Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 55 of 707

Fig. 1. Typical peening intensities achieved with solid glass beads using suction-type equipment with 1/2-in. diameter nozzle at 90o for A and H, and 85o for D. 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 sur- face. Glass beads are often used as a secondary peening medium, removing con- tamination 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 material is thrown against the target. The formula: I = MV2 /2 indicates that impact energy (I) equals one half the mass or weight (M) times the square of the velocity (V) at a 90o for other angles. nozzle angle. Correction factors should be used As a general rule, the smallest particle that will provide the desired effect on the surface is the most efficient one to use, as this gives the greatest number of impacts per pound of glass spheres. When the nozzle is at a 90o angle to the surface being treated, the bounceback of beads has a "blinding" effect. This interferes with the effectiveness of the blast stream and tends to increase the rate of bead consumption through break- age. Generally, an angle between 45 and 60o 54 will give the most effective perfor- mance. In some cleaning applications, still lower angles may help speed the

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