MK4 Probing for plate in mid-air | Issue homing Z-Axis when starting a print
Description of Issue:
My MK4 is having trouble probing the bed during the start of a print. It homes the X and Y axis just fine, but when it begins to home the Z it starts to behave erratically.
Instead of moving the nozzle down smoothly until it taps the surface of the build plate, it stops, moves back up slightly, then takes multiple small steps down. It will repeat this a few times until it finally finds the build plate (it if was not very far away to begin with) OR it pops up a collision detected screens and rehomes the X and Y and proceeds to repeat the same process on the Z.
The printer can normally start the print by repeating this process, but it is very slow, and doesn’t always work.
Troubleshooting Information / Theories:
It has never failed to detect the build plate when it does make contact. If I manually bring the nozzle close to the build plate before starting a print it will probe and start the print successfully.
It seems as if the Load Cell is registering a tap on the build plate while the nozzle is still in mid-air.
Occasionally the printer will attempt to clean the nozzle in mid-air. To me, this clearly indicates the Load Cell is registering contact with the plate. The printer would only attempt a nozzle cleaning procedure if it thought it probed the plate successfully, right?
Interestingly the ‘Auto-Home’ procedure in the settings menu does not suffer from this problem. It moves the Z-axis smoothly down until it taps the plate. I have included a reference video of this.
Prusa Chat had me verify my Z-Axis is moving smoothly. They suspected some ‘binding’ in the rods (smooth or threaded) may be causing this behaviour. I had my doubts that this would lead to this issue, but I double-checked anyways and the Z-Axis has always run smoothly.
I had considered the possibility that the filament was ‘dragging’ against the Load Cell as the axis moved down causing it to read a false positive for the plate. So in the longer example, linked above I just loaded a short length of filament into the printer. Still acted the same.
I have chucked in the nozzle nozzle/hot end assembly a variety of different ways, under more or less ‘pre-load’ aka pushing up on the nozzle more or less firmly while tightening the thumbscrews. I always re-run the Load Cell test afterwards. Doesn’t seem to affect much.
I need some help y’all. What ideas or information do you have? I’ve seen a few posts about this, or very similar behaviour, I included some references below.
===== REFERENCES =====
Links:
Similar Topic in Forum: LoadCell issue (MK4) ?
GH Issue for similar bug: [BUG] MK4 cannot complete Z-Homing
Videos of behaviour:
Smooth Operation of Z-Axis
Z-Axis Test: https://photos.app.goo.gl/MXkWDzrvvSjCr4859
Z-Axis Calibration: https://photos.app.goo.gl/2chDDchD5GLMTn1E9
Manual Jogging: https://photos.app.goo.gl/Ki2cuuC2cjqDQiuj8
Auto-Home: https://photos.app.goo.gl/QP1Y4AoepZfUK5589
False probing on print start
Shorter example
Right after 1:15: https://photos.app.goo.gl/YLh8Dw3gMvtNuGn29
Longer example, ending in nozzle cleaning failure. (with filament detached)
Right after 1:25: https://photos.app.goo.gl/eW6FUZfBYrVaWPeV9
The nozzle was dirty in this video, I cleaned the nozzle and it did the same thing, except the nozzle cleaning passed and it started the print.
RE: MK4 Probing for plate in mid-air | Issue homing Z-Axis when starting a print
This is exactly what I experience. Your videos could be mine exactly. I have rebuilt the machine several times and still have issues with it. My MK4 prints great when it finally goes but I have to spend an hour fighting with it before every print. Would love a solution to this but I find myself falling back on the 2 MK3 printers I built and upgraded. I have no issues with either of those, they just work.
RE: MK4 Probing for plate in mid-air | Issue homing Z-Axis when starting a print
Sorry to hear you're experiencing the same thing. But I am glad I am not alone 🙂
I've only seen a couple of similar instances, and no real solutions.
RE:
It sounds like the load cell is triggering for some reason.. it would be nice if Prusa had some diagnostic mode so that whenever the load cell sensed contact it might do something.. like make a beep.. Then you’d know.. not sure if a bad solder joint to the load cell could do that, or there is some pressure on the load cell and it is triggering randomly.. Can you convince Prusa to give you a new hot end, assuming the load cell is integrated.. I don’t have MK4 so not sure the electronics there. But you definitely have some fault there with the printer and replacing the load cell sounds like the best thing to try.
