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Non-Traditional Machining

ME 338: Manufacturing Processes II
Instructor: Ramesh Singh; Notes: Profs. 1
Singh/Melkote/Colton

Introduction
• Machining is a broad term to describe
removal of material from a workpiece.
• Machining categories:
– Cutting involves single-point or multipoint cutting
tools, each with a clearly defined geometry.
– Abrasive processes, such as grinding.
– Nontraditional machining, utilizing electrical,
chemical, and optical sources of energy.


Nontraditional Machining
• Ultrasonic Machining (USM)
• Water-Jet Machining & Abrasive-Jet
Machining
• Chemical Machining
• Electrochemical Machining (ECM)
• Electrical-Discharge Machining (EDM)
• High-Energy-Beam Machining
– Laser-beam machining (LBM)
– Electron-beam machining (EBM)


Traditional vs. Nontraditional
• Primary source of energy
– Traditional: mechanical.
– Nontraditional: electrical, chemical, optical
• Primary method of material removal
– Traditional: shearing
– Nontraditional: does not use shearing
(e.g., abrasive water jet cutting uses
erosion) Water jet machining

Grinding

2D cutting process

Why Nontraditional Machining?
• Situations where traditional machining processes are
unsatisfactory or uneconomical:
– Workpiece material is too hard, strong, or 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.
– Temperature rise or residual stresses are undesirable or
unacceptable.


Ultrasonic Machining (USM)
• Process description
– The tool, which is negative of the
workpiece, is vibrated at low
amplitude (0.013 to 0.08 mm) and
high frequency (about 20 kHz) in an
abrasive grit slurry at the workpiece
surface.
– The slurry also carries away the
debris from the cutting area.
– The tool is gradually moved down
maintaining a constant gap of
approximately 0.1 mm between the
tool and workpiece surface.


USM (Cont.)
• Cracks are generated due to the high stresses produced
by particles striking a surface.
• The time of contact between the particle and the surface
is given by:
1/ 5
5r  c 0 
t0 ≈   (10 − 100 µ s ) Force of a particle on surface:
c0  v 
F = d (mv) / dt
r: radius of a spherical particle
c0: workpiece elastic wave velocity = E / ρ Average force of a particle
striking the surface:
v: velocity of particle striking surface
Fave = 2mv / t 0


USM (Cont.)
• Example: Explain what change, if any, takes place in the
magnitude of the impact force of a particle in ultrasonic
machining as the temperature of the workpiece is
increased.
Solution:
Here, m and v are constant.
1/ 5
5r  c 0  1 1
t0 =   ⇒ t0 ∝ ∝
c0  v  c04 / 5 E 2/5

When temperature increases, E decreases
and t0 increases. Hence, F decreases.


USM (Cont.)
Assuming hemispherical brittle fracture

3
2π  D 
V=  
3 2
D ≈ 2 dh
2π
V= (dh) 3 / 2
3
MRR = ηV Z f
where V = volume removed by a single grain
f = frequency of operation
Z = number of particles impacting per cycle
η = efficiency


USM (Cont.)
• Applications
– USM is best suited for hard, brittle materials, such as ceramics,
carbides, glass, precious stones, and hardened steels. (Why?)

• Capability
– With fine abrasives, tolerance of 0.0125 mm or better can be held.
Ra varies between 0.2 – 1.6 µm.

• Pros & Cons:
– Pros: precise machining of brittle materials; makes tiny holes (0.3
mm); does not produce electric, thermal, chemical damage
because it removes material mechanically.
– Cons: low material removal rate (typically 0.8 cm3/min); tool wears
rapidly; machining area and depth are limited.


USM Parts

Ceramic


Water-Jet Machining (WJM)
also called hydrodynamic machining

WJM is a form of micro erosion. It The extreme pressure of the accelerated
works by forcing a large volume of water particles contacts a small area of the
water through a small orifice in the workpiece and acts like a saw and cuts a
nozzle. narrow groove in the material.
http://www.flowcorp.com/waterjet-resources.cfm?id=360

WJM (Cont.)
• Pros: no need for predrilled holes, no heat, no workpiece
deflection (hence suitable for flexible materials), minimal
burr, environmentally friendly.
• Cons: limited to material with naturally occurring small
cracks or softer material.
• Applications:
– Mostly used to cut lower strength materials such as wood,
plastics, rubber, paper, leather, composite, etc.
– Food preparation
– Good for materials that cannot withstand high temperatures of
other methods for stress distortion or metallurgical reasons.


