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Standard tolerances on zinc plating

The engineering department at my company has specified a thickness range of .00015 to .0002 on steel shafts. The coating is just standard clear or I think sometimes called blue zinc. Our supplier is having a problem achieving this thickness do to a condition called "dog boning". Dog boning is described as a build up of coating at the ends of each shaft. This is caused by the pull of current form the electrodes in the tank. The shafts are approximately 1-3 inches in diameter and can range from 4-9 feet long. Is this thickness parameter achievable? What is the standard tolerance of zinc plating?

Richard Hartbeck
- St. Louis, Missouri
2003

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First of two simultaneous responses

It is absolutely achievable, but may require the use of robbers or thieves, or shields, or rearranging the anodes so that they are directly across from only the center of the shaft.

James Watts
- Navarre, Florida
2003

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Second of two simultaneous responses

There is no "standard tolerance" but your range sounds quite restrictive in view of the fact that not only is dog-boning to be expected but egg-shaped plating as well. (Or maybe I should say cam-shaped).

With some experimentation the dog-boning can be fixed with large diameter plastic pipe acting as shields on the ends of the shafting if it is impractical to remove the anodes near the ends of the shafts due to their varying lengths.

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
2003

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Have you heard of electroless nickel. Holding this thickness in any electrodeposited coating will be nearly impossible. It will also vary with the type of zinc plating chemistry used: acid will "bone" more than cyanide and alkaline non-cyanide will "bone" the least, however, holding 50 millionths tolerance over a 4-foot to 9-foot shaft is a challenge. With 100 pieces, you might get 20 good with some fixturing and 75 with real close process controls and fixtures, however, there will always be much higher fallout than desirable.

Milt Stevenson, Jr.
Syracuse, New York
2003

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I believe that your electrolyte should be considered carefully. Alkaline Non Cyanide Zinc should give deposits with better thickness distribution than acid zinc baths.

Thieves or Robbers can be used after trial and evaluation to get the most uniform deposit with Zinc on your parts.

Ed Budman [dec]
- Pennsylvania
With deep sadness we advise that our good friend Ed passed away Nov. 24, 2018
2003