With the increased diameter and peripheral speed of the circular saw, however, a grave difficulty presented itself. The saw would heat at its periphery, and its rim portion expanding without commensurate expansion of the central portion, would cause the saw to crack and fly to pieces under the tremendous centrifugal force. This difficulty is provided for by what is known as “hammering to tension,” i. e., the saw is hammered to a gradually increasing state of compression from the rim to the center, thus causing an initial expansion or spread of the molecules of metal of the central parts of the saw, which is stored up as an elastic expansive force that accommodates itself to the tension caused by the expansion of the rim, and prevents the unequal and destructive strain, due to the expansion of the rim from the great heat of friction in passing through the log.
Mounted upon a portable frame, this machine was put to its great work upon the logs in the forests of America, and for many years this type of sawmill held its sway, and an enormous amount of work was done through its agency. Among its useful accessories were the set-works for adjusting the log holding knees to the position for a new cut, log turners for rotating the log to change the plane of the cut, and the rack and pinion feed, by which the saw carriage was run back and forth. Following the rack and pinion feed came the rope feed, in which a rope wrapped around a drum was carried at its opposite ends over pulleys and back to the opposite ends of the carriage, which was thereby carried back and forth by the forward or backward movement of the drum.
FIG. 242.—DIRECT-ACTING STEAM FEED SAWMILL CARRIAGE.
The greatest advance in sawmills in recent years, however, has been the steam feed, in which a very long steam cylinder was provided with a piston, whose long rod was directly attached to the saw carriage, and the latter moved back and forth by the admission of steam alternately to opposite sides of the piston. This type of feed, also known as the shot gun feed, from the resemblance of the long cylinder to a gun barrel, was invented about twenty-five years ago, by De Witt C. Prescott, and is covered by his patent, No. 174,004, February 22, 1876, later improvements being shown in his patent, No. 360,972, April 12, 1887. The value of the steam feed was to increase the speed and efficiency of the saw, by expediting the movement of its carriage, as many as six boards per minute being cut by its aid from a log of average length. An example of a modern steam feed for sawmill carriages is seen in [Fig. 242]. With the modern development of the art the ease and rapidity of steam action have recommended it for use in most all of the work of the sawmill, and the direct application of steam pistons working in cylinders has been utilized for canting, kicking, flipping and rolling the logs, lifting the stock, taking away the boards, etc.
FIG. 243.—METHOD OF SHAPING AND HOLDING LOG FOR QUARTER SAWING.
Beautifully finished furniture in quartered oak has always excited the pleasure, and piqued the curiosity of the uninformed as to how this result is obtained. [Fig. 243] illustrates the method of sawing to produce this effect. The log is simply divided longitudinally into four quarters, and the quarter sections are then cut by the vertical plane of the saw at an oblique angle to the sawed sides, which brings to the surface of the boards the peculiar flecks or patches of the wood’s grain so much admired when finished and polished.