Features

The recent emergence of specially designed five-axis grinding machines can now meet the highest expectations of accuracy and machine dynamics within the special demands of the medical industry, particularly the orthopedic segment. A short setup 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 multispindle lathes.

Knee joints being ground on a CNC five-axis grinding center.

A key feature on these new grinding centers is direct drive technology in all five axes. The axes x, y, and z are driven by linear motors as well. By eliminating ball screws or gears from the drive system and components, these machines have the capability to perform extremely accurate and highly dynamic movements at the same time. The onboard drive technology not only offers high dynamics, but is a backlash-free drive system that can accelerate fast and is not prone to unwanted wear or otherwise harming the drives. The advanced CNC technology complements the drives and programming options, and the drive and control concept allows an effective velocity at the part surface of over 500 inches per minute.

Orthopedic implants such as femoral knees, tibial trays, and hipstems — plus instruments such as hiprasps, medical-cutters, drills, step drills, and reamers — gain large benefits from this technology. High-speed machining is helping to reduce production costs and offers a wider variety of options and a different approach to cutting technique and tool life. But there is another aspect of machine utilization in orthopedic production work that’s highly important: how can you program all these diverse, one-off, or low-volume parts on just one machining platform?

Machine Programming

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

Builders have developed software packages in-house that combine off-line 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, then simulates production before the machining job gets started. This programming platform ensures stable production with predictable work output on one hand, and on the other hand, 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. The program information gets stored in a file that’s been sent to the machine, and gets called up at the machine control panel to start the job.

The CNC controls all axes of motion and the rotating arbor that carries various tools for machining operations.

Only a few simple 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, with no special skills needed 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, and switching over from one program to another involves only a mouse click to have all software features onscreen, where they match the programmer’s expectations.

For long, thin-geometry drills or reamers such as those 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. 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. Software typically offers an interface to CAD-CAM systems that provide the post-processing routine for such high-volume data and complex geometry tool path information.

Machining Options to Reduce Cost

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. In some cases, abrasive belting and polishing can make sense, too. These additional operations are now possible within the machining envelope, as the tool magazine on these new grinding centers can be extended from 5 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.

Robotic loading and unloading are programmed for operator-free material handling.

Not only do the development of the machine, software, and elaborate clamping systems belong to this new spectrum of machine capabilities, but so does the machining of complex or costly materials such as cobalt-chrome, titanium, stainless steel, and even ceramics, to find the right technological solution for the orthopedic application. Cast blank femoral knee implants, for example, have shown after the grinding process a part splay effect in which the inner contour has opened. The grinder must be able to reduce and eliminate the part splay effect through technology that uses the right tools, coolant supply, and programming method.

For freeform 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 offer its own built-in CAD-CAM interface. Only a blank CAD model and a drawing should be needed to get started. With a model import function, all 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 special needs for grinding and milling operations are optimized in lean programming packages for any programmer to use, even without extensive experience in grinding on such tasks.

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