The Smaller, The Better
In search of turning and milling solutions for the micro world
By Jim Lorincz
There are many reasons why manufacturers continue to chase machining solutions for the smallest parts. The biomedical industry, for one, thrives on developing the least-invasive devices for treating patients for everything from cardiovascular, orthopedic, and neurological procedures to surgical applications of every type. For industries as diverse as microelectronics, aerospace, and communications, as well, increasing demand for complex, smaller components made from difficult-to-machine and exotic materials have spurred emphasis on machines and tooling solutions scaled to the task.
When one thinks of small precision devices, it's hard not to immediately call to mind the advances in Swiss turning centers that have made them essential to manufacturing precision-engineered parts to a maximum 32–37-mm diam for literally every industry that values the benefits of miniaturization. Sliding headstock, Swiss-style CNC automatics have been a mainstay of these manufacturers, but even these precision machines are being tweaked to improve quality, finish, and accuracy for the parts they produce in a single setup.
"The orthopedic medical parts we are seeing are more complex today," explains Dan Murphy of Rem Sales Inc. (East Granby, CT), importer of Tsugami machine tools. "If you look at an implant from 20 years ago versus today's, the implants are now more contoured, more sculpted to fit better and move better within the human body. They used to look more mechanical. They are also available in a variety of sizes, again, to better fit the human body."
Size is one thing; tougher materials are another. Murphy explains: "The parts are made of tougher materials with higher tensile strength to last longer. All of these evolutions have impacted the machining of these parts. Manufacturers need machines with more tools accessible in the work zone. They need the ability to do contours and 3-D profiles. They need rigid spindles. For surgical instruments, most use pre-heat-treated stainless such as 17-4 and custom 455 stainless. These are typically heat-treated to about RC 38 or so. For trauma parts, things like bone nails and bone screws, we are seeing a lot of Nitronic 50 or 60 stainless and BioDur alloys rather than titanium and 316 stainless."
Efficiency-draining changeover time has challenged Swiss machine design. "In the past, manufacturers have made a blank on a Swiss-type machine and then put the part on a machining center to do the complex contouring, reasoning that a machining center could do a better job than an attachment on a lathe or gang-tool Swiss machine," says Murphy.
Tsugami's TMU1 Swiss type machine offers a solution. It has a tool changer. "All of the tools required for the entire family of parts stay in the machine all the time," Murphy points out. "This cuts changeover time dramatically, because operators don't have to set tools between runs. Operators can change tooling in under ten minutes. Further, the TMU1 has a machining-center spindle built in, so it can handle 3-D profile milling and similar operations efficiently right on the one machine. That improves accuracy, too, because the part doesn't have to be rechucked."
Rigidity is another important consideration in Swiss-style machining. The double-spindle design of Tsugami's TMU1, BH38, and BH20 machines is said to provide a more rigid spindle. "We typically offer about 2.5–3x the strength and rigidity over a standard Swiss machine. These machines can handle heavy turning in more difficult materials," says Murphy.
"All Swiss-type machines have a guide bushing to support the bar stock as it slides through the spindle," explains Andre Bettuz, Index Corp. (Noblesville, IN). "Most of the time, the user has to manually adjust the pressure of the bushing on the bar during the setup prior to starting the machining cycle. And that's it. Any variance in the bar quality—diameter or roundness—during machining means that the guide bushing will be tighter or looser as the bar is fed into the machine. That's why some applications require centerless ground stock to assure consistent tolerance of incoming stock. That's an added material cost."
The programmable guide bushing on Traub's TNL series CNC sliding headstock automatics offers the ability to program a series of pressures on the bar. Instead of just setting the position of the guide bushing and moving on, the TNL machines use a series of air or hydraulic pressures, depending on the machine model, that will allow the guide bushing to consistently support the bar despite variances along the length of the bar. The bushing actually moves depending on the pressure range set within the program of the machine. As the bar diameter varies, the guide bushing follows the variance.
"We know that from 2–4 bar pressure is going to be a sliding pressure, good enough to support the bar, but not to hold it too tight. But when the machine has to mill or drill or do heavier work, the program calls for a high pressure, and the guide bushing is actually going to clamp the bar as a fixed headstock machine would. Then the program calls for a lower pressure, and you're ready to turn again," says Bettuz.
"The idea remains the same; the guide bushing is always supporting the bar, and when you need to do heavier work, the bushing actually clamps the bar like a fixed headstock. This has a direct impact on machining quality," says Bettuz. "With the programmable guide bushing, the incoming bar quality is not as critical. On typical Swiss machines, the incoming bar needs to be ground or the product output will not be good enough."
