Introduction: Reprap: How to Build JBs Bowden Dual Extruder

This instructable contains a step by step guide to assemble a Bowden dual extrusion 3D printing head I designed for the Prusa i3. The design specifically uses J-heads, and is centred around making each hotend modular and having as small a footprint as possible. The main aim of the design is to reliably extrude 3mm filament in a multi-head configuration, which is something I was not able to achieve with existing designs. It's is possible this will work with 1.75mm filament, but I have not tried it and it is unlikely that I will.

Though originally intended for dual extrusion, the final design could theoretically allow for up to 4 print heads at the same time. I don't currently have the hardware to try this out, so I hope someone else will!

Step 1: You Will Need

All the printed parts are available on Thingiverse here.

Printed parts:

2x X-carriage

2x hotend holders

2x hotend blocks

Non printed parts:

2x MKV J-head Hotends (but any other with the same footprint should work)

4x LM8UU bearings (for X carriage)

2x 10mm push fittings (to connect PTFE tube to hotend block)

PTFE Tubing


16x M3 nuts

8x 35mm M3 bolts (to hold X Carriage assembly together)

4x (at least) 20mm M4 bolts (to secure hotend in block)

You will also need a Bowden drive system of your choice (and associated parts); I went with Rich's Bowden Geared Assembly (you won't need the cold end parts)

Step 2: Print the Parts!

Print out all the pieces needed: 2 X-carriage halves, 2 hotend holders, 2 hotend blocks, plus whatever you need to build your Bowden drive.

You will want the infill of the parts to be quite high, since they will need to take some stress. I recommend 100% infill for the holders, since they are relatively thin.

Step 3: Assemble the X Carriage

Remove any existing X carriage you have on the X axis. You will need to have 2 LM8UU bearings on the top bar, and 2 on the bottom bar.

Take the holders and insert 20mm M3 bolts into the holes on the top and bottom of it. Secure them with M3 nuts. These will facilitate height adjustments for the individual nozzles, and locking them in place.

The X carriages will line up with the bearings on the bars. I found the easiest way to put this together was to hold one half to the axis in place, then secure the belt to hold it in place temporarily. You can then take the other half and close the carriage.

To complete the mounting, line up the backs of the holders with a matching set of holes on the X carriage. If you are going to mount the nozzles behind each other (as has been previously pictured) do this at the same time, and secure both sides with 35mm M3 bolts and nuts.

Step 4: Fit Hotends Into the Blocks

Take just the PEEK body of the nozzle and push it all the way into the larger hole on the hotend holder. This was very tight, and I found using a vice to press it in very helpful. Secure the block with 20mm M4 screws (the holes are tight, so I did not find a longer bolt and nut necessary).

Then, take a push fitting and screw it into the smaller hole on the hotend block. Again, mine were quite tight, so I had to use a vice and pliers, but it has given a solid fit.

Now is a good time to run some filament though the assembly and make sure it's straight.

Step 5: Bringing It Together

You can attach the prepared nozzles to the PEEK body at this point. Their alignment doesn't matter, since you will measure the offset later.

To affix the hotends, simply slot the blocks into the holders with the screw heads facing outwards, and secure with the top screws on the holders. To start, leave the bottom screws completely retracted to see how level the nozzles are.

Next, assemble the Bowden drive system as necessary. If you use the system I mentioned earlier, you'll need to put it together in a similar fashion to a standard Greg's Wade Extruder. (As an aside, when it came to join a push fitting to the Bowden extruder, I held the fitting in line with the hole it needed to connect to and filled the surrounding space with epoxy resin). Finally, connect the heaters, thermistors and motors to the appropriate parts on your controller and connect the PTFE tube to the drive system, but not the hotend just yet.

Step 6: Wiring and Configuration

I use a Marlin - RAMPS 1.4 setup, so I'll briefly describe the wiring for it and software configuration. Other setups should not be too dissimilar.

I take the front extruder to be number 1. The resistor for the first extruder connects to D10, and the second to D9. The first extruders thermistor connects to T0, and the second to T2 (not T1, which is where the hot bed thermistor goes). Don't forget to connect both motors as well. Extruder 1 to E0 and Extruder 2 to E1. Make sure the power is off before doing this! That's it for the hardware.

In Marlin a few settings need changing in Configuration.h as follows (remember this is specific to RAMPS 1.4/1.3):

#define MOTHERBOARD 34 <-- change to 34
#define EXTRUDERS 2 <-- change to 2
#define TEMP_SENSOR_0 71
#define TEMP_SENSOR_1 1 <-- change to a non-zero value, depending on your thermistor
#define TEMP_SENSOR_2 0
#define INVERT_E0_DIR true
#define INVERT_E1_DIR false
//You may or may not need to flip one or both of these. You'll find out when you test things.

You will need to measure the precise offset between nozzle orifices and enter them into the firmware. I'll describe the trial and error process I used in the next step.

// You'll find these values in the next step. You'll have to come back and enter them.
#define EXTRUDER_OFFSET_X {0.0, 1.50}
#define EXTRUDER_OFFSET_Y {0.0, 57.50}

Step 7: Calibration

First of all, we want to get the nozzles at the same height. (Against general consensus, this easier to do with the nozzles cool because the adjustments are so close.) Begin by levelling the print bed to one of the nozzles.(If one of the nozzles is harder to reach than the other, it may be easier later to make is slightly lower now, and level the bed to that nozzle).

With that done, adjust the other nozzle until they are at the same height. I like to use a piece of paper to gauge this.

We now need to find the exact offset between the nozzles. To begin, a good method to try is to home one nozzle to an arbitrary point on the bed, mark it, and use M114 to get its coordinates. Then home the second extruder to the exact spot, get the coordinates and calculate the offset. (The same process is more thoroughly defined here, though in the context of the setup of Marlins ABL feature.)

Power everything on and heat up the printer. Before tightening the tensioners, feed the filament through the driver and tube so that around 30mm is visible on the other end. When the hotend is ready, carefully feed it though until you see plastic coming out and the tube lock in the fitting. Be careful not to put pressure on the block as you do this (I use a hand to support the holder, but you still may want to check your Z calibration again). Also check that both extruders extrude in the right direction, otherwise you will need to flip the variable mentioned in the last step.

Now for a calibration print!

Instead of copying the full instruction set here, I refer you to the calibration object and accompanying instructions user doctek has posted on this thing here:

There's a good chance that the print will not be perfectly aligned in the beginning. My approach was to print 2-3 layers, then stop and measure the gap/overlap, reflash the firmware with adjusted values and try again. In the end, you'll get the numbers right:

Step 8: Complete!

Hopefully, you have a working dual extruder by now. It took me A LOT of testing to get the parameters right for consistent prints, but now that I have the results are starting to improve. I have tried a couple of slicers, but personally I'm finding Slic3r to be slightly better for multi prints. Printing with a skirt as tall as the object, and infrequent, fast and compensated retraction goes a long way to removing ooze from the final print, but it is still the main problem I'm seeing with this. (Test print in picture is The idle extruder has has a tendency to print a "ghost" of the active extruders part (as can be seen in the timelapse video). Getting the settings right to eliminate this is currently my main aim with the extruder.

As I mentioned before, if you have the hardware you could add another 2 extruders to this setup and have a quad machine!

I'd be very interested to hear of other peoples results and extensions on this, so please let me know how it goes, and post some pictures/videos! Similarly, if you encounter any problems while building this do let me know as well so that I can also try to improve the design/instructions.

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