Building the Right Combination
Enhancing manufacturability while reducing costs is an ongoing challenge for machining and tooling providers.
As orthopedic companies design and engineer smaller, more intricate devices and implants, manufacturing technology must evolve to keep pace—including machining and tooling. With customer expectations for dimensional tolerances in the +/-.0001-inch range, machining and tooling companies must find the right combination of processes and technologies to deliver on these demands.
Parameters are shifting constantly. With complex geometries, tight tolerances and new and improved materials, machine shops must be on top of their games to find the most cost-effective (and often innovative) machining approach that will maximize manufacturability and keep costs down.
Miniaturization of devices is pushing machining tolerances to the limit. To meet these challenges, machining is getting smaller as well, with end mills approaching 0.002 inches in diameter and electrical discharge machining (EDM) wire diameters of 0.00078 inches.
One of the biggest challenges is maintaining control of the machine variables. For example, even the slightest changes in the tool and tool-holding, setup environment, machine vibration, cutting tools or electrodes can have a big impact on overall quality. That’s why OEMs want more quality controls on all processes to ensure high quality and repeatability—especially automation. A robust automated system uses the latest sensor technology to detect variances or other problems in real time, which greatly reduces downtime, rework or wasted material. It also helps with validating the manufacturing process and meeting U.S. Food and Drug Administration quality management system expectations.
Emuge Corp.’s Z-GF Miniature Solid Carbide Thread Mills are designed for easy machining of difficult materials. Due to their size and design, the thread mills can increase productivity, the company claims. Photo courtesy of Emuge Corp.
All of this preparation, of course, helps companies turn around customer expedites every day. Lean processes, quick changeovers and fewer machining/tooling steps are critical for keeping production flowing according to customer needs. Companies much be nimble and quick—machining and tooling equipment constantly is evolving, and shops must continue to mix and match new and existing equipment, processes, software and upgrades to find the perfect combination for each individual project (for example, using coolant-fed drills can eliminate gun drilling on some products, which saves time and money). Companies also tend to be very guarded about the processes they have fine-tuned that work best for them (for example, every source in this story refused to share any information about the grinding-wheel technologies they use).
Five- and seven-axis milling machines on the market today have the capability to complete complex orthopedic designs in a single operation. Seven-axis contour milling machines in particular enable profile milling of extremely complex geometry. Tool management controls and tool breakage detection allow uninterrupted processing until the part is complete. Tool speeds reach 40,000 rpm.
“Our seven-axis contour milling machines can completely manufacture a six-sided part (i.e., maxillofacial plate implant) from a single machine without requiring the secondary operation of turning the part over and re-fixturing it in order to machine the opposite side of the part,” said Brian Hrouda, director of sales and marketing for Norman Noble Inc., a micro-precision contract manufacturer for medical devices and orthopedic implants based in Cleveland, Ohio.
With newer Swiss turning machines, multiple operations such as milling, gun drilling and thread whirling can be completed at different stations on a single machine, enabling the production of a part in a single operation and eliminating the need for secondary operations. Tolerances of +/- .0001 inches can be accomplished for a range of materials, including PEEK, stainless steel, cobalt chrome, nitinol and tantalum.
For Swiss lathe applications in particular, tooling manufacturers are getting better at designing chipbreaker and grade combinations for inserts for specific applications, especially for smaller-sized, medical-related parts. A chipbreaker is molded or ground into an insert and shapes the material being removed into small chips or curls that do not “ball up” in the machine and can be removed easily.
“Swiss lathe processes typically have low feed rates (.001 to .003 inches per revolution), whereas chucker CNC (computer numerical control) lathes are more aggressive, with feed rates in the .005 to .020-inch per revolution range,” said Jim Rowe, technical salesperson for Mahar Tool Supply Company, a Saginaw-Mich.-based machining and cutting tool firm. “Traditional chipbreakers create a large ‘birds nest’ of chips that can especially cause problems when the part is transferred to the pickoff spindle (sub-spindle). Swiss-application insert chipbreakers are usually sharper in regards to the rake angles molded or ground into the top of the insert that create the actual chip while the material is flowing over it.”
Multi-tasking mill/turn technology incorporates two or more tools during the cut that perform multiple processes such as milling, turning, drilling, and threading on a single machine, all in a single setup, saving time and improving efficiencies. For example, producing the part or product on a single machine with opposing twin spindles with two turrets and milling capability can cut cycle times by 50 percent. Faster control processors and machine axis speeds and more accurate positioning devices also contribute toward reduced cycle times.
“Multi-tasking, multi-axis sub-spindle lathes that do mostly milling work eliminate or reduce the need for exotic fixtures and multiple set-ups,” said Rowe. “All the major machine tool manufacturers have at least one version of these machines and have recognized the need for smaller-footprint machines to make the smaller medical-implant products.”
Ten years ago, machine tool manufacturers were taking spindle speeds on CNC mills above 20,000 rpm. Today, by using electric- and air-driven spindles, machines easily can reach 50,000 to 100,000 rpm. This level of rpm is required when calculating the proper speeds for very small diameter tools (.005-inch drill or endmill) with newer coatings that dissipate heat easily.
