Thanks for the reply Jonathan. Good to hear that the design isn't all bad:)

I re-did the calcs for the assembly and it turns out it's closer to 2500g, rather than 1500g (linear bearings are almost 1kg), which means more pull downwards and less backlash. I'm sticking with a single 20mm lead nut from delrin and will work on backlash if and when it happens.

Now the only thing I need to figure out is how to put the case together and make sure everything is straight and aligns. Using wood for the prototype might be more of a challenge than I though it would be.

Regards,
dsc.