Metal Finishing Guide Book


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Page 380 of 843

Fig. 8. Cross-section of herringbone-style perforations to keep small-diameter, straight parts inside barrel. As mentioned above, there are other types of barrel perforations available to the plater. These include herringbone, screen, fine mesh, and slots. To produce herringbone perforations, the barrel panels are drilled halfway through each panel at a 45° angle relative to the inside and outside panel faces (see Fig. 8). In this way, the holes intersect at the middle of the panel in a 90° angle. Smalldiameter, straight workpieces, such as nails, pins, etc., cannot pass through the perforations because the holes are not straight. Plating solution and current can pass through the perforations, although at a reduced rate. Barrels with fine-mesh panels with very small openings are generally made of polypropylene and are used to plate very small or delicate work. Larger workpieces will tear, gouge, or wear through the mesh in an unusually short period of time. Some barrels are manufactured with thinner panels in perforated areas to aid drainage. Consideration must be made that this may come at the expense of barrel integrity and service life. Cathode Electrical Contacts The type of interior cathode electrical contacts in a barrel significantly determines the variety of work the barrel can process. Flexible-cable dangler-type contacts are the most common in barrel plating (see Fig. 9). Dangler contacts are dynamic relative to the workload because the workload rotates with the barrel and tumbles over the danglers. The danglers remain fixed to the barrel support assembly as this occurs. Other types of dynamic cathode contacts are "hairpin" and chain. These are uncoated metal, usually steel, contacts that extend into the workload to enhance cathode contact for specialized workpiece types. The best barrel plating results are achieved when the end contact surface, or "knob", of the danglers remain "submerged" in the workload as a result of optimum equipment design. This configuration causes danglers to maximize electrical contact and minimize, or eliminate: arcing, sparking, or burning of the work that could otherwise result from inconsistent/intermittent contact which is "making" and "breaking" contact. The contact knob end of each dangler should touch the bottom of the barrel one-fourth to one-third of the inside barrel length from each barrel end. To determine proper dangler length, measure the total distance from the point that the dangler contact knob should touch the inside bottom of the barrel, continuing through the barrel hub (trunion) to the outside 373

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