Tolerances on Copper Plated Threads

find your finished sizes and work backwards

Normal ISO M20 has a 2.5 pitch, your 6 pitch would increase the helix angle of the thread to the point where it is probably seizing on the thread angle as its locked up, if you neeed a fast pitch use a multi start thread with conventional dimensions or change the thread form to one suitable to fast helix angle.s

Opps! Sorry, I didn't read properly, M260 X 6P

Check the machinest handbook. Many companies such as Starrett have programs that calculate all the dimensions needed. You just give it the major diameter, lead (Pitch) and the class of fit you want.

I work for a customer fabricator. Everything we build is sanitary finished 316 or 304 ss, so we know ss. We NEVER EVER use an ss nut with an ss bolt. Try monel for the nut. Stainless will gall to itself nearly everytime. We can't use an ss bolt with a tapped ss frame work either. Then the bolt has to be a different material.

I am not as familiear with ISO Metric threads, but ANSI standards covers inch-size threads and there are several classes of fits. Tables give the maximum and minimum sizes for each class. Besides that there are also different pitches, course, fine, very fine, etc. which could have a bearing on (pardon the pun) the friction exerted on your two mating surfaces.

I like the idea of plating on one of your components. Just be sure to include the plating thickness in your machined dimensions allowences and compensate (adjust) for the extra thickness, i.e. machine off more steel if thickness will be plated back on.

As another one suggested, check The Machinists Handbook, but be sure it is a recent edition as older editions do not contain the metric thread data.

Danger here! Look up Galvanic series. Copper is Kryptonite to stainless steel's Superman if water is present. Here is excerpt from key to steel: The position of the copper alloys in the galvanic series suggests that copper alloys will not suppress crevice corrosion in stainless steels and, in fact, may accelerate crevice attack once it has started. Experience indicates copper alloys provide no useful galvanic protection for stainless steels. http://www.key-to-steel.com/default.aspx?ID=CheckA rticle&NM=160 Thou hast been warned. Why copper plate? Milo

Why copper plate? For lack of a better material that will still work in the application. Perhaps he should use Unobtainium! (grin)

Reread the original posting/question. His mating threaded parts (bolt and nut) were damaging each other, tearing away at the surface. This is common when there are two identical materials in contact under pressure. There is a friction welding or cohesion effect that resists dissembly of the mating surfaces. It is especially common in screw threads which must make repeated contact. That is why steel leadscrews are mated with bronze or plastic nuts. This damage is known as "galling". As he discovered, even oil/grease lubrication is not a complete solution, since the high pressure displaces the lubricant film. This is not a corrosion problem, but a mechanical surface problem.

Copper may not be the best choice, from a galvanic standpoint, but it would extend the life of his parts beyond what he is experiencing now, which is practically nil.

Thanks for answering my rhetorical question. I really don't need to reread, I understood what they were trying to do, but how did they come to pick that solution? Perhaps I should have left off the word "plate." I understood why he wanted to do it, just couldn't figure out why copper plate instead of ferromanurium, your unobtainium, or kryptonite, etc. Of course it galled. "So now lets add something that will give us a long term failure was my point." You are correct in saying it is a mechanical engineering problem -today- when disassembly is tough. In a few years in service, after moisture does get to these fasteners, it will be a corrosion problem; might escalate into a legal problem if bolts fail due to galvanic corrosion. Lawyers will try to determine designer of record. Question 1) are you / were you familiar with the galvanic series? No further questions your honor. We must look holistically at everything about the application, not just the immediate "problem". Engineering should be more than reacting to current difficulties, thats all. Galvanic series should be first thing anyone thinks of when thinking of combining dissimilar metals. That's the real point I tried to make. Thanx for your efforts to clarify. milo