Metal Finishing Guide Book


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Table I. Hardness of Abrasive Materials Abrasive Type Chemical Symbol Mohs��� Scale Aluminum oxide (fused) Al2O3 8-9+ Aluminum oxide (calcined) Al2O3 8-9+ Tripoli-silica SiO2 7 Silicon carbide SiC 9.6 Iron oxide (red rouge) Fe2O3 6 Chrome oxide (green rouge) Cr2O3 8-9 rescreened to its final classification (grit sizing). The difference between fused aluminum oxide and calcined alumina is that the fused oxide is of a crystalline structure that is much harder than that of the calcined alumina. Fused aluminum oxide is used mainly on abrasive belts or setup wheels for polishing. As for buffing, fused aluminum oxide is used for cutting down ferrous metals. The abrasive sizing is generally from 60 grit to -325 grit for buffing compounds. Tripoli Tripoli is considered to be microcrystalline silica, which is made naturally. It is highly suitable for buffing of aluminum, brass, copper, and zinc die cast or other white metals. Tripoli and silica can be used as a cutting abrasive or a so-called cutand-color abrasive for nonferrous metals. Tripoli should not be classified as an amorphous silica, but it is microcrystalline in nature. Crystalline silica may cause delayed lung injury for people when exposed to it over a long period. Users of products containing these abrasives should be aware of this possibility and should wear a mask and work in a ventilated area. Silicon Carbide Silicon carbide (SiC) is of a crystalline structure that is harder than fused aluminum oxide. It is formed by mixing coke and silica in an electric furnace at approximately 1,900 to 2,400oC. The material is cooled, ground, and sifted to the required grit size similar to the processing of fused aluminum oxide. The crystalline structure of SiC is a hexagonal. Red Rouge The chemical formula for rouge is Fe2O3; it is also called jeweler���s rouge. Its purity is 99% ferric oxide. The crystalline structure of ferric oxide is spherical. Rouge is used mainly on precious metals to give an exceptional high luster. Green Rouge The chemical formula for chromium green oxide is Cr2O3. The hardness of chromium oxide is 9 Mohs as opposed to iron oxide, which is 6 Mohs, and is used to produce an exceptional luster or color on ferrous as well as nonferrous metals. These abrasives mentioned represent a small percentage of material available to give a specific finish required on a particular substrate. See Table I for typical hardness values. Although the wheel speeds for buffing with grease bars will vary greatly from job to job and operator to operator, the figures in surface feet per minute given in Tables II and III will serve as a guide for hand buffing operations. Buffing wheel speeds for automatic operation may vary with the design of the machine and the contact of the work to the wheel. It can, therefore, be more definitely fixed without depending on the physical ability of the hand buffer to main22

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