CNC Machining: The Ideal Method for Creating Medical Screws

Why is CNC Machining Used For Creating Medical Screws?

CNC machining offers several benefits, including producing high quantities of parts while maintaining tight tolerances. In the medical industry, custom parts are often required, and CNC machines can be programmed to produce complex metal parts based on custom blueprints. It allows for rapid prototyping and the quick production of thousands of identical parts.

CNC machines employ various cutting tools to create unique cuts, angles, and other features with great precision and repeatability. It results in lower costs for custom-machined parts in the medical industry.

Medical equipment often contains hundreds or thousands of tiny parts that must be machined with great precision to eliminate the risk of failure. Many medical components also require harder metals like titanium and stainless steel to maintain sterility and meet strength requirements. CNC machines can cut these materials more precisely than most other machining processes.

Lastly, CNC machining offers speed and efficiency that benefits the medical industry. Once set up, CNC machines can run 24/7 without requiring much human intervention, resulting in a faster supply chain and reduced waiting times. Additionally, CNC machining is the most efficient way to produce high-quality parts, lowering costs. 

Comparison Between Medical Screws And Regular Screws

Wood screws are commonly used in furniture manufacturing, and they are usually made of carbon steel. These screws are formed through cold extrusion, which is a cost-effective and efficient manufacturing method that yields high output. However, screws used in medical procedures must be solid and corrosion-resistant. While they have a similar structure to wood screws, medical screws made of SUS321 stainless steel cannot be cold-extruded using regular machine tools. It is due to factors such as the material used, the need for special tools, and the fact that they are produced in small batches.

The Technical Difficulties of Processing Medical Screws

Medical screws are small components that are used in the medical industry. The screws are mostly made from materials stainless steel or titanium. Manufacturing medical screws with precision and tight tolerances is a difficult task due to their small size. Moreover, the manufacturing process involves strict adherence to quality control standards to ensure that the screws are safe and effective for use in surgical procedures. These factors make the processing of medical screws a challenging task.

CNC Machining Principle of Medical Screws

The key to processing qualified medical screw parts is to plan the tool path based on their geometric characteristics. For thread processing, CNC machine tools use cemented carbide-coated inserts. To ensure the cutters have a reasonable service life, the linear speed tolerance v of the inserts must be calculated, and the spindle speed for turning medical screws should be determined accordingly using the formula v = πDn/1000 (where v is the linear speed in m/min, D is the turning diameter in mm, and N is the spindle speed in r/min).

When producing medical screw threads, the main cutting force used accounts for more than 90% of the total power of the machine tool. Additionally, feed resistance consumption accounts for more than 5% of the full power of the machine tool. Using a traditional forming tool to process threads on a CNC machine tool is not ideal, as the cutting resistance increases as the cutting depth of the tool increases. It can lead to issues such as vibration, deformation, bending, and an inability to turn.

To address the issues encountered in traditional thread-forming tools, an NC 35° profiling turning tool is utilized. A macro program is created to manage the tool tip’s movement along the thread profile. Once the tool tip completes its path through the profile, it is gradually fed into the turning layer by layer. It results in a constant contact area between the tool and the workpiece, ensuring the cutting force remains small and constant. This approach overcomes the growing problem of cutting resistance in conventional thread-forming tools.

1. Tool Material Selection

Medical screws are commonly used to connect artificial joints and human bones. They require specific strength, corrosion resistance, and acid and alkali resistance. To achieve these characteristics, SUS321 stainless steel with high strength and resistance to corrosion, acid, and alkali is used.

However, this type of stainless steel is challenging to process due to its high strength, plasticity, and severe hardening during processing. During cutting, the tool may experience large cutting resistance, which can cause significant deformation of the medical screw. The high cutting temperature can also cause cutting tumors.

Since medical screws can easily undergo work hardening, making their processing difficult, it’s essential to select a blade that has good heat resistance, wear resistance, and thermal conductivity and is not easy to bond. Additionally, it should be fully cooled during processing. Therefore, using a water-based cutting fluid with good heat dissipation is reasonable.

