Problem with nozzle dragging/z-stepping

Hi there, got a MendelMax handed over to me as part of my 3rd year university project and it's been an absolute nightmare for me to get on with.

There was a Chinese guy who's now out of contact that worked on the printer before me, but when I was given the project, I knew absolutely nothing about reprap. He'd got the printer to work fairly well, with consistent prints, and had started designing a cooling fan duct, but that's where it was left at. I downloaded the Marlin software as I didn't know he'd been using Sprinter primarily, and got to know how to get the printer running. I've calibrated it and the movement and extruder rates are all near enough bang on, using the calibration calculations. However. When printing a small, simple wine glass which was on the computer I'd been given, the nozzle tends to end up dragging in the previous layers and ends up making it become a blob, and ultimately leads to poor prints.

For a while I've had my supervisor bearing down on me and telling me that it's obviously laying down too much filament. So I changed the extrusion multiplier several times, right down to 0.8, but was still encountering the problem. I lowered the movement rates of the z axis, and also the acceleration, but the problem persisted. Then just before leaving university to come home for Christmas, I noticed that sometimes when switching onto the next layer in the build, the z-rods don't turn. The problem seemed to be completely sporadic, not necessarily the same each time I did a print. I'm now at home, so can't really try anything, but I'm absolutely way behind on my project as I need the darn printer to work, and swamped with a lot of other university work.

Please, if anyone could offer any advice on making the printer work that'd be amazing. I'm really lost on this thing, it's a massive shame I'm finding it incredibly infuriating and disheartening to work with, as several things just keep going wrong and I'm going nowhere!

Oh, I'm using PLA, with a heated bed, using PVA on the glass. Quite often I've got not too much of a problem with the adhesion, but it's not always consistent (also on my long list of problems I've had!)

It sounds like you may not have the Z axis current set high enough, or too high. On layer changes can you hear the motors trying to turn? If you're using the plug in stepper drivers (e.g. Pololu) then there's a potentiometer that you can use to adjust the current. If you set the current too high this may also give you the same symptoms since the stepper driver will overheat and shutdown until it's cool enough again.

Edited 3 time(s). Last edit at 12/26/2013 12:48PM by samp20.

You can see that the z-rods turn very very slightly on some layer changes (and feel them slightly jerk if you lightly touch the coupling), though I haven't noticed it making any sounds in particular, but I may have just missed them. I've got RAMPS 1.3 and Pololu stepper drivers

The main problem is that the chap before me had the printer working to a fairly good standard, and in my majorly trial and error, reverse engineering and highly problematic approach (Glass bed smashed when endstop wasn't plugged in when I first got given the printer, loose wire of nozzle thermistor shorted and burned the whole length of the wire at one point, thermistor was split in half, nozzle melted out of PEEK holder), I've gotten so incredibly lost on this thing! It's been an absolute nightmare

Also my supervisor is an absolute terror so hasn't been very supportive of how much of a struggle I've been having and just keeps telling me I'm a pathetic excuse of an engineer...brilliant! I'm just completely swamped!