The surface of the finished parts must be free of pores or cracks, otherwise sterilizing agent residue will remain. Furthermore, the machine tool itself must be radiation-resistant, typically using PVD coatings such as titanium nitride (TiN) or zirconium nitride (ZrN) instead of traditional paint.
5 Applicable CNC Machine Tool Types and Technical Parameters
5.1 Five-Axis Machining Center
(1) Applicable Scenarios and Structural Characteristics Five-axis machining centers are core equipment for orthopedic implant manufacturing due to their ability to complete complex surface machining in a single setup. Typical applications include three-dimensional surface machining of acetabular cups and femoral stems for artificial joint prostheses, porous biomimetic structure forming for spinal fusion devices, and high-precision machining of complex cavities for surgical instruments.
The structural forms of five-axis machining centers are mainly divided into three categories based on the relationship between the rotary spindle and linear motion: double rotary table, single-rotor-swing type, and double-swing head type.
(2) Key Technical Parameters ① Spindle speed. For machining titanium alloys, the spindle speed needs to be 10,000–20,000 r/min, and for stainless steel, it needs to be 15,000–30,000 r/min to achieve high-speed cutting and reduce thermal damage.
② Positioning accuracy: Linear axis positioning accuracy ±0.005 mm, repeatability ±0.003 mm; rotary axis positioning accuracy ±5", repeatability ±3".
③ Linkage control: Supports five-axis interpolation calculations and has look-ahead control to ensure smooth machining of complex trajectories.
④ Cooling system: Requires high-pressure cooling (pressure ≥7 MPa) and a minimum quantity lubrication (MQL) system to meet the cutting requirements of difficult-to-machine materials such as titanium alloys.
5.2 Milling-Turning Machining Center
(1) Applicable Scenarios and Structural Characteristics Milling-turning machining centers are suitable for the full-process machining of shaft and disc-shaped medical device components. Typical applications include: integrated machining of orthopedic intramedullary nails and bone screws (turning-milling-drilling); thread machining and radial hole machining of surgical instrument shafts; and mass production of small implants (such as bone screws and dental implants).
Milling-turning machining centers typically employ a dual-turret, dual-spindle structure, integrating turning, milling, and additive manufacturing (laser cladding) functions. They can directly machine complex structures onto titanium alloy implant blanks, improving material utilization.
(2) Key Technical Parameters ① Spindle Speed. Turning spindle speed ≥ 5000 r/min, milling spindle speed ≥ 12000 r/min, to meet the speed requirements of different processes. ② C-axis Accuracy. Positioning accuracy ±3.6", repeatability ±1.8", ensuring the indexing accuracy of threads and radial holes. ③ Power Tool Post. Equipped with a servo-driven power cutter head, power ≥ 5kW, torque ≥ 30N·m, supporting high-speed milling. ④ Automation system. Integrated loading and unloading robots and online detection devices to achieve unmanned mass production.
5.3 Precision Grinding Machines and Special Processing Machine Tools
(1) Applications of Precision Grinding Machines Precision grinding machines (such as surface grinders, cylindrical grinders, and coordinate grinders) are used for high-precision surface processing of medical device parts. Typical scenarios include: precision grinding of surgical scalpel cutting edges to achieve a surface roughness value Ra < 0.1μm; grinding of implant mating surfaces to ensure a mating clearance ≤ 0.005mm; and surface grinding of medical device guide rail surfaces with a flatness ≤ 0.01mm/1000mm.
Precision grinding machines are typically driven by linear motors with a positioning accuracy of ±0.002mm. Combined with CBN grinding wheels, they can achieve mirror-finish grinding on titanium alloy surfaces without subsequent polishing processes.
