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Zinc Plating Thickness on I.D.

We are zinc plating hubs for blower wheel (.0002-.0004). The hubs are .72 and .98 in diameter with various hole sizes .313/.314, .3755/.3765, .5005/.5015.

We drill and ream the holes to the nominal (using a hydromat) and check ok before plating. But after plating some of the hub's ID are undersize, ex .3127 instead of .313. These hubs most always are undersized by the ends of the holes. I understand that the is more of a chance for the zinc to build up here because of the electric concentration but by a factor to two or three times?

My question is - can it build up by a factor of two/three times?

Please advise and explain.

Thank you,

Kevin Ziegman, QAM
- North Ridgeville, Ohio

Yes, thickness variations of two or three times, or even more, is not a lot. However, you should determine if your plater is using an acid or an alkaline process as the alkaline processes offer significantly more uniform thickness, and it may be possible to reduce the thickness variation without redesigning the part. You should also tell us if the parts are racked or barrel plated.

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


Because you are plating an I.D. you actually reduce the I.D. by 2x the plating thickness. We plate eye unions and we will spec the pre-plate I.D. oversize by 2x the upper specification of the plating thickness so as to be in specification after plate. However since yours is a close fit, (with I assume a shaft) you may want to plug the I.D. to prevent plating build up from changing the I.D. For your future reference, if you plate a thread your thread pitch will increase 4x the plating thickness. I hope this helps.

Kevin Davis
- Versailles, Kentucky, USA

Mr Ziegman:

I have been plating hubs similar to the ones you speak of for quite some time. I can relate to your problem because I have had parts over plate in this area. You are correct in your posting of concentrated electricity, or High Current density. Typically Chloride Zinc baths (acid zinc), because of their very high efficiency, can fluctuate in distribution well beyond the 2x you are referring to. First thing I would do is to inquire about your suppliers chemistry. If its alkaline and you can pinpoint some kind of frequency, I might have some other suggestions.

Marc Muzzarelli
- Muncie, IN, USA

Thank you all for your responses. We are looking into the plating bath make-up as you have suggested. We are also looking into reaming the holes bigger with the anticipation of the hole closing up. If you have any other suggestions I welcome them and thank you in advance.

Kevin Ziegman
- N. Ridgeville, Ohio

Ed. note: The following letters seem to offer insight into thread allowances for plated parts: 2906, 22409, 12211.

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