Abrasive Products

Abrasive delivered in a bond or matrix is often called the abrasive product or abrasive tool. The products can be grouped under three broad categories: coated/impregnated abrasives, bonded abrasives, and powders, slurries, and compounds (Fig. 1). Each of these product types can be used in rough, precision, or high-precision applications (Table 3).

Table 3 Abrasive products and their typical application categories

product type

Rough

Precision

High-precision

Bonded abrasives

Snagging surfacing cutoff grinding

Grinding (surface, creep feed, form, thread, contour, cylindrical, disc, tool, and cutter)

Honing, superfinishing, flat honing, microgrinding

Coated abrasives

Belt grinding, sanding

Belt grinding (surface, form or profile, flat stock finishing, polishing)

Belt polishing

Impregnated abrasives

Polishing, deburring, buffing

Powder, slurries, compounds, and jets

Abrasive jet machining

Polishing, buffing, liquid buffing, ultrasonic machining

Lapping, polishing, buffing

Fig. 1 Abrasive "machining" product categories

Bonded abrasives are so named because the abrasive is firmly retained in a matrix called the bond. They are available in discs or cylinders called grinding wheels. They are also available in segments of a cylinder, which are assembled into a wheel, and in rectangular blocks, nicknamed "stones," "sticks," or "hones," that are presented to the work material under various geometric configurations. The bond material can be made of phenolic resin, vitreous glass, or powder metal alloys, depending on the requirements. Every bonded abrasive product can be thought of as an engineered composite. This composite is usually of controlled pore size and structure, and its uniformity is critical for successful use of the abrasive product (Fig. 2). The size, shape, and configuration of bonded abrasives are constantly changing to meet growing needs. For example, grinding wheels are available in thicknesses from 0.02 to 500 mm (0.0008 to 20 in.). They are also available in a wide range of shapes.

Conventional Superabrasives abrasives

Fig. 2 Construction of typical bonded abrasive products

Coated/Impregnated Abrasives. Coated abrasives are single or multiple layers of abrasive products that adhere to a flexible sheet or belt. In impregnated abrasives, sold under various trade names, the abrasive is impregnated into flexible or woven material. Sometimes a layer of highly wear-resistant abrasives (superabrasives) are electroplated or brazed to a rigid steel core. Most coated and impregnated abrasives use a polymeric binder to retain the grains on the backing or the woven matrix. Most coated abrasives are available in flat sheets or belt loops of various lengths and widths.

Powders, slurries, and compounds are products in which the abrasive is delivered in a jet or in a liquid or paste configuration. When the abrasive is assisted by high-pressure water or air jet, the process is called abrasive jet machining. If the abrasive action is assisted by localized ultrasonic motion of a tool, it is called ultrasonic machining. Most abrasive powders, slurries, and compounds are delivered between the work and a wear-resistant plate called a lapping plate, flexible woven material called polishing cloth, or buffing wheels. (Lapping, polishing, and buffing are described in detail later in this article.) These products have extremely closely sized abrasive particles, often mixed in a liquid or carrier of controlled viscosity. Sometimes these abrasives remove the surface layer of work material through chemical action in addition to mechanical action.

Designations of Abrasive Products. A wide variety of products and their designations exist in industrial and consumer use. The common aspects of such designations are that:

• All abrasive products are identified by the abrasive type. The size of the abrasive used and its content are also frequently identified.

• In coated abrasives, the type of backing material (paper, cloth, etc.) and the features of the coating (open, closed, supersize, etc.) are also identified.

• In grinding wheels, the bond matrix used (resin, vitrified, metal, etc.) is designated along with its modifications, usually represented by a manufacturer code. The pore content is identified using the grade or hardness of the wheel.

• In the case of loose abrasives used in lapping or polishing, the particle size, shape, and distribution are critical and are identified in the specification. These products are also distinguished by the liquid medium or carrier used and their flow properties, such as viscosity and temperature stability. Abrasive Materials

Abrasive materials provide the cutting edges in abrasive finishing processes, so they are the essential element of any abrasive product. A variety of abrasives are used, selected for their hardness, thermal and chemical stability, and wear resistance. Figure 3 compares the hardnesses of abrasives to those of some common work materials. Figure 4 illustrates the range of abrasive sizes used and their applications.

Fig. 3 Comparison of the hardnesses of work materials and the hardnesses of the abrasives used to machine them
Fig. 4 Abrasive grain sizes vs. application type

The broad groupings of abrasives are identified in the following paragraphs, but in each category there are several versions that differ in shape, size, fracture strength, and so on. The reader is encouraged to obtain information about modifications from manufacturer literature and the references at the end of this article. This knowledge can be very useful in optimizing abrasive machining applications.

Conventional Abrasives. Emery and corundum are commonly used natural abrasives. Emery is a naturally occurring mixture containing between 57 and 75% aluminum oxide. The remainder is iron oxide and impurities. Corundum is a naturally occurring fused aluminum oxide that is tougher and harder than emery. Natural abrasives usually cost less than synthetic abrasives and are useful for polishing some softer metals. When applied to the harder metals, natural abrasives wear at a rapid rate, so they have been largely replaced by synthetic abrasives.

Synthetic Abrasives. Aluminum oxide and silicon carbide are the most widely used synthetic abrasives. They are harder, more uniform, longer lasting, and easier to control than natural abrasives. Because aluminum oxide grains are often very angular and have excellent bonding properties, they are particularly useful in finishing tougher metals, such as alloy steels, high-speed steels, and malleable and wrought iron.

Silicon carbide is harder and a better conductor of heat than aluminum oxide; it fractures easily, providing new cutting surfaces that extend the useful life of the abrasive. Silicon carbide is usually used in polishing low-strength metals, such as aluminum and copper. It is also applied in polishing hard, brittle materials, such as carbide tools, high-strength steels, and chilled and gray irons. Silicon carbide tends to decompose readily in the presence of iron and is not useful as an abrasive for grinding at high material removal rates involving ferrous work materials. However, it is used extensively in the grinding of titanium alloys.

Various types of alumina and silicon carbide are used as abrasives. The variations are derived from such factors as purity, crystal size, shape, and strength. Zirconia-alumina is a eutectic version with high impact strength and toughness, which are desirable characteristics for abrasives used in heavy or rough grinding conditions. A very pure form of aluminum oxide is produced using sol-gel techniques. In some cases, this microcrystalline alumina provides dramatic performance improvements compared to fused aluminum oxide.

Superabrasives. Diamond and CBN are the two superabrasive materials used in grinding applications. Diamond, available in both natural and synthetic forms, is the hardest known material. CBN, the second hardest material, is synthetic. Both of these superabrasive materials are synthesized at high pressures and temperatures in the presence of molten catalyst solvents.

Synthetic diamonds are available in a wide range of shapes and sizes. Weak, friable polycrystalline diamonds are generally used for grinding tough and brittle materials such as ceramics and carbides. Stronger abrasives find frequent application in the grinding of concrete, stone, and glass. The most popular form of CBN, used for grinding most steels and nonferrous high-strength alloys, is a monocrystalline variety. A tougher, polycrystalline form of CBN is also available.

Diamond and CBN are available with a coating of nickel metal. Metal-coated abrasives, predominantly used in resin-bonded wheels, increase grinding wheel life by a factor of 2 to 3. Diamond abrasives are also available with copper coatings, predominantly used in dry grinding applications.

Combinations of abrasive types (combinations of superabrasives with conventional abrasives, combinations of conventional abrasives, and combinations of superabrasives) are sometimes made to take advantage of the best properties of each.

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