When it comes to machining today’s finely tuned medical components, speed—not to mention tool life and automation—is of the essence.
Constant refinement of medical machining from tooling design to finished product requires not only the ability to handle a broad range of plastic and metal materials but also to achieve predictable results—particularly in the face of strict regulations.
Whether it’s just turning and milling or hybrid processes with lasers being teamed with Swiss systems to perform cutting and welding operations all in one machine, medical manufacturing requires machine builders and toolmakers to stay on their toes.
To machine medical parts and features as small as the hexalobes in bone screws while preserving tool life, Tsugami/Rem Sales LLC (Windsor, CT) has equipped several machines in its portfolio with custom speed-quadrupling units.
Standard spindles on Swiss-style machines typically spin live tools at 6000 to 8000 rpm, said Derek Briggs, Swiss product manager for Tsugami/Rem Sales. “When you are performing fine surfacing and five-axis contouring, engraving or etching, you need extremely high rpms because the tools are very small; you typically have an 0.125″ [3.175 mm] or smaller ball end mill that you are doing fine contouring with, and you’re cutting on center during these types of operations. You need very high spindle speeds to keep the surface footage at an acceptable level for tool life and meet surface finish requirements.”
Tsugami’s solution is an attachment using gear reduction that multiplies spindle speed by up to four times without taxing the spindle motor to reach an optimal speed, such as 20,000 rpm. Tsugami/Rem Sales can even integrate the high-speed spindles made by NSK America and run them up to 80,000 rpm.
Most Swiss-style lathes use a gear train that runs three or four spindles simultaneously when the motor is turned on, Briggs said. With just one modular spindle with reduced gearing going faster than the rest, “you’re not taxing the other spindles. It’s nice to get higher spindle speeds for that fine surfacing and small five-axis work that the medical market is looking for more and more today.”
While a standard thread whirling attachment will certainly do the job, if a machine is dedicated solely to producing bone screws all day, manufacturers should consider requesting ceramic bearings instead of more standard steel roller bearings. With ceramic bearings, operators can tighten the preload more than on steel, stiffening the system and maintaining maximum spindle speeds. Ceramic bearings dampen very fine harmonics and vibration when cutting bone screws and can also take the heat of higher rpms, dispersing heat much better.
In addition to maximizing spindle speeds, Tsugami/Rem Sales is also working to introduce more quick-change functionality to Swiss machining. The company offers live-tool attachments and gang tool plates with removable heads that let an operator take out a screw, remove the entire ER collet assembly or tool cartridge, set tool height outside the machine and install a pre-prepared spare—all in 30 seconds, Briggs noted.
Tsugami/Rem Sales is also developing quick-change turret holders for its turreted machines, in addition to its modular tool setups for face milling, angular milling and thread whirling attachments. “These holders are designed to help reduce operator error that could potentially lead to a machine crash or broken tools. This can also be done outside the machine on a Zoller or other presetting fixture.”
A gun-drilling application provided a case study for improving feeds and speeds.
When Tsugami/Rem Sales needed to create a high-frequency eye tumor removal device, the specs were challenging: the unit had to be 5″ (127 mm) long with a 0.06″ (1.52 mm) hole drilled all the way through. It employed the high-frequency pulsating TriboMAM system from M4 Sciences to do the job.
The device, made of a custom titanium, is akin to a vacuum and can suck out just the tumor, not other soft tissue, Briggs explained. Typically, a gun drill would only be able to be fed at less than one-tenth of an inch per revolution; integrating TriboMAM quadruples that speed “because the oscillation of that tool breaks chips into very small pieces, and high-pressure coolant is extracting them out of the hole.” In all, a process that would otherwise have taken several minutes was completed 75% faster.
Another innovation, low-frequency vibration (LFV) servo system technology “is one of biggest advances in the machining of tough materials used in medical component production,” said Jim Cepican, show manager for accessories sales with Marubeni Citizen-Cincom Inc. (Allendale, NJ). LFV improves tool life, chip control and processing of difficult-to-machine materials.
