Medical Machining Software
Multiaxis, HSM, and mill-turn machining strategies help turn out medical components
By Patrick Waurzyniak
Machining small, intricate parts for medical applications presents no simple task. To machine the oftentimes tiny metal parts and specialized tooling used in surgical and dental applications, manufacturers rely on multiaxis machining techniques, particularly simultaneous five-axis machining and Swiss-type lathes, as well as high-speed machining and multitasking or mill-turn machining strategies to produce the complex shapes and micro-machined parts required for today's medical industry applications.
Medical machining applications run the gamut of surgical tools, bone screws, custom metal implants for knee and hip joint replacement surgeries, components for cardiac catheterization, and pacemakers. Hard materials like titanium for surgical implants often require HSM and multiaxis techniques to produce parts with the requisite high-quality finishes for medical use.
Compared to industries like automotive, aerospace, and recreation, medical applications have some overlap, but medical still has distinct differences from other industries. "Medical industry machining does a lot of things that get implanted in the body, and the materials they use for implants are different than what they would use on your bicycle," says Bill Gibbs, president and founder, Gibbs and Associates (Moorland Park, CA). "More titanium, more exotic alloys. Materials are an issue. Size of parts are an issue. There are issues in quality assurance that are very important, because medical parts frequently are life-and-death issues."
CAD/CAM and reverse-engineering come into play with medical manufacturing where specialized parts like a hip or knee implant must be custom-machined to fit perfectly. "Medical parts are frequently machined to match a specific body, and so we're back to scanning things and then manufacturing parts based upon these 3-D scanned databases," Gibbs notes. "The hip joint might be customized to match up with your hip bone, for example, and it's interesting to see, once again, the three-dimensional scanning and machining become a factor."
Medical parts tend to be turned or milled, and produced in smaller volumes than typical machined parts. "First of all, they're mostly low-volume parts," Gibbs says. "The highest-volume parts are things like bone screws, but because so many of these other parts are customized for the size of the person or the application, they probably only use a couple hundred a year. Second, they vary from person to person, and third, they're a very developmental product, things are changing all the time. A whole lot of the medical stuff is experimental, cuttingedge, and that's kind of fun, too, working with doctors and surgeons, some of our customers, who are actually developing new things, new tools."
Unlike many CAM developers, medical manufacturing represents a majority of CAM supplier PartMaker's work, according to Hanan Fishman, president, PartMaker Inc. (Fort Washington, PA). "Medical is our biggest market, and most of our customers are doing multiaxis and Swiss turning," notes Fishman, who adds the work for medical customers such as Medtronic and Stryker is heavily CAD-oriented with a lot of 3-D modeling for making medical devices including bone screws, hip and knee implants, CT scan machines, and specialized medical tools.
"A big segment of it is tooling and implantable devices," says Fishman, noting the custom nature of many medical parts. "That's one of the big issues. You can't have a one-sizefits-all bone screw." Founded in 1991, PartMaker develops five software packages specializing in programming of Swiss-type lathes, turning and turn-mill machines. Acquired last July by Delcam, PartMaker holds key patents to its software technology for the multiaxis turn-mill and Swiss-type machines often used in machining medical parts.
"One of the biggest differentiators for us is that we have postprocessors for just about every Swiss machine and every multiaxis lathe built in the past decade and a half," Fishman says, "so we're able to offer a truly off-theshelf solution. If a guy calls us up and has all these wacky machines acquired over a number of different years with different programming requirements, we've got an off-the-shelf solution which, within a week, this guy's going to be up and running and productive on."
Advanced simulation capabilities are key to lowering setup times and proving out NC programs before taking up valuable machining time on shop floor. "The biggest benefit we give our customers is reduced setup time," Fishman says. "That's what we really attack, because especially in a medical environment, you're doing small lot sizes. The real productivity saving is more on the setup side, even more so than it is on the programming side, because they're programming offline. That's where the simulation is so important—the more that they can simulate and prove-out off-line, the faster the setup time is going to be, which means less machine downtime."
With its recently released Part-Maker 8, the company has added full machine simulation as an option, which models machines like a Citizen M with high realism, adds Fishman. "What you're literally seeing is a true virtual reality machine model comprised of the true actual components of the machine," he says. "So literally, these are all exact solid model designs of every component of every bit of the machine's componentry, so that simulation you're getting is literally as realistic as humanly possible.
