Metal Finishing Guide Book


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to plate threaded parts and fasteners properly. The tumbling action of the barrel makes and breaks the electrical contact throughout the workload, yielding the most even coverage on the root, mean diameter, and crest of the threads. Part material must not be adversely affected by any baths required in the total plating-process cycle. A trial load is a useful tool for evaluating which type of barrel equipment and technique can be utilized for plating a particular part. Long workpieces and entangling parts, such as rods, bars, or tubes, can be successfully barrel plated. Methods used to plate these parts include long barrels; longitudinal and radial compartments; rocking motion; and various, special stationary contacts (see Fig. 4). Special extra-length barrels allow long parts to fit, whereas compartmented barrels confine movement of long parts and entangling parts, helping to eliminate bridging or entanglement. Limited barrel oscillation or rocking motion (usually 180° of rotation or less) accomplishes the same task by minimizing part movement. To do this, a reversing switch, or contactor, along with an adjustable control timer can be installed on the barrel drive to rotate the cylinder alternately in each direction. The barrel interior can be equipped with stationary cathode contacts to plate small, delicate, or nesting parts (for example, small electronic components with projecting fingers). Stationary contacts rotate with the cylinder so that there is little relative movement between the workpieces and the contacts. As a result, the work cascades over or around the stationary contacts, and less abrasion or edge contact takes place, minimizing the potential for damage to the work (see Fig. 7). Disk, center-bar, cup, strip, button, hairpin, and chain are some types of stationary contact. Certain types of stationary contacts, such as strip contacts, assist tumbling of the work. Parts that are flat or lightweight should be plated in barrels with uneven interior surfaces that are not flat and smooth. A convoluted or uneven barrel interior surface, such as grooved, ribbed, or dimpled, promotes tumbling and eliminates much of the sticking of flat workpieces. When finishing recessed or cupped parts, other smaller parts, which are to be plated to the same specification, may be mixed in with the load to provide contact into recessed areas; however, the cost of the time spent to separate the smaller parts from the others after plating/processing must be acceptable. BARREL EQUIPMENT DESIGN All designs of barrel equipment, including horizontal and oblique, should include features to optimize productivity. Reduction of labor requirements and improved ease-of-maintenance are important factors for well-designed components and systems. Some of these important features are discussed in the following sections. Barrel Construction Barrels should be made of materials that are chemically and physically inert to use in each bath or piece of equipment in the plating line. It is important that the barrels be capable of operation in excess of maximum bath temperatures in the entire system. A plating barrel may expand and contract as much as 3/8 in. in total length due to the different bath temperatures in a plating line. Changes in temperature cause stresses that can "work" a barrel to pieces. This is particularly critical for barrels constructed of materials with different coefficients of expansion. The 370

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