Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 52 of 707

mechanical surface preparation MECHANICAL SURFACE PREPARATION IMPACT BLASTING WITH GLASS BEADS BY ROBERT C. MULHALL AND NICHOLAS D. NEDAS POTTERS INDUSTRIES INC., VALLEY FORGE, PA.; WWW.POTTERSBEADS.COM Glass beads were originally used for decorative applications. Their use as a medi- um in impact blasting came about largely as a result of the aerospace buildup of the 1950s. At that time, a need developed for multipurpose media that combined the advantages of coarse, organic, metallic, and fine angular abrasives. Table I shows a comparison of glass beads with other impact abrasives for cleaning, fin- ishing, peening, and deburring applications. Impact blasting with glass beads is well placed to satisfy demands of the 1990s for an energy-efficient and environmentally acceptable method of metal finishing. When properly controlled, the system is safe for workers and spent media presents no disposal problems. PROCESS BENEFITS Glass beads are virtually chemically inert. This factor, combined with their spherical shape, provides several key benefits. Media consumption is minimized; Table II compares consumption data of impacting media on different metal sur- faces of varying hardnesses. On both metals tested, glass beads offer the lowest consumption per cycle. In addition, close tolerances are maintained and glass beads remove a minimal (if any) amount of surface metal. Impacted surfaces are free of smears, contaminants, and media embedments; high points are blended and pores sealed. A wide range of finishes from matte to bright satin are achievable. The peening action of the media further acts to impart a layer of compressive stresses on the surface of the part. This increases fatigue life, decreases susceptibility of the part to stress corrosion, and enhances surface strength. PROCESS ENGINEERING Proper design of impact blasting equipment is essential for each application to achieve the full benefits of high productivity and low costs. Most important, the system should be easily controllable to produce consistent results. Key to this control is determination and maintenance of the "arc height peening intensity" of the operation. To measure the peening intensity in a par- ticular application, special steel strips are bombarded on one side only by the blast- ing media. The compressive stress induced by the peening action causes the strip to bow in the direction of the blast. A series of values of arc height versus blasting time are obtained, and when plotted on a graph, yield a saturation curve. From this curve, the arc height peening intensity can be obtained. Environmental factors, operator skill, OSHA standards, and equipment capa- bilities are the process parameters involved in all glass bead blasting opera- tions—whether they are cleaning, finishing, peening, or deburring. Once all the variables are optimized and the arc height peening intensity determined, process 51

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