Abrasive Machining Processes
Such machining processes are precision machining processes. They generate work-piece surfaces along with a high degree of surface smoothness and dimensional accuracy. Rough grinding, is occasionally used for eliminating excess material from the surfaces of castings, forgings, and same work-piece that have not been machined. During rough grinding, a rotating abrasive grinding wheel is guided manually concerning to a work-piece surface that fundamentally differs from generally identified grinding operation to acquire smooth finish and close tolerance in that grinding tool is machine guided and held. Abrasive are also utilized on belts or like powder paste.
Abrasives are hard materials along with sufficient toughness. A number of materials may be used as abrasive, but in these machining processes today the three commonly utilized abrasives are diamond or a form of pure carbon, silicon carbide or SiC, and aluminium oxide or Al2O3. Diamond is the hardest identified material, along with a hardness of 10 on the Mohs' scale. However, because of its relatively high cost, diamond is utilized only for those applications that require it employ and justify its higher cost. Various abrasives are found in the earth's surface like natural abrasives, and diamond is individual of such. Aluminium oxide along with varying proportions of impurities is also determined as a natural abrasive. Though, with the intention of control purity and secure uniformity, virtually all of the aluminium oxide and every silicon carbide employed today in the abrasive machining procedures are generated synthetically. In Table no.3, showing relative hardness, this may be noticed that silicon carbide is somewhat harder than aluminium oxide. Conversely, aluminium oxide is tougher than silicon carbide, along with the result that its grains will not fracture as like readily. Aluminium oxide is utilized for grinding steels, mallable iron and another material along with high tensile strength. Silicon carbide is utilized for grinding cast iron, aluminium, bronze casting and other materials along with low tensile strength. Green 99% SiC is employed to grind cemented carbide.
Table no.3: Relative Hardnesses of Some Materials
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Material
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Relative Hardness
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Diamond
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70
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Silicon carbide
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25
|
|
Aluminium oxide
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20
|
|
Tungsten carbide
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19
|
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Hardened steel
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08
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Common window glass
|
04
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Abrasive material whereas produced synthetically or determined in nature, is crushed to particles of needed size. These particles are termed as "grains", or grits. For several applications the crushing is continued till the abrasive becomes a fine powder, often known as "flour". Like sands, abrasive grains are passed via a set of standard-mesh sieves to separate them as per to grain size. Grain size is designated via the mesh of the finest standard sieve through that the grains will pass. The size is explained by number of meshes per linear inch. The standard meshes are from 10 to 600. The size is termed as grit size code. The code is illustrated below as:
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Grit Size
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Code
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Coarse
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10, 12, 14, 16, 20, 24
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Medium
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30, 36, 46, 54, 60
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Fine
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80, 100, 120, 150, 180
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Very Fine
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220, 240, 280, 320, 400, 500, 500, 600
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Coarse and medium size grits are utilized for soft material whether fine and very superior size are utilized for hard materials. Abrasive flours that are too fine for separating along with sieves, should be segregated via a flotation method in a liquid. Abrasive might be utilized as loose grins or flours like in buffing, lapping, and polishing. For grinding, super-finishing and, honing, the abrasive grains are bonded together into shapes called grinding wheels, stones and sticks. In any instance, the abrasive grains that suitably held and stimulated across a work-piece surface, remove material via a cutting action. This cutting action generates minute chips and it is essentially same to the cutting action of other cutting tools.
Grinding is accomplished along with the use of an abrasive grinding wheel, mounted upon a proper machine and rotated at a proper rotational velocity. During precision grinding, movements of the grinding wheels relative to the work-piece are accurately guided via the machine. Grinding is completed on a Work-piece for three principal reasons.
(a) To machine materials that are too hard for other machining methods that use cutting tools.
(b) To produce surfaces inside closer dimensional tolerances.
(c) To produce surfaces along with a higher smoothness degree.
Several grinding processes are economical for the removal of material of relatively huge amounts. For other grinding procedures, power requirements, tools costs, and labour costs are higher than for another cutting process. Materials harder than about Rockwell C45 can't be machined via the machining processes that employs cutting tools. They should be machined via grinding, via some other abrasive machining procedure, or via a suitable newly developed procedure. While a work-piece is to be hardened, the majority of the excess material must be removed by several other machining processes prior to hardening. Only a minimum of excess material must be left on the work-piece surface to be removed via grinding after the hardening. Such excess material is called as the "grinding allowance".
Grinding allowance will have to be little larger for various work-pieces that may change shape slightly throughout heat treatment. A significant advantage of grinding is the minute size of the cutting edges upon the grains. Because the cutting takes place in exceedingly minute amount it can be controlled to hold close dimensional tolerances and to generate smooth surfaces. Work-piece of moreover hard or soft materials is often ground to receive advantage of this capability of grinding. Dimensional tolerance is minute as 0.0025 µ and surface smoothness of 0.025 µ are readily acquired via grinding. Still these might be exceeded by utilizing better than average equipment, care and skill.