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.
Results and Client Feedback
We successfully met the client’s stringent requirements, achieving significant improvements in key performance indicators. The results are summarized below:
| Indicator | Before Improvement | After Improvement |
| Surface Roughness (Ra) | 1.2 μm | 0.08 μm |
| Tool Life | 15 minutes | 120 minutes |
| Delivery Time | 4 weeks | 2.5 weeks |
The client confirmed that the product’s reflectivity reached 90% (exceeding the required 85%), and the corrosion test passed the ISO 9227 standard for a 500-hour salt spray test. The project was delivered two weeks ahead of schedule, and the client commented that the “surface quality surpassed that of similar German-made products,” leading to an additional order of 200 pieces.
How to Work With Us?
This project validated three of our core competencies:
- Material Expertise: We improved the machining efficiency of 316L by 300% through the use of coated tools and optimized parameters.
- Process Integration: Seamlessly integrating CNC machining with mechanical polishing reduced post-processing time by 30%.
- Standards Compliance: We precisely met the quality traceability requirements of the UK BS/ISO systems.
For international clients with high-end needs, we offer:
- Complimentary surface treatment solution simulations
- UK certification support (UKCA/BS)
- Small-batch trial production services (MOQ = 5 pieces)
Whether you require mirror-finished stainless steel components that comply with ASTM A480 standards, or are seeking a machining solution to achieve UKCA certification, our engineering team is ready to provide a customized solution for you.
