The distances between the saws, and their parallelism with the sides of the frame, are adjusted by means of iron blocks made of the exact thickness required in the slabs of marble, the blocks and blades are placed alternately, and every blade is separately strained by its tightening wedge until it is sufficiently tense, the blocks are sustained between two transverse bars, called gage bars, and are allowed to remain between the blades to give them additional firmness.
The traverse of the saw frame is given by a jointed connecting rod, attached by an adjustable loop to a long vibrating pendulum, that is put in motion by a pair of connecting rods, placed one over the other, and leading from two cranks driven by the engine. All three connecting rods admit of vertical adjustment on the pendulum. The connecting rod of the saw frame is placed intermediately between the other two, but its exact position is regulated by the height at which the saws are working, as it is suspended by a chain and counterpoise weight, which allow it to descend gradually downwards on the pendulum, with the progress of the cut, so as always to keep the connecting rod nearly horizontal.
In the London Marble Works four of these sawing machines of different sizes are grouped together, with the driving shaft and pendulums in the middle, and so arranged that each pair of saw frames reciprocate in opposite directions at the same time, in order to balance the weight, and reduce the vibration.
Another mode of traversing the saw frame sometimes adopted, is by means of a vertical frame that is reciprocated horizontally on slides, and the connecting rod instead of being jointed, is fixed rigidly to the saw frame and slides upon a vertical rod. Various other unimportant modifications in the construction of the machines are also adopted.
One of the most difficult points in the application of these machines, was found to be the supplying of the sand and water mechanically to the whole of the cuts at the same time. This is now successfully effected by the following arrangement. Above the block of marble to be sawn, is fixed a water cistern or trough, extending across the whole width of the frame, and measuring about one foot wide and one foot deep, about 20 small cocks are arranged along each side of the cistern, and a small but constant stream from each of the cocks is received beneath in a little box, a sloping channel leads from every box across the bottom of a trough filled with sand, which mingles with the water and flows out in separate streams that are conducted to each of the saw cuts. In the first construction of this apparatus for the feed, the sloping channels were led straight across the bottom of the sand trough, but it was then found that the water excavated little tunnels in the sand, through which it flowed without carrying the sand down. This difficulty was overcome by leading the channels across the bottom of the trough in a curved line, when viewed in plan. The form of the channels is shown in fig. 1092, which represents four channels cut across the middle of their length, to show their section, from which it will be seen that the channels are made as a series of gothic shaped tunnels supported only on the one side, and open on the other for the admission of the sand; the water flows through these tunnels, and continually washing against the convex side of the channel undermines the sand, which falls into the water and is carried down; to assist this action the attendant occasionally stirs up the sand to loosen it. There is a sand trough and set of channels on each side of the water cistern, so that every saw cut receives two streams of sand and water in the course of its length.
The saws having been adjusted to the proper distances for the required slabs, the saw frame is raised by means of a windlass and the suspending chains attached to the vertical frames, and the block of marble to be sawn is mounted upon a low carriage, and drawn into its position beneath the saws, and adjusted by wedges. The saws are then lowered until they rest upon the block, the counterpoise weights are adjusted, and the mixed sand and water allowed to run upon the saw blades, which are put in motion by attaching the connecting rod to the pendulum. The sawing then proceeds mechanically until the block is divided into slabs, the weight of the saw frame and connecting rod causing them gradually to descend with the progress of the cutting.
To allow the sand and water to flow readily beneath the edges of the saw blades, it is desirable that the horizontal frame should be slightly lifted at the end of each stroke. This is effected by making the lower edges of the frame, which bear upon the guide pulleys, straight for nearly the full length of the stroke, but with a short portion at each end made as an inclined plane, which on passing over the guide pulleys lifts the frame just sufficiently to allow the feed to flow beneath the saws.
For cutting slabs of marble into narrow pieces, such as shelves, and which is effected by hand with grub saws as explained at page 1195, a machine called a ripping bed is employed, in which as many cuts as may be required in the one slab are effected simultaneously, by an equal number of circular saws with smooth edges, revolving vertically, and fed as usual with sand and water. This machine, represented in fig. 1093, consists of a bench about 12 or 14 feet long, 6 or 7 wide, and about 2 feet 6 inches high; upon the top of the bench is fixed two rails, upon which a platform mounted on pulleys is drawn slowly forward by a weight. The horizontal axis carrying the saws revolves about nine inches above the platform, and to ensure the rotation of the saws, the axis is provided with a projecting rib or feather extending its whole length. The saws are made as circular plates, about 17 inches diameter when new. The saws, or cutters, are clamped between two collars about 6 inches diameter, fitted, so as to slide upon the spindle, and be retained at any part of its length by side screws.