WJM Examples

PWB (printed wire board)


Abrasive Water-Jet Machining
(AWJM)
The water jet contains
abrasive particles such as
silicon carbide, thus
increasing MRR.
Metallic materials can be
cut. Particularly suitable for
heat-sensitive materials.


AWJM Parts

Bullet Proof Glass Part

Steel rack (75 mm thick)

Ceramic Part

Source: http://www.waterjets.org/


Abrasive-Jet Machining (AJM)

A high-velocity jet of dry air, The gas supply pressure is on
nitrogen, or carbon dioxide the order of 850 kPa (125 psi)
containing abrasive particles is and the jet velocity can be as
aimed at the workpiece surface high as 300 m/s and is
under controlled conditions. controlled by a valve.


AJM Process Capability
• Material removal
– Typical cutting speeds vary between 25 -125 mm/min

• Dimensional Tolerances
– Typical range ±2 - ±5 µm

• Surface Finish
– Typical Ra values vary from 0.3 - 2.3 µm


AJM Applications & Limitations
• Applications
– Can cut traditionally hard to cut materials, e.g., composites,
ceramics, glass
– Good for materials that cannot stand high temperatures

• Limitations
– Expensive process
– Flaring can become large
– Not suitable for mass production because of high
maintenance requirements


Chemical Machining (CM)
• Chemical machining, basically an etching process, is the oldest
nontraditional machining process.
• Material is removed from a surface by chemical dissolution using
chemical reagents, or etchants, such as acids and alkaline
solutions.
• The workpiece is immersed in a bath containing an etchant. The
area that are not required to be etched are masked with “cut and
peel” tapes, paints, or polymeric materials.
• In chemical milling, shallow cavities are produced on plates,
sheets, forgings, and extrusions for overall reduction of weight
(e.g., in aerospace industry). Depths of removal can be as much as
12 mm.


CM (Cont.)
• Chemical blanking is used to produce features which
penetrate through the material via chemical dissolution.
The metal that is to be blanked is
– thoroughly cleaned with solvents.
– coated and the image of the part is imprinted.
– soaked in a solvent that removes the coating, except in the
protected areas.
– spray etched to dissolve the unprotected areas and leave the
finished part.


CM (Cont.)
• Typical applications
– Chemical blanking: burr-free
etching of printed-circuit boards
(PCB), decorative panels, thin
sheet-metal stampings, and the
production of complex or small
shapes.
– Chemical milling: weight
reduction of space launch
vehicles.

Pros: low setup, maintenance, and tooling costs; small,
delicate parts can be machined; suitable for low production
runs on intricate designs.
Cons: slow (0.025-0.1 mm/min); surface defects; chemicals
can be extremely dangerous to health.

Electrochemical Machining (ECM)
• Process description:
– In ECM, a dc voltage (10-25 v) is
applied across the gap between a
pre-shaped cathode tool and an
anode workpiece. The workpiece
is dissolved by an
electrochemical reaction to the
shape of the tool.
– The electrolyte flows at high
speed (10-60 m/s) through the
gap (0.1-0.6 mm) to dissipate
heat and wash away the
dissolved metal.


ECM (Cont.)
• The material removal rate by ECM is given by:

MRR = C I η
where, MRR=mm3/min, I=current in amperes,
η=current efficiency, which typically ranges from 90-100%,
C is a material constant in mm3/A·min.

Feed rate (mm/min): f = MRR / A0
Assuming a cavity with uniform cross-sectional area A0


ECM (Cont.)
• Pros: high shape complexity
possible, high MRR possible, high-
strength materials, mirror surface
finish possible.

• Cons: workpiece must be
electrically conductive; very high
tooling (dedicated) and equipment
costs; high power consumption.

• Applications: complex cavities in
high-strength materials, esp. in
aerospace industry for mass
production of turbine blades.


EDM-History


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.
EDM is one of the most accurate
while quite affordable mfg process.


EDM (Cont.)
The EDM system consists
of a shaped tool or wire
electrode, and the part. The
part is connected to a
power supply to create a
potential difference between
the workpiece and the tool.
When the potential
difference is sufficiently
high, a transient spark The dielectric fluid 1) acts as an
discharges through the insulator until the potential is
fluid, removing a very small sufficiently high, 2) acts as a
amount of metal from the flushing medium, and 3) provides
workpiece. a cooling medium.


