CNC Crash Course #3 – G91 Incremental Coordinates
🎥 Video
📝 Overview
This lesson explains G91 Incremental Positioning, one of the two fundamental CNC coordinate modes. Incremental mode tells the CNC to interpret every movement relative to its current position, not from a fixed origin. Understanding G91 is essential for reading and writing G‑code, especially when troubleshooting or creating simple manual programs.
The video demonstrates how G91 behaves inside Candle (GRBL Control) so beginners can see exactly how the machine responds to incremental moves.
📌 What You’ll Learn
- What G91 Incremental Mode means
- How incremental moves differ from absolute moves
- Why G91 is useful for repetitive patterns
- How GRBL behaves when switching between G90 and G91
- How to test incremental moves safely inside Candle
📂 Tools & Software Used
- 3018‑style CNC router
- Candle (GRBL Control)
- GRBL firmware
📘 Description (based on the video content and CNC references)
G91 sets the CNC into incremental positioning, meaning all coordinates are interpreted as distances from the machine’s current location. This mode is helpful for repetitive operations like drilling a series of holes or stepping through a pattern. The video shows how to activate G91, how the machine responds, and how to avoid common mistakes when switching between G90 and G91.
💡 Key Takeaways
- G91 moves the tool relative to its current position.
- G90 and G91 are modal — they stay active until changed.
- Incremental mode is powerful but can cause unexpected moves if you forget which mode you’re in.
- Testing G91 in Candle builds confidence before using it in real toolpaths.
- Most CAM programs output G90, but knowing G91 helps with manual edits and troubleshooting.