![]() ![]() “You have to stay in the cut.”Īfter machining a few parts for a job, Oberg has captured enough data to understand the process, making it a predictable job, Hoffman pointed out. “You have to bite into the material, but you cannot sit there and dwell,” he said. To minimize workhardening and the damage it can cause to even a properly designed cutting tool, the tool must be kept loaded and continue to shear the workpiece rather than rub it, according to Hoffman. “You’re not going to go in there with a sharp-corner tool,” he said. Jim Hoffman, director of manufacturing for Oberg Medical, Freeport, Pa., emphasized the importance of a tool’s corner radius. “If you’ve got those four things figured out, you can make the tools work really well.” “The substrate, coating, geometry and edge prep are key,” he said. He added that the coating, however, can extend tool life at least 30 percent. The tools can be coated with aluminum titanium nitride or ordered uncoated if the orthopedic company has not yet validated the toolmaker’s coating, Dahms noted. They have a carbide substrate with at least 10 percent cobalt to enhance tool toughness. The endmills also have unequal index geometry to minimize vibration and corresponding chatter. They are available with corner radii for added rigidity or for matching the size of the part feature. Oak View Tool designed its Ortho-Cut “CC” series endmills for machining medical-grade cobalt chrome. ![]() “It’s so tough of a material, you have to hit it with the best of both worlds.” “The series is designed for cobalt chrome and those types of tough materials that workharden and tear up a typical tool,” Dahms said.īecause cobalt chrome can workharden, CC tools have an edge prep combined with an effective shear angle, Dahms explained. Oak View Tool also makes CC specials, such as keyway, form and dovetail cutters. To enhance efficiency when cutting medical-grade cobalt chrome, the Columbia City, Ind.-based toolmaker developed the Ortho-Cut “CC” series of endmills. “The hard spots are about 58 HRC,” said Matt Dahms, founder and president of Oak View Tool Co. The challenge of cutting cobalt chrome is compounded when the workpiece has hard, abrasive intermetallic compounds in its microstructure. “That way you don’t have to worry about a very small cutting tool that just wants to ping right off once it touches the part,” he said. When the metal’s hardness makes it too much of a pain to mill or turn, or part features are too small or delicate for those machines, the shop will wire or sinker EDM it. Moser noted the company tends to apply uncoated carbide ballnose and other endmills to cut cobalt chrome, with some tools having a titanium-nitride coating for enhanced wear resistance. “It becomes even more difficult once the hardness gets up in the 50-HRC range.” Although the material has a tendency to workharden, he said cobalt chrome’s high hardness causes the shop more problems. “Cobalt chrome is extremely challenging to machine,” concurred Russ Moser, machining manager for medical parts manufacturer Judson A. Hardness ranges from 40 to 50 HRC or higher. Cobalt chrome is stronger than stainless steel, but weighs twice as much as stainless and is brittle under impact loading. For example, CoCr28Mo6 ASTM F75 contains 58.9 to 69.5 percent cobalt, 27.0 to 30.0 percent chromium, 5.0 to 7.0 percent molybdenum, up to 1.0 percent manganese, silicon and nickel, up to 0.75 percent iron and up to 0.35 percent carbon. In addition to cobalt and chromium, the metals used in the medical industry contain various alloying elements with desirable wear- and corrosion-resistance properties for implants, such as shoulder, knee and hip replacements. The Lake-ville, Minn., medical parts manufacturer specializes in implants, surgical instruments and diagnostic equipment, as well as serving the defense and aerospace industries. “It ranks at the top in terms of difficulty,” said Steve Storlie, vice president of business development for Mendell Machine and Manufacturing Inc. Manufacturers of medical and dental parts face an array of challenging workpiece materials, but cobalt-chrome alloys might take the cake. Oberg Medical performs multiaxis machining to produce a variety of medical devices.Ĭobalt-chrome alloys are a good fit for medical and dental parts, but machining them can be a headache. ![]()
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