CNC Crash Course #4 – G90 Absolute Coordinates
🎥 Video
📝 Overview
This lesson introduces G90 Absolute Positioning, the coordinate mode used in nearly all CAM‑generated CNC programs. In absolute mode, every movement command references a fixed zero point—usually the workpiece zero you set at the start of a job. This makes toolpaths predictable, repeatable, and easy to read.
The video demonstrates how G90 behaves inside Candle (GRBL Control) and contrasts it with the incremental G91 mode from the previous lesson. Understanding this difference is essential for reading, writing, and troubleshooting G‑code.
📌 What You’ll Learn
- What G90 Absolute Mode means
- How absolute moves differ from incremental moves
- Why nearly all CAM software outputs G90
- How GRBL interprets absolute coordinates
- How to test G90 safely inside Candle
- How switching between G90 and G91 affects machine motion
📂 Tools & Software Used
- 3018‑style CNC router
- Candle (GRBL Control)
- GRBL firmware
📘 Description (from the video’s published text)
After using Incremental mode (G91), this lesson shifts to Absolute positioning with G90. In this mode, every command references the absolute zero point set on the machine, so movements are based on fixed coordinates rather than the last position. The video revisits the earlier example—this time issuing commands according to an established coordinate system—and encourages viewers to identify the positioning commands before they’re explained.
💡 Key Takeaways
- G90 interprets all coordinates from a fixed zero point.
- G90 and G91 are modal, meaning they stay active until changed.
- Absolute mode is safer and more predictable for most machining.
- CAM software almost always outputs G90 for clarity and consistency.
- Knowing both modes helps you troubleshoot unexpected machine motion.