Process-Basics


EDM (Cont.)
MRR is basically a function of the current and the melting point of
the workpiece material. An approximate empirical relationship is:
4 −1.23 MRR=mm3/min
MRR = 4 × 10 I T w
I=current in amperes
Tw=melting point of workpiece (ºC)
Wear rate of electrode:

Wt = 11× 10 3 I Tt −2.38 Wear ratio of workpiece
to electrode:
Wt=mm3/min
Tt=melting point of electrode material (ºC) R = 2.25 Tr−2.3
Tr=ratio of workpiece to
electrode melting points (ºC)

MRR - EDM

Metal removal is function of pulse energy
• Experimental Approach
and frequency:
h = K1Wn
TOOL
(-) D= K2Wn
where W = Pulse energy, J

DC VOLTAGE
h = height of crater, mm
D = diameter of crater, mm
K1, K2 = constants depending
h on electrode materials
D and dielectric
n = constant depending on
WORKPIECE work tool combination
(+) The crater volume from geometry,
π 3 
Scheme of Crater Formation Vc = h D 2 + h 2 
6 4 
MRR = Vc f η π 3 2 2 3n
where f = frequency of operation and η = efficiency Vc = K1  K 2 + K1 W
6 4 


Volume of the crater


EDM Process Capability
• MRR
– Range from 2 to 400 mm3/min. High rates produce rough finish,
having a molten and recast structure with poor surface integrity
and low fatigue properties.
• Dimensional Tolerances
– Function of the material being processed
– Typically between ±0.005 - ±0.125 mm
• Surface Finish
– Depends on current density and material being machined
– Ra varies from 0.05 – 12.5 µm
– New techniques use an oscillating electrode, providing very fine
surface finishes.


EDM Applications
Widely used in aerospace, moldmaking, and die casting to produce die
cavities, small deep holes, narrow slots, turbine blades, and intricate shapes.

Cavities produced by EDM Stepped cavities


EDM Limitations
• Limitations
– A hard skin, or recast layer is produced which may be
undesirable in some cases.
– Beneath the recast layer is a heat affected zone which
may be softer than parent material.
– Finishing cuts are needed at low MRR.
– Produces slightly tapered holes, specially if blind.


Wire EDM

A wire travels along a prescribed path,
cutting the workpiece, with the discharge
sparks acting like cutting teeth.


Wire EDM (Cont.)
MRR in Wire EDM

MRR = V f h b

where, b = d w + 2s

MRR = mm3/min
Vf = feed rate of wire into the
workpiece in mm/min
h = workpiece thickness or dw = wire diameter in mm
height in mm s = gap between wire and workpiece in mm


Wire EDM Parts


Example
• Example: You have to machine the following part from a
85mmx75mmx20mm steel block. You have to choose
between EDM and Conventional machining. Your
objective is to minimize the cutting power required, which
process will you choose?
12.5

Assumptions:
– EDM process:
40
• Wire diameter: dw=0.2 mm
• Gap: s=0.1 mm
– Conventional machining: 10
• Negative of the part has to be 12.5
removed
12.5 20 20 20 12.5

Example
Solution:
- EDM process
VEDM = lc*(dw+2s)*t = 1440 mm3
- Conventional machining
VM= Vtotal – Vpart = 99500 mm3
- Power comparison
u M VM u EDM VEDM
We will choose machining if ≤
tM t EDM
let’s assume tEDM=αtM
u EDM VEDM
then machining if α ≤
u M VM

Reverse micro-EDM
• Fabrication of high aspect ratio micro-electrode arrays

• Potential application in machining hole arrays via micro-
EDM/ECM


Arrays Fabricated via R µ-EDM @IITB

6x6 array 4x4 array

Experimental Setup


Fabricated Texture


High-Energy-Beam Machining

• Laser-Beam Machining (LBM)

• Electron-Beam Machining (EBM)

• Focused Ion-Beam Machining


Laser-Beam Machining (LBM)
• Laser Concept
– Add energy to make electrons “jump” to higher energy orbit
– Electron “relaxes” and moves to equilibrium at ground-state
energy level
– Emits a photon in this process (key laser component)
– Two mirrors reflect the photons back and forth and “excite” more
electrons
– One mirror is partially reflective to allow some light to pass
through: creates narrow laser beam


LBM (Cont.)
Excited
State

Electron Ground
State Photon

Nucleus

Orbits

Electron is Electron relaxes
energized to the to ground state
excited state and photon is
produced

Photon Emission Model

LBM (Cont.)
• More precise
• Useful with a variety of
materials: metals,
composites, plastics,
and ceramics
• Smooth, clean cuts
• Faster process
• Decreased heat-
affected zone


