Dual Extruder Using a Single Motor

Anybody that has ever built or bought a 3d printer can tell you, one of the best upgrades or capabilities it can have is known as duel extrusion. Simply put, duel extrusion is the ability to use more than one material or color in a single print. This is useful when you are designing a part that has hidden internal passages that require support. Having the ability to selectively dissolve part of your print makes creating internal features easy.

The main reason for a printer not to have dual extrusion is cost. Having extra extruders comes with the unwanted side effect of a heavier tool head. More weight in the moving head means that the overall structure of the machine has to be beefed upped to handle the added inertia and this can get expensive.

One solution to this is to drive both lines of filament with the same motor and have some kind of servo mechanism change the position of the desired filament to be extruded. Others have had this idea and I can think of two working examples of it but both of them I consider to be overly complex.  

As you can see from the animation above, a single stepper motor is used to drive both lines of filament. A hobby servo controls filament tension. An important detail to note is that the stepper motors direction to advance either line of filament is the same, clockwise. The only difference is the position of the servo that dictates which filament is pressed against the drive wheel and therefore advanced. This becomes important when we start talking about machine firmware. Marlin and Repetier both come with M-code servo control support  so no firmware modification is needed. If anybody is interested in building one and would like to see the design files, let me know and I will send them. I plan on releasing this to open source when I finish it.