V0.1 Z-motor to weak to lift the bed

[gsezz]

Hi,

I mostly finished my V0.1 LDO kit build, but I have some issues with the Z-axis. It seems the bed is too hard to lift for the motor, and I even stripped the T8 POM Nut.

Without the lead screw, the bed moves super easy on the linear rails. With the rails in a horizontal position, I can move the bed by blowing on it.
But with the default setting of run current = 0.37, the LDO motor isn't able to lift the bed against gravity. It's skipping steps, and the bed is even dropping down a little bit (screw turning backwards), while the motor struggles. Moving down works fine. Up works only when I help it a little bit with my finger. Turning the lead screw by hand, with the motor switched of, doesn't take an unreasonable amount of force either, I think. 

Since the bed moves very easy without the lead screw, I think the screw must be the problem. 

First I used a pretty stiff white lithium grease on the PTFE coated lead screw, but I couldn't get it moving upwards at all. Not even with a run current of up to 0.5. I didn't try more than that. 
I read that a PTFE coated lead screws and POM nuts are intended to work without grease. So I removed the grease, and used some PTFE spray instead. With this I got it working reliably at a run current of 0.45. 
After 30h of printing it stripped the threads of the POM Nut.

I changed the nut, even reprinted the part of the bed that holds it, tripple checked that the screw is straight, tripple checkt that the screw ist mounted parallel to the linear rails. I measured the distance from the rails to the screw at the bottom and top, which were the same. And also I mesured the distance at the top, while the bed was in the upper and lower most positions, which was consistent. So it seems there is no sideways pressure on the screw and Nut.
This time I used a softer all purpose grease, and got the same results as before with the PTFE-spray: Not able to move at run current of 0.37 at all, reliably at 0.45. This time the motor makes considerably more noise while holding. (btw I'm using a holding current of 0.35)

So, the printer is working now. But I still don't know if it stripped the nut only because I ran it without grease, or if something else is wrong. If there has been another problem, I gues I didn't solve it yet, because the motor still needs the same run current?

Is there any advice, besides just running the printer as it is and wait if it strips the nut again?

Since the provided printer.cfg (SKR Mini E3 V2.0) sets the run current to 0.37 for the LDO motor, I asume this should be enough? Is it ok to set it to 0.45? Stepper and driver aren't getting any warmer than the A & B ones. 

 

 

 

 

[gsezz]

Just an update: I think the problem was the backlash thingy. 

I removed it, and also completely removed the screws from the T8 nut, and now it seems to work. The lead screw is much easier to turn by hand, and I was able to dial the run current down to the original value. 
Without the screws, the bed is held down only by gravity. But the print quality seems fine.

But I don't think the two screws were the problem, since I already had them very loose, and the nut was able to move around before.

 

Edited April 26, 2022 by gsezz

[claudermilk]

I'm not specifically familiar with the V0, but have run into some issues on a vertical lead screw before when building my Prusa Mini. I over tightened the lead screw nut and that caused big issues. Make sure you've attached the bed to the nut just barely enough to keep it from rattling, any more will bind things up and cause the kind of issues you are seeing.

The POM nut takes no lubrication at all. I have not used any on my Mini or on my Trident. I swapped from the stock brass nuts on the Trident motor kit to POM nuts, no lubrication and they work smoothly.

Lastly my understanding is using hold current is now deprecated.See if it works without that. Also, what is the max current rating on your motor? Suggested starting current in your config is 40-50% of max.

[gsezz]

The motor is rated for 1.0A. 
I've been afraid raising the run current could overheat the driver or motor. My intention with setting a hold current has been to keep them cooler, since the Z-motor spends much more time holding than moving, during a print.

I don't thing anything is realy binding right now. With the motors off, the screw is pretty easy to turn by hand. 

I've loosend the nut a bit, it was quite tight. 

But the situation with the run current is still the same. 

At 0.37 the motor isn't able to lift the bed at all. 
At 0.4 it can barely lift it, but still skips steps when pressing against the endstop switch.
At 0.45 it can lift an adaitional 200g of weight on the bed. Which would be just enough for a large print on the V0.

If it's OK to run the motor with this current, or even a bit more, I'm OK with that. I just find it a bit strange that my motor needs that much more current than the reccomended value for this motor from the example config. I don't think, who ever came up with the config, would choose a value that would barely work. If that was the case, my problem would be pretty common. But since it's not, I'm wondering if there is still something wrong with my build. 

So, I guess I'll do some prints, and wait what happens.

 

 

[claudermilk]

My extruder motor is rated for 1.0A and I'm running it at 0.45. If you're getting good motion at 0.45 then I think you will be fine. You will probably be ok at 0.5 if needed. The default printer.cfg for my Trident notes that motors can be run up to 70% of the rated max.

[gsezz]

The T8 Nut is stripped again.

It worked for a few days with lube and a run current of 0.45. Two days ago it skipped a few steps while homing. I applied more lube, and got it working again. Same thing yesterday. Today lube didn't help anymore, so I disasambled the Z-Axis again. There is no Thread left in the nut itself. The backlash thingy looks OK. 

I don't get it. Without the screw, the bed moves super easy. 
The screw is straigth. It seems to be parallel to the bed, as far as I can measure this with a simple caliper. This time I left the nut a little bit loose. 

 

 

[ChicagoKeri]

I joined this Forum specifically to thank you for this thread!

I also built a V0.1 and was vexed by the high current and resultant hight temperature of the Z stepper to avoid missing steps.

Thanks to your helpful post, I found the same condition... high friction due to the anti-backlash thing on the POM nut.  Removing the thing dropped the friction to very low level, but instead of leaving it off, I merely clipped about 1/4 -1/3 of the spring off. The reduced spring tension reduces the friction to almost as low as leaving the thing off, but still provides some anti-backlash effect.  I was able to lower the motor current to 0.35 and it still worked, but I put it at 0.4 just in case. The motor now only gets warm in an hour of printing.

[ChicagoKeri]

Even with the Kirigami bed, there was still some backlash with the worn POM nut with anti-backlash thing. 

So, if plain POM nuts are good enough for PRUSA, I decided to try a plain NON-Anti-Backlash POM nut.

The plain nut almost threads down the Teflon coated T8 leadscrew by gravity and when assembled, the bed has no perceptible Z-axis backlash. This  a considerable improvement over the "Anti-backlash" nut which managed to have both increased drag (friction) and considerable backlash, once the spring tension is overcome. To be fair, I had shortened the spring to get the drag down to a usable level.

The Voron Cube test print came out better than usual. It still has issues, but I think the Z axis ones are solved.