While catching up on the blog, I found a really useful post by Tony (copied below):
I was able to get some very nice results from the original GM3 powered BFB extruder, all the same principles apply, keep the melt zone short and support the PTFE close to the heater. Stability of the motor speed and the quality of the motors were the only slight issues. I ran both single and twin motor versions for many months without a problem. I still believe this original extruder design is very good and it certainly proved itself reliable over that time.
For those who had problems getting them to work I strongly suggest you blueprint the design exactly as it should be, by all means sharpen the thread and undercut it with a hack saw if you must but this should be the limit of the mods. The most common error I have heard about is the omission of the small reaction washer behind the main feed screw. Miss this out and you will never grip the filament tight enough. Build the extruder exactly as the instructions and it works, mess with the PTFE add custom heaters and coach bolts for the drive screw(what!), and you are in unknown territory. Run it how it should be first, get some experience in printing, you will then start to see where modifications would pay dividends, the original one works very well and starts reliably from cold.
Coach bolts? Who would do such a thing! :-)
Time to re-check. If it's only me and one or two others having problems with the BfB drive, then it's probably either my setup or incorrect assembly.
Re-state the problem.
Initial problem: heater/motor control worked fine, but extruding at 250C and any speed only resulted in 2-3mm/minute, almost too slow to see. Tried various temps and speeds, same result.
Removing heater, I was able to manually pull the filament through (screw thread not biting).
Tightened all the bolts - still no change to extrusion. Tightened to the point the acrylic started to crack (on the 2mm plate holding the bolt/filament roller). No change. Springs fully compressed. (reaction washer in place)
Disassembled, sharpened bolt thread with die. Better grip, but no better extrusion.
Examining the heater and drive separately, found that the heater required a reasonable force, but extruded OK, but the drive system still wasn't strong enough - although it seemed more than strong enough during testing, it still wouldn't extrude.
I then experimented with some different nozzles and drive systems. I found some nozzle designs needed less force, but needed careful machining to be reliable.
Replacing the m8 thread with a coach bolt , which had a much larger, sharper thread really gripped the filament very well - but still didn't extrude. WTF?
I think the problem is the thread is very grippy - but the pitch is about 4 times larger than the m8 thread. The coach bolt drive is very strong, but drives faster than the m8 thread - a few mm per revolution. I think pushing too fast chokes the extruder, stalling the filament, and stripping the filament (confirmed by examination). It might work very well, but needs to be geared slower (or driven by a stepper).
OK, so after reading the BfB forum and posts, I stripped down my extruder to have a closer look.
A careful inspection of the extruder parts revealed this:
I'll file the channels out, reassemble to the original BfB extruder, and try again. I think it *might* just work this time (famous last words...)
As we say in computing, PICNIC (Problem in chair, not in computer)....or should that be PIANID (Problem in Assembler, Not In Design)...