Well, you can wiggle it by force, but my feeling is that the adjustment screws put enough tension on the springs.
I had this doubt as well, especially as the heated bed introduces a bit more unstability. But I clamped and taped it firmly to the platform. I´m almost sure that this is not the reason, as I was printing in the same setup without the issue.
It sure looks like something related to the Z screw - maybe the brass nut is too tight and doesn't have enough room to wiggle side to side as necessary during printing.
I get results sort of like this now if I try to print at the volume limit of the nozzle. What is your layer height, speed, and temp?
This looks familiar! It looks like bed issues. Most likely height or Z issues but it could be so many things.
Usually Z screw issues repeat every 3mm (one rotation) but not always. Did you put the green grease on it?
I bet it looks better with .2mm layers and I bet .1mm layers looks much worse. But I could be wrong on this.
So some ideas:
1) Play with moving the bed up and down - a lot! do it by hand (turn off steppers) and spin the Z axis up and down many times. See if you see the bed get stuck and then slip suddenly. See if things get tight near the bottom of travel (bottom 2 inches) which can be caused by bed not assembled quite right. Z nut (that huge nut in the bed) should be able to slide slightly from side to side, but maybe it's better to glue it in place.
2) Look at where the Z screw is connected into the coupler at the bottom. It should be all the way. Is it possible that it's loose or not all the way into the coupler? You shouldn't be able to see where the z screw gets thinner at the bottom.
3) is the bed hitting anything? Wires? steppers? Is there anything that might make the bed tilt, or get stuck slightly, or reduce it's speed.
4) Are the 4 screws agreeing? If you push on the plexiglass corners, does it wobble like a chair with one short leg? Is one screw not being used?
5) Move the z axis with the stepper. With Cura or Pronterface (not with ulticontroller!). Move 100mm at a time. Anything? Could your acceleration be set too high? Did you mess with it? The default with cura is 100mm/sec^2 you can check with the ulticontroller or if you don't have one use pronterface to query the Z acceleration. Mine is set to 200mm/sec^2. You can go quite a bit higher but 1000 or 2000 mm/sec^2 acceleration can cause this pattern due to skipped steps.
Put a ruler or something taped or clipped to the side of the UM near the bed. Put a pencil or flat ruler or something on the bed so it just touches the ruler. Move by 10mm or 1mm and look carefully. Very carefully at how much it moves exactly.
Another idea: maybe the z-motor is loose and this leads to z-wobble. You mentioned "...after several months" so it was better before. Maybe due to wear the screws of the z-motor sunk into the wood on one side and thus the whole contraption is inclined to one side.
Dear Makers!
Thanks a lot for the good input and the new ideas. Today I got a bit of freetime and will dismount and reassemble the Z axis and see if any improvement can be reached. I´ll keep you up to date, and try to track down the problem and the possible solution.
Ok, after reassembling the whole Z axis the following changes were done:
- Making some space for the brass nut of the Z screw to enable it to move.
- Checking the stepper placement and the rods and the whole table for some problems and reassembled everything properly.
- Additionally we rebuilt the heated bed to make the layers (heat insulation, heater PCB, glass plate) fit tight together, to prevent movement from there.
And finally we decided to update the firmware as it´s almost a year old now and meanwhile checked the maximum acceleration of the Z axis which is now set to 200 mm/sec^2
The funny thing is, it did not change anything in the printing results, the random wobble is still there and did not even changed. (Well that on the other side means that our Z axis was pretty well built and in order 
)
Afterwards, in desperation I played around with cura settings and tried to change flow rate and following the advise of gr5 tried different layer heights. It´s rather strange but the random wobble effect seems to disappear at a layer height of 0.24, as seen on the attached picture. (Layer height of 2.4 vs. 0.1)
It´s to late for me now to play around more with the settings, but I wonder if it something like perimeter that are to close together and build up to a specific height where they kind of collapse and produce these kind of effects on thin layers.
