Pi-Controlled Pen Plotter with GRBL

A pen plotter is a CNC machine that draws instead of cuts. An Arduino Uno running GRBL moves two stepper motors with smooth acceleration, a small servo lifts the pen, and the Pi uses vpype to optimize SVG paths and stream them over USB.

Mission steps

1. Build the frame

   Print or buy an XY gantry; belts and smooth rods give cleaner lines than threaded rods. Mount the pen holder on a hinge a servo can lift.

2. Flash GRBL

   Load GRBL onto the Uno R3 from the Arduino IDE, or a GRBL fork that drives a pen servo with M3/M5 if you skip a Z stepper.

3. Wire and set current

   Seat the A4988 drivers on the CNC shield, plug the steppers into X and Y, and set each driver's current limit with a multimeter.

4. Configure GRBL

   Set steps/mm ($100, $101), max rates and acceleration, then draw a test square and measure it.

5. Prepare the art

   On the Pi run pip install vpype vpype-gcode and convert an SVG with a profile that sets your pen-up and pen-down commands.

6. Stream and draw

   Run the streaming script with the .gcode file and watch the plotter draw.

Code

python3 -m venv ~/plot && source ~/plot/bin/activate
pip install vpype vpype-gcode pyserial
# 'myplotter' is a profile you define in ~/.vpype.toml with your pen up/down codes
vpype read drawing.svg linemerge linesort gwrite --profile myplotter drawing.gcode

# stream.py — send G-code to GRBL line by line, waiting for 'ok'
import serial, sys, time

port = serial.Serial('/dev/ttyUSB0', 115200)   # genuine Unos often show as /dev/ttyACM0
port.write(b"\r\n\r\n"); time.sleep(2); port.reset_input_buffer()

for raw in open(sys.argv[1]):
    line = raw.split(';')[0].strip()
    if not line:
        continue
    port.write((line + '\n').encode())
    reply = port.readline().decode().strip()
    if reply != 'ok':
        print(f"{line!r} -> {reply}")
print('Done')

Tip: test every script on the bench before installing it in a moving machine.

Questions