GRINDING AND CRUSHING. 290 GRINDING AND CRUSHING. shallow pan, into which the materials to be crushed are thrown. The crushing is done by the heavy wheel passing over the material. Jaio crushers, or reciprocatin;/ crushers, are one of the kinds most commonly used. They consist of two jaws, one fixed and one movable, so ar- ranged as to form a V-shaped opening between them. This V-shaped jjiw is not closed at the bottom, and one of the jaws is hinged at the top and so connected with suitable mechanism that it is alternately pushed toward and drawn away from the fixed jaw, thus alternately en- larging and reducing the V-shaped opening be- tween the two. The material to be crushed, stone or ore, is inserted into the top of the jaw open- ing and is crushed bv the reciprocating motion of the movable jaw until fine enough to run out of the bottom opening. by a vertical shaft which has simple rotary mo- tion without gyration. tiansage-miU crushers consist of a longitudinal shaft carrying radial teeth working iu.sidc .a cylindrical .shell, from whose inner surfaces pro- ject radial teeth between which the shaft teeth niesh during revolution. All of these principal txpes of crushers are made in several forms, for each of which special qualities are claimed by the manufacturers. Stamp mills are almost ex- clusiveh' used for crushing metal-bearing ores: roll and edge-runner crushers for crushing fibrous materials; and jaw and gj^ratory crushers for crushing ores, rock, cement, phosphate, and similar hard materials. Grinding Machines. In many processes crushing is simply a preliminan,' process to grinding to powder or flour. The machines for grinding are necessarily of a diff'erent character, Fig. 2. VEKTICAL SECTION OP JAW CRUSHER FOH STONE. Gyratory crushers consist in respect to the crushing elements of an inverted hollow frus- tum of a cone, inside which an upright frustum of a solid cone receives a gyratory motion from a vertical shaft liinged at tlie top and connected with suitable propelling mechanism at the bot- tom. The material is crushed in the annular Fig. 3. vertical section of gyratohy crusher for stone AND ORES. opening between the walls of the two frustums of cones. A coffee-mill crusher resembles a gyra- tory crusher except that the solid as well as the hollow frustum is inverted and is separated Fig. 4. VERTICAL section emery stone mill for fine QRIKDING. and are usually of less massive construction. Grinding machines are commonly called mills, and this term will be used here. The first form of real grinding machine was, perhaps, the mill- sione mill, and this still remains one of the most common forms in use to-day. A millstone mill consists of two circular stones set face to face, one of which revolves on a vertical axis, while the other is fixed. Usually the upper stone is the movable one or runner, and has a hole at the centre through which the material to be ground is fed, and after being subjected to attri- tion between the two stones emerges at the edges of the two stones and falls into a receiver below. The grinding faces of the two stones are dressed into grooves and ridges of various forms and arrangements to facilitate the grinding and the movement of the powder toward the edges of the stone. Sometimes the lower stone is made the runner, and for special purposes the stones arc set ver- tical instead of horizontal. Millstone mills are extensively used in grinding grain, cement, phos- phate rock, etc. IScill mills are of two prin- cipal types ; in one tyjx" balls of comparative- ly large size travel around a fixed track in a definite path : in the other the balls are loose in a rotating drum. Usually the balls are of hardened steel, but in some forms of mills flint
