Direct Drive 3mm-GRRF-Hotend Bowden Extruder for Huxley (with removable fan ;-)

by FMMT666, published

Direct Drive 3mm-GRRF-Hotend Bowden Extruder for Huxley (with removable fan ;-) by FMMT666 Jan 7, 2013
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One can never have enough 3D-printers, so I decided to build some Huxleys.
A lot of parts didn't fit or needed some improvements, so I ended up redesigning most of the machine.

Here's the result for the driver/extruder combination and a GRRF 3mm hotend ( https://shop.germanreprap.com/de/Modular-Hot-End-3mm ).

Hotend and bowden can easily be removed by only one or two screws.
The fan is detachable, which comes in handy during machine calibration or to have better sight on the first printed layers.

The design has gone through many changes, but now it works like a charm.

For the protocol:
I am in no way affiliated to GRRF and just a customer who bought some stuff from them...


  • Print all the parts from the this thing (except the one named "OVERVIEW" ;-)
      Use stiff PLA and high infill values!
  • Print this http://www.thingiverse.com/thing:34819
  • Ream all holes with a 3mm drill.


  • Screw the "Holder" to the motor using a 25mm screw through the "large part".
  • Assemble the "Arm":
      - Put 1-2 washers on each side of the ball bearing.
      - Use a 16mm screw and a nut to fix the bearing (head to countersunk part of the arm)
      - Another 16mm screw and nut goes to the hole on the other end.
      - Use a ~30mm screw and several washers to attach the arm to the holder.
         Increase the amount of washers until the screw can be tightened
        without fixing the arm.
  • Mount the driver to the filament drive bracket ( http://www.reprap.org/wiki/File:Reprappro-huxley-filament-drive-bracket.jpg ).
      and make sure that the tip of the arm sits on the threaded rod (see image).
      A 16mm scre should be sufficient.


  • Put 2 x 3mm nuts into the holes of the rear "HotendClamp".
  • Tighten them with a screw from the opposing side, pulling them into the plastic.
  • Carefully remove the screws without letting the nuts fall out.
  • Mount the rear hotend clamp and the "Plate" to the "X-Carriage" with two
      30mm screws through the holes on the bottom and two
      35mm screws for the upper two holes.
  • Screw a 6mm nut on each side of the PTFE tube.
  • Push the nut of one end of the PTFE tube in the hole of the hotend holder.
  • Push the hotend in.
  • Fix the front part of the hotend clamp with two 25mm screws and two big washers.

The other end of the PTFE tube can now be mounted to the driver.
This requires another 25mm screw.

  • Insert a 30mm screw through the hole on the side of the arm.
  • Put one nut on the other side (where the spring will reside).
      Screw it in as much as possible.
  • Put on a 3mm locking nut and screw it on until the screw pokes out a few mm (5-8).
  • Pull the screw out and make sure the nut sits in its hexagonal notch (inside the spring hole).
  • "Unscrew" it carfully until the locking screw blocks further movement.
  • Insert the spring.
  • Push on the MK7 drive gear and load some filament.
  • Align the drive gear and tighten it.


  • Hold the two parts together and use a soldering iron to weld them together.
  • Fix the fan with four zip-ties.
  • Put two locking nuts on the bottom two screws on the rear of the X-carriage.
  • Hook the fan holder in there.


Reduce the motor current to a minimum.
If it gets hot it will melt and bend the plastic parts!

Good stepper parameter starting point:

  • 97.7 steps per mm, assuming 1/8 step drivers and 2.9mm filament.

Retraction settings starting point:

  • 7mm at 50-70mm/s

BOM (hopefully complete):
1 x GRRF 3mm hotend, https://grrf.de/de/catalog/grrf-modular-hot-end
1 x NEMA17 stepper motor
1 x MK7 drive gear
1 x 623 ball bearing, (3x10x4)mm
1 x spring; (6x30)mm; d=0.81; appr.: >30-50N at 17mm
1 x (6x4)mm PTFE tube, 60cm length
2 x M6 nut
1 x fan, (50x10)mm
6 x zip-ties, standard 3.0mm width
2 x 3mm washer, large
3 x 3mm locking nut
4 x (3x16)mm screw
4 x (3x25)mm screw
4 x (3x30)mm screw
2 x (3x35)mm screw
11 x 3mm nuts
? x 3mm washer, small and thin (ball bearing and arm fixture)

CHANGES 01/2013:
  - removed double "GRRF" position from the BOM

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Nice! I'm also using a GRRF hot-end, but with 1.75mm filament. I would like to adapt your design to this size, and also for my ORD Bot Hadron.
What soft did you use to design the parts? Would you share the source files?

I am thinking about uploading all this stuff to Github.
Nevertheless I also need a 1.75mm driver, but I did not receive the hotend or filament yet. Just be patient :)
Unfortunately, there seem to be not that much resources for 1.75mm filament in Germany...