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You'll need a "holding torque" so you ideally need a gearbox or gearing of some sort as the weight of even a moderate weight CNC Z axis can spin a ball screw and run right to the bottom when the steppers aren't holding it, counterbalancing at least some of the weight with a gas ram will also help.
I've considered something similar on my lathe cross slide but it already works and with the DRO a few thou backlash isn't really a big issue on a lathe
Ball screws are about taking out friction, for manual use you will need to add friction back in, the gas strut will reduce vertical load but you still want some friction for a manual machine, that's your holding force.