UNCONVENTIONAL MACHINING PROCESSESWhat is unconventional machining process?

Unconventional machining process is machining process where the conventional methods of cutting, machining, machine tools & cutting tools are not used. Here plastic deformation may not be the only reason for material removal. Metal or non-metals are removed from the work piece by several other ways like melting and vaporizing, chemical reaction, erosion etc.

Why unconventional machining processes are required?

Unconventional machining process are required to overcome 5 major constraints of conventional machining process-

1. Conventional machining processes are difficult to use for low machinability material like stainless steel, tungsten, titanium, diamond etc.

2. Very complex geometries are difficult to achieve through conventional machining process.3. For the requirement of very high dimensional accuracy conventional machining processes

are not perfect.4. Conventional machining processes have low production rate & thus they are not much

economical.5. For machining of very low dimensions like micro-holes, conventional machining processes

are not suitable at all.

CLASSIFICATION

Energy type Mechanics of material removal

Energy source Process

Mechanical Plastic deformation

(conventional)

Mechanical motion between work & tool

Conventional machining process

Erosion Fluid motion Water Jet Machining (WJM)

Mechanical & Fluid motion Abrasive Jet Machining (AJM)

Mechanical motion Ultra Sonic Machining (USM)

Electrochemical Ion displacement Electric current Electro Chemical Machining (ECM)

Electrochemical & Mechanical

Ion displacement & plastic deformation

Electric current & Mechanical motion

Electro Chemical Grinding (ECG)

Chemical Corrosive action Corrosive agents Chemical Machining (CHM)

Thermal Fusion & vaporization Electric spark Electro-Discharge Machining (EDM)

High velocity electron Electron Beam Machining (EBM)

Powerful radiation Laser Beam Machining (LBM)

Ionized material Ionized Beam Machining (IBM)

Plasma ions Plasma Arc Machining (PAM)

ELECTRON BEAM MACHININGEBM is an unconventional thermal material removal process in which a high velocity (nearly 30% to

75% of light) focused (typically 25 as diameter) stream of electrons in high vacuum (about 10-4

torr) is made to impinge on work piece surface with a power density in excess of 108 W/cm2 (more than sufficient to instantly melt any material), on impact the kinetic energy of electron is converted into thermal energy with an efficiency of nearly 100%. And thus produces intense heating at the point of bombardment. As a result the work piece is melted, vaporized and thus removed leaving a machined surface. It can melt any known material on earth.

The operational summary of the process is listed below:

Accelerating voltage 50-200 V

Beam current 100-1000 A

Power 0.5-50 kW

Pulse 4-65 ms

Pulse frequency 0.12-16000 Hz

Vacuum pressure 10-2 -10-4 mm of Hg

Beam spot size 12-15 µm

Beam deflection range 6.5 mm2

Beam intensity 1.55x105-1.55x109 W/cm2

Depth of cut 6.5 mm (up to) (applicable for thin sheets)

Taper on hole 1”-2” (typically)

Mrr 40 mm3/s

Penetration rate 0.25 mm/s

Perforation rate 5000 holes/s (typically)

Tolerance ± (0-5) mm (typically)

Recast layer & HAZ seldom exceeds

0.025 mm

Materials Stainless steel, nickel & cobalt alloys, Cu, Al, Ti, ceramics, leather and plastics

MECHANICS OF EBM

1. Effect of Electric Field

a) ElectronAn electron is an elementary charged particle carrying a –ve charge, whose-

charge, e= 1.602 x 10-19 C

mass, me=9.109 x 10-31 kg

radius, r=2.82 x 10-15 m

So, charge to mass ratio= =1.759 x 1011 C/kg

b) Electrons in Free StateAn electron the smallest elementary particle of a matter can readily be obtained in free state that is broken free from the surface of a metal by applying a sufficient energy. Most often the source of free electron in such cases is a piece of metal called a thermo-ionic cathode. The cathode is raised to a temperature at which electrons pick up enough speed to escape in surroundings. As a result what is known as emission of electron takes place. The magnitude of thermo-ionic current depends on the temperature of the cathode, the work function of the cathode and the properties of the cathode surface by the so called Richardson-Dashman equation-

Je = Emission current density in A/cm2

A = Emission constant depending on the emitting substance & up to 40-70 A/cm2K2 for most pure metals

T = Absolute temperature of the emitter in K

eln = Base of natural logarithm

= work function of the emitter metal in Joule (min amount of work done to release

one electron)

K= Boltzmann constant = 1.38x10-23 J/K

It is seen from the above equation the magnitude of the thermal current is mostly dependent on the temperature of the emitter. However an excessive increase in temperature accelerates the evaporation of the metal and cuts down the life of the emitter.

c) Electron in MotionEffect of Electric Field: free electron can be set in motion by electric or magnetic field. Since electrons have lowest mass of all known elementary charged particles they may be imparted huge accelerations. Electrons be placed in a uniform electric field of intensity E, setup between two parallel plates of a sufficient large area, the force acting on the electron will be equal to the product of the electron charge(e) by the field intensity(F) at the location of the charge, given by

