Detailed explanation of machining principles and techniques for mechanical turned parts for the medical sector

The machining of medical turning parts requires strict principles and specific techniques to ensure that the parts meet the requirements of high precision, biocompatibility and functionality. The following is a detailed description of the processing principles and techniques of mechanical turned parts for the medical sector:

1. Processing principles of mechanical turned parts for the medical sector

(1) High precision and dimensional stability

1) Micron-level tolerance: 

Medical parts (such as orthopedic screws and dental implants) usually require tolerances within ±0.01mm, and require the use of high-rigidity machine tools (such as Swiss-type lathes) and precision fixtures.

2) Avoid deformation: 

Thin-walled parts (such as surgical instrument housings) require low cutting force processes to prevent machining deformation.

(2) Material biocompatibility

1) Preferred medical-grade materials:

● Stainless steel: 316L (low nickel, corrosion resistance), 17-4PH (precipitation hardening).

● Titanium alloy: Ti6Al4V (best biocompatibility).

● Cobalt-chromium alloy: used for high-wear-resistant parts (such as artificial joints).

2) Avoid pollution:

It is forbidden to use cutting fluids or lubricants containing toxic elements such as lead and cadmium.

(3) Surface integrity requirements

1) Low roughness (Ra < 0.8μm):

Reduce bacterial attachment and facilitate sterilization (e.g. Ra can reach 0.05μm after electrochemical polishing).

2) No burrs and microcracks:

Sharp edges need to be chamfered or polished to prevent tissue damage.

(4) Clean and pollution-free processing

1) Clean workshop:

Some implants need to be processed in ISO Class 7 (10,000) environment.

2) Special cutting fluid:

Use medical grade water-soluble cutting fluid to avoid mineral oil residue.

(5) Traceability and compliance

1) Full-process records:

Process parameters (speed, feed rate) and tool wear data need to be archived in accordance with FDA 21 CFR 820 or ISO 13485 requirements.

2) Material certificate:

ASTM F136 (titanium alloy) or ASTM F138 (stainless steel) certification of the raw materials must be provided.

2. Core skills for machining mechanical turned parts for the medical sector

(1) Tool selection and optimization

1) Material:

Carbide tools: suitable for stainless steel and titanium alloys (good wear resistance).

PCD (polycrystalline diamond) tools: used for high-silicon aluminum alloys (such as medical equipment housings).

2) Geometric parameters:

Sharp cutting edges (front angle ≥ 15°) reduce work hardening (titanium alloys are prone to produce hardened layers).

Small tool tip radius (R0.2mm) improves contour accuracy.

In actual applications, it is necessary to select appropriate tools based on the specific processing materials. For example, when machining titanium alloys, it is recommended to use sharp positive rake angle tools and cooperate with high-pressure cooling systems. Tool life management is also important, and a complete tool replacement system needs to be established.

(2) Cutting parameter optimization

In actual processing, it is necessary to find the best parameter combination through trial cutting and establish a standardized processing parameter database.

(3) Measures to reduce work hardening

1) Titanium alloy:

Use high-pressure cooling (>70bar) to avoid high temperature-induced α-phase hardening.

2) Stainless steel:

Control cutting speed to avoid built-up edge (BUE) adhesion.

3) Cobalt-chromium alloy:

Use medium cutting speed with minimal lubrication to balance cutting heat and tool wear.

Different measures are taken for different medical material characteristics - titanium alloy focuses on high-pressure cooling temperature control, stainless steel needs to suppress built-up edge, and cobalt-chromium alloy needs to balance speed and tool durability.

(4) Special structure processing techniques

1) Micro-hole (<1mm):

Use gun drill or micro-diameter boring tool with high speed (>10,000 RPM).

2) Thin-walled parts:

Precision turning in multiple times (allowance ≤0.05mm), use reverse cutting to reduce deformation.

3) Complex contours:

Use turning and milling combined machining center

For these special structures, special processing processes need to be developed. For example, when machining deep holes, internally cooled tools can be used, and high-pressure cooling systems can be used to improve chip removal.

(5) Online inspection and quality control

1) Real-time monitoring:

Use laser micrometers or pneumatic gauges to detect critical dimensions.

2) Surface roughness control:

After fine turning, Ra≈1.6μm, and after polishing/electrolysis, Ra<0.4μm can be achieved.

All inspection equipment needs to be calibrated regularly, and complete inspection records must be kept. For critical dimensions, 100% inspection must be carried out to ensure that each part meets the requirements.

3. Typical machining cases of mechanical turned parts for the medical sector

(1) Artificial joint machining

The ball head part of the artificial joint requires extremely high shape accuracy and surface quality. The following process routes are usually adopted:

Rough turning: remove most of the excess

Semi-finishing turning: control shape tolerance

Fine turning: achieve mirror effect

Polishing: further improve surface quality

During machining, it is necessary to pay attention to controlling the cutting temperature to avoid changes in the material structure. The final surface roughness requirement is Ra<0.05μm.

(2) Dental implant processing

The difficulties in processing dental implants are:

Processing of tiny threads

Precise control of neck taper

High surface treatment requirements

Usually, Swiss-type lathes are used with special thread cutters for processing. After processing, sandblasting and acid etching are required to form a microporous structure that is conducive to bone bonding.

(3) Cardiovascular stent processing

Processing characteristics of cardiovascular stents:

The wall thickness of the tube is only about 0.1 mm

It is necessary to process a complex grid structure

Extremely high surface quality requirements

This type of mechanical turned parts for the medical sector usually adopts the process route of laser cutting combined with precision turning. After processing, special surface treatment such as electrolytic polishing is required to ensure a smooth surface without burrs.

4. Summary

The processing principles of mechanical turned parts for the medical sector revolve around precision, biosafety and traceability, and the core skills are tool optimization, parameter control and special structure processing. By strictly following these principles and techniques, it can be ensured that the parts meet the demanding requirements of the medical industry.