Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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work. The work energy of the flying particles is also affected by the distance from the nozzle to the work surface. It is usually best to keep this between 4 and 8 in. to avoid loss of velocity, and to gain maximum acceleration and proper diffusion of particles into the most desirable pattern. BEAD CONSUMPTION Because beads can become broken after repeated impacts on the work surface, con- trolling 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 capa- ble 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 clean- ing 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 effi- ciently. 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 usu- ally 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 con- sideration. 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 gen- eral 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 spe- 55

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