Getting faster & more precise, especially on smaller parts
Medical manufacturing, like other industries, faces intensive demands for improved productivity. As a result, many manufacturers are focused on achieving greater efficiencies and precision in making small parts.
No technology beats Swiss turn when it comes to churning out many small parts. According to Brian Such, executive vice president of Marubeni Citizen-Cincom (MCC), Allendale, N.J., one recent improvement in Swiss machining is the use of modular gear boxes that quadruple the speed of the live tool. Such explained that as the industry tackles smaller parts, with drills going down to 0.004" (0.1016 mm) in diameter and end mills down to 0.008" (0.2032 mm), the spindle must spin at 30-40,000 rpm. But “there’s no room to bring the whole gearbox up to that level. Most Swiss machines have live tooling in the range of 6,000 to 10,000 rpm.”
Air spindles are an earlier—and still used—fix for this problem, but they lack the power for anything other than some drilling. Such said modular “4× spindles” are a better solution. “You just slide it into a standard live tool pocket, and whatever you program at 1,000 rpm will go 4,000 rpm, but with the power of the machine, not the light power of an air spindle.” And because Citizen standardized the cross spindles on its M, L, and A models, “a user can move the speeder from one machine to another,” said Such, making it a very “useful and creative” option.
If even more speed is needed, a user can add an electric spindle from a vendor like NSK. These units go up to 80,000 rpm and have more power than an air spindle, though not as much as the geared unit, explained Such. For Citizen machines, they are made to MCC specifications and fit in the same location as the standard live tools. In addition to their expense, it becomes more difficult to set up and program multiple electric spindles, “so a gear-driven spindle is a much freer and more portable solution.”
Citizen brings another easy and cost-effective option: tandem live tools. As Such explained, many Swiss machines, including some Citizen models, are limited in the number of live tools. “For example, the Cincom L12, L20, and L32 have four live and four static tools for sub-spindle work. In the past, if a customer needed more tools we would have to offer a machine with more functions and added cost.”
So Citizen’s MCC R&D group in the U.S. engineered a tandem tool that uses one live tool station to drive two tools. The gear-driven mechanism is tiny, so the added live tool takes one of the static tool positions, but no additional space. “We can put four of these units in and get eight backside live tools, doubling our offering to our customers,” said Such. “This allows customers to get more tooling into their faster Swiss lathes for better production rates.” He added that not only does this often save the customer from buying a more expensive machine, the units are modular and can be moved from machine to machine, like the speed multiplier.
Automating the Avalanche
Since Swiss turn is inherently good at spitting out many parts fast, “the hardest thing isn’t machining the part, it’s collecting it,” Such explained. “It could be a one millimeter part that you can fit by the hundreds in the palm of your hand.” The challenge is especially acute for medical parts that can’t have any blemishes or dings. These need to be handled individually in a way that protects them.
An example is producing bone screws, which vary in size, shape, and features, often including tiny, tight tolerance, hexalobe forms in the head—an excellent example of where micro tools and high-speed spindles are needed. Bone screws also have “very important finishes all over,” according to Such. So Citizen’s MCC Engineered Automation division created a system that individually grabs each bone screw with plastic fingers and moves it to a multi-axis tray center attached to the machine, placing each screw in a tray without banging it into other screws. “This tray system can be set to handle one part, 20 parts, 50 parts—it all depends on the size,” explained Such. The system moves trays in to collect parts, and then move them to storage positions inside the unit, or directly into a part washing system, if desired.
The system also facilitates SPC by setting aside a sample from each tray according to whatever part count is desired. For example, the system can be programmed to collect lot sizes of 50 per tray and set aside one per tray. “You can come back from unattended operation overnight, check the last part, and if it’s good they’re all good,” explained Such. “But if that part has a problem, you can go back to the next tray. Is that one good? And you can easily sort through a vast numbers of parts all individually placed in trays.” Such said this approach to QC has enabled customers to go from dedicating an operator to each machine to running 70 percent unattended on bone screw projects.
Citizen also implements vacuum extraction systems for tiny parts, added Such. These suck the part from the sub-spindle at cut-off and move it to the storage unit. The speed is also a challenge, as Such explained. “You’d basically be shooting a bullet into your basket, banging your parts. So we have an elaborate vacuum system that goes into a speed reducing funnel, and the part just drops nicely into the basket system.”
After that are the same options as the gripper-based handler just discussed.
