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2 Z-motors not always moving in sync?

Posted by disneytoy 
2 Z-motors not always moving in sync?
December 07, 2017 09:10AM
Heloo!

So I built a Prusa MK2 clone from scratch. I have 8mm Acme threaded rods on my Z-axis. Both motors connect to a single DRV8825 driver on Ramps 1.4. Latest Marlin Firmware.

The left motor nearest the Ramps seems to work fine, But the right motor, farthest from the Ramps some times hesitates and throws the level off. But not always.

I tuned the drivers per a video I saw.

So any thoughts on how to fix this?

Could it be my wiring,

Would increasing the driver current work?

Thanks

Max
Re: 2 Z-motors not always moving in sync?
December 07, 2017 11:33AM
If one is working reliably and the other isn't, it's probably a mechanical problem that's making it hard to turn one screw.

DRV8825 drivers have problems skipping steps at low speeds. It won't be the cause of this particular problem, but it may be the cause of others. See: www.engineerination.com/2015/02/drv8825-missing-steps.html#!


Son of MegaMax 3D printer: [www.instructables.com]
Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]
Re: 2 Z-motors not always moving in sync?
December 07, 2017 01:10PM
Easy to fix, just get rid of one of the motor, a belt and pulleys to drive the two lead screws like what I did here: [www.thingiverse.com]
It is one of the common design error seen in the Prusa and many other 3D printers.

Edited 1 time(s). Last edit at 12/07/2017 01:11PM by MKSA.

"You failed to maintain your weapon, son" (Harry Brown )
Re: 2 Z-motors not always moving in sync?
December 07, 2017 05:03PM
Should I replace the drv8825 with a a4988? I have one. I just though the drv8825 was more accurate. 1/32 stepping.

Increasing the current wont help?

Thanks
Re: 2 Z-motors not always moving in sync?
December 07, 2017 05:07PM
Makes sense MKSA. But I don't have a printer to make parts. I'll put your modification on my list.

Thanks
Re: 2 Z-motors not always moving in sync?
December 07, 2017 06:22PM
With dual motor setups, the motors have to stay in sync when the power is on and they are running, and when power is off, and they aren't. If they lose sync for any reason, the X axis will tilt relative to Z and the print bed. Until you realign the motors/screws/x axis, you may have trouble printing. 2 motors may stay in sync while they are running if everything is set up right mechanically, but they won't necessarily stay in sync when you cycle power to the printer.

Using a single motor with a looped belt guarantees that the screws stay in sync, and the X axis perpendicular to Z, whether the machine is powered up or down or cycled. Of course, both setups have failure modes. If the pulleys let go of the screws or motor, or the belt breaks or comes off the pulleys, all bets are off. Think about how often you've seen any of those failures, then think about the dual motor failure modes. With dual motors if a cable/connector fails, or a driver fails (if you're using two), or a screw comes loose from its motor shaft, or you simply cycle power to the machine (like each time you turn it on), or a curious child with nothing but bare fingers decides he wants to see what happens when he/she turns a screw, you're out of luck.

You can take all sorts of preventive measures to ensure that these failures don't occur. In either setup, you can put lock-tite on the grub screws of the pulleys and motor couplers, motor mounts, etc. (a good habit to get into), and use good construction practices (a good habit to develop) to ensure that belts don't come off pulleys, you can enclose the machine to make it difficult for curious fingers to go places they shouldn't. In the dual motor set-up you can inspect the cables and connectors, use a single driver (not without its own risks). The newer Prusa machines get around the power cyclingproblem by lifting the X axis to the top of the frame until hard stops are hit at the top of the Z axis. That forces the X axis to tilt back to perpendicular to the Z axis (assuming the frame is square), and should only have to be done at each power-up.

Either machine setup can be used with or without autoleveling.


Son of MegaMax 3D printer: [www.instructables.com]
Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]
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