There are two answers to this:

1. You work to the dimensions given in a thread chart eg Machinery's Handbook, etc.

2. You set up with the outer diameter turned to the nominal size (eg for a 6mm screw, you need a turned 6mm cylinder to start with). Then you cut a thread in the conventional way but not so deep that you cut away the outer diameter at all, you need to leave a narrow land between the grooves you cut. Then try a nut which is the same thread. If it goes on nicely, bingo. If it goes on and wobbles about, start again and make your first thread cut less deep. Most likely, the nut won't go on. So, deepen the thread a small amount* and try again. Keep doing this until the nut will go on, and you are there.

* follow the instructions for your lathe, to ensure that the second cut of the thread follows the first. This usually means leaving the lead-screw engaged until the job is finished, winding the tool-post back by reversing the lathe, and not by loosening the clasp-nut. On some machines there is a separate indicator which will tell you at what stage to engage the clasp nut, but the safest for a beginner is to leave it all engaged and wind back.

I do hope this makes sense to you

Regards, Tim

Metric threads, the male thread has a flat crest but the female thread is a deep 'V'. Looking at the thread tables you need to start the male thread a bit undersize in dia. ie 6mm, the intended bar to cut the thread should be approx 5.9 dia. Then cut the thread to allow a nut to run down the thread. Better to have female thread part cut all ready and use that as a gauge.This may seem contra to what Tim has said, but the flat crest has to be inside the female 'V', but remember the thread flanks make the thread so a nominal 6mm dia will make a poor fit! In fact if you cut a 'Vee' thread the crests will clash so the flat is very important on the male thread.

This becomes more evident when cutting larger dia. threads, say 15 or 20 dia. The flats show up clearly. I now await being shot down by all and sundry!

Clive

Many years ago, my Father who was an expert toolmaker, taught me to stop a little short of screw cutting to the full thread depth and to finish the job with a die. This will produce the correct thread form that cannot be achieved using a single point tool and it will remove the "rag" left buy the screw cutting process.

I'd recommend reading Martin Cleave's book on threading.

**LINK**

For the same reasons as Clive mentions, with a single point tool that's too pointed it's possible for full depth threads not to fit and in those circumstances I think it's better to also consider broadening the cuts rather than simply going deeper and deeper. Broadening the cuts will lead to stronger threads as well I'd have thought.

Jon