In electro-discharge machining (EDM), spark generated between the conductive tool electrode and the conductive workpiece melts and vaporizes the workpiece to realize material removal. A small gap is maintained in between the tool and workpiece and an appropriate dielectric fluid is applied in this gap. Dielectric is basically a liquid that can suddenly breakdown once a particular potential difference is applied across it. Before breakdown, it acts as insulator; whereas after breakdown, it acts as a good conductor. This breakdown is necessary for spark generation in EDM. Functions of dielectric fluid in EDM process include—(i) to restrict electron flow before breakdown, (ii) to allow smooth flow of electrons and ions after breakdown or to assist spark generation, (iii) to protect work surface from oxidation, (iv) to remove generated heat and to protect work or tool from overheating, and (v) to flush away removed debris from the sparking zone. Accordingly, the dielectric should possess high dielectric strength, minimum ignition delay time, good cooling capability and good fluidity.
Electro-Chemical Machining (ECM) is another non-traditional machining process where material is removed from work surface by controlled electro-chemical reaction following Faraday’s Law in an electrolytic cell. Here also the shaped tool electrode and the workpiece (both made of conductive materials) are connected with two terminals of the DC power source. A suitable electrolyte is applied in between them (in inter-electrode gap, IEG). This electrolyte is basically a conductive liquid that carries ions and electrons under the presence of potential difference across two electrodes. Electrolytes in ECM process serves the following functions—(i) it allows flow of current between two electrodes via itself, which is necessary for electro-chemical reaction to take place, (ii) it removes generated heat from IEG, and (iii) it displaces dissolved metallic ions and restricts accumulation of such ions in IEG. Its composition, concentration, fluidity and temperature are crucial factors that can influence capability and material dissolution rate in ECM. Various similarities and differences between dielectric and electrolyte used in EDM and ECM are given below.
Similarities between dielectric and electrolyte
- Both are liquid.
- Both of them helps in removing machined products (sludge) from the inter-electrode gap. Continuous flow of dielectric or electrolyte does not allow removed material to accumulate and precipitate in work surface.
- Both require sufficient potential difference to be applied across them for their functions.
- Both of them dissipates generated heat from the machining zone.
Differences between dielectric and electrolyte
| Dielectric | Electrolyte |
|---|---|
| Dielectric fluid is utilized in electro-discharge machining (EDM) process. | Electrolyte is utilized in electro-chemical machining (ECM) process. |
| A dielectric should be electrically non-conductor at normal condition; however, it should behave as good conductor after breakdown. | Electrolyte should be good conductor of electricity for completing the electrical circuit to allow smooth flow of ions between electrodes. |
| The dielectric should be oxygen free. Due to extreme spark heat, liberation of oxygen can oxidize the work surface. | Presence of oxygen in electrolyte does not matter as ECM process does not generate extreme heat. |
| Common dielectric fluids used in EDM include deionized water, transformer oils, kerosene, paraffin, etc. | Commonly used electrolytes in ECM process include sodium chloride (NaCl) and sodium nitrate (NaNO3) solutions. |
References
- Unconventional Machining Processes by T. Jagadeesha (I. K. International Publishing House Pvt. Ltd.).
- Nonconventional Machining by P. K. Mishra (Narosa Publishing House).