FEATURE
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Figure 1. The HPM 600 HD, shown here with an integrated pallet changer, from GF AgieCharmilles offers dynamic three-axis milling, spindle speeds that can reach 20,000 rpm, and high chip removal rates.
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Medical technology has been remarkably successful at prolonging lives and, just as importantly, improving patients’ quality of life. The phenomenal rise in the use of hip and spine implants in industrialized countries is a case in point. In Switzerland, for example, half of the population over the age of 60 suffers from joint-related ailments, according to Stama Maschinenfabrik (Schlierbach, Germany; www.stama.de). Approximately 16,000 artificial hips are implanted annually, and that number will rise dramatically in the years ahead.
To satisfy market demand, medical device manufacturers are engaged in unbridled competition. Companies are racing to develop the next breakthrough technology, which is often part and parcel with a reduction in the size of the product. “Various market leaders [in orthopaedic implants] are investing huge sums of money in the development of new products designed to treat patients quickly and less invasively,” says Philippe Charles, med-tech segment manager at Tornos S.A. (Moutier, Switzerland; www.tornos.ch). “To enable minimally invasive operations, the size of the surgical instruments and the actual implants need to be scaled down.”
Providing the means to manufacture those instruments and implant components in a cost-effective and rapid manner while maintaining extremely precise tolerances is the challenge that machine builders must face day in and day out.
Do it once . . . and for all
The ability to multitask is not just a desirable quality in employees; implant manufacturers want their capital equipment to be comfortable with multitasking, as well. Stama, for one, has embraced the concept of complete machining from the bar on one centre since it began developing equipment for use in medical manufacturing.
Multitasking eliminates idle time in the workplace, says Michael Herman, director of sales and marketing for Stama in the United States. Fabricating the implant, as well as the specific tools needed for its machining, on a single centre allows the manufacturer to produce identical or different parts in succession from the same bar quickly with no setup changes, notes Herman. “Milling and turning with a maximum of two chucking operations on one centre helps the implant manufacturer to improve the quality of his workpieces. It also saves time because there is no additional setup involved, thus lowering costs.”
Key elements in the efficient fabrication of implants, according to Gisbert Ledvon, business development manager at GF AgieCharmilles’ US plant in Lincolnshire, IL (www.gfac.com), include the use of high-performance machine tools and equipment made with advanced damping materials. Integrated automation is also a vital aspect of effective multitasking, he adds (see Figure 1).
To eliminate or reduce secondary operations, GF AgieCharmilles is introducing more-dynamic high-performance machine tools that can handle tough alloys, says Ledvon, but integrated automation and the use of so-called smart machine features are also important. “For example, a sensor in the spindle of our machines is connected to the controller, allowing the operator to check for tool balance, vibration, or thermal expansion.” The so-called smart machine module is monitoring all of these parameters, making modifications on the fly or shutting down the machine if cutting conditions reach a critical point.
Think small
The Office series of machines from Haas Automation (Zaventem, Belgium; www.haascnc.com) have garnered a great deal of attention because of their compact size. “The series was conceived as a smaller version of our larger general-purpose machines and was designed to fit through a door—literally,” says product manager Dave Hayes. But the Office mill and lathe can hold their own with the larger machines and are, in fact, being used to manufacture implants and smaller medical parts, he reports.
“Typically, people will turn a part and then carry it to a mill and maybe put it in a fixture and flip it around,” says Hayes. “With the Office lathe and the right software, you can turn and mill the little part and knock it off finished. Whether you are doing small lots or prototyping, you have the advantage of not having to go to two machines. You don’t have to refixture the part and worry about it not being clamped correctly. With everything getting so small, it’s challenging enough just to hold the parts, let alone move them around!”
