Medical Implants in One Setup
Multitasking protects margins and improves responsiveness
By James R. Koelsch
Supplying the 700 level-one trauma centers in the US is an intensively competitive business. Not only must suppliers like Smith & Nephew Orthopedics Inc. (Memphis, TN) produce very fine surface finishes on implantable devices and surgical instruments made from difficult-to-machine materials, but they also must deliver quality products, as surgeons need them. They also must satisfy hospital administrators, who like paying low prices, dealing with as few suppliers as possible, and carrying the smallest possible inventories of all the bone screws, plates, pins, and other devices that their surgeons might need.
"A level - one trauma center has got to have every single product on hand," says Marcus Gunter, team leader for Trigen screws in Smith & Nephew's Trauma Division. "So if one product is not there when they get a patient with multiple fractures from a car accident, they'll throw you out." His job is not to let this happen to his company, the fifth largest manufacturer of orthopedic implants in the world.
Because of the pressure to deliver quality products quickly, his bone-screw group is representative of a growing number of medical suppliers that are relying on advanced multitasking machine tools to produce smooth, precise parts in one operation. Multitasking machines are becoming increasingly crucial to the lean-manufacturing strategies at many medical-device suppliers, not only to help them to remain cost competitive without sacrificing quality but also to give them an edge in customer service by shortening lead times.
In bone-screw production, the most popular multitasking machine is the Swiss-style lathe. Of the nearly 60 Swiss lathes at Smith & Nephew, four of the newest models - two Deco 13a and two Deco 20a automatics from Tornos Technologies US Corp. (Brookfield, CT) - have the advanced technology necessary to make bone screws from titanium and stainless steel in one setup. Because of this ability and a few other initiatives, these machines have been instrumental in reducing 22-day lead times by approximately seven days.
Five years ago, the screws required three processes to be cut to the required size and finish. One lathe would make blanks from the barstock, creating pieces that resembled nails. Perhaps the next day, another lathe would cut the threads with a single-point tool, and yet another machine would create the other features. Now each Tornos lathe creates the nail-like blank, transfers it to the subspindle for cutting the hexagonal head, whirls the threads with a special attachment, and mills the flutes into the tip. "In most cases, we can finish a screw completely with the exception of a little wire brushing in the threads to remove burrs," says Gunter.
These machines cut costs and reduce lead times not only by performing a variety of operations but also by streamlining changeovers. The gang tools, for example, sit on a removable carrier plate. At changeover, the operator simply replaces a whole block of tools with one that has already been preset offline while the machine is completing a job.
Modern computer technology shortens changeovers in a similar way. The CNC accepts programs created and optimized off-line on Windows-based software. As the programmer writes and inserts blocks, the graphics on the screen show the tools moving and removing material. "The software allows you to detect and fix crashes before you load the programs into the machine tool," says Gunter. "Once the program is downloaded into the lathe [over an Ethernet local area network], you can crank it up and run a first piece without cringing."
Because of the quick-change tools and offline programming and verification, the machine can economically produce a mix of screws in small lots. "Lean manufacturing is reducing our inventory and releasing the monies tied up in in-process inventory," says Gunter. And we can respond to the customer order and move it through the shop in a shorter time."
In medical machining, however, speed can never come at the expense of quality, especially surface finish. Very smooth, burr-free surfaces are the norm in the medical industry for a variety of reasons. One is that sharp shards and edges can have disastrous effects in the body. Another is that smooth surfaces make implants and surgical instruments easier to clean and sterilize and, in some cases, inhibit the growth of undesirable biological films on implants. Smooth surfaces also have an important cosmetic effect: They exude a feeling of quality and reassure surgeons that they are using well-made implements.
For all of these reasons, the Swiss machines used by Smith & Nephew's Trauma Div. must produce the finest finishes possible. Fine finishes from the machines reduce the time that the stainless screws spend in the electropolishing bath that produces their largely cosmetic mirror finishes. Fine-machined finishes serve a more practical purpose on the flatter, gunmetal gray surfaces on titanium screws, however. They provide the foundation necessary for the anodizing processes that many titanium screws undergo. Some go to a vendor for a gray anodizing process that anneals the surface. "We anodize other titanium products in-house to apply color for systems that are color-coded to simplify surgery," says Gunter.
