84 



Forestry Quarterly 



With narrow saw kerfs a and b increase with the increase in 

 the amount of lumber produced, but c decreases sufficiently ta 

 compensate for the increased loss from a and b. Likewise with 

 wider kerfs a and b decrease with the decreased production of 

 lumber but their decrease is offset by the increase in c. 



Perhaps a concrete case will illuminate this somewhat. Sup- 

 pose a certain log contains 125 cubic units ; that 25 units be lost 

 in kerf ; that 20 units be necessarily allowed for edging waste 

 and shrinkage {a and b above); and that the remaining 80 units 

 be lumber. The lumber is to edging {a and b) as 80 is to 20,. 

 therefore of the 25 kerf units 20 go with the lumber and 5 

 with the edging (<:). In other words, the total allowance (a + (^ + 

 ^:) = 25 units. With different widths of kerf «, b, and c neces- 

 sarily vary individually, but their total remains practically 

 constant for the same log regardless of the thickness of the saw 

 used in its manufacture. 



This interesting and most important fact solves one of the 

 difficulties in the way of producing a universal log rule in that 

 it makes it possible to adapt a correctly computed rule to saws 

 cutting different widths of kerf. 



The standard scaling score of the International Rule is com- 

 puted for a ^-inch kerf and may be adapted to saws cutting the 

 various standard widths of kerf as follows : 



100,000 feet as scaled by the 

 rule will cut out : 



For ^^:f inch kerf flfl^flf 1.3% 101,300 ft. B. M. 



subtract ^.0^/0 95,000 



9.5% 90,500 



13.6% 86,400 



" 17.4% 82,600 



" 20.8% 79,200 



Most modern band saws cut a ^-inch kerf. Some i6-gatige 

 band saws cut as low as g^-inch kerf. Gang saws ordinarily cut 

 from y% to Y\-inch kerf. Rotary saws of the dimensions now 

 commonly in use cut a kerf of about ^-inch. Large rotary 

 saws cut kerfs varying from yg-inch to ^-inch. The old-time 

 ' ' yi and y g ' ' inch kerf is now happily practically obsolete. 



Allowance for Edging. It has already been noted that with 

 logs of varying dimensions but similar form the amount of wood 

 necessarily wasted in square-edging the lumber is in direct pro- 

 portion to the bark surfaces of the different sized logs. The 



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