The photos show the process.
- Cut the bar to rough length.
- Turn the main outer diameter and check to see that it slides snugly into the lathe spindle
- Drill out the central hole (6mm) from one end as deep as possible.
- Enlarge the central hole by drilling out to 10mm diameter - for about 40mm depth
- Flip the bar in the chuck and drill a 6mm hole to meet up with the one from the other end (hopefully).
- Form a shoulder on the unworked end of the bar (this shoulder must fit through the hole in the changewheel guard).
- Turn down the top end of the bar, ready to accept an M12 thread.
- Cut the M12 thread (I used a die).
- Cut four slots to permit the mandrel to expand (I simply used a hacksaw, having no other method).
I considered cross-drilling two through-holes at the tips of the slots (before cutting the slots). Other ones I saw on line had done this (so the slots ended with a hole). This looks good, and would be essential to stop cracks propagating from the slots in some applications. Here, I don't think it is necessary, and because I have had difficulty cross-drilling on my bench drill (the bit wanders and never seems to go across a diameter properly) I thought it best to give this a miss!