MCC’s L series machines “are used by the majority of medical manufacturers we work with” for their faster processing speeds and enhanced programming capabilities, Cepican noted. “The number of axes has increased, including a B axis that is necessary for manufacturing more complex medical components.”
The L12 is a particularly good seller for the medical market, added Regional Sales Manager Glen Crews. “The machine’s small size and high spindle speeds make it ideal for machining miniature medical device components. Options are also available on all our machines to reduce the spindle size, making material handling easier.”
Faster spindles have been developed for cross working and end drilling that are interchangeable with the machine’s standard spindles, Crews added. “Being gear driven, they are able to attain high spindle speeds with better torque than you would get with the electric or air-driven spindles many manufacturers use.”
Ultimately, using the proper standalone electric or pneumatically driven spindles is vital to optimizing feeds and speeds for today’s medical applications, said Michael Gabris, industrial sales manager at NSK America Corp. (Hoffman Estates, IL).
“You might have 20,000 rpm on a milling machine, but if you’ve got a 0.01″ [0.254-mm] diameter drill, you will not get the speeds you need to run that properly,” he noted. “You may break the tool very easily.”
In fact, 0.005″ (0.127 mm) is becoming a standard diameter for solid-carbide end mills and drills, added NSK Product Manager Mike Shea. “If you want to run 200 sfm, that’s about 76,000 rpm if you’re running it correctly,” he explained. “Even if the [tool] manufacturer recommends running 100 sfm, you’re still just under 40,000 rpm. Our spindles fit the need for machining the micro and nano applications that are becoming much more common.”
When an NSK spindle leaves the factory, it is guaranteed to have less than 1 μm of runout, Gabris said. NSK also manufactures its own precision collets, accurate to three to five microns, and offers costly-to-manufacture ceramic bearings on all high-speed spindles.
Whether outfitted on new machines or retrofitted on older machines, “our spindles do all the work,” Gabris said. Operators “have to lock the main spindle and run electric and air to our spindle, or just air if it is pneumatic,” Shea added. “The main spindle does not rotate, so there is no wear and tear on the machine spindle. Our spindles have no gears in them. By applying a small amount of air to them for cooling and purging, it is possible to run 24/7, with no thermal growth or harmonic issues. This is ideal in a constant high-production situation.”
Conversely, if using just a geared speeder head, the speeder should rest after 20 or 30 minutes of use to dissipate the heat in the gears, he advised.
With medical component materials ranging from plastics to cobalt chrome to titanium, Sandvik Coromant (Fair Lawn, NJ) has evolved its tooling considerably over the past few years.
“Yes, it is about producing parts faster and holding tolerances,” said Patrick Loughney, product manager for small part machining, “but it’s also about the predictability of the process and tooling.”For instance, with turning inserts, “we changed our technology tremendously with our Inveio coating innovation,” he noted, which allows for stronger substrates, more precise edge treatment and a new way of applying coating.
“We figured out that the crystals in the coating in the old process were random, to where they just protected the insert,” he explained. “Now we are able to align those crystals in one direction to where we can provide better tool life and insert wear.” This allows fewer adjustments to the machine and offers the predictable tool life critical to a stable machining process. After the insert is molded, edge preparation is executed to micron tolerances, he said.
“We have modified our tooling to execute different medical features,” he continued. “We have our medical boring bar especially made for producing the cuff for knees and hip joints.”
Meanwhile, the CoroMill 316 exchangeable-head milling system has been modified with extended cutting depth, especially for producing the knee trays that support implants.
Sandvik Coromant has also created special turning inserts for medical plastics like polyether ether ketone. “Most of the inserts are blasted on top so the plastic doesn’t stick,” Loughney explained.In October, a new drill made especially for heat-resistant materials in medical and aerospace applications will be part of the company’s biannual new product releases.
Sandvik Coromant’s biggest hurdle, however, is thread-whirling inserts. “The majority of the time, a bone screw has a special thread form,” Loughney noted. “It seems that every doctor who has come out with a bone screw has his own signature thread form. So we can’t sell just standard inserts for that; they all have to be specially engineered. We have to be really flexible in grinding these inserts on a case-by-case basis.”
