Fig. 3151.
The construction of the saw is shown in [Fig. 3151], and it is seen that for half its length, the teeth are formed to cut when the saw moves in one direction, while for the other half the teeth slope in the opposite direction, and are therefore arranged to cut when the saw is on the opposite or return stroke, and the construction whereby the saw is enabled to cut on both strokes is obtained as follows:
Referring to [Fig. 3149], the two slides e, e′, on which the saw-carrying frame f f slides, are not in line or parallel one with the other, but each slide is at an angle of about 85 degrees to the line of feed, so that as frame f is reciprocated at each stroke, one end of the saw advances towards the cut, and the other recedes from it, thus causing the saw to cut first on one half and then on the other of its length, one half cutting on the forward, and the other on the return stroke.
The studs or saw-buckles for attaching the saw to the frame are shown in [Fig. 3151], in place on the ends of the saw, the part i, that fits in the frame f, [Fig. 3149], being squared so that the saw cannot be twisted in tightening up the nuts of the saw-buckle.
The belt works for driving the saw are arranged as follows: at t are the fast and loose pulleys for driving pulley r, the belt passing from t over two pulleys (shown dotted in, [Fig. 3149]), u, u′, whence it stretches to the crank driving pulley r, whose bearing is provided on the cross-head, so that the two move together when the cross-head is altered in height from the work-table or carriage, to accommodate different thicknesses or diameters of logs.
It is obvious that in proportion as the cross-head is set nearer to the carriage, the belt from t to u, u′ would become slack; provision is made however, to prevent this as follows:
Pulley u, is carried on a frame or swing lever x, to which is attached by rope v the weight w, which therefore regulates the tension of the belt.
The cross-head d may be raised or lowered by belt power or by hand, as occasion may require, the usual course being to move it to nearly the required position by belt power, and then complete the adjustment by hand, a graduated scale being provided as shown, whereby the rack can be set to cut the required thickness of plank without measuring the timber.
The belt motion for raising or lowering the cross-head is obtained by the pulleys at y, the wheel for the hand adjustment being shown at y′. In either case the bevel gear wheels z, z′ operate, respectively, a vertical screw engaging a nut on the cross-head.