Avoid machining malpractice

Cutting components for the medical industry is a lot like surgery: Success is rewarding, but the operation can be difficult. Complications stem from challenging part designs and specifications, as well as stringent requirements from regulatory bodies, such as the Food and Drug Administration. So like surgeons, shops need special knowledge, tools, techniques and practices to bring medical operations to a satisfactory conclusion.

One of the main challenges facing shops that machine medical parts is how to deal with different materials. For example, spinal implants used to be made of common materials like titanium and stainless steel, said Mike MacArthur, vice president of engineering at RobbJack Corp., a maker of solid-carbide cutting tools in Lincoln, California. Now, however, the materials for these implants include polymers like polyether ether ketone and transparent options that allow doctors to see through them to check the progress of healing tissue.

“So we develop specialized geometries to cut these unique materials,” he said.

RobbJack also makes tools to machine intraocular lenses implanted in eyes. These lenses used to be made of hard, rigid plastics. But today, MacArthur said the materials of choice are soft, pliable polymers that enable the lenses to be folded, which makes them easier to place in the eyes of patients.

Custom-made, small-diameter, solid-carbide step and twist drills from Star Cutter are used to machine medical components. Image couresay of Star Cutter

“That’s beneficial for the patient,” he said. “But it’s very difficult to cut this soft, rubbery type of
material.”

To meet the requirements of this application, RobbJack makes tools in the 0.254 mm (0.01″) dia. range “with ultrapolished finishes and ultrasharp cutting edges,” MacArthur said.

As for metals, those gaining popularity in the medical industry in recent years include flexible nitinol and corrosion-resistant BioDur. While the properties of these materials are useful in medical instruments and implants, they are also more abrasive than conventional alternatives, said Steve Easterday, Swiss applications manager at NTK Cutting Tools USA in Wixom, Michigan.

Coating Considerations

NTK Cutting Tools focuses on multilayer physical vapor deposition tool coatings for hard-to-cut materials. Although a thick titanium nitride coating might do the job for titanium thread whirling, Easterday said it wouldn’t suffice for cutting abrasive BioDur or nitinol.

For tools that will be used to machine these materials, “we might do a TiN coating and then layer it with a titanium aluminum nitride coating to give it more wear resistance,” he said.

PVD coatings can be relatively thick, but Easterday pointed out that the PVD process still produces thinner layers than chemical vapor deposition coating. This makes PVD coatings more suitable for the sharp-edged inserts needed to cut small medical parts.

When turning tiny parts, insert “sharpness is everything because you want less tool pressure and freer cutting,” he said.

In addition to coatings, Easterday advises shops engaged in medical machining to pay close attention to feeds and speeds. When switching from cutting titanium bone screws to medical parts made of harder materials like nitinol and BioDur, for instance, he said the surface feet per minute and feed rate have to change to get the right amount of heat into chips. When turning nitinol, he recommends starting with an sfm and a feed rate significantly lower than those used for titanium. And in some cases, depending on the length and diameter of the material, as well as the depth of cut, the initial parameters might have to be adjusted as the process goes along.

“Nothing’s really written in stone with feeds and speeds on these exotic materials,” he said. “You have to get in there and tweak it in order to get it right.”

As the machining process proceeds, most materials used for medical parts tend to workharden, said Gary McCarel, application engineer at Star Cutter Co., a toolmaker in Farmington Hills, Michigan. Therefore, he said it’s important to have a stable process — that is, one in which there is a consistent feed rate. For example, there should be no dwells in the cut.

Keeping Your Cool

McCarel also recommends “a consistently clean coolant stream” for cutting medical parts.

In addition, he said shops should do research to make sure they’re using top-notch coolants and ones developed specifically for machining medical components.

In some cases, MacArthur pointed out, conventional coolants may have to be ruled out for medical machining because of stringent purity standards. Implants, for instance, might not be allowed to come into contact with potential contaminants. In these situations, he said one option is to make sure the programmed cutting
technique is not generating excessive amounts of heat. For example, he suggested that shops could make lighter cuts rather than heavy ones but at faster feed rates.

In medical and other applications, high-pressure coolant directed at the cutting edge helps evacuate chips and increase tool life. Image courtesy of NTK Cutting Tools

Another possibility is to use supercritical carbon dioxide for cooling and lubrication. A fluid state of CO₂ held above its critical temperature and pressure, supercritical CO₂ looks like a gas but is as dense as a liquid. When used as a coolant, supercritical CO₂ leaves no residual contamination on medical parts, said MacArthur, whose company has partnered with firms that use supercritical CO₂ to machine medical parts.