# Dual extruder with gear shifting

I am planning to upgrade my printer with a second extruder. Since my printer is a Tronxy X8 it's frame is not exactly vibration resistant, so I'd like to keep the print head weight down. At the same time I really don't want a Bowden setup.

I came up with the idea of making a dual extruder driven only by a single stepper with a gear shift setup that switches the stepper between the two extruders.

The idea seems simple, but googleing didn't turn up anything else.

Is there anything I am missing that would would make such a setup unfeasible?

Did anyone else build something like that?

A clarification, because it came up in an answer:

What I imagine is this:

One stepper motor is connected to a gear shift system that is either connected to Extruder A or Extruder B, depending on the gear. When shifting it just connects the stepper motor to the other extruder. So it is still similar to a regular direct driven dual extruder system, except that it only uses a single stepper to drive two extruders, each connected to it's own hotend.

That is perfectly viable these days in Marlin firmware, there are options for setting this up using the configuration file, e.g.:

// :[0, 1, 2, 3, 4, 5, 6, 7, 8]
#define EXTRUDERS 1
...
...
...
// A dual extruder that uses a single stepper motor
//#define SWITCHING_EXTRUDER
#if ENABLED(SWITCHING_EXTRUDER)
#define SWITCHING_EXTRUDER_SERVO_NR 0
#define SWITCHING_EXTRUDER_SERVO_ANGLES { 0, 90 } // Angles for E0, E1[, E2, E3]
#if EXTRUDERS > 3
#define SWITCHING_EXTRUDER_E23_SERVO_NR 1
#endif
#endif


You'll need a custom firmware.

Yur custom firmware will have to react to the "Change extruder" command differently than a normal firmware: instead of just swapping to a different extruder, you'll need to perform some operations to alter the gearing (possibly a solenoid?), and possibly include some kind of "break" to make sure that the filament is not slipping back without the extruder attached. However, there already is a setup that pretty much does this: the Prusa MMU2 uses something similar. The MMU does use a Bowden setup, but you could use Bowden and direct drive in combination, especially if both motors run in sync.

• I think you misunderstood me. I would still have two extruder gears that stay with the filament. I would just have a gear shifting system that would connect the stepper to either of the extruders. That way no filament cutting is involved. All it needs is a servo or something to move the stepper's gear to one or the other extruders. Marlin should be able to handle that. Dec 22 '20 at 23:34
• @Dakkaron Solenoid. You want to trigger a solenoid. But in either case: you need to make your own firmware, and you can look into how the MMU2 gets away with doing a "tool swap" without using more than one extruder motor. Dec 23 '20 at 1:47
• Marlin has built-in support for servos etc. to be triggered kn tool change. I am programmer, so I am not worried about the software side. It's more the hardware side that I am worried about. The solution sounds simple, but I have never seen it implemented, so I am wondering what I am missing. Dec 23 '20 at 23:24
• @Dakkaron hm... how about start with a sketch? one or two hotends? Dec 23 '20 at 23:26