"This is a massive undertaking for us, and this is really where things are going, having a true machine simulation inside of the software. It'll help in the medical market, just because complex programming that really sort of drives the purchase of the CAM system is most prevalent in the medical field. Medical isn't the only field that requires Swiss machines and multiaxis lathes, but it is the field that most typically requires the most complex programming of those machines."
Custom dental parts can be machined using Delcam's (Birmingham, UK) DentMILL specialized machining software that uses knowledge-based machining technology. Acquired from Protech, a Delcam reseller in Sweden, the DentMILL package has been incorporated into PowerMILL, Delcam's multiaxis machining system, to provide a knowledge-based machining process for making caps, bridges, and implant bridges in ceramics and titanium.
"It's a custom-made software to simplify things for dental manufacturers," notes Glenn McMinn, president of Delcam USA (Salt Lake City). "It's milling these dental products directly with four-axis milling. US customers are just starting to come around now. One of the questions is who will use it. Is it the dentist, or the dental lab, and what kind of expertise do they have? We're just getting started with it so it's pretty new for us."
Used by large dental companies in Scandinavia and now available from Delcam's international subsidiaries and resellers, the DentMILL software can accept geometry from dental design software or from dental 3-D scanners in most point-cloud and triangulated data formats. The Dent-MILL process begins by splitting the model into the areas to be machined from above and from below the parting line. The user then specifies the positions for the pins to hold the part during machining. Software shading provides a warning if the pin positions will lead to any machining of undercuts. The positions can then be adjusted to eliminate the problem.
Once the correct orientation has been set, toolpaths can be generated automatically for the appropriate type of part and material. As with all PowerMILL programs, the results are fully checked to prevent gouges or collisions, and will run on virtually all CNC milling machines, including specialist equipment for the dental industry.
Simultaneous five-axis machining and other multiaxis strategies are most common for medical manufacturing markets that are showing signs of consolidation with large medical companies buying up smaller medical manufacturing shops, notes Steve Bertrand of CNC Software Inc. (Tolland, CT). "The manufacturers are becoming conglomerates—they're buying up small manufacturing companies that specialized in the medical field," Bertrand notes, "and because of that, think they can afford the technology more. They're now looking at improving their processes, and improving their prototyping, which is kind of the black box of the medical industry in the prototyping area. They have the ability to invest in more sophisticated machinery, which absolutely demands software. The multiaxis machinery is one of the things that, even though they're becoming more and more affordable, it may have been out of reach for some of the smaller manufacturers."
In the highly competitive medical field, manufacturers are also trying to make prototypes of the latest products as quickly as possible. "It's always simultaneous five-axis. They'll run through prototypes day in and day out," Bertrand says, "and once they get to the manufacturing aspect, they're going to prove-out the process. Maybe they'll send it out, but many times they don't because it's such a competitive industry—they try to keep their cards close to their chest."
Multiaxis and simultaneous five-axis are popular with medical machinists in order to minimize setups. "If they've got 3+2 machining, it's for big parts, and they can use anything for that, and of course we support that," Bertrand says. "It's the more intricate parts, for example, a knee replacement. You start thinking about the components that go into that. Those are not easily machined in three axis. It's almost not cost-effective to do those parts in three axis with multiple setups, so the reality is you invest in a five-axis machine, you can get away with much fewer setups, maybe as little as one setup to do the entire part. We see a lot of new technology for manufacturing vertebrae parts.
Other factors in medical are traceability of parts, and cleanliness of any metal implants. "Titanium is preferred," notes Bertrand. "Titanium is used because of its durability, its strength, it doesn't rust. It's a bear to cut, and it's expensive. It's resistant to rejection, too, that's one of the things. They don't want anything that's going to rust, it's got to be a specific alloy. They're experimenting. There are new metals coming on to the forefront now."
Such implants are called appliances by the medical industry, Bertrand notes. "How do you get a leg up? It might be types, it might be types of market," he adds. "It might be ease of implementation, which is a big issue. Customization is also a thing that we're seeing now, particularly with knees. People are getting partial implants, so they're getting many different styles and sizes of the partial implant device.
