Z-axis repeatability problem?

[stephenbrooks 1]

My Ultimaker 2 had been printing quite happily for a while but today when I tried to print, the bed would stop 1-2mm short of the nozzle, so of course the filament didn't stick.

I tried re-levelling the bed and it was time to lubricate the Z-screw (6 months), so I did that and put the bed up and down a few times. However, no matter how many times I re-level the bed, when I go to actually print, it stops 1-2mm short of how high I want the bed to be!

The only thing I can think of is there's some debris around the screw, but there's this box with "Ultimaker 2" written on it at the back of the platform that hides part of the screw, maybe something dropped down there? Any ideas would be appreciated.

[Melka 57]

You can take a look at the platform attachment to the bearings by simply removing the two screws that secure the cover (where "Ultimaker 2" is written).

To locate those two screws, take a look below the plate.

Edited October 19, 2016 by Guest
typo

[stephenbrooks 1]

You can take a look at the platform attachment to the bearings by simply removing the two screws that secure the cover (where "Ultimaker 2" is written).

To locate those two screws, take a look below the plate.

OK, undid the screws. The plate took a bit of prying to get up but underneath there didn't seem to be any problems:

The other place I looked is the Z switch recessed down in the bottom. I wondered if some debris had got in there but it doesn't seem like it.

Could it even be some weird software problem where it's not remembering the number of Z steps to the top? I doubt it because it happened spontaneously.

The only other thing I can think of is that when the plate rises it's fairly quiet but when it lowers there's a bit of crackling noise. I haven't had a *good* look around the Z stepper motor and Z switch since I've not removed the base plate yet.

[stephenbrooks 1]

I wonder if it's possible to hack the .gcode files to put in some additional Z steps (upwards) at the beginning of the print? At least that way I could still use my printer.

[peggyb 169]

Can you move your bed by hand up and down and feel irregularities or hear some 'grinding' noise from the bearings?

Can you also check the z-nut for play? Reach under the build plate at the back and try to push the build plate up, there should be no movement pushing the bed up..

There is an interaction in the two parts: when the z-nut has some play, the buil plate can get stuck, hang at some point, but if it has no play it will force the buil plate to move.

[stephenbrooks 1]

This is getting interesting:

 

Can you move your bed by hand up and down and feel irregularities or hear some 'grinding' noise from the bearings?

I have a grinding noise when I move the plate down (and the Z screw rotates), but not when it goes up.

 

Can you also check the z-nut for play?  Reach under the build plate at the back and try to push the build plate up, there should be no movement pushing the bed up..

There is some play when I push underneath the plate - of this sort of millimetre level too. The plate moves up a small amount before the Z screw starts to move.

Where is the Z nut so I can tighten it? Do I have to remove that back plate again?

[peggyb 169]

Both the bearings and the z-nut are behind the white cover. There are several posts about this issue on the forum, I ended up replacing the z-nut and bearings, but you can start by cleaning the bearings.

Like I said: if your bearings are good, the build plate lowers itself just on its own weight, there is no need to replace the z-nut.

If your z-nut is good, it will force the bearing to move the right amount, even if they grind.

https://ultimaker.com/en/community/20050-how-to-properly-clean-z-axis

https://ultimaker.com/en/community/18394-sounds-like-theres-sand-on-the-z-screw

[stephenbrooks 1]

Both the bearings and the z-nut are behind the white cover.

I'm guessing the brass one in the middle (screwed in from below) is the z-nut and the sleeves on either sides are the bearings.

I unscrewed the right-hand bearing and it snags near the top of the rod if I try and move it up and down (haven't tried the left one yet).  Feels like ball bearings jamming up.

I see gr5 in one of your linked threads recommending the fix here is to take the faulty bearing(s) off and clean them out?

The z-nut screws feel very tight, even though there's perhaps 0.3-0.5mm play in the plate if I push it from below.  So I don't think I can tighten those 4 screws from below any more than they are already.

[Later...] OK, got the bearing off one of the Z rods (and the rod slid out of the bottom of the machine). I can run it up and down and can feel the bearing snagging periodically. How exactly do I clean it out? (I'm a little afraid to introduce moisture). Alternatively, I guess I could order replacements.

Edited October 23, 2016 by Guest

[peggyb 169]

more info and pictures here: https://ultimaker.com/en/community/18335-uneven-layers-although-i-print-at-1-mm-i-am-still-able-to-see-every-layerhelp-please?page=1

hope this helps..

[stephenbrooks 1]

Well, I disassembled it, did my best to clean the Z-nut and bearings.  Then tonight I reassembled it but now the plate won't go all the way to the bottom.  When it gets about an inch above the lower limit the Z stepper motor makes a loud hum/whirr noise for a few seconds and then the printer says "Error: Z switch broken".

It's completely jammed and I can't get it to move. Is there a risk of this from over-tightening the screws? I made sure I did them up tight because I thought that would help. I guess I could unscrew the bearings and maybe the nut again.

Edited October 28, 2016 by Guest

[peggyb 169]

Did you move the bed with the knob all the way down when this happened?

Turning of the machine lets you move the bed by hand, grab it close the rods. Can you move it manually up and down?

When reassembling the bearings and z-nut, you could leaf the screws loose a bit, moving the assembly up and down a few times, and gradually tightening everything.

[stephenbrooks 1]

Hi,

Got it moving again.  Loosened the screws on the bearings and it turns out the plate can slide quite a bit back and forth (by a millimetre or so) relative to those screws.  So I moved it towards the front of the machine and away from the back: I noticed there's hardly any clearance between the build plate and the back of the machine, so I pulled it forwards as much as possible before re-tightening those 8 screws.

Now the build plate can raise and lower just fine.  However, I just tried printing again (after re-levelling the plate) and the plate still stops the same ~1.5mm short of the nozzle when it rises!

I'm starting to wonder if it's a weird software error that misses steps.

[edit] Oops! Wait, this is my fault!

I was about to try and hack the .gcode file to force it to move the Z to the correct place (and along the way found this nice manual), when I saw this:

So that looks like my ~mm gap. Looking more carefully at the model in Cura, I then saw this:

So long story short, my model was at fault. The printer is now printing another model just fine!

The fundamental reason for the error was that the software I use to generate the STL files sometimes leaves rogue vertices after cut operations. These are usually joined to zero-area triangles that don't show up in renders unless I specifically plot them:

So, Cura was seeing that vertex and thinking it was the base of my model, and trying to plot the rest floating in thin air.

Thanks to everyone who helped. I now know how the Z stage works and how to disassemble it, which might come in useful later anyway.

Edited October 29, 2016 by Guest

[peggyb 169]

pfff, glad you found the issue, but like you said, you get to know your printer better..