FEATURE

Companies involved in the design and manufacture of orthopaedic products face a daunting set of challenges. They are constantly seeking ways to accelerate production processes and lower costs while maintaining compliance with the requirements of a highly regulated industry. On the technical front, these firms are often called upon to produce small parts with complex geometries and tight tolerances. And they must do so using difficult-to-machine materials such as titanium. We asked some makers of precision machining equipment how they are helping medical device manufacturers to meet these challenges. Here is what they told us.

Spindle speed

Figure 1: The Office Mill series from Haas Automation Inc. offers full CNC capabilities in a compact unit that can be wheeled through a typical doorway.

“The trends in medical device manufacturing are for smaller parts, smaller lot sizes, more complex geometries, tighter tolerances, and complex multi-axis contouring,” says Jeff Law, product manager, horizontal machining centres and rotary products, at Haas Automation Inc. (Zaventem, Belgium). From a machine tool perspective, he comments, meeting those demands calls for improved servo control systems with enhanced motion control algorithms and faster processors. High spindle speeds are also necessary to take advantage of today’s high-speed cutting tools, he adds.

Machines with multiple axes offer several advantages in satisfying industry demands, notes Law. They make it possible to perform multiple operations on a workpiece without changing its position. This enables a reduction in machining time and minimises handling of the workpiece while safeguarding its dimensional accuracy. However, multi-axis jobs take longer to set up and require input from skilled programmers and operators. Production quantities need to be large enough to amortise the additional costs, and multi-axis machining generally requires a more advanced CAD/CAM system.

Haas offers a small-footprint OM-1 Office Mill, which is available with fourth- or fifth-axis rotary tables, for medical device manufacturing (Figure 1). Designed for high production or rapid prototyping of precision 2-D and 3-D parts, the machine offers full CNC capabilities, yet is small enough to fit through a typical doorway.

The CNC mill reportedly achieves spindle speeds of up to 50000 rpm, facilitating part drilling and milling using extremely small tools. High-speed spindles are vital to maintaining the proper chip load on small cutting tools. The machines also feature stiffness, which contributes to maintaining dimensional accuracy and extending tooling life, and thermal stability, which is important when working with difficult-to-machine materials.

Haas also offers vertical machining centres (VMCs) for precision medical device, mould-making, and tool-and-die applications. Available in three frame sizes, the VM series of VMCs come with a 12000-rpm spindle driven by a 30-hp vector dual-drive system, a 24-pocket side-mounting tool changer, a high-speed control with full look-ahead capability, and a multifixturing table. All of the high-performance machines feature cast-iron construction and include extensive internal ribbing to maximise rigidity and damp vibrations.

High-precision linear guides on all axes reinforce machine rigidity while minimising friction and offering long-term reliability. A combination of high-speed brushless servomotors, fine-pitch ball screws, and high-resolution encoders ensures precise positioning and repeatability. Also, the in-line direct-drive system minimises heat, vibration and noise for smooth, quiet operation and good thermal stability.

For its turning centres, Haas offers the Intuitive Programming System (IPS), which simplifies programming for the production of one-offs and short runs. A disk drive and Ethernet interface
option makes it easier to run programmes involving complex part geometries without needing to “drip feed” the data through an RS-232 interface.

A high-speed-machining (HSM) option is also available. It provides full look ahead and enhances contouring performance and surface finish, according to Law. A stop-jog-resume function makes it possible to back the tool away from the workpiece, change a worn cutter, and resume the job for a finish pass. This feature is especially useful when machining hard materials and super alloys.

Bone screws

Figure 2: Swiss equipment maker Monnier + Zahner is focused on designing machines that can accommodate growing demand for the fabrication of complex bone-screw geometries.

For Monnier + Zahner AG (Safnern, Switzerland), one of the more interesting developments in orthopaedic parts involves bone screws. “We are seeing more complex bone screw designs come on the market, in conjunction with a significant reduction in batch quantities,” says Daniel Luder who heads the R&D Department. Designers currently favour screw shapes with variable leads, ramps, and radii, as well as additional threads in the head and shaft area, he explains. Machining centres from Monnier + Zahner are designed to fulfil these evolving requirements (Figure 2).

Machining centres M544 and M600 automatically load, unload and clamp complex parts. The control system, which is based on the proven dialogue approach to programming, has been enhanced with a number of features to improve flexibility. For example, one of the tools allows users to write some of the sequence programmes. The software can combine user-defined sequences with the more rigid predefined sequences. More flexibility enhancing features are in the pipeline, says Luder.

