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Grinding Machine Technology for the Medical Industry

"The recent emergence of specially designed 5-axis grinding machines can now meet the highest expectations of accuracy and machine dynamics in a wide range of applications found in various market segments such as tool and cutter grinding, as well as the special demands of the medical industry, particularly the orthopedic segment," said a Schütte spokesperson. "A short set-up time and high flexibility in equipment and software changeover are among these new machines' many advantages for end users. To achieve this performance level, many companies have relied on their experience in building various machine tools, which include grinding machines and multi-spindle lathes."

A key feature on these new grinding centers is direct drive technology in all five axes. The XYZ axes are driven by linear motors as well. "By eliminating ball screws or gears from the drive system and components, these machine have the capability to perform accurate and highly dynamic movements at the same time," said the spokesperson. "The onboard drive technology not only offers high dynamics, it is a backlash-free drive system that can accelerate faster than any machine tool on the market and is not prone to unwanted wear or otherwise harm to the drives. The advanced CNC technology being supplied complements the drives and programming options to perform at a state-of-the-art level. This drive and control concept allows an effective velocity at the part surface of over 500 IPM."

Orthopedic implants such as femoral knees, tibia trays and hip stems plus instruments such as hip rasps, medical cutters, drills, step drills and reamers gain benefits from this technology. High speed machining is helping to reduce production costs and offers a wider variety and different approach in cutting technique options and tool life.

"But there is another aspect of machine utilization in orthopedic production work that is highly important," said the spokesperson. "Namely, how can one program all of these diverse, one-off or low-volume parts on just one machining platform?"

Each class of workpiece would typically need a special software solution to address its characteristics and to start off from a simple drawing supported by parameter input pages, DXF contour import functionality or very complex geometry 3-D CAD models. With standard geometry features combining with demanding free-form surfaces, such as femoral knee implants, the programming challenge can seem quite extreme.

Builders have developed different software packages in-house combined with the full capability of offline programming and simulation capabilities of the entire workpiece from the blank to the finished product. The software incorporates machine and accessory environments and collision checks, and then simulates production before the machining job gets started. "This programming platform assures stable production with predictable work output, and it is an absolute time saver for the production engineering department to get the CNC code prepared and ready for the machine without losing precious production time," said the spokesperson. The program information gets stored in a file that has been sent to the machine and gets called up at the machine control panel to start the job. Only a few mouse clicks are necessary to create such a program, combining part handling, part probing, tool measurement, wheel dressing, grinding, milling, abrasive belting and polishing operations, without special skills to understand and program the code. The software concept is designed as a typical one-page screen. All software features can be placed individually on the screen and easily changed to a different arrangement. Each arrangement can then be stored under a separate profile name. To switch over from one program to another involves only a mouse click to have all software features on-screen. The CNC control panel and software call-up are ergonomically designed, with many intuitive functionalities to operate the various features on the machine.

All software packages are constantly being improved and extended to add or improve existing features, or find solutions for customers who are having a demand aside from what has been used as the standard of operation in the field. The software offers, along with other option packages, a large assortment of various drill bits. For drill bit geometries unique to a certain product, the software can import a DXF contour of a special feature and immediately generate a CNC program. For long, thin geometry drills or reamers, such as those quite often used in orthopedic surgeries, it is necessary to support the part while machining exactly at the grinding point. To accomplish this task, the machine can be equipped with a dual auxiliary slide system carrying a tailstock to hold the part on center and a steady rest slide to absorb the grinding forces and hold the part where needed. These slides can be independently programmed, positioned, activated or deactivated while machining is in progress.

"On parts such as femoral knee implants, dealing with free-form surfaces is mandatory," said the spokesperson. "Achieving a constant but highly effective velocity at the part surface, without losing the accuracy by grinding hundreds of parallel lines, is a unique challenge. This new software typically offers an interface to CAD/CAM systems that provides the post-processing routine for such high-volume data and complex geometry tool path information. The software and machine are capable of performing the grinding job within 4-8 minutes typically, depending on the part size and fineness of the desired surface quality."

For operations where milling is an expedient substitution for conventional grinding, the machine can switch the drive parameter setting to perform an optimum cutting path for this type of machining. Also, live radius compensation can be applied to milling applications in full five axes of motion to control and offset part dimensions, or to compensate for diverse cutter dimensions. "But grinding and milling operations are not the end of the machining sequence option list," said the spokesperson. "In some cases, abrasive belting and polishing can make sense, too. These additional operations are now possible within the machining envelope, since the tool magazine on these new grinding centers can be extended from five up to 12 or 24 magazine places to store specially designed belting assemblies and polishing arbors. This magazine and spindle concept is designed to carry the tool and its coolant manifold in the same place, but optimized for each application. An integrated polishing spray nozzle can provide the polishing compound into the finishing process."

"Not only does the development of the machine, software and elaborate clamping systems belong to this new spectrum of machine capabilities, but also does the machining of complex or costly materials such as CoCr, titanium, stainless steel and even ceramics, to find the right technological solution for the orthopedic application," continued the spokesperson. "Cast blank femoral knee implants, for example, have shown in the past, after the grinding process, a part splay effect in which the inner contour has opened. Experiencing this effect spurred additional efforts by the engineers to maintain this critical profile feature. The grinder must be able to reduce and eliminate the part splay effect by special technology using the right tools, coolant supply and programming method.

"For free-form surfaces, a CAD/CAM system needs to be in place to provide the complex tool path information. For instruments such as surgical bone rasps, the software must offers its own built-in CAD/CAM interface. Only a blank CAD model and a drawing should be needed to get started. With its model import function, all different kinds of toothing systems can then be programmed, simulated and measured within the same programming platform, from straight, angled, helical and fan style toothing to chip breakers. The software packages now being offered in the market provide pre-defined input parameter masks to create programs within a few minutes. The software also knows the special needs for grinding and milling operations. These are optimized and feature lean programming packages for any programmer to use, even without having extensive experience in grinding on such tasks.

"Reducing production costs is an ongoing challenge for all orthopedic market players. Big and small companies alike are facing this fact in the medical field and, as a result, it is our challenge as machine builders, too. Automation therefore becomes more significant to provide a constant and predictable workflow. For low-, mid- or high-volume part batches or a higher diversity on the part spectrum, automation solutions need to address this challenge." Depending on part or batch size, a chain loader with 50 up to 160 part stations can be equipped onto a machine. For extended needs, part volumes, more diverse mixes of different parts or other applications such as part measurement, finishing options and part storage, a robotic cell is a very open system that can be attached to the grinding machine and programmed to interact seamlessly, with all the necessary big data capture via the CNC and the proper bus communication.

For all machining applications, part clamping is a final, critical necessity. For clamping round and cylindrical instruments, automated collets or hydraulic chucks are available. To clamp and hold workpieces such as femoral knees, tibia trays, hip stems or hip rasps, builders are offering a wide range of clamping solutions, fully automated or for manual loading.

"Process engineering and consulting, from the early stages of a project until the installation of the equipment, accompanied by training programs for machine operator, programmer and maintenance staff alike, are other important hallmarks of today's medical grinding machine builder's value proposition," said the spokesperson.

Schütte is a medical machine tool provider capable of delivering machines, filtration systems, auxiliary equipment, clamping solutions, software, process development and automation solutions from a single source. "It is our long-standing commitment to stay in close touch with market demands and our customers' needs in the critical medical instrument and orthopedic implant segments," said the spokesperson.

For more information contact:

Schütte, LLC

4055 Morrill Road

Jackson, MI 49201


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