• Topic 57493 "Zinc Plating Internal Threads: dimensional changes & coverage issues"

A. 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.

A. 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,

My kind friend Khozem treated my response too charitably :-)

I missed the facts that studs will be hardened steel, not mild steel, and it is indeed difficult to alkaline zinc plate hardened material, so switching from acid to alkaline seems more likely to compound your problems than to simplify them as he warns. More importantly, his hands-on experience easily trumps my book knowledge!

Regards,

A. You need to have undercut in threads. This means threads are deeper before plating. The nominal diameter is reduced to compensate for the plating thickness depending on the plating thickness. Some also drill larger hole and tap accordingly in matching part if you are buying standard bolts. Have you tried minimum zinc plating on the bolt? Some standard companies sell plated bolts which will fit correctly as they have undercut on bolts.

A. Hi Brijesh. Yes!

Regards,

A. Hi Anna. I don't think that your interpretation is correct. Looking through earlier revisions it appears that Class C is 0.0015" thickness persists across all of them. Further evidence, besides the spec itself, is that an earlier revision had Class C deleted, and called for substituting Class B (0.0001") for it. Working to canceled specs is always a problem :-(

Luck & Regards,

A. Hi Mike

This is an age old problem. The answer should be clearly shown on the drawing - but it rarely is. The only person who can clarify the position is the designer - his name should be on the dwg. Don't waste time speaking to the buyer. He will not have the authority or probably the knowledge to make the decision. Make the point that you cannot risk your company's reputation on a wrong guess and that the parts will not be plated until you have written confirmation (guess who will be blamed if you get it wrong). If you make enough fuss you may save yourself and others having the same problem on future orders.
It is possible to buy tap and die sets with "built in" plating allowance but by the time the parts hit the plating shop it is far too late.

A. I have worked in aircraft fastener quality for 40+ years. The parts as finished must meet all drawing dimensions within the tolerance limits listed on the drawing. Usually the procurement specification will have that statement within it.

A. - 6h ring gauge

A. Hi Himanshu. Although the chromate conversion coating will have no effect on the dimensions, someone must specify the thickness of the zinc plating before your question can be answered because the plating can range from 5µm nominal for mild service to 25µm for very severe service.

Once the thickness is known, guidelines like MIL-S-7742 can be applied, wherein the short answer is that plating adds 4X the plating thickness to the pitch diameter. Good luck!

Regards,