Gunpowders now in actual use may be divided into grain powders and compressed powders. The granulation of grain powder is effected by passing the press cake between revolving toothed rollers of gun-metal ; in the machine, Figs. 608, 609, 610, 611, and 612, A is the hopper with raising arrangement, B endless band, C C rollers, D D short screens, E E long screens, F G boxes for dust and grain, H box for "chucks." The four pairs of rollers C are arranged in a slanting direc tion, one above the other. These are set in the two strong side frames of gun-metal. Each pair is adjusted at the proper distance apart by set screws ; but the back roller of each pair works in a sliding bearing, kept up by a weighted lever to admit of the rollers opening out and admitting an excess of material to pass through without injury to the machine ; the two upper pairs of rollers have coarser teeth than the lower pairs. Slanting rectangular screens D'D aro placed from underneath each of the three upper pairs of rollers C C to the top of the next, to convey any fragments which escape proper crushing in one pair into the teeth of the next pair. Underneath the whole is a long rectangular frame carrying two long screens E E, Fig. 612, to separate the proper size of powder, and a board E' underneath to receive the dust and carry it down into a tub placed to receive it. Both the abort screens and the long frame are attached to the framework of the machine by strips of lanoewood, and receive a vibratory motion by means of a polygonal wheel, which works against a circular wheel running loose on an ash) fixed ou the screen frames.
The press cake is placed in a hopper at the hack of the machine, and carried up by means of an endless band, as in the breaking-down machine. The first pair of rollers have pyramidal teeth in. high ; the second pair, similar teeth, in. high ; the two lower pairs have chisel-shaped teeth, formed by cutting longitudinal V-shaped grooves and circumferential rectangular ones. The two lower pairs are replaced by smooth ones. When fine grain gunpowder has to be granulated, the arrangement and sizes of mesh of the screens may be easily remem bered, as only one size of powder is required, and the size is defined by the fact that it must pass through one mesh, and not pass through another. The short screens therefore between the rollers are covered with the size of mesh through which the grains must pass, to allow all fragments of the right size to fall through, but which carry on the pieces which are too large to the next pair.
The upper screen of the long frame underneath is covered with the same wire ; the lower one with the size of mesh wire on which the grain required must be retained.
Attention must be paid to the angles at which the different screens are placed. This varies in different machines, and the proper inclination can only be ascertained by experiment. Some frag
ments of too large a size will escape all the rollers, and consequently require a third box to receive them. These pieces, or "chucks," require to be passed through the machine again. When the hopper has reached the limit of its travel upwards, a clutch is relieved which stops it, and a hell is rung in the watchhouse, where the workmen then re-enter and place the grain and dust in tubs ready for transmission, the for mer to the dusting houses, the latter to the mills for rework ing.
The machine used for making compressed powder is shown in Figs. 613, 614, and 615. There are two rams, an upper one A to press the pellets, and a lower one B to raise and lower the cross-head C. The upper cross head is connected with a lower one by powerful side rods D D of wrought iron. On these rods there are stops E E, so adjusted as to admit of the upper cross head being brought down only far enough to admit the upper punches to touch closely the mould plate, and close the openings of the moulds. F is the upper plate fixed to the cross-head, and carrying the upper punches. G G is the lower double plate, which is fixed in the revolving table, and which carries the lower punches hang ing loose in it. A (Fig. 614) is a circular revolving table carry ing four mould plates, each containing 200 moulds. B B are the top cross-heads of the rams. There being four mould plates, two are being pressed at the time the other two are being filled. Thus 400 pellets are pressed at one time. The revolving table consists of a framework of gun-metal, having the top boarded over to prevent the powder falling down into the machine. It is traversed round by hand by means of a rack and pinion. The mould plates are of gun-metal, being made double, as in Fig. 615. Here A is a small charge of powder placed in the mould, the bottom of which is closed by a tightly-fitting steel punch B fitting it accurately. This punch hes a shoulder on which it rests loose on a plate C. The lower end of B rests on the upper surface of the hydraulic ram D, and an upper punch E of larger diameter than the mould brought down on the surface of the mould plate ; the punches are blued to prevent oxidation ; a pellet is made by bringing the top punch down on the plate and fixing it there so as to confine the powder, then by raising the lower punch by means of the ram till a proper amount of compression has been given to the powder, the pressure is stopped from beneath, and the upper punch raised, the finished pellet being raised out of the mould by the pressure of the ram underneath. Any form can be given to the pellets by altering the shape of the moulds and punches, and hol lows or perforations can be made in the pellet if required.