I recently finished building a Prusa I3 Mendel Reprap from a Geetech kit, with a .4 mm nozzle and 1.75 filament. I am using ABS, but had the same problem with PLA.
The filament does not feed straight into the threaded rod that leads to the hot end. See in the photo it hits the edge of the threaded rod. I have to remove the fan and heat sink to load the filament, and sometimes when the filament pulls back it comes completely out and wont feed back in, thus destroying the print.
Am I missing some kind of guide to feed it into the hole? or should there be a chamfer or counterbore to guide in the filament? Or am I doing something wrong? Any help greatly appreciated,
Paul

This can be fixed by straightening your filament between the spool and the extruder with one of these:

[www.thingiverse.com]
[www.thingiverse.com]
[www.thingiverse.com]

I have had success using a modified extruder base with a little wedge that goes between the tube and the gears, like this:

[www.thingiverse.com]

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-David

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Thanks, I'll get one and try it.