Conventional machining processes (such as turning, milling, facing, drilling, etc.) cannot provide high surface finish and better dimensional accuracy. To improve the surface integrity of machined features, sometimes abrasive finishing is carried out. There exist several abrasive finishing processes like grinding, honing, abrasive flow finishing, magneto-abrasive finishing, etc. All these processes utilize grain abrasives either in the form of a bonded device or in the form of loose abrasive slurry. Whatever be the case, the hard abrasive particles act as cutting tool and slowly removes material from the work surface. In traditional grinding, a disc type wheel made of abrasive particles bonded in a harder medium is rotated against the workpiece to remove material in the form of solid micro-chips. Common abrasives that are used to fabricate wheel include alumina, silica, diamond, cBN, etc. Grinding is not usually suitable for high material removal rate or bulk removal of material; instead, it offers a smooth surface with improved level of integrity.
Electric Discharge Grinding (EDG) is categorized as a non-traditional machining process. It is fundamentally different from conventional grinding (the only similarity exists in their apparently similar construction). Similar to electric discharge machining (EDM), EDG also utilizes electric current to repeatedly constitute sparks between the grinding wheel and workpiece. So EDG wheel must be made of conductive materials (like graphite) and can be applied for electrically conductive work materials only. Similar to grinding, this EDG wheel is also rotated; however, a small gap is maintaining between the wheel and workpiece. Suitable dielectric is applied in this gap. Rotating wheel inherently facilitates dielectric flushing; thus no additional flushing strategy is required. Whereas material is removed in the form of solid micro-chips in grinding, EDG follows the principle of melting and vaporizing the material with the assistance of high intensity spark. Various similarities and differences between traditional grinding and electro discharge grinding (EDG) are given below in table format.
Similarities between traditional grinding and electro discharge grinding
- Both the traditional grinding and the electro discharge grinding (EDG) follow same principle of subtractive manufacturing (where layers of material is removed from a solid blank to obtain desired object).
- Both the processes employ a disc type wheel (however, the wheel material differs).
- Both of them offer higher level of surface finish (as low as few tens of nm) and improved tolerance.
Differences between traditional grinding and electro discharge grinding
| Traditional Grinding | Electro Discharge Grinding |
|---|---|
| It can be applied for electrically conductive as well as non-conductive workpiece materials. | It can be applied only for electrically conductive workpiece materials. |
| The grinding wheel remains in continuous contact with the workpiece during machining. | The rotating disk type electrode (grinding wheel) does not physically touch the workpiece. A small gap (called inter-electrode gap) always exist between them. |
| No dielectric fluid is used here. Suitable cutting fluid can be applied; however, its function is different from dielectric fluid. | An appropriate dielectric fluid should be applied in inter-electrode gap to assist spark formation and debris removal. |
| The grinding wheel can be made of electrically conductive or non-conductive material (like alumina, silica, diamond, metal, etc.). | The grinding wheel must be made of electrically conductive material (lie graphite). |
| Material is removed in the form of solid micro-chips. | Material is removed in molten and vapour phases. |
| In grinding, mechanical energy is directly utilized to remove excess material from workpiece in a controlled way. | Material removal in EDG is based on thermal energy. So heat is primarily used to remove material from workpiece. |
| Size and sharpness of the abrasives are crucial factors as they determines MRR and finish level. | Such parameters don’t influence machining capability in EDG. |
| Due to physical contact between rotating wheel and workpiece, significant cutting heat is generated undesirably. | Although wheel does not touch the workpiece, this process is based on thermal energy so heat is intentionally generated. |
| Excessive cutting heat has numerous drawbacks. It hampers surface finish, changes surface properties, reduces MRR, accelerates wheel wear. | Although localized temperature reaches as high as 10000°C, heat does not spread over a wide area. Accordingly, heat related problems are not significant here. |
| Here chip is formed primarily by shearing and assisted by ploughing and rubbing. | Material removal mechanism in EDG is nothing but melting and vaporization due to extreme spark heat. |
| Chip accumulation in inter-grip space at wheel periphery occurs frequently. Thus dressing is required to carry out regularly. | No chip accumulation on wheel periphery occurs here. Thus no need to carry out dressing. |
References
- Mishra et al. (2018). Experimental investigations into electric discharge grinding and ultrasonic vibration-assisted electric discharge grinding of Inconel 601. Materials and Manufacturing Processes. 33(14): 1518-1530.
- Unconventional Machining Processes by T. Jagadeesha (I. K. International Publishing House Pvt. Ltd.).
- Advanced Machining Processes by V. K. Jain (Allied Publishers Private Limited).