MINDA Z offset issue  

RE: MINDA Z offset issue

My ideal Z-offset for the textured sheet is around -1.080mm, and roughly -0.7080mm with the smooth sheet. I usually don't need to change my z-offset between prints.

With the nozzle resting against the build plate, the probe should be roughly 0.8mm-1.0mm off the bed. Further instructions can be found here: https://manual.prusa3d.com/Guide/Troubleshooting+the+MINI/1507?lang=en#s40940

RE: MINDA Z offset issue

Posted by: @jonathan-v

.. my Z offset is set to about -0.540mm, but ...

No idea what´s the reason for this massive shifting of z-value but what I can say is:

My z offset is -0,660 (very close to yourse and that makes absolutely no problem. So I don´t think that it is necessary to change sensor height to come closer to the middle of the PRUSA-ideal-z-offest-value.

Best regards, Clemens

RE: MINDA Z offset issue

@clemens-m

Thanks, I actually adjusted it so it would be 0.540 since it was 2.000 originally.

I just can't seem to figure out why it varies so much.

RE: MINDA Z offset issue

I believe it is related to temperature. Take a look at this thread for more detail: https://forum.prusa3d.com/forum/hardware-firmware-and-software-help/z-level-calibration-varies-with-temperature/

RE: MINDA Z offset issue

Posted by: @drkmstr

First Layer Z offset keeps changing.

Is anyone else having this issue?

I'm having this issue exactly! My MINDA Probe seems to be installed just right, a width of a tie zip (1 mm) above the surface. I'm experimenting with heatup as we speak, not there yet.

-Izmet Fekali

RE: MINDA Z offset issue

@jonas-1

@drkmstr

I've never had this problem, but I modified my start g-code very early to always preheat before the mesh bed level to avoid any problems.  Maybe this will work for you.  In my Prusa Slicer I have saved a modified Prusa Mini printer profile that I always use.

I found the workaround gcode on github in this issue although I use 120 seconds.  Good luck!

RE: MINDA Z offset issue

Posted by: @drkmstr

First Layer Z offset keeps changing.

My first issue was the probe was set too high, causing an "ideal" Z offset to be at -2.000mm, I moved the pinda probe by loosening the clamp, carefully moving it upwards, then re-tightening the clamp. 

I finally got it to a point where my Z offset is set to about -0.540mm, but every time I print it changes and I have to manually adjust between 0.4mm and 0.7mm for EVERY print.

 

Is anyone else having this issue?

 

I'm having this issue as well. On the third print since assembling the printer, the nozzle scored my print surface (smooth sheet) despite the fact that the previous print had the proper z-offset. It was then that I started recording my mesh bed leveling values and z-offset values. They are different on almost every single print. I've been talking with support about it and they are sending me a new MINDA probe. However, I'm not confident it's going to solve the problem.

On 8 consecutive prints, I had to perform a live z-adjust at the start of the print to the following values:

1. -0.200 
2. -0.430
3. -0.120
4. -0.120 
5. -0.537
6. -0.350
7. -1.327
8. -1.590

MINDA probe aside, it seems obvious this is caused by the design of the YZ-axis. Even with everything assembled properly, there is too much play in the Z-axis. Of course I was expecting this to be the case when I bought the printer because it's very hard to make a cantilever structure completely rigid. It's also another reason why the mesh bed leveling is so important. I really hope Prusa rethinks the MINDA all together, especially if the heat from the bed can throw it off.

With that said, once I ensure the z-offset is correct at the start of every print, the print quality is amazing. Also, understand that this printer is going to have some growing pains since it's the first version of its class. I'm judging Prusa by their customer service more than anything and so far I'm pleased. 

I've also had a problem with the USB drive that came with the printer. It just stopped working after a few prints. The printer and both of my computers won't recognize it. It's just completely dead. 