The Tornos Swiss-style lathes have the ability to produce the fine finishes required by these final finishing processes. One reason is that, as Swiss machines, they have sliding headstocks. "As you are feeding out of the bushing on a Swiss machine, the bushing always supports the work where the tools are cutting," explains Tom Dierks, president, Tornos Technologies. A Swiss machine shines when the length-to-diameter ratio is large, especially when it exceeds 3:1, as it does on most bone screws and pins.
Another reason for the performance is that the combination of gang tools, driven tools, and subspindle allow the machine to perform a number of operations on the front and back of a screw in one chucking. "As part specifications get tighter, it becomes increasingly difficult to manufacture a part to size and finish if you handle it more than once," says Dierks. If you have to perform a second operation, you now have to re-chuck the part, which always introduces a certain amount of error - not to mention the risk of nicking or scratching it when you move parts, tools, and fixtures among processes.
An advantage that fixed-headstock machines have over their sliding-headstock counterparts is rigidity. As this Hardinge collet-lathe spindle (top) shows, there is very little overhang from the spindle bearings, which reduces the chance of vibration occurring.
Gunter at Smith & Nephew prefers whirling over conventional threading techniques because the process nibbles at the material, not only cutting faster but also requiring less chip management and deburring. "When the thread is whirled, the OD is clean and to size," he says. "When you single-point the thread, the edges are generally too sharp and can vary slightly in size based on variations in the raw material." So it requires more attention during the manual buffing operation that must follow.
Whirling produces better threads than single-point threading because, besides using a nibbling action, whirling allows the bushing in the sliding headstock to support the workpiece throughout the process. "Chasing a thread with a single-point insert works satisfactorily for a normal length to diameter ratio," explains Dierks. "When the screw is longer than five or six-times diameter, however, you lose all your support as you turn down the diameter for the thread and pull it back into the bushing. So you tend to get chatter and tears." The whirling attachment avoids this problem by threading the bar as the machine feeds the raw barstock.
Another important factor contributing to the ability to produce fine finishes is the 8000 - 12,000-rpm spindle speeds on many of today's machines. Given the small diameters, surface speeds are quite high, meaning that the tool not only cuts faster but also reduces chip loads. Consequently, users can exploit coated-carbide, diamond, and CBN cutting tools, as well as sharp edges with highly positive rakes to increase the shearing action, reduce friction, and produce smoother cuts. "Typically our machines will turn 16-µin. [0.4-µm] finishes without any problem, and 8 µin. [0.2-µm] when they have to," says Dierks.
The ability to produce quality surfaces, as well as to hold dimensional tolerances, depends on machine quality. "Burrs and bad surface finishes are the result of the rigidity of the machine and the tooling," says Olaf Tessarzyk, president, Index Corp. (Noblesville, IN). "An interrupted cut on a machine that is not rigid enough will cause chatter and create a burr along the crest of the first and last threads of a bone screw, for example. And these burrs are not easy to remove. If you were to try to recut them on the machine, you would just fold them left and right." Although grinding them solves the problem, the better solution is to avoid the problem altogether by using a rigid setup on a rigid machine.
Rigidity starts with the casting, which should be dense and have ribs to damp vibration. Tessarzyk believes that sturdy slides also contribute to rigidity, and urges buyers to compare sizes on the machines that they consider. He also recommends paying attention to the spindles, looking for rigidity, trueness, and power. This advice goes for the subspindle, driven tools, and whirling attachments, as well as for the main spindle.
Spindle power is especially important in screw machines because they typically have more than one tool in the cut at a time. Index's Traub TNL26 screw machines, for example, can put as many as four tools in the cut at once. To avoid creating the vibration that can occur when a spindle is slowing down, both the sliding and fixed-headstock versions of this machine come with spindles that generate as much as 14.6 hp (11 kW). The main spindle and subspindle on these machines are the same size and have equal power to eliminate this source of chatter. The motors on the driven tools are as large as 8 hp (6 kW) to avoid the problem on cross drilling and milling.
