The X axis bearings constraining the lead screw failed in an odd way. I replaced one with a double angular contact bearing that I've used successfully elsewhere.
Most ballscrews you buy come with a fixed side and a floating side. The floating side allows axial motion, the fixed side does not. The fixed side is implemented on those by machining the end of the ballscrew with a shoulder plus a bolt thread. The shoulder constrains the leadscrew axially against the bearing, while the bolt thread allows you to use a locking nut with a set screw to constrain it axially the other direction with an easy way to preload out any backlash. I don't have a lathe, so I use two locking collars plus lots of pliers and a few tries to get the axial constraint plus preload right.
