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Electro Chemical Machining (ECM)

Electro Chemical Machining (ECM)

Electrochemical machining (ECM) is a non-traditional machining process uses the principle of Faraday to remove metal from the workpiece. Electrolysis is based on Faraday laws of electrolysis which is stated as
weight of substance produced during electrolysis is proportional to current passing, length of time the process used and the equivalent weight of material which is deposited.
Michael Faraday discovered that if two electrodes are placed in a bath containing liquid and when a direct potential is applied across electrodes, The metal can be depleted from the anode and plated on the cathode. This process is universally used in electroplating by making the workpiece the cathode.
Electrochemical Machining (ECM) is performed by reversing the process of electroplating, ECM utilizes the principle of electrolysis for metal removal. The high rate of electrolyte movement in the tool-workpiece gap washes metal ions away by anodic dissolution from the electrically conductive workpiece (anode: positive pole) before they have a chance to plate onto the tool (cathode: negative pole).
Tool work-gap in ECM needs to be maintained at a very small value of the order of 0.25 mm for satisfactory metal removal rates. The electrolyte needs to be pumped through the gap 0.25 mm at high pressures (0.7 – 3.0 MPa). For the recirculation of electrolyte, it is necessary to clean the electrolyte of the debris formed due to metal removal.
Electro Chemical Machining
ECM Metal Removal Rate is independent of the hardness of work.
Material removal rate in ECM process
Where, F = faraday’s constant = 96,500 Columns = 26.8 amp-hours,
I = current flowing in amperes,
Z = Valances of metal dissolved,
A = atomic weight of material in grams,
MRR = Material removal rate in grams per second.

Electro Chemical Machining consists of following basic elements:

Electrolyte: In Electro Chemical Machining an electrolyte acts as a current carrier. The electrolyte in Ecm should have high electrical conductivity, low viscosity, high specific heat, chemical stability, resistance to form a passivating film on the workpiece surface, non-corrosives and non-toxicity. The conductivity of electrolyte depends on salt concentration, dissolved gases, machined debris and temperature. Inorganic salt solutions satisfy these requirements, such as sodium chloride (NaCl) or Potassium chloride (KCl) is mixed in water or sodium citrate.
The electrolyte enters the gap between the electrode and the workpiece a pressure ranging from 1.4 to 2.4 MPa. The flow of electrolysis serves the function of removing heat and hydrogen bubbles created in the chemical reactions of the process. The flow should be without cavitation, stagnation and vortex formation. This can be achieved by avoiding sharp corners in the flow path.
Tool material: Tool materials for ECM should be electrically and thermally conductive and highly resistant to corrosion. The surface finish of the electrode tool is generally made of aluminium, copper, brass, bronze, titanium, cupronickel or stainless steel.
The electrode is generally made smaller than the cavity desired because the erosion action progressing outward from the electrode always produces a cavity larger than the electrode.
Power supply: In ECM a DC power supply is used in the range of 5 to 25 Volts to maintain current densities in the range of 1.5-8.0 A/sq.mm. The voltage is kept relatively low to minimize arcing across the gap.

Advantages:

  • Complex, concave curvature components can be produced easily by using convex and concave tools.
  • Tool wear is zero, same tool can be used for producing infinite number of components.
  • No direct contact between tool and work material so there are no forces, residual stresses.
  • The surface finish produced is excellent.

Limitations:

  • Out of all the unconventional machining methods, electro chemical machining requires high specific cutting energy.
  • Sharp edges and corners are not possible to produce.
  • Work material must be electrically conducting.
  • Generally preferable for producing contours only.

Applications:

Electro chemical machining technique removes material by atomic level dissolution of the same by electro chemical action. Thus the material removal rate (MRR) is independent on the mechanical or physical properties of the work material. ECM can machine any electrically conductive work material irrespective of their hardness, strength or even thermal properties. Moreover as ECM leads to atomic level dissolution, the surface finish is excellent with almost stress free machined surface and without any thermal damage. Mainly ECM is used for producing complex shapes of compound like turbine blades.
ECM is used commonly for operations like
  • Die sinking
  • Profiling and contouring
  • Trepanning
  • Grinding
  • Drilling
  • Micro-machining

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