(2) Application of Specialized Machine Tools Specialized machine tools are indispensable in the microstructure processing of medical devices, mainly including: ① Electrical Discharge Machining (EDM) machines, used for processing micro-holes (diameter ≤ 0.1 mm) and narrow grooves (width ≤ 0.05 mm) in medical devices, such as the mesh structure forming of cardiac stents, with a processing accuracy of ±0.003 mm. ② Laser processing machines, suitable for cutting and drilling medical polymer materials, such as rapid prototyping of PEEK parts, with a heat-affected zone ≤ 50 μm. ③ Ultrasonic processing machines, used for processing hard and brittle materials (such as ceramic implants), capable of processing micro-holes with a diameter of less than 0.1 mm, and a surface roughness value Ra ≤ 0.4 μm.
5.4 Composite Specialized Machine Tools
Composite specialized machine tools exhibit unique advantages in the production of customized medical devices.
(1) Five-axis linkage + ultrasonic vibration composite machine tool: Positioning accuracy ±0.003mm, amplitude control within ±0.001mm. In the machining of zirconia ceramic dentures, the surface roughness value Ra≤0.2μm, and the machining efficiency is 2 times higher than that of traditional equipment.
(2) Femtosecond laser + electrolysis composite machining system: Pulse width 350fs, electrolysis voltage adjustable from 0 to 30V. In the machining of 316LVM stainless steel microcatheters, array machining with a hole diameter of 100μm and a hole spacing of 150μm can be achieved.
6. Optimization of CNC machining process
6.1 Machining of titanium alloy parts
(1) Optimization of cutting parameters: The key parameters for machining titanium alloy (Ti-6Al-4V) are controlled as follows: ① Cutting speed: 80-120m/min for roughing, 120-180m/min for finishing. Excessive cutting speed will lead to tool overheating and wear. ② Feed rate: 0.1～0.3mm/r for roughing, 0.05～0.10mm/r for finishing. Small feed rates can reduce surface roughness. ③ Depth of cut: 0.5~2mm for roughing, 0.1～0.5mm for finishing. For thin-walled parts, the depth of cut should be controlled to ≤0.2mm. ④ Cooling method: Use high-pressure internal cooling (pressure 8~10MPa) combined with extreme pressure cutting fluid, or micro-lubrication (MQL) + low-temperature cold air (-30℃) to reduce cutting temperature and reduce titanium alloy adhesion.
(2) Tool selection and wear control ① Tool material: CBN (cubic boron nitride) or ceramic tools are preferred, with a hardness ≥3000HV and good high-temperature resistance; secondly, coated cemented carbide (such as TiAlN coating) can be selected, with an oxidation resistance temperature ≥1100℃. ② Tool geometry parameters: rake angle 5°～10°, clearance angle 10°~15°. Increasing the clearance angle can reduce friction between the tool and the workpiece; helix angle 30°～45° to enhance chip removal capability. ③ Wear monitoring: Tool wear is monitored in real time by a cutting force sensor (sampling frequency ≥10kHz). When the cutting force increases by more than 20%, the tool is automatically changed to avoid scrapping parts due to tool wear.
6.2 Thin-walled parts and microstructure machining
(1) Deformation control technology for thin-walled parts Key points of machining process for thin-walled parts (wall thickness ≤0.5mm) of medical devices: ① Fixture design. Vacuum adsorption fixtures or multi-point support fixtures are used, with a contact area ≥60% of the surface area of the part to reduce local stress concentration. For example, when machining a 0.3mm thick titanium alloy biopsy forceps head, a silicone-filled fixture is used, and the deformation is reduced from 0.05mm to 0.01mm. ② Cutting strategy. The "layered cutting + symmetrical machining" method is adopted, with a cutting depth of ≤0.1mm for each layer. The symmetrical surface is machined first to balance the cutting force. Spindle speed ≥ 15000 r/min, using centrifugal force to reduce cutting vibration. ③ Auxiliary support. For slender, thin-walled parts (such as instrument rods with a diameter of 1 mm), ultrasonic vibration is used to assist cutting, with a vibration frequency of 20-40 kHz, reducing cutting force by 30%, and the amplitude controlled at 5-10 μm to avoid resonance deformation.