Engineered attachments for Citizen CNC Swiss turning centers from Citizen Machinery America Inc., Tooling & Accessories Div. (Agawam, MA), are designed to improve productivity and the quality of finished parts. They include thread whirling, high-pressure coolant systems, and OD and ID toolholders.
Thread whirling is recognized as one of the most efficient ways to produce difficult OD threads for bone screws. These complex threaded components usually require long hours of process development, setup, debugging, and cycle time. Inserts in cutter heads have ±25° maximum helix angle. Thread variations possible include deep Acme and buttress threads, taper threads, variable lead threads, extra long threads, and miniature threads. Thread whirling packages are available for Citizen machines.
Cool Blaster high-pressure coolant systems on Citizen Swiss machines are well-suited to performing deep-hole operations such as gundrilling and boring, and for effective chip control for high-speed drilling, turning, and grooving. Direct high-pressure coolant can either break or control the chips by moving them away from the tool and workpiece, increasing tool life and improving finishes. Cycle time for drilling can be reduced 40–70% with direct high-pressure coolant. Pressures range from 0 to 137 bar. Cool Blaster-equipped machines provide high-pressure coolant for a variety of OD and ID toolholders. The Cool Blaster system includes a custom mechanical and electrical interface for each different model machine.
The adaptive guide bushing for Citizen Swiss-style lathes is a double-taper bushing with constant holding pressure along the full length of the bushing. It enables the manufacturer to use nonground bar stock, and allows for bar deviation to 0.008" (0.2 mm) and improved TIR, as constant pressure provides greater rigidity.
The ECAS 20T 12-axis Swiss-style turning center from Star CNC Machine Tool Corp. (Roslyn Hts, NY) was shown at SME's WESTEC exposition, and will also be on display at EASTEC. The ECAS 20T features a completely independent three-turret design, allowing for three tools to be working simultaneously in the cut at any time. Each station can have from one to six cutting tools, and each turret has a Y axis, moving vertically so that it can accept vertical tooling. One turret has a Z axis, which allows different operations to be done at the same time.
The advantages provided by the three-turret design and the ability to put multiple tools in the cut simultaneously are fast cycle times and the possibility of having opposing tools working on the part at the same time as a support. A turret is assigned to the subspindle that can use any kind of tool; ID and OD machining, face work, off-center work, and angular work are all possible.
Normally, with this many tools you can make any kind of part, and probably do it in a bare minimum of cycle time. You can make very complex parts because you won't be short of tooling. This is especially important in medical device machining for machining complex parts such as spinal devices.
Micromachining figures prominently in mold and die making and hard milling, especially for smaller, intricate plastic parts. The trend toward product miniaturization is characterized by miniature, micron, and submicron manufacturing technologies as manufacturers strive to meet the increasingly tight tolerances of parts that are becoming smaller. According to Makino Inc. (Mason, OH), micromachining is "the manufacturing process for small or miniature components, and is simply defined as the manufacturing of a part, or the mold or die to produce that part, wherein the part itself has a size of 10 mm or less, or has feature sizes of 0.1 mm or less."
Makino's most recent entry for small-part machining, the V22 VMC, is designed to machine complex materials, including ceramics, as well as intricate cores and cavities for precision parts manufacturing, such as medical equipment and small and intricate molds for plastic injection-molding operations. The V22 features an 11.3 hp (8.4-kW) continuous 40,000-rpm spindle with core cooling and under-race lubrication. Travels are 320 x 280 x 300 mm (in X, Y, Z) in a work zone, and table size of 450 x 350 mm that can handle a 100-kg load and maximum workpiece size of 450 x 475 x 200 mm. Rapid traverse and cutting feed rates are 20 m/min.
If you can still see the part or feature with the naked eye, perhaps you're still operating in a macro world. At least it seems that way with machine tools from Kern Precision Inc. (Webster, MA) that can literally split a hair (about 60 µm diam) from a human's head. "Our product lineup is geared toward micromachining parts that can be held in the palm of the hand, something that could fit on the head of a pin to a 4" [101.6-mm] cube," says Gary Zurek, president of Kern Precision. Examples include an endoscopy tool 4.5 mm long for tissue removal, a graphite electrode for an electric razor with blade thickness of 0.1 mm, and brass spacer blocks drilled with a depth tolerance of ±0.01 mm, machined untended in an automatic pallet system. Processes performed include four-axis fine cutting and engraving 50 µm wide to a maximum thickness of 3 mm in materials such as stainless, copper, titanium, and precious metals.
Kern's machines include the Evo Ultra Precision machining center, which can achieve positioning tolerance of ±1 µm and Ra of 0.1 µm. It offers a 50-kg capacity and X, Y, Z travel of 300 x 280 x 250 mm. The Micro High Precision machining center has a positioning tolerance of ±1 µm, and Ra of 0.2 µm, with a 30 kg capacity and X, Y, Z travel of 250 x 220 x 200 mm. For still larger parts, the Pyramid Nano machining center is designed with hydrostatic drives and guideways to handle workpieces to 250 kg with X, Y, Z travels of 500 x 500 x 300 mm.
