Syntax
G3 [X] [Y] [Z] [I] [J] [R] [F]
Parameters
| Parameter | Description |
|---|
| X/Y | End point of the arc |
| I/J | Arc center offset from start in X and Y |
| R | Arc radius |
| F | Feed rate |
Examples
G3 X0 Y50 I0 J25 F600
↑ CCW quarter arc from (0,0) to (0,50), center at (0,25)
G3 X0 Y0 I0 J20 F800
↑ CCW full circle, center 20mm in Y
G3 X-30 Y30 R30 F500
↑ CCW arc using radius
Important Notes
- G3 is used for counter-clockwise arcs in the active plane (G17=XY, G18=XZ, G19=YZ).
- Climb milling (G3 on conventional mills) gives better surface finish on finishing passes.
- Helical arcs: add Z movement alongside I/J — creates a spiral ramp for safe pocket entry.
Common Mistakes
- ❌ Confusing G2/G3 direction — CW vs CCW is relative to viewing from positive Z axis (from above on vertical mills).
- ❌ Using G3 for full circles with R format — always use I/J for full circles with G2/G3.
Frequently Asked Questions
Q: Why is G3 preferred for finishing passes?
G3 (CCW) in conventional right-hand-spindle mills produces climb milling — the cutter engages material such that the chip is thicker at the start and thinner at exit. This reduces deflection, improves surface finish, and is the standard for finish passes.
Q: How do I make a helical entry with G3?
Add Z movement to a G3 arc command: G3 X0 Y0 I10 J0 Z-5 F400 — this creates a helical path descending 5mm over one full circle. Used for safe entry into pockets without plunging.
GCodex
← Open Viewer