Non-conventional machining process
In recent years many new materials have been developed. These include titanium alloys, hast alloys, nimonic alloys etc. these materials cannot be machined accurately with the help of conventional machining process so a different method of machining has to adopted. These new method of machining is called as non-conventional type or unconventional machining process. This type of process generally uses ultrasonic, plasma arcs, laser beam, chemical and electro chemical for machining purposes.
Electrical Discharge machining
In this process metal is removed from the surface of the work piece by erosion. The erosion is caused because of the electric spark produced between the work piece and the tool. This process consists of a tank in which a fixture is placed inside. The work piece is held in the tank with the help of fixture. The tank is filled di-electric fluid such as kerosene, white spirit, mineral oil, paraffin. These di-electric fluids do not conduct electricity.
The work piece is connected to the positive terminal of the battery (usually a D.C supply). The tool is held vertically over the work piece. Graphite, copper, brass or tungsten is generally used for making tools. The tool is hollow. It is connected to the negative terminal of the D.C supply. This makes the tool to act as cathode. A gap in the range of micron is generally kept between the work piece and the tool. In this gap the di-electric fluid is passed through in a very high pressure.
When the power supply is given an electric spark is produced in the gap between the tool and the work piece. Because of the spark heat is generated in the gap. The heat starts makes the surface of the work piece to melt. Thousands of sparks occur per second across the gap. The forces due to the spark tear thee particle of molten metal from the work piece. The tool is connected to the negative terminal to minimize wear on the tool.
Servo mechanism is used to feed the tool and maintain a constant gap between the tool and the work piece. The di-electric fluid also acts like coolant and carries away the eroded metal particle.
· All types of material which can conduct electricity can be machined using this method.
· The mechanical properties of the work piece are not affected by this process.
· Surface finish in levels of micron is possible using this method.
· All types of complicated shape can be made on the tool which can again be obtained on the work piece.
· It is less time operation.
· The hardness of the work piece is not a factor as long as the material conducts current it can be machined.
· No stress is applied on the work piece so thin work pieces can also be machined.
· Very high power consumption.
· Only electrically conductive metals can be machined.
· Perfect square corners cannot be manufactured.
· Redressing of tools is necessary for deep holes.
· Used for die sinking.
· Used to cut off rods.
· Used for making intricate shapes.
· Used for polishing of tools, cutters and broaches.
· Used for accurate drilling of very small holes, slots, etc.
· Profiles and cavities can be formed on hard and brittle like Brittle materials tungsten carbide using this method.