I put a ballscrew on the x, and am now putting one on the z.
A ballscrew is very, very much better than an acme screw.
I used an unusual overconstrained solution, with the screw in tension. This makes it much more stiff, which is the ultimate goal.
1. Supporting both ends, makes the free length 1/2. This doubles the rgidity.
1.1. A tensioned screw has double the rigidity of a std screw, according to standard lit from screw manufacturers.
2. I also went from a 16 mm screw to a 19.05 (3/4"), approximately doubling the rigidity.
In total its 14x stiffer, and 6x more accurate (=6x less stiction).
There is no issue with freewheeling the screws.
I simply disable the drive, which leaves an ideal slight drag on it.
If I want to lock an axis, I just enable the drive again.
I just put 2 small electric switches leading to the drive.
My step sizes are about 0.1 and 0.2 microns, on z and x respectively.
30.000 servo counts on z, at 1:3, with 15 mm wide HTD 5 mm profile belts.
Actual mechanical resolution is better than 1 micron, according to DTIs.
I am now changing the z axis to a 32 mm ground class 0 ballscrew with nut.
And to AC brushless servos, as the geckos have all sorts of problems (after 8 years).
(Common 5V vs common ground mixup, horrible start-up current surge ie no soft-start, cannot be freewheeld as they dont have back-emf protection, single ended vs differential wiring, hissing ie noisy vs silent. New drives are just that, new).
I am fitting the z screw it behind the lathe, where it belongs, out of view/bother/swarf.
I have also kept the original manual operations, although the electronic cnc infinitely adjustable powerfeeds are betterr.
Haas also fits the screws behind the lathe.
My lathe is a 12x24" light industria model, fwiw.