Prusa Mini - Bondtech LGX Adapter

Get the benefits of the Bondtech LGX extruder on the Prusa Mini without having to do a direct drive mod. What happens when you throw a $100 extruder on a $400 printer?

Features

• Has a replica of the cable routing feature on the stock extruder so you can secure the umbilical cable away from the extruder.
• Has a cutout for the Bondtech logo so you don't have to remove it from the LGX.
• Screw lengths were carefully measure on the LGX so they don't bottom out in the extruder motor. Selected screw lengths are widely available.
• Prints with no supports

CAD Model

OnShape Cad Model

The CAD model is original but it uses imports of the Prusa parts to extract critical dimensions. As the Prusa Mini is under GLP 3.0, so too is this work.

Printing

Standard 0.4mm QUALITY settings (3 perimeters and 15% infill). PETG filament is recommended.

Parts Required

Assembly

1. Orient the extruder and the adapter so the extruder can be attached with 4 screws and the adaper can be attached to the printer with 2 screws.
2. Start by inserting a 1 x 16mm screw into the hole that is behind the adjustment arm on the LGX. This will be the lower hole when the adapter is attached to the printer. You cant get the screw past there after the extruder is attached.
3. Mount the extruder to the adapter with 3 x 40mm screws and 1 x 25mm screw.
4. Insert the other 16mm SHCS into the adapter and install onto the Z axis carriage. You may need a ball driver to access the lower of the two mounting screws.
5. Connect the cable to the LGX motor and Mini Buddy board. Pass the cable through the umbilical sleeve as in the stock configuration.
6. Attach the umbilical with 3 zip ties through the provided slots. Make sure the zip ties are flush and not protruding out towards the extruder.
7. Install the bowden tube between the LGX and the hot end. Re-attach the filament sensor to the LGX if using.
8. Use additional zip ties as you see fit to secure the umbilical and bowden tube.

Wiring

The stepper motor needs to be wired up. The LGX doesn't come with a cable. I strongly recommend buying a pre-wired cable so you can easily revert the mod.

(TK: This section could use some reference links/images for the LGX pinout and the Mini Buddy pinout)

Firmware Adjustments

Set E-Steps to 400

The E-steps must to be updated to 400, this can be done in the 'experimental’ menu. Go to LCD Menu, enter option Settings, scroll down to highlight HW Setup then long-press the knob. A new “Experimental” submenu will appear on the screen. From there select Extruder step and set the value to 400.

Z Axis Length

If you run the XYZ axis self test it will fail the Z axis. The stock extruder takes up slightly more room than this one. Change the Z-axis length to 190mm and run the XYZ self test to verify correct operation.

Z Motor Reversing

In the same experimental menu there is an option to reverse the extruder motor if need be (depends how your cable is wired). Test moving the extruder motor to see if this is required.

Other Considerations

Motor Current

Bondtech recommends a motor current between 450mA and 650mA. The Prusa Mini defaults its extruder motor current to 400mA (at least that's the best info that I can find, please let me know if you know better).

So the good news is the mini wont fry your motor by default and so far 400mA is working fine in my (limited) testing. If you find that you need to, you can boost the current for printing from Prusa Slicer in the printer's start G-Code:

; Use more current on the Bondtech LGX
M907 E500

There isn't currently a way to change this permanently. The M500 gcode was disabled on the Mini, building custom firmware is a pain due to signing requirements and motor current isn't an available setting in the experimental menu. Maybe Prusa will help us out here ?

Cable Routing

The cable routing is slightly different because now the umbilical cable is forward of the bowden tube instead of aft. I haven't found this to be a problem in my testing.