Bolt Hobbing Tool
Made by David_J, uploaded
I have used this to successfully make a hobbed bolt for my new geared extruder. It's a bit fiddly to get the height right in the lathe's tool holder, but a bit of persistence paid off. I found that it helped to rotate the bolt gently when the tap starts to bite (mind your fingers!) until the cuts are deep enough to turn it automatically.
It's best to work with the knowledge that the lower bearing is going to get trashed by swarf - I used sealed bearings, but the lower one felt very gritty after use, even when brushed out carefully. If you have a suspect bearing in your parts box then it would be a good idea to reserve it specially for this job. It doesn't need to be a smooth-running bearing - anything that turns easily would do, even if it's a bit rough.
G & C CoreXY printer