Wait wait wait…
I was just looking at my anti-backlash nuts, and considering how the threaded rod is used on an MPCNC, and in fact on the LowRider2.
Do not use anti-backlash nuts on these platforms
anti-backlash nuts are engineered to put pressure on the nuts such as to force them to the thread side closest to the mounting face. This is done by applying spring pressure against another nut locked into rotational coincidence with the mounting face of the nut.
The MPCNC and LowRider2 both avoid backlash by “sitting on” this face. Therefore the spring pressure would be in opposition to gravity, actually attempting to suspend the gantry against the force of gravity. This means that the spring is reducing the anti-backlash force of gravity (Or gravity is reducing the anti-backlash force of the spring, depending on which is greater.) Either way, you are getting LESS anti-backlash force than you would get with only one of the two in play. Given the designs of both, gravity is the obvious answer as to which of the two you want.
If you insist that gravity alone is not providing enough anti-backlash, then you will have to go with a homebrew anti-backlash nut, similar to what I described in the previous post, but the force of the spring must be set up to attempt to pull the 2 nuts together, not to press them apart, as the anti-backlash nuts are designed to do normally. You could modify the anti-backlash nuts to work this way by clipping the spring that they come with and hooking one end through a mounting hole, and the other around the back side of the flange meant to retain the spring. Perhaps drilling a fine hole on the back of the flange would be effective. This is less reliable long term, but would increase pressure in the desired direction for the nut on the threads.
For this to have any desirable and measurable effect, you would also need to increase the clamping force between the core of an MPCNC and the face of the nut as well. As it is, those screw holes are only meant to prevent the nut from rotating.
Edit: actually, there’s an easier solution, which is to add a spring tensioned between the core and the Z motor mounts, pulling the Z axis downwards. This would more firmly seat the mounting nut into the core and reduce available backlash with the threads. It would increase the tendency of the spindle to fall when the steppers are turned off though.