Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Table I. Hardness of Abrasive Materials Abrasive Type Aluminum oxide (fused) Aluminum oxide (calcined) Tripoli-silica Silicon carbide Iron oxide (red rouge) Chrome oxide (green rouge) 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 approxi- mately 1,900 to 2,400o C. The material is cooled, ground, and sifted to the required grit size similar to the processing of fused aluminum oxide. The crystalline struc- ture of SiC is a hexagonal. 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. Red Rouge The chemical formula for rouge is Fe2 O3 um oxide is 9 Mohs as opposed to iron oxide, which is 6 Mohs, and is used to pro- duce an exceptional luster or color on ferrous as well as nonferrous metals. These abrasives mentioned represent a small percentage of material available to Green Rouge The chemical formula for chromium green oxide is Cr2 O3 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 def- initely fixed without depending on the physical ability of the hand buffer to main- tain the correct position and pressure against the wheel. LIQUID SPRAY BUFFING Liquid spray buffing compositions have largely replaced bar buffing composi- tions on automatic buffing machines. Unlike the bar compound previously dis- cussed, liquid buffing compound is a water-based product. The liquid buffing compound has three main constituents: water, binder, and abrasive. Water is used as the vehicle to transport the binder and abrasive to a buffing wheel through a spray system. This water-based liquid is an oil/water emulsion. In this emulsion the abrasive particle is suspended and could be thought of as particles coated with a binder material. The emulsifying materials act as a device to hold the oil-soluble mol- ecules onto the water molecules. Larger abrasive particles offer less surface area (when compared with the weight of that particle) than several smaller particles. Surface area and density play an important role in the suspension of any liquid emulsion. Stability is the ability to 22 . The hardness of chromi- ; it is also called jeweler's rouge. Its Chemical Symbol Al2 Al2 O3 O3 SiO2 SiC Fe2 Cr2 O3 O3 Mohs' Scale 8-9+ 8-9+ 7 9.6 6 8-9

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