Using Z_DUAL_ENDSTOPS for build plate leveling on Z_MAX with probing on Z_MIN

I've built CoreXY printer with dual Z steppers. I have BLTocuh as my probe on Z MIN.
Sometimes Z steppers get "out of sync" and build plate is not parallel any more to XY plane.
I would like to add two endstops on Z MAX (bottom of printer) and use them to make build plate parallel with XY plane.
Marlin does have Z_DUAL_ENDSTOPS, but from what I can see they must be on homing direction (MIN in my case).
Is it posible to setup marlin to have homing in MIN direction and this Z_DUAL_ENDSTOPS leveling in MAX direction?

I feel like this is working around the problem. Sorry for not providing an answer, but wouldn't it be a better idea to fix the steppers going out of sync?

What could be the cause of that? Binding? An axis leadscrew slipping in relation to the stepper shaft?

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http://www.marinusdebeer.nl/

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Ohmarinus
I feel like this is working around the problem. Sorry for not providing an answer, but wouldn't it be a better idea to fix the steppers going out of sync?

What could be the cause of that? Binding? An axis leadscrew slipping in relation to the stepper shaft?

Maybe my choice of words wasn't the best. It's not problem while printing. But when I remove print surface (flexible) or poke around printer, then I sometimes accidentally move one lead screw. When that happens I need to manually level it again. I would like to have a way to do it automatically before every print.

Correct this basic mistake: two steppers and screws for Z.
Besides, a well made cartesian machine doesn't need auto bed leveling.
It is explained in the CoreXY thread.

Edited 1 time(s). Last edit at 06/01/2019 10:17AM by MKSA.

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"A comical prototype doesn't mean a dumb idea is possible" (Thunderf00t)

I have used auto homing (Syncing) using dual Z steppers motors on two of my printers and it is a very nice feature to have.
You need a home sensor for each Z stepper motor.

My first printer which has separate stepper driver boards (eg doesn't use a ramps board), I made an additional PCB board which has a circuit that when homing the Z axis, it drives both z stepper motors until the first Z stepper detects home (Min), it then disables that stepper driver but still allows the other Z stepper drive to continue driving until that stepper motor sensor (Z Min) is detected, then a Z home signal is sent back to the Arduino.

My second printer a CoreXY which uses a ramps board, I was able to modify the firmware to have homing with two z stepper motors which of coarse requires two Z home sensors - one for each Z stepper motor.
It wasn't that easy a task to modify the firmware and there was one issue that it takes a set time to home the Z axis no matter where the Z axis may be. The firmware that I modified is not the latest version. Haven't had time to look at the latest version to see whether it is possible.
With this printer I use Min and Max sensors for the X and Y axis and two Max and one Min for the Z axis. Total of 7 endstop sensor switch's. One of the Z stepper motors drives 2 lead screw. All up I use 3 lead screws being driven by 2 motors for my print bed.
I don't use a print bed probe sensor..

You simply don't have to worry about having to trying to keep the Z stepper motors in sync with this nice feature.
I can see why you have asked this question....

Cheers

Edited 1 time(s). Last edit at 06/02/2019 06:43PM by RepRot.

Indeed, a complex solution to try to correct a problem that could be eliminated.

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"A comical prototype doesn't mean a dumb idea is possible" (Thunderf00t)

Just a thought, but do you think it would be an idea to add a belt around both axes or a string that is looped around then a couple of times so that the axes also keep each other physically in sync?

Edited 1 time(s). Last edit at 06/04/2019 09:41AM by Ohmarinus.

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http://www.marinusdebeer.nl/

I use a low-tech solution for this problem - I have a couple of 3D printed jigs that fit around the Z axis rods. I put those in place, turn the lead screws by hand until the axis hits the jig on each side, pull the jigs out, and presto - the axis is level and ready to go.

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Ohmarinus
Just a thought, but do you think it would be an idea to add a belt around both axes or a sting that is looped around then a couple of times so that the axes also keep each other physically in sync?

It is the way to go. One motor plus closed loop belt. Synchronous belts have been "invented" decades ago to address this.

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"A comical prototype doesn't mean a dumb idea is possible" (Thunderf00t)

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MKSA

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Ohmarinus
Just a thought, but do you think it would be an idea to add a belt around both axes or a sting that is looped around then a couple of times so that the axes also keep each other physically in sync?

It is the way to go. One motor plus closed loop belt. Synchronous belts have been "invented" decades ago to address this.

Okay, I'm happy it's not a dumb idea haha. Thanks for the feedback!

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http://www.marinusdebeer.nl/

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MMcLure
I use a low-tech solution for this problem - I have a couple of 3D printed jigs that fit around the Z axis rods. I put those in place, turn the lead screws by hand until the axis hits the jig on each side, pull the jigs out, and presto - the axis is level and ready to go.

Currently I'm using similar approach, but I would like to have it automated in the future.

It turns out in Marlin 2 there is a support for exactly what I need. It's called "Z Steppers Auto-Alignment". It adds the G34 command to align multiple Z steppers using a bed probe.
It can be enabled in Configuration_adv.h by uncommenting Z_STEPPER_AUTO_ALIGN.
I wasn't able to make it work at this time as it looks like it's still work in progress (there are few opened bug reports about it).