Electrochemical Machining – Parts, Construction, Working, Advantages, Disadvantages and Applications

Electrochemical machining is a non-traditional machining process. In the electrochemical machining process, a combination of electrical energy and chemical energy is used to remove material from the workpiece.
This process is characterized as a reverse electroplating process but this process removes material instead of depositing it.

This process is a non-contact process in which the cavity obtained is the replica of the tool shape.
The ECM process is not affected by the strength, hardness, or toughness of the workpiece material. Unlike conventional machining processes, we have to select tool material which is harder than the workpiece material.
Complex cavities can also be machined in high strength material using the ECM process.

This process work on the basis of Faraday’s Law of Electrolysis.
The mechanism of the ECM process is very similar to that of the EDM process. In both cases, the tool is used as a cathode and the workpiece is used as the anode. The difference between two is that a conductive electrolyte is used in ECM instead of the dielectric which is used in EDM.
The electrolyte is responsible for the creation of the plasma which ultimately helps in evaporating the material from the workpiece. The electrolyte is a conductor of electricity.

Equipment used in ECM Process:

DC Power Supply:
The DC power supply used in this process is from 3 to 30V. The DC Power Supply is used to create a potential difference between the workpiece and the tool and make them act as anode and cathode respectively.

Table:
The table is used to hold the workpiece on top of it.

Work Piece:
This is the object from which material is to be removed. The workpiece is connected to the positive terminal of the DC power supply and acts as an anode.

Tool:
This tool is connected to the negative terminal and acts as a cathode.
The tool (cathode) should have good thermal conductivity and electrical conductivity and also should be corrosion proof. The most commonly used tools in the ECM process are stainless steel, brass, copper, etc.

Feed Unit:
It is used to constantly move the tool downward towards the workpiece.

Electrolyte:
The electrolyte is used to establish an electrical connection between the anode and the cathode so that positive and negative ions can flow from these two.
The electrolyte is also used to remove the heat produced from the passage of the current.
The most commonly used electrolyte used in the ECM process are salt electrolytes like sodium chloride and sodium nitrate.

Pump:
This pump is used to pump the electrolyte towards the gap between cathode and anode.

Filter:
The filter is used to filter the electrolyte and remove the unwanted particles from the electrolyte.

Pressure Gauge:
The pressure gauge is used to control the pressure at which the electrolyte is supplied to the system of electrochemical machining.

Flow Meter:
Flow Meter is used to regulate the flow of electrolyte which is supplied to the system.

Construction:

The basic setup of the ECM process is like this,
The workpiece is always dipped in a fluid which is called an electrolyte.
This electrolyte always flows through the gap between the tool and the workpiece.
The gap between the workpiece and the tool through which the electrolyte pass is known as the inter-electrode gap. Very high current with low voltage is passed through this gap.
In this process, the workpiece is connected to the positive terminal and works as anode and the tool is connected to the negative terminal and works as a cathode.
As very high current is used, the tool and the workpiece are isolated from the system to prevent any short-circuiting or leakage of the current.
The electrolyte is filtered and made to flow through the workpiece continuously.
The pressure at which the electrolyte is pumped depends on the application or requirement of the process.

The electrolyte which can be used in this process are:
1) Sodium Chloride 2) Sodium Nitrate 3) Sodium Hydroxide. 4) Sodium Sulphate.
Any electrolyte used should have the capability to establish electrical contact between the cathode and anode.
The gap which is maintained between the tool and the workpiece is nearly 0.5 mm.

Working:

The electrolyte is supplied to the system of anode and cathode i.e workpiece and tool using the pump. In between the pump and the system a filter is placed which is used to filter the electrolyte before reaching the system.
After passing through the filter and flow regulator, the electrolyte reaches the space between the workpiece and the tool.
As the electrolyte reaches the gap between anode and cathode, electrical contact is established between the anode and the cathode. After the electrical contact is established, the positive ions start flowing from the workpiece towards the tool and negative ions start flowing from the tool towards the workpiece.
When the positive ions flow from the workpiece towards the tool, the electrolyte carries away the positive ions with it and stops it from reaching the tool.
So the materials start removing from the workpiece as positive ions from the workpiece are dissolve and carried away by electrolyte and materials are also removed due to the striking power of the negative ions coming from the tool.
This material removal process from the workpiece is continued until a cavity is created in the workpiece which is a replica of the tool used in this process.
So, in this way material is removed in the electrochemical machining process.

Let us try to understand with the help of an example:
Let us take an example of the machining of low carbon steel. For the machining of any ferrous alloy generally, the electrolyte taken is the neutral salt solution of sodium chloride.
When the potential is applied the neutral salt (NaCl) and water (H₂O) undergoes ionic dissociation as shown below:

NaCl ↔ Na⁺ + Cl^–
H₂O ↔ H⁺ + (OH)^–

After the potential difference is applied, the positive ions move towards the tool (cathode) and negative ions move towards the workpiece (anode).

The hydrogen ions will combine with electrons coming from the cathode (tool) and form hydrogen gas as:
2 H⁺ + 2 e^– = H₂ ↑
Also, iron atoms will come out of anode in form positive ions as:
Fe = Fe⁺⁺ + 2e^–

When the iron ions mix with electrolyte, it will combine with chloride ions to form iron chloride, and also sodium ions will combine with hydroxyl ions to form sodium hydroxide.

Some iron ions combine with hydroxyl ions to form iron hydroxide.

So the iron ions coming from the workpiece ( anode ) will combine with chloride ions and hydroxyl ions to form FeCl₂ and Fe(OH)₂ respectively. The FeCl₂ and Fe(OH)₂ will get precipitated in the form of sludge and this sludge will be carried away with the electrolyte.
Also, there is no coating formed on the tool, only hydrogen gas evolves at the tool.

Advantages:

1) It can cut any materials irrespective of their hardness, strength or toughness.
2) Complex cavities can be produced using the ECM process.
3) Tool wear is zero as there is no direct contact between the tool and the workpiece.
4) A very good surface finish is produced in this process as the material removal takes place due to atomic level dissociation
5) Very less heat is generated during this process.
6) No residual stresses as there is no direct contact between the tool and the workpiece.
7) There is no thermal damage to the workpiece.

Disadvantages:

1) Due to the use of saline or acidic electrolyte, there is a chance of corrosion of the tool or the workpiece or any other equipment.
2) It cannot be used for machining of soft materials and non-metals.
3) Using this process, only those materials can be machined which are conductors of electricity.

Applications:

1) It is used in Die Sinking Operations.
2) It is also used in drilling.
3) Used for trepanning.
4) Profile and contouring.
5) Micro Machining.
6) Multiple hole drilling.