Because smooth, burr-free finishes also depend on tool sharpness, screw-machine builders have developed ways to handle chips and allow putting fresh tools in the cut without stopping production. For example, many builders now offer removable plates that users can load with tools off-line, and even store a complete setup on a nearby rack. When the time comes for changing the tools, the operator replaces entire plates of tools at once. Other builders design their machines to hold a large inventory of tools.
For example, the Traub TNL26 from Index can hold as many as 70 tools in its carriers and turrets. "If tool life is only a few hours, you can load several of these tools in the turret so you don't have to stop the machine to change them," says Tessarzyk. The large capacity allows tooling-up the machine for several recurring jobs or size variations to reduce setup time for the frequent changeovers that characterize the business. Turrets also index rapidly to throw off the nests of long, stringy chips that gather around tools, especially when machining titanium and stainless steel. This technique helps to prevent the welding that can shorten tool life and mar finishes.
Because of the performance that Swiss-style screw machines offer for difficult-to-hold parts, many machine shops in the medical industry are in a habit of putting all of their precise small parts on Swiss machines automatically. A number of machine builders advise against this practice. They argue that fixed-headstock machines can create finer finishes and cut to tighter tolerances for much less money in certain size ranges.
Consider the Quest GT27SP, a gang-tooled lathe built by Hardinge Inc. (Elmira, NY) that has produced cranial cutters for drilling into skulls, tiny impeller-like cutters for clearing cholesterol from arteries, and tiny parts for pacemakers. This machine comes with an 8000-rpm spindle and a subspindle capable of processing bar stock less than 1/8" (3.2-mm) diam and as large as 1" (25.4 mm), a size range comparable to the bar capacity of many Swiss machines.
Its spindle speed also is comparable. So, like its moving-headstock counterparts, it can exploit high speeds to not only cut faster but also reduce chip loads. The low chip loads allow users to run tools made of diamond and CBN, and those made with sharp edges with highly positive rakes, which increase the shearing action, reduce friction, and produce smoother cuts.
The basic difference between this machine and a Swiss-style lathe, of course, is the headstock, which can be a blessing and a curse, depending on the application. "Obviously, very long, slender parts, like a long needle, would have to be done on a Swiss machine," admits Bob Allington, senior applications engineer. "But for parts that have, say, a 3:1 length-to-diameter ratio or less, the gang-tooled lathe can do just as well as, if not better than, the Swiss machine."
To support his claim, he points to the 0.00001" (0.25-µm) resolution of the axes on his gang-tooled lathe. "Our 10-millionths resolution translates into better surface finish because the slide moves in such tiny increments," explains Allington. Finishes are finer on small details, such as on corners and edges." Moreover, Hardinge's spindle is ground within 0.000015" (0.4-µm). So the features require either less polishing or none afterward, and the size and form tolerances are within 0.0002" (0.005 mm).
At Index, Tessarzyk agrees in principle that a good fixed-headstock machine can produce parts with greater precision and finer finishes when length-to-diameter ratios are relatively small. He points to a more fundamental reason for their better performance, however, namely the physics of the machines. The guide bushing in a sliding-headstock machine has a slight clearance to allow it to slide over the bar. Although this design is extremely accurate for turning, the clearance allows the bar to move and vibrate slightly when a milling or other cross-tool applies forces in the radial direction of the bar.
"The collet cannot clamp the material in a fixed position," he adds. "Therefore, milling [on a sliding headstock] is not as precise as with a fixed headstock." Since medical applications typically involve a large amount of milling, he, like Allington, recommends considering a fixed-headstock for short parts that require precision.
Another advantage of fixed-headstock machines is that they tend to be cheaper to buy and operate. According to Allington, the typical gang-tooled fixed-headstock machine costs between $80,000 and $90,000, and Swiss-style machines carry a price tag of $200,000 - $250,000. So machine shops that favor Swiss-machines out of habit often can get a huge cost advantage simply by transferring work from a very expensive machine to a cheaper one - which might be better suited for it anyway.
Fixed-headstock machines sometimes offer an additional advantage in operating costs by being less complex. "On a Swiss machine, you work with a guide bushing, which is not the friendliest device," says Allington. "Operators tend to struggle with the guide bushings because Swiss machines are usually very small, congested, and quite difficult to work on."
