I have had to pause work on my robots as I have not been able to get any reliable prints out of my old Prusa Mendel 3D printer. The frame of the printer had twisted again making auto bed leveling impossible while the extruder drive was failing. After moaning to my brother about the troubles he gave me a “new” printer. The “new” printer being an old Up Plus 2 that had previously been written off and stripped for parts.
The first thing I did when it arrived was pull all the covers off to see how bad it was. The mechanical faults seemed to be limited to a broken Y belt clamp and a destroyed hotend. The Y belt clamp had been broken when the printer was pushed against a wall while opperating and there may be an accompanying electrical fault that I can’t assess without first fixing the mechanical issues. So first I designed and printed a new belt clamp using the old printer. Fortunately it worked the first try.
Next I needed to replace the hotend. The only hotend I had available was the E3D V6 on my current printer. So I downloaded some drawings of the E3D V6 and measured the Ups hotend and extruder and made a new mount for the hotend. The trick was to try and get the nozzle of the new hotend to be in the same position as the original nozzle. Since the E3D hotend is much higher than the Up hotend the new mount needed to raise the extruder a fair way. This new mount was the last print ever done on the old Prusa Mendel as I pulled the hotend out after it was finished.
With the mechanicals sorted I could test the electronics. I downloaded the Up software and connected the printer to my laptop. The first problem came when homing the motors. The X and Z axis worked fine but the Y axis whould crash into the front after homing to the rear. It seemed that the Y stepper driver IC was damaged and was stepping a different distance when moving one way than the other. That was very strange but not worth looking into after finding out about the thermistor problem.
The E3D V6 comes with a very common 100k ohm thermistor. A 100k thermistor is used on just about all 3D printers everywhere, except for the Up. The Up uses a much more uncommon 100 ohm thermistor. The different thermistors require different pull up resistors on the control electronics for them to work. Because of this the Up software would report a hotend temperature of over 300 degrees and refuse to run. To fix this I would need to replace the pull up resistor and then work on the Y stepper driver fault. Or replace the control board with my own. So I pulled the RAMPS 1.4 board out of my old printer to replace the Ups damaged controller.
The next step was to completly re-wire the printer and setup the firmware from scratch.
After struggling to get auto bed levelling working on my Prusa Mendel I have fixed the problem.
This is the latest in the never ending series of repairs and upgrades to my Prusa Mendel 3D printer. The previous upgrade was to add auto bed leveling after replacing the X carriage to mount the new hot end. The auto bed leveling is Z probe “micro switch on the end of a servo” type. The problem was that it never seemed to work.
When I say it didn’t work I mean that it would successfully probe the bed and calculated the plane of the bed and then adjust the Z height during X and Y movements but the calculated plane did not match the bed. The calculated plane was always higher on the left rear and lower front right. After the printer had probed the bed I would need to adjust the bed height for the printer to work. No matter how many times I adjusted or measured the Z probe or checked the firmware settings the calculated plane was always off. I kept using the printer and manually adjusted the bed height after probing. But now several months later I had an idea and fixed the problem.
While watching the printer go about its probing yesterday I had a look at the X rails and noticed that the two rails were not parallel but we’re twisted. The motor end was sagging under the weight of the stepper motor and causing the X rails to warp. The warping meant that the X carriage would roll as it traversed its axis. The rolling did not affect the hot end nozzle noticeably but the Z probe is a fair way off centre to the rear of the carriage and this would cause the probe height to vary by several millimetres over the length of the X axis. This variation accounted for the difference in the calculated plane at the Z probe and the plane of the bed at the nozzle.
I immediately printed up a new pair of X ends from Thingiverse and swapped them over. Once I had removed the old motor X End it became very obvious what the problem was. The clamps for the linear bearings had cracked and was allowing the weight of the stepper motor the twist the X End. After installing the new X ends the auto bed leveling worked perfectly the first time.