GRINDING, the operation of shaping any hard substance by rubbing away its surface with a rough stone or with a cutting powder. It is similar to filing, and is used in cases where, from the hardness of the material, or for other reasons, filing is inap plicable. Thus cutting-tools and other steel instruments may be filed before hardening and tempering; but after this, if further abrasion is required, they must be ground. Glass lenses and metal specula are ground to shape with emery-powder laid upon a metal tool. Ornamental glass is ground into facets or otherwise by means of stones and lap-wheels. Diamonds and other gems are ground or cut with diamond-dust imbed ded in soft iron. When large flat surfaces are required, they are obtained by first working two pieces of the substance nearly flat, and then laying one upon the other, and grinding their surfaces together with sand, emery, or other suitable cutting powder. Plate-glass is flattened in this manner; also surfaces of cast-iron where accurate fitting is required, the iron surface being either prepared with a planing-machine, or by turn ing in a lathe with a slide-rest. Sockets and other bearings which require to be fitted with great accuracy are usually finished by grinding together. For brass and bell metal powdered pumice-stone is best adapted for such purposes, as emery is liable to imbed itself in the metal, and give it a permanent Cutting action upon the bearings.
Dry grinding is the term applied to the grinding of steel with dry grindstones. Its principal applications are in the grinding of the points of needles and forks, the sur faces of gun-barrels, and in finishing steel-pens. This kind of work produces painful irritation in the throat and nostrils of the men and women who follow it; and although the distressing effects have been very much diminished of late by the introduction of currents of air to carry away the particles of steel, and mouth-pieces of damp cloth, the evil is not entirely obviated; in some branches, such as gun-barrel grinding, it is still very great. Besides this evil, the stones used for gun-barrel grinding. which are
very large, revolve with great rapidity, and occasionally break with great force while revolving, and seriously endanger the lives of the men.
Another kind of grinding, quite distinct from the above, is that of crushing and rub bing a substance into a fine powder. This is effected by passing the substance between rough stones, as in the common flour-mill, or between rollers, either smooth or toothed, according to the degree of fineness required, or by a heavy stone,or iron cylinder revolv ing upon a smooth plate. Colors are ground in small quantities with a miller end ,10?). The mailer is a heavy piece of stone of somewhat conical shape, and which rests on its base upon the slab of stone, and is grasped by the hands, and the color is mixed to a pasty consistence with the required medium of oil or water, and rubbed between the two surfaces until smooth and impalpable. On a larger scale iron or stone cylinders revolve on a slab in such a manner that they shall not merely roll but shall also rub upon the surface of the slab. A knife or scoop follows one cylinder and precedes the other, scooping the paste into the position required to come fairly under the cylinder which follows it. Chocolate, spices, plumbago for crucibles, and a variety of other sub stances, are ground in this manner.