Precision prescriptions: Medical Manufacturing

Ensuring optimal grinding of stainless steel medical instruments.

Courtesy of United Grinding Technologies

A surgical drill being ground on an EWAG Rotoline machine.

There’s no way around it, said Brian McKahan. When you talk about making the jump from grinding conventional carbide tools to grinding stainless steel medical instruments, you face an array of new hurdles.

McKahan and his wife, Christina, own True Tool Innovations, Craydon, N.H., where they make medical devices such as burs, reamers and saw blades along with standard carbide cutting tools like endmills, drills and routers. They’re at once well-versed in the manufacturing requirements of both stainless and carbide and well aware of their peers’ growing interest in adding medical parts to their product offerings. “You have a lot of companies that have been riding the carbide wagon for a long time making cutting tools, and I know of some that are trying to switch into the medical field, but it’s difficult,” said Brian McKahan.

The technological challenges newcomers and veterans alike face when grinding stainless medical instruments range from keeping the grinding wheel clean when working with the soft, gummy material to fixturing, surface measurement and minimizing burns and burs.

For McKahan, the strategy to head-off burning is three-fold. “We have three Rollomatic CNC tool and cutter grinders that use a full synthetic oil coolant,” he said. “Critical to our operation is the Transor filter system. We filter cutting oil to 1μm and chill the oil to within ±1° F. All of our machines are run on the filter system to help [prevent] burning.”

In addition, he uses advanced coolant nozzle technology with orifices that bar the introduction of oxygen into the stream, and thereby deliver a concentrated flow of fluid. Unlike nozzles that spray coolant, the focused stream can break the air barrier created by the rotating grinding wheel, and effectively cool the wheel/workpiece interface.

Courtesy of Rollomatic

Rollomatic Nano5 grinding of a tree-shaped rotary carbide bur with brazed shank using five CNC axes.

“We also use Regal Diamond Products’ copper-bond grinding wheels for all of our medical applications,” Mc- Kahan said. “The copper bond is bound with a copper base that goes to the core of the wheel and displaces the heat up to the core. That wheel is one of the biggest advances I’ve found to combat burning and burring. And I have yet to run into an instance when it has failed.”

As for deburring, McKahan reserves a wheel slot on the CNC to do the operation online with a brush wheel. “I’m able to program it to mimic the grinding in a deburring fashion,” he said. “People have a tendency to think you have to bury the wheel to debur, but it’s just the opposite,” he added. “If you grind the parts right, the burrs should be very light and flaky, and flick off easily. If you bury the wheel in there, it can push the burr back down into the flute or hole.”

Grinding Wheels

Aside from preventing burns, coolant is essential for keeping grinding wheels clean, especially when working with gummy stainless material. This is best achieved by ensuring that coolant delivery is appropriate for the specific grinding conditions, said David Drechsler, vice president of marketing for grinding machine manufacturer Huffman Corp., Clover, S.C. “For example,” he said, “for CBN wheels, ideally the coolant velocity will match the wheel peripheral speed. In other cases, a very high-pressure [1,000 psi to 1,200 psi] wheel scrubber is required to achieve acceptable removal rates without loading the grinding wheel.”

One recent advance that’s played a large part in boosting grinding production has been the introduction of automatic wheel changers. “After visiting shops, I’ve become a big believer in wheel changers,” said Ed Sinkora, marketing manager for United Grinding Technologies Inc., Fredericksburg, Va. He cited the example of a grinder with 12-wheel packs with various sized wheels on each pack. “Because you know all the wheels are available, you can design a whole host of tools that use different combinations of those wheels and never have to recombine or build new wheel packs,” he said. “When making a complex tool, you don’t have to worry about interference from other wheels. For example, you can only cram six or seven wheels on a two-spindle machine. And when you do that, there’s a tendency for one wheel to interfere with an adjoining wheel for some operations. It’s much easier to set it up with a wheel changer, where you’ve got only one or two wheels per pack, because the machine can automatically switch to another pack.”

Workholding

One perennial challenge of grinding stainless medical parts is fixturing. The awkwardness of some tools, combined with the material’s propensity to bend, sometimes calls for solutions as novel as the demands.

“The difficult parts are the very small, long drills; they’ll flex and move,” said Simon Manns, tool grinding applications manager for United Grinding Technologies. “Because it’s gummy, the part will try to pull itself up into the wheel.” He recommended using a steady-rest workholder with a bushing that provides more than 180° of encapsulation.

At True Tool Innovations, McKahan often uses customized fixturing. He cited the example of a drill design calling for a 6 “-long flute from a 6½ “-long blank. “You come into a problem with fixturing there because you can’t get close enough to the actual clamping mechanism when fluting the drill,” McKahan said. “So you have to create special fixturing, like using extra-long steady rests or you have to have customized wire EDMed workholding made to run that particular tool. I do have a company I partner with that does all my fixturing. I design the fixtures but they build them. Without them, I wouldn’t be able to make a lot of components.”

3-D Simulation Software

Not all of the advances aimed at ensuring optimal grinding results are focused solely on the machining process, per se. Among the more effective technologies has been the introduction of tools used before grinding even begins. “Simulation and visualization software enables the end user to see the part before actually grinding, thereby reducing trial and error, reducing scrap and saving time and money,” said Huffman’s Drechsler.