In disc grinding the abrasive is commonly glued on cloth discs, and these are attached with cement to the face of the metal disc wheels of the machine. For the finer-grained abrasives paper backing is frequently employed. The speed of a disc grinder is designated by that of the circle at three-fourths of the radius of the revolving disc—outside of this, of course the speed is greater, inside the speed is less. The customary speed for the three-quarter radius ranges from 4,000 to 7,000 feet per min ute, according to the work — the slower speeds for wood finishing, the highest for very soft materials, such as artificial rubber, the metals taking. the intermediate speeds.
Lapping is a grinding operation between the technical grinding and polishing, and is per formed either by wheel-shaped or disc laps. The lap is a piece of soft metal, usually lead into the surface of which has been pressed, usually by rolling, a very fine abrasive, as, for example, diamond dust. After the machine work has been brought by the ordinary methods of grinding to a finish within a few tbonsa[tdrhs of an inch, the exact dimension is secured by the lap. The operation is employed chiefly in the accurate finishing of the interior of bores, as for delicate bearings.
Polishing wheels are often of wood covered with leather to which the abrasive is affixed with glue, or they may be made up wheels of solid leather, canvas, or linen discs clamped together. Felt and paper are also used in the same for special purposes. Wheels for polishing irregular surfaces are made with the outer part pliable so as to follow into cavities readily. Buffing wheels are usually made of cloth, and the abrasive material is applied to them in a mixture of tallow or grease paste.
Sand-jet grinding is a remarkable process, in which abrasion is effected by the percussion of small hard particles on a plain surface. Sharp
silicious sand, varying in hardness and fineness according to the kind of work to be done, is employed in most cases. This sand is impelled by a blast of steam or of air. A hole 1% inch in diameter by deep, has been bored throligh a solid of corundum (the hardest mineral known except the diamond) in 25 minutes by sand driven with steam-power at 300 pounds pressure on the square inch. A diamond has been sensibly reduced in weight, and a topaz altogether dissipated by a sand-jet in one min ute. These results are obtained by causing a sand-stream to mix with a steam jet. The sand passes through a central tube, and steam through an annular tube which surrounds it; a kind of suction acts at the end of the con centric tubes, which draws the sand into the steam jet, and both dash with great force against the stone or other substance to be acted upon, which is placed at about an inch from the mouth of the tube. By the use of flexible jointed connecting tubes the jet can be nifried in any direction, and Grooves, nioldistitV etc., can be produced instead of merely straight cuts or cavities. • By using an air jet instead of steam, and varying the pressure, a design can be engraved on glass, the parts not to be acted upon being covered with the pattern,de "ra of paper, lace, india-rubber or oil paint. See ABRASIVES.
Consult Mown & Sharpe Manufacturing Company's 'Construction and Use of Universal Grinding Machines' and 'Cylindrical Grinding? (Providence, R. L 1914); Colvin, F. IL and Stanley, F. A., 'American Machinist Grinding Book' (New York 1912) ; Guest,J. J. 'Grind ing Machinery) (London 1915) ; Hamilton, D. T, and Jones, F. D., 'Advanced Grinding Prac tie& (New York 1915); Woodworth, J. V., 'Grinding and Lapping' (New York 1907).