Until recently, I was struggling with printing in ABS. Some advice from the forum sorted me out and I was able to produce acceptable prints more consistently. But the quality of the printed parts in my Mendel90 kit, and many of the photos of prints posted here on the Mendel90 forum, were still much better than what I have produced. Bridging was hit-and-miss (mostly miss) and the first six or seven layer of my prints always seemed to stick out, giving every print a little lip at the bottom that tapered away after half-a-dozen layers. So my focus shifted to calibration, accurate bed leveling and a proper Z_HOME_POS setting.
I quickly noticed that my Z_HOME_POS needed to be set often. Like every couple of prints. It would need to be adjusted as much as 0.3mm either way after a print. Searching for loose fasteners that could affect the height of the bed turned up nothing. Everything looked fine on the Z-axis as well. Then I noticed that there was plastic in the threads at the bottom of the left lead screw. It looked sort of like a hobbed bolt that needed cleaning. I poked and prodded, and eventually held the coupler while turning the lead screw. It was as if the top of the coupler was a clearance hole. The lead screw spun very close to freely, with virtually no friction. I honestly don't know how it worked at all. My guess is that, a year-or-so ago, I had thought that the coupler was loose on the motor side, and tightened the bottom bolts, freeing up the lead screw side to move a small amount. Over time, this movement ground away the plastic in the coupler until the problem became bad enough that I noticed.
Funnily enough, I was preparing to replace the couplers anyway, and had been printing off couplers while trying to track down the problem.
After replacing the coupler, my prints are finally getting close to the quality I imagined when committing to the Mendel90 kit. I just about have settings in Skeinforge and Slic3r for acceptable prints at a layer height of 0.2mm, and think that 0.1mm might be possible as well.