Kern guarantees the performance of its machines, and tests them under strict VDI standards. Spindles with speeds from 20,000 to 160,000 rpm are available. Automatic measuring of the workpiece by touch probe with data transfer by infrared beam is available with vector-controlled or oriented spindles. Automatic measuring of tool length and radius with a laser system is also available.
Machining small high-precision parts requires high-performance machines, specifically designed for the purpose. "Micromachining is fundamentally different from macro-scale machining," says Onik Bhattacharyya of Microlution Inc. (Chicago). "In microscale cutting, you are removing chips that are on the same order of magnitude as the cutting-edge radius of the tool. Because of this phenomenon, it is important that the tools maintain a minimum feed rate when machining small, high precision features. If the tool drops below this feed rate, the tool will not remove material in the form of chips. Instead it will burnish or rub the surface. This results in poor surface quality and increased tool wear.
"In microscale machining, the tool is never moving in one direction for long and is constantly changing direction, i.e. driving tight corners, interpolating small holes, etc. To maintain the required feed rate and keep the tool cutting through the material, the machine must have high acceleration capabilities. Our company's model 363-S has 5 g of acceleration capabilities along each of the three axes of motion to meet this requirement," Bhattacharyya explains.
Microlution's technology fits in well with the objectives of biomedical product development, Bhattacharyya asserts. "Parts are getting smaller and smaller by the day. Surgeries are getting less invasive; implants, smaller. The whole industry is driven by miniaturization. End users can be found in a variety of R&D venues, including biomed and aerospace OEMs, Tier one contract manufacturers, and university and government laboratories.
"In biomedical product development, our machine is a tool to test out design ideas for prototyping and research. One biomed company is using our machine in the development phase, and will be tooling up to bring more of our machines in when they go into production. That's the advantage of being involved early in the development and prototyping stage where design for manufacturability [DFM] can figure in a company's future capital equipment planning."
As might be expected, workholding for small precision parts and for untended operation present their own challenges. Microlution has developed its own proprietary workholding system with submicron repeatability for removing pallets and replacing them. The company also offers the quick-change palletizing system from Erowa Technology Inc. (Arlington Hts, IL) integrated with the machine. "The Erowa palletizing system is widely used for machining centers and EDMs among the contract manufacturers and biomedical manufacturers we work with," says Bhattacharyya.
Still, machines don't always have to be the smallest to machine the smallest workpieces and features from the hardest or thinnest and most brittle materials. The HS430L high-speed mill from Sodick Inc. (Schaumburg, IL) features linear motor drives on the X, Y, Z axes, making it well-suited for manufacturing electrodes, die and mold components, and for high-precision medical parts in materials to RC 70.
Programming according to Sodick's HSM method proceeds from a 3-D solid model to an on-network PC, and to the machine tool via a server connected through Ethernet. The solid part model is imported into the CAM system design, a tool list and process strategy based on part geometry and material hardness are mapped out, and the post program for the machine that the part will run on is developed.
The machining method uses shallow cuts, high speeds, and rapid feed rate with small-diameter end mills. No heavy load is applied to the cutting edge because of the higher-rpm shallow cutting, and the height of the tooling mark on the machined surface is low, resulting in improved accuracy, surface finish, and tool life. Other benefits of HSM include shorter cutting time and standardization of tooling; semi-dry cutting may be applied.
Typical applications for the Sodick HS430L include machining thinribbed copper electrodes, micro pins, microhole machining in PEEK 450 polymer, and microtext machining on brittle materials. Outer ribs of the copper rib electrodes feature a 0.1-mm tip, 3-mm width, and 15-mm height. An extra-fine pin in the center has a tip 0.2 mm in diam that is 15-mm high. Machining is done with one end mill. Microholes can be drilled to 0.05-mm diam in 0.5-mm thick PEEK. In a demonstration, Sodick has even machined microtext on a single grain of rice with text 0.16-mm high and 0.015-mm deep, with 110 characters (Japanese, of course). Graphite machining extends to high-precision machining in material 0.05-mm thick for an electrode 0.5-mm high. A pencil lead was drilled with 60 holes 0.5 mm in diam in a rectangular pattern. The HS430L features a 40,000-rpm oil-air spindle with HSK-E32 shrink-fit toolholders. Heidenhain 7 nanometer glass scales are mounted on the moving X, Y, Z axes, positioned to measure true machine position.
This article was first published in the May 2008 edition of Manufacturing Engineering magazine.