RE: MINDA Z offset issue

I also have this issue. Being a noobie, it took a while to identify the problem. I tried to do first layer calibration and on cold machine, I slowly increased the Z offset. I had to try several times before the filament stuck and offset was around -1.750. I didn't print anything at that time. Next session I had I again started with cold printer and I again tried first layer calibration. Filament barely grabbed the surface and I desperately increase z-offset to full -2.000. And is does not stick. Second run, it sticks, but filament does not spread at all. On third run it suddenly is too close and I end up  to somewhere -1.76 or so, like on the first session.

Eventually I determined that it must be related to expansion when the printer is warming up. So I tightened the YZ-axis screws (just in case I was over causious), switched to textured sheet and started over. I finally resorted to preheating the printer with PLA-temperatures for about 10 minutes (5 is not enough). This way my z-offset is correct (good enough) and I can start printing without any more fiddling. I'm using PET-G exclusively at the moment.

Without preheating, the -2 max z-offset is not enough to get fist layer to stick. My "working" Z-offset is -1.890, but requires preheated printer.

RE: MINDA Z offset issue

@jari-holpainen

you can move the MINDA up about 1mm, then your working offset will be closer to -0.890. I had to do this because I sometimes needed values greater than the -2.00 limit.

Has anyone insulated their MINDA with a silicon sock or a printed piece? I suspect it wouldn't fix the temperature problem but would at least stabilize the variation and make it more reliable.

RE: MINDA Z offset issue

@jari-holpainen

I’ve been going back and forth with support for two weeks now. A new MINDA did not fix the problem. I kept noticing something off about the bed. It just looked warped. I put the edge of a metal ruler on the bed and it was a little bowed. I also did this to the y-carriage and is was warped too. I sent the picture to support and he said the bed should not be warped. He asked me to take off the bed, carriage, and y-axis rods and take a picture of them on a flat surface to show they are warped. Funny thing is, when I did this nothing was warped anymore.

While attempting to put the y-carriage back on I noticed that it was not sitting squarely in the linear bearings. It turns out that the y-axis rails are a little closer together than they should be. The carriage binds slightly toward the back and front of the printer. I printed a template on my Mk3S and the rails are closer together at the rear of the printer. Right now I’m printing another y-axis rear face on my Mk3S to see if anything improves. 

I also found out that the bed and carriage were not as warped after I put everything back together. That is, until I printed something. The heat and stress to the carriage from the y-axis rails being too close together seems to create a condition where the warping if the need varies slightly. This would explain why the z-offset has to be changed so often. It also may be why others are seeing better results if they preheat the printer. That allows everything a chance to settle down.

I think it’s possible that the 3D printed front/back faces on the y-axis have poor tolerances on some of the printers. For example, I measured the distance between the holes for the y-axis rods on both faces and got 127.49mm. On the template I printed on my Mk3S, I got 128.04mm. This is a small difference but my template makes it more clear that the rods are too close together. 

RE: MINDA Z offset issue

Its a known issue unfortunately... It is caused by the temperature of the MINDA sensor while performing the calibration. Different temperature values will produce different results. The only known fix for this time is to preheat your MINDA with a custom gcode in the beginning of each print. This way you will achieve even values in each print. I have found this doesn't work so good when changing from PLA to PETG probably because of the difference in nozzle temperature so every time I let it rest before starting a new print. I haven't tried this yet but maybe wrapping the whole MINDA assembly in kapton tape will solve this problem? I am trying to get hold of some tape here but I am still searching.

RE: MINDA Z offset issue

@geoper2

I have seen this suggested before and I have not had any success with it. I have even done tests where I don't even let the printer cool down between prints and it still yields a different z-offset. I have been working with support for 3 weeks on this and they are ready to recall my printer for warranty service. We have already replaced the MINDA and gone through mechanical issues, more specifically with the bed, and y-carriage. I recently printed new front and rear y-axis faces because the smooth rods that the y-carriage rides on were at least 0.2mm too close together. This was causing some binding of the linear bears that the carriage rides on when the carriage was near either end of full travel. Now the binding is gone but the problem is still there.

