Modern Machine Shop

FEB 2018

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Page 69 of 147

MMS FEBRUARY 2018 68 EDM TECHNOLOGY This mold insert is used to mold micro-rotors with an overmolded magnetic core. The micro-rotors are used in stepper motors to drive miniscule metering pumps to inject very small quantities of active agents in medical applications. Photo: PKT This rotor made of PA 12 is only a few millimeters long and weighs 0.002 gram. Photo: Klaus Vollrath tips that could also provide the required spark gap of 7 to 10 microns. To wire-EDM this insert, the insert would have had to be divided on each tooth tip, Mr. Rapp explains. "This is virtually impossible with gears like this one, which feature 60 to 100 teeth, because the mold would become an impossibly complicated, high-precision puzzle of tiny parts," he says. In addition, he says the use of wire EDM is further restricted if the workpiece geometry also requires three-axis machining. To mill the gear geometry would require a milling cutter with a maximum diameter of 10 microns, and this type of cutter is not available for the materials to be machined. Laser ablation was also considered, but Mr. Rapp says that process was rejected because of the conicity generated by the ablation saturation during laser cutting. In its quest for suitable technologies, PKT became aware of the micro-EDM process devel- oped by Swiss company Sarix (Sarix equipment is available in the United States from Alouette Tool Company in Fairport, New York). Its 3D micro-milling process is similar to die-sinking EDM, except that the electrode does not repre- sent a geometric negative of the desired mold cavity. Instead, the electrode is an extremely thin rotating rod that, like a milling tool, moves along the workpiece, thereby removing material by spark erosion. The rotation of the rod is said to

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