How to Assemble the Extruder and Hot-End on Your Prusa i3 3D Printer - dummies

How to Assemble the Extruder and Hot-End on Your Prusa i3 3D Printer

By Kalani Kirk Hausman, Richard Horne

The final procedure for building your own Prusa i3 3D printer is to assemble the extruder and hot-end — in this case, a modern, compact, geared extruder with a J-head hot-end.


The process can be divided into five general steps, as follows:

  1. Fitting the filament drive to the motor shaft.

  2. Assembling the extruder idler pressure bearing.

  3. Fitting the J-head hot-end.

  4. Fitting the assembled extruder to the X carriage.

  5. Wiring up the Hot-end heater and thermistor to RAMPS.

How to fit the filament drive to the motor shaft

Fitting the filament drive to the motor shaft is pretty straightforward. Our stepper motor already has a gearbox fitted to it; this is a very compact and lightweight way to make a small, powerful extruder.

Other types of extruder assembly use printed gears to do a similar job, but a more popular option is to use an off-the-shelf gearbox-and-motor assembly to improve operational life of the printer and to increase print quality. So all you need to do is fit the drive wheel onto the motor gearbox shaft and tighten with an Allen wrench.

Another type is fitted to the geared motor. They all perform the same job.

You may have to mount a bracket or printed adapter as well, depending on the type of motor you’re using and where it was sourced.

How to assemble the extruder idler pressure bearing

The idler pressure bearing performs a very important job of firmly pushing the filament toward the drive wheel so the rotating motor can force the filament down into the hot-end. These general steps provide an overview:

  1. A 3D-printed lever is usually supplied for the idler; to this you fit a small 623-size bearing.

  2. The idler assembly is attached to the motor body, forming a lever.

  3. A spring is pushed in between the idler assembly and the mounting bracket to lever the idler bearing into the filament drive wheel.

  4. A small, 3D-printed guide is attached to the motor body with an M3 x 10mm bolt; this helps guide the filament into the gap between the drive wheel and the idler bearing.

How to fit the J-head hot-end

The J-head hot-end will most likely come as a ready-assembled unit. Fortunately, other compatible hot-ends such as the Pico from B3 Innovations, the Prusa V2 nozzle, or the V5 from E3D can also be used in exactly the same fitting. Here’s what to look for while fitting the J-head hot-end:

  • The J-head (or compatible) hot-end will have a groove mount at the top of the unit. Use this to clamp into a matching recession of the metal plate or 3D-printed adapter found on most extruder assemblies.

    Make sure you don’t trap any wires — and be careful with the fine thermistor wiring attached to the J-head body.

  • Some mounts will just slot in, others will require either M4 x 16mm bolts or M3 x 20mm bolts to lock them in place.

Fitting the assembled extruder to the X carriage

This is usually a simple task, requiring two M3 or M4 bolts to fit the extruder body onto the X carriage of your machine. For the Prusa i3 design, various mounts can be downloaded from github or from Thingiverse.

Wiring the extruder to RAMPS

The last part of our build is to wire up the extruder motor, hot-end heater, and thermistor sensor to our set of RAMPS electronics. Remember to check the RAMPS wiring guide.

Then follow these general steps:

  1. Wiring the Extruder motor is exactly the same procedure used for wiring up the X, Y, and Z motors.

  2. When the Extruder motor is wired up, fit it to Extruder E0 on the RAMPS board.

  3. Connect the thermistor. Its two-way header can go in either orientation, and connects to T0.

  4. Connect the hot-end heater can to the D10 screw-terminal connectors, in either orientation.

    Use ferrules on the wire ends if possible.

You can now calibrate your machine. When that’s done, you can perform your first 3D print.