Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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rotates. Turntables can also be situated on carts to move a heavier part into and out of the cabinet. Automation packages may consist of powered rotary turntables, multiple blast guns, oscillating movement for the blast guns, and timer controls. They cov- er more area faster, enable the operator to perform other tasks, and often increase part-to-part consistency. Ergonomic designs are relatively new to the market. They conform the machine to the operator, rather than the other way around. Blast cabinet operators can therefore perform at peak efficiency longer and turn out higher quality finishes because distractions have been eliminated. Common ergonomic modifications include a "sit-down" cabinet, padded arm rests, a positionable foot rest, and sound- suppressing devices. Custom modifications build the machine around a particular part or process. For example, an extra tall part may require a higher cabinet ceiling and two operator positions to allow access to the full height of the part. The same part may be conveyed around the plant on an overhead monorail, so an overhead cable slot may be cut into the roof of the cabinet. Automatic Blast Systems Automatic blast systems can increase production and part-to-part consistency when the expense can be justified. They are dedicated to a single workpiece or fam- ily of parts. The basic elements of an automated system include material handling (conveyor, rotary, indexing satellites), fixturing, blast (suction or pressure, multi- ple-oscillating guns), dust collection, media conveying and conditioning, and controls. Wheel Blast Mechanically propelled blasting machines differ from air-blast systems in that they apply the media to the workpiece by centrifugal force from a power-driven, high- velocity bladed wheel. They also lend themselves to automatic and semiautomat- ic production techniques. Cabinet mechanical blast finishing is the most common, but tumbling equip- ment is also used. The wheel is enclosed in a protective housing, so there is no dan- ger of stray abrasives. Considerable wear can be expected; therefore, the parts are designed for ease of replacement. Heavy rubber mats are also used to pad worktables and prevent damage from abrasive shot. Wheel blast equipment covers a wider blast swath and can impact harder than air-blast equipment. Media used are usually limited to steel shot and steel grit because they are durable, less erosive to the equipment, and have maximum "throw weight." Wet Blast Wet blasting is a precision finishing operation. It normally consists of an air- blasted slurry of fine abrasive suspended in chemically treated water. Wet blasting can be controlled to avoid metal removal and hold dimensional tolerance to with- in 0.0001 in. It is also used to hone multitooth hobs and finish fragile items such as hypodermic needles. Wet-blasting equipment usually incorporates a cabinet. It is frequently modified with auxiliary strippers, take-off conveyors, and wash-rinse-dry stations. Although wet blasting is usually reserved for small, delicate workpieces, it can be used to remove light surface residues, blend scratches, and correct other surface defects on large pieces. In addition, wet blasting is used to reveal scores, heat- 47

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