(2) Microstructure Machining Machining methods for medical device microstructures (feature size ≤ 1 mm): ① Micro-milling. Using a micro end mill with a diameter of 0.1-1 mm, machining is performed on a high-speed machining center (spindle speed ≥ 40000 r/min). For example, machining a 0.5 mm diameter medical titanium alloy microhole, with a feed rate of 100-200 mm/min and a feed per tooth ≤ 0.005 mm/z. ② Electrical discharge machining (EDM). CNC EDM is used, with copper or tungsten steel as electrode materials. The processing parameters are pulse width 1–10 μs, pulse gap 5–20 μs, and processing voltage 60–120 V. Narrow slit processing of 0.05 mm is possible, with a surface roughness Ra ≤ 0.8 μm. ③ Laser micromachining. Femtosecond lasers (pulse width < 100 fs) are used, achieving a processing accuracy of ±1 μm and a heat-affected zone < 5 μm. This is suitable for microstructure forming of PEEK materials, such as processing micro-hole arrays with a diameter of 0.1 mm.
6.3 Surface Treatment Processes
Surface treatment of medical device components directly affects biocompatibility and functional performance.
(1) Titanium Alloy Surfaces: Commonly used methods include sandblasting + acid etching (SLA) to form a rough, porous surface with a thickness of 10–50 μm, promoting bone cell adhesion; or anodizing to generate a TiO ceramic layer with a thickness of 5–10 μm, improving wear resistance.
(2) Stainless steel surfaces: Electrolytic polishing (voltage 10-20V, temperature 50-70℃) can reduce the surface roughness Ra from 0.4μm to 0.1μm, while simultaneously forming a passivation film to enhance corrosion resistance.
(3) Polymer material surfaces: Plasma treatment (power 50-100W, pressure 10-100Pa) can improve surface hydrophilicity and promote cell adhesion.
6.4 Process Integration and Automated Production Lines
(1) Multi-process integrated processes: ① Turning-milling-grinding composite process. For example, in dental implant processing, an integrated process of "turning to form - milling threads - grinding the surface" is used, completed in one go on a machine tool, reducing the production cycle from 20min/piece in the traditional process to 8min/piece. ② Additive manufacturing + subtractive manufacturing composite process. For complex porous implants, a blank is first 3D printed, and then precision milling and grinding are performed on a five-axis machine tool to improve material utilization. (2) Automated Production Line Design ① Flexible Manufacturing Unit (FMC). For example, a surgical instrument production line consists of two five-axis machining centers, one milling and turning machine, and a robotic loading and unloading system. Through MES system scheduling, it enables rapid switching between multiple product types and small batch production, with a changeover time ≤ 30 minutes. ② Intelligent Inspection Unit. Integrating visual inspection (accuracy ±0.01mm) and laser vibration meter, it monitors part deformation and tool wear in real time during processing, automatically stopping and alarming in case of abnormalities.
7 Challenges Facing the Development of the Medical Device Industry
7.1 Technological Bottlenecks
(1) Foreign Monopoly on High-End CNC Systems Most of my country's five-axis machine tool CNC systems rely on imports, primarily from brands such as Siemens (Germany) and FANUC (Japan), which have long held a dominant market position. Currently, CNC systems incorporate artificial intelligence technology, achieving intelligent programming, adaptive machining, and real-time fault diagnosis, resulting in stable performance and reliable operation. However, there are many technological barriers, and prices remain high.
Domestic five-axis machine tool control technology started relatively late. Domestic enterprises and research institutions have gradually mastered basic technologies by learning from advanced foreign experience. In recent years, significant breakthroughs have been made in high-end CNC systems in China. Some companies have developed high-performance CNC systems that have made important progress in key technologies such as high-precision interpolation algorithms and error compensation techniques, meeting the CNC requirements of high-end five-axis machine tools and significantly narrowing the gap with foreign countries [2].