RE: MK4 Probing for plate in mid-air | Issue homing Z-Axis when starting a print
I agree. I love the idea of some debugging tools -- might help prusa support, too. I think I wasted a good deal of people's of time messing around with the Z-Axis, making sure it wasn't binding.
Yeah, I the load cell is integrated in the heatsink, it comes glued in. I'd love to do some troubleshooting around this to verify the Load Cell is at fault, either by swapping parts or some other process.
RE:
yeah.. if there aren't proper diagnostics built into the software to assist users in finding the issue, then Prusa should replace parts until the problem is identified. Obviously at least 2 of you have this issue, which probably translates to a significant enough number that Prusa should add some diagnostic testing to the load cell so they can identify the error without wasting a bunch of time testing working hardware. I was a design & test engineer in another life.. today's lack of meaningful error messages drives me nuts. Take a short video of what happens and then insist they send some replacement parts or replace the machine as you obviously have some hardware failure.
RE:
The very unlike possibility could be , your filament could be stacked somehow and pulls the the nozzle backwards and acts against the load cell.
Is that a bad idea?
For diagnostics I am using this Prusa-Firmware-Buddy/doc/metrics.md at master · prusa3d/Prusa-Firmware-Buddy · GitHub
RE: MK4 Probing for plate in mid-air | Issue homing Z-Axis when starting a print
I've tried to eliminate filament drag being the cause of the false positive reading (I've tested with only a small length of filament loaded). It's possible something is dragging internally?
Wow! Thanks for sharing that metrics doc. I will definitely be going down that rabbit hole. That is so cool.
RE: MK4 Probing for plate in mid-air | Issue homing Z-Axis when starting a print
@iamjcole
it is easier to install and run it on a Linux machine. I did not try it on Windows.
I am using an old PC for it. On Linux there is also simpler to compile the MK3 and MMU FW. It is running as a server with Influx DB and Grafana. The client can run on the same or another as well as on Windows machine. Do not forget set the parameters on the printer. Settings/Network/Metrics on FW 5.1 and higher.
RE: MK4 Probing for plate in mid-air | Issue homing Z-Axis when starting a print
I’m having the exact same issue on my MK4. I’ve been able to eliminate the “mid-air homing” and crash detection by paying out ample filament prior to initiating a print. When I do this, the z axis proceeds directly to the plate.
RE:
That's really interesting. So in your case it does seem to be filament tension related, which does not help the other folks who have tested with only short bits of filament. I do wonder if most people are using a friction reducing roller on the spool holder (if you are using the spool holder at all)? The very first thing I did was print up some rollers in PETG for my spool holder because I could tell there'd be more drag during printing than I was comfortable with.
There does seem to be something else going on here for the OP and kaelin, but if filament tension is playing a role, the solution to that piece of it is a very quick print away.
-J
RE: MK4 Probing for plate in mid-air | Issue homing Z-Axis when starting a print
I am spooling from a filament drying container using PETG. Sounds like I must have a different problem.
RE:
I've just (hopefully) solved my MK4 mid-air homing. The issue for me wasn't Z-axis or filament related - it was screws on the rear of the Nextruder backplate and Z-bearings. They weren't even loose, but tightening them solved it. Belts were fine.
I've got it in a V1 Lack enclosure with a slot in the top and PTFE tube feeding filament. Plenty of slack, no catching on the slot, I even payed out filament to try to get it to home. Nada. None of that worked. The mid-air homing would sometimes be accompanied by repeated X-bashing (often failing) and Z travel had a terrible resonance. Tightening screws on the backplate and Z bearings instantly caused it to home normally.
After watching a couple of your videos, your Y axis has four or so taps to home - it should have maximum two. More than two seems to indicate an issue. For me I don't think the X-axis was catching or anything, I think it was the resonance from being too loose messing with the (possibly too sensitive?) load cell just enough to mess with homing.
Review your Y-bearing mounts and belt tension (and if you get X-bashing, Nextruder backplate screws and X belt). Half the issues I've had with my MK4 often aren't related to the symptom itself which I find bizarre.
RE: MK4 Probing for plate in mid-air | Issue homing Z-Axis when starting a print
iamjcole describes the same exact problem that has been driving me crazy. I just tried the solution digital_realist proposed and so far this seems to have fixed the problem.
