THE SAW. 85 



fixed for the spacing, which ought never to be exceeded. In the case 

 of hard woods the necessity of close spacing is further accentuated 

 by the fact that each single tooth can do comparatively little work, 

 and that consequently the more numerous they are, i.e., the closer 

 the teeth, the more effective is the saw. Hence the superiority of 

 M teeth and other similar forms, which increase the number of teeth 

 for a given length of blade. 



The angle between the two profiles of a tooth may vary between 

 70° and 45°, being about 45° to 50° for very soft woods and 65° 

 to 70° for very hard woods. Hence, since we know that the num- 

 ber of teeth must increase with the hardness of the wood, saws 

 for very hard woods should always have teeth in the form of 

 isosceles triangles, and should hence cut when drawn in either 

 direction. 



The line passing through the points of the teeth (clearance teeth 

 alone obviously excepted) should be an even line ; that is to say, 

 some teeth should not project beyond others, otherwise the former 

 alone will do the work and the cutting power of the saw will 

 thereby be diminished. This proviso being satisfied, the length of 

 the teeth need not always be the same. There are many woods so 

 constituted that in cutting them the saw can be moved only with 

 difficulty at the commencement of each cut, and there is much 

 splintering and tearing of the wood if the cut is commenced with 

 coarse teeth. To obviate this drawback, the size of the teeth is 

 gradually increased, so that the finest commence the cut and the 

 coarsest finish it. 



The teeth should be filed away on one side to a sharp edge. If 

 the saw is to cut in one direction only, the face alone should be so 

 filed (see Fips. 26 and 27) ; if in both directions, both profiles of 

 the teeth should be sharpened (see Figs. 28, 29, and 30). Alternate 

 teeth should have their sharp edges on opposite sides. The Indian 

 sawyer nearly always neglects to give the teeth of his saw any 

 sharp edge at all, probably in order to diminish wear ; but against 

 this diminished wear must be set the much greater loss he suffers 

 from the smaller quantity of work he turns out. 



Set of the teeth. — The teeth are given a set in order to enable 

 them to clear in the wood a passage wide enough for the blade of 

 the saw .to pass through without any tendency to buckling. The 

 softer or more coarse-fibred or gummy or resinous the wood, the 

 stronger must be the set ; but it should be just strong enough to 

 serve its purpose, otherwise there is waste of wood due to too thick 



