Metal Finishing Guide Book


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Table II. Consumption Studies of Various Impacting Media Direct Pressure Dry Blasting System Average Consumption per Cycle (%) Blasting Material Target Material Hardness, Rockwell Glass beads (106-153 μm) Al 2024 Al 2024 CRS CRS 75 B 75 B 30 C 30 C Al 2024 Al 2024 CRS CRS 75 B 75 B 30 C 30 C 6.0 Al 2024 Al 2024 CRS CRS 75 B 75 B 30 C 30 C 10.6 Al 2024 Al 2024 CRS CRS 75 B 75 B 30 C 30 C 11.4 Aluminum oxide (180 grit) Soda-lime silicate ground glass (125 μm and finer) Borosilicate ground glass (125 μm and finer) 2.1 3.5 6.7 12.9 13.9 CRS = cold-rolled steel 3 /16-in. I.D. nozzle; 1/8>-in. I.D. grit stem; 90o target angle; 6-in. target distance; 0.006 Npeening intensity; and 25 psi blasting pressure. System control via arc height peening intensity is applicable to all cleaning, finishing, peening, and deburring operations. In cleaning, the arc height technique can be used to maintain process speed. In finishing, profilometer measurements of root mean square (rms) microinch finish can be correlated to peening intensity, thereby eliminating any subjective evaluation of performance. In peening, the degree of compressive stress induced is directly related to the arc height peening intensity. By such control, significant benefits are achieved in terms of labor productivity, reduced supervision requirements, and decline in the number of rejected parts. As indicated in Table I, both steel shot and glass beads are available for peening applications. Steel shot with its heavier density offers a deeper depth of compression, but requires more energy to propel while leaving dissimilar metallic smears (i.e., various forms of contamination) on the part's surface. Glass beads are often used as a secondary peening medium, removing contamination while improving surface texture and finish (lower rms) of the part. Glass beads are also used extensively as a peening medium, achieving a wide range of arc height peening intensities in a variety of applications and industries (see Fig. 1). Typical glass bead peening applications take place before plating and after grinding and welding on aerospace, automotive, and machine tool components. KEY FACTORS IN USE OF GLASS BEADS There are a few key considerations that will help the user to enjoy the benefits of glass bead impact media to the fullest. Whether for cleaning, finishing, peening, or deburring, the work actually done depends upon the amount or weight of abrasive thrown against the target surface in a given time. It also depends upon the speed with which the 50

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