Technology in Action Green will agree that certain aspects of Exactech’s medical
device manufacturing require dedicated systems/machines. But he and his team wanted the manufacturing flexibility to grind different components if needed, in addition to simply boosting output. Te system that met both criteria, and then some, turned out to be a Walter Helitronic Vision five-axis grinding machine from United Grinding North America. “When deciding on a system, we determined that there was
a level of service and application support we definitely needed from the supplier of that equipment,” explained Green. “We weren’t just trying to increase capacity, but also implement production improvements and add versatility. Te implant targeted for the equipment was a next-generation design, and on the manufacturing side, we wanted to streamline how we processed those parts.” Te Helitronic Vision, featuring grinding and milling ca-
pability as well as self-contained automation, pumps out about 18,000 of the femoral knee implant components annually. Exactech’s existing dedicated equipment produces only 7000 similar femoral parts per year. In the time it takes the old equipment to produce one fem-
oral component, the Helitronic Vision generates two within the same cycle time to double output. Te machine’s self- contained automation is standard design, so when needed, Exactech can easily switch the unit over to handle other parts. Te Helitronic Vision uses linear motor technology as
opposed to ballscrews that typically must be replaced every six to eight months because of wear. With the long-life linear motors, the Helitronic Vision has already run for well over 7500 hours without need of maintenance. Plus, the machine is stable and repeatable thanks to its low-vibration, solid mineral cast, gantry-type construction. Te machine accommodates workpieces ranging from 3 to 320 mm in diameter, up to 370- mm long and weighing up to 110 lb (50 kg).
individual femorals, enough for the machine to run nonstop and untended all day. In operation, a gantry-type robot with grippers transports
one arbor into the working area and places it in the machine’s A-axis/rotating C-axis unit. As the part is being ground or milled, the A axis provides high-precision positioning/rotation for generating accurate contours and superior surface finishes. Te most challenging aspect of the femorals is their contours and tolerances that must be held to exacting standards. Te Helitronic Vision grinds what are known as the
femoral’s condyle and patella relief sections and mills using a carbide end mill—the two portions referred to as the tip and the cam. Te machine grinds away the gates, and cast scale from the castings using solid, pre-profiled, dressable roughing wheels and pre-profiled non-dress vitrified plated wheels from Saint Gobain. Te machine automatically dresses roughing wheels and compensates for changes in wheel size. Finish
grinding wheels generate surface finishes between 8 and 12 Ra and grind about a thousand parts before wearing. Once com- pleted on the Helitronic Vision, femorals move to automated hand polishing/buffing operations. Exactech produces knee replacements in a variety of sizes
designated from zero to six. Te parts are also made in leſt and right-knee configurations, basically mirror images of each other. For a particular size, the company machines all the right side femorals together and all the leſt side femorals separately. Since the femoral is just one component of a total knee replacement, the parts must each be perfect, function properly and deliver unfaltering performance. And Green said the Helitronic Vision’s rigidity and capability has been a great asset in maintaining the relationship of these components. Besides processing versatility, the Helitronic Vision, ac-
cording to Green, also offers increased flexibility in terms of open programming. As part of its application support, United
Since the femoral is just one component of a total knee replacement, the parts must each be perfect.
Using the Helitronic Vision’s milling capability, Exactech
machines femoral features that are impossible to reach with a grinding wheel. Te milling cutters are carbide and run in the machine’s 10,000-rpm, 59-hp (44-kW) milling spindle. During production, lot sizes of 12 parts each will circulate
through Exactech’s manufacturing process. From raw to fin- ished femoral takes about two weeks, with the bulk of the time being spent on the post-grind finishing operations. Femorals begin as raw cobalt chrome castings that get
mounted onto special custom designed fixture/arbors that hold two parts each for the grinding and milling operations on the Helitronic Vision. Machinists then load the machine’s three pallets with multiple arbors—15 each for a total of 30
74 Medical Manufacturing 2014
Grinding developed Exactech’s specific grinding process to program as a conventional milling one. “Our reasoning behind this approach,” explained Robby
Faulkner, medical accounts manager at United Grinding, “was that learning to program for grinding can be a challenge. While on the other hand, most machinists, especially those at Exactech, are familiar and comfortable with programming for milling operations. With grinding machine soſtware as conventional machining soſtware, the same Siemens PLM programming format used for milling implements seamlessly into the Helitronic Vision. So if an Exactech machinist can program a machining center, they can easily program the Helitronic Vision.”
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