Z-axis not moving up accurately when printing

I was having problems with the print becoming “squished”. It resulted in a part (say) 10mm high ending up (say) 8mm high with one or more layers being chewed up by the nozzle because the Z-axis wasn’t moving up the correct distance between layers. The fault was intermittent and seemed to occur about 40% of the time I ran the printer. After trying all sorts of software fixes (none of which worked), greasing the threaded Z-axis rods (lithium grease) and cleaning the smooth rods with IPA, the problem remained. However, I think I have finally found the problem. I carefully removed the plugs that go into the base of the two Z-axis stepping motors and cleaned them (these white plugs simply pull out). I then repeated the cleaning operation on the black z-axis plugs that go onto the Rambo control board. (You’ll have to open up the case).Fortunately the cables are labeled. (Note to remove the two black Z-axis plugs on the Rambo board you have to depress a tab under the plug (you can’t see it easily, but you can feel it). Don’t just keep pulling at the plug, and certainly do NOT pull the wires entering the plug). Work slowly and carefully. Cleaning method: Add a few drops of 91% IPA to the pins in the socket, and a few drops into the holes in the plug. Wait 10 - 20 seconds and blow away any remaining liquid with compressed air. Repeat. Carefully replace the plug in the correct socket. Be gentle and don’t force the plug or you might bend a pin which could mean a new Rambo board! The plugs go in very easily.
So far this seems to have resolved the problem.
Here is a" before and after" photo of part of a key holder:

At first I thought this “Squishing problem” was related to the way the printing head retracted after the print had finished (in multiple vertical steps, rather than one continuous action), but it doesn’t seem to be. For more details on this see my posting of about a week ago.

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Having a similar issue on the Workhorse :frowning:

Let me know if cleaning the Z-axis connectors on the Rambo board and at the motors’ end works for you. So far it has worked for me. I live in downtown Cincinnati, sandwiched between two freeways, so the air is pretty polluted and with the fan pulling in air to keep the circuitry cool, I figure it pulls in enough oily pollutants to compromise the connections between plug and board. Thus, the Z-axis misses a few pulses each time, and the missed pulses accumulate, which is why the problem got worse as the height of the print increased. The plugs and pins are, incidentally, budget priced nickel plated, not gold plated, the former being more prone to poor connection. My Mini is about 2 years old and the only modification is the addition of the GLCD. I was using PLA as the print medium.

Interesting info. Thanks.
What is the GLCD mod that you mention?
Also where I live IPA refers to beer but I doubt it would be the best thing to clean contacts with :slight_smile:

If you compare the Mini 1 with the newer Mini 2, you will see that the Mini 2 has a small black box fixed to the top right of the frame. This is the GLCD (Graphical LCD), and is available as a bolt-on to the Mini 1 model. If you buy the GLCD (available from Lulzbot) you no longer need to connect your Printer to your computer. You can download your design in G-code to an SD card and stick that in the side the the GLCD box. Much more convenient. (The GLCD also allows you to control the printer in other ways too).
IPA is short for Isopropyl alcohol and can normally be bought from any pharmacy as either 70% solution or 91% solution. It is also known as rubbing alcohol. If you drink it, it will probably kill you; and if you clean the contacts in your printer with India Pale Ale (IPA) it will probably kill your printer. Take care you choose the right one! :open_mouth:
By the way, Cleaning both ends of my Z-axis contacts seems to have completely fixed my printer - not a failed print after cleaning. :grinning:

Thanks. I did consider adding the control box to my mini when they were on sale a while back but I had just setup a RaspberryPI with Octopi to do the same thing more or less and for me I think it works better. I hooked up an old webcam to the pi which already had wifi so now I’m able to view what’s going on from any device in my house basically.
The finish on some of my prints are a bit like your ‘before’ shot but since I’d never seen anything better I assumed that to be normal. I purchased my mini used and not working. Had to fix a broken wire going to the X axis micro switch. Maybe I will clean up the contacts and see what happens.

Finally fixed my Z-axis problem.
The problem was that the Z-axis was not indexing up properly once the printer (Mini 1 + GLCD) had been running for long enough to lay down about 10mm vertical height of a print. Furthermore (and I felt it was somehow related) once the print was complete, the hot end would return to home position in a series of stop-go moves, not in the smooth, continuous, mellifluous way that it should have done.
The reason was that the Z-axis stepper motors control chip (Allegro A4982QFN) on the Mini Rambo 1.3a board was overheating. This chip has a thermal shutoff circuit built into it (to protect itself from melt-down), and when the chip is over some preset temperature, it shuts down for a second, or even less, to cool. When it shuts down, the Z-axis fails to index the programmed distance. Thus, the head comes in and smashes into the previous layer of hot polymer. The result is a lousy squashed print that is useless.
The X and Y axes are both driven by single stepper motors, each pulling 1.5 amps through individual stepper motor control chips (A4982QFN). However, the Z-axis (with its twin threaded screws) has two identical motors wired in parallel - thus, the Z-axis motors together pull twice the current (3 amps) from its controller chip (A4982QFN). Unfortunately the A4982QFN controller chip is only rated for 2 amps, so no wonder the Z-axis was faltering. I am assuming the design reasoning was that the single X-axis and Y-axis motors run more or less continuously so pulling 1.5 amps through a control chip rated for 2 amps is fine. And so it is. Although the two Z-axis motors exceed the current rating of the control chip every time they run, they run so infrequently, (for a fraction of a second whenever a new layer is sent for), that with enough cooling, things would be OK. So what when wrong? My cooling fan (Fan 2) in the enclosure wherein lives the Rambo board, had become unplugged from the circuit board. It is only a push fit, and there is no latch on the plug to hold it in place. It had wiggled free from loose fitting and dirty pins. I cleaned the pins with IPA, then very carefully spread them apart slightly (needle nose pliers), and reinserted the Fan 2 plug. Now my Mini is as good as new - it never misses a beat. To ensure that cooling of the controller chips is more than barely enough, I have ordered some tiny 8x8mm heat sinks via eBay, and will glue these to the top of all the controller chip.
IMG_0337 copy
The part on the left is when the Z-axis controller chip stays cool, the one on the right is when it overheats.
The part is a light fixture on a boat. The polymer is Overture PETG.
Finally, now, once the part is finished, the hot-end glides smoothly, without stopping to the home position. Clearly the two faults came from the same source.

Glad to hear that you got to the bottom of this problem.