(2) Configuration of a Electro-machining system

A typical electro-machining system configuration is shown in [Fig.1]. As seen in [Fig.1], the work and the tool (electrode) is held very closely (0.02~0.7mm typical) to each other. A DC current is applied to the work as an anode (positive) and the tool as a cathode (negative) through the electrolyte.

Normally, the voltage used is 5~20V, and current density is 30~200A/cm2. This voltage/current level is quite high in comparison to Electrolytic-polishing and plating. The higher voltage and current results in faster process speed, improved accuracy and better surface roughness.

In order to constantly maintain a fixed positive/negative electrode distance (process gap), the cathode is fed into the work piece. The feed speed is 0.5~10mm/min based on the current density specified. As the result, the work piece is milled in a shape inverse of the tool (cathode) shape.

When the current is applied, hydrogen gas is formed from the cathode surface, and dissolved product from the anode surface. Also, the electrolyte temperature rises due to Joule heating by the electrical current application. If the electrolyte temperature reaches a boiling point, controlling of the electrical current becomes impossible. It is important to remove these negative factors immediately. To do this, the electrolyte is forced to flow between the electrodes at high speeds by the recirculation pump at 6~60m/S, typically.

The used electrolyte falls into the storage tank, then the impurities are let sedimented, centrifuged, filtered, and cooled down. The clean electrolyte is reused.

(3) Electrolyte

Desired properties for the electrolyte used are: (1) does not promote generation of passive solubles that may passivate the metal surfaces; (2) positive ions in the solution do not electro-deposit on the electrodes; (3) high conductivity and low viscosity; (4) produces good accuracy and finished surfaces; (5) low corrosiveness and toxicity; (6) stable solution composition and low cost, easy to obtain; etc.

Neutral salt solution has a lower conductivity in comparison with acids and alkali, but is often used. For instance, sodium chloride (NaCl) solution is used for steel alloys, nickel-chrome alloys, and most other metals. This solution contains chloride ions and prevents anode passivation. Also, the positive Na ions prevents electrodeposition on cathodes.