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I'm considering removing the driver motor from the extruder assembly, and placing it on a stationary mount point instead, and then using a flex-shaft type connector from the motor to the extruder assembly to actually drive the extruder. The motivation for this change is to reduce the overall weight of the extruder driver and hot end assembly, allowing for quicker movement of the carriage on it's associated axis(one of X,Y).

Would it be better to run a pair of drivers (one for each direction) to manage reversing the filament pressure or would it be better to use just one driver and reverse the motor as usual?

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    $\begingroup$ The configuration you suggest is in place and called Flex3Drive. A quick search will present many links to the same web site for this product. I cannot vouch for the product, having cancelled my delivery. $\endgroup$ – fred_dot_u Apr 12 '17 at 0:31
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Very cool idea,

One motor would definitely be more than capable of producing the required torques even through a flexshaft connector. For any normal sized 3D-printer the torques required, and the speeds you'll need for rapid response are well within the capabilities of any off-the-shelf stepper motor.

Just a note on the idea though, with a normal, 'rigid', connection there is essentially no winding or unwinding, and only the backlash between the gears and the filament to consider, and that is effectively zero. With a flex-shaft though, the stack-up of twists and flexing will be much greater. The system will require more rotations at the source to effect the same amount of torque at the end effector as the flex shaft flexes and bends under the load. For tiny torques with short flex shafts, this wont be an issue as filament pressure is pretty minimal. But if you scale up this project or start working at much higher speeds, you may run into some issues with this design.

I have no idea how big or fast you'd need to be working at for this to begin to become a problem. I'm imagining pretty big though. Just something you might want to keep in mind if you try turning this into a huge, super fast 3D-printer.

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If you're talking about a common 3D printer stepper motor and driver, then one driver is perfectly capable of driving the motor both forward and backward.

The flex tube setup you describe sounds like a Bowden extruder.

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    $\begingroup$ What he described is not a bowden extruder, but something more similar to the Flex3Drive as noted by fred. $\endgroup$ – Tom van der Zanden Apr 12 '17 at 10:35
  • $\begingroup$ @TomvanderZanden Ah, I understand now. Thanks for pointing that out. $\endgroup$ – ctag Apr 12 '17 at 13:57
  • $\begingroup$ But the Bowden is an approach that solves the same problem in a similar but different way. Moving the motor off the printhead, and connecting a "thing that transmits force", (the filament captured in the tube, for the Bowden), to the printhead from where the force is generated at the motor. I suspect the Bowden is lighter, based on flex-shaft tools I have owned. $\endgroup$ – Ecnerwal Apr 22 '17 at 3:52

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