1. 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,

- electrode potential.

- 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.

- 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,

- selective dissolution

- sporadic breakdown of the anodic film

- flow separation and formation of eddies

- evolution of hydrogen

 

• Typical electrolytes are,

 

 

• Summary of ECM characteristics,

- mechanics of material removal - electrolysis

- medium - conducting electrolyte

- tool material - Cu, brass, steel

- material/tool wear - infinite

- gap 50 to 300 μm

- maximum mrr 15*103 mm3/min

- 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.

 

 

1.1 REFERENCES

1.2 PRACTICE PROBLEMS