GRINDING. The operation of shaping or smoothing 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 eases where, from the hardness of the material, or for other reasons, filing is inapplicable. 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 into 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 imbedded in soft iron. When large flat surfaces are required, they are obtained by first working two pieces of the sub stance 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 finished in this manner; also sur faces of cast iron where accurate fitting is re quired, the iron surface being either prepared with a planing-machine or by turning in a lathe. Sockets and other bearings which require to be fitted with great accuracy are usually finished by grinding together. For brass and hell-metal pow dered pumice-stone is best adapted for such pur poses. 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 grind stones. Its principal applications arc in the grinding of the points of needles and forks, the surfaces of gun-barrels, and in finishing steel pens. See GRINDING AND CRUSHING MACHINERY; CIITLERY.