Back of the saw, in a log-mill, there is, or should be, what is known as a spreader or split ter wheel, which in the best makes is thinner in the middle than near the edge, to lessen fric tion. The shaft bearing the splitter is supported in hangers, and on it is a large roll, which supports the lumber passing over the frame ; but the roll and the splitter plate rotate inde pendently of each other, this arrangement preventing accident by reason of a heavy stick of timber resting on the shaft, preventing the splitter-wheel from turning.
The carriage of a circular saw-mill of the first class consists essentially of two long side sills or timbers, framed together by iron cross beams above, and which bear on its under side iron facing pieces, which bear on rollers placed at suitable distance on cells in the floor of the mill. Carriages are usually made in sections of about 15 ft. in length, and fastened together by rods and dowels. The side piece nearest the saw bears on its under surface a rack that engages with a pinion by which lengthwise feed of the carriage and log are given, driving the saw through the log.
In some mills this rack-and-pinion feed is dispensed with and a rope feed is used ; in others the carriage is connected to the piston-rod of a long steam-cylinder, and admission of steam drives out the piston and forces the carriage along by direct action at a marvellous rate of speed ; this constitutes what is known as a " shot-gun feed." Lengthwise of the carriage. on the side furthest from the saw, is what is known as the set-beam, which is prevented from springing up by suitable projections engaging with the under sides of the cross pieces of the carriage. To this set-beam there are attached the various head and side blocks and uprights to which the log is attached or against which it rests. The set-beam, blocks, uprights, and log are given traverse across the carriage by slight advances each time that the saw has made a cut and the carriage is drawn back ; the rate of withdrawal being much more rapid than that of feed, even with the shot-gun feed. The set-beam is advanced only a slight degree after each cut ; and in large mills it is retired by power to make room for the next large log after one has been sawed down to the last board.
The rack-and-pinion carriage feed has the disadvantage that the teeth of the rack and pinions are liable to break, causing annoyance and delay. 'l'o lessen this trouble, it is necessary to increase the width of face of the gears, which of course adds to the weight of carriage. Where rope feed is used, there are several ways of effecting the winding up of the rope. In one of them, which may properly be called a rope and gear feed, the rope sheave is made in the form of an internal gear, having the cogs or teeth on the inside and the spiral groove for the rope outside. This sheave is keyed to a short shaft, which runs in boxes bolted to the timbers underneath the carriage and directly opposite to the mill frame. It is rotated IT a feed pinion which runs in the internal gear in the same manner as it would in the rack of the carriage.
Sonic sawyers prefer trucks on the carriage and tracks on the floor, but this has disad vantages, in that tracks on the floor obstruct the floor itself, and dirt on them is readily accumulated and is likely to throw the carriage otf the track or lift it on one side, thus making an irregular cut. A carriage with the track on its under side is lighter than one bearing trucks; it runs more easily; the rolls may lie more readily kept in line and level than a track ; the chairs which bear them may be set on a level with the floor of the mill, enabling it to be crossed with barrows, et c. ; they are more durable, because only such rolls as the car riage passes over rotate, while where they arc on the carriage every one turns ; they are more readily replaced when worn, and are more economical, because when those opposite the saw frame, which arc most used, are worn, they can be exchanged for those nearer the ends ; and the back rolls being finished the same as the front ones, can he changed to the front and made to do service as guide rolls.
In the best mills the head blocks and horizontal rests on the carriage are at intervals of 3 to 4 ft. the entire length of the carriage, and uprights which add side support are placed on the set-beam directly over, and at right angles to, the head blocks. This arrange ment does away with the necessity of moving the head blocks when sawing logs which vary in length.