McKENZIE LOG RULE, 1915 63 



58. McKenzie Log Rule, 1915. This log rule is a universal formula 

 and not a commercial standard or true log rule. It is intended to 

 reduce all the variable factors in the production of sawed lumber to 

 elements in a formula, which will permit the determination of a local 

 rule that will accurately measure the sawed output in the log for any 

 condition, and eliminate over-run. 



The factor of taper is treated by building up the log in 8-foot sections, 

 permitting the use of whatever actual average taper coincides with that 

 of the logs sawed. The allowance for slabs, edging and crook is made by 

 the first method, that of deduction from the diameter previous to sub- 

 tracting saw kerf. Shrinkage could be included with saw kerf, if neces- 

 sary, but the author does not mention it. 



The formula is the one already shown to be correct and universal for board-foot 

 log rules, 



B.M. = (1 -6) 7854(D-a) L . 



The saw kerf allowance, b, is computed to include width as well as thickness of 

 lumber sawed (§53). To this general formula the author adds a constant, c, to 

 offset excessive taper on small logs. 



The principal utility of this log rule will be found in determining, in advance of 

 sawing, the amount of over-run which may be obtained from logs scaled by a com- 

 mercial rule, or to test the results in over-run to be expected by the use of different 

 log rules and different methods of manufacture. The objections to adopting it as a 

 standard of measure are stated in § 49. 



Reference 

 Bui. 5, California State Board of Forestry, by H. E. McKenzie. 



59. International Log Rule for |-inch Kerf, Judson F. Clark, 1900. 



In constructing this rule, modern conditions of manufacture in large 

 mills were presupposed. The values of the rule as published are for a 

 band saw cutting a J-inch kerf and are rounded off to 5 and 10 board 

 feet, thus approaching the principle of a decimal rule. Saw kerf is 

 first subtracted, allowing j^-inch for shrinkage, or a total of j^ inch. 

 The deduction for slabs and edging, including a normal crook of from 

 1 to I5 inches is then made in the form of a plank measuring 2.12D. 



The formula reads: 



B.M. = (.66fl 2 -2.12D)— . 



12 



The rule was constructed as follows: Since the per cent of waste in saw kerf plus 

 TC ^ 



shrinkage is - — — this becomes for inch boards or 3 parts in 19, which gives 



\-\-K. 16+3 



.158, and the factor for residual volume is .842. Then, 



.842(.7854D 2 ) = .66D2. 



