Unacceptable Plating Thickness on Double Ended Studs
I am having a problem with electroplating zinc on steel. I am using the barrel plating process. The parts are all 3 and 1/2 inches or longer and are usually double ended studs with the hex at various lengths along the shank. I need at least 5-10 microns coverage. The problem is when I get the minimum required at the center of the hex, the both ends are overplated to the point that I can not even get the appropriate ring gage on the parts. If I plate to the gage, the shank has no plating at all. I have tried variations in the process including changes in load size, tumble time, bath concentrations, and current adjustments. Nothing seems to work at giving me an acceptable part. Is this the infamous "dogbone effect?" All advice is greatly appreciated. Thank you for your help.William Travis Boyles
- Altoona, Pennsylvania
Assuming you are using an acid zinc process (about 90 percent of zinc barrel plating is acid zinc), you can probably reduce the dogbone effect by going to a plating shop that offers alkaline zinc or cyanide zinc plating.
The thickness variations you are seeing are a direct reflection of the primary current distribution, which is what you will get in an efficient (acid zinc) process. You need a process which is less efficient at high current densities, so that some of the electricity going to the ends of the studs releases hydrogen and oxygen instead of depositing metal.
Ted Mooney, P.E. RET
Pine Beach, New Jersey
Mr. Mooney is correct in recommendation of the process change but you have to remember that the shank is a non-machined portion and baths which generate more hydrogen and plate evenly may leave the shank portion unplated as well on account of the plating depositing faster in the cut or rolled threads.
In my experience only Chloride Zinc baths are able to deposit on Stud shanks . The only remedy to this will be to ask for a greater dimensional tolerance before and after plating or use yellow or olive chromate to compensate for the reduced plating thickness . regards,
Bangalore, Karnataka, India
I am having same problem on double-ended studs except they are brass and bright nickel plated .0002-.0005. Parts are 2 inches long with 1-inch hex between the 2 10-32 threaded ends. One threaded end is 1/4-inch and the other 3/4-inch. Before plating, appropriate sample size passed QC ring gauge inspection. After plating, about 40% of studs at the short end either strip out when the nut is applied or strip the nut itself out. Some of them won't go through the "Go" gauge at all and some will only go part way. The long end that is threaded has not presented a problem. Can anyone explain this? Thank you in advance.Jere Wright
- Dallas, Texas
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