Here's some pics primarily for Dave. I could have sworn that I posted these pics before, but I can't find them on here.

This is an indexing jig for doing the cuts in the green pen that Dave showed.
It is simpler than it looks. One end has just a pointed bit of thread from a bolt which acts as a tailstock.
The other end has a 24 tooth gear with another bit of bolt which fastens it to a roller bearing inside the block.
The blank is prepped by tapping a thread in one end and a small centre drill dimple in the other end. The blank is then screwed onto the 'headstock' end and locked with the lock nut.
The slitting saw ( or slot saw if you prefer ) has its own rigid mandrel and is held in the lathe chuck.
The jig is mounted at the desired angle on a compound table, which is used to pass the blank under the blade.
Because the cuts are just less than half way through the blank, I could cut them all and glue the aluminium in afterwards.
I moved the jig to do the second set of cuts. Slighty steeper angle too. After trimming with heavy duty scissors, the aluminium was filed flush with the blank. It was then drilled for the tube(s) and then turned to size for the pen.
I use the same jig on my metal lathe when using tougher material.
I hope this makes sense

PS. Of course, I just put the finished blank back in the jig for the pics

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