After my previous success drilling a tiny but deep hole through some brass rod, I got thinking about how to machine the outsides. Referring back to my diagram the idea was to take a 4mm rod and machine most of it down to 2mm.
But on reflection this seemed like a bad idea so I decided instead to start with a thinner rod and add a bush to mount the gear wheel. I found some 2.4mm copper coated welding rod. I initially tried machining the copper coating but the rod was bending in the lathe and did not produce a good finish. However I discovered that the soft copper coating was quite thin and was easily removed when polished with some emery cloth.
A brass bush was drilled and turned so that it was a tight fit on the welding rod and also a good fit for the plastic gear.
Here’s the redesigned shafts, the lengths have still to be determined.