Issues Z axis

Hello everyone, I've just finished the build of a big size 3D printer (400x400x400).
However, I'm having some isues with my Z axis. I have 4 nema17 motors(84oz) with 8mm threaded rod lifting the heatbed (attached on a piece of light wood). The motors can lift the bed, but with the time, the bed has a tendency to break the "leveled state", so before every print I have to level the bed (most of the time because one of the rods moves that corner down).

I was able to print after a little fight trying to keep the bed leveled, but today it was imposible.

I've read some treads with similar issues and people suggest to add a worm gear reducer to the motor. Is this the best option or it's also viable to use a pulley?

Here is what I found: Worm Gear Reducer for Nema17 motors

Edited 1 time(s). Last edit at 01/27/2021 04:58PM by adrian_rg.

4 screws are over constraint, as are 4 guide rails. Two guide rails and 3 screws should be ideal.

The motors get out of sync whenever power is cycled. They jump a little. Some will jump forward and some jump back. The bed tilts and in the case of 4 screws, the bed bends and/or the support does, and if it get far enough out of whack, it will bind and won't move.

It's much better to use one motor driving 3 screws with a looped belt. The screws will never get out of sync and you shouldn't have to keep releveling the bed.

If you are going to use a worm gear drive, get one that has the motor attached.

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Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]

Quote
the_digital_dentist
4 screws are over constraint, as are 4 guide rails. Two guide rails and 3 screws should be ideal.

The motors get out of sync whenever power is cycled. They jump a little. Some will jump forward and some jump back. The bed tilts and in the case of 4 screws, the bed bends and/or the support does, and if it get far enough out of whack, it will bind and won't move.

It's much better to use one motor driving 3 screws with a looped belt. The screws will never get out of sync and you shouldn't have to keep releveling the bed.

If you are going to use a worm gear drive, get one that has the motor attached.

Thank you for the advice! (I believe the comment about Worm gear drive on that post was from you).

So the ideal configuration would be one big motor (Nema 23?) moving 3 leadscrews and then adding 2 guided rails for extra stability? (like HevORT ZR axis)

I'll atach some photos of the actual config, but I'm aslo worried that I'll need bigger lead screws (change the 8mm with 16mm).

General build
Z axis

Edited 1 time(s). Last edit at 01/28/2021 09:12AM by adrian_rg.

You need some sort of guide rails for the Z axis- the screws that lift the bed will not provide the precision and accuracy of guidance that are needed.

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Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]

Thanks again for all this useful info.
After doing some research of the parts needed I became with this bill of materials:

• Nema 23 - 2.4Nm and 1.8A (replacing the 4 nema 17 motors)
• 2-4 linear rails (was thinking on putting one linear rail on each corner of the bed/frame)
• 3 GT2 pulleys (40 teeth is enough?)
• Worm Motor Reductor for Nema 23 (dont know if its needed with this configuration) The smallest reduction is 5/1
• Looped belt (don't know where to find such a big looped belt)

For tensioning the belt I have some spare rods that can be usefuL. I'll also reuse the 8mm lead screws I already have.

Edited 1 time(s). Last edit at 01/28/2021 11:44AM by adrian_rg.

Linear guides or rails in the Z axis are there to restrict lateral motion while allowing vertical motion. In theory, a single linear guide would do that (there are two ball races, so it's actually two guides close to each other) but the rail is a little flexible and whatever it is mounted on probably flexes, too, so a second rail will usually be adequate to prevent the twisting. The more linear guides you use, the harder it becomes to align them parallel to each other and to keep them that way when you move the printer or through temperature changes. Linear guides are very good at what they do but are very intolerant of misalignment. It is easy to cause them to bind, so in spite of what you see on Thingiverse, don't use 4 linear guides.

You don't necessarily need a NEMA-23 motor- the lead screws provide some mechanical advantage, and if you use a small drive pulley and larger pulleys on the screws the advantage will be increased. I'd try it with the NEMA-17 motor first and see if it works. If not, you can always switch to a gear reduced motor. The little motor driver modules on your RAMPS board will not handle the current a NEMA-23 motor will need. Buy some spare motor modules and keep them handy- you're going to need them.

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Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]

If you use single-start T8 * 2 rod, it won't back drive, and you get lots of mechanical advantage. Using the 4-start T8 * 8 rod back drives easily and then the worm gear comes into play as an anti-back drive mechanism, but that's simply not necessary if you stick to single-start.

I'm in the process of mounting an 850x1500mm bed (laser, not printer) on four T8*2 screws. (There four makes sense because that frame can flex diagonally; it's not monolithic like a printer bed.) It won't back-drive, and I expect to drive a bed that large with a NEMA23 motor. My 330mm x 330mm cast aluminum corexy printer bed is lifted by two T8 * 2 single-start screws. It is not only driven by a single 1A NEMA17, but also I'm using a 60-tooth gear on the motor to 20-tooth gears on the screws, so 3:1 torque disadvantage. I would absolutely expect 1:1 with NEMA17 to work fine for a 400x400; that would be three times the torque for only half again the area. My Z is 420mm so the area comparison is actually the right proportion; no volume confusion here.