Filament Cutter for RatRig V-Core 3.1 Toolhead (Orbiter V2)

Inspired by the Pico MMU filament cutters, designed by RPi.

This cutter is designed to work on the RatRig Toolhead, without having to reprint any existing parts.

Update 2026/09/13 (Rev 6)

I have made further updates to the cutter to improve reliability.  The most significant changes are fitting aluminum spacers and moving the pin slot in the bracket to the center to keep it from getting mangled due to off-center pressure.  The actuator block was also revised to allow for finer adjustment.

With this revision, I've had well over 1000 consecutive successful filament changes, so I'm pretty satisfied with this design.

Update 2026/07/19 (Rev 5)

I have made some updates to the cutter in order to improve reliability.  The changes allow the cutting edge to sweep wider, preventing it from getting in the way of the filament path when not cutting, and potentially allowing it to move further past the filament path, avoiding incomplete cuts.

I also changed to using two lighter springs, which can compress more (allowing the blade to cut deeper).

Note: One of the biggest causes for failures with this cutter design is if the blade does not fully retract after cutting.  After installing the cutter, make sure that the lever pops all the way back out after pushing down on it.  If it doesn't, you need to fiddle with the assembly until it does or you will end up with failed prints.

Additional Hardware

• 2x M3x12mm screw (cap head or user preference)

• 2x M2x12mm screw + lock nuts (alternately, nylon screws and nuts, to be glued)

• 1x M3x6mm screw (optional, flat head, length doesn't need to be exactly 6mm)

• 2x Compression spring, 0.625" length, 0.120" OD, 0.084" ID. McMaster-Carr P/N 9657K637

• PTFE tubing.

• A 9 mm box cutter snap-off blade (preferably with a handle, making it safer to load).  I used this.

Additional Hardware - Rev 6

• 1x M5x16mm screw (flat head)

• 1x M4x30mm screw (I use a cap head, but it's your choice; longer is fine)

• 1x M4 heat-set insert (up to 10mm length)

• 1x M3x6mm set screw (length can vary somewhat)

• 1x M3 heat-set insert (up to 4mm length)

• 2x M3x6mm aluminum unthreaded spacer, 4.5mm OD (McMaster-Carr P/N 94669A101

Additional Hardware - Rev 5 and Earlier

• 2x M3 washers (optional, can be used to reduce pressure of the extruder on the cutting bracket)

• 1x M5x16mm screw (cap head)

• 1x M5 washer

Recommended Print Settings

The same as the Pico MMU printed parts, or (abbreviated, just use the Pico MMU 3mf file).

Use ASA, though you can probably get away with PETG if you never print in an enclosure.

• 0.2 mm layer height

• 0.45 mm line width (first layer)

• 0.4 mm line width (inner wall, sparse infill)

• 97% infill, rectilinear

• 2 top and bottom layers

• Organic supports on build plate only

The above is for a 0.4 mm nozzle.

Post Processing

If needed for smooth movement, use sandpaper to smooth the bottom of the bracket.  I have used 320 grit followed by 600 grit, but most recently used only what was needed to remove elephant's foot effects on the bracket. You may also want to use the sandpaper on the bottom extruder support piece, particularly if the bracket still doesn't slide well.

Carefully fit the blade into the bracket.  While the extruder support is designed to allow replacing the blade without removing the extruder or extruder support, the slot for the blade is going to be a bit tight immediately after printing.  See below for more.

Cutter Assembly

1. Remove the parts from the top of the toolhead (umbilical and extruder)

2. Place the umbilical support on the fan grill.  It should fit in the impression in the grill.

3. Secure the umbilical onto the support with the original screws.  They should be long enough.

4. (Optional, but strongly recommended) Replace the original cap-head screw for the stand-off to the extruder with a flat-head screw.  This makes it possible to remove the umbilical and extruder in one piece when needed.  You may need to make the hole a little larger and/or provide a countersink.  You can use a drill bit for this, but be very careful. It's probably best to do this by rotating the bit by hand.

5. Secure the extruder stand-off to the umbilical with either the flat head screw or the original cap head screw.

6. With the extruder upside down, fit the M3 screws through the mounting holes.

7. (Optional for Rev 5 and earlier, skip for Rev 6+) Put the washers on the M3 screws, the underside of the extruder.  I did this to reduce the pressure on the cutter bracket, allowing it to slide more easily, but you may find it unnecessary.