The Human Touch in Small Part Assembly
Assembling medical components presents its own set of unique challenges, according to John Lytle, engineering manager for Promess Inc., Brighton, Mich. “They’re usually small parts with light loads, often put together by hand.” Automating such a process requires the ability to detect and react to nuances in force or motion with precision and speed. And the servo-presses in which Promess specializes are designed to do exactly that.
One example is a hand-held injectable. As Lytle described the assembly process, “a plastic housing has a groove that the injector mechanism must snap into. It needs to overcome that groove to snap it into place, but if it goes just a few thou too far, it destroys the whole assembly because it’ll hit the other side, breaking the injectable part.”
Promess solves this problem by detecting the rate of change in force as the housing approaches the groove, and graphing this rate relative to the position. Then, based on modeling from test runs, Promess knows where to press “a little bit more.” The approach delivers the precise control and repeatability needed to automate the process, so that the assembler becomes a machine operator, as Stephanie Price, senior application engineer, put it. The operator puts the components into the Promess press and removes finished parts. But the machine does all the rest, including providing feedback.
A titanium catheter assembly provides another example. Two halves needed to be crimped together so that the “business end” maintained a tight tolerance. And the crimp was so small it required a microscope to inspect, explained Lytle. The required load was so light that Promess also needed to control the force to within less than one newton.
Given its work on such projects, it is not surprising that Promess sees medical companies “migrating more towards higher level automation and servo control technology,” as Lytle observed. Price added that “the medical field, probably more than any other industry now, has really embraced our stand-alone press units,” as opposed to integrating a Promess press into someone else’s larger mechanism. A stand-alone Promess Electric Press Work Station features a light curtain for operator safety and offers an easy way to add sophisticated press technology to a production line.
Medical manufacturing systems also require documentation. As a result, Promess systems for medical applications measure forces and movements with great precision, and record it all in detail. And “it’s not just that it gives data; we allow you to add in multiple sensors,” explained Price. “So if [a system is] using external probes or lasers, or external load cells, within a process, we can take in all those different sensors. Not only can we control our motion using those different sensors, we can also collect all the data from those sensors in one centralized location. So instead of needing data management for that machine, it’s all stored in one local database.”
Machining Instruments and Implants
As the folks at SW North America, New Hudson, Mich., see it, making larger, lower volume, parts like surgical instruments usually requires prioritizing flexibility in retooling. But, for a single-use product like neurosurgery forceps, the high volumes justify a specialized solution based on a machine like its twin-spindle BA 321 horizontal machining center.
“These forceps are made of aluminum so that they can grasp tissue components while simultaneously preventing minor bleeding with electrical pulses,” said Frank Pauschert, SW’s regional sales manager in Waldmössingen, Germany. “Minute tubes are integrated into each arm of the forceps to direct irrigation fluid into the operating area. Despite its straightforward appearance, the instrument requires time-consuming manufacturing involving many mechanical and manual work cycles—all while adhering to strict quality standards. Many of these work cycles must be carried out under a microscope.”
The BA 321 machines the forceps halves from special press-drawn sections without separating the two halves until the last work cycle. This avoids having to clamp the slender and delicate segments from the beginning. The final cut to separate the halves actually leaves a minimal burr that prevents the pieces from falling into the machine, which would damage the tips.
Instead, special adapters loaded into the spindles from the tool magazine grab the parts and break off the remaining burrs. They then deposit the parts individually into a drawer, which travels via a conveyor belt to a removal station, where the parts are manually inserted into basket racks for the next work cycle. Owing to its 17,500 rpm, HSK A63 twin spindles and the ability to change tools with a 2.5 sec. chip-to-chip time, the BA 321 delivers two milled forceps halves in 3 min. And, thanks to the automation system, it can do this around the clock, largely without supervision. “The system sometimes runs for up to a week with no human interaction other than supplying material and removing finished milled parts,” said Pauschert.
Achieving high material removal rates on the tougher titanium and cobalt chromium used for orthopedic implants takes more than a fast machine. It takes tough tools like the solid-carbide, high-feed end mills from Cole Tooling Systems/Millstar, Orion Township, Mich. Vice President Ron Field recounted that the tools were first developed for moldmaking work, but then an observant applications engineer surmised they’d be effective on titanium implants.
At first, recalled Field, “the titanium guys said ‘Have you ever seen an uglier tool than this? How’s that going to cut titanium?’ ” They thought it was “ugly” because the tool features a large radius across the end that blends into a smaller radius around the corner, and then a back-taper. The result, said Field, is a positive cutting edge with no tangent point to induce wear. “We put it in the machine, and instead of roughing titanium in the 300 sfm [91.4 m/min] range at a fairly slow feed rate, we ran at 600 sfm [182.9 m/min] with a feed rate of 200-300 ipm [5-7.62 m/min]. It basically did the roughing in one third the time.”
