Electrical Discharge Machine: Principle,Component

Introduction

• Sometimes it is called spark machining, spark eroding, burning, die sinking or wire erosion
• In there a desired shape is obtained using electrical discharges (sparks).
• Material is removed from the work-piece by a series of  current discharges between two electrodes, separated by a dielectric liquid and subject to an electric voltage.
• One  electrodes – ‘tool-electrode’ or ‘tool’ or ‘electrode’.
• Other electrode - work-piece-electrode or ‘work-piece’.

General Aspects Of EDM

EDM is a machining a hard metals or those that would be very difficult to machine with traditional techniques. 

• EDM  works with materials that are electrically conductive, although methods for machining insulating ceramics with EDM have been proposed.
• EDM can cut  contours or cavities in hardened steel without the need for heat treatment to soften and re-harden them.
• Same method can be used with any other metal or metal alloy such as titanium, hastelloy, kovar, and inconel.
• Applications of this process to shape poly-crystalline diamond tools have been reported

EDM-System

EDM-Component

1.         Electric current
2.         Dielectric medium
3.         Work-piece and tool
4.         Servo moter

• Work-piece & tool are electrically connected to a DC power supply.
• The density of current in the discharge of the channel is of the order of 10000 A/cm2 and power density is nearly 500 MW/cm2.
• The range of spark gap is from 0.005 mm to 0.05 mm is maintained between the work piece and the tool.

Working Principle & How its Work

• Metal removal based on the principle of material removal by an interrupted electric spark discharge between the electrode tool and the work piece.

• Tool and work material are be conductors.
• Tool and work material are present  in a dielectric medium.
• Kerosene or deionised water is used as the dielectric medium. 
• Gap is establish between the tool and the work-piece.
• Gap between the tool and work-piece, an electric field would be established.
• Mostly tool is connected to the negative terminal (cathode) of the generator and the work-piece is connected to positive terminal (anode).
• When electric field is established between the tool and the job, the free electrons on the tool are subjected to electrostatic forces.
• When bonding energy of the electrons is less, electrons would be emitted from the tool.
• Emission of electrons are called or termed as ‘cold emission’.
• “cold emitted” electrons are then accelerated towards the job through the dielectric medium.
• Electron gain velocity and energy, and start moving towards the job, there would be collisions between the electrons and dielectric molecules.
• Electron will be collision and may result in ionization of the dielectric molecule.

• When electrons get accelerated, more positive ions and electrons would get generated due to collisions.
• This  process will increase the concentration of electrons and ions in the dielectric medium between the tool and the job at the spark gap. 
• Concentration would be so high that the matter existing in that channel could be characterise as “plasma”.

• A large number of electrons will flow from tool to job and ions from job to tool.
• This is called avalanche motion of electrons.
• The movement of electrons and ions can be visually seen as a spark.
• The electrical energy is dissolute as the thermal energy of the spark.
• And the high speed electrons then impinge on the job and ions on the tool.
• The Kinetic energy of the electrons and ions on impact with the surface of the job and tool respectively would be converted into thermal energy or heat flux.
• Temperature which would be in excess of 10,000oC.
• Rise in temperature leads to material removal.

EDM-Working Principle

• Withdrawal of potential difference, plasma channel collapses.
• This  creates compression shock waves on both the electrode surface.
• At high spots on work piece surface, which are closest to the tool.
• This molten material and forms a crater around the site of the spark.

EDM - Schematic

EDM-Working Principle

EDM-Electrode Material

• When Electrode material is impinged by positive ions then it would not undergo much tool wear.
• The tool should be easily workable.

Characteristics Of Electrode Material

• Higher density 
• High melting point 
• Easy manufacturability
• Cost – cheap
• High electrical conductivity 
• High thermal conductivity 

Used Commonly In The Industry

The different electrode materials which are used commonly in the industry:

• Graphite
• Electrolytic oxygen free copper
• Tellurium copper – 99% Cu + 0.5% tellurium
• Brass

Function Of Dielectric

There are three important functions of a dielectric medium.

