21 February 2017

What is electric discharge machining

Electric discharge machining also is known as spark erosion.
It is the process of material removal based on the principle of metals by an interrupted electric spark discharge between the electrode tool and workpiece.

Working principle :

The main components of the EDM process are : 
  • Electric power supply
  • The dielectric medium 
  • The workpiece and tool
  • A servomotor

Electric discharge machining

The basic process of EDM is really quite simple. The workpiece and the tool are electrically connected to D.C electric power supply. The workpiece is connected to the positive terminal of the electric source so that becomes the anode. The tool is a cathode. A gap is known as the 'spark-gap' in the range of 0.005 to 0.05 mm is maintained between the work piece and the tool.
In the spark gap produced spark is visible evidence of the flow of electricity. This electric spark produces intense heat with temperatures reaching 8000 to 12000 degree Celsius. The spark is controlled by very carefully and localized so that it affects the surface of the material only. 
The EDM process can be used in two different ways: 
1. A preshaped or formed electrode used as tool usually made from graphite is shaped to the form of the cavity it is to reproduce. The formed electrode is fed vertically down and the reverse shape of the electrode is eroded (burned) into the solid workpiece. 
2. A continuous-travelling vertical-wire electrode used as a tool and its diameter of a small needle or less is controlled by the computer. It follows a programmed path to erode the workpiece or cut a narrow slot to produce the required shape. 

The electrode ( Tools ) :

The shape of the tool will be basically the same as that of the product desired except that an allowance is made for side clearance and overcut.
The electrode materials generally used can be classified as metallic materials, non-metallic materials and a combination of metallic and non-metallic materials. Usually, copper, yellow brass, zinc and graphite are used for tools. Some low wearing tools are also used like silver-tungsten, copper-tungsten and metallized graphite. For commercial applications copper is best suited for fine machining. Aluminium is used for die-sinking while cast iron for rough machining.
One of the advantages of EDM is due to the fact that a tool made of a material softer than the workpiece material and which is a good conductor of electricity can be used to machine material of any hardness.
The wear of the tool in the EDM process due to electron bombardment is inevitable. 
The tool wears rates to determine the machining accuracy, tool movement, and tool consumption. 
The tool wear is the function of the rate of metal removal, the material of the workpiece, current setting, machining area, gap between the tool and workpiece and the polarity of the tool.

Wear ratio = Volume of work material removed / Volume of electrode consumed

Wear ratio = Depth of cut / Decrease in usable of the electrode

The wear ratio for carbon electrodes is up to 100:1.
For copper 2:1 
For brass 1:1
For copper tungsten 8:1

Purpose of Dielectric fluids :
  • It used as a coolant for the workpiece and the tool. 
  • It works as an insulating material during the charging of the condenser, as a result, perfect condition for efficacious spark discharge and its conduction when ionized is obtained. 
  • The eroded materials are carried away by this medium. 
  • It is a coolant in quenching the spark and prevents the arcing. 

Requirements of Dielectric fluids :
  • Remain electrically nonconducting until the required break-down voltage has been reached.
  • Breakdown electrically in the shortest possible time once the breakdown voltage has been reached.
  • Have a good degree of fluidity.
  • Be cheap and easily available.
  • Be capable of carrying away the swarf particles.
  • Inflammable. 
  • It should be a hydrocarbon compound. 
  • It should not produce toxic gases or vapours during the operation. 

Accuracy :
Tolerance value ( +- 0.05 ) mm could be easily achieved by EDM normal production.
By close control of the several variables tolerance ( +-0.003 ), mm could also be achieved.
Taper value id about to 0.005 to 0.05 mm per 100 mm depth.
An overcut of 5 to 100 micron is produced.
In no wear machining using graphite electrode a surface finish within 3.2 microns can be achieved.

Application of EDM :
The EDM provides economic advantages for making stamping tools, wire drawing and extrusion dies, header dies, intricate mould cavities.
It extremely used in aerospace industries, refractory metals, hard carbides and hardenable steels.
Some of its applications are following below :
  • Drilling of micro-holes.
  • Thread cutting.
  • Helical profile milling.
  • Wire-cutting EDM.
  • Rotary forming.
  • Curved hole drilling.
  • Vacuum tubes.