MILLS WITH GRINDING ACTION.
Conical 7 (pi. 7) shows a modification of the ancient Roman mill: a massive conical stone revolves in a hollow cone, the mate rial to be comminuted being conveyed through the narrow spaces between the two surfaces either by gravitation or by centrifugal force. A is the hopper, and B is a shaking-spout for introducing the material with some regularity in the space between the runner (C) and the collar-stone (D). This space narrows from above downward, and can be contracted at will by screws, as seen in the Figure. The smaller the particles of material become by attrition, the farther they sink down, until they filially arrive in the annular collecting-reservoir, which (at E) is provided with a gutter for the discharge of the ground product. The runner (C) sits firmly upon the spindle (a), which is rotated by the bevel-gearing (c, a') from a horizontal driving-shaft, which, as indicated iu the Figure, can be moved by means of a crank (e, j), or by a pulley, in place of the crank, driven by a belt from a transmitting-shaft. In the first case it is advisable to provide the fly-wheel (b) in order to obtain a uniform motion.
The revolving-shaft of the runner may also be arranged horizontally. In the latter case, however, the velocity of revolution must be sufficiently great to enable the resulting centrifugal force to convey the particles of material from the thinner to the thicker end of the runner. In the same instance the driving by means of belt and pulley is also presupposed. Ver tical conical mills were used by the Greeks and Romans (p. 31), though the arrangement was the reverse—that is, with a stationary central cone and a revolving hollow cone. At the present time conical mills are much used for grinding colors, coal, coffee, and other materials. Instead of stone, the two grinding bodies are frequently constructed of wrought iron, steel, or cast iron (chilled), and are generally provided with sharp-edged ribs; so that the grinding effect is supplemented by a shear-like action.
Portable of the numerous portable mills largely used on the farm for grinding the usual grains into feed is the mill (6/. 8, fig. 6). The cut shows it as having a connection with a horse power, and is geared 56 to r. It will grind from fifteen to twenty bushels per hour by one pair of horses. It is fitted with patent double-reduction hardened-iron grinding-burrs, each pair capable of grinding' from one thousand to three thousand bushels of grain before renewal becomes necessary. The arrangement, position, and purpose of the parts are so clearly set forth in the illustration that a description iu detail is needless.
Thtsc mills, constructed chiefly of iron and steel, are simple, and are, there fore, easy to operate. The grinding-plates are protected against injury from hard foreign substances by safety-pins of wood easily replaced. The dress of the burrs is such as to avoid clogging, and at the same time prevents the passage of ungiound grain.
The "Eccentric" mills (/l. 8, 1-5), invented half a century ago by James Bogardus, have superseded millstone-mills fur a variety of purposes. Their introduction opened up new possibilities in the milling industries and accomplished results never before attempted. The name "eccen tric" is derived from the action of the grinding-plates. These plates revolve in the same direction with nearly equal speed, but as their axes are not on the same centre, there is secured an unlimited variety of move ments over their entire working-surfaces. Figure r shows the concen trically-grooved face of the plate (b) surrounding the ingathering curves (c) at the centre. Figure 2 is an edge-view of the plates, exhibiting also the pins by which they are driven, and Figure 3 is a plan of the superposed plates. This disposition of the grinding-surfaces admits of an equal pres sure over all parts of the plates, which, on account of the longer travel and the free discharge of the materials, give better results in the finished product than do stones run concentrically. When worn, the plates, which are made on the interchangeable system, can be readily replaced by new ones without stopping the machine every few days to recut the stone, as is necessary in millstone-mills. Figure 4 shows a machine which is intended for grinding dry substances; it is called the "dry" mill, in contradistinction from the " wet " mill exhibited in Figure 5. In the former the materials pass through the eye (c) of the upper plate (fig. I) to the grinding-surfaces, and escape at the peripheries of the plates within the case, and thence through the spout at the front of the machine. The grinding adjustment is effected by a hand-screw and lever. A weighted lever is further pro vided for the purpose of relieving the plates should there pass unnoticed substances too hard to In ground. In the wet mill (fig. 5) the position of some of the parts is reversed. The top plate is driven and a cylindrical hopper is formed on the upper part of the shaft, which is perforated, and through which the fluid is conducted by gravity from the hopper to the grinding-plates. The material escapes at the periphery of the plates, from which, as it oozes, it is removed by a scraper crossing the joints, and falls into a receptacle beneath.