MINI 2: Z height issue with varying XY for new machine

Hey Lulzbot & forum members…

I was quite excited to get the new mini 2, and printing smaller things and test cubes in the center of the bed worked fine, but on my first print of a larger object I noticed a weird pattern: the front left side of the bed would squish the first layer, the front right would barely adhere thus failing to print well (and reverse problem on rear of the plate). Cura settings were almost default for Polymaker PLA (to avoid the prior elephant’s foot: 65c bed temp, first layer size reduced down to 0.18).

Went to all-defaults standard quality calibration print of 25 small flat squares to check the pattern on the whole bed and my obvservation was confirmed: https://imgur.com/a/NqKsDQY

Doing x-axis calibration from the menu didn’t help, I moved the axes every which way, used the movement menu to place the head in various places, confirming that at front right the head is way higher than at the front left for the same z. Reading up on how the belts should be, they are are tight and make the twang noise…

Is this a bad unit needing a return or a design issue from moving to a belt system for the Z axis? Those carriages are easy to shift up or down separately…

Help!

Perhaps a long shot, but if the X-axis is level, and the probing for the corners is working correctly, then it may be that your print surface is not level with respect to the 4 washers that the nozzle probes.

Check to see that the four washers are snug, and not tilted up from the spring-loaded pressure that’s pushing the glass and PEI up from beneath. The best way to do this is the use your thumb and forefinger to squeeze the glass to the bed beneath, releasing the pressure on the washer – the washer should not be able to be rotated by hand when you’ve done this. If it can easily spin, or if it visibly wobbles when you do the squeeze thing, then use an allen wrench to snug it up (don’t overtighten, the aluminum threads are easy to strip).

If the washers are ok, then check the bed for other issues – is your glass flat? Or (more likely) is your PEI layer level and properly adhered to the glass all around?

I have had the same issue and have been working with support but have not got anywhere to resolving yet. I have been too busy this week to continue the troubleshooting.

@mwester

Thanks for the suggestions, I tried to check out the washers and they are flat against the glass. The auto-leveler definitely does go and touch them, it’s just failing to actually do the leveling…

@Keebie81

What have they suggested so far? I’m not as patient as you lol, if I can’t get it fixed in a few days it’s going back to Amazon…

We have been working closely with marlin devs to try to get backlash compensation code added to upstream: https://github.com/MarlinFirmware/Marlin/pull/11061

While we are working with upstream to get it more standardized across various platforms, we have included Z backlash compensation within Cura LE 3.2.23 for the Mini 2. We are planning to release this version as the next stable release later today or tomorrow, but can currently be downloaded here: http://devel.lulzbot.com/software/cura-lulzbot/

This implementation will measure any potential backlash is in your system for the Z drive, and compensate accordingly. We have found this is ideal to help with the first couple of layers, but is not required for the full part build. We fix this by “fading” out backlash compensation over a specified height on each print. We currently have all default profiles set to turn this compensation off after 1mm of height for the Mini 2 in Cura 3.2.23. This will insert a M425 F at each layer, to slowly reduce the how much it is compensating. How long this compensation lasts, or if it is activated is now within the quality as “backlash fading distance.”

In order to ensure that the start gcode changes, and the cura engine changes are working properly please be sure to clear your cache and add a new machine once installed. I will have some photos up in about 2 hours showing some squares with and without this activated for reference.

Cura LulzBot Edition has been updated to version 3.2.23 today and can now be downloaded from LulzBot.com/Cura. The changelog can be viewed here: https://code.alephobjects.com/w/cura-lulzbot/

I upgraded to Cura 3.2.24 deleting the profile directory, upgraded the firmware (restoring z-offset, extruder steps didn’t change), readded the printer (did not usb connect) and tried the same test from before - 25 squares at .25mm standard defaults and here are the results: https://imgur.com/a/v9pNvrs

Hard to capture details in a such a photo, but the fix does look like a bit of an improvement, especially the front-right part. The center sweetspot appears wider and more consistent with squishier diagonals, perhaps allowing rotation of longer thinner pieces appropriately… that said, the very bottom-right and top-left corners are still drastically different from their anti-axis counterparts and so I suspect using the full area will remain problematic for difficult prints, especially given how squished the extremes are, I will most likely have to raise the global z-offeset to avoid elephant foot there, thus again lowering adhesion in other places…

Will try re-making my failed piece, aligning it front-left to front-right as before just to test it out, but this is honestly not great for a $1500 machine. Is the defect present on all MINI 2s or just individual units? If the latter I’m thinking of trying a replacement, as utilizing the whole area at once is a big requirement for getting this printer in the first place.

Some of the issues might just be also because of the automatic leveling. I have 4 mini’s and the leveling is nice but when printing something that needs a consistent appearance on the part touching bedplate I always go to one of my taz 5’s with manual leveling since I can fine tune each corner and it stays consistent through each print restart.

I do wish that in the future there could be some way to manually fine tune the leveling on the mini and my taz 6

So Lulzbot, no other suggestions? I’m not sure I understand backlash in the first place to even know what to try to fix… is the difference in Z over XY caused by proximity to one or the other Z belts? Would tightening them to identical tension help?

Is this issue occurring on some individual machines or all mini 2s due to the z belts?

Backlash is present in moving mechanical designs. Think of backlash as the delay between you saying “Move!” and when it actually starts moving. This can be caused by gears within a motor, play between leadscrew drive systems, the stretch of a belt, and even any binding in a specific direction.

The LulzBot Mini 2 uses the G29 auto bed compensation feature to measure the print surface plane. Auto Bed Level compensation will slightly move your tool head axis up and down across the calculated print surface plane help get the perfect first layer.