When traditional machining alone will not fill the bill, adding one or more lasers to the process greatly enhances manufacturing capability.
For instance, Tsugami/Rem Sales’ LaserSwiss machine tool line was created for the medical industry. LaserSwiss combines traditional CNC Swiss turning and laser cutting in one machine, explained Tina Carnelli, marketing manager for Tsugami LaserSwiss. “This means medical manufacturers can produce complex medical parts, such as stents, with one setup and one part program,” she said. “We recently added a second laser head, for welding, to the LaserSwiss machine to effectively combine three operations on one machine.”
At Marubeni Citizen-Cincom, “the introduction of laser cutting and welding on our machines is one of the most significant modifications we have made,” said Regional Sales Manager Rich Kuhn. “The ability to do machining work and laser cutting in the same machine has a major effect on reducing the number of operations required to produce a component. Adding automation and doing laser welding has allowed our medical customers additional choices for solving some of the ever-increasing high-tolerance requirements.”
Of course, lasers have been a standalone medical manufacturing solution for some time. For instance, Trumpf’s portfolio is widely used by medical device manufacturers, from laser welding and fine cutting applications to additive manufacturing and laser marking for traceability, according to Salay Quaranta, industry manager for Trumpf Inc. (Farmington, CT).
Additive manufacturing, or 3D printing, has afforded numerous opportunities for Trumpf, she added. “The building of various dental crowns, bridges and RPDs using our TruPrint 1000 enables manufacturers’ generative production of the smallest single-batch parts and series on the plate. With a multilaser option, production can increase up to 80% while still maintaining flexibility to process customer orders. Even complex shapes can be quickly and easily converted from the CAD design to a 3D metallic component with top quality.”
It is that design flexibility that can set lasers apart from traditional machining when it comes to metal-based, layer-by-layer powder bed production.
“By harnessing the techniques for design with 3D printing, a manufacturer or lab can build a product to near net shape,” Quaranta said. “There is very little scrap as seen with traditional machining, and the metallurgy is sound. Manufacturers can leverage these platforms to build a variety of components from implants and dental crowns to scaffolds and constructs for many applications.”
Strict regulations governing every aspect of medical component production and the need to ensure a stable, predictable process would seem to make it ideal for automation. However, that rigid oversight presents a conundrum, according to Sandvik Coromant’s Loughney. Once a process is settled upon, it can be extremely difficult and too time- or cost-ineffective to change—even with a material or process innovation waiting in the wings.
“We are seeing more automation in medical manufacturing, but the majority is on the secondary operations that are not controlled as much, like grinding a gate off a forging,” he said. “We are seeing newer things like hip stems being produced with automation.”
With medical components usually produced in batches with frequent changeovers, accountability and traceability are paramount, he continued. “It’s a little bit harder to automate something like that. But it is changing; the systems and software are improving to where it can be tracked better.”Regarding another trend, the continued push toward Industry 4.0, machine builders are responding with numerous innovations.
On request, Rem Sales will integrate Tsugami Swiss machines with FANUC robotics for loading and unloading parts, Keyence vision systems for in-process gaging and auto compensation for automating offset adjustments on the fly, Briggs said. “We’re pulling a part off the conveyor from the machine with a robot, orienting it and placing it into a fixture on the measuring device. If any measured dimensions are out of tolerance, the Caron Engineering AutoComp system will automatically compensate in the machine as it is running, and the robot will place the bad part in a separate bin to ensure no bad parts are mixed with good.”
For today’s customers, “we cannot simply provide a single machine to process parts,” explained MCC’s Cepican. “We have to provide complete processing systems. This includes highly technical automation systems.” MCC has designed several automatic loading and unloading systems—for instance, systems that load blank parts into machines to be laser welded to a component that has been machined.
Added George Bursac, general manager of Star CNC Machine Tool Corp. (Roslyn Heights, NY), “Automation has been part of medical manufacturing for some time and is expanding based on new requirements.” While medical components like bone screws, dental implants, surgical instruments and other components associated with those products “are staying the same without many changes,” he said, “our team is responding to any new challenges manufacturing companies may require.”