This is what happens when you’re trying to design a part to fit something that you don’t have a good model of. These are all the prototypes of the fan duct I made for my printrbot jrv2, starting on the right with the one where I hadn’t learned how to use Openscad’s offset function, and ending on the left with the one where I forgot that extruding something for a different distance changes the angles of the sides.
The final version (!) fits pretty well, and clears both the heated bed and the protruding heated-bed adjustment nut about 1cm outboard of the extruder’s default home position. It blows enough air that those little wisps of filament from transport moves trail out on the downwind side of the nozzle, and there seems to be a noticeable increase in print quality for standard PLA. (Albeit I will need to print more than a marvin or three before the verdict is in.) Files at Youmagine for anyone with a jrv2 or a yen to hack bad openscad code for some other machine.
An interesting thing I discovered during the prototyping process: the printhead isn’t quite vertically aligned. Probably because of all of the weight of the stepper motor on one side of the cantilever, there are a few degrees of twist to the head (which means that a duct that’s flat with respect to the head will bottom out on the bed slightly earlier on one side than the other). As long as it’s consistent, I can’t see it making any difference in the final prints, but still something I hadn’t expected.