Electrochemical Machining (ECM) Principles

Electrochemical Machining (ECM) is based upon Faraday's law of electrolysis. Faraday's law states that the the mass of a metal altered by the electrode is proportional to the quantity of electrical charges transferred to that electrode.

• In ECM the removal of metal is controlled by the anodic dissolution in the electrolyte. 

In ECM -

•  The workpiece acts as the anode
• The tool act as cathode.
• The electrodes should be placed closely with a gap of about 0.5 mm .
• The anodes and cathodes should be immersed into electrolyte. (Here Sodium Chloride)

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Schematic illustration of the electrochemical-machining (ECM) process

The main principles of ECM are as follows - 

• A potential difference is maintained between the electrodes as a result ions existing in the electrolytes migrate towards the electrodes. 
• Conventionally positively charged ions are attracted towards the cathode and negative ions are attached towards to the anode. And thus flow of current is initiated in the electrolyte. 
• The set-up is kept stationary and tool is fed linearly. 
• The desired amount of metal is removed because of ion migration towards the tool. 
• For keeping the tool safe from damage a continuous supply of electrolyte is ensured by pumping it at high pressure (15kg/cm2). 
• In this process the temperature generated is very low and no spark is produced and thus there isn't any scope of metallurgical changes in the job. 
• When electricity is supplied to the metallic ions of the w/p is pulled out. The positive ions of the metal reacts with the negative ions present on the electrolytic solution and hydroxides of metal and other components. This hydroxides are precipitated and washed away by the electrolytic solutions.  
• In Electrochemical Machining the tool and workpiece doesn't come in direct contact with each other so negligible wear and tear is observed. 
• Metal removal rate is high and voltage supplied is very low. 
• The metallic workpiece is not damaged due to thermal stresses. 
• Dimensions up to 0.05 mm can easily machined . 

Examples of parts made by ECM 

Different parts made by electrochemical machining.  (a) Turbine blade material :  nickel alloy (b) Thin slots on a 4340-steel roller-bearing cage.  (c) Integral airfoils on a compressor disk.

 Electrochemical Grinding  (ECG) Principles  

(a) Schematic illustration of the electrochemical-grinding process.  (b) Thin slot produced on a round
nickel-alloy tube by this process.

        Functions of Electrolyte

•           The current is carried between the tool and the w/p through electrolyte.
•           The produced heat is dissipated by the liquid electrolytic solution.
•           The product of machining is removed by the solution.               
•            It keeps the reactions continuous by supplying the elements necessary for the reaction.  

What Should be The Criteria of Selecting Electrolyte in Electrochemical Machining  (ECM) and Grinding (ECG)

The selection of the electrolyte should be done by considering the following matters - 

·         Required Machining rate 

·         Required Dimensional Accuracy 

·         Surface Texture and Integrity  

            

      The properties of Electrolytes 

•            High Electrical Conductivity 
•            High Current Efficiency for machining 
•            Good Surface finish and integrity is necessary
•            Composition of the electrolyte and structure of the material controls the final surface texture. 

 Flow Arrangement of the Electrolyte in  ECM / ECG 

       Perfect electrolyte flow across the machining tool is mandatory for proper machining.

      cavitation is likely to be occurred in the tool so proper care is necessary to keep the 

      tool in shape. Tool design must ensure the uniform flow of electrolytic solution in all

      the machining areas. Optimum flow of the electrolyte is desired because excessive

      flow can cause erosion of the tool. 
      
     Mainly two types of flows are used –

     1.     Divergent flow

     2.     Convergent flow

     Convergent flow provides a smoother flow of electrolytes. At first the electrolyte has to pass a

     chamber known as ‘dam’. The dam is used to pressurize the are outside  the working tool. 

   Advantages of convergent flow system.  

  

•         Improved Surface finish 
•         Improved Uniform and predictable side over cut as well as front machining gap. 
•         Less prone to arcing. 
•         Clean operating environment 
•         Stray currents make it possible to eliminated unwanted machining.  

But it is also to be mentioned that machining in convergent flow is much expensive than divergent flow. 

Advantages of Electrochemical Machining (ECM)

1. Accurate Machining 
2. No direct contact between tool and job. 
3. Negligible wear and tear of the tool. 
4. Environment friendly 
5. no thermal or mechanical stress is induced on the tool.
6. There is no contact between worpiece and the tool so its is possible to machine non-rigid and open w/p. 
7. Jobs with complex geometric shapes can be machined with ease accurately and repeatedly. 
8. ECM is a time saver when compared to conventional machining. 
9. During drilling several holes can be done at once.  
10. Deburring can be done in hard to access areas. 
11. Fragile and brittle materials which are prone to damage can be machined easily in ECM without cracking or breaking. 
12. Surface finish up to 25 μ in can be achieved. 

Disadvantages of Electrochemical Machining (ECM)

1. Sometimes this process is costly because the equipments are expensive. 
2. Continuous supply of electrolytic solution is mandatory. 
3. Steady voltage or potential difference should be maintained. 
4. Rigid fixturing is required to withstood the high flow rate of electrolytes. 
5. Designing the tool is arduous because it must be insulated to maintain the perfect conducting paths towards the workpiece. 
6. Corrosion free material is needed for the structure and the electrolyte handling unit. 
7. If hydrogen is liberated at the tool surface then it is possible to suffer from hydrogen-embitterment of the surface. 
8. There is possibility of damages because of sparks. 
9. Conventional machining techniques produce more improved fatigue properties than ECM.  

Applications of Electrochemical Machining ECM 

• ECM is mainly used in the areas where conventional machinig techniques are not feasible . One of the main applications of ECM is found in the aerospace industries where accuracy is very important when complex shaped difficult to machine materials are needed to be machined. 
• Different Industrial techniques have been developed on the basis of Electrochemical Machining Such as 

                a. Electrochemical Cutting 

                b. Electrochemica ECM 

                c. Electrochemica broaching 

                d. Electrochemica drilling 

                e. Electrochemica deburring 

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