This is the MK4 version of my geared extruder series for Prusa i3 clones.

The advantage to this design over others, to my mind, is that it contains all of the features a printer should have:

• Based around the venerable J-head, from http://hotends.com. This design should work with all genuine J-heads from the MK V on up, including the MK8, and of course the v8, which is what I use.
• Greg/Wade style geared extrusion to prevent skipping E steps at higher flow rates.
• Optional Inductive sensor mount for Z endstop and auto bed leveling (LJ18A3-8-Z/BX or equivalent). It should be possible to make mounts for other types of sensors (e.g. BLTouch, IR, etc). The included mount puts the center of the sensor at X +43 mm, Y -20 mm (relative to the nozzle orifice).
• Quick-release filament drive with adjustable tension.
• Blower fan with shroud for layer cooling (sized for a standard 5015 fan).
• Adjustable motor position to control gear backlash and to compensate for wear.
• Mounts for LEDs around the hotend.
• Off-the-shelf hardware to put it all together.
• Wide mounting holes, to allow the extruder to fit any X carriage hole spacing from 24 to 31 mm, inclusive.
• Uses normal 5-mm-bore hobbed drive gears, so you can buy precision engineered parts instead of hobbing bolts by hand.

I first designed my original "MK1" extruder as an upgrade over the X-carriage-mounted non-geared extruders (e.g. "Stepstruder" clones) that a lot of Prusa i3 clones come with, which included my own bot. Three versions later, I guess one can say the design has evolved somewhat. :-)

Besides being thinned-down, the gears have been modified from upstream to take an M5 bolt (rather than the usual M8). These have a ratio of 47:9, which is unchanged from upstream. The effective diameter of your hobbed gear will of course affect your E steps per millimeter setting.

To support multiple types of hobbed drive gears, this extruder uses replaceable gear "cartridges", which are cut both to position the drive assembly so that the hobbed gear's teeth overlap the filament path properly (by about 0.3 mm, assuming a straight piece of 1.70-1.75 mm filament centered in the filament path), and to avoid having too much clearance around the hobbed gear and pressure bearing. As each gear is different, each requires a cartridge cut to fit it. I've included cartridges for MK7, MK8, plain 11 mm OD ("26 tooth"/"36 tooth"), and plain 12 mm OD ("40 tooth") gears. Most other gears should be easy to accommodate, by starting from the "uncut" cartridge in the .blend work file (so long as the cartridge can be cut to allow the gear to be inserted from the front without requiring excessive clearance around the hobbed portion). There's enough space to fit gears up to roughly 13 mm OD and perhaps 14 mm long, maybe a bit more.

This also means that if you need or want to change do a different gear, you can simply print and install the proper cartridge instead of re-printing the entire frame, and perhaps change the idler arm's bearing, if appropriate.

While I haven't designed for it, it should be trivial to adapt this extruder to fit 3 mm filament.

This extruder has not been tested with other hot ends or so-called J-head clones.

Update 2018-03-24: I've altered the sensor mount to put the sensor further to the right and closer to the X rods. This makes it possible to remove the large gear/drive assembly for routine maintenance, without taking the sensor off first. I also added little supports under the thin part of the sensor mount to keep it from being knocked over, and added a flat spot just above the side mounting screw to give a reliable spot to bump against your X-max endstop, if you're using one.

Update 2018-03-23.1: The J-head clamp's countersinks were still too shallow - deepened them by 2 mm. Re-cut all four cartridges to deepen the idler arm bearing recesses a bit. Reduced the width/diameter of the large gear's hex recess by about 0.2mm (it's supposed to be a really tight fit, requiring you to force the bolt into it).

Update 2018-03-23: Increased the clearance between the "top" of the sensor mount's slot (where it fits over the blower shroud mount) to account for bridging sag, and beveled some of the edges that face the plate, to account for elephant's foot.

Update 2018-03-22: I widened the bearing retainer barriers on the back of the frame a bit and increased the offset between the horizontal and vertical parts, to help them print better (PETG really just doesn't bridge well without some "help" from the model itself).

Update 2018-03-21.1: I re-cut the cartridges to minimize the clearance around the idler pressure bearing area (too big of a gap, particularly below the bearing, makes the filament tend to curve out of the pinch area instead of going straight down, during filament changes).

Update 2018-03-21: I made the clamp's countersinks too shallow, enough so that even 20 mm screws couldn't reach far enough into the frame, because while the nut traps are in the correct spots, I was actually using the corresponding nuts as my reference point, and they were in the wrong #%$@ing places.

Update 2018-03-20.3: I noticed my J-head clamp was wiggling a bit during retracts, so I've taken the opportunity to tighten and strengthen the model a bit, with shallower screw countersinks and a slightly thicker top (which should now butt-up against the bottom of the frame solidly; the screws may be a tight fit). I also thinned-out the support membranes over the holes (to 0.2 mm, which creates a single-layer bridge over each, as I originally intended).

