ABRASIVES, are natural and synthetic substances that are used to remove material from workpieces made of metal, wood, plastic, ceramic, or glass. The material is removed by a fracturing or shearing action. Removal of chips by shear or fracture is attained by forcing hard abrasive particles through a softer workpiece layer or by causing abrasive particles to strike the workpiece. Efficient chip removal requires a proper application and selection of abrasives to obtain* high material removal rates, achieve precise dimensional and geometrical accuracy, and produce controlled finishes. In theory the sharp cutting edges of the abrasive must be presented to the workpiece material in a definite orientation. Abrasive particle orientation is accomplished by producing grinding wheels and sticks, coated abrasive belts and paper, and loose abrasive particles.
Effective abrasive manufacturing operations can be achieved when a processed abrasive is used in conjunction with hand methods or machine tools. Common abrasive manufacturing operations are surface and cylindrical grinding, impact grinding, honing, lapping, superfinishing, polishing, buffing, sanding, and tumbling. Abrasive manufacturing operations play a key role in the manufacture of products for home, business, and industry. Abrasives are used directly in the manufacture of products or in the building of machines and tools used to make products.
History. The development of abrasives parallels man’s progress in technology. Until the 20th century, man’s technological progress was slow. A contributing factor was a lack of adequate abrasive materials to take advantage of advances in metallurgy, machine design, and manufacturing ideas and techniques. Without abrasives of good fracture strength, hardness, and wear resistance, toolmakers and machinists had great difficulty in producing spindles, bearings, bearing surfaces, machine ways, and other structural elements of machine tools. Early machine tools could not meet the high speed, heavy cutting, and mass production requirements of modern methods.
Archaeological evidence indicates that sand polishing and stone rubbing of weapons and utensils were attempted between 25,000 and 15,000 b.c. In the Iron Age, man fashioned implements by rubbing stone against iron and bronze.
Thousands of years later other natural abrasives were discovered. Diamonds, which have better hardness and wear properties than sandstone or sand, were mined first in India about 700 b.c. Emery was probably first used as an abrasive about 100 a.d., but it may have been used hundreds of years earlier.
During the Middle Ages, scholars recorded that naturally bonded grinding wheels of quartz and flint were used aud that gemstones were lapped with natural abrasives between metal plates.
Çorundum was discovered in India about 1825. Of ali natural abrasives, çorundum is closest to diamonds in hardness. About this time, the first use of an artificially bonded, natural-abrasive grinding wheel was reported in India. It contained çorundum bonded with gum resin.
Advances in engineering material sciences and manufacturing process applications led to a need for improved abrasives. In 1891, Edward G. Acheson commercially produced silicon-carbide crystals (carborundum). These crystals are second only to diamonds in hardness. Synthetically produced silicon-carbide crystals are superior to emery and çorundum in hardness, toughness, and fracture strength. The use of general-purpose, artifîcially bonded grinding wheels and loose abrasive grains of silicon carbide led to great improvements in abrasive manufacturing operations.
In 1900, Charles Jacobs manufactured aluminum oxide from bauxite and developed a tempering technique that provided a synthetic abrasive with hardness and strength suitable for abrasive processes.
Manmade diamonds, which became available to industry in 1955, are widely used as abrasive grains bonded in thin layers to metal or resinoid wheels. The cost of natural and man-made diamonds stili is a serious limitation. Silicon carbide, aluminum oxide, and natural and man-made diamonds are the major abrasives used by modern industry.
Physical Description. Each abrasive has defînite hardness, toughness, and fracture characteristics. The abrading performance of synthetic and natural diamond abrasives is predictable. This is not true of other natural abrasives because impurities are present. Diamonds are the hardest abrasive material; silicon carbide is second in hardness; and aluminum oxide is third. The hardness property permits penetration into workpiece material. Diamonds are used to measure the hardness (scratch resistance) of other abrasive materials. Silicon carbide generally has less body strength (toughness) and more friability. These properties provide less resistance to grain fracture. Aluminum oxide, which generally has more body strength and less friability, resists dulling when used on tough, high-tensile-strength materials.
Body strength and friability properties vary, depending on grain size, grain shape, and crystal structure. Natural abrasive grains such as emery, crocus, or pumice, do not have high hardness, high strength of grain cutting edge, or high body strength. In use, they progressively break down. However, these abrasives are well suited for buffing and polishing under light load pressures.
Grain size is of utmost importance for rapid removal rates or finish. Grain sizes range from 10 mesh size for fast material removal to 600 and 1,000 mesh size for lapping and polishing. Abrasive grain size is determined by screening grains through a series of sieves of progressively smaller hole size. The lower the grain size number, the coarser is the abrasive grain.
Abrasive Forms. Abrasive grain bonding secures the grain in a desirable shape, as in wheels, stones, sticks, or endless belts. The bonding material permits the release of dull grains and permits grain fracture to expose new sharp cutting edges to the workpiece. Grinding wheels commonly have a vitrified, rubber, or resinoid bonded construction. In coated abrasive belts, discs, and papers, the abrasive is applied to a backing surface in a thin layer and is bonded to the surface. Loose abrasive grains carried in air, water, or oil media are examples of a non-bonded form. In ali abrasive forms, abrasive grain size and type are important in achieving a desired result from abrasive processes.
Abrasives are grouped as natural or synthetic abrasives. Synthetic abrasives are more important because of their usefulness in industry.
Synthetic Abrasives—Silicon Carbide. A compound of silicon and carbon has hard, friable properties suitable for grinding dense, hard, or brittle workpieces made of cast-iron, ceramic, carbide, nonmetallic, or nonferrous materials. A variety of abrasive forms can be used to produce flat surfaces, accurately sized external and internal diameters, and fine finishes by any of the common abrasive manufacturing operations.
