I'm back!
My apologies for the long silence. I have been busy with many things.
First of al, let me correct something.
chumm wrote: ↑Mon Aug 28, 2017 10:37 pm
Bent lathe or not, creating rigidity between the motor shaft and the leadscrew will predictably lead to more wobbling, it's the whole reason we use flexible couplers in the first place. Unless you can ensure that your leadscrew is perfectly straight and every component with which it interacts is perfectly lined up (which has probably never happened on a 3d printer), trying to center your shaft and screw will amplify every imperfection the screw encounters.
This is not true. I will explain you why.
Here is a picture of the normal situation. The problem is friction.

Lets assume my print bed ways 3kg and the coefficient of friction between the motor axle and spindle is 0.2. The force necessary to move the spindle on the motor axis in this situation will be 0.6kg!

This force will eventually be generated by the print bed when the print bed comes really close to the coupler. This will create one heck of a lot Z wobble issues! Remember that my flexible coupling actually creates a miss alignment itself by means of bad tolerances on the holes.
This problem can been solved by removing the misalignment of the spindle by means the solution mentioned above. Can this solution create alignment issues? Yes it could. But it will be less due to the better alignment. You definitely don't want the XY force of 0.6kg!
By now you can probably guess what I have been up to all this time. I found another lathe and I successfully implemented my idea. Did it work? Yes and no. I'm still confident that this solution works but it is not visible on the print. The quality is still unchanged. Probably mainly because I'm printing small objects which are far away from the coupler. The influence of the coupler modification will be minimal on those objects.
I have been busy printing several test prints to figure out what the reason for those line are. This is what I tested.
- Better coupler mount (as mentioned above) --> no improvement
- Reduced Z axis acceleration from 600 to 100mm/sec2 --> no improvement
- Extrusion multiplier 8% lower (crazy test

) --> Lines are better hidden but still there. No improvement
- Jerk Z axis from 0.4 to 0 mm/sec --> no improvement
- Print rotated 45 degrees (square print is no longer aligned with X and Y axis) --> no improvement
- Different brand of PLA filament --> no improvement
No matter what I try the lines remain there. I'm running out of ideas.
Edit:
Could coupling stiffness be an issue? I will investigate this

Any other ideas?