Category Archives: Unconventional Machining Process

Chemical Machining Process | Electro Chemical Machining Process

CHEMICAL MACHINING

In chemical machining, some chemicals are used to remove material from the required portions of the workpiece. This process is also called chemical milling. Chemical milling is done in the following steps.

01 - CHEMICAL MACHINING PROCESS - UNCONVENTIONAL MACHINING PROCESS

Cleaning

The workpiece surface is carefully cleaned.

Masking

The portions of the workpiece which do not require machiningis covered with masking sheets. The sheet is cut and taken from the area where machining is required. Templates are used for this purpose. If the entire area of the workpiece is to be machined, masking is not necessary. Usually vinyl, neoprene and rubber based materials are used as mask sheets.

Etching

After masking, the workpiece is submerged in a hot chemical solution. This solution is called the etchant. For Aluminium Caustic soda is used as etchant. Acids are used for steel, magnesium and titanium alloys. The etchant removes the metal from workpiece by chemical action. The rate of metal removal is about 0.025mm per minute. The rate of metal removal depends upon the concentration and the temperature of the etchant. For higher concentration and higher temperature, the rate of metal removal is more. The amount of metal removal also depends upon the time duration for which the workpiece is immersed in the etchant.

Demasking

After etching, the workpiece is taken out from the etchant. The workpiece is cleaned with the help of water. Then the masking is removed.

ELECTRO CHEMICAL MACHINING (ECM)

Electro chemical machining process is the reverse of electro plating process. The workpiece becomes anode) and the tool becomes cathode. Therefore the workpiece looses metal. Normally, the metal will flow through the electrolyte and get deposited on the cathode. Here the tool is the cathode. Therefore the metal gets deposited on the cathode. The dissolved metal is forced away with the electrolyte.

01 - ELECTRO CHEMICAL MACHINING PROCESS - UNCONVENTIONAL MACHINING PROCESS.

The workpiece is held in a suitable fixture inside a tank. The workpiece is connected to the +ve terminal (anode) of a 20 V D.C. supply. The tool is held in position over the workpiece. The tool is hollow one. It is connected to the –ve terminal (cathode) of the supply.

The shape of the tool depends on the shape to be produced on the workpiece. A small gap of about 0.2mm is maintained between the workpiece and the tool. The sides of the tool are insulated. So the sides of the tool will not machine the workpiece, this avoids taper in the hole machined.

An electrolyte, usually sodium chloride, sodium nitrate or sodium chlorate is passed through the hollow tool.

When the D.C. supply is given, the current flows through the circuit. Electrons are removed from the surface of the workpiece (anode). These ions will attempt to reach the cutting tool (cathode). But these ions are carried away by the fast flowing electrolyte. The tool is fed towards the workpiece the workpiece automatically to maintain the gap between the workpiece and tool surface. The machining rate and surface finish are directly proportional to the current. The electrolyte is filtered and recirculated using a pump. The temperature of the electrolyte is maintained between 25 to 60°C.

Electro Chemical Grinding | Non-Traditional Machining Process

Electro Chemical Grinding (ECG)

Electro chemical grinding is also called electrolytic grinding. Metal is removed from the surface of the work piece by electro chemical action and also by abrasive action of a grinding wheel. 90% of metal is removed by electro chemical action and 10% of metal is removed by the abrasive action of the grinding wheel.

01 - WORKING OF ECG - UNCONVENTIONAL MACHINING PROCESS

The equipment has a metal bonded grinding wheel. Brass, bronze and copper are bonded with abrasive grains in the grinding wheel. Diamond abrasive is used for grinding tungsten. Aluminium oxide abrasive is used for other metals.

The wheel is held in a horizontal spindle. The spindle is supported on insulated bearings. The work piece is held in a fixture against the grinding wheel. A gap of about 0.01mm is maintained between the wheel and the surface of the work piece. The work piece is connected to the positive terminal of a D.C. supply. The grinding wheel is connected to the negative terminal. 4 to 16V, 300 to 1000 Amps D.C supply is applied. A mixture of sodium chlorite, sodium chlorate or sodium nitrate and water is used as the electrolyte. The electrolytic solution is made to flow between the work piece and the grinding wheel. Electro chemical action takes place. Metal from surface of the work piece is removed in small particles. In addition to this, the rotating grinding wheel also removes metal from the work surface by abrasion. The small particles of metal removed from the work piece are carried away by the electrolyte. The electrolyte is collected in a reservoir. It is filtered and recirculated by a pump. Electrolyte also acts as coolant. The work piece is slowly fed towards the grinding wheel maintaining a constant gap between the work piece and the grinding wheel.

01 - CONSTRUCTION OF ECG - NON TRADITIONAL MACHINING PROCESS

Applications:

· Used for machining hard materials which are conductive to electricity

· Used for grinding of tungsten carbide tool tips and hard steels.

·  Used to grind thin section.

· Cylindrical grinding, form grinding, plunge grinding and surface grinding operations are done using this process.

· Used for machining refractory materials, high strength steels, nickel and cobalt base alloys etc.,

Advantages:

· Very fine finish is obtained

· Suitable for machining very hard materials like carbides. Carbides are difficult to machine by other processes.

· No heat is generated during the process.

· No distortion to the work piece

· No burrs are produced.

· Fast operation

· Thin materials can be ground without deflection as the grinding wheel does not press the work piece.

· Wheel wear is drastically reduced.

· No heat is generated so there is no danger of burning or heat distortion.

Disadvantages:

· This process can be used to machine only metals which are conductive.

· Sharp corners of the work piece cannot be machined.

· Electrolytic solution is corrosive.

· Initial cost of the equipment is high when equipped with larger power supplies.

· Intricate shapes may not be formed.