Electrical Discharge Machining (EDM)

Electrical discharge machining (EDM) is one of the most widely used non-traditional machining processes. The main attraction of EDM over traditional machining processes such as metal cutting using different tools and grinding is that this technique utilises thermoelectric process to erode undesired materials from the work piece by a series of discrete electrical sparks between the work piece and the electrode.

The traditional machining processes rely on harder tool or abrasive material to remove the softer material whereas non-traditional machining processes such as EDM uses electrical spark or thermal energy to erode unwanted material in order to create desired shape.

So, the hardness of the material is no longer a dominating factor for EDM process. A schematic of an EDM process is shown in Figure , where the tool and the workpiece are immersed in a dielectric fluid.

Schematic of EDM process

EDM removes material by discharging an electrical current, normally stored in a capacitor bank, across a small gap between the tool (cathode) and the workpiece (anode) typically in the order of 50 volts/10amps.

Working principle of EDM

As shown in Figure, at the beginning of EDM operation, a high voltage is applied across the narrow gap between the electrode and the workpiece. This high voltage induces an electric field in the insulating dielectric that is present in narrow gap between electrode and workpiece. This cause conducting particles suspended in the dielectric to concentrate at the points of strongest electrical field. When the potential difference between the electrode and the workpiece is sufficiently high, the dielectric breaks down and a transient spark discharges through the dielectric fluid, removing small amount of material from the workpiece surface.

EDM – Components

The main components in EDM:

Electric power supply

Dielectric medium

Work piece & tool

Servo control unit.

The work piece and tool are electrically connected to a DC power supply.

The current density in the discharge of the channel is of the order of 10000

A/cm2 and power density is nearly 500 MW/cm2.

A gap, known as SPARK GAP in the range, from 0.005 mm to 0.05 mm is maintained between the work piece and the tool.

Dielectric slurry is forced through this gap at a pressure of 2 kgf/cm2 or lesser.

EDM – Working Principle

It is a process of metal removal based on the principle of material removal by an interrupted electric spark discharge between theelectrode tool and the work piece.

In EDM, a potential difference is applied between the tool and workpiece.

Essential - Both tool and work material are to be conductors.

The tool and work material are immersed in a dielectric medium.

Generally kerosene or deionised water is used as the dielectric medium.

A gap is maintained between the tool and the workpiece.

Depending upon the applied potential difference (50 to 450 V) and the gap between the tool and workpiece, an electric field would be established.

Generally the tool is connected to the negative terminal (cathode) of the generator and the workpiece is connected to positive terminal (anode).

As the electrons get accelerated, more positive ions and electrons would get generated due to collisions.

This cyclic process would increase the concentration of electrons and ions in the dielectric medium between the tool and the job at the spark gap.

The concentration would be so high that the matter existing in that channel could be characterised as “plasma”.

The electrical resistance of such plasma channel would be very less.

Thus all of a sudden, a large number of electrons will flow from tool to job and ions from job to tool.

This is called avalanche motion of electrons.

Such movement of electrons and ions can be visually seen as a spark.

Thus the electrical energy is dissipated as the thermal energy of the spark.

The high speed electrons then impinge on the job and ions on the tool.

The kinetic energy of the electrons and ions on impact with the surface of the job and tool respectively would be converted into thermal energy or heat flux.

Such intense localized heat flux leads to extreme instantaneous confined rise in temperature which would be in excess of 10,000oC.

Such localized extreme rise in temperature leads to material removal.

Material removal occurs due to instant vaporization of the material as well as due to melting.

The molten metal is not removed completely but only partially.

Electrode Material

Electrode material should be such that it would not undergo much tool wear when it is impinged by positive ions. Thus the localized temperature rise has to be less by tailoring or properly choosing its properties or even when temperature increases, there would be less melting. Further, the tool should be easily workable as intricate shaped geometric features are machined in EDM.

Thus the basic characteristics of electrode materials are:

High electrical conductivity – electrons are cold emitted more easily and there is less bulk electrical heating

High thermal conductivity

Higher density

High melting point – high melting point leads to less tool wear due to less tool material melting for the same heat load

Easy manufacturability

Cost – cheape.g, Graphite ,Electrolytic oxygen free copper ,Tellurium copper – 99% Cu +0.5% tellurium ,Brass

Application of EDM

The EDM process has the ability to machine hard, difficult-to-machine materials. Parts with complex, precise and irregular shapes for forging, press tools, extrusion dies, difficult internal shapes for aerospace and medical applications can be made by EDM process. Some of the shapes made by EDM process are shown in Figure

Difficult internal parts made by EDM process

Advantages of EDM

The main advantages of DM are:

By this process, materials of any hardness can be machined;

No burrs are left in machined surface;

One of the main advantages of this process is that thin and fragile/brittle components can be machined without distortion;

Complex internal shapes can be machined

Limitations of EDM

The main limitations of this process are:

This process can only be employed in electrically conductive materials;

Material removal rate is low and the process overall is slow compared to conventional machining processes;

Unwanted erosion and over cutting of material can occur;

Rough surface finish when at high rates of material removal.

Dielectric fluids

Dielectric fluids used in EDM process are hydrocarbon oils, kerosene and deionised water.

The functions of the dielectric fluid are to:

Act as an insulator between the tool and the workpiece.

Act as coolant.

Act as a flushing medium for the removal of the chips.

The electrodes for EDM process usually are made of graphite, brass, copper and coppertungsten alloys.

Design considerations for EDM process are as follows

Deep slots and narrow openings should be avoided.

The surface smoothness value should not be specified too fine.

Rough cut should be done by other machining process. Only finishing operation should be done in this process as MRR for this process is low.

Wire EDM

EDM, primarily, exists commercially in the form of die-sinking machines and wire-cutting machines (Wire EDM). The concept of wire EDM is shown in Figure . In this process, a slowly moving wire travels along a prescribed path and removes material from the workpiece. Wire EDM uses electro-thermal mechanisms to cut electrically conductive materials. The material is removed by a series of discrete discharges between the wire electrode and the workpiece in the presence of dieelectirc fluid, which creates a path for each discharge as the fluid becomes ionized in the gap. The area where discharge takes place is heated to extremely high temperature, so that the surface is melted and removed. The removed particles are flushed away by the flowing dielectric fluids.

Wire erosion of an extrusion die

The wire EDM process can cut intricate components for the electric and aerospace industries.

This non-traditional machining process is widely used to pattern tool steel for die manufacturing.

The wires for wire EDM is made of brass, copper, tungsten, molybdenum. Zinc or brass coated wires are also used extensively in this process. The wire used in this process should posses high tensile strength and good electrical conductivity. Wire EDM can also employ to cut cylindrical objects with high precision. The sparked eroded extrusion dies are presented in Figure

Sparked eroded extrusion dies

This process is usually used in conjunction with CNC and will only work when a part is to be cut completely through. The melting temperature of the parts to be machined is an important parameter for this process rather than strength or hardness. The surface quality and MRR of the machined surface by wire EDM will depend on different machining parameters such as applied peak current, and wire materials.