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E-coating threads, bores and pins - What is the effect?

I am a black belt working for a large construction equipment manufacturer and we are currently e-coat and top coating the majority of our parts. However we mask most pin shafts, bores and threads against e-coat and top coat paint. We would like to mask only against top coat paints. However we are receiving resistance from our engineers. Who are saying that e-coated threads suffer from a clamp force loss between 20 and 45%. Also that e-coated threads will suffer from e-coat pooling depending on the attitude of the boss. Can anyone provide me with information to help persuade the engineers to allow e-coat into threaded holes? Also our engineers believe e-coated joints will suffer from joint slop as the e-coat wears away in bore and pin joint. They are therefore prepared to only e-coat with the pin or the bore but not both. Is this a valid argument? Any help would really be appreciated.

Andrea Rees
Construction equipment supplier - Leicester, Leicestershire, England
2003

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As an engineer, I'll side with the engineers, Andrea :-)

Your letter doesn't mention any actual advantage to e-coating these areas, and all I can infer is that it would be "easier". That's probably not sufficient reason to justify the sloppy joints. Try to tell the engineers about some advantage to the e-coating these areas, not just "the equipment will be cheaper". If you can't, well, cheaper and crummier isn't enough justification :-)

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

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Andrea, your engineers are correct. E-coat will pool and fill thread surfaces, and greatly affect the joint characteristics (tightening behavior, resistance to vibration, etc.).
If I were them, I would not relent to your request.

Toby Padfield
automotive parts/module supplier - Michigan
2003

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December 28, 2009

Hi Toby, do you have any specifics or references regarding the effects on joint characteristics, and of pooling (final thread dimension). I am dealing with this question as a design issue as well and would really appreciate some additional information.

Thank you,
Paul

Paul Borloz
- Petaluma, California

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Paul,

This is such a well-known issue in the automotive industry, that I'm not sure I would be able to find a reference that describes the issue in detail. What I can tell you is that there are many specifications developed for thread masking products (essentially fluorocarbon coatings) that are routinely applied to millions of fasteners in order to prevent E-coat from depositing on them. Nylok and ND Industries are two suppliers of these types of coatings. A Google search using "GM6076M" (their specification for fluorocarbon masking coatings) will provide some information on the subject. Searching Google or Google Patent using the keywords "fluorocarbon thread masking" will also provide some background.

Toby Padfield
automotive parts/module supplier - Michigan
January 13, 2010