One company that can speak intimately about the challenges of producing just-in-time (JIT) components for the implantable medical device market is FPD of McMurray, Pennsylvania. FPD is a fully integrated manufacturing and engineering company that provides high-quality forgings, machined components and assemblies to the aerospace and medical industries.
Recently, FPD was challenged to produce an especially difficult part and found that they needed added capacity and capabilities. “The customer told us that the surface finish and geometries were so critical, you couldn’t even touch the final part with your bare hands after the final polish,” said David Tenison, General Manager & Senior Vice President, FPD.
Dedicated to precision
The company’s dedication to precision is evident in its finishing and assembly disciplines, including hand belting and polishing, multi-media surface texturing, high-energy media finishing and polishing and laser marking. FPD uses advanced engineering to design quality into its processes, leveraging a proprietary design and manufacturing system based on a central database containing 3-D solid models of parts and all associated tools. Advanced forging simulation software is used to validate the forging and machining process and vital information, such as press loads, material flow, temperature profiles and die loads. In addition, CNC programs are optimised to reduce cycle times with tool path simulation programs.
FPD has purchased several Makino machines since its first in 2004; it currently operates three a81Ms and five a61 machines. “We first approached Makino because of their reputation for fast, reliable and accurate machines,” said Tenison and added, “We compete on a global basis and it is critical that we are the best.”
Since they are designed for high-torque, high-speed machining, FPD has special applications for the a81Ms to run titanium parts for its medical and aerospace customers. The a61s are utilised in precise production work due to their ability to quickly machine parts with close tolerances and fine finishes.
Artificial knee implant
FPD was awarded a contract to manufacture the femoral component of a leading artificial knee system. The knee system is a long-term, quality implant based on a clinically proven design. It is available in both posterior stabilised and anatomically correct cruciate retaining configurations, which provides the flexibility needed to address patients’ needs. In addition, instrument standardisation and ease of identification keep the end-user in mind and promote functional efficiency.
The design of this femoral component is unique in that it has 96 per cent femoral/tibial congruency, a deep patella groove and a flared anterior flange to improve patella tracking. All of these characteristics promote greater flexibility compared to other artificial knee applications, but they also created several manufacturing challenges for FPD.
The part is made of cobalt chromium-molybendum alloy, which is often used in surgical implants due to its high strength and good polish. The part is extremely complex and employs difficult geometries, especially in the deep patella groove, which has to be precisely machined in order to reduce contact stresses against the tibial insert.
“The patella groove is difficult to machine because it has steep, constantly changing angles and smooth lines,” explained Thomas Bugay, FPD’s production superintendent. “Adding to the difficulty is the smooth, sweeping geometry of the profile tolerances and the fact that the final assembly is actually completed by the surgeon during the implantation procedure,” he added, running his gloved finger over the mirror-like surface of the part.
Inaccurate tolerances could also increase the wear rate of the implant, causing additional medical problems for the patient. “Before bidding the job, we knew we didn’t have the capacity or the machinery to efficiently produce this complex part,” said Bugay. “We had to come up with a way to produce the part more quickly and more accurately than our competitors.”
Machining instead of grinding
FPD was aware that the other manufacturers use a grinding process on the part to ensure the surface finish is achieved. “Grinding is suitable for parts with simple surfaces but it can lead to incorrect geometries,” said Bugay. “This was not an option for us. Our process would have to allow for accuracy and surface finish, without sacrificing one for the other.”
FPD engineers understood that in order to achieve accurate tolerances, the part would have to be machined. Machining is much more accurate than grinding because, over time, grinding wheels wear down, changing the geometry of the part. “Typically, ground parts have many rough blend areas that need to be worked by hand to meet the required tolerances and geometries,” said Bugay. “While this is fine for certain parts, it wouldn’t work well for a medical component that is produced on demand, such as this.”
“We have a very capable engineering and design process,” commented Tenison. “However, since the femoral component presented such complex challenges, we decided to engage Makino’s Turnkey Engineering Services to determine the most efficient design and manufacturing process.”
New process yields results
“The goal of the project was to deliver a complete process that accurately and timely machined the entire outside surface of the knee component in just one operation,” said Mike Boyd, Application Engineer, Makino, who was in charge of the FPD Turnkey. The finish had to be at 16 Ra and the cycle time of less than 40 minutes per part. In order to achieve both, they needed the complete system to include the proper machine, options, tooling, fixturing, coolant and CNC program.
“Tool life was a concern and, after many tests, a Hanita Vision series carbide endmill that yielded the best tool life was chosen,” added Boyd. “They also utilise a shrink-fit holder to improve run-out and clamping power. As getting production up and running quickly was a concern, we chose a fixture and CAD/CAM software that FPD was already using.”
A Parlec tombstone provides quick-change tooling plates to accommodate the six different part sizes, while allowing FPD to mount eight parts at once, either all the same size or different sizes. While the previous manufacturer would grind the part to desired tolerances and then finish the part by hand, using the a61, FPD can machine the component to ±0.0005 inch profile and ±0.0025 inch datum tolerances on one machine.
Ensuring proper machining
FPD’s a61 was configured with several options to ensure proper machining of the knee component’s sweeping geometries, including Makino’s Super Geometric Intelligence. This option allows precise axis motion using small block, point-to-point movements, while maintaining faster feed-rates and quality. In addition, the dynamic fixture offset option was selected, eliminating the need to re-post the CNC programs for any minor positional fixture adjustments. This, too, is an advanced function that calculates the delta between the theoretical location of the part and the actual location, all “on the fly” and transparent to the operator.
After machining, the part is hand-polished to a mirror-like surface finish of 4 Ra or better. The resulting surface finish is so fine that the part can be handled only with gloves, as contact with bare hands could scratch the part and force rework. Using this new process, FPD is able to meet the production on-demand needs of the medical device manufacturer and can deliver finished components on time, every time.
The article was originally published in Competitive Production – The
Publication from Makino