Milling tooling developments increase quality and productivity in the machining of orthopedic replacement components
The manufacturing of medical components must meet standards of accuracy, reliability, quality, and traceability that equal and sometimes exceed those required for aerospace and nuclear parts. In addition, global competition and efforts to restrain health care expense create great pressure to maximize productivity and reduce manufacturing costs. Tooling manufacturers are helping medical partmakers meet these challenges with a selection of milling tools custom-engineered for the machining of complex orthopedic replacement components.Introduction
The manufacturing of medical components must meet standards of accuracy, reliability, quality, and traceability that equal and sometimes exceed those required for aerospace and nuclear parts. In addition, global competition and efforts to restrain health care expense create great pressure to maximize productivity and reduce manufacturing costs. Tooling manufacturers are helping medical partmakers meet these challenges with a selection of milling tools custom-engineered for the machining of complex orthopedic replacement components.
Replacing hips and knees
Demand for replacement and reconstructive parts for the human body is growing rapidly. When considering components for knee and hip replacements, trauma reconstruction and orthobiologics, sales of the parts exceed $25.2 billion worldwide. More than 50 percent of the total consists of knee and hip components, with five major medical OEMs taking almost 90 percent of the business. Two main factors spur continuing growth. First, the world’s population is staying alive longer, resulting in a gradual increase in the average age. The most rapid growth, about 3.5 percent a year, is in those 65 years and above. Coincidentally, the average age for knee surgery is 65. The other major trend contributing to a surge in orthopedic implants is the growing number of persons who are overweight or obese. Approximately 1.57 billion of the world’s 7.2 billion people are overweight, and 0.53 billion are classed as clinically obese (BMI > 30%). Excess weight increases the likelihood of the development of osteoarthritis, a major reason for joint replacement.
Parts of a replacement knee
Typically, a total knee replacement consists of three subcomponents: the femoral component, which replaces the rounded bottom end of the femur bone; the tibial tray, which replaces the top end of the tibia bone; and the tibial or bearing insert, which fits between and cushions the other two parts. The bearing insert usually is produced from UHMWPE (Ultra High Molecular Weight Polyethylene, an engineering polymer), whereas the femoral component and tibial tray are in most cases produced from cobalt chrome (Co-Cr) alloy or in some cases a titanium alloy. These alloys are strong and hard, biocompatible materials with high stiffness (Youngs modulus) and abrasiveness when being machined.
Machining the femoral component
Machining techniques for femoral components include both grinding and milling. The challenges are to achieve a burr-free profile with superior surface finish that minimizes the need for manual polishing, and at the same time maximize productivity and tool life. For these tough milling operations, Seco has developed specially designed tapered ball nose cutters and modified JHP770 high-performance cutters that feature differential flute spacing to minimize vibration during operation. Among the machining methods employed are corner plunging, periphery machining, box roughing and finishing, cam finishing and box blend machining.
The femoral component has rounded contours that mimic the condyle bone formation at the end of the femur. The shape has traditionally been produced via grinding, but that operation can generate high temperatures that may distort the part. Seco has developed tools and performed tests to replace the grinding process with milling. A large medical OEM performed trials with the tools, finishing a cast Co-Cr femoral component with a copy milling strategy that employed a special solid carbide ball end mill. The result was cycle time reductions of up to 11 minutes per part, representing 50 percent less time compared to the grinding method used previously. Tool life exceeded 12 hours, enabling one cutter to machine more than 80 parts. Excellent control of radial depth of cut on a 5-axis milling machine contributed to the extended tool life. In 4-axis applications without such control, tool life reached 6-8 hours. The change from grinding to milling also eliminated the possibility of scrap parts due to distortion. The details on the application can be found in the table 1 below.
Machining the tibial tray
Machining the Co-Cr tibial tray also presents challenges in terms of surface finish and productivity requirements. In addition, the part has right-angle locking details that must be produced burr-free. Machining the part typically can take up to seven separate machining operations, and those are summed up in table 2.
The latest developments in the machining of the tibial tray are in operations 3 and 6. To achieve a superior finish on the base where the tibial insert is seated, a new Seco multi-flute cutter with special wiper geometry was applied in operation 3. The tool has produced Ra values of below 0.1 µm. In operation 6, Seco implemented a combined wall finish/chamfer cutter. The combination of finish and chamfer tools provides a controlled way of mechanical edge profiling (MEP) and prevents secondary burrs while eliminating manual rework and reducing tool costs.
Machining the bearing insert
Bearing inserts for replacement knees typically are made of an engineered polymer (plastic) commonly known as UHMWPE. This material is relatively soft and therefore generates low cutting forces, but surface roughness requirements of 0.10 µm Ra demand that it be machined with sharp, top quality finishing tools. Under its brand, Seco developed the ‘Premier Finish’ solid end mill designed to meet the specific requirements of a leading global medical OEM. The six steps to machine this part are described in table 3, and steps 4 and 5 are discussed in more detail below.
