192 FELLING AND CONVERSION OF TIMBER. 



as the original teeth become worn-out. Fig. 112 shows how 

 the perforations are made in a straight cross-cut saw. 



Saws are set in order that the hlade may be drawn easily 

 backwards and forwards in the wood without buckling ; this is 

 effected by forcing over the successive teeth alternately to 'the 

 right and left of the axis of the blade. In order that setting- 

 may be effected properly, the metal must be sufficiently soft 

 for the teeth to bend without breaking, but the blade must 

 not be too soft, or the teeth will retain neither their edge nor 

 their set. 



By use, the teeth lose their sharpness and come back into 



Fig. 114. Key. Fig. 115. Earth's setting-iron. 



their original position. The chief excellence of cast-steel saws 

 consists in the fact that they retain their edge and set, much 

 better than old-fashioned saws. If any of the teeth are too 

 hard they can be softened by holding them for a few seconds 

 between a pair of red-hot pincers. For setting the teeth of 

 saws, a key, usually of the shape shown in Fig. 114, is used, 

 the teeth being held in one of the grooves, and then bent over. 

 Fig. 115 is a mechanical apparatus for setting the teeth of 

 saws in a very regular manner. The blade m n (shown in 

 section) rests on the adjustable screw dp, which may be raised 

 or lowered, and on the anvil o o, so that the teeth pass succes- 

 sively between a a, and are bent by the hammer k. The 

 apparatus is fixed firmly to a solid basis by the spike <. 



