MILLS WITH CRUSHING ACTION.
primitive mealing implement is not only the earliest known grinding device, but is also the simplest form of a ball-mill, whose action is chiefly reciprocatory, though slightly rotary. To grind gold-ore the Coreans of to-day spread it on a level bed of rock, and over it roll huge cobble-stones or boulders. The development of this simple principle of pulverizing is exhibited by the modern ball-mills, which, however, are driven by motive-power through the medium of gears, or pulley and belt, or both, and depend on gravity only for the requisite pressure upon the materials.
The ball-mill (p. 6, figs. i, 2) consists of a number of metal balls confined in a slowly revolving case, which also contains the material to be ground. In the side of the case there is an opening, through which the material is both introduced and removed, and which during the ope ration of grinding is closed by a door held firmly in place by a bar and bolt. By gravitation the balls seek the lowest position in the case, but the revolving of the case imparts to than a rolling; motion, and their weight exerts a effect on the material beneath and between them, and at the same time a grinding action by a sliding contact with one another and with the inner surface of the casing. The quantity of mate rial to be ground at one time is limited, but by prolonging the process any degree of fineness may be secured.
Modified ingenious modification of the ball-mill is shown in Figure 3 (p1. 6), in which a pocketed ring replaces the balls in a revolving case. As the case revolves the ring follows its revolutions, rolling upon the material, which at the same time, entering the pockets of the ring, is scooped up, carried over, and discharged in front of the ring, and is thus repeatedly acted upon until the desired reduction is obtained. A door or opening in the periphery of the case is provided for the introduction and discharge of the material. This machine is called the "silent" mill, because it is driven without gearing, the motive-power being applied by means of a spool on the driving-shaft, in which spool the case fits and on which it revolves by contact.
In the indigo-mill (fig. 4) the pulverization of indigo or other sub stances of like nature is effected by the pressure of a number of smooth cast-iron balls, which, being rolled over and on the indigo, crush it into powder by their weight, and mix it also into paste when the proper liquid is added. Through the medium of the bevel-wheels (E) on a hori zontal and a vertical shaft rotary motion is imparted to a driver (C, C), whose pendent fingers push the balls around. Motion is imparted to the machine by hand applied to the crank (B), and is regulated by the arm (F) with heavy balls at its ends, the whole being properly supported and car ried on a wooden framework.
Mills, or Chasers, as they are generally called, are more effective in the pulverization of material, and with continuity of action they have a larger capacity of production than ball-mills. The early mills of this kind were turned by men or animals (p. 3l); the force was applied directly to the shaft, which extended beyond the grinding-bed, as shown in Figure i (p1. 2); they, were used for grinding grain and for crushing olives. The so-called " Chilian mill " 6, fig. 5) exhibits the rude construction of a chaser still used in sonic of the more remote districts of Mexico, Central America, and Peru. Modern and more effective forms of chasers are shown on the Plate. Figure 7 shows two heavy cylinders, differently distanced from the centre, which revolve upon a horizontal shaft (a) and are guided in their circuit by the vertical shaft (b) by which they are driven. A pin in the slot of the vertical shaft prevents the shaft (a) from moving laterally, but it is free to move vertically in the box formed on the shaft b. Thk freedom of motion permits the chasers to rise over the material during the action of crushing.