EDM, primarily, exists commercially in the form of die-sinking machines and wire-cutting machines (Wire EDM). The concept of wire EDM is shown in Figure1. In this process, a slowly moving wire travels along a prescribed path and removes material from the work piece. Wire EDM uses electro-thermal mechanisms to cut electrically conductive materials. The material is removed by a sequence of discrete discharges between the wire electrode and the work piece in the presence of dielectric fluid, which creates a path for each discharge as the fluid becomes ionized in the gap. The area where discharge takes place is heated to tremendously high temperature, so that the surface is melted and removed. The removed particles are flushed away by the flowing dielectric fluids.
Fig.1 Schematic representation of Wire cut EDM process
The wires for wire EDM is made of brass, copper, tungsten, molybdenum. Zinc or brass coated wires are also used extensively in this process. The wire used in this process should possess high tensile strength and good electrical conductivity. Wire EDM can also employ to cut cylindrical objects with high precision.
Fig.2 Wire cut EDM machine
Wire cut EDM process is usually used in conjunction with CNC and will only work when a part is to be cut absolutely through. The melting temperature of the parts to be machined is a significant parameter for this process rather than strength or hardness. The surface quality and Material Removal Rate [MRR] of the machined surface by wire EDM will depend on different machining parameters such as applied peak current, and wire materials.
Main Advantages of EDM:
• All electrical conducting materials can be machined.
• High accuracy can be obtained.
Fig.3 EDM wire material
Limitations of EDM
The main limitations of this process are
This process can only be employed in electrically conductive materials;
· Material removal rate is low and the process overall is slow compared to conventional machining processes;
· Unwanted erosion and over cutting of material can occur;
· Rough surface finish when at high rates of material removal.
Dielectric fluids used in EDM process are hydrocarbon oils, kerosene and deionized water. The functions of the dielectric fluid are to:
· Act as an insulator between the tool and the work piece.
· Act as coolant.
· Act as a flushing medium for the removal of the chips.
The electrodes for EDM process usually are made of graphite, brass, copper and copper-tungsten alloys.
Design considerations for EDM process are as follows:
· Deep slots and narrow openings should be avoided.
· The surface smoothness value should not be specified too fine.
Wire cut EDM Process