plastic intermediate gears or metal for mini-lathe?

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Joules:
Right I figured out what I should do with that part I made, make a thread forming tap. Since the mini lathe was setup for threading I found some silver steel on the bench that was close to the diameter I needed, about 4mm. A quick bit of turning to size, round the end off with a needle file and polish with the fine diamond stone. This time I took half the cut depth I used on the brass and used paraffin (kerosene) as cutting fluid, wondering if my tool tip would hold up. It did and I got the tool below, it has 0.05mm larger diameter thread than the brass part .

This was put into the drill press and pulled firmly down onto the bit of brass I had drilled a 3.6mm hole in. The brass nut being in the drill vice, I turned the drill chuck manually whilst keeping the pressure on to force the form tap through the brass. Don't let the pressure off or back the tap out before you have completed the threading as you will never get it realigned.

Test fit the part. I had taken my measurement of the brass threaded part from the lathe dials and forgot about the brass deforming so my brass part wasn't out at all. A very fine curl of brass came off the part as it was screwed into the half hard brass sheet. It's a snug fit but I could fit them together with finger pressure. A couple of times screwing it together produced a silky smooth fit. I forgot to mention I de burred the nut with a drill bit as the form tool raised quite an edge as it exited the brass sheet.

Final shot of both parts together. It serves no purpose other than to test I could make a tool to form the thread and that the fit was good. I am very pleased as my thread cutting tool is still as sharp as when I made it. The thread forming tool is only really practical for sheet metal, as a small billet would impart too much force on the tool and most likely snap it at the neck, could be OK on soft alloy though. Another tool made another technique learnt, it's good to play.

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