I have been following this thread for quite a while and have read most of the posts and seen a lot of awesome engineering!
I got my Ultimaker2 back in September and have not started having under extrusion problems until now (I've only gone through 3/4th of a reel of filament (too much school work..)).
After thinking about this problem for quite a while and reviewing the results from other extruder designs I it is clear the problem is more caused by friction in the bowden tube than anything else. Additionally I have noticed that the filament conforms to the shape of the reel that it is on and even when unspooled returns to a tightly coiled shape (which gets more tightly coiled the further down the spool you go).
I believe if we created some type of device that could remove this coiling from the filament (basically just straighten it out like this machine:
) that we would be able to greatly reduce the amount of work the extruder thus (hopefully) solving under extrusion and grinding. Anyone know if something like this has been implemented or have any thoughts?