182 FELLING AND CONVERSION OF TIMBER. 



Formerly forest saws were rolled out of wrought iron, and 

 the rolled blade was then hammered cold to make it hard 

 and elastic. At present, saws are made of cast steel, and work 

 more easily than the older implements. Owing to the superior 

 toughness of the steel they retain their edge and set better, 

 and, owing to their smooth sides, they cause much less friction 

 in use than the iron saws. 



Saws have to overcome not only the resistance of the wood, 

 but also the friction of their sides against the rough surface of 

 the wood which is being sawn and the sawdust that collects 

 between the teeth. Their teeth act chiefly by tearing the 

 wood-fibres asunder*, and so much the more, the more porous 

 the wood and the longer and tougher the wood-fibres ; this is 

 therefore especially the case with soft, broadleaved and coni- 

 ferous woods. In the case of hard broadleaved woods, this 

 tearing action is superseded partly by a cutting action. The 

 more a saw tears the fibres apart the greater the amount 

 of sawdust, which is therefore most abundant in the case of 

 softwoods. 



(b) Mode of Construction of Saws. Saws are constructed 

 differently according to the uses for which they are intended ; 

 they vary in shape, length, weight, and shape of teeth. 



They may be used either for large or small timber. In the 

 former case they cut both ways, are worked by two men 

 and termed two-handed saws. In the latter case they cut only 

 one way and are one-handed : such saws rarely exceed 1 J to 

 2J feet in length, whilst the two-handed saws may be 3J to 6J 

 feet long, their length being determined by the diameter of 

 the piece to be sawn, and the distance through which the 

 arm moves. The weight of a saw depends also on its 

 length. 



The construction of the teeth of saws varies considerably. 

 Kach tooth may be cither symmetrical or unsymmetrical, and 

 tlio tooth vary- in depth, thickness and distance from one 

 smother, each of which points affects the working of the saw. 



As regards the shape of the teeth, a distinction must be 

 made between those cutting one or both ways. In the former 

 case they are shaped usually as in Fig. 87, that of a right 



* I "nle p. 115. 