The desire to consolidate processes without sacrificing finishes and dimensional tolerances is not confined to small workpieces. Manufacturers of larger implants, such as hip, knee, and shoulder replacements, also are showing interest in advanced machining technology that can give them this ability. For these manufacturers, however, smooth finishes are crucial for function, not just for sterility and aesthetics. The mating members of a joint need as little friction as possible to reduce the wear that can occur over 20 to 30 years of use. The goal is to avoid the surgery that's necessary to replace worn parts.
To strike a balance between machine time and a suitable surface finish, some are deploying the technologies that Mikron Bostomatic Corp. (Milford, MA) offers through its Smart Machining program. In the builder's Mikron HSM 400U machining center, this program combines automation and five-axis, high-speed milling to optimize machining performance and allow untended production. According to Gary Zurek, applications engineering manager, the Smart Machining program includes such modules as a system for monitoring and recording spindle vibrations, a thermal control system to compensate for thermal deviations, and an interface that allows experts to optimize the machining process for a specific workpiece.
Implant manufacturers also have learned the benefits of multitasking from operations such as Smith & Nephew's Trauma Div., which uses modern Swiss machines. They are beginning to explore how they can also reap lower costs and shorter lead times by using multitasking machine tools. Mazak Corp. (Florence, KY) reports that it has been developing several applications over the last year or so for making hip and shoulder implants on its Integrex multitasking machines. Because these machines are lathes with a full-size milling spindle in place of a motorized tool, they can produce almost complete devices from round stock in one setup.
Moreover, twin-spindle models can do the work without using special fixtures, yet create 16-µin. (0.0004-mm) Ra finishes. The secret is holding workpieces with three-jaw chucks to support them on both sides with the opposing spindles. "Rather than just hanging on a part in space on one side, which vibrates like a tuning fork as you start cutting, holding onto the part with both chucks gives us a much more stable process," says Mike Finn, an applications engineer at Mazak. "It enables us to hold onto the part more securely. And the more securely you hold a part, the less vibration and higher quality surface finish you're going to have."
According to Finn, the setup enabled production of one implant entirely, except for polishing, from a round bar of high-temperature alloy. He claims that cutting small lots of the complex part this way is more efficient than putting it on a five-axis machining center. He admits that having to machine more material away increases cycle time, but notes that the short setup time and the ability to feed the bar through the spindle more than makes up for it. "Because the only setup needed was the chuck jaws, I was able to get rid of a bulky fixture that might be expensive and interfere with access to the part," Finn says. So the technique shines in small-lot production and will save the user time and money.
Other implant manufacturers are looking for fine finishes from only one setup on multitasking machines based on grinding. Schutte TGM LLC (Jackson, MI) reports that a number of implant manufacturers are using its five-axis tool grinders to eliminate most of the hand polishing on mating surfaces of joints. The multitasking machines are capable of creating 3-µin. (0.08-µm) rms finishes-automatically under computer control.
"There has always been a lot of hand polishing in the medical industry," says David Brigham of Schutte TGM. "Even when they began using robotic polishers and buffers, they still need the hand work after they come off the robot." Because the tool grinder can mill, grind, sand, and polish intricate workpieces in multiple axes, it usually can cut and polish implants from castings and forgings in one setup. Hand polishing in a few tight spots would be the only work--if any--that might be necessary afterwards.
In knee-joint production, Brigham estimates that a five-axis, multitasking tool grinder can perform 99% of the cutting and polishing. In one application, the machine cuts and polishes a knee from a cobalt-chromium alloy forging in only 18 min. Moreover, the operator needs only a minute to setup the machine and load the appropriate program. If the user had wanted to employ the machine's probing package, on-machine inspection would have added another few minutes to the cycle time.
Using conventional methods, the same knee would have required five operations: milling; two robotic belt-sandings; robotic polishing; and manual polishing. The total cycle time of all five processes would have been approximately 1 hr 40 min, and setups and the time spent between operations would have extended the time that the piece would spend in production.
Because of the advantages inherent to using multitasking and other advanced machining technology, machine tool builders believe that medical manufacturers of all kinds will continue to embrace it. Combining several operations into one is a new concept that many users struggle with at first, but one that they profit from as they learn to use it.
This article was first published in the May 2005 edition of Manufacturing Engineering magazine.