Banding; software or hardware

It is mechanical and there is no doubt software mis-configurations such as speed (user, sw bug, or system bug initiated) can amplify the issue.  I have not seen a single sd2 printed part posted anywhere, I see bands on everyone's prints to some degree and have yet seen a print free of banding; you may need to change the angle that you look at the print, even the printed parts used to build the printer are banded. I will test using ultra-slow external perimeters, this should not be needed...I never noticed a a difference in my quality when using my .3, .2., .1 settings at different reasonable speeds.

The best .stl (http://www.thingiverse.com/download:17273) imo and the one I use to test is the simple .5mm single-walled cube; I multiply the height in RH to 6x or so. Your level of banding will show up and the print will be thin enough to be opaque allowing you to shine light through which really enhances the ability to see the issue.

That looked somewhat better than one of my best efforts. I have not seen this print before today.  It makes me want to remove my rod and use a Upvc or (printed+suguru) coupling.  Kendall also has a coupling and his prints look very banded; mine is just glued on a bit crooked.  What about looking at the object upside down or from the sides?

Someone said it is easy to get the screw off the motor shaft, it was not in my case...so I wouldn't go for their threaded rods;  I would buy 1 good LS and try a soft/flexible coupling.  I need another motor (ordering now) or I would have already done this months ago.  (as you can see I don't have terrible issues either)

Removing the lead screw takes about 15 minutes... most of that for dis-assembly.  I had to use 3 jam nuts (5/16-18's) to get it off the motor.  Screw two to the bottom, grab a couple of pliers (It took me 2 as I had to hold the screw with one and the nuts with the other), then rotate to apply pressure to the motor.  After a couple of turns, both nuts were spinning freely, but the shaft was still not free.  I added another nut and did some more turning and the shaft literally popped off.  Then I just had to superglue the new screw onto the motor shaft and wait.  Re-assembly took another 5 minutes.  Total time was about 30 minutes to remove, replace, re-assemble.

I noticed immediate improvement in the quality of the prints.  Minimal banding at best and I managed to reduce that even further by slowing down the perimeters.

As for 3, if you can afford it, sure, but you should only need the one, unless you were sent another bent one which shouldn't be happening.  If you really want to test, get one new screw and a coupling.  Test the old screw with the coupling, then the new screw.  Would be a full afternoons work, but you'd get your answer.

Edit:
Adding a pic of before/after replacement.  Same STL file used for both prints, .3 mm layer height.

As you can see, the left one shows considerably less banding than the one on the right.  This is the degree of bend that was in the lead screw that I replaced.  I get some pretty respectable prints now and I'm working on getting the .1 mm layer height configs to work.

Just use some explosives of course.

I have been thinking about mounting a laser pointer to the bed. To measure the X/Y wobble while moving the Z-Axis up and down.

With a few meters to the wall, with some blue tacked paper to take measurements. It should really amplify any wobble, and allow you to try and remove it it without having to waste time on constant prints.

good work man keep it up