This extruder is designed for E3D v6 3mm Direct Hotend. Lightly modified extruder body and idler were made by request for 1.75mm Universal (Direct) hotend - these parts have not been tested yet. The extruder can be mounted into Prusa i3 Rework -type X-carriage, which is included in the files.

The extruder uses 2mm pitch GT2 belt drive to get rid of backlash and degeneration of 3D-printed gears (got the idea from clough42's 389105). I chose 20:80 pulley-ratio, that will give approximately 600 E-steps/mm with Hobb-Goblin's 7mm effective diameter and 1/16 microstepping (A4988 stepper driver).

Highly recommended inductive proximity switch is installed approximately 30mm right from the hotend nozzle. 30mm (or 40mm) print cooling fan is mounted without additional duct as it seems to work for ABS printing.

Compared to other extruders I have used (Mak-a-face's 257811 and 725082), the space between hobbed bolt and heatsink's filament entry is narrower and completely open. This makes removal of heatsink easier if filament gets stuck and cannot be pulled away. The center of the mass is also designed to be closer to X-carriage. The trade-off is that the bottom of X-carriage comes closer to print bed, so make sure there's no contact with heated bed cables when print bed moves to the front.

Idler without springs:

I have designed idler to be used without spring load. Just tighten the 35mm long M4 screws to the end and reliable grip to the filament is obtained. Idler's filament guiding arm prevents over tightening. With high-quality ABS I have not had any problems with this mechanism (so far over 260 hours of printing).

Additional metal parts:

The extruder has four small metal parts that should be easy to made with basic tools: hacksaw, tin snips, pliers, metal file and hand drill. Only 3mm and 12mm drill bits are needed. See attached images for reference drawings.

1. Inductive proximity sensor is mounted by aluminium L-bracket. Cut a piece from 20 x 20 x 2mm L-profile.
2. Small metal arm for cable's strain relief. Made from sheet metal.
3. Bottom plate to direct heatsink's cooling blow away from hotend. Made from sheet metal.
4. Idler's 608ZZ axle is cut from 8mm steel bar, 17-18mm length. Alternatively alum. pipe or M8 threaded rod can be used.

Extruder is designed to work with these parts:

• Hotend: E3D v6 3mm Direct or 1.75mm Universal (Direct)
• Heatsink cooling fan: 30x30mm E3D fan (2 ball bearings)
• Print cooling fan: 30mm or 40mm
• Stepper: NEMA17 (34mm long version works fine)
• Driver pulley: 20 tooth GT2 pulley
• Driven pulley: 3D-printed 80 tooth GT2 pulley (STL included)
• Belt: Neoprene Rubber 188-2GT-6 (link to Aliexpress.com)
• Hobbed bolt: E3D's Hobb-Goblin
• Z-Probe: LJ12A3-4-Z/BX Inductive proximity switch, NPN, normally open (link to Aliexpress.com). Works without any additional electronics.

Assembly hardware:

2 x 688ZZ 8x16x5mm ball bearing (hobbed bolt)2 x 8x12x1mm copper sealing washer (hobbed bolt)1 x 608ZZ 8x22x7mm ball bearing (idler)1 x 8x18mm steel axle / alum. tube / M8 thread bar for 608ZZ bearing (idler)4 x 20mm M4 socket/machine screw (mount to X-carriage)4 x M4 hex nut (mount to X-carriage)3 x 12mm M3 socket/machine screw (stepper)3 x M3 washer (stepper)2 x 35mm M4 socket/machine screw (idler)2 x M4 washer (idler)2 x M4 hex nut (idler)1 x Ø3mm 25mm self-tapping screw for plastic OR M3 socket/machine screw (idler's bracket)2 x Ø3mm 20mm self-tapping screw for plastic (hotend clamp)1 x Ø3mm 16mm self-tapping screw for plastic (fan arm)4 x Ø3mm 8mm self-tapping screw for plastic (small metal parts)1 x 2.5mm zip-tie (cable's strain relief)

The self-tapping screws I used have been salvaged from broken stuff.

https://www.boltdepot.com/fastener-information/type-chart.aspx
http://de.screwerk.com/en/products/stp/stp32.html

At the bottom of this page can be found the printing parameters I use with this extruder.