How to cut threads on a lathe

Assuming your leadscrew is 8 tpi, cutting 10 tpi means you need to feed the carriage back to the start using the leadscrew. You can't use the thread dial unless the tpi is a multiple of 4. For the 20 tpi, you can use the thread dial. Since it's a multiple of 4 but not 8, use either all even numbers or all odd numbers.

The compound should be at 29.5 degrees, not "about 30 degrees".

1/2-20 threads have a minor diameter of ., so the depth to cut is (.5-.)/2 = .. At 29.5 the infeed is .. You can take a bigger DOC at the beginning of the thread and reduce it as you go in. Rather than relying totally on the compound dial, I would use a nut to try the fit.

You need to have the cutting bit aligned perpendicular to the work or the threads will be malformed. Use a 60 degree "center gauge" for this. You can order one from Enco for $6. The center gauge also is useful in grinding a HSS tool to verify the 60 degree angle at the tip. To save the hassle you can get carbide inserts for threading that are precise. The tip needs to be at the midpoint of the work.

The other way to get a good final thread is to single point it most of the way and use a die to finish. The single point assures that the dies will thread on straight. This is what I like to do on small diameters especially.

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