Just come across this thread and really enjoying it
If I may I will share my experiences of creating a metal tap. I wanted to create a tap to cut 2 1/2" dia, 2 TPI thread as utilised on numerous benches as sold be benchcrafted, Richard Maguire et al
The tap would need a big piece of steel so this is how I solved the problem, in addition I do not possess a mill
Initially I cut the steel bar like this
carrying out the procedure on the lathe like this
Once cut the pieces were fitted together like this
And welded
This was then mounted in the lathe via a revolving centre in the tailstock and a four jaw independent in the headstock. A suitable cutter placed in the toolpost and off we go taking numerous shallow cuts (the topslide was angled at 30 degrees so only the leading face of the 60 degree cutter was removing metal)
Finally I ended up with this
And a trial use in some offcuts
As for the thread in the timber again I utilise the lathe with a router mounted in a plate specifically made for the job
Not used it in anger yet but trials in some offcuts are here