How to Program Fillet in CNC: Syntax and Example

To program a fillet in CNC, use the G02 or G03 commands for circular interpolation with the radius specified by R. This creates a smooth rounded corner between two lines by defining the arc's endpoint and radius.

Syntax

The fillet in CNC is programmed using circular interpolation commands:

• G02 for clockwise arcs
• G03 for counterclockwise arcs

The key parts are:

• X and Y: coordinates of the arc end point
• R: radius of the fillet (arc)
• F: feed rate (optional, controls speed)

Example syntax:

G02 Xx Yy Rr Ff

This moves the tool in a circular arc from the current position to (Xx, Yy) with radius r.

gcode

G02 X10 Y10 R5 F100

Example

This example shows how to program a fillet with a radius of 5 units between two straight lines:

gcode

G00 X0 Y0
G01 X10 Y0 F150
G02 X10 Y10 R5 F150
G01 X0 Y10 F150

Output

Moves from (0,0) to (10,0) in a straight line, then arcs clockwise with radius 5 to (10,10), then moves straight to (0,10).

Common Pitfalls

Common mistakes when programming fillets include:

• Using R radius with the wrong sign or value, causing the arc to be too large or small.
• Confusing G02 (clockwise) and G03 (counterclockwise), which changes arc direction.
• Not setting the correct start point before the arc command, leading to unexpected tool paths.
• Omitting feed rate F, which may cause the machine to use a default speed that is too fast or slow.

Example of wrong and right usage:

// Wrong: radius too large
G02 X10 Y10 R20

// Right: correct radius
G02 X10 Y10 R5

Quick Reference

Command	Description	Example
G02	Clockwise arc (fillet)	G02 X10 Y10 R5 F100
G03	Counterclockwise arc (fillet)	G03 X10 Y10 R5 F100
X, Y	Arc end coordinates	X10 Y10
R	Radius of fillet arc	R5
F	Feed rate (speed)	F150

Key Takeaways

Use G02 or G03 with R to program fillet arcs in CNC.

Set correct arc end coordinates (X, Y) and radius (R) for smooth corners.

Choose G02 for clockwise and G03 for counterclockwise arcs.

Always verify start position before arc commands to avoid errors.

Include feed rate (F) to control machining speed during fillet.