CNC Course (11) Boring an 8 mm Circular Magnet Pocket With a .25″ End Mill
Video
Overview
In this CNC Course lesson, we cut a precise 8mm circular magnet pocket using a ¼-inch end mill. This method is perfect when your tool is smaller than the final hole diameter. By combining incremental positioning (G91) with circular interpolation, you can create clean, accurate pockets for magnets, inlays, jigs, and fixtures.
This is a foundational CNC technique you’ll use constantly in woodworking and machining projects.
What You’ll Learn
- How to bore a hole larger than your tool diameter
- How G91 incremental mode simplifies offsetting
- How to use G2/G3 for circular interpolation
- Why a spring pass improves accuracy and finish
- How to cut a clean, repeatable 8 mm pocket with a 0.25″ end mill
Key G-Code Concepts
- G91 — Incremental positioning
- G1 — Linear moves for stepping outward
- G2/G3 — Circular interpolation for the pocket
- Spring pass — A repeat finishing pass to clean up the wall
Project Notes
- Using a smaller tool gives you flexibility and cleaner results.
- Incremental mode makes it easy to step outward without recalculating absolute coordinates.
- A spring pass is especially helpful in wood or softer materials where deflection can occur.
- This technique works on CNC routers, mills, and hobby machines running GRBL or similar controllers.
Tools & Materials
- CNC router or CNC mill
- ¼-inch end mill
- Workpiece (wood, plastic, or aluminum)
- CAM software or manual G-code entry
G-Code Snippet (Simplified Example)
(Paraphrased for clarity — not a verbatim copy from the video)
Code
G91 ; incremental mode
G1 Z-3 F200 ; plunge to depth
G1 X0.25 ; step outward
G2 I-0.25 ; circular interpolation
G2 I-0.25 ; spring pass
Watch the Lesson
This short video demonstrates the exact sequence of moves and shows how cleanly the 8 mm pocket comes out using this method. It’s a quick but essential CNC skill-builder.