"1. Is it really a big deal to have two ball nuts connected by a preload adjustable coupler? They say it helps with backlash. Worth the extra money?"
Backlash is a problem not only because it worsens your accuracy. If you cut metal with backlash you will get more noise, worse surface quality, and you might get considerably shortened tool life. This is because with backlash, there's nothing to dampen vibrations.
Preloaded nuts actually totally eliminates backlash in the screw/nut interface. They might not be of much use unless the rest of the transmission is also backlash free: preloaded angular contact ball bearings for the screw, backlash-free gearing and couplings etc...
Arvid |