This is not the extruder you're looking for. If you're looking for the extruder referenced in Josef Prusa's i3 design, see http://www.thingiverse.com/thing:24219

A direct drive extruder designed for use with a Minebea geared stepper motor, a Makerbot drive wheel, and a MakerGear GrooveMount (or similar) hot end.

All Electronics is out of the required motor - MPJA has a supply of them at a slightly higher price - http://www.mpja.com/prodinfo.asp?number=18840+MS

From the very beginning with my printer, I've been fighting clearance issues with the extruder, which has prompted me to attempt a long series of different extruder designs. I tried internal (ring) gears, a printed planetary gearbox, and a Legoâ„¢ (!!!) worm gear. None were completely satisfactory.

I was about to break down and spend $33 apiece (plus shipping) on gearbox motors from robotshop.com, when ezmobius linked a Minebea gearbox stepper in IRC which was selling for $6.50 + shipping (Many thanks, ezmobius).

So, here you have my best effort for a compact extruder.

My goals for it are (in order):

1. Torque. It must be able to drive 3mm filament.     So far, I've only tested it with PLA, but I've had no difficulty     extruding at average speeds.

2. Reliability. It must never let the filament slip, or lose steps.     Once I figured out that I needed to reduce the retraction     speed to 10mm/s, it became quite reliable.

3. Size. It must give me use of the full width of my print bed.     On the newer of my printers, which was built with an extra     15mm width in the frame, I've achieved that goal. On the     older printer, the frame will limit my width at high print     heights by around 10mm.

4. Ease of setup. I don't like adjusting bolt tension whenever I     change filament.     The makerbot drive wheel is designed for use with fixed idler    tension, and (when tested with other extruder designs)     allowed me to switch between PLA and ABS filament, with     .1mm difference in diameter, without having to readjust.

5. Ease of maintenance. I don't like taking an extruder apart to     clean it.     The back side of the extruder body is open, so that you can     clean the teeth of the drive wheel without even loosening     the idler.     The filament guide runs close between the idler and drive     wheel, to reduce the likelihood of jamming the the     extruder when switching filaments.

After designing it, I found that arcol had started an indiegogo campaign to build drive wheels in various sizes, so I added another goal.

6. It needs to accommodate either the makerbot MK7 drive     wheel, or the arcol hyena 11x5 drive wheel.     The current version adds a taper to the lower filament guide, which allows it to be used with either drive wheel.

Motor:      http://www.eminebea.com/en/product/rotary/steppingmotor/pm/gearbox/pg35l-d48.shtml     (available at http://www.mpja.com/prodinfo.asp?number=18840+MS , but you'll need a small gear puller or faucet puller to remove the vestigial gear).

Drive wheel:      http://store.makerbot.com/mk7-drive-gear.html or      http://www.indiegogo.com/mini-hyena (the 11x5 version).

Two variations are provided: One which fits a compact carriage (45mm in in the x-axis), and another which fits the standard Prusa carriage.

Revisions:     Fixed the bridges in the carriage belt clips.     Increased the thickness of the carriage mount.     Increased the diameter of the igus bushing holes.     Added an (experimental) pocket for an LM8UU bearing to         each of the carriage rails. I haven't tested this, and I'm         not sure if it will work to use only 2 LM8UU on a         carriage, but it saves plastic - consider yourself         warned.     Added bolt holes for a 40mm fan to the carriage.     Widened the openings in the LM8UU pockets. Hopefully         it'll now actually be possible to get a bearing into them.     Moved the filament channel a bit closer to the drive wheel, and tapered the lower filament channel, in order to support arcol's "mini hyena" drive wheel.     Closed the outside of the extruder body, to prevent it from flexing as the filament is driven.

Vitamins: Extruder body     1 Geared stepper motor (Minbea PG35L-D48).     1 MakerBot MK7 Drive Wheel (MakerBot calls it a drive gear).     1 624 bearing (4x13x5).     2 M3 x 25mm Socket head screws.     4 M3 x 35mm Socket head screws.     1 M4 x 20mm Socket head screw.     6 M3 washers.     6 M3 nuts.     1 M4 nut.

Vitamins: Compact carriage     4 igus bushings ( https://sdp-si.com/eStore/PartDetail.asp?Opener=Group&PartID=64165&GroupID=830 ).     3 M3 x 25mm Socket head screws.     2 M3 x 20mm Socket head screws.     2 M3 washers.     5 M3 nuts.     1 M3 locknut.

Choose your carriage style, and print the parts.

Note: I had to use SFACT to slice the model, because slic3r doesn't generate good toolpaths for many of the bridges, and because it leaves out the support for the belt clamp on the carriage.

Extruder Body     Insert 6 M3 nuts into the nut traps on the extruder body. I find it useful to epoxy them in place, but it's usually not required.     Attach the motor to the extruder body with 2 M3 x 25mm Socket Head Screws and washers.     Attach the MK7 drive wheel to the motor, using the supplied M3 set screw. I run a piece of filament through the filament channel to hold the drive wheel in place while tigtening the set screw.     Insert the 624 bearing into the idler bracket.     Install the M4 bolt and nut into the idler bracket.     Attach the idler bracket to the extruder body with 4 M3 x 35 Socket Head Screws and washers.

Calibration     With the hot end removed, connect the extruder motor to your printer, and feed a length of filament into it. If it runs backwards, either reverse your motor wires, or change your firmware's configuration.     Mark the filament where it enters the extruder body.     Tell your host software to extrude 100mm of filament.     Mark the filament where it enters the extruder body.     Tell your host software to reverse the filament out. I remove all of it to make measuring easier.     Measure the distance between your marks.     Use http://calculator.josefprusa.cz/ to determine your new steps per mm, and change your firmware's configuration to match.     Repeat calibration until you achieve zen.

    Note: because of the gearing on the minbea motor, you'll have to use a fairly slow retraction speed - 10mm/second or so

Compact Carriage     Using the hot end, align the extruder's mounting plate with the bottom of extruder body. Using a sharp pencil (or needle), mark the locations of the mounting holes, and the outline of the extruder body's bottom.     Drill holes in the mounting plate for 3mm bolts, at the marked locations. Cut the mounting plate down to fit the extruder body's bottom.     Insert 2 M3 nuts into the nut traps on the underside of the carriage.     Insert 2 M3 nuts into the outside nut traps on the topside of the carriage.     Insert 1 M3 nut into the inside nut trap on one of the belt clamps (whichever side you want the belt tensioner on).     Install 1 M3 x 25 bolt into the tensioner nut, with the bolt head facing the outside of the carriage.     Glue 1 M3 locknut onto the inside end of the tensioner bolt. The rounded end of the locknut will be used to protect your belt from the tensioner bolt.

Final assembly     Install the carriage on your x axis.     Clamp the ends of the belt to the carriage using M3 x 20 bolts.     Adjust the belt tension using the tensioner bolt.     Install the extruder and hot end on the carriage using M3 x 25 bolts and washers.