Metal Finishing Guide Book

2013

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BEAD CONSUMPTION Because beads can become broken after repeated impacts on the work surface, controlling bead consumption is of critical importance. It is affected by five key factors: 1. Bead size—the larger the bead, the more durable and resistant to breakage it is at a given impact intensity. This preference for larger beads must be balanced against the greater efficiency of smaller size beads, which are capable of the work required. 2. Uniformity of size—proper sizing also affects efficiency of operations. The wider the range of bead sizes in a particular "charge," the higher the rate of consumption at given conditions. 3. Roundness or sphericity of beads—the more spherical the individual beads, and the freer the "charge" from nonspherical particles, the lower the rate of bead consumption. 4. Surface hardness of material being treated—the harder the surface being treated, the higher the rate of bead consumption. 5. Angle of impingement—the closer to 90o the stream of beads is to the work surface at a given arc height peening intensity, the greater the rate of bead consumption. APPLICATION NOTES Cleaning Because of the wide variety of different materials that must be removed in cleaning operations—including mill scale, rust, carbon buildup, and the like—it is often best to experiment with different nozzle angles to find which works most efficiently. Where there are internal recesses and other difficult areas, the use of the smaller bead sizes may be particularly helpful. Because a high cleaning speed usually minimizes labor cost, bead size and nozzle angle are the key considerations. Normally, a velocity that optimizes cleaning speed with a given size of bead will optimize consumption, to give the lowest total cost. Finishing Where appearance is of prime importance, bead size is normally the key consideration. Velocity, nozzle angle, and other factors should be adjusted, first to give maximum finishing speed, and second, to minimize consumption. This will provide the lowest total labor and material cost per unit of production. As a general rule, large beads at high intensities provide a deep matte; at low intensities large beads give a smooth, bright surface; small beads at high intensity give a dull matte, and at low intensities a bright satin. Selective masking of surfaces, the use of multiple nozzles, and a "painting'' motion may be employed for highly specialized decorative effects. Automated machines are generally used for finishing. Peening Peening to increase fatigue resistance or to increase stress corrosion resistance is essentially a uniform "hammering" operation. Uniformity of bead size and control of the number of nonround and angular particles included is critical to process performance. The key consideration is impact intensity, which must be specified as minimum and maximum. Nozzle angles should be as close to a right angle as possible without excess bead consumption. In general, the larger bead sizes, because of their resistance to breakdown, will prove most cost effective. In peening fillet areas, it is a standard rule that beads no larger than one half the radius should be used. 52

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