Machine Design

Advanced Manufacturing: A different twist to electrochemical machining

Resources: PEM Technologies, Edited by Leslie Gordon

ECM, or electrochemical machining, is considered a form of nonconventional metalcutting because it machines and deburrs parts without the cutting tool touching the workpiece. The advantage: No thermal or mechanical stresses are transferred to parts. However, traditional ECM cannot hold tolerances better than thousands of an inch. Fortunately, a process called precision electrochemical machining (PEM) from PEM Technologies, Natrona Heights, Pa., advances ECM, using a pulsed dc current and an oscillating cathode. PEM can machine hard and exotic alloys such as Hasteloy and Nitinol to less than a thousandth of an inch and produce fine features and small edge radii. It can also handle roughing and finishing in one operation, producing surface finishes down to 0.05 micron Ra.

Recall that ECM is often described as “reverse electroplating” because it removes material instead of adding it. The method works like this: The workpiece (anode) and tool (cathode) are exposed to electrolyte (a salt solution), which conducts electricity. As the tool moves towards the workpiece, the dc circuit closes and current flows. This dissolves the anode electrolytically, with the amount of material removed determined by tool shape and placement. The rate at which metal is removed is a function of current (I) × time (T), and is approximately inversely proportional to the distance between electrodes.

Similar to ECM, PEM also advances the cathode into the workpiece. But when the tool and part are close (typically a 10 to 20–micron gap), the metal-removal current pulses in sync with the oscillating head of the machine. During the pulse period, pressurized electrolyte is injected at a set temperature through the gap (typically, at a rate of 5 to 80 fps) to flush away metal hydroxide waste.

In addition to removing waste, flushing reduces gas-bubble generation and electrical heating. PEM dissolution rates are not influenced by hardness or any other attribute of the metal. In addition, the cathode shape remains unchanged. Applications include gears, manifolds, surgical instruments and implants, and metal-stamping dies.

The PEM machine has a granite frame and machining table with built-in stainless-steel T-slots. It uses a patented oscillation mechanism driven by a frequency-regulated servomotor. The power source delivers pulse currents ranging from 2,000 to 8,000 A. The electrolyte supply unit micro-filters the electrolyte and provides a variable flow rate, while controlling the pH, conductivity, and temperature. Last, the controller’s touchscreen lets operators program workpiece-oriented sequences by selecting on-screen parameters.

© 2010 Penton Media, Inc.

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