"Since some of these materials are bears to machine, it requires new technology and strategies, so you'll hear from the cutting-tool manufacturers about cutting tools that are specifically made for the materials, hardened materials. High-speed machining is a key. That's where we're seeing a big growth in the high-speed machining, and multitask machines, mill-turn types of applications are becoming more and more popular."
Controlled-engagement cutting techniques also can help medical manufacturers obtain high-quality finishes on medical parts that demand it. "Controlled engagement can prolong the length of your tool life, and the way that you do that is by having a constant engagement, not having light cut, heavy cut, light cut, heavy cut, where you introduce a lot of chatter, a lot of vibration. You wear the tool away quicker, and tool life becomes shorter. The finish is poor. There are many different advantages to having this controlled. The idea is you want to get as close to near-net-shape as possible. With the technology available today, you should be able to do that."
Mill-Turn Programming Helps Shop Speed Machining
Medical implant maker Triangle Precision Industries Inc. (Kettering, OH) sought true art-to-part production with one setup for manufacturing medical implants and surgical tools to very tight tolerances on mill-turn machining centers. The insistence by surgeons on delicate, flawless surfaces favored an automated, hands-off CAD/CAM solution.
Part geometry sent in to Triangle by medical supply companies, surgical hospitals, and research institutions is readied for machining with virtually no changes in EdgeCAM software from Pathtrace Systems Inc. (Southfield, MI). EdgeCAM is then used to program machining operations to avoid all manual handling—all sides of the part are machined in a single setup.
At Triangle, programmer Bryan Hunt uses CNC machines with integrated milling and drilling capabilities on additional spindles. The mill-turn machines, a Mori-Seiki SL253BMC and a Star Turn SR20, can be programmed as either three-axis mills, as two or four-axis turning centers, or both. EdgeCAM was chosen in part for its developments in programming mill-turns, which have up to nine axes to be coordinated, and for fully automated programming of machined solid models—adding up to hands-off machining.
Most mill-turn jobs are programmed quickly starting with the solid, says Hunt, noting EdgeCAM's Mill/Turn package's ease of use. "Solid machining is strongest in round parts and it is very quick to set up the part, right on the centerline, oriented to the machine tool's axes," Hunt adds. Tolerances on critical surfaces of surgical parts are 0.001" (0.03 mm), which results in shiny finishes surgeons insist on. For parts with staggered fits, tolerances get as tight as 0.0003" (0.008 mm). Staggered fits are an error-prevention technique that is used to ensure parts can be assembled only one way, and EdgeCAM programs those with no special effort.
Triangle Precision was named 2005 Supplier of the Year by its biggest medical/surgical customer, whose products include implants and instruments for hips, knees, shoulders, elbows, wrists, hands and feet. Orders are often for a complete package of surgical hardware, assemblies, and instruments. The medical business, along with machining aerospace parts, is central to Triangle Precision's diversification. Medical machining of implants, fittings such as screws and staples, cut guides, and cut-guide fixtures account for about a quarter of the shop's business. At one time a large portion of Triangle's business was machining special printing heads, but as that work went overseas, Triangle sought diversification and new markets.
Founded about 25 years ago, the 50-employee shop is owned by experienced model makers Gerald Schriml and Paul Holzinger. Most jobs are prismatic and relatively simple to program and machine. In contrast, most of the new medical and aerospace work has compound surfaces that are curved in two directions like saddles, and organic surfaces, as in implants, that are entirely freeform. This work is common in the aerospace and medical industries.
In medical, the challenge is visually perfect surfaces. Because of the ever-present risk of infection, surgeons reject any implant or cut guide with any visible flaw, regardless of specifications and tolerances. "Because surgeons will reject anything that might harbor an infection, the aesthetics of the implants are life-and-death to us," Hunt says.
Many surface flaws are due to rough handling between machining setups. At Triangle, avoiding dings in delicate finishes was a major justification for the mill-turns, which can rough and finish-machine every surface of a part in a single setup, without any handling. Deadlines are tight and engineering changes are frequent. Hunt praises EdgeCAM for handling changes in a matter of minutes. "We are able to reprogram so quickly and so accurately," he adds, "there is no need to treat prototype and production jobs differently."
This article was first published in the May 2007 edition of Manufacturing Engineering magazine.