The immense velocity with which the wheels re volve, and the great weight with which their periphe ries are loaded, would make it dangerous to the workmen or byestanders, if by the violence of the centrifugal force, any of the blocks should happen to be thrown off from the rim of the wheels ; to prevent the possibility of such an accident, an iron cage or guard isplacedbetween the workman and the machine.
The shell of the block being now mortised and completely shapen, the last operation is performed by the Scoring Machine, which, by means of cutters, scoops out a groove round the longer diameter of the block, deepest at the ends and vanishing to the central hole for the pin on which the sheave turns. The intention of this groove or channel is to receive the hempen or iron strap which surrounds the block. The only thing that now remains for completing the shell, is the removal of the little roughnesses from the surface, and giving to it a kind of polish, which is done the band.
The SHEAVES. The wood generally used for making sheaves is lignum vitae ; but iron or bell metal have occasionally been substituted for this wood. An attempt was made to introduce sheaves of a kind of porcelain, which answered well enough for some particular purposes, but were not to be trusted in situations where they were liable to sud den jerks and irregular motions. In the navy they are almost invariably of lignum vitae, a few perhaps of ebony. The machinery employed for making this part of the block, consists of a Circular Saw by which the log is cut into plates of the thickness re f quwed for the sheaves, according to their several dia meters. These plates are next carried to a Crown Saw which bores the central hole, and, at the same time, reduces them to a perfect circle of the assign ed diameter. The sheave, thus shaped, is next brought to the Conking Machine, a piece of me chanism, not inferior in ingenuity to the Shaping Machine, for the shells. It would be useless to at ' tempt to describe, by words, the movements of this engine, but the effect of the operation is singularly curious. A small cutter, in traversing round the central hole of the sheave, forms a groove for the insertion of the coak or bush, the shape of which is that of three semicircles not concentric with each other, nor with the sheave, but each having a centre equally distant from that of the sheave. The manner in which the cutter traverses from the first to the second, and from this to the third semi circle, after finishing each of them, is exceedingly curious, and never fails to attract the particular notice of visitors. So very exact and accurate is this
groove cut for the reception of the metal coak, and so uniform in their shape and size are the latter cast in moulds, that they are invariably found to fit each other so nicely and without preparation, that the tap of a hammer is sufficient to fix the coak in its place. The soaks are cast with small grooves or channels in the inside of their tubes, which serve to retain the oil or grease, without which it would soon ooze out, and the pin become dry.
The sheave, with its coak thus fitted in, is now taken to the drilling-machine, which is kept in con stant motion. In casting the coaks a mark is left in the centre of eaoh of the three semicircles. This mark is applied by a boy to the point of the moving drill, which speedily goes through the two cooks and the intermediate wood of the sheave. A copper pin, cut from wire, of the proper length and thickness, is inserted into the holes thus drilled. And the sheave is then taken to the rivetting-hammer, which is some • thing like a small tilt-hammer, and can easily be made to strike on the pin with greater or less velo city, according as the workman presses with more or less force on the treadle. The rivetting being perform ed, the next operation is that of broaching the central hole on which the sheave turns, by means of a steel drill or cutter.
The last process is that of turning a groove for the rope to run in round the periphery of the sheave, and this operation is performed by a lathe, which is so constructed, that while this grove is cutting round the rim of the sheave, another part of the engine is turning smooth the two surfaces or faces of the .sheave; and this lathe can be made to adapt itself to sheaves of different diameters.
The shell and the sheave being now completed, there remains only the iron pin, which, passing through the two aides of the former, serves as the axis on which the latter turns within the mortice. These pins are also made, turned, and polished by engines for the purpose, so that, with the exception of strapping by rope or iron, the whole block is completed at the wood-mills. It may here be re marked, that the French, in the dock-yard of Brest, have long been in the practice of making blocks by machinery ; but they have not attempted anything like a Shaping-Machine, nor any substitute for it ; the external shape of the shell being made entirely by hand; nor have they such a Cooking Machine as that invented by Brunell. The machinery at Brest is put in motion by horses. (See Docx-Yaan.)