The basic plastic printerbot frequently produces bands on the sides of the prints which match the z-threaded rod pitch. This is most obvious on prints using .2mm or thinner slices.
In order to find the source of the problem I made custom parts to lock the bearings and rods in place. I also made an external bracing.
The source of the banding appears to be a rocking in the z-motors.
This causes uneven z-lift which in turn causes some layers to be squashed and buldge on the sides.
This is easiely fixed with a few wedges but the other parts may be of some use for people wanting a more rigid bot or to re-purpose for something else.
Also the z-motors are .5mm too far out on my bot. This is an error in the orginal CAD files.
I have a corrected but untest version here. http://www.thingiverse.com/thing:26542
I will be publishing more modified parts when I find time.
I have done the CAD for a modified base with extra motor support but this is overkill for most people.
You can print these wedges if you like but you might want to make them out of something else - I used clothes-peg parts originaly.
As stated earlier - you probably don't need this frame to get good prints.
My bot lives in a bamboo serving tray so my bottom bracket is designed to mate with the corners of the tray. It could be modified to work with a sheet of ply or something else.
I use the same sort of 5/16 threaded rod as is used in the PB kit.
The smooth rod joiner and the V joiner at at the top mate using a tapered fitting. It was intended that the two nut-knobs could be unscrewed without changing the other adjustments and the bot could easily be removed. Mines are too tight to come apart easily and some sanding is needed. It is possible my taper is too shallow.
(PS it is sort of working).
I have longer z-smooth rods but I'm not using the extra travel ATM. I may have to modify the design to get full height as the e-motor may (or may not) conflict.
If I want full height and width I will need to increase the length of the top joiner rod and remove the front part of the spool holder A-frame.