SAWMILL. 603 SAWMILL. mill was the sash sawmill, whoso opnpral con- struetiou and operation aro shown hy Fifj. I. In this the saw, which is simply a ]iropcrly toothod straight band of steel, is strained taut by means Cross Head '■■■Suides Copnec-Hrxj Rod Fig. 1. SASH sawmill. of the rectangular frame or sash, and this sash is given a vertical reciprocating movement between upright guide timbers by means of a connecting rod whose lever end extends to a crank on one end of an engine or water-wheol shaft. The log to be cut is fed endwise against the saw by means of a traveling carriage. In usual practice the sash sawmill makes about 150 strokes per minute and produces about 2000 feet. Ixiard measure, in ten hours. The next de- velojiment in sawmills was the invention of the 'muley' sawmill (Fig. 2), the chief merit of which, compared with the sash sawmill, was the great reduction in the weight of the reciprocating parts. The saw is clamped to two light cross- heads, one at each end, which work up and down, but is not strained or kept taut by tension as it was in the sash in the earlier sash sawmill. To keep the saw straight in its movement, up])er and lower guides, aided by the cross-heads and the log itself, were depended upon. The muley sawmill was followed by cir- cular sawmills. In the circular sawmill the saw is a circular disk of steel with teeth on its edge. This is mounted on a shaft which is given rapid rotary motion by gearing or belting operated hy a water-wheel or steam engine. The saw jjrojects some- thing less than half its diameter above the frame or carriage on which the log is placed and fed endwise against the teeth. The circular sawmill gave a con- tinuous cutting motion of from GOOD to 9000 feet per minute, with which great advantage, however, it combined a num- ber of disadvantages. Its rigidity or capacity to maintain a true plane of rotation decreases with the diameter of the saw, or, in other words, with the depth of cut, and this is obviously just the reverse •of the requirements. The only wa.y to increase its rigidity is to increase the thickness of the Vol. XVII.— 3». di.sk, and this means the cutting of a wider gash or kerf and a waste of a greater portion of the log in sawdust. All things consiilered, it was found impracticable to employ a circular saw much exceeding six feet in diameter. Less than half of this diameter is the cutting depth of the saw. To saw logs of gi'eater diameter than about two ftet, there- fore, it is necessary to emi>loy two saws, one mounted above so as to cut a kerf downward into the log and the other mounted in the ordi- nary way to cut a kerf upward to meet the kerf formed by the upper saw. It has been estimated by reliable authori- ties that tlie kerf waste with circular saws is about 20 ])cr cent, greater than with the band sawmill, which succeeded them in the order of development. The band sawmill (Fig. 3) was known hing before the circular sawmill had come into general use, but its adoption was delayed for many years Carriaofff _a J[ "Cross Head Fia 2. 'MULEY' SAn'. LL. FlO. 3. AMEBICAK BAND SAWMILL. by the dilTiculty of making saws which would endure under the severe service. When once it was possible to secure durable saws the develop- ment of the band sawmill was exceedingly rapid.
