Dual Drive Bowden Extruder

UPDATE 14-01-16:
New files added for use with MK8 drive gears. Unfortunately I would have had to compromise the design too much to make a version which can do both MK7 and MK8 gears, so theres two slightly different parts with slight changes to the geometry. The smaller diameter MK8 drive gears will give a slightly higher gear ratio, so more force on the filament, however they will also have slightly less grip as there will be less teeth in contact.

UPDATE 06-01-16:
Some updates to the V2 extruder frame and pivot arm. All the geometry is the same but there are now better mounting holes, a filament guide above the drive gears, some levers to grab to make it easier to change filament!

UPDATE 13-10-15:
I have left the MK1 files there in case anyone wants to remix or modify them, but MK2 is superior in many ways so if your building one for the first time, MK2 should be the one to build.

Lots of updates, now using off the shelf MK7 drive gears so you don't need to hob anything (unless you want to), its easier to print, there are bearings on the pivot arm, and the drive gears are now located in between the shaft bearings so you can apply much more clamping force without the arm or frame twisting!

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There are many extruder designs out there already, why do we need another one?

Well, you might not. If you are running a small printer, with a direct extruder, then this is not for you. If you are running a small printer with a direct drive bowden extruder, then its still probably not for you. This is aimed at BIG printers, with the aim of going FAST.

I run a relatively large delta printer, not the largest out there by any means, but large enough to get some pretty large volume prints on the plate. And the problem you soon discover with large prints is that they take AAAGGGEEESSS. My problem is also compounded because I run 3mm filament into a 0.35mm j-head hot end. Not the highest flowing hot end, and a small nozzle to be squeezing plastic out of.

The second thing that plagues large bowden extruder printers is retraction. Because my bowden tube is 800mm long, I need a lot of retraction to avoid stringing. Lots of retraction means lots of running back and forward over the same poor piece of filament, which leads to grinding, slipping, and under extrusion. Ideally I would be running about 8mm of retraction, with 3mm filament 8mm is a lot of extrusion out the hot end before I get a fresh piece of filament to grip.

I soon established that my primary limiting factor was grip on the filament. To print fast I had to put a lot of spring pressure on the drive roller, which means when you retract back over the same piece of filament it gets destroyed. The fix? More teeth. By gripping the filament from both sides you get twice as many teeth holding it so can use less pressure and get the same grip. With nice sharp hobbed bolts I can make my stepper slip before the filament does, grip is not a problem with this arrangement!

The gear ratio is only 1:2.5 which is less than most Wades extruders, but there is a trade off between gear ratio and how big those gears get. With a decent spec motor you should still have plenty of torque (I use a 42BYGHM809).

This is now the second version, clearly the first one worked well enough that I decided to keep working on the concept. The second one works even better! I can print faster, retract faster, and have less problems grinding filament with lots of retraction on small models. I would love to see a few more out there and see if they work well for anyone else too.

It was intended for use with 3mm filament and a bowden setup. I guess theres no reason you couldn't use it as a direct extruder, but you would need a fairly large printer for it not to look silly (its pretty bulky). If you run 1.75mm filament then you probably dont need it, but I dont know anything about printing with 1.75 filament so feel free to correct me and give it a go!

Bear with me, this is a work in progress. Any questions, just post a comment I'll do my best to answer.

For anyone who wants to push their extruder to its limits I consider this article to be mandatory reading:
http://www.extrudable.me/2013/04/18/exploring-extrusion-variability-and-limits/

Note that he is using a 0.6 nozzle, so his results may not be directly comparable to your setup. I intend to perform a similar test for my setup, but I haven't had time yet.

https://www.youtube.com/watch?v=0AV3KpuHrKA
https://www.youtube.com/watch?v=WPuSdkbF-h0