Schematic of LBM Device


Laser Setup
• Laser Processing Center
– 100 W SPI single mode fiber laser (Power and frequency modulated)
– Optics for variable intensity distribution and spot size
– 3 axis (Z decoupled) translational stages and controls

“Method and device for generating laser
– Provides uniform/Gaussian intensity beam of variable intensity distribution
– 7 µm -900 µm spot size possible and variable spot size”,
– Hardening/Cladding/Texturing/Brazing Indian Patent Application No.
442/MUM/2011.
Machine Tools Laboratory
Micromachining Cell

LBM (Cont.)
• Important physical parameters in LBM
– Reflectivity
– Thermal conductivity of workpiece surface
– Specific heat and latent heats of melting and evaporation
• The lower these quantities, the more efficient the
process.

• The cutting depth t: t = P / vd
P is the power input, v is the cutting speed,
and d is the laser-beam-spot diameter.


Heat Source Modeling
• Solution for stationary point using Green’s Theorem:
The differential equation for the conduction of heat in a
stationary medium assuming no convection or radiation, is

This is satisfied by the solution for infinite body,
δq ( x − x ') 2 + ( y − y ') 2 + ( z − z ') 2
dT '( x, y, z, t ) = 3
ex p [ − ]
4 a ( t − t ')
ρ C (4 π a ( t − t ')) 2

sem i − in f in ite
2δ q ( x − x ') 2 + ( y − y ') 2 + ( z − z ') 2
dT '( x, y, z, t ) = 3
ex p [ − ]
4 a ( t − t ')
ρ C (4 π a ( t − t ')) 2

δq = instantaneous heat generated, C = sp. heat capacity, α =
diffusivity, ρ = Density, t = time, K = thermal conductivity.
gives the temperature increment at position (x, y, z) and time
t due to an instantaneous heat source δq applied at position
(x’, y’, z’) and time t’. ME 338: Manufacturing Processes II

Moving point heat source in semi-infinite body

In moving coordinate system:
2δ q ( X − x ') 2 + (Y − y ') 2 + ( Z − z ') 2
dT '( x, y, z, t ) = 3
exp [ − ]
4 a ( t − t ')
ρ C (4 π a ( t − t ')) 2

In fixed coordinate system:
2δ q ( x − vt '− x ') 2 + ( y − y ') 2 + ( z − z ') 2
dT '( x, y, z, t ) = 3
ex p [ − ]
4 a ( t − t ')
ρ C (4 π a ( t − t ')) 2

Note that δ q = Pdt ' ME 338: Manufacturing Processes II

Moving point heat source:
Consider point heat source P heat units per unit time moving with velocity v on semi-
infinite body from time t’= 0 to t’= t. During a very short time heat released at the
surface is dQ = Pdt’. This will result in infinitesimal rise in temperature at point (x, y, z)
at time t given by,

t '= t
2 Pdt ' ( x − vt '− x ') 2 + ( y − y ') 2 + ( z − z ') 2
dT '( x, y, z, t ) = ∫
t '= 0
3
ex p [ −
4 a ( t − t ')
]
ρ C (4 π a ( t − t ')) 2

The total rise in of the temperature can be obtained by
integrating from t’=0 to t’= t

Gaussian Circular
• Gaussian beam distribution
2P 2( x '2 + y '2 )
I ( x ', y ') = 2
exp[− 2
]
πσ σ

• Gaussian circular heat source
2dt ' ( X − x ') 2 + (Y − y ') 2 + ( Z − z ') 2
dT '( X , Y , Z , t ) = 3
I ( x ', y ')dx ' dy 'exp[− ]
4a(t − t ')
ρ C (4π a(t − t ')) 2

∞ ∞
2dt ' 2P 2( x '2 + y '2 ) ( X − x ')2 + (Y − y ')2 + ( Z − z ')2
dT ' = 3
πσ 2 ∫ ∫
−∞ −∞
exp[−
σ2
]dx ' dy 'exp[−
4a(t − t ')
]
ρ C (4π a(t − t ')) 2

In fixed coordinate sytem
t '= t
4P dt '(t − t ') −0.5 2(( x − vt ') 2 + y 2 ) z2
T − T0 =
ρ C π 4 aπ ∫0 σ 2 + 8a (t − t ') exp[ − σ 2 + 8a (t − t ') − 4 a (t − t ') ]
t '=


