Solving Surface Defects in CNC Milling for Better Quality
In CNC milling, surface quality is not just about appearance. It directly affects performance, functionality, and product life. A poor surface finish can cause premature wear, higher friction, and even part failure in electromechanical assemblies.
To achieve a high-quality finish, engineers must focus on three key areas: optimising cutting parameters, selecting the right tooling, and applying strict quality control.
This article explores the main causes of surface defects in CNC milling, with a focus on vibration-related problems, and explains how to solve them using practical workshop strategies.
Four Practical Solutions for Resolving Milling Surface Defects
Below are four common surface defects in CNC milling and the practical solutions to solve them.
Defect 1: Annoying Chatter Marks
Symptom: Regular wave-like patterns on the surface of the workpiece.
Practical Solutions:
- Increase spindle speed first. Raise spindle speed by 20–30% (within tool and machine limits). This is often the fastest and most effective solution.
- Reduce cutting depth/width. Limit depth of cut (Ap) or width of cut (Ae) to 30–50% of tool diameter.
- Use rigid toolholders. Choose the shortest and stiffest toolholder and tool possible. Excessive overhang is the main cause of chatter.
- Check clamping. Make sure both the tool and the workpiece are fully secured. Any looseness increases vibration.
- Adjust feed rate. A slight reduction in feed (F) may help in some cases.
Defect 2: Persistent Burrs On the Edges & Hole Entrances
Symptom: Sharp, unwanted metal burrs on edges or hole entrances.
Practical Solutions:
- Keep tools sharp. Dull tools are the main cause of burrs. Always use undamaged cutting edges.
- Add a finishing pass. Use a light cut with low depth, low feed rate, and high spindle speed at the end of contouring or hole machining to smooth edges.
- Use chamfering or deburring tools. Dedicated tools ensure consistent, efficient results.
- Optimise toolpath direction. Plan finishing cuts so the tool moves from the inside outward, using cutting forces to suppress burr formation.
Defect 3: Poor Surface Finish on Curved Surfaces with Ball-Nose Cutter
Symptom: Visible tool marks on curved areas, local spikes in Ra values, and uneven surface quality such as light/dark streaks.
Practical Solutions:
1. High spindle speed with micro-feed. This reduces vibration.
- Cutting speed (Vc): 150–220 m/min
- Spindle speed (n): 12,000–18,000 rpm
- Feed per tooth (Fz): 0.03–0.06 mm/z
2. Optimise toolpath strategy to remove tool marks
First Options:
- Helical contouring (3D contour) for continuous cutting without tool marks (steep surfaces).
- Contour finishing + shallow undercut for steep (>45°) and flat (≤45°) areas.
Advanced Settings:
- Enable smooth transitions (G2/G5 continuity) so feed rates stay constant at junctions.
- Extend toolpath by 0.2 mm beyond surface edges to avoid edge rebound.
3. Select the right ball-nose cutter
- Cutting edge runout ≤0.005 mm (check with tool presetter).
- Use 2-flute ball-nose cutters for better chip evacuation than 4-flute.
- Choose uncoated, polished cutting edges for aluminium alloys.
Defect 4: Poor Hole Wall Quality
Symptom: Rough hole walls with scratches, taper, or out-of-tolerance diameters.
Root Causes:
- Poor chip evacuation → clogging and scratches
- Tool vibration → vibration marks/taper
- Inadequate cooling → built-up edges
- Tool runout → hole diameter errors
Practical Solutions:
1. Choose the right cutting tool
| Hole Type | Recommended Tool | Core Advantage |
| Through holes (L/D < 5) | Solid carbide drill | High rigidity, runout ≤0.02 mm |
| Deep holes (L/D > 8) | Internal-coolant drill | Effective chip evacuation, cooling at the edge |
| Precision holes (H6) | Reamer + diamond finishing tool | Achieves Ra0.2 surface roughness |
| Difficult materials | TiAlN-coated drill | 50% higher heat resistance |
2. Reference drilling parameters
| Material | Pecking Pitch (Q) | Rubbing Height | Feed Correction |
| Aluminium alloy | 2.5×D | 2 mm | F × 120% |
| Steel | 0.8×D | 1 mm | F × 80% |
| Stainless steel | 0.6×D | 0.5 mm | F × 70% |
| Titanium alloy | 0.3×D | 0.3 mm | F × 60% |
G-code example (D10 drill, Grade 45 steel, depth 50 mm):
G83 X0 Y0 Z-50. Q8. R1. F80. (Q = 0.8×D, retract to R1 height)
3. Suppress vibration with three methods
- Use hydraulic toolholders (runout ≤0.003 mm).
- Keep overhang ≤4×D (e.g., D10 drill → overhang ≤40 mm).
- Choose unequal-angle cutting edge designs (10°–15°) to avoid resonance.
4. Apply step-by-step drilling for deep holes
Centre drill → Pilot hole → Rough drilling (0.1 mm allowance) → Chip clearing → Finish reaming (Ra0.4).
Example for D10, depth 85 mm, 45 steel:
- Centre drill: 1.5 mm deep
- Guide hole: φ9.8 × 20 mm
- Rough drilling: φ9.7, Q=10 mm, F=80 mm/min
- Finish reaming: φ10H7 reamer, F=50–60 mm/min, n=300 rpm
Partnering with a Professional Manufacturer for Better Milling Quality
Surface defects have many causes, but each can be solved with precise diagnosis and targeted action. By improving tooling, adjusting parameters, optimising toolpaths, enhancing cooling, and ensuring rigidity, engineers can greatly improve CNC surface finish.
If milling defects are still troubling your parts, JILI can help. We provide one-stop CNC milling services, supported by experienced engineers and advanced equipment. Our team specialises in solving complex milling challenges and delivering parts with consistent, high-quality finishes. Get in touch with us to discuss your project and find the most effective solution.