The –ve sign indicates that the force acts in direction opposite to the field intensity vector because the electron carries –ve charge. The work expended by the electric field to move a charge from one point to another is equal to the product of the charge and the potential difference between the two point s and given by

where V is the potential difference between the two points 1 & 4 as shown in volts. This work is expended to impart the electron an amount of kinetic energy given by

where u and u0 are the velocities that the electron has at point 1 and 5. Since the sum of the kinetic energy and potential energy of an electron in motion in an electric field remains constant. So we mat write

(Since the electron acquires kinetic energy at the expense of potential energy)

If the initial velocity of an electron is zero,

i.e. u0=0, then

Or,

Substituting the values of charge & mass of the electron in this equation we can deduce an approximate formula for the velocity of an electron,

km/s

Thus the velocity that an electron can pick up in moving through an accelerating field solely depends on the potential difference applied. From the above equation it is seen that electron can be brought up to a very high velocity comparable to light, for e.g.

At V=150 kV, u=228478 km/s whereas the velocity of light =300000 km/s i.e. nearly 76.16% of light.

By adjusting the magnitude and direction of the initial velocity and also the magnitude and velocity of field intensity it is possible to force electron to follow a predetermined path or trajectory. In this way we are in position to control the motion of electrons, their energy etc.

2. Effect of Magnetic Field

The ability of a magnetic field to control the paths of electrons is utilized, focused and deflect the electron beam. The process is explained later.

3. Effect of Impingement

When a fast moving electron impinges on a work piece material surface, it penetrates through a layer undisturbed (the reason is still not clearly understood). The layer through which the electron penetrates undisturbed is called transparent layer (TL). Then it starts colliding with the molecule and experiences multiple scattering and loses its energy gradually rather than at once, as it collides repeatedly with atomic nuclei and lattice electrons and thus heat is generated. As a result the electrons entering the material experience change in both the velocity and directional motion and ultimately brought to rest. The electronic heating takes place inside the material itself below the transparent layer.

The total range to which the

electron can penetrate ( )

before it is brought to rest which is often termed as mean free path, depends on the kinetic energy, i.e. is on the accelerating voltage.

It has been found that

mm

Where = Accelerating voltage in volts

= Density of the material in kg/mm3

Thus we found that the maximum temperature will occur at a layer below the surface. However analysis of heat conduction equation shows that an increase in the pulse duration causes the layer having

a maximum temperature( ) to

move towards the surface due to heat conduction as shown. And that given certain pulse duration this layer will ultimately rise up to the surface.

Physically the external factors accompanying the effect of an electron beam impinging on a metal include extra emission of x-ray, heat radiation, visible light,

scattered and secondary electrons, evaporation of some metals as atoms and ions, and may be depicted as shown in the figure.

1. X-ray

2. Thermal radiation

3. Visible light

4. Ions

5. Scattered & secondary thermo-ionic electrons

6. Molecules

4. Formation of Penetration zone

Observations and theoretical calculations seem to favour the following mechanism of deep fusion:At first when the electron beam is stationary, the beam is raised to a predetermined power and focused so that all beam power is concentrated in the surface layer of the work material with a thickness equal to the electron penetration depth and within a hot spot with a diameter equal to that of the beam. Since energy density at the metal surface is high, metal particles are carried away form the heating zone at a high rate leaving behind them a cone shaped depression with the sides exceeding the cone base in area. As the cone is formed the beam energy density on its side surfaces decreases i.e. electrons heating per unit area on the side surfaces decreases with increase in depth of penetration and it decreases until it attends a steady state value at which the crater does not change its size any longer.

Source: courtesy, Messer Griesheim GmbH, Puchheim, W. Ger.

5. Dimensional analysis for establishing co-relation among various parameters

For rough estimation of the function of characteristics during EBM process, the dimensional analysis is found to be quite useful.

The quantities of interest are-

i. Beam power Pii. Beam diameter d

iii. Velocity of beam with respect to work piece v

iv. Thermal conductivity of the work piece material k

v. Volume specific heat of the work material

vi. Melting temperature of the work piece material

vii. Depth of penetration zFrom dimensional analysis it can be found that

Since f ( ) = 0

Experimentally it is found

Therefore

Again experimentally it is found that

The equation gives us the value of the depth of Penetration of melting temperature in terms of other quantities.

