The gear-train Delrin (I think) gearwheels on my Chester Mini-Multi lathe/mill unit have keyed bushes that couple the gearwheels to the drive shafts. Chester no longer stock these bushes as replacement parts. And one of mine has partially sheared (either just through age or something I've done wrong). Which means I don't currently have either screw-cutting or automatic lead-screw movement facilities. Neither are particularly important to me at present but I wondered if replacement bushes could be made on the lathe/mill . The bushes are 10mm OD, 6mm ID, and of length 16mm. The key which is an integral part of the bush runs the length of the bush and is 3mm wide by 1.5mm height on top of the 10mm OD.

My naive current idea is to get some solid delrin rod 12mm in diameter, create the key using an end mill, and then gradually rotate the rod by small degrees running a suitable end mill along the length to make a multi-sided approximation to the 10mm OD circle and then just smooth the circle until it was a push fit into the gearwheel. Any other suggestions bearing in mind my current lathe inexperience would be appreciated.

To Frank: yes I thought about that, but as the key is meant to break (I presume) if something causes the gear train to snarl up, I would still want the key to break rather than carve up the inner part of the gear wheel (or if that didn't happen) to shear off several of the gear wheel teeth.

To Jason: The 1.5mm key sits on top of the 10mm OD bush – so the outer diameter of the top of the key is 11.5mm. But I take the point regarding making the actual bush out of brass or steel and make a channel for a key to fit in. Now do I make that out of plastic or lead or something else. See my reply to Frank above – I need the key to break if it happens again rather than the gear-wheel. So it needs to be equal or softer than the delrin of the gearwheel.

Hi John,

What a good idea – the 3D printer. Saw some at the Model Engineering Exhibition. Nearly bought one – but couldn't think of anything to do with it that would provide an acceptable reason for buying it to my wife. Now just need to investigate exactly what plastics you can use and whether these are suitable. Thank you John.