I have seen a lot of forum posts regarding Z axis issues. This is an effort to consolidate these posts into one try-this-first post.
Problem: Z axis does not move, only moves down, jitters, or does not have a lot of power
Mechanical issues:
Cables catching - check to make sure the cables have clear movement at all points on the table. Rearrange as necessary
Z axis not lubed up - lube axis
Z axis nut too tight - The Z axis nut needs a little wriggle room to compensate for any misalignment from the gantry. Loosening the small screws that hold the nut onto the Lowrider will help immensely. 1/4 turn to a full turn to loosen things up should be sufficient.
X/Y misalignment - Missing steps in the other axis’ can through the table out of square. When the table is out of square things tend to bind up. Check to make sure the machine is not rubbing on the sides of the table. Make sure the belts for the other axis are tight.
Z axis guide rails out of square - measure the distance from each z axis guide rail to its complimentary guide rail at the top and bottom. Ensure that they run as square as possible to allow for smooth movement. Common areas for error here are tightening the Z rails into the small locating lip of the printed piece used to join it to the x axis. Make sure that there is equal gap on all screw points where the printed pieces join to mate the z and x axis guide rails together.
Table not level or square - Check the gantry at several points on the table and ensure that it is level to keep the Z axis from binding.
Coupler Misaligned - if the coupler isn’t tightened onto both shafts and seated squarely issues can arise. Especially look where the grub screws bite into the threaded rod, if they make the coupler sit cock-eyed on the shaft, rotate the shaft and retry until you get the coupler to sit square.
Gantry too heavy - this should NOT be a problem unless you are running a very large configuration or using motors with low torque. Consider using springs to help counter balance the load. Consider re-arranging your wiring to eliminate weight on the gantry
Threaded rod dirty/damaged - disconnect the threaded rod from the coupler and see how easily you can spin it through the Z axis nut. Are there any places where it hangs up?
Electrical Problems and programming issues
Make sure the motors are wired in series, not parallel. Seems like a lot of people make this mistake.
Make sure all connections to the motors are strong - test different motors in the same place. If a different motor does well in the same place, you may have a bad connection to the original motor.
Make sure the motors you are using are correct for the firmware you are running - 1.8° or .9°?
Do the motors you are using have enough torque?
If you are using RAMPS, do you have the little potentiometer adjusted correctly for the motors you are trying to run?
Do you have one or both motors plugged in upside down?
Do all unions have good connections? Use as few unions and as little wire as possible. This will limit the amount of places a wire can get snagged.
Make sure you have the right firmware installed for the board you are trying to use. Check the electronics web page for the Lowrider on this same site for more info, this forum won’t let me post a link.
Is there a defect in the board? Try plugging the Z axis wires into the X or Y terminal on the board. Can you then move Z by moving x or y?
Use slower movements from your programmer, especially rapid commands
Use the same motors from the same manufacturer. Unless you are running dual Z axis from the board, any stepper motor mis-match will cause issues
Further things to try to isolate issues:
Try one motor at a time.
Do both motors work simultaneously without load?
spin the z axis thread by hand. Does the gantry lift smoothly? Are there any points where it is harder to spin?
I am sure I missed some things, so feel free to post below