Thread Choice On Larger Diameter

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Robert
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I would like to do some single point threading on 3/4" dia stock to thread into another part. So there will be both internal and external threads. I would like reasonably fine threads but not so fine that my amateur tolerances become an issue. The screw standard of 3/4-10 is obviously too coarse. I was thinking about doing 20 or 24 TPI. What would be a good choice? Is there a rule of thumb for deciding this? Is there a common thread for this size work?

Robert
 
You are in the driver's seat for designing and using your own choices. There are other reasons for choosing coarse or fine threads, even for going smaller or larger than standard thread choices. It depends on what you are after. Fine threads are good for fine adjustment and for leaving more cross section in the parts. Coarse threads are quicker to screw together. If you are a raw beginner at making threads, and there are no real reasons to pick something different, I would start out with standard threads. 3/4-16 is a standard UNF thread. 3/4-20 is the UNEF thread. Since you are practicing and learning, you might also try a M20-1.5 thread as well, it is about the same size. Try to keep your results within the usual tolerances for the size. Use Machinery's Handbook or look them up on the web. Threading often needs to be done accurately, so strive for results that are within a chosen standard, and also for a nice surface finish. It will take practice and experimentation, so ignore the bad ones, the stock can still be used for another project, and bask in the joy of well fitting, nice looking threads that fit together nicely.

Speaking of fine threads, I will be trying to make 1/2"-32 TPI panel nuts today, 3/32" thick, for a Servo power feed toggle circuit breaker. They are an oddball thread but available, 3000 miles away, $25.00 minimum, plus freight. I think I can find a couple of them in a piece of 5/8" O-1 drill rod from my metal rack. It is chucked up in the lathe now, the 9/16" hex was formed on the mill using a hex collet block, and rough drilled, now I am ready to bore and thread the I.D., then part them off to length. They will be delicate toward the end...
 
Bob Korves said:
Speaking of fine threads, I will be trying to make 1/2"-32 TPI panel nuts today, 3/32" thick, for a Servo power feed toggle circuit breaker. They are an oddball thread but available, 3000 miles away, $25.00 minimum, plus freight. I think I can find a couple of them in a piece of 5/8" O-1 drill rod from my metal rack. It is chucked up in the lathe now, the 9/16" hex was formed on the mill using a hex collet block, and rough drilled, now I am ready to bore and thread the I.D., then part them off to length. They will be delicate toward the end...
Click to expand...
Post some pics here for educational purposes if you wish. I have just enough experience threading large diameters to be dangerous.
Sounds like 20 TPI would be a good choice. I will first make a test plug with a threaded OD and try to get it perfect. Then use that to test the internal threads.
I need to make or buy an internal threading tool.
R
 
The advantage of making a standard thread is you can buy an off the shelf nut and bolt to easily check the fit of your OD and ID threads on your parts...

Glenn
 
bigger threads are stronger

but make the bolt weaker

find your happy spot
 
When you call 3/4" "large", it leads me to think that you have a small lathe. If that is the case, you are wise to choose a finer thread to learn on, and even to regularly use on projects that you design or adapt. The standard thread, 10 TPI, requires removal of a fair amount of material, proportionally. It can be done, obviously, but takes more care simply because your machine is not rigid enough to take a heavy cut. You must "nibble" away at it taking light cuts. That makes for a nice, accurate, pretty thread most of the time, but is time consuming and requires patience and diligent care during the process.

In comparison, a UNF (16 TPI) takes considerably less material removal, and the UNEF even less. On a small lathe, that is a good pitch to practice on. Besides, the carriage moves a lot slower making the half nut disengagement timing much easier. Even at full depth, a 20 pitch thread is pretty shallow and is not much of a load on the machine.

Have fun!!
 
Fine threads are stronger than coarse threads, all other things being equal.

Size for size a fine thread is stronger than a coarse thread . This is both in tension (because of the larger stress area) and shear (because of their larger minor diameter)
 
Thanks guys. Very helpful. I guess I should not have implied 3/4" was "large" LOL. Just larger than the typical bolt or screw I work with. I have a SB 10K. I definitely like the idea of the slower carriage travel since my lathe has no brake and takes a while to stop.
R
 
Do you have a threading dial? If so, make a relief groove at the end of the threads and disengage the half nuts when you get there. It can all be done with the spindle turning, no big deal. Practice first on external threads, then try the internal ones. Put some tape or a mark on the shank of the internal tool so you know when to disengage the half nuts. If no threading dial, use your slowest back gear speed and keep the half nuts engaged. A lathe without a threading dial is a real pain when threading, and is pretty easily fixable...
 
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