Building a RatRig V-Core 4: CoreXY Kit Assembly, Wiring, and Klipper Commissioning

We've covered the Voron line extensively on this site — the 0.2, the 2.4, and the Trident — but Voron isn't the only open-design CoreXY kit worth building. RatRig's V-Core 4 is the other major player in this space: a purpose-engineered, actively developed CoreXY platform with its own community, its own take on gantry design, and print speeds that push well past what most Voron builds target out of the box. If you want a large-format CoreXY machine and you're choosing between the two ecosystems, this build walks through what the V-Core 4 actually involves.

What You're Building

The V-Core 4 ships as a kit — extrusion, printed and machined parts, electronics, and hardware — not a finished printer. RatRig sells it in multiple bed sizes (300, 400, and 500mm) and several drive configurations including single-extruder, IDEX (independent dual extrusion), and a hybrid variant. This guide covers the 400mm single-extruder build, the most common starting configuration, but the assembly sequence is the same across sizes.

SpecValue Build volume (400 variant)400 × 400 × 300mm KinematicsCoreXY ControllerKlipper Max hotend temp300°C Rated print speedUp to 700mm/s (frame and motion system capable; actual sustained speed depends on tuning, cooling, and hotend) MaterialsPLA, PETG, ABS, ASA, TPU, PA, PC

Frame Assembly

Start with the extrusion frame — RatRig's instructions specify torque values for the corner brackets and gusset plates, and it's worth actually using a torque driver rather than eyeballing "snug" the way you might on a smaller printer. A slightly out-of-square frame at this stage compounds into gantry racking and Z-axis binding later, and it's far easier to fix before you've mounted rails and belts than after. Check diagonal measurements corner-to-corner as you go; they should match within a millimeter or two on a 400mm frame.

Mount the linear rails to the frame next, following RatRig's specified rail spacing exactly — this is a kit designed around specific tolerances, and rails mounted even slightly off-spec will bind or introduce play in the CoreXY motion system that's difficult to diagnose later.

CoreXY Gantry and Belt Path

The CoreXY motion system is the heart of this build and the part most different from a bed-slinger or standard three-axis printer, but it should already be familiar if you've built a Voron — the belt routing logic is the same family of kinematics, just RatRig's own gantry geometry and toolhead carriage. Route both belts through the full path specified in the manual, get tension even and symmetric on both sides, and confirm smooth gantry motion by hand across the entire XY range before wiring anything electrically. A belt that's tensioned unevenly between the two loops will show up as diagonal ("staircase") artifacts in prints rather than clean vertical and horizontal lines — worth catching now rather than after your first print.

Bed and Z-Axis

The V-Core 4's bed mount uses a kinematic three-point or four-point leveling system depending on your kit configuration — set this up per RatRig's documentation and expect to fine-tune it further once Klipper's bed mesh calibration is running. Z-axis lead screws need to be installed plumb and synchronized (if your configuration uses dual Z motors) before you power on; unsynchronized dual-Z is one of the more common sources of a racked, uneven first layer on this style of machine.

Wiring and Electronics

RatRig kits typically pair with a Klipper-compatible mainboard (an SKR-family board or RatRig's own electronics depending on what you ordered) plus a Raspberry Pi or equivalent SBC running Klipper's host software. Route wiring through the frame's cable channels rather than letting cables hang loose near the belt path — the same discipline covered in our Voron build guides applies directly here. Double-check thermistor and heater wiring polarity and connections before first power-on; a loose hotend thermistor connection is a common cause of thermal runaway faults on a freshly wired machine.

Klipper Commissioning

With frame, motion, and electronics assembled, flash Klipper firmware to your mainboard and set up your printer.cfg starting from RatRig's published configuration for your specific kit variant rather than a generic template — the pin mappings, stepper current settings, and kinematic definitions are specific to this frame and drive configuration. Run through first-boot checks in this order: confirm each stepper moves the correct axis in the correct direction, verify endstops trigger correctly, run PID_CALIBRATE for both hotend and bed, then run Klipper's BED_MESH_CALIBRATE and QUAD_GANTRY_LEVEL (or equivalent gantry leveling routine for your bed mount type) before attempting a first print. Our Klipper quad gantry leveling guide and Klipper macros guide both apply directly once you're at this stage, since the underlying firmware is identical to any other Klipper machine.

Input Shaper and Speed Tuning

The V-Core 4's frame and motion system are built to handle far more speed and acceleration than a stock Ender or Bambu-class printer, but you won't safely access that headroom without running Klipper's input shaper calibration using an accelerometer (an ADXL345 mounted to the toolhead is the standard approach). Skipping this step and simply cranking acceleration values in your config will produce ringing and ghosting artifacts that get worse, not better, as you push the frame's mechanical capability. Follow the same calibration workflow from our input shaper and pressure advance guide.

Safety Notes

This is a kit build involving mains-voltage wiring to the bed heater and power supply on larger configurations — if you're not confident with mains wiring, have someone experienced check your power supply and heated bed connections before first power-on. Keep a fire extinguisher rated for electrical fires in the room during initial commissioning runs, and don't leave a freshly built printer completely unattended for its first several prints while you're still confirming thermal runaway protection and general behavior are working correctly.

Parts List

• RatRig V-Core 4 kit (frame, motion system, and printed parts)
• Raspberry Pi 4 or equivalent SBC for running Klipper host software
• ADXL345 accelerometer module for input shaper calibration
• Torque driver set for extrusion frame assembly
• Digital calipers for belt tension and frame squaring checks
• Metric hex key set
• microSD card for the SBC running Klipper/Mainsail or Fluidd
• Isopropyl alcohol for bed surface cleaning before first prints

Building a V-Core 4 is a multi-day project even for someone who's assembled a Voron before — budget real time for the frame, wiring, and especially the Klipper tuning phase, since rushing the input shaper and bed mesh steps is how a mechanically excellent frame ends up printing no better than a budget machine. Done properly, though, it's one of the more capable open-hardware CoreXY platforms available, and the active RatRig community is a genuine resource when you hit a kit-specific snag the general Klipper documentation doesn't cover.