These codes set the working plane for circular interpolation (G02/G03) and cutter compensation. They tell the control which two axes an arc or offset lives in.
| Code | Plane | Typical use |
|---|---|---|
| G17 | XY | Standard milling (default) |
| G18 | XZ | Lathe work, vertical arcs |
| G19 | YZ | Side-axis arcs |
Most mill programs run in G17 and never change it. Switch planes only when an arc or compensated move lies in a different pair of axes.
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