Machinery's Handbook, 31st Edition
1280
ABRASIVE PROCESSES GRINDING AND OTHER ABRASIVE PROCESSES
Processes and equipment discussed under this heading use abrasive grains for shaping workpieces by means of machining or related methods. Abrasive grains are hard crystals either found in nature or manufactured. The most commonly used materials are alumi- num oxide, silicon carbide, cubic boron nitride and diamond. Other materials such as garnet, zirconia, glass and even walnut shells are used for some applications. Abrasive products are used in three basic forms by industry: a) Bonded to form solid-shaped tools, such as disks (the basic shape of grinding wheels), cylinders, rings, cups, segments, or sticks to name a few. b) Coated on backings made of paper or cloth, in the form of sheets, strips, or belts. c) Loose, held in some liquid or solid carrier (for lapping, polishing, tumbling), or pro pelled by centrifugal force, air, or water pressure against the work surface (blast cleaning). The applications for abrasive processes are multiple and varied. They include: a) Cleaning of surfaces, also the coarse removal of excess material—such as rough off- hand grinding in foundries to remove gates and risers. b) Shaping, such as in form grinding and tool sharpening. c) Sizing, a general objective, but of primary importance in precision grinding. d) Surface finish improvement, either primarily as in lapping, honing, and polishing or as a secondary objective in other types of abrasive processes. e) Separating, as in cut-off or slicing operations. The main field of application of abrasive processes is in metalworking, because of the capacity of abrasive grains to penetrate into even the hardest metals and alloys. However, the great hardness of the abrasive grains also makes the process preferred for working other hard materials, such as stones, glass, and certain types of plastics. Abrasive processes are also chosen for working relatively soft materials, such as wood, rubber, etc., for such reasons as high stock removal rates, long-lasting cutting ability, good form control, and fine finish of the worked surface. Grinding Wheels Abrasive Materials.— In earlier times, only natural abrasives were available. From about the beginning of this century, however, manufactured abrasives, primarily silicon carbide and aluminum oxide, have replaced the natural materials; even natural diamonds have been almost completely supplanted by synthetics. Superior and controllable proper- ties, and dependable uniformity characterize the manufactured abrasives. Both silicon carbide and aluminum oxide abrasives are very hard and brittle. This brittleness, called friability, is controllable for different applications. Friable abrasives break easily, thus forming sharp edges. This decreases the force needed to penetrate into the work material and the heat generated during cutting. Friable abrasives are most commonly used for precision and finish grinding. Tough abrasives resist fracture and last longer. They are used for rough grinding, snagging, and off-hand grinding. As a general rule, although subject to variation: 1) Aluminum oxide abrasives are used for grinding plain and alloyed steel in a soft or hardened condition. 2) Silicon carbide abrasives are selected for cast iron, nonferrous metals, and nonmetallic materials. 3) Diamond is the best type of abrasive for grinding cemented carbides. It is also used for grinding glass, ceramics, and hardened tool steel.
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