114 THE SAWMILL 



Each additional horse -power should add 1,000 board feet to amount cut. 



The amount depends, naturally, on size of logs and on hardness of fiber. To give an approximation, 



there is required :- 



8 horse-power for a 20-inch to 30-inch saw 



12 horse-power for a 30-inch to 40-inch saw 



15 horse-power for a 48- inch to 50-inch saw 



25 horse-power for a 50-inch to 62-inch saw 



Softwoods require less power than hardwoods; thin gauges less than heavy gauges. 



(b) Right-hand and left-hand mills, if the carriage is to the left of the observer while the saw 

 runs toward him, the mill is a left-hand mill, and vice-versa. A right-hand saw is screwed to the arbor 

 by a left-hand nut, and is usually driven by a left-hand steam engine. 



Center crank engines can be used for either right or left hand mills. 



(c) Speed. Disregarding the power available, the proper speed at the rim of a circular saw is 

 9,000 feet per minute. Reducing the number of teeth, the speed must be reduced also, and the power 

 can be reduced proportionately. 



There should be a speed indicator to control the saw's speed. It costs 75 cents. 



If the power is too light to run the mill at standard speed, the sawyers in portable mills usually 

 increase the speed of the engine, and put a larger receiving pulley on the saw mandrel. 



Mills running with small horse-power, should run the rim of the saw at a speed not e.xceeding 

 360 feet per minute to each horse-power, and the teeth should be 5 or 6 inches apart. Obviously, speed 

 requires power; and many small mills consume all the power at hand to produce speed, having none 

 left for the saw to cut. 



More speed is required for sawing hardwoods than for sawing softwoods. 



(d) Proper qualities of a saw. 



1. The usual thickness is 7, 8, or 9 gauge. Finer gauges (10 or 11) are compatible with first-class 

 equipment. Frequently the center is heavier than the rim. 



2. There should be a sufficient number of teeth for the amount of feed. 



Each tooth must cut as much as is offered to it at a revolution. 



To cut a gash of one inch into the log, one may use, for example, either: — 



8 teeth, cutting Vs inch each, or 

 16 teeth, cutting Vm inch each, or 

 32 teeth, cutting Vnj inch each 



The number of teeth for one inch of feed should be, in hard timber, 12 teeth, and in soft 

 timber, 8 teeth. 



The distance between the points of consecutive teeth varies between 7 inches (maximum) 

 and 3 inches (minimum possible). 



The usual feed is from 1 inch to 6 inches per revolution. The quicker the feed, the more 

 teeth and the more power is required to do the work. 



3. The saw must be hung perpendicularly, and must slip easily on the mandrel against the fast collar, 

 so as not to twist out of true when the loose collar is tightened up, thus causing it to buckle. 



The loose collar is hollow at the center (small saws excepted) and has about 6 inches 

 diameter and 74 inch rim. 



By pressing a layer of writing paper between the collar and the saw, the latter may be 

 slightly bent toward or away from the carriage. 



4. The saw must be evenly set (either spring or swage set). The teeth, filed square (not to a 

 point but to a cutting edge), must form an exact circle and must retain that form in the course 

 of operation. 



