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Universal G-Code Sender — Machine Week

This is a transcript of my conversation with Claude while setting up Universal G-Code Sender (UGS) to run our cookie decorator pen plotter for machine week.

Installing UGS on Mac

I downloaded UGS version 2.1.22 (MacOSX x64 with bundled Java) from the UGS website.

UGS download page

To open the .dmg: - Double-click the .dmg to mount it - Drag Universal Gcode Sender into the Applications alias - On first launch, Mac blocked it: "can't be opened because Apple cannot check it for malicious software" - Fix: System Settings → Privacy & Security → Open Anyway - A second prompt appeared asking Java to control the computer via accessibility — clicked Deny (not needed for G-code)

How UGS Works With the Machine

The flow: 1. Arduino Uno — has the xycore_grbl firmware uploaded (stays there permanently) 2. UGS — connects to the Arduino via USB and streams G-code commands in real time 3. UGS does not create Arduino code — it sends G-code line by line to the firmware already on the board

To send a G-code file in UGS: 1. Connect UGS to the Arduino (select port + baud 115200 + Connect) 2. Confirm Grbl 1.1h ['$' for help] appears in the console 3. Click the folder icon to load your .gcode file 4. Click Play to start sending it to the machine

Our Machine Hardware

Our machine is a CoreXY pen plotter (cookie decorator) built with: - Arduino Uno - CNC Shield v3 - A4988 stepper motor drivers

Note: DRV8825 and A4988 drivers look interchangeable but should not be mixed — their pinout diagrams are reversed. We burned out a DRV8825 finding this out. We switched to all A4988 drivers on the advice of Global Open Time instructors.

CNC Shield with A4988 drivers

Firmware: xycore_grbl

The firmware (xycore_grbl) is a GRBL-compatible G-code interpreter written for our machine. It was originally written by the team and used on a PC. It works on Mac via UGS with no changes.

Pin definitions (CNC Shield layout): 

X_STEP_PIN   2
X_DIR_PIN    5
Y_STEP_PIN   3
Y_DIR_PIN    6
ENABLE_PIN   8

Default config: 

stepsPerMmX = 80.0   // tune with $100=value
stepsPerMmY = 80.0   // tune with $101=value
feedRate    = 1000.0 // mm/min
rapidRate   = 3000.0 // mm/min for G0

Problem: Servo Pen Lift Not Working With G-Code Files

Our machine uses a servo to lift the pen instead of a Z-axis stepper. This worked when manually typing commands (servo 90, servo 0), but when sending a G-code file, Z commands were ignored because the firmware didn't handle them.

Solution: Modify the firmware to intercept Z moves and translate them to servo angles: - G0 Z1 (any Z > 0) → servo 90° → pen up - G0 Z0 (any Z ≤ 0) → servo 0° → pen down

Additions to the firmware (marked with Dorian addition comments):

At the top (includes and pin definition): 

// start Dorian addition
#include <Servo.h>
#define SERVO_PIN 11
Servo penServo;
// end Dorian addition

In setup(): 

// start Dorian addition
penServo.attach(SERVO_PIN);
penServo.write(90);  // start with pen up
// end Dorian addition

In handleLine(), after extracting X, Y, F word values: 

// start Dorian addition
float zVal = extractWord(line, 'Z', 999);
if (zVal != 999) {
  penServo.write(zVal > 0 ? 90 : 0);
  delayMicroseconds(300000);  // give servo time to move
}
// end Dorian addition

Important: declare zVal only once, after the other word extractions. Declaring it twice in the same function scope causes a compile error.

Servo Wiring on CNC Shield

On the CNC Shield v3: - SpnDir = pin 11 - SpnEn = pin 12

Either pin works for a servo since both output PWM. Our servo is wired to pin 11 (SpnDir), so #define SERVO_PIN 11 is correct.

Connect the servo signal wire to the SpnDir pin. Power the servo separately using the shield's 5V and GND pins — do not power it from the signal pin.

Do not plug the servo into the Z motor driver area — that is designed for a stepper motor, not a servo.

SVG to G-Code

We are plotting the Fab Academy logo, which has 3 colors (red, green, blue). Each color was exported from CorelDraw 2026 as a separate SVG file: - logo-red.svg - logo-green.svg - logo-blue.svg

Each SVG is converted to its own G-code file and run separately with a pen swap between runs.

Converting SVG to G-Code with jscut.org

  1. Go to jscut.org
  2. Click Open SVG and load the SVG file
  3. Click the shape in the preview to select it
  4. Click Create Operation → set type to Pocket (fills the shape with parallel lines)
  5. Settings for a 1mm pen:
    • Tool Units: mm
    • Diameter: 1
    • Step Over: 0.9 (90% overlap = full coverage with no gaps)
    • Rapid: 1000 mm/min
    • Cut: 500 mm/min
    • Pass Depth: 1
    • Material Clearance: 5 (pen up height)
    • Plunge: same as Cut speed (servo moves pen, not a motor — not critical)
  6. Click Simulate GCODE to preview
  7. Click Save GCODE to download

Running the Files

Run one G-code file at a time in UGS, swapping pens between colors: 1. Load and run logo-red.gcode → draw all red 2. Swap to green pen → load and run logo-green.gcode 3. Swap to blue pen → load and run logo-blue.gcode

Important: do not move the cookie/paper between runs — the colors will only line up if the surface stays in place.

G-Code vs 3D Printer Files

G-code is the same language used by 3D printers, which is why Bambu Studio opened the .gcode file. However the commands are different: - 3D printer G-code — includes temperature, extrusion, layer height - CNC/pen plotter G-code — only XY moves, Z up/down, feed rates

STL, OBJ, and 3MF are 3D geometry files, not G-code. Bambu converts those to G-code internally when slicing. UGS only accepts plain G-code files.

Previewing G-Code Before Running

To simulate the toolpath before sending to the machine, upload the .gcode file to ncviewer.com — it shows a visual preview of every move the machine will make.

See full word-for-word transcript: universal-G-Code-transcript.md

This document will be updated as work continues.