Unconventional or Nontraditional Machining - II

Electrical Discharge Machining (EDM)

EDM is a thermal erosion process whereby material is melted and vaporized from an electrically conducive workpiece immersed in a liquid dielectric with a series of spark discharges between the tool electrode and the workpiece created by a power supply. Positive terminal erodes faster so workpiece is made of anode.

EDM Tool is also erodes due to spark hence it should have low erosion rate or good work to tool wear ratio. Other properties of EDM tool is electrically conductive, high melting point, and have good machinability. The usual choice of tool material are Cu, brass, Tungsten alloy, hardened plain carbon steel, copper graphite, and graphite.

For optimum machining efficiency, this gap between tool and work should be maintained constant. This is done by servo- mechanism which controls the movement of the electrode.

The dielectric fluid

1. acts as an insulator until the potential is sufficiently high,
2. acts as a flushing medium,
3. and provides a cooling medium.

• The voltage across the gab at any time t, $V = V_o (1-e^{-\frac{t}{RC}})$
• Time constant, $\tau = RC$
• Charging time, $t_c = RC ln(\frac{V_o}{V_o - V_d})$
• Discharge Voltage, $V_d = V_o(1-e^{-\frac{t_c}{RC}})$
• Energy Released per spark, $E = \frac{CV_d^2}{2}$
• Average power, $P = \frac{E}{t_c}$

Applications: Widely used in aerospace, mold making, and die casting to produce die cavities, small deep holes, narrow slots, turbine blades, and intricate shapes

Wire EDM

It is a special form of EDM wherein the electrode is a continuously moving conductive wire. A thin wire of brass, tungsten, or copper is used as an electrode. Deionized water is used as the dielectric.

Material removal rate MRR = V*h*b   $mm^3/min$

where, $b = d_w + 2s$                                        

$d_w$ : Wire diameter in mm                              

s: gap between wire and workpiece in mm          

V: feed rate of wire into the workpiece (mm/min)

h : workpiece thickness or height in mm             

Advantages: This process is much faster than EDM. Geometrically accurate but moderately finished straight toothed metallic spur gears, both external and internal type, can be produced by wire type Electro discharge Machining (EDM).

Laser beam Machining (LBM)

Schematic of LBM Device

• Laser beam machines can be used for cutting, surface hardening, welding, drilling, blanking, engraving and trimming.
• Workpiece need not be conductive.
• It is used to drill micro holes. 
• Used to produce cooling holes in blades/vanes for jet engines
• It is costly method and used only when it is not feasible to machine with other processes.

Electron-Beam Machining (EBM)

• The setup consist of electron gun, a high DC power source, electromagnetic focusing lens and deflecting coils.
• It uses a very high velocity beam of electrons.
• Workpiece placed in vacuum chamber to minimize electron collision with air molecules.
• Material melts and vaporizes due to electron beam energy.
• Used for drilling very fine holes, cutting, contours and very nerrow slots.

Plasma Arc Machining (PAM)

• Plasma is a stream of high temperature ionized gas that cuts by melting and removing material from the workpiece.
• Power requirements depend on material being cut, plus depth of cut.
• Cutting operation with plasma is frequently performed by means of CNC (computer numeric control) cutting machines.
• Recast layer is deeper than with other processes.

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