RepRap Z-Wobble Woes

While I am happy with almost all aspects of my Mendel90 that I built, I have never really been happy with one aspect of the print quality. That aspect of print quality is z-wobble. Or so I thought it was, as I have so far found out, it must be a much more difficult to remedy Z-artifact problem.

This is how prints looked when I first made my printer:

Very typical Z-Wobble - it has the same pitch as the leadscrews of the Z axis and is easily fixed in theory by removing a bit of constraint. However, even at this point in time, the Z-artifacts were inconsistent - they could be horrible one print and then hard to spot on the next.

The first thing I tried was to install a new type of nut trap (pictured below) which transferred virtually no movement except for movement in the Z direction - a desirable thing. Along with this initial modification I then drilled out the holes that the Z rods pass through to give them a bit more room to turn without knocking about, pressing against the X ends and to make up for inevitable misalignments. But if you looked closely there were still artifacts, and now they were looking different and more irregular, I guess the Z-wobble, which was now reduced by a great deal, had been concealing these other artifacts.

I then tried re-seating the rods in the old couplings and not over-tightening them in an attempt to align them more accurately and make sure they couldn't exert too much force in the X and Y directions, but this still didn't grant results I was happy with. I eventually moved to another coupling method - using my lathe to turn down the ends of the rods to 5mm so a flexible section of tubing could be used as a coupling. Alternatively I could have just used 5mm threaded rod.

The coupling system installed. I may have to replace
the cable-ties with hose clamps because the tube
softens up a bit due to heat - no problems so
far though.

The turned down end, I was very impressed given that
it was done using my first home-made/ground HSS
lathe tool. Cutting stainless-steel too!
In the background is half of an old coupling, a nut trap
and also a badly heat-deformed (from the motors) piece of
tubing that went inside the old coupling.

After all these mods, done over much time, I got this print quality on an X Motor End for a Mendel90 - unsatisfactory to say the least:

However, I was now certain that the Z-wobble was gone and that this was something else. This is due to the fact that the misalignment does not happen at regular intervals and does not look like a sinusoidal wave of  X-Y layer displacement. Instead it happens very randomly and can go from the displacement varying every layer to happening every 10. The displacement is also very slight - about 0.1mm most of the time, however under certain lighting and angles, and with my highly-opaque white plastic, I can either hide it completely or make it look ugly. I suspect that if I put my plastic into an UP! or a Makerbot Replicator, the prints would come out with a bunch of highly-visible ugly artifacts like this too, but the photos of their parts sure do look nice. Having actually seen UP! prints I can say that the slight transparency that their white plastic has hides a lot, for it is very, very hard to distinguish layers by eye even at large layer heights. Holding hard, 90 degree edges up to light gives away a lot of small, but visible layer alignment issues on the UP! too though.

So, still looking for answers, I thought it could be backlash or skipped micro-steps as per Nophead's excellent article on micro-stepping. So I tightened up my belts and changed a few things with how my StepSticks were set up. I didn't read too in-depth into his article to know how it all related to my motors as they are low-current and high-resistance and I didn't really bother with the maths. So I didn't calculate the off times to get the PWM right for small-current portions of the micro-step cycle given current decay in the windings, let alone how to set this on the driver. But reading through the data sheets and his post, I realised that if I set my current too high with the potentiometer, say 0.4A (hey, I thought having a bit of juice available to them was a good thing) when my motors only draw 0.3A per winding at the drive voltage, the motors would have very bad and uneven micro-stepping. This is because the potentiometer does not directly limit the current to the motor by being in the path of current travelling into the winding, it is instead a user-set way of telling the driver at what current the 100% current level should be. This 100% current level is when all the current is in one winding and the other is completely off - a full step. If you set it above the current a motor can actually draw due to its resistance, the motor may actually reach full current in one winding while the driver is on the 13th out of 16 micro-steps or whatever, causing some really dodgy stepping.

I was in fact able to see and hear this, the motor would, at very slow speeds, change its sound and start and stop when motion should have been continuous. After setting the current properly, movement was smooth and the tone of the motor didn't change nearly as much. I might still only be getting effectively 1/8 or 1/4 micro-stepping for all that I know though, but at least it's smooth. That is something to look into another time. Here's the resulting print, it's still got all the same problems so it looks like the micro-stepping or belt backlash wasn't the big culprit:

Ah well, after all that effort investigating the motors I at least have gained some knowledge on stepper motors and hopefully eliminated that visible artifact on shallow curves and low-angle X-Y lines where it looks like the line is uneven with little bumps due to jerky movement.

There are a few more things to test including an unevenly hobbed bolt and badly-meshing gears on the extruder, my bed changing height as it expands and contracts with temperature swings and also the filament pulling the carriage around a tiny bit - for which I have ordered a PTFE guide tube which will arrive soon for me to test, assuming free shipping from China on eBay is quick.

The RepRap magazine has an article on Z-artifacts if you are having trouble with them like I am, and I hope that you have gained something from this post.