2. Medical Screw Size

The medical screw in the figure below has the specifications M6-2.5mm x 55mm. It has an external diameter of 6mm, a pitch of 2.5mm, a bottom groove width of 0.4mm, a top groove width of 0.05mm, and a tooth angle of 60 degrees. The length of the screw is 55mm, and the max diameter of the right end is 11mm. However, due to the low rigidity of the components and the large screw pitch relative to its diameter, there are certain challenges in enhancing the rigidity of the workpiece clamping and in preparing a reasonable NC macro program.

3. Processing Technology

Medical screws are manufactured in small batches. If the conventional “one clip, one top” method is used to create threads, the workpiece’s poor rigidity cannot withstand the cutting force, leading to a bending deformation problem in the middle of the workpiece. Therefore, a specialized thread-turning support fixture must be designed to stabilize and support the workpiece throughout the process, preventing deformation and ensuring the workpiece’s reliability. 

Furthermore, a 35° profiling turning tool with carbide titanium carbide coating is selected to prevent the workpiece from deforming and reduce cutting resistance when cutting threads. The program uses path synthesis to cut the thread layer by layer. This method significantly reduces cutting resistance while maintaining a relatively constant level during the thread turning.

4. Steps to Process medical screws

The following are the steps to process medical screws:

1. The two parts are processed together, leaving an additional 15mm at the center for self-centering chuck clamping and an additional 7mm at both ends for drilling the center hole.

2. One clamp and one top turn 6mm and 11mm excircles.

3. Clamp the 6mm excircle and turn it to remove the process head. It also removes the center hole. Then, turn the two ends of the part into a complete taper.

4. Clamp the 11mm excircle and use a support fixture to support the 6mm excircle of the medical screw. Use a macro program to turn the thread.

5. Cut the two screws that are connected to ensure that the length is even and the size is 60mm.

6. clamp the blanked part on a horizontal milling machine using a vertical rotary worktable. Use a saw blade milling cutter to mill a 1.5mm wide groove. Drill center holes at both ends of the blanking. Process the head for turning the excircle, turn the taper, support the turning thread with a fixture, and flatten the length after cutting.

5. Macro Programming

Macro programming plays a crucial role in all aspects of part processing. Macro programming is utilized for thread processing on a FANUC0i CNC lathe. The principle of macro programming is as follows: Whenever the tool identifies a point in the thread profile shape, it moves a pitch. The tool returns to the starting point, drives a point along the thread profile, and carries a pitch again. This process is repeated until the entire tooth shape is turned. After turning the whole tooth shape, the layered feed turning is achieved through a coordinate system offset, ultimately turning the complete tooth shape.

6. Processing Precautions 

When processing medical screws, it’s important to control the error in the size of the excircle diameter of 6mm at around 0.04mm. If the error is too large, the semicircle hole with a diameter of 6mm in the supporting fixture won’t align well with the excircle of the screw. It will weaken the supporting fixture’s role, leading to vibration or workpiece deformation during turning. It’s also crucial to keep the blade sharp when turning the thread. Avoid any changing the tool in the middle of the process, as it may disorderly buckle the thread.

7. Quality Checking

To ensure the parts meet the size requirements, use a micrometer to check the outer circle size of the medical screw, a caliper to measure the thread pitch, and a surface roughness comparator to check if the surface roughness value of the thread Ra3.2 μ M is up to the standard. Once tested, you can confirm that the parts meet the size requirements and can also meet the use requirements.

Summary

By analyzing the principle of processing for medical screws, we have completed the design of a support fixture for medical screws as well as macro programming. It has allowed us to carry out CNC processing for medical screws, which fills in the gaps of cold extrusion that is used for ordinary screws. With this method, we can complete small batch processing of medical screws at a lower cost. Furthermore, this method of using CNC machine tools to process medical screws can serve as a reference for processing similar screws made of unique materials.