One is z-wobble (visible here) caused by a rigid yet slightly non-coaxial coupler between that and z-axis drive stepper motor, and a slightly bent leadscrew.
My obscure printer uses the same leadscrew as version 1 of the common MonoPrice Select Mini, and a few folk have had similar issues with them, and this project takes inspiration from some of the fixes they created or found.
Fitting a flexible coupler (see photo) is said to help as, I have learned, that only really well made machines are accurate enough for solid couplings between motor and leadscrew – any misalignment or bend makes the driven item wobble around as much as its guiding structures will allow (two 6mm rods, in the Fab v2). For 3D printers, the ideal answer is a flexible coupler to the motor, and no bearing at the other end of the leadscrew.
Read more: Two days before the lathe