So I've been doing a bit more design work on this, taking on board Maddox10's advice about the shaft diameter. I have upped it to 20mm now, which hopefully will be ok (given I am running less than 1/4 of the KE of Valkiri 3). I have also been having a think about how to mount it robustly and within my (albeit self imposed) tight space envelope. I am thinking of making the bottom hub and the shaft one monolithic component, welded together from water-cut bits then tidied up on a worktop lathe, which should hopefully achieve ~H9D9 fit tolerances. The exploded view below shows more.

I will use off the shelf oilite bushings instead of bearings. These will fit inside a rotating drum that also houses the blade. I then have a top running plate for the bushings to run against. The last piece in the puzzle is what to do for the top hub... Still hoping to find something off the shelf here, but I may have to concoct my own if needs be.

weapon_hub v2.PNG
weapon_hub_exploded.PNG

Best of luck to anyone competing in Manchester this weekend. I will be an avid viewer on Sunday. Hope to join in next year!