Taz 6 print head problem

I have an older Taz 6 - I think it is 6 years old, maybe 7. Anyway, it just started having a problem. The last two prints progress to about layer 10 and then drops down and to the very back, beyond the build plate and lower, and keeps printing. As it prints it rubs the back of the build plate. I’m very hesitant to print anything else until I figure out how to diagnose the problem.

Any ideas?

Did you make any changes like update the firmware or update your slicer? How do you get the gcode on the printer (via USB or SD card)?

What you describe is quite an odd failure as lowering the tool head below the build plate involves sending stepper commands to the Z motors. What does the LCD screen say for X, Y, and Z when this happens?

Maybe post a video. Slice something small like a 5x5x5mm cube. You can “print” it without loading any filament. I just want to see the failure.

No updates have been made since the print head was updated to the SE 2.85mm 4 months ago. Prints are sliced by the Lulzbot Cura v 3.6.37 and sent via Octoprint and USB.

There has been a further development. I decided to print a short item (5.4x44x29mm) and video record the entire process. The first layer started well but after about half done the print head started rubbing the bed. So the height changed in the middle of the layer. I cancelled the print and all further attempts to move the Z axis manually from the printer display result in just loud humming and no movement. I didn’t read the display screen coordinates. I’m guessing this is probably a controller problem since there are two steppers and neither are moving. It seems to me that for both to fail at the same time would be unusual.

Can you manually lift the Z axis before a print? It sounds like the lead screws may be dry. Lube with white lithium and manually twist the lead screws to help the motors, move it up and down a few times with the motors disabled/power off.

You’ll need to manually align the gantry after doing it.

Both Z motors are plugged into the same driver, so they appear to fail together.

Thanks for the feedback. This seems to have been the problem. I have been using white lithium since purchase and I started to lube again and found that the lead screws were gummy. I used isopropyl alcohol to clean and then re-lube and things are working fine again. I am wondering what the condition is inside the (nor sure what they are called) carriage housings and if they can be cleaned. I am talking about the cone shaped things that move up and down the lead screws. I’m pretty sure some of the alcohol seeped into these and probably loosened the grease inside but it would be nice to clean out any old gunk in there. I’m good with this kind of work. I’ve been running the FIRST robotics team at Brewster for 7 years.

Thanks again for the help.

For any disassembly/reassembly, the first place to go is the OHAI: Open Hardware Assembly Instructions