My nephew has an external-frame backpack (yes they still exist) which uses aluminum attachment pins holding the pack to the frame, along with a weird single stainless steel cotter pin threaded through the tips of two aluminum pins together to lock them in place. Somehow, while the pack was being cleaned after a trip, one set of pins with its associated cotter pin went missing.
Fortunately, there are two sets of identical hardware, one each for the right and left sides, and the other side was present in its entirety, ready to copy.
I was really glad to have put an ER40 collet chuck in my lathe, because it made the project a lot easier. The aluminum pins were easy to copy from a length of ⅝" 6061 aluminum, but I didn’t have 2mm stainless steel wire.
Silly me, I didn’t precisely measure the wire before ordering wire. It looked to me like 300mm (~12") would be more than plenty long, and a bundle of 20 was cheaper than a single wire (because of course it was). It turns out that a larger bundle is also valuable for packing for shipment, because 20 wires tightly bundled together are stiffer than a single wire and don’t get bent in shipping.
I got really lucky. I needed exactly 300mm of wire to make the cotter pin.
I didn’t attend with any precision to the thickness of the heads on the pins, and you can see that one of the replacement pins has a slightly thicker head than the originals, but that doesn’t matter much. The diameter and length are spot-on.
The trickiest part was that the shorter aluminum pin wasn’t long enough to hold through the ER40 collet in a collet block in the mill vice for drilling the cotter pin hole. I ended up putting a pair of 1-2-3 blocks in the vice, with the new pin right at the top of the blocks, and the original pin at the bottom to keep them parallel, and then gingerly spotted and drilled. I should have taken a picture of that fixture, but didn’t think about it at the time.