Lots to see here, folks! The video https://www.youtube.com/watch?feature=player_detailpage&v=gR1vrykA90E shows http://www.thingiverse.com/thing:34821 in action, printing its first version of the Dual Extruder Calibration Piece (soon to be another Thing). It's taken nearly eight months of work since the original posting, but the dual extruders are now working reliably.
Building this dual extruder is still not a project for beginners, but the careful builder will be rewarded with a compact dual extruder that mounts directly on an X-carriage ÃƒÂ¢Ã‚Â€Ã‚Â“ no bowden arrangement. The complete build instructions are in the Instructable that accompanied the original Thing. Posted here are the details that have evolved during the course of making this work.
It's worth noting http://www.thingiverse.com/thing:93513 is an X-carriage for a Prusa that will mount Jim's Flying Motor Mount Dual Extruder. Included with that is a fan mount for the nozzle cooling fans. If you have a Prusa, you can get going quickly. These pieces were used for all my development so far. I have also designed a part cooling duct that I will post soon. My initial work did not use it, but I will mount it now for all future printing.
One of the first challenges I faced was that Marlin for the Printrboard did not fully support dual extruders. Fortunately, I found a version of Marlin which included support for dual extruders, but it was for an earlier Marlin version and had not been merged. So I merged it into Lincomatic's Printrboard version. I will post this to Thingiverse so anyone who wants it can use it.
The modified Marlin can independently control the dual extruders (independent temperature settings and stepper settings), but there was still some effort involved in making Pronterface handle dual extruders in a reasonable manner. Instructions for using Pronterface to control dual extrusion will be included with the Marlin code posting.
Once I could extrude plastic independently using Marlin and Pronterface, I needed a way to calibrate the extruders. So I designed a simple calibration block in OpenSCAD. Of course, the next step was to slice it so that the two extruders were used to print it. Fortunately, Slic3r now has the capability to handle dual extruders, either printing support material or two different colors. Full instructions for creating the calibration block and slicing it with Slic3r will be the subject of another Thing.
With calibration working, the next challenge was to try longer prints to see what problems might occur. And, yup, some occurred. The main problem was that the motors heated up. While the heat did not cause any permanent deformation of the PLA extruder bodies, it softened them enough to allow them to flex (I think) and the drive gears disengaged. This caused the flow of plastic to stop.
Four possible solutions occurred to me:
Print the extruder bodies in ABS which is more heat resistant.
Put cooling fans on the motors.
Use a hypocycloid reduction gear http://www.thingiverse.com/thing:79231 on each motor.
Re-design the extruder body to increase its strength and rigidity.
ABS bodies were offered for testing, and so it was easy to try them. The results were mixed: While the flexing problem was completely solved, the motors still heated up and started missing steps. This might have been happening earlier, and not noticed.
Putting on the cooling fans http://www.thingiverse.com/thing:32994 cured the heat problem completely. The motors barely heat up now. So I have not tried the other two solutions. I doubt if making the bodies stronger would help, since the ABS bodies didn't defect and it doesn't help the heating problem. Using the hypocycloid gear reducers would work (the motors run cooler since they run at higher rpm more efficiently and heat up less), but would complicate things and require development effort. The only down side is that the cooling fans take up some height from the build area.
So with the fans in place, Jim's FMM Dual works reliably. Not to say there won't be any tweaking, but it's ready for use. Build one, get the tools and techniques as I post them, and explore the world of dual extrusion. Have fun!
For the PLA I was using, I set the Retraction Length (on the Printer Settings/ Extruder 1 page) to 5mm and the Extra Length on Restart to 1mm. (Same for Extruder 2.) This seemed to give decent results, but can certainly stand more tuning. I haven't experimented with settings on that page yet.