Letter 22409

Electrodeposited Zinc Plating Thickness and Tolerance  

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What is a reasonable thickness and tolerance to require for electodeposited zinc on parts with external threads? We have experienced substantial fallout on external threaded parts requiring zinc plating that do not pass after-plate thread ring gages. Our preplate gages have been setup to allow for .0001-.00035" plate thickness. Our current plating supplier is asking for us to loosen the tolerance to .0002"-.0005" stating that this is as good as the industry can provide. I have several concerns with doing this: the cost of new pre-plate gaging, the effect of .0005" plating thickness on the 60 deg. thread form, extensive print changes, and conflict with our existing customer prints.

Gary Buell
- Fond du Lac, Wisconsin, USA

The answer will depend, in part, on what average or minimum plating thickness you are specifying. It is unusual to specify less than .0002" average plating thickness, so .0002"-.0005" doesn't sound at all unreasonable to me.

Ted Mooney, P.E.  finishing.com Brick, New Jersey

Gary & Ted, This might sound as a very stupid comment, but we have encountered this problems in the past and some clients don't consider that the thickness of the deposit has to be counted double when considering a round diameter. Thickness in each side of the part. Also, in special applications the thickness on the threads is considered that it afects 4 parts of the thread; the right hand side of the thread (creast), the left side of the same thread and the same happens on the other side of the fastener.

Now, what are you using as a nut? Have you considered the thickness on this part also? All of these increase the original diameter. I hope this helps Enrique

Enrique Segoviare
- Monterrey, Mexico

To shed more light on our zinc plating thickness issue, most of our in-house thread ring gages are class 2A gages. In the past we have attempted to produce threads to a undersize pitch diameter (less 4 times plating thickness) and use the 2A gages to check afterplate condition. If I'm reading SAE J476A correctly we should be allowed to go to class 3A gages to allow for the thread growth on external threads. Is the class 3A gaging option more forgiving for a thicker plating allowance (.0002-.0005")?

Gary Buell
- Fond du Lac, Wisconsin, USA

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Ed. note: The following letters also seem to offer insight into thread allowances for plated parts: 2906, 31694.

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