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Author Topic: [Q] On Bronze Rods, and Choosing a Thread Diameter  (Read 5263 times)
von Corax
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« on: March 19, 2013, 08:05:17 am »

I have a package of 1/8" bronze welding (brazing?) rod which I intend to use in the construction of a series of electrostatic generator thingumadoodgers (the correct technical term. Tongue) Specifically, I plan to thread the ends of the rods to allow the attachment of brass/bronze discharge spheres, insulated handles &c. The problem is that my thread-cutting dies in this size range are not marked in inches, but in "number" sizes, and I have no experience in converting.

Could someone with more (or, indeed, any) knowledge in this area please advise me on the correct choice of thread diameter for these rods?

Much obliged,
Prof. Darwin Prætorius von Corax
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Narsil
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« Reply #1 on: March 19, 2013, 10:12:18 am »

There are quite a few different thread standards, if you are cutting both the male and female threads yourself with taps and dies from the same set it shouldn't matter too much. Do the numbers marked on your small dies increase or decrease with size ? it it possible that they are marked in 16ths or 32nds of an inch for clarity. ie number 2 might be 2/16ths ie 1/8th . There are also various arbitary number conventions (#) for small diameter threads where fractions are impractical.

The most common non-metric standard in the US  is UTS in which case 1/8" corresponds to a number #5 tap with a #38 or #37 drill for coarse and fine respectively. But is would be best to check as there are other possibilities.

Threads are almost always specified by the outside diameter of the male fastener (the rod in this case) so you definitely want a 1/8th die. The female thread will require a hole which is slightly undersized.
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von Corax
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« Reply #2 on: March 20, 2013, 12:18:55 am »

I have both taps and dies for #4-40NC, #6-32NC and #8-32NC machine screw threads (although I can of course buy other sizes.) I'd like to know what diameter die I should use so I can look for balls with the same diameter thread; I can then choose a thread pitch to match the balls. (ie. Match hole diameter to rod diameter, then match rod pitch to hole pitch.)

As I plan to purchase the balls, not make them, I want a readily-available size, and I don't recall ever seeing a #5-threaded anything anywhere.
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Narsil
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« Reply #3 on: March 20, 2013, 12:54:54 am »

OK, I think that the answer to your question is that the nominal diameter of the thread is the diameter of your rod. I'm not very familiar with UST threads but this is the normal convention.

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von Corax
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« Reply #4 on: March 20, 2013, 01:17:06 am »

I think you may be right about 1/8" being #5. However, as I don't expect to find other parts in a #5 thread, would you recommend I go down to #4, or up to #6?
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Narsil
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« Reply #5 on: March 20, 2013, 01:26:56 am »

Do you have taps as well as dies ?

Given that the application of for an electrical connection i would suggest that the priority is a large contact area rather than mechanical strength in which case I would probably favor forcing an oversize male thread into an undersized female one.

On the other hand If you have good soldering facilities (ie gas torch) I would go for an over-sized female thread and solder or braze the joint, or if you can mechanically support the joint use conductive paste.

I admit that this is distinctly elvish advice but there are ways that you could make both work .
« Last Edit: March 20, 2013, 01:41:20 am by Narsil » Logged
von Corax
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« Reply #6 on: March 20, 2013, 08:31:07 am »

It actually did occur to me (after my last post) that I could just solder the thing together; I have a butane microtorch, and I'm fairly sure there's a propane plumber's torch around here somewhere.

Nevertheless, it still might happen that I need to thread for readily-available hardware (eg. hex nuts, t-nuts, acorn nuts &c.), so the question stands: would I be better off threading it for a #6 or a #4 thread?
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« Reply #7 on: March 20, 2013, 09:10:22 am »

Maybe this will help. The Unified Thread Standard (UTS) is in Wikipedia:

http://en.wikipedia.org/wiki/Unified_Thread_Standard

The Major Diameter (outer thread size) is identical to the rod diameter.  I would not deviate from the #5 if I could avoid it (you have a whopping 0.3mm diameter difference in either larger or lower size.  Doesn't sound like much, but I'm doubtful), but your hardware options will be limited.  The only other reasonable option is to thread to #6, at the expense of mechanical strength, and if it's too loose (you will have partial teeth on the rods), then solder like Narsil suggests.    


http://www.appropedia.org/Cutting_screw_threads

Quote
The best plan, when both tapping and threading have to be done, is to tap the hole first, as this is of fixed size, and then cut the bolt-thread to fit. The rod should be slightly tapered near the end to give the die a start, and the dies can then be adjusted to fit the rod and the two halves tightened sufficiently to hold the stock and dies in position. Be sure to keep the stock at right angles to the rod which is to be threaded.

Turn the stock until a shallow thread of the required length has, been cut. Next, turn it back to the end and tighten the dies a little more; repeat the process until the thread has been cut to the correct depth. Test by using the nut previously tapped. If the nut will not go on when the thread has been fully cut on the rod, the rod is too large in diameter and part of the threads must be filed off before the dies are run down again. It is not wise to follow the practice of some metalworkers of using the dies to reduce the diameter of the rod. This will not only injure the die but it is almost certain to damage the threads or completely strip them. If the rod is of small diameter, it will probably be twisted off.


You could try to reduce the diameter to #4, but just know you are sacrificing the die...

Also note size 6-32  (the same size as American light switch cover screws) is more common than 4-40.  Most brass hardware comes in size 6.  Size 4 nuts are usually in 40 threads per inch (fine) and are pretty much the smallest you can get in your regular hardware store (only Lowe's in the Americas carries size 4-40 brass nuts.  Acorn brass nuts in 4-40 and 6-32 are obtainable only from speciality stores and they are as rare as diamonds; often times I have to substitute for regular brass nuts if the stores run out).  I would go for #6-32 just for availability of parts.
« Last Edit: March 20, 2013, 10:01:21 am by J. Wilhelm » Logged

Narsil
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« Reply #8 on: March 22, 2013, 10:47:43 am »


Yeah trying to mix different thread sizes won't really work at all unless the male thread is undersized and you reinforce the joint with adhesive or solder, in which case there isn't that much point cutting a thread in the first place.

You could try reducing the diameter of the rod at the end and using a smaller diameter thread but this is not an easy task.

One option would be to obtain some screws of the correct diameter, solder or braze them onto the end of the rod and then file the joint flush.

As mentioned already, when cutting threads with dies the male thread size is determined by the diameter of the rod you start with, you can't really cut a smaller or larger thread than the appropriate one unless you change the rod diameter.
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