The mill’s spindle speed achieves 30,000 rpm and higher, according to Hayes, and the gang-tool lathe attains 6000 rpm. “The lathe has a c axis and can accommodate driven tools,” adds Hayes. “It allows you to do a lot of small parts that require turning and light milling and drilling in one setup. The Swiss-type machines that are typically used for medical parts are expensive and time-consuming to set up,” he adds. “The Office lathe is less costly, but still has good capabilities and a lot of flexibility.”
The EDM alternative
As medical parts continue to get smaller, more manufacturers are looking at electrical discharge machining (EDM). The technology offers distinct advantages in med-tech manufacturing, not the least of which is the level of precision it can achieve.
In the EDM universe, “we are used to speaking in microns,” says Hans-Jürgen Pelzers of Mitsubishi Electric (Ratingen, Germany; mitsubishielectric.de). “Five micrometers is big for us!” In addition, EDM can produce finished parts in one shot.
Conventional machining techniques physically cut into the workpiece, leaving behind any number of imperfections that must be removed by secondary operations. EDM, however, uses cutting wires that can be as small as 0.02 mm in diameter. An electrode produces a spark that vapourizes a metal surface layer. Because the electrode never touches the part, it leaves no burrs, scratches, or cracks on the material’s surface.
The technique is being used by researchers at the Institute of Microtechnik (IMM; www.imm-mainz.de) in Mainz, Germany, to fabricate teeth bores. “The bores are flexible, but must have sharp tips,” explains Pelzers. “IMM researchers tried to make the tips by grinding, but it was difficult to ensure accuracy because the parts bend when they are touched.” EDM equipment from Mitsubishi proved to be the perfect tool for this application, notes Pelzers, because the workpiece does not touch the part and, therefore, applies no pressure that would cause it to bend.
Ledvon also champions EDM for certain applications partly because of rising materials costs. “The prices are getting out of hand, and wire EDM cuts slugs, not chips, which can be used to machine something else,” he explains.
One of the technology’s shortcomings—its slowness—is being addressed by GF AgieCharmilles, adds Ledvon. “Twin-wire technology, which is available on our Vertrex FI 2050TW and FI 6050TW machines, switches automatically from a 0.25-mm wire for high-speed cutting, for example, to a much smaller 0.1-mm wire to finish the details. Cutting time can be reduced by as much as 40% compared with conventional EDM equipment,” says Ledvon.
Accelerating time to market
Optimizing fabrication methods to enhance accuracy is only part of the battle, though. Getting the product to market fast is equally important. That requires machines with built-in versatility.
“In addition to high dimensional precision and products with an impeccable finish, the machine tool needs to be designed to be able to machine different materials such as titanium, stainless steel, and even PEEK, which are used in the manufacture of implants,” says Charles of Tornos. The company’s Deco range of machines are designed to boost productivity in this regard, he adds.
When it comes to high-speed milling, it is not uncommon for manufacturers to think that simply getting a tool with a 20,000 to 30,000 rpm spindle speed will increase their throughput, says Ledvon. But that is shortsighted.
“If your part has a lot of details, which are typical in the medical industry, and has sharp corners, the machine has to slow down when it gets to those corners, slowly move around them, and then accelerate quickly to benefit from a high-speed spindle. If your acceleration and deceleration speeds do not exceed 1 G, you won’t be able to achieve faster cutting times, even with a high-speed spindle,” says Ledvon. You have to look at the entire process of what high-speed and high-performance cutting means, he adds. “If you can’t accelerate fast enough, you won’t be able to keep a consistent chip load on your cutter, causing it to wear out faster. This will affect your turnaround time and tooling costs because you have to change your tooling more often,” he explains.
Hayes cautions manufacturers from trying to run small parts on ill-suited big machines. “They are trying to whittle away with spindles that don’t have the necessary rpm,” he says, “and then they are left to deal with [unsatisfactory] surface finishes and tool-life issues. With the right spindle rpm, though, you can machine not just at the normal speed, but in some cases even faster,” says Hayes.
That’s good news when your focus is on shaving time off your production schedule to beat your competition to market. Sound familiar?