Overcoming condyle contour machining difficulties
The condyle shape of both the femoral component and the bearing insert can be difficult to machine. Operations 4 and 5 of the above table describe the machining of the condyle contours of the bearing insert.
Previous to the development of the Premier Finish endmills, condyle surfaces were machined using polished HSS form cutters or conventional solid carbide tools. Both methods have several disadvantages.
- Form tools often create visible cusps on the part surface, especially when the machine tool control is not quick enough to generate a smooth cutting path.
- The zero rake angle and low helix angle of the HSS cutters make it harder to achieve appropriate surface results.
- Use of conventional carbide tools allows only product forms with a radius. In addition, not all radii can be generated due to design limits of the cutter body.
When the shortcomings of the tooling made the required surface roughness unachievable, additional less-reliable operations such as manual polishing or soda blasting were necessary. Those operations were unpredictable in terms of time, costs and quality.
To overcome these problems, the Premier Finisher (figure 5) design is based on concave and convex sections either tangent or connected with a straight line. Compared to mold and die tools the profile tolerances of the tools are quite generous. However, the manufacturing of these cutters requires special care regarding the cutting edge geometry and the overlap between the concave and convex shapes, areas where the contour starts or ends with a small contour radii, and considerations regarding the tools’ largest diameter.
Manufacturing must be controlled to avoid sudden changes in the pressure of the tool grinding wheel or generation of excessive heat, which may produce areas on the cutting edge that are not sharp enough for the required operation, resulting in a shearing instead of a cutting action. Clean cutting is essential in producing fine finishes in the UHMWPE workpiece.
Seco has refined the manufacturing grinding operations for Premier Finish tool and eliminated any problem areas to produce a constant rake over the whole cutting length with a cutting edge radius of around 5 µm.
The Premier Finish tools can be applied between 100 and 200 m/min, dependent on the quality of the CNC control. Generally, with better control systems, higher feed rates are possible. The feed per flute per revolution is normally between 0.004-0.006 * the cutter diameter: for example, between 0,02 and 0.10 mm/flute for a 20 mm-dia. cutter.
Cutter care
Normal tool life for a Premier Finish form cutter is between 1,000 and 2,000 parts. It is common practice to leave the cutter in the machine until it is worn: taking it out and storing it causes too much risk of damage.
Other care and maintenance procedures for these tools are recommended as well and discussed in more detail below:
Handling: The tools should be handled with extreme care as the cutting edge is easily damaged. Metal to metal contact should also be avoided, taking into account that even fingernail contact could result in light wear on the cutting edge.
Logistics: Protection during shipment and transit is indispensable. Tools are covered with a protective wax and sealed inside the packaging tubes. Stickers are applied with special written instruction saying: “Handle with Care” and “Do not repack”.
Measurements and detection: Within the medical industry, traceability is a very important requirement to ensure quality in sealed (validated) processes. Therefore all tools are provided with unique laser marking. Reliability and quality standards are set by contactless measurement techniques and sealed grinding procedures. Results are reported and included in the packaging tubes to prove quality assurance to the customer.
Tool-setting: Operators should be instructed on how to remove the protective wax from the cutters and find correct diameters on the reports supplied. The tools can be reconditioned up to five times; when significant wear is present, safe packaging is necessary to prevent uncontrolled wear or damage while in transit during the reconditioning process.
Cleanliness: When the tools are applied on a milling machine that is also used for cutting metals common for orthopedic implants (titanium/cobalt chrome alloys & stainless steels) attention should be paid to clean the machine thoroughly. Metal chips will harm the cutting edge of the tool when making direct contact. Therefore it is necessary to have machines equipped with coolant filtering systems sufficient to ensure clean coolant that is free from chips. The implant supplier should also take into account the quality of the UHMWPE material. Imperfections in the material could result in tool wear due to inclusions (figure 6). Premature wear of the cutter could be a sign that the machined polyethylene is not clean. In this case, quality in procurement procedures should be point of attention.
Conclusion
To productively and profitably fulfill the increasing demand for high-precision orthopedic components and other medical parts, manufacturers of the parts must take advantage of every opportunity to enhance their production technology. A key contributor is tooling technology, such as that provided by Seco for medical component milling operations. Sophisticated tools, of course, command a higher price than the basic tools of the past. For example, Premier Finish tools are eight times as expensive as the ball nose cutters formerly used in to machine UHMWPE. However, given the features of the cutters and their outstanding capabilities in regard to quality, productivity and consistency, as well as the fact that they can reduce cost per part by up to five times, investment in these cutters is a truly worthwhile strategy.
