printrbot and adjusted so that I can print parts reliably I decided to start making some changes (partially to fix some problems/limitations and partially to learn how to get sketchup to work best for designing parts for some other projects).
On the printrbot the carriage for the x-axis uses two parallel linear bearings and binds the drive belt from the back of the machine to the extrusion mount. This is the test fit for my prototype extruder mount and the carriage.
What annoys me most is that the attachment point for the belts is too narrow and effectively acts as a point connection for the belts -> it acts as the center point around which force applied (in this case because of changes in velocity of the entire print assembly) will rotate around. Couple of ways to sole this -> attach the ends of the belt farther apart so the effective circle of rotation is much much larger -> add a limiter to the amount of rotation possible (longer or multiple bearings). I decided to do both.
I tried to directly modify the original sketchups of the parts but that didn't go well so I did it over from scratch keeping the horizontal alignment slot concept, the screw locations and the separation distance for the bearings. I didn't like the bearing compression idea where tightening the center screw adds pressure to the bearings so I didn't include that. Other parameters I changed a lot to increase the rigidity and strength of the carriage.
The extrusion mount has a wider attachment face for the the carriage and provides surfaces for attaching devices other than the extruder. It ought to attach to the standard printrbot carriage (but with less surface area than the modified carriage).
Misumi hardend rods and bearings. One of the companies that never disappoints - really great quality parts.
If you are interested I've put the parts here:modified x-carriage, modified extruder mount, extruder cooling duct. You may want to adjust the diameter of the bearing holes for the exact type you have on hand (and of course your preferred printing/slicing parameters). I prefer a nice press-fit rather than adjustable compression since you ought not to need very much force to keep the bearings aligned and working correctly if all the other pieces are properly configured.