Machinery's Handbook, 31st Edition
GRINDING WHEELS 1281 4) Cubic Boron Nitride (CBN) is known by several trade names including Borazon (General Electric Co.), ABN (De Beers), Sho-bon (Showa-Denko), and Elbor (USSR). CBN is a synthetic superabrasive used for grinding hardened steels and wear-resistant superalloys. (See Cubic Boron Nitride (CBN) starting on page 1081.) CBN grinding wheels have long lives and can maintain close tolerances with superior surface finishes. Bond Properties and Grinding Wheel Grades.— The four main types of bonds used for grinding wheels are the vitrified, resinoid, rubber, and metal. Vitrified bonds are used for more than half of all grinding wheels made, and are preferred because of their strength and other desirable qualities. Being inert, glass-like materials, vitrified bonds are not affected by water or by the chemical composition of different grinding fluids. Vitrified bonds also withstand the high temperatures generated during normal grinding operations. The structure of vitrified wheels can be controlled over a wide range of strength and porosity. Vitrified wheels, however, are more sensitive to impact than those made with organic bonds. Resinoid bonds are selected for wheels subjected to impact, or sudden loads, or very high operating speeds. They are preferred for snagging, portable grinder uses, or roughing operations. The higher flexibility of this type of bond—essentially a filled thermosetting plastic—helps it withstand rough treatment. Rubber bonds are even more flexible than the resinoid type, and for that reason are used for producing a high finish and for resisting sudden rises in load. Rubber bonded wheels are commonly used for wet cut-off wheels because of the nearly burr-free cuts they pro duce, and for centerless grinder regulating wheels to provide a stronger grip and more reliable workpiece control. Metal bonds are used in CBN and diamond wheels. In metal bonds produced by electro deposition, a single layer of superabrasive material (diamond or CBN) is bonded to a metal core by a matrix of metal, usually nickel. The process is so controlled that about 30–40 percent of each abrasive particle projects above the deposited surface, giving the wheel a very aggressive and free-cutting action. With proper use, such wheels have remarkably long lives. When dulled, or worn down, the abrasive can be stripped off and the wheel renewed by a further deposit process. These wheels are also used in electrical discharge grinding and electrochemical grinding where an electrically conductive wheel is needed. In addition to the basic properties of the various bond materials, each can also be applied in different proportions, thereby controlling the grade of the grinding wheel. Grinding wheel grades commonly associated with hardness, express the amount of bond material in a grinding wheel, and hence the strength by which the bond retains the individual grains. During grinding, the forces generated when cutting the work material tend to dislodge the abrasive grains. As the grains get dull and if they don’t fracture to resharpen themselves, the cutting forces will eventually tear the grains from their supporting bond. For a “soft” wheel the cutting forces will dislodge the abrasive grains before they have an opportunity to fracture. When a “hard” wheel is used, the situation is reversed. Because of the extra bond in the wheel the grains are so firmly held that they never break loose and the wheel becomes glazed. During most grinding operations it is desirable to have an intermediate wheel where there is a continual slow wearing process composed of both grain fracture and dislodgement. The grades of the grinding wheels are designated by capital letters used in alphabetical order to express increasing “hardness” from A to Z. Grinding Wheel Structure.— The individual grains, which are encased and held to- gether by the bond material, do not fill the entire volume of the grinding wheel; the inter mediate open space is needed for several functional purposes such as heat dissipation, coolant application, and particularly, for the temporary storage of chips. It follows that the spacing of the grains must be greater for coarse grains which cut thicker chips and
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