FWIW (and hopefully someone from Prusa Research reads this)........I purchased a MK4 kit since I was looking for a faster 3D printer. I had previously assembled a MK3S kit approximately three years ago and it was flawless. Never an issue with it. IMHO the MK4 should have never been released until the engineering problems were resolved. I have had issues with tension on the X and Y belts, multiple head crashes, failure to load files from the USB port, and now failure of the Z-Axis not homing at the start of a print. Wonder what issues will come up next? I'm really sorry that I sold my MK3S. If I had to make a decision on my next 3D printer knowing the hours I have spent trying to get this crazy printer to work, I would have purchased a Bambu. Now I know why All3DP no longer lists Prusa (and specifically the MK4) as their top pick.
RE: MK4 Probing for plate in mid-air | Issue homing Z-Axis when starting a print
if you want prusa to see your comments you would be better linking to them on the prusa mk4 github page
regards Joan
I try to make safe suggestions,You should understand the context and ensure you are happy that they are safe before attempting to apply my suggestions, what you do, is YOUR responsibility. Location Halifax UK
RE: MK4 Probing for plate in mid-air | Issue homing Z-Axis when starting a print
Hi! Thanks for the advice!
I will surely take a look at those and see if I can get those to affect my bed homing issue.
I would echo some of that behaviour/hearing resonance on some moves. Honestly, I have had a bear of a time with belt tension on this machine. The belt tension app didn't work well for me :). Prusa support advised me to use the GT2 belt tensioner thingy and that really helped my x-axis tension woes.
Just to confirm, which of these 6 screws did you need to tighten to fix your issue? Did I miss any? 
I've just (hopefully) solved my MK4 mid-air homing. The issue for me wasn't Z-axis or filament related - it was screws on the rear of the Nextruder backplate and Z-bearings. They weren't even loose, but tightening them solved it. Belts were fine.
I've got it in a V1 Lack enclosure with a slot in the top and PTFE tube feeding filament. Plenty of slack, no catching on the slot, I even payed out filament to try to get it to home. Nada. None of that worked. The mid-air homing would sometimes be accompanied by repeated X-bashing (often failing) and Z travel had a terrible resonance. Tightening screws on the backplate and Z bearings instantly caused it to home normally.
After watching a couple of your videos, your Y axis has four or so taps to home - it should have maximum two. More than two seems to indicate an issue. For me I don't think the X-axis was catching or anything, I think it was the resonance from being too loose messing with the (possibly too sensitive?) load cell just enough to mess with homing.
Review your Y-bearing mounts and belt tension (and if you get X-bashing, Nextruder backplate screws and X belt). Half the issues I've had with my MK4 often aren't related to the symptom itself which I find bizarre.
RE: MK4 Probing for plate in mid-air | Issue homing Z-Axis when starting a print
I would also like to add, I have been in constant contact with Prusa support about this issue. We are actively working on it and will post updates after any cause is verified.
We have swapped out the heat-sink w/ loadcell and it made no difference to the behaviour.
RE: MK4 Probing for plate in mid-air | Issue homing Z-Axis when starting a print
The suggestion by digital_realist seems to work. Even though the screws noted as 1,2,3,4 were not loose, I tightened them slightly and it seems to have solved the issue. I'm printing a Super Chipmunk R/C airplane by 3DAeroventures and have printed 8 pieces in the last few days without a single failure. I had previously run the calibration routine from a complete reset multiple times and it had passed the tests consistently. It appears the settings used for the calibration routine are less sensitive than the settings used to check X, Y and Z homing prior to initiating a print. I would suggest giving digital_realist's suggestion a try if you are continuing to have issues with Z-Axis homing. Simple fix for a very annoying problem.
RE: MK4 Probing for plate in mid-air | Issue homing Z-Axis when starting a print
I've had my mk4 (kit) running flawlessly for over 6 months, never had a belt tension or homing issue. I just added the MMU3 this afternoon and now can't print at all due to homing failures. The nextruder changes are first on my list to check as nothing else changed about the mk4. I'll be testing some of these theories and will post results.
X marks the spot.
RE: MK4 Probing for plate in mid-air | Issue homing Z-Axis when starting a print
Hi, after a first print, ending at a relatively high z, my MK4 used to probe mid air like described if i started a new print. However a G29 on the serial port was always concluded with a successful homing.The only difference between the two scenario, seems to be the preheating (M140,M104,M106) before the homing (G29). So i added a first G29 before the preheating in the start gcode. This way the printer homes then preheat and homes again always successfully in my case.
I hope this will help.