(2) Insufficient Synergy in New Material Processing Technology: The processing of biodegradable magnesium alloys (such as AZ91D) faces the challenge of synergistic control of "cutting-corrosion". Experiments show that when using emulsion cutting, magnesium ions react with fatty acids in the cutting fluid to form soaps, leading to deterioration of the cutting fluid and accelerating intergranular corrosion of the magnesium alloy. However, the cooling systems of existing machine tools are not designed for the characteristics of magnesium alloys. One company used traditional water-based cutting fluid to process magnesium alloy bone nails, and after sterilization, 15% of the products showed pitting corrosion, which was found to be caused by residual cutting fluid. The processing of ultra-high molecular weight polyethylene (UHMWPE) artificial joints presents a "processing-wear" contradiction. Chip entanglement is easily generated during cutting, and the cutting parameters of traditional machine tools (cutting speed 100 m/min, feed rate 0.1 mm/r) can lead to micro-cracks on the surface. These cracks accelerate wear during joint movement. However, optimizing cutting parameters requires machine tools with higher spindle speeds (above 20,000 r/min) and sufficient rigidity, areas where domestically produced machine tools have significant shortcomings.
(3) Lack of Precision Retention in Micro-Nano Machining At the micro-nano scale (<100 μm), the impact of machine tool thermal deformation and vibration on precision is amplified. Swiss-imported machine tools, through liquid-cooled spindles, achieve temperature fluctuations ≤0.5℃; employing a thermally symmetrical structural design, they can control thermal deformation within 0.3 μm, a feat difficult for domestically produced machine tools to achieve. Micro-nano machining also faces process failures due to scale effects. When the tool diameter is <0.1 mm, the scale effect of cutting forces renders traditional cutting parameters inapplicable. 7.2 Industrial Ecosystem Dimension
(1) Cross-Industry Technology Integration Gap: A professional barrier exists between medical device companies and machine tool manufacturers. Medical device companies focus on medical characteristics such as biocompatibility and sterilization adaptability, while machine tool manufacturers excel in manufacturing indicators such as mechanical precision and processing efficiency. Their technical languages are not aligned. For example, an orthopedic implant company required a surface roughness value Ra < 0.2 μm for titanium alloys processed by machine tools, but failed to clarify the impact of surface texture (such as groove direction) on bone cell adhesion. This resulted in a situation where, although the surface processed by the machine tool manufacturer using conventional processes met the precision requirements, cell experiments showed a lower-than-expected bone cell adhesion rate.
This gap is also reflected in the differences in standard systems. Medical devices follow the ISO 13485 quality management system, requiring processing equipment to have traceable process parameter records. However, machine tool industry standards (such as the ISO 230 series standards) focus on precision testing and lack alignment with medical standards.
(2) Economic Contradiction in Small-Batch Production: The small-batch production (usually < 50 pieces) of customized medical devices creates a sharp contradiction with the high investment costs of machine tools. An imported five-axis machining center is expensive. Based on an 8-hour shift and a 60% utilization rate, equipment depreciation accounts for a high percentage of the cost per unit. Small-batch production leads to low machine tool utilization.
Small-batch production also presents economic challenges in process validation. Medical device registration requires at least three batches of full-size inspection data. However, in small-batch production, changing batches requires re-adjusting the machine tool, which incurs high costs each time, severely impacting company profits.
(3) Special Requirements for GMP Certification GMP certification is the entry threshold for medical device companies entering the pharmaceutical industry and a prerequisite for product market launch. Major global economies have independent GMP standards. GMP certification imposes strict requirements on the clean design of machine tools: surface requirements-no right-angle corners, corner radius ≥3mm to avoid dust accumulation; lubrication requirements-a fully enclosed lubrication system with a leakage rate ≤0.1mL/h; validation requirements-a clean validation plan must be provided to prove that residual contaminants can be controlled below 10ppm. This requires machine tools used for medical device processing to comply with GMP requirements during the design phase, increasing costs, adopting dead-angle welding and food-grade coatings, and effectively passing certification.
8 Development Trends in the Medical Device Industry
8.1 High-Precision and Intelligent Machining
(1) Nanoscale Machining Technology Technical directions of machine tool spindles: ① Air-bearing spindles, through...
