Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 19 of 707

mechanical surface preparation MECHANICAL SURFACE PREPARATION THE SCIENCE OF SCRATCHES—POLISHING AND BUFFING MECHANICAL SURFACE PREPARATION BY ALEXANDER DICKMAN, JR. ALEXANDER DICKMAN, JR. CONSULTANT, LLC, SOUTHBURY, CT. POLISHING Mechanical finishing refers to an operation that alters the surface of a sub- strate by physical means such as polishing and buffing. Polishing plays a vital role in the development of a quality product. The term pol- ishing is not to be confused with buffing. The definition of polishing is surface enhancement by means of metal removal and is generally done by an abrasive belt, grinding wheel, setup wheel, and other abrasive media. A definite coarse line pattern remains after such a polishing operation. This polishing effect removes large amounts of metal from a particular surface. Buffing is the processing of a metal surface to give a specific or desired finish. The range is from semibright to mirror bright or high luster. Polishing refers to an abrading operation that follows grinding and precedes buff- ing. The two main reasons for polishing are to remove considerable amounts of met- al or nonmetallics and smooth a particular surface. This operation is usually fol- lowed by buffing to refine a metallic or nonmetallic surface. POLISHING WHEELS Polishing wheels can be made up of a different variety of substrates such as muslin, can- vas, felt, and leather. Cotton fabric wheels as a class are the most commonly used medi- um for general all-round polishing due to their versatility and relatively modest cost. Polishing wheels can have a hard consistency, such as canvas disks, or a soft consistency, such as muslin, sewn together. The most popular wheels are composed of sewn sections of muslin disks held together by adhesives. The types of adhesives used include those with a base of silicate of soda and the animal-hide glue type. Felt wheels are available in hard densities to ultrasoft densities. The outside periph- ery or face of the wheel must be kept true and be absolutely uniform in density over its entire surface. Felt wheels can be easily contoured to fit irregularly shaped dimensions. Felt wheels are generally restricted to use with finer abrasive grain sizes. In general, the more rigid polishing wheels are indicated where there is either a need for rapid metal removal, or where there are no contours and a flat surface is to be maintained. Conversely, the softer types with flexibility do not remove met- al at such a high rate. In addition to polishing wheels, precoated abrasive belts can be obtained in any grit size ready for polishing operations. Metallic and nonmetallic articles are polished on such belts running over a cushioned contact wheel with the prop- er tension being put on them by means of a backstand idler. Where a wet pol- ishing operation is desired, the use of abrasive belts in wet operations needs to have a synthetic adhesive holding the abrasive particles to the belt backing. This synthetic adhesive must have a waterproof characteristic. When determining the belt's grit size, the condition of the surface is what will dictate the aggresiveness of a belt. Too aggresive belt can put in larger imperfections than those initially in the surface. 18

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