Non-Traditional Machining (NTM) processes employ different forms of energy in their direct form to removal material from the workpiece. Such energy forms include mechanical energy (AJM, AWJM, USM), chemical energy (CHM), electro-chemical energy (ECM), thermal energy (EDM, LBM, EBM, PAM), etc. The mechanism of material removal also varies accordingly. As the name suggests, chemical machining employs an appropriate chemical solution to etch away material from work surface. Such chemical solution is called etchant and is highly corrosive. No separate cutting tool, shaped tool or similar device is used here. The etchant itself reacts with work material and thus the later one gets dissolved into the etchant solution. Thus it is mandatory to select etchant composition based on the work material. His process is independent of mechanical, electrical and thermal properties of work material; however, relies on its chemical reactivity.
In Electro-Chemical Machining (ECM), material is removed by ionic dissolution but under the assistance of electro-chemical reaction. No corrosive etchant is used here; instead, a suitable electrolyte is employed. ECM requires a shaped tool made of electrically conductive material that is placed close to the workpiece (a small gap is maintained). Both tool and workpiece are connected with two polarities of a DC power source and are immersed partially into the electrolyte. Under the presence of sufficient potential difference, electro-chemical reaction initiates following the principles of Faraday’s Law. Here the tool material and the electrolyte are chosen in such a way that material from worksurface only gets dissolved into the electrolyte (tool and electrolyte remain intact). The ECM tool must have profile based on intended profile because inverse shape of the tool is imparted on the workpiece. Moreover, ECM can be applied for electrically conductive materials only. Various similarities and differences between CHM and ECM are given below in table format.
Similarities between CHM and ECM
- Both CHM and ECM are considered as advanced manufacturing processes (AMP). However, none of them is hybrid process.
- None of them causes burr or thermal damages to the machine surface.
- Both processes are quite (unlike AJM or PAM that generates excessive noise).
- Both processes are independent of mechanical properties of the work material. Thus ductility, hardness and strength don’t possess any limitation on the application of CHM and ECM.
- Both offer similar levels of surface finish (0.1 – 2.0 micron).
Differences between CHM and ECM
| Chemical Machining (CHM) | Electro-Chemical Machining (ECM) |
|---|---|
| As the name suggests, CHM is one chemical energy based NTM process. | It is one electrical energy based NTM process. |
| Here material is removed in ionic form due to controlled dissolution by chemical etchant. Etchant slowly dissolves metal to realize machining. | Here also material is removed in ionic form but due to controlled electro-chemical dissolution of work metal. |
| It is independent of electrical conductivity of workpiece metal. So it can be applied for conductive and non-conductive materials. | The workpiece must be electrically conductive as it is used as anode of the electrolytic cell. |
| No additional electrode is necessary as the chemical reaction takes place between etchant and workpiece. | An electrode having curvature according to intended shape is absolutely required to complete the electrical circuit. |
| No power supply is necessary as the process is not related to electrical energy. | One DC power supply is required to maintain desired potential difference between cathode (tool electrode) and anode (workpiece). |
| Here suitable etchant (based on workpiece material) is employed. Typical etchant include FeCl3, CuCl3, FeNO3, HNO3 and HF. | Instead of etchant, appropriate electrolyte is applied in ECM. Typical electrolyte include NaCl and NaNO3. |
| The rate of chemical dissolution is very hard to control. Once the process is initiated, the rate cannot be controlled accurately anymore. | Rate of electro-chemical dissolution can be controlled during machining simply by adjusting inter-electrode gap and current. |
| It cannot inherently control the area from where material should be dissolved. Material is removed wherever the worksurface is exposed to etchant. Thus mask can be separately applied to control exposure. | This process can inherently control the area from where material should be removed (dissolution occurs only from the areas where tool is in close proximity). |
| Chemical etchants are highly corrosive and thus the process possesses a risk to the operator. | The etchant is not such corrosive and thus the process is somewhat risk-free towards operator. |
References
- Unconventional Machining Processes by T. Jagadeesha (I. K. International Publishing House Pvt. Ltd.).
- Nonconventional Machining by P. K. Mishra (Narosa Publishing House).