“The smaller-diameter tool requiring the high rpm also requires less spindle-to-tool runout, or the tool will not last long and most likely break,” added Rowe. “Simple-style collets have been promoted over the last two decades, like ER or TG style; now there are many more effective ways to ensure a more accurate rotation of the tool with hydraulic, shrink-fit or special versions of collets by manufacturers like Big Kaiser that enable a more successful machining experience.”
Spindle temperature control also is available on some machines and can enhance the micromachining performance. For example, spindle-core cooling and under-race lubrication system from Makino Inc. (a metal cutting and manufacturing technology manufacturer in Mason, Ohio) is effective for suppressing spindle thermal distortion on high-speed spindles. The spindle core cooling system circulates a large volume of temperature-controlled cooling oil through the center of the rotating spindle to cool it directly from the inside. With under-race lubrication, the cooling oil circulated through the spindle flows through holes in the inner bearing races to lubricate the bearings as well.
The chemistry and bonds within carbide structures also continue to advance. Grain sizes of the chemical elements that comprise the substrate now are less than a micron in size, creating a more compact and durable carbide mixture. This results in longer tool life and less fracturing of the carbide when machining tough materials such as stainless steel and titanium.
“By creating grains of chemical elements as small as 0.3 to 0.5 microns, the strength of the cutting tools are more suitable for the high-strength/longer-lasting raw materials being used in medical implants today,” said Rowe.
Over the years, taps have been designed using advanced cutting geometries, base materials and coatings for faster cutting of threads. Thread mills today can perform multiple functions with the same tool and produce threads faster and more accurately than ever before.
“The latest thread mill designs offer more benefits to manufacturers of orthopedic components compared to the more conventional method of cutting screw threads with taps,” said Peter Matysiak, president of Emuge Corp., a manufacturer of cutting tools based in West Boylston, Mass. “Due to the high-precision nature of the orthopedic components, which in most cases are made of alloys that are difficult to machine, very high demands are exerted on all the variables in the machining process. Manufacturers therefore must be very well equipped with state-of-the-art machines, operated by top notch machinists and programmers.”
Although it is costly material, nitinol is being used by more OEMs because of its unique physical properties. This metal alloy of nickel and titanium exhibits two remarkable properties: shape memory and superelasticity. Shape memory is the ability to undergo deformation at one temperature and then regain original dimensions when it is heated above the transformation temperature. A good example is a deformed (collapsed) nitinol stent that can be inserted into a vein and then heated to expand to its normal shape, reinforcing the vein to improve blood flow. Nitinol also can become superelastic (up to 30 times more than normal metal) at a temperature that slightly exceeds the transformation temperature. Combining these properties with nitinol’s very low thermal conductivity makes machining very challenging. The best solutions are low-heat processes such as certain grinding methods (abrasive cutting), EDM and laser cutting.
“OEM design engineers who are developing next-generation implants require a heat-free machining process that does not induce thermal damage into the raw material,” said Hrouda. “This is especially true for nitinol and bioabsorbable polymer-based cardiovascular and neurovascular stents.”
For example, Norman Noble’s UltraLight athermal laser machining process was developed to cut high-precision features into materials such as bioabsorbable polymers, shape memory metals and other exotic alloys without producing any heat-affected zones. According to the company, the system’s ultra-short laser pulses do not damage or weaken the material’s structure, which saves on costly deburring and post-processing steps and increases product quality and yield.
David Cabral, president at Five Star Companies in New Bedford, Mass., a manufacturer of general and orthopedic instruments, see continued interest in the use of polymers and carbon fiber in orthopedic devices.
“OEMs are increasingly interested in these compounds because they are lightweight and radiolucent,” said Cabral.
The medical industry tends to be slow to embrace new materials, preferring to rely on compounds that already work and are well understood. OEMs gradually are becoming more aware of how polymer and carbon fiber blends can be a good alternative to conventional implant materials such as stainless steel and titanium. Carbon fibers are embedded in polymer and woven in specific orientations at the core of the implant, making it stronger. Carbon-fiber implants also have an elasticity module very similar to that of cortical bone. Unlike the distortions created by conventional implants, carbon-fiber implants provide a clearer image of the implants and make it easier for surgeons to monitor fracture healing.
Carbon-fiber implants also are more fatigue-resistant than metal implants. Carbofix Orthopedics Ltd., an Israel-based provider of carbon fiber-polymer implants, fatigue-tested its Piccolo Composite material against stainless steel and titanium with 1800N at 5Hz. The company reported the titanium rod broke after 40,000 cycles, the stainless steel rod after 440,000 cycles and the composite material was still intact after 4 million cycles.
Acetal is a durable, heat-resistant, high-performance engineered polymer that often is used for parts that would otherwise be made of metal. Its physical properties make it an excellent material for both machining and injection molding. Acetal is strong, stiff, abrasion-resistant and presents a low coefficient of friction against metals and other plastics. In general it is resistant to most chemicals; however, it can be affected by strong acids and oxidizers and may degrade when exposed to ultraviolet light.
