Solved New problem: Z-axis sometimes not raising

Ok, I've got a new problem now...

When I'm making a print, sometimes it works fine, but other times the Z-axis stops rising up at new steps and the printer starts mashing plastic over and over again on the same layer. This started happening after I replaced the hot end fan all the way to the RAMPS board. It's very obvious when it starts happening, because you can hear the extruder dragging across the infill hatches and makes a terrible sound and you can see the carriage bouncing across.

The motors are getting signals to move, because they keep moving around while printing from the auto level. Normally, I can see the Z-axis rods slowly moving around as it prints a layer, then see a quick jerk to the new level. When this happens, there's never a jerk to the next layer.

It doesn't happen on every print, only about 20% of them. I think it might try recovering itself, because I saw it happening on a calibration cube, yet the cube was about the right size. It looked relatively good, but it was fragile and pulled apart very easily.

I've tried the two "latest" firmware files in case it was something caused by this, and also completely uninstalled and reinstalled MatterControl in case it was caused my one of my many slicer tweaks.

It doesn't seem to happen on low quality, only on medium or high. It happens when using MatterSlicer or Slic3r.

 

I'll try to print a high quality print from the SD card and see if it might be the computer.

 

Well your description of the issue and the print results don't really coincide.

There's a number of factors that might show your results.

The most common reason I've seen for the nozzle dragging over previous layers, especially after around 3mm, is because the wrong firmware was flashed. If you're running M8 rods then you have a step/mm of 2560. For 5/16 rods you have a step/mm of 2267.72. That means if you have M8 rods on your printer and are running a 5/16" firmware you won't move enough per step and you'll drag over filament.

This will result in a print to be shorter than expected however. Since yours did not come out at the wrong height it's probably not this. You will also get dense and squished layers and/or jamming

The second could be caused by over extrusion. If a print puts down too much material per layer it will continue to stack up until it starts to bulge up and the nozzle may drag over. This seems unlikely as it wouldn't create brittle prints but would rather be bulgy and strong.

The third is underextrusion. This will create brittle and weak parts while still being the right size. Though generally you don't see layers dragging over each other, but it's possible. Your print description makes me think this is more likely.

The fourth would be torque stall. This is caused by excessive stiction in the movement of the threaded rod over the nut. This could cause layer movement if it tries to move too fast (or slow) to be prevented. Lubricating the rods can fix this. This and the combination of underextrusion could be the cause, though it would result in shorter than expected print.

The fifth reason I can think of would be stepper overheating. Since you mentioned replacing the ramps fan this was my first instinct, as I've experienced it. If you're running too many amps on the stepper driver it has the potential to overheat. When this happens it basically shuts off for a moment and won't perform movements. This seems unlikely after the description you gave because it wouldn't stall any other motors nor cause under extrusion. It would also results in parts being too short.

 

Yeah, I realize that this is a strange problem. The intermittent behavior makes it even more difficult to track down.

I checked the firmware a couple of times, since it was my first instinct as I'd recently updated it. I checked and double-checked, and I've definitely got the correct firmware settings for my rods. I flashed it with both the file that MatterControl says is current and also the one from the Robo3D site. On both, the z steps are correct at 2560.

I've replaced the 20mm fan on the extruder head, but not the RAMPS fan. I did, however, disconnect that fan when I removed the bottom plate. It's concievable that I might have plugged it in backwards when I reassembled it since the polarity isn't indicated on the PCB. I'll pull the bottom off again tomorrow and make sure that the fan is blowing on the board and not out the bottom.

I've checked my extrusion using the 100mm test, and it seemed to be pretty much spot-on. The plastic is extruded from the nozzle in a nice, steady stream straight down, so I don't think extrusion is the problem, either.

When it is happening, and the extruder is dragging against the previous infill layers, you can see the carriage bouncing up and down as it's being dragged across, so much so that it almost goes far enough to trip the auto level switches. It is definitely not moving up for each layer when it gets in this mode, though the motors do run to keep up with the auto leveling. Since plastic is still being deposited, the piece is still growing, but I can't tell how much. I'm also printing with black filament, so it's difficult to make out much detail while its building.

There's no trouble at all when raising or lowering the Z-axis using the interface, but I haven't measured if it raises in the correct 10mm steps. For all I know, it might raise 9mm once in a while...

I could see it being torque stall. When it happens, it usually seems to be in a rather consistent distance from the bed, so there may be more friction at certain angles. My threaded rods are not perfectly straight, and I've minimized my Z wobble by turning the rods so that the wobble is cancelled out for the most part by making the bends point in opposite directions. I'm going to get some new, straight M8 rods tomorrow from a local screw shop and I'll try to get my Z axis taken care of before I try anything else.

 

Before going out and buying M8 rods try and lubricate the ones you have. That could be the cause.

It might also help to enable lift on movements so your nozzle only passes over print areas when printing.

 

I took the original rods off, and they were straighter than I'd assumed based on the crazy wobbling that they had. I added a ball bearing to each coupler, cleaned the original rods, got everything straight as I could, reassembled and lubricated them. They seem better now, so I'll try some prints and see what happens.

I also checked my RAMPS fan - it was unplugged after all. I just hooked everything back up and I'll see if it's behaving better now.