ABRASIVES (EN: abrasives, abrasive materials; DE: Schleifmittel, Schleifstoffe; FR: produits abrasifs; ES: materiales abrasivos; RU: абразивные материалы) are the substances of the high hardness for the machining of the rocks, minerals, etc.
The natural abrasives (flint, emery, garnet, pumice, corundum, and diamond) were used since the most ancient times, the artificial abrasives (corundum, silicon carbide, boron carbide, monocorundum, synthetic diamond, etc.) were used since the late 19th century. The main natural abrasive is the diamond, and there are other natural abrasives of the great importance, such as corundum, emery, garnet, flint pebbles, pumice, tripoli; the silica sand and the red sandstone are used also.
The main characteristics of the abrasives are the hardness, the strength and the wear, the shape of the abrasive grain, the abrasive ability, and the graininess. With the increase of the strength of the abrasives, their resistance to the cutting forces improves, at the same time the resistance of the abrasives to the compression is several times greater than their resistance to the tension. The strength of the abrasives in relation to the tension and the compression decreases with the increase of the temperature of the grinding.
The processing with the help of the abrasives is characterized by the participation in the process of the large number of the randomly placed cutting edges of the grains at the same time. The abrasive grain is the crystal shard (crystallite), or less often the single crystal, or the aggregate which consists of the many small crystals (polycrystal), may have the approximately equal dimensions in height, width and thickness (isometric shape) or may have the shapes of the sword and the plate. The shape of the grain which is isometric or close to it is rational, because every grain is a cutting tool. The needle shaped grain is the least profitable.
The abrasive ability is characterized by the mass of the material, which is removed at the time of the grinding up to the bluntness of the grain (see Abrasiveness). The grit characterizes the size and the uniformity of the grains of the abrasives; it is defined by the classification of the grains according to their linear parameters by the method of the sieve analysis, by the sedimentation in a liquid, or others. The number of the grit is assigned in accordance with the linear sizes of the grain of the main fraction. The more uniform are the abrasives in the shape and the size of their grains, the better are their exploitation qualities. The grit of the abrasives is regulated by the standard.
The abrasives are used in the form of the grains, which are fixed (bonded) by the binding substance into the abrasive tools with various shapes and purposes, and in the loose state in the form of the powders, pastes, and slurries. The abrasives have the most of the practical application in the drilling, where they are used to reinforce the rock breaking tool. They are also used in the working devices of the mining machines for the extraction and the tunneling, etc.
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