Z-axis referencing table detection problems
Hello,
I am reaching out to you here in the hope that you may be able to assist me in identifying and resolving the issue more quickly.
Some time ago, I noticed a problem with the Z axis, namely that the rear screw had about 1 mm of play and the prints on the rear part came out terribly. Support did their job, I got a new motor and nut. In addition, I updated the software during the replacement. And here the problem begins.
1. Z-axis calibration - done
2. Head referencing - problem
The printer moved the table up to a height of about 7-8 cm from the head and stopped, slowly moving up bit by bit, constantly stopping. The nozzle touched the table - hurrah! But not so fast - a collision was detected. Printer reset, table lowered, homing and collision again... after 4 minutes it finally homed. First print - homing, collision detected, homing, table search, collision, homing - works. Second test print, same story.
After contacting support, they suggested that the hotend, which I hadn't touched, was too tight. I went through the entire load cell troubleshooting procedure, and it got worse. I tightened everything back up my own way with a torque screwdriver, and it's a little better, but every print has at least one collision during homing.I'm slowly starting to regret my purchase because for the price of the printer + MMU + camera, I could have bought the same set from a competitor. My friend has a Chinese printer for a fraction of the price and doesn't have as many problems as I do with my Prusa.Has anyone else encountered this problem and managed to solve it? Or is selling the printer and replacing it with something else the only solution?
RE:
The erratic Z homing is due to false readings from the load cell -- it reports touches where there are none. Two reasons for this have been discussed in various forum threads:
- Electrical interference from the nozzle heater. Since the heater current is modulated (PWM), its pulse edges can interfere with the load cell signal. Only a few printers seem to be affected to such a large extent that Z sensing becomes unreliable. We can discuss countermeasures if needed -- but I understand that your printer shows problems also when you select "Auto Homing" from the menu, without the nozzle heater being on, right? In that case this can't be the issue.
- Mechanical strain on the print head. Prusa's theory about the hotend being tightened too much points in that direction. More common causes I have seen discussed here are: (a) nozzle being pushed in too far, such that its rear end pushed against some internal stop in the print head, or (b) cable and PTFE tube tugging on the print head because they don't have enough free length. See here in the assembly instruction, and also the subsequent steps.
Could you confirm that Auto Homing also has problems while the nozzle heater is off (to rule out cause 1), and have a look at potential mechanical issues (cause 2)?
You may have found this already: You can see the current load cell readings under Control → Calibration & Tests → Loadcell Test. That may be helpful to see whether e.g. the load cell responds strongly to touching the cable harness, or whether it becomes more quiet when you pull the nozzle out a tiny bit.
RE: Z-axis referencing table detection problems
Thank you for your quick reply.
1. It's better with the heater turned off. I don't actually use Autohome every day, only every few prints.
2. I replaced the nozzle with a new one and moved it slightly closer, without applying any special pressure or resistance. The PTFE tube is different from the original because I have an MMU3, but the printer was working normally before and I did not notice any particular problems with homing.
3. I did a cell test where you have to touch it with your hand, and it passed.
When I get home, I will try to do homing with Z-axis calibration without heating the nozzle, and then I will try to run the programme so that the printer is based with the nozzle turned off.