Thursday, June 11, 2009


BitsFromBytes extruder

Reading a comment on the RepRap forums I *finally* realised that BitsFromBytes had a Forum and a separate Blog. Since I've got an older v2 kit, I hadn't checked back there for a while.

While catching up on the blog, I found a really useful post by Tony (copied below):
MK2 extruder
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:
From BfBExtruder
The heater bolts are *slightly* too large for the channels in the extruder drive, causing a gap between the plates. This meant when I removed the heater to check the drive system, it closes up - and the drive works properly. When I screwed in the heater to check the extrusion and bolt to the reprap, the bolts screw in - the gap opens - and the pressure is taken off the screw thread. The screw doesn't bite the filament, and doesn't pull it through.

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)...

Labels: , , ,

PICNIC also known as "Error 24" (Error 24 inches from monitor)
Or the older
PEBCAK ("Problem Exists Between Chair And Keyboard")
When I was building mine, I got a small file (5mm wide triangular one) and filed down the holes for the bolts of wherever it needed it. I figured that anything that needed to be held in place the thing was designed to do with the nuts. So everywhere where a bolt seems tight, just give it a quick file down. not too much, but enough that it'll all go in easily. If the thread is biting the acrilic, then you need to file down more.

My BfB gen 2 extruders only major problem is that the extrusion seems to be a little bit non linear. It's output rate changes like 5mm....6mm...5mm...6mm. But I think thats because the bolt I got was a little bent. The small gear I also had to glue on, but otherwise thats about it for me
I did something similar, even with these holes: I just didn't do a very good job.

I'd ended up filing the channel at a slight angle, so the entry hole was larger than the exit (near the captive nut). This meant that when I put a ruler across the entrance with the bolts, it just had clearance - it was the exit hole that was tight and pushed it up.

My fault!
Hi Renoir,
Just spotted where you are in the world! I live near Stroud, I have an old extruder here you are welcome to have a go with, either way we can swap notes.
Hi Tony!
Thanks for the great posts - full of info and very helpful.

I'm not that far away - and I used to live in Newent, even closer!

Now I've found the problem, I'm going to have another go this weekend.
If I get into difficulties, I might have to take up your kind offer to compare notes...:-)

I have a BfB mark 2 extruder which seems to be working fine fine. I've not successfully printed anything yet but I have laid down several messes of ABS. I don't think the mess part is the extruder's fault, it's either 'error 24', the frame isn't quite right, the settings are off or most likely a combination of all three ;-}

This is from memory, which given it's now about a year since I started trying to make plastic cups, could be a bit off.

Starting from the top:

The bolt is super-glued into it's gear as just held by a nut it wasn't at exactly 90Deg and so because of it pushing into the motor gear it turned at an uneven rate even when no filament was present.

I drilled where the 4 end on bolts went, filing would have been much more sensible though.

I sanded both the top and bottom once the two halves were joined to get a flat 90Deg edge to join to.

I re-arranged the bearings to increase the distance between them.

I sharpened the bolt with a die, but didn't go as far as using a hacksaw.

I filed the filament guide just after the bolt into a funnel shape to 'catch' the filament after it has past over the bolt's thread.

I have an aluminium sleeve on the PTFE tube that goes from the top to 1mm short of the nut on the heaters top.

I have a washer between the PTFE tube and the main body to increase the 'squish' on the PTFE tube.

The spring bolts are only tightened until their ends are flush with the bolts. If it's not feeding the extruder it even works without the pinch wheel, I used to run it like that to line up the filament's exit before I had filed the funnel for it.
Thanks for all the advice.

I've applied most of the tips, filed my holes properly, and now the screw bites *really* well.

I can't pull it out by hand, and winding the gear by hand feeds the filament slowly through.

I'm much happier with it now.

Now off to reconstruct the extruder...
I've tried building it according to the instructions. Only when it didn't work was when I resorted to different setups. It was my third extruder already that didn't work reliably. I made a screw cut with a dye from a piece of 8mm rod. That should have improved things. It never really worked. Now that we've designed a new extruder I'm finally printing again after fiddling for more than half a year with the BfB version (no offence meant, though! It just never worked for me).

There must be something specific that I've been missing. The fact that so many people start modding the BfB design gives me the impression that it didn't work on the first go.
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