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.
The workpiece surface is carefully cleaned.
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.
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.
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.
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.