The problem was getting a suitable harmonic drive.
I was aiming for about a 6" chuck, but with as big a bore as possible, and such harmonic couplings were pricey, unless you want to take the gamble with the plethora of used offerings on eBay.

I came to the conclusion that many CNC 4th axis use worms, and they generally don't have many problems with backlash, as they are designed to be adjusted, which is key. The problem is making it adjustable, while having a big motor attached.

For inspiration, you may want to check out the original InTurn design thread over on the Mach forums. It used belts, and a brake - https://www.machsupport.com/forum/in...72469#msg72469