Machine Design

"Liquid electrode" finishes alloy parts

A special electrochemicalmachining (ECM) technology uses an electrolyte solution with proprietary chemistry as a "tool" to perform finishing operations on alloy parts instead of the more-typical, conformal, metal electrode.

Close-up samples of a medical implant show the part before (top) and after undergoing the special CoolPulse finishing process.

Extrude Hone, Irwin, Pa., licenses the special technology from a Swedish developer and calls the process CoolPulse. The process handles deburring, polishing, cleaning, passivating, and stress-relieving. Medical applications include clamps, scissors, stents, cannulas, guide-wire catheters, and orthopedic implants such as knees and hips.

Extrude Hone ECM manager Jim Koroskenyi says traditional ECM is similar to electricaldischarge machining (EDM) in that a conforming electrode placed near a part creates the required shape. "The difference is ECM doesn't affect the underlying metal structure. The process removes metal with an electrochemical reaction instead of spark erosion — kind of like a reverse electroplating," he says. "The part is the plus side, or anode, of a low-voltage dc circuit and the shaped tool is the negative side or cathode. A complex reaction takes place in the small gap between part and tool. The metal forms a hydroxide that can be filtered from the solution."

CoolPulse takes this a step further, performing finishing operations on complex shapes without requiring a conforming electrode. Parts are immersed in the high-resistance solution which operates at lower temperatures (32 to 60°F) than conventional electrolytes (room temperature to about 80°F). The cathode can be several inches, or even a foot, away from the part.

"The high resistance causes the current to move slowly through the solution," says Koroskenyi. "This, in turn, makes the ions change their trajectory and migrate towards part edges, which have the highest current density. A diffusion layer fills the valleys of the part surface and takes the peaks away." The company uses an "antenna" to direct ions to the inside of parts with internal features.

CoolPulse handles aluminum, cobalt chrome and stainless-steel parts in sizes ranging from small handheld ones to those as big as a PC cabinet, with runs ranging from a few hundred to tens of thousands.

Extrude Hone,

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