The underiven end is supported with a bearing block. The z axis backing plate on which the stepper is mounted has vertical ribs to add ridigity.

The lovejoy coupling looks similar to the jaw couplings i'm using but with more spider teeth. The Oldham looks like a superior solution. I'll investigate.

I'm going to re-align the two shafts again and see if I can improve on yesterdays attempt. If no improvement i'll look to other areas and investigate the pulley's & belt option, which I think I have 'in stock' in the workshop.