Table falling down in a middle of a print

[morganfree]

Hello to all!
Well, I had this issue two or three times already...

The table is falling down (-Z) in a middle of a print, but the print continues... therefore the nozzle continues delivering plastic to the air... the table completely down, forming a nest of plastic above the table. I couldn't see if the table continues to move in Y axis once down... because I only note this happened once the program is over.
This happened 2-3 times in the last 2 weeks, but in the middle of the programs that went wrong I could print many parts w/o this issue. So it seems to be something that happens ocasionally.

Any sugestion of what might be the problem???

Thank you very much in advance!!!

[sthone]

This happens to the best of us from time to time.... (I printed a nice new bird nest just the other night)

The table dropping usually happens when the z-motor is over stressed (so to speak) when the motor is over stressed I think it basically turns off (or resets?) to keep from causing damage to anything else.

The most common reason it can get over stress is when the nozzle hits the print. This can happen for a number of reasons but the most common is the print releases from the bed for lack of bed adhesion and then the nozzle runs into it. Unfortunately the printer doesn't know this happened and continues on with the g-code which results with the printer printing in the air.

[ednisley]

it basically turns off (or resets?) to keep from causing damage to anything else

There's no feedback at all, so the G-Code continues to tick along as if nothing happened ... because, as far as Marlin can tell, nothing has.

A non-manifold model could produce bogus G-Code, but that seems unlikely on the Z-axis; if the same thing happens at the same place on the same model with the same G-Code, then I'd should worry. Previewing the G-Code is always a Good Idea, nonetheless.

If this is an old-old M2, then the Z-axis motor has high-resistance windings that aren't suitable for microstep drivers; it's under-powered for the task. Replacing the motor with one having low-resistance windings will cure the problem.

[morganfree]

Amazing! Both of you! Thanks a lot!

Well, I wasn't that worried since I'm still printing, and as I said, it seems to be something that happens rarely.
@Ednisley, I do have an old Z motor (black-silver) so I guess that should be one problem, even though I'm printing parts that I already printed, MANY times... so I guess there´s no manifold issue on those models. I don't understand where do I have micro-steps if those parts were printed before with no issues. Anyway, I'll keep in mind the switch of the Z-motor (I suppose that if it was modified, its because there were some issues or enhancements).
And I have to agree with Sthone's comment about some parts being unglued from the table, so the nozzle might be hitting those parts causing some stress to the whole system (that's something I noticed, some parts were unglued). That might be the beginning of the issue...

On the other half, if it's something known and some of you have the same issue... I'm even with that! hahaha

Thanks again!

[ednisley]

where do I have micro-steps if those parts were printed before with no issues

Over time, the mechanical force required to move the big lump of hardware hanging on the Z-axis leadscrew will increase, even if you've conscientiously lubed the Z-axis rails & leadscrew, due to normal mechanical wear and the inevitable accumulation of dust inside the bearings.

The required force now occasionally exceeds the torque available from the known-weak stepper motor turning the leadscrew and, when that happens, the motor stalls and the platform drops.

The old-new and new-new Z-axis motors (there were at least two iterations) produce more torque, although upgrading from an old-old motor to a new-new motor may be difficult due to other changes. A chat with MG support should identify the least-awful upgrade path.

Here's how I did a DIY transplant, with linkies to some background info:
https://softsolder.com/2013/06/28/maker ... ransplant/

[morganfree]

@Ed, thanks a lot for your explanation!

I'll follow your links and contact MG support in order to make this upgrade immediatelly. Thank you very much again!

[b8smat]

I've just been experiencing a very similar problem but very early in the print (after just a few layers)

The key difference is I see that the hot end had turned off also, so the even though the X/Y movement continues, nothing is extruded.

Even though the bed had fallen to the bottom, the Z motor was still energized.

I have the older Z motor.

So while the explanation kinda makes sense for the Z behavior, I don't understand why the hot end would also have turned off. (although it was nice that it hadn't produced a rat's nest of extruded filament!)

[b8smat]

Doh! Nevermind. My problem is (mostly) just me being stupid.

We sliced the part wrong and the raft and model were printing with different extruders. The first extruder (for the model) was at the wrong temperature (40C) so the firmware prevented cold extrusion after it finished the raft. It still doesn't really explain why the bed dropped and it continued X/Y movements but I guess that is not really a problem.

[insta]

You still need to replace the motor, unfortunately. It fell because a Z-hop smacked a nozzle or something similar, and you're so close to the motor's threshold that it couldn't handle the tiny bit of extra strain. A new Z motor will fix it -- and when you take the old one apart, you'll find all the phenolic around the windings to be charred from heat...

[willnewton]

Cleanliness and lubricity is the first thing you should look at.

First let's assume you were formerly printing fine.

1. Home the Z.

2. Disable the motors

3. What does your Z do?

• a. Nothin', just sits
  
  b. Staaarts toooo droooopp reeeeaaalll slooooowww.
  
  c. Falls quickly to the bottom within a couple seconds.

4. If your answer is anything other than c. then you need to clean and lube the screw.

5. Wipe off the screw with a paper towel. Place a brass brush against the screw near the brass nut and home your Z while riding the brush up and then jog your Z back down and ride the brush down the screw from underneath. You may need to do this a few times to degunk the screw. Do NOT scrub or use abrasives to clean the screw. Hopefully the Z will start falling faster after a few of these cycles.

6. I apply about a thin pencil line of the grease along the screw and smear it in a bit. Then run the Z up and down under power and then see how it is dropping. You may need to apply lithium grease sparingly to flush out the old grease and repeat step five if it is slow in a spot.

7. You should be able to get back to printing now. If not, you may have alignment issues or electronic gremlins or unhappy steppers.

I have the small Z-motor and this has solved every Z-drop issue I have had until I notice that the build plate is starting to drop slowly again. I sometimes let it get too far, but it is easy enough to see the need for cleaning as it arises.

Maybe it will help out with your issue.