Electrochemical machining and the related process of electrodischarge machining use the passage of an electrical current to remove material from the surfaces of metal workpieces. In the case of electrochemical machining (ECM), an electrolyte carries metal ions away from the workpiece; in the case of electrodischarge machining (EDM), heat generated by an electrical discharge vaporizes atoms from the surface of a metal workpiece.
Versus mechanical machining
ECM and EDM benefit by not suffering the limitations of more conventional mechanical machining techniques. Mechanical machining uses sharp-edged tools to form objects by cutting slivers of material from the surfaces of workpieces. The cutting edge of the tool must be harder than the surface being machined, which limits the availability of tool materials for machining extremely hard workpiece materials.
Mechanical machining subjects the workpiece to significant mechanical stress, which can cause defects in the finished object. Tools wear and become blunt with use, and there is a lower limit to the amount of metal that can be removed by a single cut. Below this limit, a tool simply skids across the surface of the workpiece without cutting into its surface.
Electrochemical machining
In electrochemical machining, a metal workpiece is connected to the positive terminal of a direct-current power supply and immersed in a suitable electrolyte, such as sodium chloride solution. The tool is a cathode, or negative electrode, which is also immersed in the electrolyte. It is held very close to the anode—at distances of a few hundredths of an inch (around 1 mm)—which minimizes power loss and ensures that the electrolytic action is confined to the part of the surface that is to be machined. Part of the cathode may be shielded with an insulator to the same effect.
When the electric current flows, the metal atoms at the anode’s surface lose electrons and become transformed into positively charged metal ions, which dissolve in the electrolyte. Atoms close to the electrode are preferentially removed, so any spikes in the surface are dissolved away and a smooth surface results.
In some ECM applications, a shaped cathode acts as a die for the finished product: the workpiece erodes most quickly near ridges in the die, so it recedes until the profile of the workpiece surface matches that of the die, when the rate of erosion becomes uniform across the surface.
In some applications, electromechanical machining is used for shaping articles that must be finished to precise dimensions. The amount of metal removed from the surface obeys Faraday’s laws, so it depends on the quantity of electric charge that passes through the workpiece, and the depth of the cut can be controlled accurately.
ECM is most useful for forming objects out of extremely hard metals. The electrolyte is pumped through the gap between the anode and cathode at high speed so as to rapidly remove the dissolved metal. This flushing action prevents sludge formation in the gap, which would cause uneven machining. Instead, the sludge forms subsequently, and it is removed by filtering before the electrolyte is recirculated through the gap.
In order to remove surface metal at a convenient rate, current densities as great as 50,000 amps per sq. ft. (around 500,000 amps per m2) are used. The great bulk and high cost of the equipment used to produce strong direct currents places a practical limit on the area that can be machined at any one time. Considerable heat develops as the current passes through the workpiece and tool, and the ability of associated cooling equipment to dissipate that heat also places a limit on the rate of machining.
A typical application of electrochemical machining is in the shaping of blades for gas turbines. Such components are typically made of hard, oxidation-resistant nickel-based alloys.
Electrochemical machining is sometimes used to smoothe or polish workpieces that have been roughly formed by mechanical machining. In such cases, strongly acidic electrolytes are used with typical current densities of around 500 amps per sq. ft. (5,000 amps per m2). An example of such a combination of techniques is sometimes used to produce gear wheels: ECM removes any burrs left by mechanical machining. Another example is in the finishing of stainless-steel drums for automatic washing machines. Electrochemical treatment gives a pleasing bright appearance as well as removing any ragged edges that would damage laundry items during wash cycles.
