GRINDING, a mechanical process in which the desired results are produced by frictional abrasion. The term includes also polishing, huffing and lapping. This process is of exten sive use in various mechanical arts, as in grind ing corn, ores and colors, in which cases the object is to reduce the materials to a fine powder; or in grinding the metals, glass and other hard substances for the purpose of giv ing them a certain shape, figure, or polish, or a. sharp cutting edge. In the first case the grinding is effected by passing the material be tween rough stones, as in the common flour mill (see Fioua) or by a heavy stone or iron cylinder revolving upon a smooth plate. Chicory, chocolate, plwnbago for pencils and a variety of other substances are ground by pairs of iron or stone rollers revolving on a slab in such a manner that they not only merely roll but also rub. on the surface of the slab. A knife or scraper follows one roller and precedes the other, scooping the paste into the position required to come fairly under the roller which follows it. Colors are ground in small quan tities with a muller and slab. The muller is a heavy piece of stone of conical shape, and which rests its base on the slab and is grasped by the hands; the color is mixed to a pasty consistence with the desired medium of oil or water, and rubbed between the two surfaces until smooth and impalpable. The grinding of cutlery.and tools is effected by means of the grindstone; glass lenses and metal specula are ground to shape with emery-powder laid on a Metal tool. Ornamental glass is ground into facets or otherwise by means of stones and lap wheels. Diamonds and other precious stones are cut or ground with diamond dust embedded in soft iron. Large flat surfaces are obtained by first working two pieces of the material nearly flat and then laying the one upon the other and grinding their surfaces together with sharp sand, emery or other cutting powder. Plate-glass is flattened in this way; also sur faces of cast-iron, where accurate fitting is re quired. Sockets and other bearings which re quire to be fitted with great nicety are usually finished by being ground together with a fine abrasive. For brass or bell-metal pumice-stone is employed in such cases, as emery is apt to embed itself in the metal and give it a perma nent abrading action on the bearings. Dry grinding is the term applied to the grinding of steel with dry grindstones. The points of
needles and forks are produced by this means, also the finishing of steel pens and the surface of gun barrels. The men and women engaged on this kind of work suffer painfully from irritation of the throat and nostrils caused by the fine, dust-like particles that fly off from the work and float in the air. These difficulties have been mitigated in recent years by the use of mouth-pieces of damp cloth, and the provi sion of air-blasts to dispose of the dust. The tools which are most commonly employed in grinding are the wheel, the disc and the lap. They may be made either of natural abrasive material, as the grindstone and whetstone, or built up out of crushed abrasives, either natural or artificial, molded into the desired shape with a bond. The wheel has an advantage over the disc, in that the cutting surface moves at a uniform speed, which can be suited exactly to the work in hand: with the disc, the outer edge moves fastest, and the section at the central part very much slower. Grind-stones are run generally so that the cutting face moves at a speed of from 500 to 600 feet per minute for grinding axes and carpenters' tools, and from 800 to 1,000 feet peripheral speed for machin ists' tools. For grinding cutlery, speeds of 2,500 to 4,500 feet per minute are not uncom mon. Molded grinding wheels are usually run so as to give their peripheral surface a speed of 5,000 to 7,000 feet per minute. A small wheel must obviously make more revolutions than a large one to give an equal cutting effect. For fast cutting, a soft wheel of comparatively coarse is used, and is run at a lower speed: in the case of soft steel it is sometimes run as slow as 25 feet per minute. Too high a speed for the work in hand will cause the wheel to glaze, and then to heat. This heat is imparted to the work, and softens it, lead ing to irregular cutting. Moreover, a glazed wheel is likely to be applied with increased pressure to secure the cut required, and this, with the heating, tends to distortion. It is customary in grinding to use some liquid to increase the efficiency of the tool, and so to reduce the time of operation. Water is used on the common grind-stone. With other abrasives various liquids are employed — lard oil, machine oil, kerosene, gasoline, turpentine, alcohol, soda water, etc.