Uniform Circular/Rectangular
• Circular
t σ
2P dt ' (( x − vt ') − x ') 2
T − T0 =
8 π 2σ 2 K ∫
0
( t − t ') ∫
−σ
ex p [ −
4 a ( t − t ')
]d x ' ×

y − σ 2 − x '2 y + σ 2 − x '2
[ − erf ( ) + erf ( )]
2 a ( t − t ') 2 a ( t − t ')

• Rectangular
2δ q d t ' z2
T − T0 = 3
exp[− ]
4 a ( t − t ')
4 b l ρ C ( 4 π a ( t − t ') ) 2

l b
( ( x − v t ') − x ') 2 ( y − y ') 2
∫l
−
exp[−
4 a ( t − t ')
]d x ' ∫ e x p [ −
−b
4 a ( t − t ')
]d y '


LBM Capability
• MRR
– Cutting speed can be as high as 4 m/min.
– Typical material removal rate is 5 mm3/min.

• Dimensional Tolerance
– Typical ranges from ±0.015 - ±0.125 mm

• Surface Finish
– Ra varies between 0.4 – 6.3 µm.


LBM (Cont.)
• Process Variations

– Laser beam machines can be used for cutting, surface hardening,
welding, drilling, blanking, engraving and trimming.

– Types of lasers used: pulsed and CW CO2, Nd:YAG, Nd:glass,
ruby and excimer.

– High-pressure gas streams are used to enhance the process by
aiding the exothermic reaction process, to cool and blow away the
vaporized or molten material and slag.


LBM (Cont.)
• Applications
– Multiple holes in very thin and thick materials
– Non-standard shaped holes and slots
– Prototype parts
– Trimming, scribing and engraving of hard materials
– Small diameter lubrication holes

• Limitations
– Localized thermal stresses, heat affected zones, recast layer and
thermal distribution in thin parts
– Difficulty of material processing depends on how close materials
boiling and melting points are
– Hole wall geometry can be irregular
– The cutting of flammable materials is usually inert gas assisted


Electron-Beam Machining (EBM)
How it Works
• A stream of electrons is started by
a voltage differential at the
cathode. The concave shape of
the cathode grid concentrates the
stream through the anode.
• The anode applies a potential field
that accelerates the electrons.
• The electron stream is then forced
through a valve in the electron
beam machine.
• The beam is focused onto the
surface of the work material,
heating, melting, and vaporizing
the material.


EBM (Cont.)
The entire process occurs in a vacuum chamber because a collision
between an electron and an air molecule causes the electrons to veer
off course. LBM doesn’t need vacuum because the size and mass of a
photon is numerous times smaller than the size of an electron.


EBM Characteristics
• Mechanics of material removal – melting, vaporization
• Medium – vacuum
• Tool – beam of electrons moving at very high velocity
• Maximum MRR = 10 mm3/min
• Specific power consumption = 450 W/mm3/min
• Critical parameters – accelerating voltage, beam
diameter, work speed, melting temperature
• Materials application – all materials
• Shape application – drilling fine holes, cutting contours in
sheets, cutting narrow slots
• Limitations – very high specific energy consumption,
necessity of vacuum, expensive machine


Comparative Performance


Focused Ion Beam Technologies
• Ga+ ion beam raster over the
surface similar to SEM
• Milling of small holes and
modifications in the structures
can be done
• Most instruments combine
nowadays a SEM and FIB for
imaging with high resolution,
and accurate control of the
progress of the milling
• Process is performed in
vacuum


Mechanism

Rate of etch depth
Where, Y is sputter yield of surface atoms per
incoming ion, using a probe of current I is given by
A is the etched area, ρ is the density of target
material, M is atomic mass of target material and
e is charge

Dual Beam System


Focused Ion Beam Technologies
• FIB finds application in:
– Ablation of hard materials:
diamond, WC
– Polishing of single crystals
– Deposition
– Site-specific analysis
– FIB lithography
– TEM samples

• Capital investment ~ 5 Crore


Process Capabilities of FIB
• Deposition
• Etching
• Low material removal
• Very high cost
• Nanometric imaging resolution
• Can process conducting and non conducting materials


Summary
• Process description and capability
– Ultrasonic Machining (USM)
– Water-Jet Machining & Abrasive-Jet Machining
– Chemical Machining
– Electrochemical Machining (ECM)
– Electrical-Discharge Machining (EDM)
• High-Energy-Beam Machining
– Laser-beam machining (LBM)
– Electron-beam machining (EBM)
– Focused Ion Beam (FIB)


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