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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.

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2 Answers 2

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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
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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.

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    $\begingroup$ 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. $\endgroup$
    – Dakkaron
    Dec 22, 2020 at 23:34
  • $\begingroup$ @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. $\endgroup$
    – Trish
    Dec 23, 2020 at 1:47
  • $\begingroup$ 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. $\endgroup$
    – Dakkaron
    Dec 23, 2020 at 23:24
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    $\begingroup$ @Dakkaron hm... how about start with a sketch? one or two hotends? $\endgroup$
    – Trish
    Dec 23, 2020 at 23:26

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