Traditional electrolytic, or electrochemical, machining uses electricity and chemistry to erode material and produce the desired end product. This method either requires an expanding gap between a machining cathode and the workpiece or an equilibrium gap established during a 'sinking' action. A large gap, however, can weaken the cathode's ability to focus current to the workpiece.

Addressing this problem, PEM Technologies has developed precision electrolytic machining (PEM), a method that ensures a very small gap during metal removal and an expanded gap between electrical pulses. The resulting focused current enables users to produce the tight tolerances, intricate features, and burr-free surfaces crucial to many medical devices.

"The machining gap in electrolytic machining determines the degree of focus of the machining current and, hence, the degree of precision of mirror-image material removal," explains Don Risko, vice president of PEM Technologies. "A larger gap causes the current to spread out relative to the distance of the negative-electrode cathode tool to the workpiece, while a smaller gap enables more-precise focusing of the current."

Based on Faraday's laws of electrolysis, the PEM process removes material by breaking atomic bonds at the surface of metal workpieces. This action causes metal ions to leave the surface and enter the electrolytic solution used in the process. Electrolytic dissolution of surface metal can also break bonds of higher-energy states. "Since PEM's material-removal mechanism is a dissolution process without mechanical action, electrical arcing, or a high-temperature effect, it does not result in mechanical or thermal stress to the workpiece," Risko adds.

The PEM process can perform roughing, finishing, and polishing operations. And while machining rates depend on the material, feature size, dimensional requirements, surface finish, and tool-electrode configuration, a full-form-electrode z-axis movement can typically machine at a rate of 1.0 to 0.5 mm/min.

Although the feature sizes achievable by the machine depend on geometry and are sometimes governed by the ability to fabricate microelectrodes, the unit can typically produce features 100 µm or larger in one direction and millimeters in another. In addition, grooves a few microns deep by tens of microns wide are possible, Risko states.

"Components with small cavities, grooves, slots, or irregular shapes are typical applications for PEM," Risko comments. "This means that a variety of implants and surgical devices are candidates for PEM machining. Stainless-steel, nitinol, and even some cobalt-chrome components are good examples of what the system can machine."
PEM Technologies
Natrona Heights, PA
www.pemtechnologies.com