Acetal is highly machinable and can be further reinforced with glass fiber for added strength. Medical-grade acetal copolymer retains its dimensional integrity across a range of temperatures, is available in multiple colors and is stable under ethylene oxide and steam autoclaving sterilization methods. It also does not react with disinfectants and germicides.
Polyetheretherketone (PEEK) continues to make headlines as researchers find new ways to enhance its properties. Invibio, for example, has created a porous PEEK material that is being engineered as a substrate for enhanced bone growth. Implantable PEEK applications include spinal implants, fusion devices, pacemakers, shoulder anchors and dental caps and plates. Antimicrobial agents can be added to PEEK implants to decrease the risk of post-surgery infection. The addition of glass fiber also greatly improves performance and mechanical strength at elevated temperatures. In fact, with PEEK being so strong, machine guidelines essentially are the same as those for metal. Because it is such a hard material it can wear out tools quickly—up to 20 times faster compared to fluoropolymer resins, for instance. It often is recommended that PEEK be annealed to relieve stress before it is machined; cracks may develop during the machining process if the PEEK is improperly annealed. A cooling fluid (typically water) also should be used to remove some of the heat generated during machining.
Raw material consistency also is a constant challenge that can’t be taken lightly. As a process is set up and validated, if the material varies even a small percentage in one chemical element it can change the cutting tool results dramatically.
“For example,” said Rowe, “a 1 percent increase in a chemical element such as chromium can cause an entire machining process to fail due to the increased hardness of the overall batch.”
A key concern for machining and tooling companies is having enough qualified workers to run their sophisticated equipment.
“Training has been a valuable investment as we look for new, more cost-effective methods to manufacture,” said Cabral. “Machine training has allowed our technicians to learn more about the machine’s capabilities and how we can better utilize these to make a product better, faster and with less cost.”
“The greatest challenge for a manufacturer is carving out time in his ever-tighter work schedule to test new tools and processes,” he said. “To this end Emuge has established a technology center fully equipped with a 5-axis CNC machine and all related machinery and testing equipment to perform development work for manufacturers. Now they can engage us when they want to establish cutting parameters for a new component, or if they want to make process improvements on an ongoing manufacturing operation. We can go on site, or bring the customer’s component into the technology center, where our applications engineers test it and recommend the best machining option.”
Rowe said education of the next generation of machinists will be critical in the coming years.
“It seems as if developing the skills for the proper use of cutting tools is being overtaken by the emphasis of having machine operators know how to read statistical process control results and only adjusting a cutting tool’s wear offset,” he said. “Getting people excited about manufacturing in any manner is getting more difficult. Once they are working, it is important to keep them motivated and not allow them to stop thinking.”
Another challenge to machining and tooling comes from another technology itself—additive manufacturing. Processes such as direct metal laser sintering (DMLS) are becoming increasingly popular for building working prototypes from 3-D CAD files. The technology takes a CAD file and slices the object into thin 20-micron (.0007-inch) or 40-micron (.0015-inch) layers. The machine then uses those layers to build the part using a 200-watt fiber optic laser. This locally melts each metal powder layer onto the previous layer, eliminating the need for a binder. The result is a fully dense, operational metal part with tolerances in the 0.002- to 0.005-inch range.
In his article “Additive Manufacturing Advances” in Manufacturing Engineering (April 2012), Terry Wohlers wrote that “metal parts from some AM [additive manufacturing] systems are already on par with their cast or wrought counterparts. As organizations qualify and certify these and other materials and processes, the industry will grow very large. In fact, additive manufacturing is poised to become the most important, the most strategic and the most used manufacturing technology ever.”
Why does additive manufacturing pose a challenge? It most likely will reduce the need for machining and tooling services in the future because more OEMs will be using DMLS and other additive manufacturing technologies not only to make their prototypes, but end products as well—especially as additive manufacturing prices drop.
For example, EOS, a Munich, Germany-based manufacturer of DMLS equipment, has developed a DMLS system that can produce innovative medical products and prototypes used for instrumentation as well as spinal, joint and cranial surgeries. These can be made from a wide variety of materials, including chromium steel and titanium.
“An entire new world of orthopedic treatment and procedures has opened up,” said Martin Bullemer, EOS manager for medical business development. “Because our laser-sintering systems can cost-effectively manufacture any imaginable geometry, and any variation on it, they are changing the way we think about medical products.”
Laser sintering, according to EOS, involves next-to-no tooling, molding or machining costs.
“As a result, devices can be economically mass-customized to conform to the requirements of individual doctors or patients,” according to recent press material from EOS. “Orthopedic suppliers can use DMLS and plastics laser sintering to create a diverse array of drill guides, clamps, implants and surgical instruments.”
What held additive manufacturing back in the past was that, although it could build acceptable prototypes, it could not build the final product. Today a wide range of materials can be used with DMLS that are approved by the American Society for Testing and Materials. As DMLS technologies improve and operational costs come down, the ability to build completely functional, ready-to-go final products will become more competitive with the traditional approach of machining and tooling.
Mark Crawford is a full-time freelance business and marketing/communications writer based in Madison, Wis. He can be reached at firstname.lastname@example.org.