EBM Gun

The cathode emits electrons due to thermo-ionic effect and due to a huge voltage difference between cathode and anode the electron are accelerated towards the anode. As the environment is vacuum and electrons are very light and the anode is placed perpendicularly with the electrons motion path, electrons cannot touch the anode due to their high momentum and moves straight downwards. A beam alignment coil is placed after anode to restrict the dispersion of electron and to align the beam. The tungsten diaphragm has a small hole. It blocks the dispersed electrons (even after passing beam alignment coil) and allows only the concentrated electrons to pass. Magnetic

field is used in magnetic lens to focus the beam and to adjust the focal point. The current beam can be moved within its small vicinity of 6.5 mm2 by beam deflection coil using magnetic field. Beam deflection coil also controls the aberration of electrons. There is a light microscope between tungsten diaphragm and magnetic lens by which the point of bombardment can be seen. The electron beam emits no visible light so the beam can not be seen, so our only option is to see at the point of machining.

Grid bias is a very important thing. Grid bias voltage is more –ve than cathode, by which we can control the pulse duration or frequency. The –ve charge of grid bias opposes the emission of electron and the number of electron that will pass depends upon the –ve voltage of the grid bias. The more –ve the grid will be, the fewer amounts of electrons will pass. If grid bias voltage is too high (-ve) then no current will pass. This pulsating nature of beam is necessary, because after one pulse the current flow is needed to stop to flush out the evaporated material. Another thing is controlling the pulse duration and focal point the maximum temperature surface can be shifted and which is very essential because if vaporization of material occurs below the transparent layer before the vaporization of surface layer than the surface material will break into small pieces and will burst out.

A slotted disk rotates between beam deflection coil and work piece at a speed synchronized with the frequency of the beam, in such a way that whenever the electron beam is on then it will pass through the slots in the disk and whenever the beam is off the solid part of the disk comes between the work piece and the beam deflection coil protecting the machined portion of the work piece from the solidified debris of vaporized work piece material.

Necessity of vacuum The whole EBM setup should be in vacuum because –

1) Due to the high temperature of cathode there is a change of oxidation of cathode at the presence of air.

2) If the electrons have to pass through air then resistance will be more and electron will lose more energy in the way.

3) If air is present between cathode and anode then due to the high voltage difference the air will be ionized and an arc will form between cathode and anode.

Process Parameters

Beam CurrentIncrease in beam current increases the energy per pulse delivered to the work piece. EBM machining systems are available that can generate pulse energies in excess of 120 joules /pulse, a value that is 200-400 % than that available from industrial laser-drilling systems. Beam current generally varies from 100𝜇amp to 1amp.

Pulse DurationThe longer the pulse duration, the wider the diameter & the deeper the drilling depth capability will be. Shorter pulse durations will allow less interaction time for thermal affects

to materialize. Typically the electron systems can generate pulses as short as 50 or as

long as 10 msec.

Lens CurrentThe lens current parameter determines the distance between the focal point and the electron beam gun (the working distance) also determines the size of the focused spot on the workspace. The diameter of the focused electron beam spot on the work piece will, in turn, determine the diameter of the hole produced. By selecting different focal positions, the hole produced can be tapered, straight, inversely tapered, bell-shaped, or center-bowed. Electron beam deflection coil has tracking criteria. It can scan 0.1 mm in 1msec, which mean an effective tracking with the work piece motions fast as100mm/sec.

Advantages A wide range or materials can be processed.

E.g. Stainless steel, nickel & cobalt alloys, Cu, Al, Ti, ceramics, leather and plastics

Due to high power density EBM can melt any known material on earth.

Work material 25-50 𝜇m away from the machining spot remains at the room temperature. No thermal distortion occurs in the rest of the work piece.

No burr formation occurs at the exit of the hole.

HAZ (Heat Affected Zone) seldom exceeds .025 mm, i.e. the effect of beam power does not generally exceed beyond 0.25mm.

It has high depth to diameter ratio, nearly 15:1.

EBM has very high productivity ratio, 5000 holes /sec.

Chances of contamination of the work piece are also less due to vacuum.

It does not apply any force to the work piece, and thereby no mechanical distortion of the work piece

Disadvantages Very high specific energy required, typically 450W/mm3-min for EBM. It is difficult to generate

this much of energy and also costs lot of money.

The whole EBM setup is too much of costly.

Use of vacuum restricts the size of the job. We can not increase the size much because for that case we have to evacuate a big chamber which is again costly and difficult to achieve.

A small lip of solidified material may remain around the rim of the hole. As EBM is used mostly for small scale machining, this lip of material sometimes causes problem.

High level of operator skill required as the whole process is complicated though in this era of automation.

EBM can not machine holes on work piece materials thicker than 10mm.

Holes & slots are tapered (10 to 20) when the sheet thickness is more than 0.1mm.

Application

1) High speed perforations or drilling of small diameter holes in work pieces for food processing, aerospace, insulation & cloth industry, such as

• Perforated screens & filters used in food processing industry

• Drilling of turbine engine combustor dome used in aerospace industry

• A large number of holes in spinning head used in insulation industry for the production of glass fiber

• Perforation of artificial leather used in shoe and cloth industry

2) Drilling metering holes in injector nozzles of diesel engine, light ray orifice, wire drawing dies etc.

3) Pattern generation associated with integrated circuit fabrication in electronic industry