Machining parts for the orthopedics industry is demanding. Whether it’s joint implants, surgical instruments, molds, or forging dies, the materials are difficult, the geometry is complex, and the quality standards are second to none. Moreover, the business grows more competitive by the day, and manufacturers must be both technically proficient and production efficiency to remain viable suppliers to this market.
Achieving High-Quality Surface Finishes
Achieving dimensional accuracy is important to a point with orthopedic implants but the biggest challenge is usually the surface finish. For this reason, post-machining hand finishing is still usually required with belt grinding, polishing, and buffing. And it’s time-consuming. These finishing processes can take 45 minutes for a single part. Unfortunately, no practical machining process today eliminates the need for hand finishing of these parts, but a high-performance machine and stable machining process can dramatically cut that time and machining cycle time as well.
How do you do that? Improved surface quality can be achieved through the combination of engineered cutting parameters and proper toolpath strategies. For example, on a performance-based machining center, moving from a 25 Ra surface finish up to a 32 Ra could reduce your cycle time by 50% for a femoral implant, providing consistent, predictable results. Providing a precise and predictable surface finish allows end-users to develop more efficient and effective secondary finishing processes, which can reduce these operations by as much as 50%.
Five-Axis Machining is a Given
Generating 3D parts and tooling shapes common in the orthopedics industry essentially dictates the need for five-axis machining. This can be accomplished with a 3-axis mill fit with a tilt/rotary table in a 3+2 configuration, but they're typical will be a sacrifice in speed, surface finish, and precision compared to a fully integrated five-axis machine.
On a good two-axis auxiliary table, you might typically see positioning accuracy of ±3-5 arcsec. and 30 to 50 rpm in speed. By comparison, Makino’s DA300 five-axis machining center, which is used widely in the medical industry, uses direct drive servo technology on its tilt/trunnion table and can achieve positioning accuracy and repeatability of ±2 arc. sec. on the tilt and a contouring speed of 100 rpm. The rotary axis is accurate to ±1 arc. sec. and can achieve 150 rpm. Combine that with X-Y-Z axes that can feed up to 1968 ipm and a 20,000 rpm spindle and you have a much more capable platform for high-speed 3D machining.
Being able to accurately feed and articulate the tool to the workpiece so quickly also enables much more efficient use of cutting tools. With simultaneous five-axis machining, you can maintain a constant tool vector to a 3D workpiece surface, which with a ball nose end mill should not be a vertical orientation. When feeding a ball mill vertical to a workpiece surface, you are essentially dragging the tip of the tool across that surface at zero rpm, which is hardly an efficient cutting process.
By tilting the tool relative to the surface you utilize the effective flute length, of the tool efficiently. This effectively increases the surface footage of the tool and can boost metal removal rates by as much as 40-50%, yet still generate better surface finishes and blends.
It’s also important not to scrimp on the quality of the tooling. Precision ground tools generate less vibration to produce better surfaces, and last longer too. Toolholders are also extremely important to minimize tool runout, which also degrades tool life and finish quality. For high-speed machining in these difficult materials, a shrink-fit may not be mandatory, but it is recommended. You can get a payback in the range of 20% improvement in tool life.
Maintaining Process Stability
It is one thing to the machine to high standards under optimal conditions, but quite another to do it all day long and over extended periods of time. For successful medical machining, that means achieving process stability that is maintainable without undue human intervention. Two critical and interrelated factors here are the inherent geometric accuracy of the machine and the thermal stability of the machine.
A common practice for machine tool builders is to address geometric inaccuracies in the machine with electronic compensation in the control. For example, say the run of an X-axis isn’t perfectly straight or there is pitch, yaw, or roll as the axis traverses from one limit to the other. These inaccuracies can be mapped and compensated for in the CNC’s motion. However, this methodology can become increasingly unstable as the machine heats up because the compensation may no longer be accurate in a state of thermal distortion with some machines.
A variety of factors combine to deliver high positioning accuracy (±0.000060") in the DA300 and other models designed for medical machining, including core-cooled, fine-pitch ballscrews with 0.05-micron-scale feedback. The high-speed spindle enables fast machining of fine features with small tools. The spindle is also thermally controlled providing optimum performance over extended periods, ensuring exacting cutting conditions from start to finish to produce tight-tolerance, high-quality workpieces.
Stable Processes Enable Unattended Machining
Today’s medical manufactures are under increasing pressure to become more efficient without sacrificing quality. Doing that effectively requires investing in more capable equipment, which, of course, comes at a cost. From a financial perspective, the top companies are thinking much more like high-volume production manufacturers with greater consideration of factors such as return on investment, equipment utilization rates, and labor costs. You can’t just be good at making parts anymore; you must also be efficient to be successful as a business.