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I'm noticing that only in certain directions, there are gaps between concentric perimeter loops. i.e. lines along the Y-axis. Photos: http://imgur.com/a/pRT6o

Other print defects:
- nut pockets are not properly shaped hexagons. They are longer in some directions than others.
- layer alignment is off near beginning, then evens out later

E3D Volcano, 0.8mm nozzle, 0.6mm layer height.
Sliced with Slic3r at 1.2mm extrusion width, 30 mm/s, 3 loops, external perimeters first.
PLA @ 220 C, (bed at 42 C)

Any idea on how to remedy these problems?

All u-joints lubed with Tri-Flow (PTFE-based dry lube). Belts are tightened pretty much as far as esbowman's belt tensioners will allow. In fact, there seem to be belt grinding noises and some black crumbs, but better tight than allowing slop, right?
http://youtu.be/QyA0mZ66-Rc

I noticed a fair amount of slop in the effector platform. Installing rubber bands didn't seem to really change that.
https://youtu.be/ML-ELfqjaiM

Belts that are too tight can be just as bad or worse than belts that are too loose.

g.

Thanks Gene, in the video I linked above I show the amount of tension on each belt by pulling outwards on it. Does that look too tight?

How much do other users of the "Easily adjustable belt tensioners" tighten down the screws?

I realized that the belt noise is due to the Astrosyn dampers allowing the motor to slightly rotate because of the belt tension, so the motor axle and pulley are no longer perpendicular to the belt. When I push on the stepper motor with my hand while moving the carriages, there's no grinding. I guess I'll try reducing the tension; unfortunately the noise reduction from the dampers is quite important for me so removing them isn't really an option.

I've got dampers on White MAX and they work great. What you might want to do is lower the top bearing to the bottom of the slot and then pull the belt tight at the cheapskate. Then lift the bearing up by about 1/16-1/8". That should result in good tension.

g.