Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 332 of 707

Fig. 8. Cross-section of herringbone-style perforations to keep small-diameter, straight parts inside barrel. 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 deter- mines the variety of work the barrel can process. Flexible-cable dangler-type con- tacts 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 assem- bly 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 opti- mum 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 mounting point of the danglers. For short barrels or stiff dangler cable, the danglers can be extended beyond the midpoint of the barrel to provide contact at the opposite end of the barrel to insure that they remain submerged in the load. Special dangler contact knobs have been developed to help maximize perfor- mance when a standard configuration is not totally adequate. Custom knobs that are heavier can be specified to help ensure they remain submerged in the workload. Also, special knobs with larger contact surface area are available where improved conductivity is important. Special dangler contacts can be ordered with the knobs made of stainless steel, titanium, or other materials. This is important when the mild steel knobs of standard danglers would be negatively affected by the type of plating chemistry used. Be aware that the alternate materials will probably exhibit lower conductivity. 331

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