THB SAW. 83 



and b is then shaved off by the broad edge of the tooth C, which 

 is hence designated a clearance tooth. In the preceding explana- 

 tion we have supposed only one tooth of each kind acting, but 

 actually the opening a may be made by two or more teeth following 

 in succession ; and similarly the opening b by as many teeth filed 

 away in the other direction, while a single clearance tooth suffices 

 to remove the section of the fibres between a and b. The action 

 of the saw is greatly facilitated if each clearing tooth shaves off 

 only a portion of the section left by the cutting teeth that precede 

 it. This end is secured by making the clearing teeth slightly 

 shorter than the other teeth. In India the faces of the teeth are 

 never filed to an edge, and the' action of the saw is consequently 

 reduced to simply tearing and shaving, or, to use a simple and 

 more expressive term, to rasping. 



The portions of fibre torn and shaved off (the sawdust), unless 

 they were at once removed out of the way of the saw, would inter- 

 fere with its passage, and, by coming between the blade and the 

 cut surfaces of the wood, eventually cause it to jam (buckle). 

 Hence the necessity of making the gullet large enough to afford 

 sufficient room to hold the sawdust until it falls out as the saw 

 continues to advance. 



Shape of the teeth. — A great variety of shapes have been 

 devised, especially in America, where the saw is better understood 

 than in any other part of the world. For us, who have to work 

 in a backward country like India, it will suffice to note only a few 

 of the principal forms. 



If a saw is required to cut in one direction only, the teeth have 

 the well-known form approaching more or less nearly that of a 

 right-angled triangle. The pitch of the teeth may vary from 80° 



to 100°, according to the softness of the 



rig. Z/. wood. It is usually high in circular 



Saws on account of their great speed. 



If the quantity of sawdust is large, the 



gullet is enlarged by hollowing out the 



back of the teeth and giving the bottom 



of the gullet a curved outline (Fig. 27). 



Such an outline is an advantage under 



all circumstances, as it prevents any tendency of the blade or 



teeth to crack. 



If the saw has to cut in both directions, the teeth must as- 

 sume the form of isosceles triangles, the bottom of the gullet 

 being, according to the quantity of dust to be cleared, either 



