G02 and G03 Explained: CNC Circular Interpolation with Practical G-Code Examples

Meta Description: Learn how to program circular arcs using G02 and G03 in CNC. Includes real-world examples, IJK parameter breakdown, diagrams, and best practices for clockwise and counterclockwise arc movement.

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🔄 What Is Circular Interpolation?

Circular interpolation refers to tool motion along an arc. G02 and G03 are the core G-code commands used to machine circular features such as holes, fillets, radii, and pockets.

• G02: Clockwise (CW) arc
• G03: Counterclockwise (CCW) arc

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⚙️ G02 / G03 Format

G02 X__ Y__ I__ J__ F__;
G03 X__ Y__ I__ J__ F__;

• X, Y: End point of the arc
• I, J: Arc center offset from current position
• F: Feedrate

Tip: All values must be in the same plane (typically XY when G17 is active).

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📐 G02 Clockwise Arc Example

G17 G90 G21
G0 X0 Y0
G1 Z-2 F100
G02 X20 Y0 I10 J0 F200

This cuts a clockwise arc from (0,0) to (20,0) with a center at (10,0) — a 180° arc with radius 10 mm.

🔁 G03 Counterclockwise Arc Example

G17 G90 G21
G0 X0 Y0
G1 Z-2 F100
G03 X20 Y0 I10 J0 F200

This performs the same arc but in the counterclockwise direction.

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🎯 How I and J Work (Center Point Offsets)

Parameter	Description
I	Distance from current X to arc center X
J	Distance from current Y to arc center Y

💡 Arc Example Diagram

Start: (0, 0) → End: (20, 0) → Center: (10, 0)

So: I10 J0

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🔢 Using Radius Instead of IJK

Some controllers (like Fanuc, Haas) allow the use of R instead of I and J:

G02 X20 Y0 R10 F200

Note: Using R is simpler, but ambiguous when arc > 180°.

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🧰 Real CNC Milling Arc Sample

G90 G21 G17
G0 X0 Y0
G1 Z-2 F150
G03 X30 Y30 I0 J15 F300
G01 X60 Y30
G02 X90 Y0 I0 J-15 F300

This code machines a “rounded U-shape” by combining arcs and lines.

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⚠️ Common Mistakes

• Wrong I/J values causing the arc to loop
• Missing G17 (XY plane) declaration
• Incorrect feedrate for arc motion
• Using R on arcs > 180° can cause ambiguity

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📊 Arc Interpolation Comparison

Parameter	G02	G03
Direction	Clockwise	Counterclockwise
Use	Pockets, fillets	Contouring, chamfers
Common Format	I, J or R	I, J or R

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✅ Best Practices

• 🧪 Simulate the toolpath before running
• 🎯 Double-check your I and J values
• 🚫 Avoid negative feedrates
• 🛡 Use G17 to lock to XY plane before arc

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📚 Summary

G02 and G03 are essential G-codes for efficient and precise circular tool movement in CNC programming. Mastering both the IJK method and the R method will give you full control over arcs, contours, fillets, and pockets. Always test with simulation and maintain a clean toolpath logic for safe and accurate cuts.