Aluminum Oxide. Aluminum oxide (alumina) is less hard and more tough than silicon carbide. Because of its desirable grainfracture characteristics, aluminum oxide is suitable for grinding tough, high-strength materials, such as alloy and tool steels. This abrasive is suitable for obtaining fine finishes and for removing small amounts of material by lapping, honing, superfinishing, polishing, or buffing.
Man-made Diamonds. Synthetically produced diamonds, which are nearly pure carbon, are very hard. Major applications for synthetic diamond abrasives are wheel grinding and honing of tungsten-carbide and aluminum-oxide cutting tools and truing of silicon-carbide and aluminum-oxide grinding wheels.
Applications for Synthetic Abrasives. Silicon carbide, aluminum oxide, and synthetic diamonds are used in a variety of forms to manufacture crankshafts, cutting tools, bearings, and functional surfaces of machines and workpiece holding devices. Honing of cylinder-block bore diameters to improve roundness, straightness, and taper is an example of a corrective operation. Cutoff and snag grinding with tough, bonded abrasive wheels are important preparatory operations on cast, welded, and forged parts. Lapping with loose abrasives suspended in oil can provide extremely accurate dimensions, correct shape imperfections, refine finish, and produce close fits. Mating gears, gage blocks, and ball and roller bearings are lapped in this way. Buffing and polishing operations make use of fine-grained synthetic abrasives in loose, coated, and compounded forms to prepare surfaces for plating or painting.
Miscellaneous Abrasives. Abrasive materials are available as steel wool, steel shot, and glass pellets. Steel wool is used to remove corrosion and to smooth metal workpieces. Steel shot and glass pellets are used to change surface characteristics by striking the workpiece surface.
Natural Abrasives. Silica, which is silicon dioxide, occurs in the form of quartz, flint, and agate. This abrasive may be used loose or bonded to paper, fabric drums, or belts.
Emery, which chiefly consists of aluminum oxide and iron oxide, is used to polish and buff. It usually is bonded to fabric, coated on rolls, or compounded into solid bars or liquid. Emery is placed on fabric, on sheepskin, on fiber discs, and on fiber wheels.
Tripoli is an amorphous silica that contains iron compounds. Die castings, aluminum, copper, and brass are buffed with tripoli. It is not used on ferrous metals because it is too soft for them.
Rouge is a ferric oxide material used to obtain a high coloring on gold, copper, and brass. It also is used to polish and buff other metals.
Crocus consists chiefly of iron oxide in a less refined form than rouge. It is used in buffing and polishing cutlery. Crocus cloth is available for light metal removal and polishing.
Pumice is a porous, spongy form of volcanic glass. In loose or block forms it is used for polishing and buffing plastics, glass, and wood.
Gamet is a hard, glasslike mineral that consists of calcium, iron, and magnesium silicates. Garnet paper, disc, and roll forms are used to sand wood products.
Diamonds, which are nearly 100 percent carbon, are mined as gemstone and bort. Bort, whieh is low-quality, flawed diamond stones and fragments, is crushed to graded grain sizes for grinding and honing applications. Mounted diamond fragments and flawed stones are used for truing grinding wheels. As eutting tools, they are used for material removal proeesses. Bonded diamond wheels are used to saw stone, granite, and concrete for highways and aireraft runways.
Industrial Uses. Synthetie abrasives and natural abrasives, such as emery, tripoli, pumice, and rouge, are used to brighten and improve a work-piece surface before plating or painting. Diecast plumbing fixtures, automobile hardware, molded plastic products, and finish conditioning of industrial tools are important applications. Endless belts and bufEng wheels are widely used. Impact grinding, blasting, and tumbling operations are performed with synthetie or natural abrasives. Silica and granite stones are used where abrasive loss is high, as in masonry building cleaning and in blasting cast parts to remove scale and oxides. The abrasive, in loose form, is moved in a blast of air or liquid. An ultrasonic impact grinding machine is used when very accurate size is important.
Home Uses. Sandpaper, a common abrasive, primarily is used in finishing wood products. A graded silica is coated on paper or fabric to form sheets, belts, and dises. Similar but more expensive abrasives, such as emery, garnet, or crocus, are available in paper, belt, or eloth form. Steel wool can be used in finishing wood products and cleaning metal implements. Pumice is used to refine the finish of cabinetry in the home workshop. Silicon-carbide sticks, aluminum-oxide sticks, and small grinding wheels are essential for maintaining sharp eutting edges on the home craftsman’s tools.
Household eleaners, polishing agents, and dentifrice may contam highly refined pumice.
Preparation of Natural and Synthetie Abrasives. Manufacture of synthetie diamonds is accom-plished by using an electric furnace and high pres-sure to bring carbon to diamond properties. Sizing and grading of diamond particles ars accomplished by passing the particles through a series of sereens. The Bureau of Standards adopted a sieve Standard in 1931. The Abrasive Grain Association modified these standards, and they remain in use.
Natural abrasives primarily are mined from the earth’s surface. Some natural abrasives, such as pumice and garnet, require little further proc-essing except crushing and grading by the sieve technique. Separating a natural abrasive material from impurities, as in the case of natural diamonds, requires an extensive wash-precipita-tion eyele. Diamonds, which are mined princi-pally in Africa, have nonwetting properties and are allowed to stick to greased rolls. The impurities or slag suspended in a water medium wets easily and will not stick to greased rolls. Diamond stone and bort are seraped from the rolls and separated from the grease.