Here’s the new built-in Z-axis backlash compensation process:

  1. Establish an electrical connection between the washer and the hot end nozzle.
  2. Send the move Z-axis up command (this happens automatically after each probe.)
  3. Measure the steps needed for the Z-axis stepper motor movements to break the electrical connection.
  4. This machine-specific backlash amount is added to your Z-axis movements, 100% at first, and is faded out after the first few layers.


    Cura LulzBot Edition version 3.2.23 and newer uses this backlash compensation to put in very small extra movements by the motor to compensate for this lash. As this backlash gets worked out after a few layers, we use less and less backlash compensation as the print progress. This allows for accurate prints while ensuring solid adhesion on the first layer. We have seen excellent results with the LulzBot Mini 2 and with our 0.25mm tool head (still in development) where we need to use a 0.2mm initial layer height.


This issue occurs on all moving machines (not just 3d printers) since you have to allow room for movement. We are exploring applying this backlash compensation to the X- and Y-axis as well, follow the progress here: https://github.com/MarlinFirmware/Marlin/pull/11061. So far, individual users across many different types of machinery are reporting benefits from this marlin inclusion.

There can be slight variances with probes. For example, if you have a slight amount of plastic on your nozzle, it will push through it before making contact with the washer. This will cause the printer to read that corner as lower than what it actually is, and affect your base layer across the build volume. A clean wiping pad will help mitigate this. Update your wiping and probing temperatures if you are using a filament brand that is not included within Cura LE. This will help ensure longer wiper pad life, cleaner nozzles, and better probes.


We tighten our Z belts to a range of 35N (Newtons) - 45N. Directions for this process can be found in step 5 in our Open Hardware Assembly Instructions (OHAI): https://ohai.lulzbot.com/project/final-mechanical-assembly-mini2/mini-2/. During the final calibration, we run a test print (attached) to measure the skirt height to determine flatness and Z offset. When measuring the height of the skirt we expect the X+, X-, Y+, and Y- measurements to be between 0.3mm and 0.5mm. If the skirt measures a thickness outside of this range, we will diagnose and re-work the printer to ensure it meets our quality checks and tolerances.

Would mind running the rocktopus test print and share your results?
octopus_rev05_callibration.gcode (1.61 MB)
We’d also like to check the dimensional accuracy from prints. Would you be willing to print 5 calibration cubes (one in each corner and one in the center) and compare their overall dimensional accuracy?

Life interfered, but I’m back to debugging this…

I did the calibration test you asked for, the results are here: https://imgur.com/a/7UcdtuE

I even did it on another Lulzbot Mini 2… which has the same issue as mine, except slightly less pronounced on the top-left side. This is in the second image in the album above with both the cubes and the paper in one shot.

I found this post that confirms that the MINI2 (and likely all other Z-belt printers) suffer from this problem… and the fix is likely insufficient, given my results.

https://github.com/MarlinFirmware/Marlin/issues/10826

I know this is an old thread but I’ve been having the exact same issue.

I’m a newbie so I didn’t know what was happening with my prints - with Alloy 910 and PC-MAX I was getting lifted corners, but mostly on the right side of the bed.
I’ve re-seated and cleaned the bed and washers, been cleaning my nozzle and watching the levelling process, I’ve also tightened the belts up and checked the surface with a straight edge (it’s ok, no rocking of a ruler) but still getting the same results:
Lm2BLP - 0001.jpg
Lm2BLP - 0002.jpg
Lm2BLP - 0003.jpg
Lm2BLP - 0004.jpg


Skirt on left side is nice and smooshed together but on the right side there is a gap in the skirt lines, not to mention the same on the actual first layer.

From the last post it seems like this is a design issue and nothing that can be done except to offset the z-axis even more, so one side is super smooshed to compensate for the other.
Has there been any update to this issue?

Other than that the printer seems to be working fine, and PLA sticks well. But I’m trying to print functional parts with PC and Nylon and this is kinda screwing up my prints and eating $$$ in filament.

Is there anything else I can do to “fix” this issue? Machine (1.1.9.34) and CURA (3.6.3) updated to latest

Perhaps in a future firmware there can be a tweak to adjust the left and right z offset so one side can be lower than the other to compensate for this?
Printer does this consistently so if there was a way to adjust how it prints the first layer left vs right that would probably fix it?

Thank you for any help.

1 Like

I am getting this same issue with the smooshed left side and the right side being too high from the plate. Did anyone ever find a solution to this? Technical support has been little to no help with the issue and thus far I truly regret purchasing this machine.

A stupid hack I did that made it “better” but not eliminate the problem was to simply shim the other 3 corners and keep printing the corner tests until it improved.

Since the right side is “too high”, you can “lower” the bed by shimming the corners at the spot under the washer and the bed (you want to introduce some space between the washer and the bed only, NOT the washer and the black plastic mount it is screwed into (I mean I guess you can if you can find a way to do this evenly, but I didn’t).

What I did was simply put a small piece of paper between the shim and the bed/print surface for the 3 better corners (I used post it notes). No paper under the worst corner. Then I printed the test. Checked the results. If no improvement, shim some more (I just folded the piece of paper over). Repeat until it looks “better”. Obviously each corner can have different thicknesses of shims.

Once I got a good result I trimmed the paper as small as I could to not interfere with printing.

Stupid solution but helped for me, I’m not a pro so I don’t know if this screwed anything else up but it “makes sense” to me, you’re introducing more space in the other 3 corners to compensate for the “bad” corner not being “close enough” since you can’t make any software adjustments, and since you can’t raise the bad corner you have to lower the other 3 corners.

Basically you want each corner to look relatively even (obviously). Then readjust the Z offset value for a nice first layer.