Monnier + Zahner also manufactures systems to hone and polish hip implants, which Luder describes as a dynamic market that is feeding demand for related production equipment. Researchers are experimenting with a range of materials and alloys that may extend implant life, and this raises expectations of what machining centres should be able to do, according to the company. Manufacturers have come to expect parts with a roundness tolerance of <5 µm and a surface roughness of <0.05 Ra, regardless of the material that is being machined. The company has developed a control system that facilitates achievement of these results while complying with stringent traceability and process validation requirements.

In addition, the company has introduced a ma­chine that grinds bone-pin drills. The M648 is equipped with 15 CNC-controlled axes and can simultaneously machine parts in four stations. It can reportedly produce a bone-pin drill in just over one minute; by contrast, conventional grinders typically require 4½ minutes.

Integrated systems

Figure 3: Thread-whirling attachments developed by Tornos SA are designed to facilitate machining of the larger threads, often without secondary operations, found on hip-bone screws.

Machine tool manufacturers eyeing the medical device market must offer integ-rated solutions if they want to succeed, says Philippe Charles, Med-Tech Market Segment Manager at Tornos SA (Moutier, Switzerland). The machining centres need to be capable of delivering completely finished complex parts (Figure 3). A thread-whirling attachment designed by Tornos illustrates this concept.

The attachment is designed to facilitate whirling of the larger threads usually found on hip-bone screws. Tornos’ Deco 20a or 26a machine so equipped can completely manufacture such parts in one setup. Drilling, turning, gun drilling, milling, broaching, deburring, tapping and part cutoff are performed with the main spindle; and on the opposite end of the part, the counterspindle takes care of drilling, thread whirling, thread deburring and milling.

The ability to machine the thread in counter operations often allows the user to manufacture screws without secondary operations or the need for any other machine tool. The thread of one screw can be cut in the subspindle while the main spindle produces another part, which is a further productivity advantage.

Also available is a thread-whirling device that employs carbide form tools and replaceable tooling inserts. It adapts easily to the Tornos whirling units. This Schwanog-made device is designed to maximise cutting speed and tool life while imparting fine finishes to titanium and stainless-steel parts.

Generally speaking, turnkey solutions represent the future for equipment makers, stresses Charles, and this approach extends well beyond the machine itself. Forward-thinking companies are bringing together people with different yet complementary skill sets to examine a project in its entirety. These design teams integrate elements of the entire supply chain, he adds, bundling know-how from equipment manufacturers and operators as well as med-tech design engineers. “The machine builder needs to become a facilitator passing along the know-how of partners across the value chain,” says Charles. For example, Tornos recently was able to increase a machine’s feed rate by 10% while decreasing tooling wear. How did it do this? Simply by using a different metal-working fluid. That option may not have emerged if the machine builder had not been open to a multidisciplinary team approach.

Machining centre rigidity

Figure 4: To prevent machine vibration from affecting the production of precise parts, Kern Micro- und Feinwerktechnik builds its systems on a polymer-concrete base reinforced with steel bars.

“The medical device industry is characterised by an extreme bandwidth of requirements,” says Burkhard K. Rother, Managing Director of Kern Micro- und Feinwerktechnik GmbH & Co. KG (Eschenlohe, Germany). One requirement is the fabrication of small parts with intricate geometries. To achieve the geometrical precision and surface quality that medical device manufacturers demand, machining centres must be exceptionally rigid, says Rother. Kern’s 3- and 5-axis CNC equipment features an Armorith polymer concrete base reinforced with steel bars for maximum rigidity and stiffness (Figure 4).

The vibration-dampening properties of this design are as much as 10 times greater than conventional cast iron bodies, according to the company.

To further meet user needs, Kern’s systems include hydrostatic guidings, integrated automatic temperature management, and specially developed software that compensates for spindle aberration. The machines can accommodate milling tools as small as 0.1 mm diameter (0.03 mm for drills), and they can achieve tolerances of ≤2 µm and 0.09 Ra surface roughness.

To help customers boost productivity, Kern offers high-throughput vector-controlled spindles that achieve speeds up to 50000 rpm (other spindles are available that can attain 160000 rpm), automatic workpiece and tool changers, and automated integrated measuring equipment for unmanned machining.

Klaus Vollrath is a freelance writer living in Allschwil, Switzerland. e-mail: kvollrath@bluewin.ch