Update 2018-03-20.2: Altered the frame and sensor mount so that you can print them separately, in case you want to get the extruder going as quickly as possible, but decide later that you want to add a sensor mount. Added little barriers behind the cartridge receptacle to keep the rear bearing from falling out if you should need to take the drive assembly off (e.g. to replace a worn-out hobbed gear). Also, shrunk the clearance around the motor hub and its mounting screws and made their countersinks shallower, to increase the overall strength of the motor mount.

Update 2018-03-20.1: Forgot to make the access holes wide to match the frame. Also, they were slightly too low, though that wouldn't have caused an issue. Fixed, in any case.

Update 2018-03-20: Added small holes to the large gear and J-head clamp, to allow access to the main X-carriage mounting screws at any time. This makes it possible to assemble the extruder first, then install the completed assembly onto your X carriage. The holes are only 3.5 mm diameter, as I assumed Allen-head screws with ~2 mm sockets. Smaller Phillips screwdrivers will probably fit, too, if that's all you have.

Changes from the MK3 design:

• The universal filament path didn't work too well in practice - it being so wide made it prone to jams. So I've gone back to a normal filament path.
• MUCH stronger, more reliable grip on the filament.
• Should work fine with flexible filaments now (I haven't tried).
• The upper filament guide has been enlarged and replaced with a loop (aside from keeping the filament straight, its purpose is to keep your filament dust filter or oiler, if any, supported up and away from the gears).
• The sensor mount is now an optional part that you print and attach to the frame, rather than having separate versions of the frame with and without it.
• Minor changes here and there to improve the printed quality of the frame.
• Added a support "ear" to the frame's cold end fan mount, I kept having problems with it lifting when I was tinkering with this design and the last few revisions of the MK3 design.
• Moved the sensor mount a few millimeters further to the right, to increase the clearance around the large Wade gear (particularly important if you're using a MK8 or some other hobbed gear with a small effective diameter).
• Nicer contours on the front of the large gear :-)

Changes between the MK2 and MK3:

• Lighter-weight "open-frame" design, with a mass similar to my MK1.
• Narrower design, the nozzle now reaches beyond both edges of the bed easily.
• More generous filament path and gear clearance to allow for many common hobbed gears up to 13 mm diameter and/or 13 mm length.
• 5 mm LED mounts instead of 3 mm.
• Better placement of inductive sensor - easier to install and adjust
• Motor now moves vertically for adjustment.
• New, slimmer printed gears.
• The idler pressure bearing is now held from both sides, so the mount area can't bend sideways.
• A version of the frame is included that lacks the sensor mount.
• Should fit any arbitrary X carriage with hole spacing from 24 to 31 mm

Changes between the MK1 and MK2:

• The motor position is now adjustable.
• The tension spring has a much larger pressure range - from 0 to basically "insane".
• The idler bearing now exerts force exactly square/perpendicular to the filament path and the drive shaft, even at extreme tension.
• The airflow from the cold-end fan is now better targeted at the J-head's cold end.
• The blower shroud should now have a much lower-resistance air path.
• Tie-down loops/holes have been added for securing/bundling up your wires/cables.
• Where appropriate, downward-facing holes are beveled to help compensate for excessive first-layer squish or elephant's foot.
• A little inset "corner" has been added to some of the bearing mounts to give your slicer a good place to put the perimeter seams (for better diameter accuracy)
• An initially unrealized side effect of moving the motor forward is that there should no longer be any interference with the printer frame's top corner braces, so you get slightly more print volume.
• The blower shroud has a mount on the inlet as well, to keep the fan inserted properly, and to prevent the inlet end sagging if it should overheat.
• Fits X carriages with 24 or 31 mm hole spacing.

Note: the old MK1, MK2, and MK3 blends can be found in the "files" section, and are included here for reference only. I do not recommend printing them, as the MK4 design supersedes all of them.

Vitamins

Bearings:
3, 623ZZ, or two 623ZZ and one 623VV shallow V-groove (1 mm depth or less).
2, 625ZZ

Screws:
6, M3 x 6 mm
1, M3 x 8 mm
3, M3 x 10 mm
4, M3 x 12 mm (3 optional)
1, M3 x 16 mm
3, M3 x 20 mm
3, M3 x 40-50 mm
1, M4 x 4-6 mm, cap head, Allen, Torx, square, or some other wide drive socket
2, M4 x 20-25 mm
1, M4 x 35-40 mm
1, M5 x 35-40 mm, hex head
5, 3.5 x 15 mm wood/sheet metal/drywall/plastic or similar, one with a narrow tip

Nuts:
15, M3 (2 of which should be nylock, if possible; 3 optional)
2, M4
1, M5
1, M5 nylock

Washers:
3, M3-sized flat (2 optional)
2, M3-ish-sized, fairly thick, with a wide OD (10 mm or so). SAE-#6-sized flat washers are also a good choice here.
2, M4-sized flat (optional/if needed)
1, M5-sized flat

Other:
1 stiff spring, approx 9 x 20 mm, suitable for hob/bearing idler drive pressure
A suitable 5-mm-bore hobbed gear to fit one of the available cartridges