I would pretty much like to print in thin layers without the effect and will probably have to find the right settings in Flow, Wall Thickness + Layer Height.
Does that make sense to you?
The thicker layers is just nicely masking the problem. I think.
The problem is with z movement. You tell it to move .1 mm and sometimes it moves less than that and you get overextrusion and fatter walls, and sometimes it moves more than that and you get underextrusion and thinner walls. And this pattern alternates in a semi random pattern.
I still think it is Z movement related - I just don't know how to prove it. I guess you need to put a piece of paper taped to the side of your UM and put a mechanical pencil on the bed. Move 1mm at a time and make a mark each time. You should see the same/similar pattern with some marks closer together and some marks farther apart.
Watch the Z nut very carefully as you move .1mm at a time.
Did you grease the Z screw?
It´s funny, after testing almost everything, including a new nozzle, it came out, the tension of the belts was not proper.
After a year of printing they have lost tension, but it felt still stiff. I probably did not notice the difference as the change was so slow.
Anyway, after including these nice belt tensioner http://www.thingiverse.com/thing:19892 the print´s looks as awesome as the beginning.
Now I´ve a spare nozzle, new cooling fans, a rebuilt heatbed and probably one month of mad while searching for solutions
Thanks for all the good help anyway!
well here I am, reporting of a Z wobble problem too.
after months (18) of use, my UM started to print with a significant wobble on the Z axis.
Read the forum, analyzed the hardware, scream like mad and tried several things:
1: free XY play for the brass nut. some file work on the wooden parts and the Z nut can move freely in ther XY domain (weird.. when I built the UM the z nut was blocked but the printing was really good with no wobble)
2: printed a add-on to guide the z thread and eliminate axis wobble, worked quite good (i will place it on thingiverse ASAP) but not resolved
3: quit CURA and get back to netfabb. NO WOBBLE at all. Now that's the problem...
It's hard to me to believe that it's CURA that helps the wobble, but the same test object (a 10mmx15mmx25mm cube) shows no signs of wobble when sliced with netfabb and printed with printrun but shows wobble (every 3-4mm ) when sliced with cura ((13.6.3 or 13.11) and printed with printrun.
still not tried anything on belts but I will ( I love cura and hate netfabb... so messy...)
any idea?
in the meantime my extruder seems to starts failing and ordered a new v2 extruder head...
That´s interesting and I have a theory on that, which relates to the missing belt tension.
I have watched Cura to produce printing patterns that are not following always a similar movement pattern. For example some layers will be printed clockwise and then counterclockwise. This is quite nice for path optimiziation, regarding ooze and other issues. But if your belts are loose, the printing head will randomly get a little offset, depending on acceleration and movement.
Other slicers will make super identical moves on every layer, which prevents this issue.
I´m gonna try another slicer the next days and see what happens.
Thank´s for the input!
Netfabb also (if you select random start in a submenu) can start every layer in a different spot. (the problem with netfabb is still the retraction than still isn't working good).
On my last print yesterday night, the test cube was quite good, no more 3mm wobble but a lot of ooze from every layer starting point.
So, even starting layers in different spots (the way you told cura works) netfabb seems to produce a cleaner print.
I do not understand. really.
I have a theory! I think Cura is shaking your machine harder. It may have found a resonant frequency. If you put some bricks under your machine or move your machine to a different (lighter or heavier) table, or more likely clamp a heavy weight to your print bed or if you change your XY acceleration settings (Cut them in half) Cura might be okay.
Resonant frequency would be when the head moves back and forth at the same speed as your entire machine shakes (or just the platform). I would use strong paper clips and clip something to the wood of your print bed to change the resonant frequency to a lower frequency.
I'd like to see if your theory is valid. I am too thinking about where to place the UM2 when it arrives. I will add some cushioned feet for sure, regardless of the position.
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Are you able to wiggle the bed due to adjusting screws being too loose?
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