Achieving a mirror-like finish in CNC machining requires more than just sharp tools; it demands precise control over how the tool enters and exits the material. Tool marks, or "witness marks," often occur during abrupt movements. Here are the essential G-Code command sequences to eliminate these imperfections.
1. The Power of Tangential Lead-In (Arc Entry)
Plunging a tool directly into the workpiece often leaves a visible "dent." Instead, use a circular move (G02/G03) to blend the tool into the path tangentially.
G0 X0 Y0 ; Rapid to start
G01 Z-5.0 F100 ; Feed to depth
G03 X10.0 Y10.0 R10.0 ; Tangential arc lead-in
G01 X50.0 ; Cutting motion
2. Implementing the "Overlap" Technique
To prevent a mark at the start/stop point of a closed contour, overlap your toolpath. By continuing the cut slightly past the entry point before retracting, you ensure a seamless finish.
3. Exit Strategies: Lead-Out and Ramp
Abruptly stopping and retracting a tool can leave a dwell mark. Always use a Tangential Lead-Out or a Ramp-Out (moving Z up while still moving in X or Y) to dissipate cutting pressure.
G01 X100.0 ; Finish the cut
G03 X110.0 Y10.0 R10.0 ; Tangential arc lead-out
G00 Z10.0 ; Safe retract
4. Controlling Feed Rates at Corners
Using G61 (Exact Stop Check) can cause the machine to pause at every junction, causing heat marks. For smoother finishes, use G64 (Continuous Mode) with a small tolerance to maintain constant velocity.
Summary Checklist for Smooth Finishes:
- Avoid G00 near the part: Always use G01 for final approaches.
- Use Arcs: Replace linear entries with G02/G03 arcs.
- Check Dwell: Minimize
G04commands on visible surfaces.
