G-code & toolpath basics

Companion notes for the dual-view visualizer: modal motion, arcs, feeds, time estimates, and what the risk flags mean.

Coordinates & modality

• G21 millimetres · G20 inches
• G90 absolute · G91 incremental
• G17 XY plane (arcs); G18/G19 supported only as chord approximations
• Motion mode is modal: after G1, further axis words keep feeding until another G0/G1/G2/G3

Motion words

G0  — rapid (non-cutting transit; time uses a high effective F)
G1  — linear feed at F (typically mm/min or in/min)
G2  — clockwise arc (XY): centre via I J (relative to start) or radius R
G3  — counter-clockwise arc
F   — feed rate (modal)
I J — arc centre offsets from start point (common CNC convention)
R   — arc radius (negative R selects the major arc)

The visualizer tessellates arcs into many short chords, colours by mode / feed / Z / time, and animates on a time-based timeline (scrubbable). Dual panes: 3D isometric + XY top.

Time estimate

t_segment ≈ length / (F_eff / 60)

Rapids use a high effective feed (≥ 3000 unit/min) when F is low. Real machines also accelerate and decelerate — wall-clock time is longer. No jerk model here.

Risk flags (heuristics)

• Rapid ending below Z = 0 (stock plane assumption)
• XY rapid while Z is negative (classic “crash while down”)
• Rapid plunge toward low Z
• Feed in stock shortly after a low rapid approach
• Extreme or tiny feed values; zero-length moves

These are not a CAM collision engine: no tool diameter, holder, clamps, or stock solid model.

SVG import

Paste d="M … L … Z" or a bare path. Strokes become feed moves at F800 for length/time demos.

Keyboard (visualizer)

Space  play/pause
R      rewind
F      fit camera
Ctrl+Enter  parse