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Ender 3 Max with Creality's dual Z motor upgrade - Won't home consistently

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GeraldM:
Hi, all:

I have a Creality Ender 3 Max printer, to which I have added Creality's 'official' Dual Z-Axis upgrade.
This consists of a second stepper, second lead screw & associated hardware, plus a splitter cable which runs the two steppers in parallel.
Since I've done the upgrade, I've not been able to get the Z axis to home consistently.  It shows up as the bed being out of level, in different directions, each time I start the machine.

I've gone through the usual remedies; re-aligning the Z and X gantrys, checking for binds, bumps, or other things.  All good.
I checked out the Vrefs for all the steppers.
E:  1.064V
Z:  0.883V
Y:  1.390V
X:  1.184V

Bumped the Z Vref to 1.60V.
It homes and prints fine if I disable the second Z axis (unplug the stepper and remove the nut attaching the second Z rod to the X gantry)

I've done some further research, and it seems that this particular product upgrade may be the epitome of bad ideas.

It seems that because the second Z motor isn't synched with the first. It will start just wherever it happens to be, without regard to where the first is.  Second, even should they be synched prior to power up,  the motors may move up to four steps in any direction, again depending on where they are.

My solution so far is to manually level the X gantry with blocks of plywood prior to homing.  I'm currently testing this out, to see how it performs.

Other solutions that come to mind are:
1 - Go back to the single Z lead screw.
2 - Get the kit that uses a belt to drive the second Z axis.
3 - McGyver the machine and the gcode so that it will find a reference spot for the Z axis screws between prints - possibly removing the M84 command at the end of a print…
4 - Add an extra stepper motor driver to the machine, use this as a driver for the second Z, and figure out how to recompile Marlin and use the G34 and M422 automatic Z axis alignment function…

Has anyone been able to get this dual Z axis kit working?

I often wonder whether I bought this printer to print stuff, or to tinker with!

MarkLiu:
It seems that the condition of the stepper motor on this printer is not very good, I have bought this kind of stepper motor, which is not bad for the price-performance ratio.

Ian.M:
Generally for 3D printers with twin parallel motor, twin screw Z axis mechanisms, you should carefully level the X axis assembly before first powerup, align the couplings in the same orientation and mark them (so you can see if it looses sync), and power up initially with one coupling not tightened on its screw.  Then the first thing you need to do is a small manual jog (1mm and back again) to power up the Z driver, and only then tighten the coupling so the motors are synced with the X axis level and not fighting each other. 

Keeping them synced and the X axis level is more problematic.  Any overload, excessive acceleration or binding will cause loss of sync, resulting in a Z axis jam and excessive stress on the mechanism.   *SOME* firmware versions are aware of the parallel motors issue and attempt to resynchronize by driving the motors to the axis limits then gently past them so that if one is ahead of the other it will hit the mechanical end stop and loose steps till the other catches up. 

GeraldM:
Thanks!

Ian, that seems to be a good idea to try for the initial power up.  I'll try it out!

dunkemhigh:
For twin screw axis, wouldn't it be better (practically and technically) to have a belt drive from the primary screw (which has the stepper) to the secondary? Having independent motors is pretty much asking for issues, particularly when they are quarter-stepping or worse.

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