Version 1.0, August 31, 2001, Copyright, Hugh Jack 1993-2001

59. ELECTROCHEMICAL MACHINING (ECM)

 

· The physics - an electrode and workpiece (conductor) are placed in an electrolyte, and a potential voltage is applied. On the anode (+ve) side the metal molecules ionize (lose electrons) break free of the workpiece, and travel through the electrolyte to the electrode (a cathode; has a -ve charge; a surplus of electrons).

 

· NOTE: in EDM an arc was used to heat metal, here the metal dissolves chemically.

 

 

· Variation in the current density will result in work taking the electrodes shape.

 

· The electrode is fed with a constant velocity, and the electrolyte is fed through the tool. The tool is designed to eliminate deposition of the ionized metal on the electrode.

 

 

· Supply V = 8 to 20V, I = >1000A.

 

· Electrode gap is typically 0.1 to 0.2 mm.

 

· mrr is about 1600mm3/min. per 1000A, OR 3KWhr for 16000 mm3 (not very efficient, 30 times more than standard machining techniques).

 

· mrr is independent of material hardness.

 

· Good for low machinability, or complicated shapes.

 

· Very little tool wear,

 

· Forces are large with this method because of fluid pumping forces.

 

· Faraday's laws state that,

 

 

· The basic principle is shown below

 

 

· The chemical reaction between an electrode and the electrolyte leads to electrons being added, or removed from the electrode metal. This addition/subtraction leads to a voltage potential.

 

 

· To make a battery.

 

 

 

 

 

· To do electrolysis.

 

 

· The mrr is,

 

 

· e.g.

 

 

· Actual rates may vary from theory as other factors come into effect.

 

 

· The table below shows various materials and relevant properties,

 

 

· e.g.

 

 

· While the current required is related to the metal removed, the voltage required depends upon,

  1. - electrode potential.
  2. - the current flow in and about the electrodes will disturb the normal distribution of voltage. Extra potential is required to overcome the effects.
  3. - Ion collect near electrodes and impede ion transfer from the electrode to the electrolyte, also adding a potential.
  4. - Some solid film forms on the surface of the electrode, also increasing resistance.
  5. - electrolyte resistance,

 

 

 

· The feed of the electrodes has the following effects

 

 

 

 

· The ECM process will erode material in a radial direction, so care must be made in tooling design.

 

 

· As current flows through the electrolyte, it is heated, and conductivity decreases.

 

· Surface finish is affected by,

  1. - selective dissolution
  2. - sporadic breakdown of the anodic film
  3. - flow separation and formation of eddies
  4. - evolution of hydrogen

 

· Typical electrolytes are,

 

 

· Summary of ECM characteristics,

  1. - mechanics of material removal - electrolysis
  2. - medium - conducting electrolyte
  3. - tool material - Cu, brass, steel
  4. - material/tool wear - infinite
  5. - gap 50 to 300 μm
  6. - maximum mrr 15*103 mm3/min
  7. - specific power consumption 7W/mm3/min
  8. - critical parameters - voltage, current, feed rate, electrolyte, electrolyte conductivity
  9. - materials application - all conducting metals and alloys
  10. - shape application - blind complex cavities, curved surfaces, through cutting, large through cavities.
  11. - limitations - high specific energy consumption (about 150 times that required for conventional processes), not applicable with electrically non-conducting materials and jobs with very small dimensions, expensive machines.
  12. - surface finishes down to 25 μin.

 

· This technique has been combined with a metal grinding wheel in a process called Electrolytic drilling. The wheel does not touch the work, and gives a surface finish from 8 to 20 μin.

 

 

59.1 REFERENCES

59.2 PRACTICE PROBLEMS