In die-sinking electro-discharge machining (EDM), the tool electrode and the workpiece (both made of electrically conductive material) are connected with two terminals of a DC power source. A small gap (in micron range) is maintained between these two electrodes. This gap is immersed with appropriate dielectric fluid that behaves as an insulator in initial state. When sufficient potential difference is applied across the two electrodes, the dielectric breaks down to make the channel superconductive. Hence, spark is generated. This spark increases the localized temperature to as high as 10,000°C, which leads to removal of material by melting and vaporization leading to the creation of a small cavity on the work surface. If such removed materials are allowed to stay in the sparking zone (i.e. in between tool and workpiece) for longer duration then short circuiting occurs. It makes the dielectric breakdown and spark generation very difficult. Since spark is utmost important in EDM, so it is indispensably necessary to continuously flush away the removed material from the sparking zone.
Flushing is the method of supplying fresh dielectric in the inter-electrode gap and subsequently removing the existing dielectric. There are several flushing strategies used in EDM—each offers certain benefits over others. Pressure flushing, vacuum flushing, jet flushing, side flushing, reciprocating electrode flushing, centrifugal flushing, etc. are typical examples of flushing techniques. In pressure flushing (also called injection flushing), fresh dielectric liquid is pumped into the inter-electrode gap, which automatically displaces the used contaminated dielectric. On the other hand, in vacuum flushing (also called suction flushing), the used dielectric is directly sucked from the inter-electrode gap and is delivered for cleaning and recycling. When used dielectric is sucked, a vacuum pressure is created; and hence, fresh dielectric from the surroundings in the machining chamber automatically comes within the inter-electrode gap. Various similarities and differences between pressure flushing and vacuum flushing are given below in table format.
Similarities between pressure flushing and vacuum flushing
- Both the flushing techniques aim towards continuous supply of fresh dielectric in the inter-electrode gap between the cathode and anode. However, this is performed in different ways.
- Presence of drilled hole within the tool electrode or the workpiece is absolutely necessary for both the techniques (for sucking or pumping dielectric).
- Dielectric breakdown, spark formation and material removal mechanism (melting and vaporization) in EDM are independent of flushing technique.
- Basic purposes of dielectric in EDM (assistance in spark formation, cooling the tool and workpiece, and removing of eroded materials) remain unchanged irrespective of flushing technique.
Differences between pressure flushing and vacuum flushing
| Pressure Flushing | Vacuum Flushing |
|---|---|
| In this flushing technique, pressurized clean dielectric fluid is pumped into the inter-electrode gap (IEG). | In this flushing technique, used dielectric fluid is sucked from the inter-electrode gap (IEG). |
| This flushing technique works on fresh dielectric. It directly delivers fresh dielectric into IEG. Thus used dielectric is automatically displaced from IEG. | This flushing technique works on used dielectric. It directly sucks used dielectric from IEG, which results drawing of fresh dielectric into IEG. |
| Undesired tapering effect is observed in external side walls owing to debris build up and evacuation discharges at the downstream of dielectric flow. | Tapering effect is observed either in internal side walls or at bottom surface. |
| Due to tapering effect, side gap does not remain constant. Thus no need to alter dielectric flow rate. | Here side gap usually remains fixed. Thus no adjustment in flow rate is required. |
References
- Makenzi et al. (2012). Proceedings of the 2012 Mechanical Engineering Conference on Sustainable Research and Innovation. Vol. 4.
- Nonconventional Machining by P. K. Mishra (Narosa Publishing House).