cut, double-cut, and rasps 

 (Fig. 127). The files in each of these classes vary in length, 

 in shape, and in coarseness of teeth. 



A single-cut file has the teeth all running diagonally 

 across the face in one direction only. A double-cut file 

 has the teeth criss-crossing or running across the face in 

 two directions, making a surface covered with small, sharp 

 points. Each style or shape of single-cut and double-cut 

 file has several grades of coarseness. These grades are called 

 coarse, bastard, second-cut, and smooth, the coarseness 

 varying with the length of the file. The longer the file, the 

 coarser the teeth and the cut. Single-cut files are generally 

 used for cutting soft metals and for lathe work. Their 

 coarssr grades are sometimes called float files, or " floats." 



