Brilliant design. Thanks for posting. It would make a great article for MEW. A new and innovative tool you can make yourself. Getting harder and harder to find ones that have not been done before in MEW. The two-speed thread idea is brilliant. So is the locking screws.
I would maybe make a couple of changes. I reckon an 8TPI single start thread would be good enough, maybe even a Acme thread a la Myford leadscrew. Yes it would take twice as many turns to raise the same distance but stlll be quick enough for me, and a lot easier for most beginners to deal with than a three start thread.
And the trouble he had getting the two threads to fit together is due to something I have never seen done before, cutting the male thread first and then cutting the female thread to fit it. Conventional method is to make your nut first, to the theoretical dimensions, then make your male thread to fit the existing female. It is just easier to skim a few thou off here and there on the flanks of the male thread thread for final fitting.
I would also mill a V groove across the top of the pad like his old commercial one had so you can support round bars etc with it with some sort of location.
Everyone is an expert after the fact! He has done a great job nonetheless. I see a new project coming up in the shed. I was just thinking I need an excuse to shelter from the summer heat in there in the newly air-conditioned comfort.