Dubbed HFM4 (for high-feed mill, four flutes), the tools have also proven themselves in hardened steels and cobalt chrome, reported Field, all with the same grade carbide and Millstar’s proprietary HSN coating. “The coating has silicon in it, so it’s like an AlTiSiN. The slipperiness of the silicon helps prevent the workpiece from welding to the tool.” He added that while the ideal depth of cut would be “fairly shallow,” the feed per tooth would be high. “For example, with a 3/8" [9.53-mm] diameter tool, the depth of cut in titanium would be about 0.014" [0.3556 mm], but the feed per tooth would be around 0.012" [0.3048 mm] per flute, and it’s a four-flute tool.”
Greenleaf Corp., Saegertown, Pa., is also a player in this field, even designing a proprietary holder to enable its tools to cut especially challenging orthopedic features. Joe Presits, sales and service engineer, has recently concentrated on machining shoulder balls and joints in cobalt chrome. “Those ball joints have to be perfect spheres,” explained Presits. “You can’t have more than a minuscule protrusion, even at the bottom of the bowl, because that would require extra labor in finishing the part. Too much of a protrusion and they’ll end up scrapping the part. I try to achieve a protrusion that is so minimal you can hardly feel it with your fingernails.” Otherwise, both the ball and the cup have a mirror finish, he said. “When you run your finger across you can’t feel anything.” Accuracies are in the range of 5 μm, he added.
Although Greenleaf is known for its ceramic tooling, Presits said these parts are typically roughed with the company’s abrasion-resistant PVD coated G-925 carbide inserts, which have a “tough edge and are highly wear resistant … and then we’ll use ceramics to finish. In some cases we will rough with ceramic, depending on the size of the inside diameter and what is required, and then it’s pretty much a question of technique.”
If a customer opts for ceramic on these parts, it’s the whisker-reinforced WG-600 insert with a proprietary coating. “It’s more heat resistant, which allows 20 percent more surface footage,” said Presits. By way of comparison, he said he’d run carbide at 230-250 sfm (70.1-76.2 m/min) and ceramic at 1,700 sfm (518 m/min). He said the chip load is the same whether the tool is carbide or ceramic, and all machines can be programmed to maintain the chip load throughout the cut, so ceramic offers a time savings.
Presits also advised using the largest possible radius when cutting the hip socket, generally a ¼" (6.35-mm) round V-bottom insert. “Because you’re constantly changing the edge as you’re rolling around that ID, it’s constantly changing the point of contact on the insert, which makes the insert last longer,” he said.
New Inspection Options
For the final inspection of small medical components, even those below 100 µm in length, Marposs Corp., Auburn Hills, Mich., introduced the Optoflash XS30. With easy-to-use software and high-resolution optics, Marposs says the Optoflash captures more than 100 static measurements within 2 sec. at U95 = 1 µm + D/200 for diameter measurement and 2 µm + L/200 for length measurements.
Measurements could include small changes in outer diameter, perpendicularity, through-hole diameters, and other features. The device also rotates the part to measure runout, or features in different axial positions. As Eduardo Bolivar, product manager, explained, the Optoflash can capture a profile image of the part and compare it to a nominal DXF profile for a quick good/not good determination within a given tolerance.
In describing its potential impact on a shop’s throughput, Bolivar cited a case in which a customer needed to measure a number of critical distances, radii, and chamfer angles. “Before getting the Optoflash, they had to use a combination of a shop floor CMM and a SURFCOM, and it took 10 to 15 minutes per part. With the Optoflash, they can perform all the required measurements in two minutes. That saved between 45 to 50 minutes per shift.”
The Optoflash XS30 itself is 2 × 1.75 × 1.25' (609.6 × 533.4 × 381 mm) in size and measures parts up to 1.2" (30.48 mm) in length and ¾" (19.05 mm) in diameter. (Marposs had already introduced similar technology for larger parts.) Bolivar added that Marposs offers a variety of workholding options, including clamping between centers, chucking the part, or mounted on a face plate, and all three can be used to rotate the part if needed. Plus the device accommodates both manual and automatic loading via robot, and it’s designed for both the production floor and lab.
Finally, Bolivar also pointed out that Marposs recently added Tecna to the Marposs Group. Tecna specializes in leak testing and solvent dispensing products, so as Bolivar explained, they “can now do leak and flow testing of dialysis equipment, bags, catheters—anything that has to hold any kind of fluid or air.” |||