1. The gap will be Insulates  between the tool and work.
2. Cools the electrode.
3. Flush metal particles out of the working gap.

EDM-Flushing

• Flushing is a process of introducing clean filtered dielectric fluid into spark gap
• One of the important factors  is the removal of debris (chips) from the working gap.
• Flushing the work material  particles out of the working gap is very important, to prevent them from forming bridges that cause short circuits.
• If flushing is not applied properly, it can result in erratic cutting and poor machining conditions.
• Flushing of dielectric plays a major role in the maintenance of stable machining. 

Four methods:

Normal flow (Majority)

• Dielectric is under pressure, through one or more passages in the tool and is forced to flow through the gap between tool and work.
• Flushing generally placed in those areas where the cuts are deepest.
• Sometimes normal flow is undesirable because it produces a tapered opening in the work-piece.

Reverse flow

• It is useful in machining deep cavity dies, where the taper produced using the normal flow mode can be reduced.

• Fluid flowing between the work-piece and the tool, there is no side sparking and, therefore, no taper is produced.

Jet flushing

• Machining of desire work-piece can be achieved by using a spray or jet of fluid directed against the machining gap.
• The time of machining  is always longer with jet flushing than with the normal and reverse flow modes.

Immersion flushing

• Simple immersion of the discharge gap is sufficient.
• Debris removal can be enhanced during immersion cutting by providing relative motion between the tool and work-piece.
• Cycle interruption comprises periodic reciprocation of the tool relative to the work-piece to effect a pumping action of the dielectric. 

Application-EDM Drilling

 Uses of tool electrode where the dielectric is flushed.

• Whenever solid rods are used; dielectric is fed to the machining zone by either suction or injection through pre-drilled holes.
• Tapered, curved can be produced by EDM.
• Creating cooling channels in turbine blades made of hard alloys is a typical application of EDM drilling.

Application-Wdm Sawing

• Tool cuts at a rate that is twice that of the conventional abrasive sawing method.
• Cutting of billets and bars - has a smaller kerf & free from burrs.
• Fine finish of 6.3 to 10 μm with a recast layer of 0.025 to 0.130 mm
  

Application-Machining Of Spheres

• Coworkers (1995) used simple tubular electrodes in EDM machining of spheres, to a dimensional accuracy of ±1 μm and Ra < 0.1 μm.
• Spherical shapes in conducting ceramics using the tool and workpiece arrangement as shown below, can be machining by rotary EDM. 
  

Application-Machining Of Dies

• EDM milling uses standard cylindrical electrodes.
• Simple-shaped electrode (Fig. 1) is rotated at high speeds and follows specified paths in the workpiece like the conventional end mills.
• Makes EDM very versatile like mechanical milling process.
• Complex-shaped electrodes for die sinking (Fig. 2) of three-dimensional cavities.
  
• Dielectric flushing due to high-speed electrode rotation.
• Electrode wear can be optimized due to contouring motions.
• Main limitation in EDM milling - Complex shapes with sharp corners cannot be machined because of the rotating tool electrode.
• EDM milling replaces conventional die making that requires variety of machines such as milling, wire cutting, and EDM die sinking machines.

Application-Wire EDM

• Uses a continuously moving conductive wire electrode.
• Spark erosion as the wire electrode is fed, from a fresh wire spool, through the work-piece causes to material removel.
• Horizontal movement of the worktable (CNC) determines the path of the cut.
• During the machining of superhard materials like polycrystalline diamond (PCD) and cubic boron nitride (CBN) blanks, and other composites.

• In (1995) used wire EDM for accurately shaping these materials, without distortion or burrs

Application-EDM Of Insulators

• A metal sheet is placed over the ceramic material.
• Spark discharges will occur between the negative tool electrode and the metal mesh.
• So sparks are transmitted through the metal mesh to its interface with the ceramic surface, which is then eroded.