8. PTFE tubing should fit in the extruder and extend enough to fit in the extruder support upper piece without bending it when pushed firmly against the extruder.  Proper clearance is essential for reliable function of the cutter.

9. Assemble the cutter.  Insert the two springs into their slots in the lower support piece and carefully add the blade bracket with the blade already inserted.  Put the upper support piece on top (this may require a bit of finesse to keep the springs and bracket inside).  Put the arm over the two pivot holes and use the M2 screws and nuts to attach it.  These should not be tight, but the nuts should be secure either because they are lock nuts or because you glued them (lock nuts are obviously preferable since they can be removed).

10. (Rev 6+) Insert the aluminum spacers into the two holes in the cutter assembly.

11. Use the lower extruder support to get the PTFE tubing leading to the hot end cut to the right length. As in the case with the upper piece, the tubing should stick out just a little bit - enough to fit into the support without requiring deforming the tubing or the support to mate the support to the toolhead plate.

12. Fit the cutter assembly onto the extruder mount screws.

13. Flip the extruder and cutter assembly over and screw it into the toolhead plate.

14. Make sure that the bracket moves easily by pushing on it and releasing it to verify that the spring pushes it all the way back out.  If it does not, you need to further trim the PTFE tubing leading in and out of the cutter assembly as that (as of Rev 6) is the only remaining source of pressure on the blade.

15. Reattach the extruder to the stand-off on the umbilical.

Actuator Assembly

WARNING

You are about to make changes to your printer configuration that could have unpleasant results. It is strongly recommended that you keep an emergency stop button within reach for these steps.

First, find your real X limit.  Do this by 

1. Editing your printer.cfg file and changing the position_max value in the stepper_x section to 20 mm more than it currently is.

2. Save printer.cfg and reboot.

3. Re-home using G28 or the UI button.  This shouldn't move the print head beyond existing boundaries, but it's a good idea to keep the e-stop handy anyway.

4. Move to the original X limit using the UI, or using G0 X400 (where 400 is your original limit.  Maybe yours was 300 or 500).

5. Stepping 1 mm at a time in the positive X direction, figure out the actual hardware limit.  In my case, it ended up being 410 mm.

6. Update your printer.cfg file, changing the stepper_x position_max to the new found value.

7. Save printer.cfg and reboot again.

Next, install the actuator block.

1. (Rev 6+) Add the M4 heat-set insert to the thicker side of the block, with the larger hole.  The opposite side of this is left empty to support the screw without putting any tension on it.

2. (Rev 6+) Add the M3 heat-set insert to the top of the mounting block.  This is used to hold the large M4 screw in place as you adjust it.

3. Remove the screw from the top of the right-hand (maximum X) joiner.  This is the one in the center of the joiner, not the ones that hold the idlers in place.

4. Place the actuator block on top of the joiner, and secure it with an M5 x 16mm screw and washer (washer for Rev 5-, not for Rev 6+).

5. (Rev 5-) The block should be pushed as far to the right as it will go.  It has a slot for adjustment but it's unlikely you'll need to push it to the left at all.

6. (Rev 6+) The actuation position can be adjusted through the M4 screw.  Once a good position is reached, use the M3 set screw to hold the M4 adjustment screw in place.

Add the tip forming code.

1. Paste the contents of tip_form.txt into the sp_mmu.cfg file, replacing the existing _SP_TIP_FORM macro.

2. Save and reboot.

3. There are three variables you'll likely adjust. They are: variable_clear_position_x, variable_clear_position_y and cut_position_x.

4. The variable_clear_position_y is up to you.  You can set it to a value that has meaning in your build, or you can set it to -99.  I chose to use 200, which is the middle of the build surface, because I have the most success at filament swaps at that position.

5. The variable_clear_position_x should be set to a position just beyond where the lever makes contact with the actuator block.

6. The cut_position_x setting will require experimentation on your part.  What you want is to find the X position where you reliably cut filament without going so far that it results in the belts skipping cogs on the steppers. I don't have a step-by-step process for this. My only suggestion is to start from the X clear position and start moving right until the filament starts successfully cutting.  Do test prints, make sure it doesn't layer shift, fine tune until you get something that works. This took me weeks to get right, unfortunately.