THE SAWMILL 



119 



carriage or toward the wheels, while the guard rolls, standing about 2 inches back of the saw, prevent it 

 from slipping backward at the approach of the log. 



The strain on the saw, by which slipping- off is prevented, is obtained by raising the upper wheel 

 through worms or gears. 



The strain should be, approximately : - 



For 8-inch blade, 14 gauge, 7,000 lbs. 

 For 15-inch blade, 14 gauge, 12,500 lbs., &c. 



The giant band saws at the C. A. Smith mill, Marshfield, Oregon, are worked with a tension of 22,000 lbs. 



The maximum diameter of logs that can 

 be handled by band saws is about 90 inches. 



The weight of a band sawmill com- 

 plete is 20,000 to 40,000 pounds. 



(e) The double cutting band saw. 

 The saw blade has teeth on both edges, 

 so that a board is obtained at each trip 

 of the carriage. 



In the "telescopic" double band mill, 

 the entire machinery is raised or lowered by 

 hydraulic pressure with a view to bringing 

 the top of the logs immediately underneath 

 the upper wheel. 



IV. Conclusions. 



(a) The superiority of the band 

 OVER the circular SAW lies in a saving of 

 approximately 1,000 board feet in every 

 16,000 feet of ' ^-inch boards obtained. In 

 heavier planks the saving is less, in lighter 

 boards more. The boards obtained have a 

 better width. Logs over four feet thick can- 

 not be handled to advantage by circular 

 saws. Further, the band saw allows of a 

 more rapid feed. Hence it is used preemi- 

 nently for valuable logs, for big logs, and 

 for large output. 



The band saw yields, on the other hand, 

 a larger percentage of mis-sawn boards 

 than the circular saw, under otherwise equal 

 conditions. Circular sawn lumber runs 

 more smoothly through the planers than 

 does band sawn lumber. 



Frequently mills of large output employ 

 simultaneously band, circular, and gang saws, 

 allotting the logs according to their quality, the best to the band saw and the poorest to the gang saw. 



Two edgers and one trimmer can take care of such a combined output. 



(b) Mammoth mills are now considered uneconomical, since it is difficult to take care of the output 

 of boards at the outlet from the mill floor. 



The output per mill hand in big concerns is up to 7,500 board feet daily. 



Four acres of mill pond hold up to 1,000,000 board feet of logs. 



Double cutting telescopic band mill. Allis- Chalmers Co., Milwaukee, Wis. 



