Update

Update: Single Motor Dual Extruder

My new extruder design has been working great. It has almost doubled the speed of my printer. Below 50 mm/s the quality looks about the same as my old extruder but crank up the speed and it really starts to shine. With my old design, I was capped at about 55 mm/s due to the vibrations of slinging two stepper motors around the X/Y plane. Now, I print by default at 80 or 90 mm/s and it looks amazing. I could push it faster but my axis motors will start to overheat. One problem that I did encounter when printing at higher speeds and layer height greater than 0.15mm was under extrusion. Even with 260 deg. C nozzle temps, I just couldn't seem to push enough plastic out of the hot end, I would end up with striped filament a caked up drive wheel. I eventually tracked this down to the apparently counterfeit EDv6 hot ends that I got off EBay. Most of the parts of these hot ends were discarded or re-purposed and the issue was ultimately remedied by installing a set of nozzles made by Micro Swiss. Their coated nozzles are outstanding and will change your life if you are under extruding.

Component Selection

As you can see from the videos and the diagrams, most of the parts of the new extruder design are printed in ABS but there are a few off the shelf items. The most important one of note, would be the servo that switches the filament. I started out with a standard servo that I had picked up from Radio Shack (yes, these stores still exist) but I soon discovered that for anything other than testing, it just wasn't strong enough. I think that it had a torque rating of about 3 Kg.cm and that was ok to test the movement concept and the throw angle but not enough to generate the needed clamping force between the filament and the drive wheel. After burning out this servo and a few others that I had laying around, I settled on an HD Power LF13MG servo and this worked out well. I chose this servo for a few reasons, the first being it's high torque rating. At 13 Kg, it is a beast and the second being that it is digital. Using a digital servo is important because most home built 3D printers are an absolute mess of signal noise. Even if your printer functions as expected, you still are likely to have very noisy ground lines. This will make an analog servo jitter and dance and fail to maintain the drive wheel contact pressure.  Another factor it keep in mind when you car selecting your components is their max working temperature. Since these will be living within your build chamber, they need to be able to withstand the heat. I like to keep my chamber between 50 and 60 deg. C but most servos don't like to stay this hot for long periods of time. My solution to this was to add a heat sink. The main body of the HD Power LF13MG is made of aluminum so adding heat sinks to both sides was very easy and the main extruder fan also blows air over them.

Firmware Modification For Marlin

Changing the firmware so that this extruder system can be used was pretty easy, most of the work had been done and is outlined in a pdf from thingverse. Unfortunately, the code demonstrated in the file didn't work for me but it gave me a starting point. Eventually I would like to make a real time menu option for adjusting the servo position during a print.