Yesterday, I attached a pin to the top of the x-axis belt pulley housing (far left end of x-axis) and placed a ruler behind it. I took photos of its position at the end of a first layer calibration (just after setting the z-offset). I then recorded the position at the beginning of a second calibration and again just after adjusting the z-offset (because the previous value did not hold). The pin was about 0.12mm (my best estimate using my photos and CAD software) higher at the start of the second calibration. After I had to adjust the z-offset to be 0.077mm lower (closer to the bed) than the previous value the photo showed that the pin was at the same height as it was at the end of the first calibration. It's almost as if the z-stepper motor is not microstepping properly. Either that or the printer is not controlling/counting the microsteps properly. I don't know, but it looks like my printer is going to have to be sent back to Prusa for them to figure it out.

Personally, I think the MINDA temp issue is valid however. I think a lot of things need to be improved on the next iteration and I am making sure to inform support of my findings. 

RE: MINDA Z offset issue

@ryan-p26

Huh, I was not thinking about printer not keeping track of stepper motor. Another thing is whether MBL works at all with last few firmwares... there are few GitHub issues opened where MINI does not compensate for uneven surface at all. Few were due to bad MINDA but others have mINDA working fine and still not compensating for tram.

RE: MINDA Z offset issue

Thanks to you all who responded. To my mind it sounds like there are two issues here: temperature sensitive MINDA probe and printer which warps when it heats up.

I know I can udjust the probe's height to get the nozzle closer to the bed (more useable z-offset range). Haven't done it yet, but might do.  So far as I stay in PET-G and textured sheet, preheating seems to do the trick. It might go haywire if I try to print another piece right after the first one is finished.

Likely I add the g-code sequense so that the printer will automatically run preheating before printing and be content with it for a while. Might try some measurements later.

@ryan-p26: thanks for the information. It confirms my suspicion that the structure of the printer warps too much when it heats up. I wonder is I could switch some of the plastic for Aluminium or at least add few slabs of it to make the printer more rigid.

RE: MINDA Z offset issue

@jari-holpainen

The Mk2 Prusa has has a non temperature compensated Pinda since it was launched. 

I't stall a good printer, you just get used to managing the pinda!

regards Joan

RE: MINDA Z offset issue

@joantabb

Other than having to make sure the z-offset is correct at the start of every print, the Mini seems to be a good printer (especially for the price). However, having to manage something that should not need managing is still an issue that Prusa needs to address. My Mk3S has spoiled me when it comes to "set-and-forget". I hit print and leave it to do it's thing without worrying about the nozzle height. 

@jari-holpainen: It could be worth trying. Unfortunately, you can only make the xz-axes so stable. I think the best place to start is with the electronics box. The unsupported corner is easy to address and I'm a little surprised at Prusa's decision to design it that way. Especially when supporting it would add stability to an inherently unstable cantilever design.

RE: MINDA Z offset issue

Well, I've been asked to send my Mini in to Prusa for service. I'm anxious to see what they find out but I hope it doesn't take too long. I'll update this forum when I have answers. 

RE: MINDA Z offset issue

@ryan-p26: Ok, will be interesting to hear what they find. I'll investigate mine, but currently don't have the time for it.

I chose mini from Prusa as the price was palatable and to get 3D printer which would work out of the box without endless tinkering. Got 1 out of 2, so far. Perhaps we will see "upgrade" for the hardware as well.

RE: MINDA Z offset issue

Posted by: @joantabb

@jari-holpainen

The Mk2 Prusa has has a non temperature compensated Pinda since it was launched. 

I't stall a good printer, you just get used to managing the pinda!

 

regards Joan

MK2 has at least support on both sides for Z axis and flame skew compensation. MINI is cantilever design, it is more challengingsetting it up perfectly perpendicular (especially when one mounting point is in plastic part). So now you are fighting not only temperature and surface shape, but also the fact that whole printer is skewed, and X is not parallel with print surface (which more often than not is the case, and MBL should compensate for it within its sensitivity limit but it does not). And judging from bugs on GitHub, in recent firmware releases it is possible that compensation is not working as it should.