GRINDING MACHINERY 411 GRINDING MACHINERY The surfaces may be smooth, as in the pestle and mortar; abrasive, as in some dise machines; or one or both surfaces may be corrugated and so exert a cut- ting action, as in the ordinary household coffee mill. On a large scale, all these methods of reducing material to a more or less fine state of division are utilized, and the various mechanical devices on the mar- ket merely carry out these processes on a lar^e scale, the particular merit of any machine depending upon the efficiency with which it accomplishes its purpose and the amount of energy it absorbs in doing so. For reducing rocks, coal and similar material from large lumps to fragments of 1 inch mesh breakers are used. These are of various types, the simplest being the Jaw Breaker, which, as the name implies, consists of two jaws, one fixed and the other movable. The movable jaw is attached to an eccentric, and moves backward and for- ward as the eccentric revolves. When the jaw moves backward a V-shaped opening is formed, the rock is fed in, and then the V closes, crushing the rock between the heavy jaws. The Gyratory Breaker consists of a gyrating head on to which the rock is fed and crushed against stationary con- caves. The gyrating head is attached to a vertical shaft, fixed to an eccentric at the base. When the eccentric re- volves, the resulting action is similar to that of a pestle being moved around a mortar by the hand, except that both pestle and mortar are inverted. A modification of the same principle is shown in the Symons Disc Breaker. In this machine, two discs are employee', of very hard steel. The discs are concave, and are set at an angle to each other, with their hollow sides facing. Both discs are rotated in the same direction and at the same speed, but a gyrating motion is imparted to one of them by means of an eccentric, and owing to the angle at which they are set their outer edges are wider apart at one point, and close together at another. The rock is fed through the center of one disc, and as the discs come together it is crushed and discharged. Crushers are usually of the roller type and are much used for reducing ores from 2 inch to ^/4 inch size, or even smaller. The crushers are heavy steel rollers, both of which revolve, but one of which is on fixed bearings while the other is supported on bearings which are held in place by heavy springs and there- fore have a certain amount of "give." The earliest type of pulverizing ma- chine was the "stamp mill," and even to- day it is commonly used. The prin- ciple upon which it works is the raising of a heavy weight by means of a cane, and then letting it fall by gravity on to the material to be crushed. The stamps sometimes weigh as much as a ton and are usually arranged in a battery of five, or more. Some of the modern stamping machines impart crushing force to the stamps by means of steam instead of re- lying on gravity. At the present time the commonest form of pulverizer is the Ball Mill or Tube Mill in one or other of its many modifications. The principle of all these mills is the same. The mil! consists of a revolving cylinder, partially filled with steel balls or flint pebbles. The material to be pulverized is fed into the mill, and as the cylinder revolves the balls or pebbles are carried round and continu- ally fall over one another. The crush- ing action is partly due to the impact of one ball falling against another, partly to the grinding action of the constantly moving surfaces of the balls. Ball mills are of the intermittent or "batch" type; tube mills are continuous, the coarse ma- terial being fed in one end, and the fine material discharged at the other. The Hardinge Conical Mill is a special type of tube mill, and consists of two cones connected by a short cylinder. The feed cone is of very wide angle, the discharge cone of much smaller angle. Pebbles of mixed sizes are commonly used in this mill, and the conical shape brings about a segregating action on the pebbles, the larger sizes being carried to that part of the mill having the greatest diameter, and vice versa. The same segregation occurs, of course, with the material being ground, and the result is that the coarse particles automatically collect in that part of the mill where grinding action is the greatest. The Raymond Pulverizer is of an entirely different type. It de- pends for its grinding action on revolv- ing rollers, which themselves rotate alx»ut a central axis. It also makes use of the principle of air-separation, the finer particles being continuously carried away oy a circulating current of air, and collected in a dust collector, while the coarser particles fall back for further grinding. By this device a very finely- ground product can be obtained, and high grinding efficiency is also claimed for the machine. Another type of mechanical grinder which finds application in the paint and drug trade is the Buhr Stone Mill, which consists of two horizontal stone discs, the lower one of which revolves, while the upper one is stationary. The sur- faces of the stones are furrowed, and the material is fed through the center of the upper stone and is collected at the pe-
