Implant innovation: Turning Performance

Though certainly good for patients, modern medical implant designs are causing complications for manufacturers in the medical industry. As the designs have become more complex to facilitate attachment to the body and make implants less invasive, they have also become more difficult and expensive to manufacture. So a need has arisen for processes capable of producing the required shapes in an efficient and economical manner.

Development of such processes was the goal of the ZykloMed project launched by four Germany-based organizations. The results, demonstrated at EMO Hannover earlier this year, are innovations that hold great promise not only for machining implant components but also for manufacturing complex parts in other industries.

Polygon turning can produce Torx-like profiles much faster than conventional machining methods. Image courtesy of Horn

Challenges for makers of modern implant parts include machining shapes that are not circular or square and/or have more curved surfaces than older components of their kind. Machining the new parts can require several steps, driving up manufacturing time and costs.

Project Partners

To effectively deal with these issues, the ZykloMed partners — Index-Werke GmbH & Co. KG Hahn and Tessky, Paul Horn GmbH, the wbk Institute for Production Science at the Karlsruhe Institute of Technology, and Beutter Präzisions-Komponenten GmbH — focused on three different machining processes: non-circular rotary turning (NCRT), polygon turning and high-speed whirl milling. ZykloMed engineers had to come up with the right machine and tool technologies for each of the three processes, which were tested in both laboratory and near-application environments.

All three processes are based on the kinematics of rotating spindle axes during machining. Specifically, the processes require “synchronized motion of your workpiece and tool,” noted Tassilo Arndt, a wbk research associate. “Without this kinematic coupling, these processes won’t be possible.”

Depending on the application requirements, the rotational speeds of the tool and workpiece spindles could be the same or different, according to Matthias Luik, head of research and development at Horn. For ZykloMed, Horn worked on tasks such as tool design, materials and coatings.

Index and wbk also played an important role in tool design, developing a simulation program that determines the shape of the non-round tools needed for the processes, as well as the right clearance angles and number of inserts. Additionally, Index and wbk developed simulation software that comes up with the proper speed ratio for the tool and workpiece spindles. Index also supplied the machine technology for the project, as well as the special tool holders required for each of the processes, noted Volker Sellmeier, the company’s head of technology development.

As for the role of Beutter, a maker of implant components, ZykloMed engineers used the company’s facilities for testing under real manufacturing conditions. According to Luik, this testing provided critical information about the project’s simulation software, as well as tool life and materials.

Turning Technologies

As the names suggest, two of the ZykloMed processes are innovative turning techniques. In NCRT, a rotating non-circular tool is moved along the Z-axis of a rotating workpiece until the non-round tool shape is produced on the component. The speeds of the perpendicular spindles are coupled at a ratio of 1:1. According to Sellmeier, this process machines the desired non-round shapes approximately 13 times faster than conventional turning. In addition, he noted that the tools last more than 10 times longer than those normally used in turning titanium. The reason, he explained, is that the new process uses the entire circumference of the tool, while only a small part of the circumference of a normal cutting insert would be used in conventional turning. This reduces the thermal load at the cutting edge.

ZykloMed processes have been successfully tested in a near-series production environment. Image courtesy of Horn

“As the tool is rotating, the point of contact on the tool (constantly) changes,” he said. “So as a point moves out (of the cut), you can cool it down with your cooling liquid. This is what gives you such an increase in tool life.”

One application for NCRT is the production of bone nails with non-round external contours that improve the fit of the nails in bone marrow. In conventional manufacturing processes, bone nails are “basically round” after turning, Arndt said, “and you’d have to mill afterwards to get a non-round contour, which is time-consuming and therefore also expensive. NCRT enables you to produce this non-round external contour in a really short amount of time.”