Can't Seem To Decide: Coarse Or Fine Thread Into Copper Bar

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Hello All!
I tried searching to see if this has been asked here in the past but came up empty. Please pardon the question if it is here & my not-so-great computer skills caused me to miss it. It regards choosing between fine or coarse threads for a particular project.

I can't seem to make a decision regarding whether to use coarse or fine threads for tapped holes in copper bars. Specifically, the project involves bolting tin plated aluminum 'lugs' onto silver plated C11000 copper bus bars. ("C110")

Both the lugs and the bars are 0.250" thick. The fasteners will be Number 10 screw size x 0.625" long. An external tooth lock washer of zinc plated steel will be employed under the hex head of the fastener. Zinc based anti-oxidation paste will be used on the threaded fastener and under the lug. No thread locking compounds will be allowed and no nut will be used on the screw.

The application is electrical and the bars will be subject to ambient storage & use temperatures from about (-) 17 degrees-C to perhaps as much as 38-C. In use, the bars are allowed a 30 degree-C rise above ambient. So there will be expansion & contraction to consider. The bars will be energized in the IEEE low voltage bracket (600 vac & below).

If the bars were made of a steel alloy, I'd default to 10-32 without question, but I am threading relatively soft copper, thus my 'pause to reflect'. In the way too many decades of work, I have always been able to use a nut or captured threaded zinc-steel plate on the back of the bar when using small fasteners, but in this situation a nut or plate would be in the 'arc gap zone'.

Both fine threads and coarse threads are often seen with small diameter fasteners threaded into copper on listed electrical equipment and the "AHJ" (Authority Having Jurisdiction) is remaining silent on the matter. However, I care because: A) Safety is paramount and B) It is important to me to provide my clients with equipment that will provide them with years of trouble free service.

However, since the bars are copper, I thought perhaps coarse threads would be more appropriate since the thread depth is deeper with coarse and the copper is soft.

Part Two of my brain teaser is: I was taught (40+ years ago) that coarse threads should only be used with material whose thickness is 1.5 or more times the diameter of the fastener. With #10 screws at 0.190" major diameter, I can't meet the requirements for coarse threads with 0.250" base material to thread. (If that is, indeed, a requirement & not a suggestion.) Flipping the coin; will fine threads hold sufficiently in the gooey copper? (And, I wonder how often, per hole, times 72, I will have to clean my tap. Better get a gallon of buttermilk!)

What do you all think is the better choice?

Thanks very much for sharing you experience with these materials. I appreciate your help!
Enjoy This Day!
Paul
 
You have plenty of thickness for 10-24 screws to hold in threaded holes in the bus bars. Not too keen on you using zinc plated screws though, even with the zinc grease! I would find some brass screws, I've even seen hard copper screws in use, too. I get mine at the local True Value Hardware store. Also have a bolt store in town that keeps a collection of brass fasteners on hand in sizes up to around 3/8".
 
I have always been taught ( many moons ago ) to use coarse threads in soft materials as they tend to hold up better from stripping out. The exception to this is if the material is not thick enough , then fine threads would be in order to get enough threads to hold well. This is only my opinion and what I was taught.
 
I haven't seen the "1-1/2 times the diameter" rule applied to threads before, just structural components. Anyway, I would use 10-32 and use a roll tap. It will give you very good thread fit and strength and the tap will probably last you all 72 holes.

I used this tap application in a copper bus bar but it was for a car audio application so my real world experience is very limited. Still, it has lasted since 2003 without loosening or any signs of an issue, even in a car with all its vibration. I use No-Oxid conducting grease on my threads.
 
I'd use the 10-32: I'd rather have more threads than bigger ones. You might also want to consider a class 3 fit.

[Edit] I also suggest that you calculate the correct torque for these screws and install them with a torque wrench.
 
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This may not apply to copper, but the diameter of a shear cylinder is larger with fine thread than it is with coarse thread, making stripping harder.
 
Thank You Everyone for your great replies. I sure appreciate you all sharing your knowledge and experience.

Zinc Steel was selected over other materials, 4Gsr, as a listing agency requirement in the project's specifications. In a non-electrical project, I'd go brass, too.
Usually electrical components in switchgear, load centers and panelboards get zinc steel or, as appropriate, stainless steel. For an example, if you have bolt-on circuit breakers in your home, the bus bars are most likely zinc plated aluminum or copper and the screws are zinc plated steel (and in many cases 10-32 into 0.125" material).

In the way-back days of early plug fuses, hard-drawn copper screws were the norm. By coincidence, I just did a project in Henry Ford's Model T plant and many of the original load centers are still in use at the now museum. Thomas Edison was the chief electrician & system designer on that job. It was kind of cool to work on that stuff- with copper bus screws and terminal screws. (And something that Edison worked on.)

Field purchasing stainless, copper or brass, fasteners gets a bit sticky with the listing agencies. There's a ton of paperwork to have the alloy approved. Usually on a job like this one, the fasteners are clearly specified as to length, diameter, pitch and alloy.
I'm scared of hardware store purchased brass for a critical job. I bought some 'brass' screws from the big box store for a decorative project in my yard, spilled them on the workbench & the magnetic tray grabbed them. Magic brass?

I never even thought about a roll tap, Mikey. Great plan to be sure!
I have a great old procunier chuck which, combined with the roll tap, should speed things along handily.

I mis-wrote about the zinc paste on the threads. It should have read 'under the head of..."
It will not be on the threads, just pressure points. The 'no paste rule' is because we have to use a torque driver to install most fasteners on this job and the paste would change the value, risking strip-out or snapping. Lots of specifications on this job- but not thread pitch! Crazy, ain't it?

But, thank you mentioning torquing, John. I do cheat in the field, sometimes. If the unit is subject to corrosive mist, such as a diary wash-down area or salt spray, everything gets paste-even the threads & I use a judgement call on torque. My truck's kind of cool in that respect. If a fastener calls for thread locking compound, the manufacturer publishes the torque value with their specified compound applied.

Thanks Again everyone for your thoughtful replies. I think 32 TPI will be my choice. I'll get the advantages that John & T Bredehoft mentioned. There's a fantastic fastener supplier in Detroit that's been here since the 40's who can not only supply quality fasteners at super prices, but the specifications as well (just in case it becomes a question later...)

Enjoy Tomorrow's Day In This Swiftly Passing Summer!
Paul
 
As far as fastener materials go, I would check to see if silicon bronze was acceptable. Usually is for electrical work, and is much stronger than your base copper bar. I too would recommend NF on the threads.
 
I don't think there would be a wrong thread choice in this situation. I think the thread fit and fastener torque are more critical.
 
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