CNC Milling UK Market: Mirror Finish on 316L Stainless Steel Machined Parts

Material properties of 316L stainless steel

• Composition and corrosion resistance
  • Austenitic stainless with molybdenum for enhanced chloride resistance.
  • Low carbon designation (the “L”) reduces sensitisation during welding.
• Mechanical characteristics
  • Tough and ductile rather than brittle.
  • Moderate hardness but strong tendency to work harden under cutting.
• Thermal and chemical behaviour
  • Lower thermal conductivity than steels, so heat concentrates at the cut.
  • Forms a passive oxide layer that affects polishing and chemical finishing.

Project Background

The company, who manufactures minimally invasive surgical instruments, required the processing of complex-geometry laparoscopy components with a tolerance of ±0.01 mm and a surface roughness that met optical reflectivity standards. 316L stainless steel was the material of choice due to its excellent biocompatibility and resistance to corrosion from bodily fluids. However, the material’s 2-3% molybdenum content exacerbated its tendency for work-hardening, posing stringent demands on both tooling and processing techniques.

The Technical Challenges in Machining

The primary difficulties included:

• Material Properties: Machining 316L tends to produce long, continuous chips, leading to tool adhesion and edge chipping.
• Thermal Management: High-speed cutting can cause local temperatures to exceed 800°C, triggering material phase changes that affect the consistency of the polishing.
• Tool Wear: Uncoated tools had a lifespan of only 15 minutes, requiring frequent replacement which impacted precision.
• Surface Integrity: Mechanical polishing could potentially mask micro-cracks, which would compromise the safety of the medical components.

Our Systematic Solution

To address the challenges above, we implemented a systematic approach:

• Equipment Selection: We utilized a 5-axis high-speed CNC machine with a spindle speed of 24,000 rpm, equipped with an active cooling system.
• Tool Optimization: We selected multi-coated cemented carbide ball-end mills with a diameter of 0.5 mm, increasing the helix angle to 45° to improve chip evacuation.
• Process Parameters:
  • Cutting Speed: 110 m/min
  • Feed Rate: 0.03 mm/tooth
  • Depth of Cut: 0.1 mm (during the finishing stage)
• Tool Path Planning: A radial spiral tool path was used to eliminate tool marks, bringing the initial surface roughness down to an Ra value of 0.4 μm.

Post-Processing and Quality Control

Post-processing involved a four-stage progressive polishing method:

• Rough Grinding: An 80# silicon carbide abrasive belt was used to remove machining marks.
• Intermediate Grinding: We used 400# and then 800# diamond lapping paste for progressive refinement.
• Mechanical Polishing: A wool felt wheel with 2.5 μm aluminum oxide paste was applied to achieve a pre-mirror finish.
• Electropolishing: This final step was used to eliminate microscopic defects, bringing the Ra value down to 0.08 μm.

Quality Control:

• Roughness was measured for each batch using a white light interferometer.
• 100% full inspection was conducted according to the BS EN ISO 4288 standard.
• The defect rate was controlled to be ≤0.1%, meeting medical regulations.