Implantable medical components are difficult to machine. They have unusual shapes, close tolerances, free-form surfaces, tough angles, tight dimensional features, and fine finishes. It takes a lot of precision, accuracy, and attention to detail to cut such parts.

Over the last few years, we have seen a steady rise in the demand for implantable medical devices including knee and hip components, trauma devices such as bone plates, screws, and pins, and bio-active implants that encourage the fusion of medical devices with living tissue.

As these devices become more sophisticated, implant manufacturers must keep pace. Machining solutions that were effective for more simple components might not suit the machining of intricate, precision parts. Medical manufacturers that want to add implantables to their list of capabilities while being a legitimate competitor in the field should carefully consider current machine tool options.

Difficult materials

Implantable components are often made from titanium, cobalt chromium, or stainless steel. These materials provide good mechanical performance and are light weight. They exhibit low toxicity and resist the aggressive environment of the human body. However, these properties — combined with the need for precision and accuracy — present several machining challenges.

One challenge: machining speed. The faster a manufacturer can get finished components out the door, the more work it can take on and the more likely the company will be profitable. However, decreasing cycle times is difficult because of the complex parts and tough materials involved when machining implantables.

To maximize accuracy, companies typically machine medical components at slow speeds. Many machine tools aren't capable of maintaining accuracy at high feedrates and spindle speeds, particularly when machining materials such as stainless steel and titanium. Also, higher cutting speeds can overheat tools, which reduces tool life.

Another difficulty is maintaining control of cutter paths to get precise part geometry and repeatable results. This is critical in the production of implantables with steep angles, smooth lines, and fine finishes. Flawless finishes are important because surgeons assemble components during the implant procedure. Surgeons also play an important role in device selection. If they are not 100% satisfied with the finish and overall appearance of a part, it could mean the end to a lucrative contract.

Lastly, machining implantables can be time-consuming and labor-intensive. Depending on the machining method used, manufacturers might have to cut components in multiple runs or rely on secondary manual handwork. This adds more steps to the manufacturing process and increases the likelihood of human error.

Machine tool innovations

Fortunately, advancements in machine-tool technology are helping manufacturers surmount these challenges. For example, machines that operate at speeds to 40,000 rpm can cut components faster and more accurately than in the past. Equipment such as Makino's horizontal and vertical machining centers are designed to efficiently produce complex 3D metal parts at high speeds. The machines use high-torque, high-rpm spindles and advanced cooling, chip extraction, and high-speed control to improve speed without sacrificing accuracy.

Another advance comes from more rigid and stable machine tools and workholding devices. During machining, stainless steel and titanium can vibrate under heavy rough cutting or high-speed finishing. Therefore, it's critical that the work piece remain secure and damp potential vibration. Rigid and stable machine tools provide better control for more accurate components.

Similar to high-speed machining centers, five-axis machining centers are suitable for the manufacture of complexly shaped implantables. The machines provide simultaneous motion of five axes — the linear X, Y, and Z axes, and rotary B and C axes — which improves tool accessibility to the part-geometry and limits the number of manual setups.

High speed “point milling” with ball nose end mills, also known as “flush fine” machining, is a way to quickly and accurately machine parts with complex surfaces. Point milling combines advances in cutting technology with high-performance machine tool control and spindle technology. The method minimizes tool deflection, eliminates the re-cutting of chips through efficient chip removal, and reduces tool and workpiece temperatures. Point milling produces high-quality surface finishes, and can reduce or eliminate polishing, EDM finishing, or manual finishing.

Machine tool manufacturers have also improved control technology. For example, Makino's Super Geometric Intelligence v.4 (SGI.4) control is intended for the cutting of complex shapes at high speeds. The control software reduces milling time by up to 30% in most complex 3D parts such as orthopedic implants and bone plates.