Update 28/05/18: Added v2 body which includes adjustable spring tensioner, improvements to the filament path and rear of the body, and a "DD" logo which stands for Dual Drive. Also added a V2 stepper gear for if you want to attempt to print the gears from TPU. I have tried it, 100% concentric infill using 98A Surreal TPU. It works very well and allows more pressure on the filament from the extruder gears. Also added some pictures to show how to assemble the parts
Update 26/05/18: Added 18t gear set. The teeth are thicker than the original gears and also allow for the extruder gears to get a little closer.
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So finally here is my design for a dual drive extruder, a la Bondtech.
I printed the whole thing in 25% cubic infill. 0.2mm layer height and the parts in the orientations they are saved in the STL files. I used PLA, you can use whatever you like. Nylon is probably best for the gears at least. Use supports touching buildplate only for the main body, I also suggest support interface / a roof. Supports everywhere for the arm and again a support interface / roof to get the best finish.
The "Arm Gear" is designed to push onto the smooth end of a brass extruder gear (like the Anet A8 comes with as stock). You may need to remove the grub screw to get it in, at this point you can either use a longer grub screw if you have one to go straight through the "Arm Gear" grub hole and the extruder gears grub screw hole and / or you could glue the extruder gear into the recess in the "Arm Gear" part. You will need to use the "Drive gear shim" to go inside the extruder gear as it has a 5mm bore whereas you will be putting it on an M3 bolt.
The "Stepper Gear" should be quite difficult to push onto the stepper shaft. It is supposed to be like this so it does not move once installed. It will go on with some encouragement.
You may want to use a 10mm long 5mm wide aluminiumstandoff (generally used for quacopters) instead of the 3mm to 5mm spacer.
1x M3x30mm pan head bolt for the gear axle on the arm with a nylock nut backwards. A 35mm bolt would be better here but I don't have any.
2x M3x20mm Cap head bolts to go through the left side of the body, into the stepper mount and then finally into the stepper. This may vary depending which stepper mount you use.
1x M3x40mm Cap head bolt for the arm hinge (I am actually using some M3 rod I cut to length at the moment)
2x M3 Washers + 2x M3 Nuts for the arm shaft to hold the top extruder gear in place.
4x F623zz bearings in the arm (Anet A8 belt idler bearings) - https://amzn.to/2LvNS75
2x Brass extruder gears like the ones that come with the Anet A8 - https://amzn.to/2GNmgGQ or https://amzn.to/2IJIpeZ
2x PC4M6 Couplings. These will take a bit of work to screw into the body, use a spanner or something... - https://amzn.to/2seeT5V
As of v1.1 you no longer need the M3 washer behind the arm!
Regarding the spring. I have found that using the stock Anet A8 extruder spring works best for me. I have some other springs with thicker wire which are just far too stiff for it and cause jamming of the gears.