Thanks to @Jason_Gullickson for this excellent example of why you should NEVER use SAE threaded rods for your Z axis. Your layer height needs to be a multiple of your full-step distance, or you’ll get artifacts like this in your prints (which are easy to mistake for Z wobble). To choose an acceptable layer height for the screws you’re using, take the pitch of your screws (I recommend M6, with a pitch of 1mm) and divide by the number of full-steps per rotation on your motors (usually 200). Microsteps are not reliably accurate enough, so ignore them for this calculation (though using microstepping will still make them smoother and quieter). For my recommended M6 screws, this comes out to 5 microns. It’s 4 microns for the M5 screws used by the i3, and 6.25 microns for the M8 rods used by most other repraps. For the 10-24 screws used in this example, the pitch is 1/24 inch (1.05833333333mm), so all layer heights must be multiples of .529166666667 microns, and the repeating decimals tell you that you are really screwed (so to speak). In this case, you can get rid of the repeating decimal by multiplying by 3 to get 15.875 microns, which is not nearly as pretty as the 5 micron figure, but will let you get somewhere. If you wanted a .2mm layer height for example, with this screw, you would instead have to use 0.206375mm (39 full steps per layer), or you would see the effect demonstrated here.
Originally shared by Jason Gullickson
Solved the circular overhang problem (cooling was the answer) but even my latest changes (http://www.thingiverse.com/thing:34140) to the z-screws haven’t solved the ribbing problem. To some degree the improvement is simular to the previous change (http://www.thingiverse.com/thing:20147) I made which makes me think perhaps only part of the problem is z-wobble and I should be looking elsewhere to find the rest of the issue.
Regardless, I’m planning to try some proper leadscrews next to further reduce wobble in the Z.