Any question answered...

Weird question, possibly:

Does anyone know if there's some way you can use a tool's thread (instead of a tap) to cut a new thread? Like using some kind of lubricant, and gradually cutting a bit, undoing, cleaning out the waste, and re-threading to cut the next bit? or something?

This is a trapezoidal tap I made this way, because they're bastard expensive. I milled couple of slots slightly off-centre and tapered the ends, which is pretty simple if you can get in a machine shop.