By these machines the shells of the blocks are com pletely formed, and they are next polished and finish ed by hand labour ; but as this is performed by tools and methods which are well known, it is needless to enter into any explanation : the finishing required being only a smoothing of the surfaces. The machines cut so perfectly true, as to require,no wood to be removed in the finishing ; but as they cut without regard to the irregularity of the grain, knots, &c. it happens that many parts are not so smooth as might be wished, and for this purpose manual labour alone can be em ployed.
The lignum vita' for the sheaves of the blocks, is cut across the grain of the wood by two cross cutting saws, a circular and straight saw, as before mentioned. These machines do not essentially differ in their prin ciple from the great cross cutting saws we have de scribed, except that the wood revolves while it is cut ting, so that a small saw will reach the centre of a large tree, and at the same time cut it truly flat. As the limits prescribed our Plates will not admit of giving drawings of these machines, and as the idea which could be derived from a verbal description would not be materially different from the cross cutting saws be fore mentioned, we shall defer any farther account of them till a future opportunity. These machines cut off their plates for the end of the tree, which are ex duly the thickness for the intended sheave. These pieces are of an irregular figure, and must be rounded and centered in the crown saw.
The crown saw is represented in Plate LX. Fig. 2. ( where A is a pulley revolving by means of an endless 6 strap. It has the crown or trepan saw a fixed to it, by I a screw cut within the piece, upon which the saw is fixed, and which gives the ring or hoop of the saw I sufficient stability to perform its office. Both the pul ley and saw revolve together upon a truly cylindrical tube b, which is stationary, being attached by a flaunch c to a fixed puppet B, and on' this tube as an axis, the saw and pulley turn, and may be slid endwise by a collar fitted found the centre piece of the pulley, and having two iron rods only one of which can be seen at d in the figure,) passing through holes made through the flaunch and puppet B. When the saw is drawn back upon its central tube, the end of the latter pro jects beyond the teeth of the saw. It is by means of this fixed ring or tube within the saw, that the piece of wood e is supported during the operation of saw ing, being,pressed forcibly against it by a screw D, acting through a puppet fixed to the frame of the machine. At the end of this screw, is a cup or bason
which applies itself to the piece of wood, so as to form a kind of vice, one side being the end of the fixed tube, the other the cup at the end of the screw D. Within the tube b, is a collar for supporting a central axis, which is perfectly cylindrical. The other end of this axis (seen at";) turns in a collar of the fixed puppet E. The central axis has a pulley F, fixed on it, and giving it motion by a strap similar to the other. Close to the latter pulley a collar g is fitted on the centre piece of the pulley, so as to slip round freely, but at the same time confined to move end wise with the pulley and its axis. This collar receives the ends of the two iron rods d. The opposite ends of these rods are, as above mentioned, connected by a si milar collar with the pulley A of the saw a. By this connection, both the centre bit, which is screwed into the end of the central axis f, and the saw sliding upon the fixed tube b, are brought forward to the wood at the same time, both being in rapid motion by their respective pulleys. The power to bring them for ward is communicated to the machine by a bent le ver H, having a handle h at the end, and at the other end a fork, which receives the two ears or pins pro jecting from a collar i, fitted on the central axis, so that the rotatory motion is not interrupted ; but the collar cannot slip endwise upon the spindle. At k is a spring of sufficient strength to counterbalance the weight of the handle, and draw both the saw and cen tre bit back. In 'this state, the workman takes a piece of wood and places it against the end of the fix ed tube b, so as to be nearly concentric with the saw ; then by turning the screw D, he advances the cup at the end of that screw, so as tb hold the wood fast ; this being done, he depresses the handle h, and, as above de scribed, brings the centre bit and the saw both to gether against the wood. The former bores the cen tral hole, while the latter pits out the circular peri phery of intended sheave : then raising the han dle h, the saw and borer retreat, and the round piece of wood may be removed to make way for another rough scantling.