G-Code for CNC Foam Cutting: Beginner's Complete Tutorial

If you own or plan to buy a CNC hot wire foam cutting machine, understanding G-code is essential. G-code is the programming language that tells your machine exactly where to move, how fast to move, and when to start or stop the cutting wire. Even if you use software that generates G-code automatically, knowing the basics helps you troubleshoot problems, optimize cuts, and customize your workflow.

This tutorial starts from absolute zero — no prior programming or CNC experience required. By the end, you'll understand every G-code command used in foam cutting and be able to write simple programs by hand.

What is G-Code?

G-code (also written as G code or gcode) is a standardized programming language used to control CNC (Computer Numerical Control) machines. It was developed in the 1950s and is used worldwide for CNC mills, lathes, 3D printers, laser cutters, and of course, hot wire foam cutters.

A G-code program is simply a text file containing a list of commands, executed one line at a time. Each line tells the machine to perform a specific action — move to a position, set a speed, turn on the wire, etc. Here's what a simple G-code program looks like:

G90           ; Use absolute coordinates
G28           ; Home all axes
M3            ; Turn on hot wire
G1 X100 F300  ; Move to X=100mm at 300mm/min
G1 Y50        ; Move to Y=50mm
G1 X0         ; Move back to X=0
G1 Y0         ; Move back to Y=0
M5            ; Turn off hot wire
G28           ; Return home

That's it — nine lines that tell the machine to cut a 100mm × 50mm rectangle. Let's break down every part.

Essential G-Code Commands for Foam Cutting

CNC foam cutters use a subset of the full G-code standard. Here are the commands you'll use 95% of the time:

Movement Commands

Command	Name	What It Does	Example
G0	Rapid Move	Moves to position at maximum speed (no cutting). Used for positioning before or after a cut.	G0 X50 Y30
G1	Linear Move	Moves in a straight line at a controlled feed rate. This is the primary cutting command.	G1 X100 Y50 F300
G2	Clockwise Arc	Moves in a clockwise arc. Uses I,J to define arc center offset or R for radius.	G2 X50 Y50 R25
G3	Counter-clockwise Arc	Moves in a counter-clockwise arc. Same parameters as G2.	G3 X50 Y50 I25 J0

Positioning Modes

Command	Name	What It Does
G90	Absolute Mode	Coordinates are relative to the origin (0,0). G1 X100 means "go to position X=100mm". This is the most common mode for foam cutting.
G91	Incremental Mode	Coordinates are relative to current position. G1 X100 means "move 100mm in the X direction from wherever you are now".

Machine Control Commands

Command	Name	What It Does
G28	Home	Returns all axes to their home (zero) position. Always use at the start and end of a program.
M3	Wire On	Turns on the hot wire heating. Must be called before making cuts.
M5	Wire Off	Turns off the hot wire. Always turn off when done cutting.
F	Feed Rate	Sets the cutting speed in mm/min. F300 = 300 millimeters per minute. Once set, it applies to all subsequent G1/G2/G3 moves.

Understanding Feed Rate: The Key to Quality Cuts

The feed rate (F value) is the single most important parameter you'll adjust. It controls how fast the wire moves through the foam, which directly affects both cutting time and edge quality.

Foam Type	Density	Recommended Feed Rate	Notes
Low-density EPS	10–15 kg/m³	400–600 mm/min	Very soft, cuts easily at high speed
Standard EPS (Thermocol)	15–25 kg/m³	200–400 mm/min	Most common material, medium speed
High-density EPS	25–35 kg/m³	150–250 mm/min	Slower for clean edges
XPS (Styrofoam)	30–40 kg/m³	100–250 mm/min	Denser, needs more melting time

4-Axis G-Code: Controlling Two Towers Independently

On a 4-axis CNC foam cutter, each tower has its own X and Y axis. The standard convention is:

• X, Y — Left tower position
• U, V (or A, B) — Right tower position

When both towers receive the same coordinates, the wire moves as one unit (2D profile cut). When they receive different coordinates, the wire tilts and creates 3D tapered shapes.

; Example: Tapered cut (circle → square)
; Left tower traces a circle, right tower traces a square

G90
G28
M3

; Left tower: circle path | Right tower: square path
G1 X50 Y0 U0 V0 F200
G1 X50 Y0 U100 V0
G3 X0 Y50 I0 J50 U100 V0      ; Left: arc, Right: straight
G1 X0 Y50 U100 V100
G3 X-50 Y0 I-50 J0 U100 V100  ; continues...
G1 X-50 Y0 U0 V100
G3 X0 Y-50 I0 J-50 U0 V100
G1 X0 Y-50 U0 V0

M5
G28

This capability is what makes 4-axis machines so powerful — and why they're the preferred choice for professional work.

The Complete Workflow: From CAD to Finished Cut

Here's the end-to-end process for going from an idea to a finished foam piece:

1. Design your shape in CAD software — Use any 2D/3D design tool: AutoCAD, SolidWorks, Inkscape (free), FreeCAD (free), or even CorelDRAW. Create the cutting profile as a 2D vector path.
2. Export the path — Export your design as DXF, SVG, or a similar vector format that preserves the cutting path coordinates.
3. Generate G-code using CAM software — Import the vector file into foam-cutting CAM software:
   • Jedicut — Free, designed specifically for 4-axis foam cutting
   • GMFC — Free G-code maker for foam cutting
   • FoamWorks — Specialized foam cutting software
   • DevFoam — Professional foam cutting CAM
4. Set parameters — Configure feed rate, wire temperature, and cutting dimensions in the CAM software.
5. Transfer G-code to machine — Send the generated G-code file to the CNC controller via USB.
6. Setup and calibrate — Place the thermocol block, home the machine, and verify wire tension and temperature.
7. Run the program — Execute the G-code and monitor the first cut. Adjust feed rate if needed.
8. Remove finished piece — The cut piece is complete with smooth, sealed edges — no post-processing required.

Common G-Code Mistakes and How to Fix Them

1. Forgetting to Set the Coordinate Mode

Always start your program with G90 (absolute mode) unless you specifically need incremental mode. If the mode isn't set, the machine may use whatever mode was active from the previous program, causing unpredictable movement.

2. Missing Feed Rate

If you use G1 without specifying F, the machine uses the last set feed rate — which might be from a previous program. Always set F explicitly for your first cutting move.

3. Not Homing Before Cutting

Always use G28 at the start of every program. If the machine doesn't home first, all coordinates will be offset by however far the wire is from the origin, ruining the cut.

4. Feed Rate Too High for Material

If the wire bends during cutting or the edges are rough and uneven, your feed rate is too high for the foam density. Reduce F by 25–50% and test again. Check our precision cutting guide for detailed tips.

5. Not Turning Off the Wire

Always end your program with M5 to turn off the hot wire. Leaving it on can overheat the wire, damage foam near the end position, and pose a safety risk.