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Abrasive Cleaning | Abrasive | Manmade Materials

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ABRASIVE CLEANING An abrasive is a material, often a mineral, that is used to shape or finish a workpiece through rubbing which leads to part of the workpiece being worn away. While finishing a material often means polishing it to gain a smooth, reflective surface it can also involve roughening as in satin, matte or beaded finishes. Abrasives are extremely commonplace and are used very extensively in a wide variety of industrial, domestic, and technological applications. This gives rise to a la
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  ABRASIVE CLEANING An abrasive is a material, often a mineral,that is used to shape or finish a workpiece through rubbing which leads to part of the workpiece being worn away. While finishing amaterial often means  polishing it to gain a smooth, reflective surface it can also involve roughening as in satin, matte or beaded finishes.Abrasives are extremely commonplace and are used very extensively in a wide variety of industrial, domestic, and technological applications. This gives rise to a large variation inthe physical and chemical composition of abrasives as well as the shape of the abrasive.Common uses for abrasives includegrinding, polishing,  buffing,honing, cutting,drilling, sharpening, andsanding(seeabrasive machining   ). (For simplicity, mineral in thisarticle will be used loosely to refer to both minerals and mineral-like substances whether man-made or not.)Filesact by abrasion but are not classed as abrasives as they are a shaped bar of metal.However, diamond files are a form of coated abrasive (as they are metal rods coated withdiamond powder).Abrasives give rise to a form of wound called an abrasion or even anexcoriation.  Abrasions may arise following strong contract with surfaces made things such asconcrete, stone,wood,carpet, androads,though these surfaces are not intended for use as abrasives. Mechanics of abrasion Abrasives generally rely upon a difference in hardness between the abrasive and thematerial being worked upon, the abrasive being the harder of the two substances.However, this is not necessary as any two solid materials that repeatedly rub against eachother will tend to wear each other away (such as softer shoe soles wearing away woodenor stone steps over decades or centuries or glaciers abrading stone valleys). Typically, materials used as abrasives are either hardminerals (rated at 7 or above on Mohs scale of mineral hardness   ) or are synthetic stones, some of which may bechemically and physically identical to naturally occurring minerals but which cannot becalled minerals as they did not arise naturally. (While useful for comparative purposes,the Mohs scale is of limited value to materials engineers as it is an arbitrary, ordinal,irregular scale.)Diamond, a common abrasive, for instance occurs both naturally and isindustrially produced , as iscorundumwhich occurs naturally but which is nowadays  more commonly manufactured from bauxite. [1] However, even softer minerals likecalcium carbonateare used as abrasives, such as polishing agents in toothpaste.Grit size ranging from 2 mm (the large grain) (about F 10 using FEPA standards) to about40 micrometres (about F 240 or P 360).These minerals are either crushed or are already of a sufficiently small size (anywherefrom macroscopic grains as large as about 2 mm to microscopic grains about 0.001 mmin diameter) to permit their use as an abrasive. These grains, commonly calledgrit, haverough edges, often terminating in points which will decrease the surface area in contactand increase the localised contact  pressure. The abrasive and the material to be worked are brought into contact while in relative motion to each other. Force applied through thegrains causes fragments of the worked material to break away while simultaneouslysmoothing the abrasive grain and/or causing the grain to work loose from the rest of theabrasive.Some factors which will affect how quickly a substance is abraded include: ã Difference in hardness between the two substances: a much harder abrasive willcut faster and deeper  ã Grain size (grit size): larger grains will cut faster as they also cut deeper  ã Adhesion between grains, between grains and backing, between grains andmatrix: determines how quickly grains are lost from the abrasive and how soonfresh grains, if present, are exposed ã Contact force: more force will cause faster abrasion ã Loading: worn abrasive and cast off work material tends to fill spaces betweenabrasive grains so reducing cutting efficiency while increasing friction ã Use of lubricant/coolant/metalworking fluid: Can carry awayswarf (preventingloading), transport heat (which may affect the physical properties of theworkpiece or the abrasive), decrease friction (with the substrate or matrix),suspend worn work material and abrasives allowing for a finer finish, conductstress to the workpiece. Abrasive minerals Abrasives may be classified as either natural or synthetic. When discussingsharpeningstones, natural stones have long been considered superior but advances in materialtechnology are seeing this distinction become less distinct. Many synthetic abrasives are  effectively identical to a natural mineral, differing only in that the synthetic mineral has been manufactured rather than been mined. Impurities in the natural mineral may make itless effective.Some naturally occurring abrasives are: ã Calcite(calcium carbonate) ã Emery(impure corundum) ã Diamonddust (synthetic diamonds are used extensively) ã  Novaculite ã Pumicedust ã Rouge ã SandSome abrasive minerals (such as zirconia alumina) occur naturally but are sufficiently rare or sufficiently more difficult/costly to obtain such that a synthetic stone is usedindustrially. These and other artificial abrasives include: ã Borazon(cubic boron nitrideor CBN) ã Ceramic ã Corundum(alumina or aluminium oxide   ) ã Dry ice ã Glass powder  ã Silicon carbide(carborundum) ã Zirconia alumina Manufactured abrasives Abrasives are shaped for various purposes. Natural abrasives are often sold as dressedstones, usually in the from of a rectangular block. Both natural and synthetic abrasivesare commonly available in a wide variety of shapes, often coming as bonded or coatedabrasives, including blocks, belts, discs, wheels, sheets, rods and loose grains. Bonded abrasives Assorted grinding wheels as examples of bonded abrasives.  A grinding wheel with a reservoir to hold water as a lubricant and coolant.A bonded abrasive is composed of an abrasive material contained within amatrix,  although very fine aluminium oxide abrasive may comprisesintered material. This matrix is called a binder and is often aclay, aresin, aglassor arubber . This mixture of binder  and abrasive is typically shaped into blocks, sticks, or wheels. The most usual abrasiveused isaluminium oxide. Also common aresilicon carbide, tungsten carbide and garnet. Artificialsharpening stonesare often a bonded abrasive and are readily available as a twosided block, each side being a different grade of grit.Grinding wheelsare cylinders that are rotated at high speed. While once worked with afoot pedal or hand crank, the introduction of electric motors has made it necessary toconstruct the wheel to withstand greater radial stress to prevent the wheel flying apart asit spins. Similar issues arise withcutting wheelswhich are often structurally reinforcedwith impregnated fibres. High relative speed between abrasive and workpiece oftenmakes necessary the use of a lubricant of some kind. Traditionally they were calledcoolantsas they were used to prevent frictional heat build up which could damage theworkpiece (such as ruining thetemper of a blade). Some research suggests that the heattransport property of a lubricant is less important when dealing with metals as the metalwill quickly conduct heat from the work surface. More important are their effects uponlessening tensile stresses while increasing some compressive stresses and reducing thermal and mechanical stresses during chip formation .  [2] Various shapes are also used as heads onrotary tools used in precision work, such as scale modelling.Bonded abrasives need to be trued and dressed after they are used. Dressing is cleaningthe waste material (swarf and loose abrasive) from the surface and exposing fresh grit.Depending upon the abrasive and how it was used, dressing may involve the abrasive being simply placed under running water and brushed with a stiff brush for a soft stone or the abrasive being ground against another abrasive, such as aluminium oxide used todress a grinding wheel.
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