FOREST SAW-MILLS. 681 



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. 302, the cutting side of the teeth being somewhat 

 out of the horizontal line. Fig. 303 shows the old German 



Fig. 302. Fig. 303. 



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 resistance and friction. Too 

 thin a blade, on the contrary, is not sufficiently stiff, easily 

 becomes 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. 



Saws for hardwoods and for resinous, knotty wood of manj' 

 conifers should be thicker than those used for soft, clean-grained 



