488 AUXILIARY FOREST INDUSTRIES. 



suspended vertically, the first tooth of the saw which strikes 

 the butt would do all the work in sawing, the other teeth 

 passing uselessly through the cut made by it. In order then 

 to divide the work equally among the teeth and afford room 

 for the butt to come forward during the up- stroke of the saw, 

 the crank gives a forward motion to the blade in its downward 

 cutting stroke and a retreating motion as it rises from the 

 cut. The distance by which the topmost overhangs the lowest 

 tooth is termed the slope of the saw. On this depends the 

 cleanness of the cut. 



4. Kind of Teeth Used. 



The most usual mode of construction of the teeth is that 

 shown in Fig. 298, the cutting side of the teeth being some- 

 what out of the horizontal line. Fig. 299 shows the old 



Fig. 298. Usual form of teeth. Fig. 299. Old form of teeth. 



German pattern of teeth which is still sometimes employed. 

 The area of the teeth is usually in a ratio 1 : 2 to that of the 

 spaces between them, but in the case of saws used throughout 

 the year for sawing coniferous wood, this ratio may be as low 

 as 1 : 3. 



5. Thickness of Blade. 



It is highly important for saws to have a proper thickness 

 of blade. Too thick a blade wastes much wood and motive 

 power, for the latter must be greater the more sawdust is 

 produced and the broader the cut. When, however, a stronger 

 motive power is used the tension of the blade must be greater, 

 this involves a heavier frame and increased strength in all 

 the other parts of the mill. All this causes increased resist- 

 ance and friction. Too thin a blade, on the contrary, is not 

 sufficiently stiff, easily gets heated, its tension becomes slack 

 and it then cuts in a wavy manner ; it may also fail to cut 

 through hard knots or annual zones in the wood. 



