Second z axis for Microcube

Add a second z axis on your Microcube (on the front) to improve bed stability.

This modification is compatible with the original Microcube: you can reuse your existing bed plate, as you juse need to replace the two original bed lever by those provided, and print again some original parts.

You will need:

• Printed parts:
  • 2 x dual_axes_bed_lever.stl provided in this thing (replaces the original bed lever)
  • 1 x new Z_axis_platform_mount from the original Microcube parts
  • 1 x new z_axis_motor_mount_rod from the original Microcube parts
  • 1 x new z_axis_bearing_mount_rod from the original Microcube parts
  • 2 x new z_motorshaft_shaft_coupler from the original Microcube parts
• Metal parts:
  • 2 x LM8UU linear bearings
  • 2 x 8mm smooth rod 200mm
  • 1 x M5 threaded rod 160mm + M5 nut
  • 1 x motor for the new z axis
  • nuts and bolts
• An alternative LCD mount if you use a LCD screen, as the original one is incompatible with a new z axis on the front side.

Instructions:

• Print the plastic parts
• Remove your bed plate and the bed lever (you can't put the lever away, you don't need the). Keep the screw on your bed as is, it's a drop in replacement
• If you use an LCD, remove it, and put the original mounting parts away.
• Mount the new z axis on the front side of the printer, as explained in the original Microcube build guide (of course there is no need for Z endstop screw on the new z bed platform).
• Fix your bed on the two z plateform using the new bed levers.
• If you use and LCD, fix it using the alternate mount

Tips:

To reduce friction and ajust the z axis, loosen all the screw of the Microcube structure.
Then, manually turn one of the axis to have each platform (not the bed) at exactly the same height (use a rule or a caliper to measure the distance between the platform top and its z_axis_bearing_mount_rod top).
Once they are at the same side, use the motors to make the bed full up and full down a few times.
While moving, ensure the structure is loosen: the rod will move a bit, having the 2 z axis falling in place to be perfectly parallel for each other. Help the structure by and if necessary.
At the end, gently tighen all the screws.

Beware:

If you are using 28BYJ-48 motors, you may need a new A4988 motor stepper driver. On RAMPS, z axis have 2 connectors, for two motors like in our case. But the power repartition between two connected motors is passive: it needs the motors to have the same winding resistance, so that they will received the exact amount of current.
Unfortunately, based on my experience, 28BYJ-48 are not accuratly made, and their winding resistance are really different from one motor to another. So, if 2 motors are connected to one driver, the one with the lower resistance will receive more current than the other: at best the higher resistance motor will loose step, at worst it will not turn at all and the other motor will fry receiving all the current.

Measure the resistance of each coil of your motors: if it is too different, you will need to connect the second z axis motor to a dedicated motor driver (use the second extruder motor of the RAMPS), and configure your firmware to replicate z motor move to E1 motor driver.