Compressed air forms a micron-level air film (thickness 1-3μm), with radial runout ≤0.05μm, suitable for nanoscale mirror processing, such as LED lens mold polishing, with a rotation speed of 160,000 r/min. ② Magnetic levitation spindle, non-contact support, critical speed up to 300,000 r/min, has been used for nanoscale grinding of semiconductor wafers. ③ Hybrid ceramic bearing spindle, silicon nitride ceramic ball combined with steel inner ring, with a rotation speed of 120,000 r/min. Combining the future technology application direction of the spindle, an ultra-high-speed machining center with a spindle speed ≥100,000 r/min can be developed to realize nanoscale texture processing on the implant surface and promote the directional growth of bone cells.
(2) Intelligent process optimization Based on machine learning algorithms, a cutting parameter prediction model is established, and an AI-integrated adaptive machining system is used to adjust parameters in real time to achieve the optimal cutting speed and extend tool life [3]. (3) Digital Twin Technology: Constructing a virtual model of the machine tool, cutting tool, and workpiece to simulate deformation and stress distribution during processing, enabling pre-optimization of process parameters and allowing for pre-simulation of the processing process in a virtual environment.
8.2 Green Manufacturing and Efficient Processing
(1) Dry Cutting and Micro-Lubrication: Promoting MQL + low-temperature cold air technology to achieve zero cutting fluid discharge in titanium alloy processing, reducing cutting fluid costs.
(2) Additive and Subtractive Material Composite Manufacturing: Combining 3D printing and five-axis milling to achieve near-net-shape forming of complex implants, reducing material waste.
(3) Automated Production Lines: Deploying collaborative robots and AGV logistics systems to build smart factories and improve production efficiency.
8.3 Integration of Micro/Nano Fabrication and Biomanufacturing
(1) Integrated Micro/Nano Structure Processing: Developing a five-axis linkage + femtosecond laser composite machine tool to achieve one-time forming of micro/nano multi-level structures (micrometer-level grooves + nanometer-level pores) on the implant surface, improving bone integration efficiency.
(2) Biomanufacturing Processes: Combining CNC machining with bio-3D printing, such as printing a hydroxyapatite coating on the surface of titanium alloy implants, with a coating bonding strength ≥50MPa.
(3) Flexible Electronics Manufacturing: Utilizing CNC micromachining technology to fabricate flexible circuits for implantable medical devices, with a linewidth accuracy ≤10μm, meeting biocompatibility requirements.
(4) Bionic Surface Processing Technology: Bionic surface processing simulates the microstructure of biological tissues, improving the biocompatibility of medical devices.
9 Conclusion
This paper systematically analyzes the CNC machining technology of typical medical device components and draws the following conclusions:
1) Component characteristics determine machine tool selection. Titanium alloy implants require five-axis linkage + ultrasonic vibration machining; thin-walled surgical instruments rely on high-speed milling and turning composite machining; and microstructure parts cannot function without EDM or laser processing.
2) Optimization of process parameters is key. Titanium alloy cutting speeds are controlled between 80 and 180 m/min, coupled with high-pressure cooling; thin-walled parts employ a high-speed, shallow-depth-of-cut strategy, combined with flexible fixtures; surface treatment must meet biofunctional requirements.
3) Quality control requires full-process management. Based on the ISO 13485 standard, online inspection and traceability systems ensure machining accuracy and safety.
4) Technological integration is the development direction. The integration of additive and subtractive manufacturing technologies, intelligent process optimization, and micro-nano manufacturing will drive medical device processing towards high precision, high efficiency, and green manufacturing.
5) Intelligent equipment configuration and enhanced intelligent interaction are guarantees for the intelligent transformation and upgrading of the medical device manufacturing industry.
Currently, CNC machining of typical medical device components has evolved from single-technology applications to multi-disciplinary integration. Technologies such as five-axis linkage and femtosecond lasers have significantly improved machining accuracy and efficiency. However, problems such as reliance on imported high-end CNC systems and insufficient synergy between new materials and processes still need to be addressed. In the future, intelligentization, greening, and domestic substitution will become the core directions for industry development. Through technological innovation and policy support, precision manufacturing of medical devices is expected to achieve a leap from "following" to "keeping pace".
