ÿþ<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> <html> <head> <title>Advanced Concepts in Large-Scale Network Simulation</title> <meta http-equiv="content-type" content="text/html;charset=windows-1251"> <link rel="stylesheet" type="text/css" href="../master_style.css"> <meta name="ROBOTS" content="noindex, nofollow"> </head> <body> <h3 align="center"><b>ARTICLE from the Encyclopadia Britannica </b></h3> <h3 align="center">Primary Contributors: D. Joseph Bodin, Thomas O. Mason</h3> <HR> <p><b>Source of information</b>: <a href="http://www.britannica.com/EBchecked/topic/1615/abrasive">http://www.britannica.com/EBchecked/topic/1615/abrasive</a> <HR> <p> Share abrasive, sharp, hard material used to wear away the surface of softer, less resistant materials. Included within the term are both natural and synthetic substances, ranging from the relatively soft particles used in household cleansers and jeweler s polish to the hardest known material, the diamond. Abrasives are indispensable to the manufacture of nearly every product made today. Abrasives are used in the form of grinding wheels, sandpapers, honing stones, polishes, cutoff wheels, tumbling and vibratory mass-finishing media, sandblasting, pulpstones, ball mills, and still other tools and products. Only through the use of abrasives is industry able to produce the highly precise components and ultrasmooth surfaces required in the manufacture of automobiles, airplanes and space vehicles, mechanical and electrical appliances, and machine tools. This article surveys the principal materials used in abrasives, the properties of those materials, and their processing into industrial products. Most abrasive products are made of ceramics, which include some of the hardest materials known. The origins of hardness (and other properties) in ceramic materials are described in the article ceramic composition and properties. <br><br><b>History<br><br></b> The use of abrasives goes back to earliest man s rubbing of one hard stone against another to shape a weapon or a tool. The Bible mentions a stone called shamir that was very probably emery, a natural abrasive still in use today. Ancient Egyptian drawings show abrasives being used to polish jewelry and vases. A statue of a Scythian slave, called  The Grinder, in the Uffizi Gallery in Florence, shows an irregularly shaped natural sharpening stone used to whet a knife. Sand and pieces of flexible hide were early man s sandpaper. Later, craftsmen tried to fix abrasive grains to flexible backings with crude adhesives. A 13th-century Chinese document describes the use of natural gums to fix bits of seashell to parchment. About two centuries later, the Swiss began coating crushed glass on a paper backing. <br><br><b>Abrasive materials: their composition and properties<br><br></b> The materials used to make abrasives can be broadly classified as either natural or synthetic. Natural abrasives include diamond, corundum, and emery; they occur in natural deposits and can be mined and processed for use with little alteration. Synthetic abrasives, on the other hand, are the product of considerable processing of raw materials or chemical precursors; they include silicon carbide, synthetic diamond, and alumina (a synthetic form of corundum). Most natural abrasives have been replaced by synthetic materials because nearly all industrial applications demand consistent properties. With the exception of natural diamond, most of nature s abrasives are too variable in their properties. <br><br><b>Fabrication into useful forms</b> <br><br><b>Preparation and sizing <br><br></b> All abrasives, with the exception of the naturally appearing fine powders such as talc, must be crushed to the particle size required for use. Sizes in use vary from 4 grit, which measures about 6 millimetres (1/4 inch) in diameter, to as fine as 900 grit, which measures about six microns (0.00024 inch) or about one-tenth the thickness of a human hair. In some cases, even finer powders are required when used for the polishing of scratch-free surfaces on high-quality optical lenses and mirrors for high-power telescopes. For the coarser sizes the crushed grain is measured by a series of test screens as established in most countries by government standards. For sizes 240 grit and smaller, the size of the grains is usually measured by a settling rate or sedimentation test. <br><br><b>Abrasive-product manufacture<br><br></b> <b>Grinding wheels<br><br></b> The most important abrasive product manufactured is the grinding wheel. Made of abrasive grain and a binder, or  bond, it is a self-sharpening cutting tool. As the grains on the periphery become dull, they are shed from the surface of the grinding wheel, and fresh, sharp cutting edges are exposed. The ability to resharpen is controlled by the nature of the bond used and the ratio of abrasive to bond, measured by volume. <br><br><b>Forming and firing<br><br></b> In the manufacture of grinding wheels, abrasives of the proper size and bonding materials are weighed and mixed together in a power mixing machine. When thoroughly mixed, measured amounts of the abrasive and bond mixture are evenly distributed in steel molds. The mold is placed in a powerful , and the mixture is compressed to the desired wheel size, allowing some oversize in dimensions for finishing operations. Pressures vary with wheel size and may be upwards of several hundred tons. Some shaving and shaping of the wheels is done before they are baked or fired. <br><br><b>Coating <br><br></b> Manufacture begins with huge rolls of backing material, either paper, cloth, or a combination of the two. The backing is fed to the making machine where the first layer of adhesive is applied. Next, the layer of abrasive is applied, either by gravity or electrostatically. The electrostatic method orients the slivery type of abrasive used, with the sharp ends facing out. The process can also control the spacing of the grains. This is an advantage in that wide spacing helps to alleviate loading problems when grinding soft, stringy, or gummy materials. <br><br><b>Shaping</b> <br><br><b>Other abrasive products<br><br></b> Other products use the abrasive in the form of grains or powders. In addition to the sizing operation, many types are specifically treated, by calcining, acid, or heating, to make them more suitable for use as lapping abrasive or perhaps as sandblasting grain. For use in lapping and polishing, the abrasive is usually mixed with a vehicle such as mineral or seal oil. Polishing sticks consist of waxes or greases impregnated with various-sized abrasive grains, depending on the particular requirements of the work. Two materials used for cleaning rather than grinding are still abrasive in nature. Glass beads, pressure blasted onto a surface, remove rust, scale, and carbon. These have replaced much hand cleaning with steel wool. Steel wool still has some applications. <br><br><b>Industrial applications</b> <br><br><b>Grinding <br><br> </b> Grinding, the most important abrasive application, is in some way involved in the manufacture of almost every product. This use may be direct, as when the product requires pieces that must be made within close dimensional tolerance limits, or a very smooth surface, or when used on materials too hard to be machined by conventional cutting tools; or indirect, as when, for example, grinding wheels are used to sharpen cutting tools. The materials that are used to make cutting tools must of course be hard in order for the cutting tool to cut and retain its sharp edge. Abrasive grinding wheels are the only means for sharpening the dull edges of such tools. <br><br><b>Cutting wheels<br><br></b> Abrasive wheels have replaced steel saws in many places. Thin, abrasive cutoff wheels are capable of sawing through nearly every material known, at rates faster than those of metal saws, while generating less heat and producing a better cut surface. Some space-age metals, because of their hardnesses, can be cut only with abrasive wheels. Granite, marble, slate, and various building blocks are cut to size with diamond abrasive wheels. <br><br><b>Tool sharpening<br><br></b> The sharpening of all types of tools continues to be a major grinding operation. Drills, saws, reamers, milling cutters, broaches, and the great spectrum of knives are kept sharp by abrasives. Coarser-grit products are used for their initial shaping. Finer-grit abrasives produce keener cutting edges. Ultrasharp tools must be hand-honed on natural sharpening or honing stones. Even grinding wheels themselves may require some sharpening. Specially designed steel disks or diamond tools are used to remove dull abrasive cutting edges and create a sharp cutting surface. </p> </body> </html> <br> <br> <br>