• 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.
• Forces are large with this method because of fluid pumping forces.
• 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.
• Actual rates may vary from theory as other factors come into effect.
• The table below shows various materials and relevant properties,
• While the current required is related to the metal removed, the voltage required depends upon,
- the current flow in and about the electrodes will disturb the normal distribution of voltage. Extra potential is required to overcome the effects.
- Ion collect near electrodes and impede ion transfer from the electrode to the electrolyte, also adding a potential.
- Some solid film forms on the surface of the electrode, also increasing 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,
- sporadic breakdown of the anodic film
- flow separation and formation of eddies
• Summary of ECM characteristics,
- mechanics of material removal - electrolysis
- medium - conducting electrolyte
- tool material - Cu, brass, steel
- material/tool wear - infinite
- specific power consumption 7W/mm3/min
- critical parameters - voltage, current, feed rate, electrolyte, electrolyte conductivity
- materials application - all conducting metals and alloys
- shape application - blind complex cavities, curved surfaces, through cutting, large through cavities.
- 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.
- 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.