Unconventional or Nontraditional Machining - III

Ultrasonic Machining (USM)

• In ultrasonic machining, a tool which is negative of desired shape, vibrates at low amplitude (0.013 to 0.08 mm) and ultrasonic frequency (about 20 kHz) in an abrasive grit slurry at the workpiece surface.

• The tool is gradually moved down maintaining a constant gap between the tool and workpiece surface.

• As the tool vibrates over the workpiece, the abrasive particles act as the indenters and indent both the work material and the tool. The abrasive particles, as they indent, the work material, would remove the same, particularly if the work material is brittle, due to crack initiation, propagation and brittle fracture of the material. 

At full indentation, the indentation depth 'h' and assuming brittle fracture takes place leading to hemi-spherical fracture of diameter ‘D’ under the contact zone.

Volume removed by the single grain is

\[V = \frac{2 \pi}{3}(\frac{D}{2})^3\]

\[D \approx \sqrt{dh}\]

\[V = \frac{2 \pi}{3}(dh)^{3/2}\]

 If number of particles impacting per cycle is n, frequency of operation f and efficiency is η then Material Removal Rate can be expressed as:

\[MRR = η V Zf = η \frac{2 \pi}{3}(dh)^{3/2} nf\]

Applications: USM is best suited for hard, brittle materials, such as ceramics, carbides, glass, precious stones, and hardened steels.

Water Jet Machining (WJM)

WJM works by forcing a large volume of water through a small orifice in the nozzle, at high pressure and velocity against work surface. 

This jet of water erodes the surface of workpiece. 

Applications: Mostly used to cut lower strength materials such as wood, plastics, rubber, paper, leather, composite, etc. 

>Good for materials that cannot withstand high temperatures.

Abrasive Water-Jet Machining (AWJM)

The water jet also contains abrasive particles such as SiC, hence material removal rate is higher than WJM.

Abrasive Jet Machining (AJM)

A high-velocity jet of gas containing abrasive particles is directed at the workpiece surface under controlled conditions. It removes material through the eroding action of a high velocity stream of abrasive-laden gas. Abrasive particles are generally of Al2O3, SiC with particle size 10 to 50 µm. The gas supply pressure is in order of 7 atm and the jet velocity about 300 m/s. It is used to cut materials which are hard to cut, e.g., composites, ceramics, glass.

General Observations

• ECM has the highest material removal rate (MRR).
• EDM has the lowest specific power requirement.
• USM and AJM have low MRR and combined with high tool wear, are used for non-metal cutting.
• LBM and EBM have high penetration rates with low MRR and, therefore, are commonly used for micro drilling, sheet cutting, and welding. 

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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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Unconventional or Nontraditional Machining

These are known as non-traditional/unconventional method because compared to conventional method there is no contact between tool and workpiece, specific power consumption is very large, MRR is low and used in situations where traditional/conventional machining processes are unsatisfactory or uneconomical:

• Workpiece is too hard, ot tough.
• Workpiece is too flexible to resist cutting forces or too difficult to clamp
• Part shape is very complex with internal or external profiles or small holes
• Requirements for surface finish and tolerances are very high

Classification Based on Energy Source

1. Electro-Chemical Processes

• Electro-Chemical Machining (ECM) 
• Electro-Chemical grinding (ECG) 
• Electro-Chemical Honing (ECH) 
• Electro-Chemical Deburring (ECD)

2. Chemical Processes

• Chemical Machining Method (CHM)
• Photochemical Machining (PCM)

3. Electro-Thermal Processes

• Electrical discharge machining (EDM)
• Laser beam Machining (LBM) 
• Plasma Arc Machining (PAM) 
• Electron Beam Machining(EBM) 
• Ion Beam Machining (IBM)

4. Mechanical Processes

• Ultra Sonic Machining (USM) 
• Abrasive Jet Machining (AJM) 
• Water Jet Machining (WJM)

Electro-Chemical Machining (ECM)

The work-piece is made the anode, which is placed in close proximity to an electrode (cathode), and a high-amperage direct current is passed between them through an electrolyte, such as salt water, flowing in the anode-cathode gap.

The tool is fed with constant velocity towards the workpiece and the electrolyte pumped at high pressure through the small gap between the tool and work.

The mechanism of material removal is anodic dissolution.

The electrolyte is so chosen that the anode (workpiece) is dissolved but no deposition takes place on the tool.

Properties of electrolyte:

• High electrical and thermal conductivity 
• Low viscosity
• High specific heat
• Non corrosive and notoxic
• Chemically stable

Material Removal Rate is given by Faraday's law:

\[\boxed{MRR = \frac{AI}{\rho Z F} = \frac{EI}{\rho F}} cm^3/s\]

where, A : gram atomic weight of workpiece (anode)

E = A/Z : equivalence weight

Z : Valency

I : Current (amp.)

$\rho$ : density of workpiece ($g/cm^3$)

F: Faraday's constant = 96500 columbs

Electro-Chemical grinding (ECG)

• The tool as electrode is rotating, metal bonded, diamond grit grinding wheel and workpiece as anode.
• As the current flows the surface metal is changed to oxide film and this oxide film is removed.
• The abrasive particles are non-conductive material such as aluminum oxide, diamond and borazon (BCN).  

Applications:

• Shaping and sharpening carbide cutting tool
• Fragile parts (honeycomb structures), surgical needles, and tips of assembled turbine blades have been ECG-processed successfully.

Chemical Machining Method (CHM)

• Chemical machining is basically an etching process, it is the oldest nontraditional machining process.
• Material is removed from by chemical dissolution using chemical reagents, or etchants, such as acids and alkaline solutions.
• The workpiece is immersed in a bath containing an etchant. Special coating called maskant protects area from which area is not to be removed.
• Cutting speed (0.0025-0.1 mm/min) is very slow.

Photochemical Machining (PCM)

• This process is also known as photochemical milling or photo etching, is a photo chemical blanking.
• Coat both sides of the plate with photoresist which is a polymer that adheres to the metal when exposed to UV light. 
• Spray metal with etchant or dip it in hot acidic solution to etch all material other than part covered with photoresist.
• This process is burr free and high precision.

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