PRECISION MACHINING

UNIT II PRECISION MACHINING 9

Introduction - Ductile mode machining of hard and brittle materials - Ultra
precision grinding and selection of grinding wheels - Electrolytic in process
dressing -Chemical mechanical polishing - Diamond turn machining - High speed
machining -Magneto rheological finishing processes.

Dr.M.Pradeep , CEG,AU
Brittle material like glass, silicon, tungsten carbide (WC), germanium and silicon nitride
have been widely employed in the industries such as precision engineering, optics,
instruments, semi-conductor and micro-electromechanical systems (MEMS) because of
its excellent mechanical optical, physical and chemical properties.

There are rapid growing demands on manufacturing of brittle materials achieving

a good quality surface finish,
stringent geometry accuracy, and
surface integrity with less or free of subsurface damage.

Meanwhile, to reduce the manufacturing cost in the production of these components and
devices made by brittle materials, effectively machining of these materials is very much
demanded.
.

Dr.M.Pradeep , CEG,AU
Traditionally, abrasive processes such as grinding, lapping and polishing have
been widely used for the final surface finishing of these brittle materials.
The demerits associated with these processes include
• poor grindability,
• high manufacturing cost, and
• subsurface damage.
• the abrasive processes will cause surface flatness deviation due to its
uncontrollable material removal resulting in the machined profile inaccuracy.
• after grinding and lapping processes, the chemical-mechanical polishing
(CMP) is essential to remove the subsurface damage layer caused by the
hard abrasive particles, which makes a very costly production . Also, these
abrasive processes especially like CMP are extremely slow, whiles grinding
and lapping processes would impart subsurface damage leading to a
degraded surface integrity
Dr.M.Pradeep , CEG,AU
Machining of hard and brittle materials always poses problems and is uneconomical due to

• short tool life,

• low material removal rate,
• poor surface quality and
• high damage to the surface near layer .

However, under certain controlled conditions, it is possible to machine brittle materials using
single- or multi-point diamond tools so that the material can be removed by plastic flow, leaving
a crack-free surface. This condition is called ductile mode machining.

According to this concept, all materials will deform plastically if the scale of deformation is very
small during machining.

Several terms are used to indicate the plastic deformation phenomenon on machined surfaces
such as ductile regime turning, ductile mode machining , ductile machining , ductile regime
grinding, microcrack-free or damage free grinding and partial ductile mode grinding.

Dr.M.Pradeep , CEG,AU
A schematic comparison
between DMC and BMC
(brittle mode cutting) of brittle
materials helps to reveal the
underlying mechanisms as
shown in Fig. 1. The
fundamental premise of ductile
mode cutting states that all
brittle material will experience
a transition from DMC to BMC
when cutting from zero depth
of cut (DoC) to a large value
regardless its hardness and
brittleness. When cutting
below the critical undeformed
chip thickness (UCT), the
energy consumed for crack
prorogation is larger than that
for plastic deformation, DMC
will be achieved in brittle
materials successfully.

Dr.M.Pradeep , CEG,AU
It has been thought that a material is less brittle below a certain depth of cut value; this has
therefore given rise to the term “ductile” mode cutting/grinding. Under this machining
condition, it is feasible to remove material without initiating a residual crack at or near the
surface leaving essentially microcrack-free surfaces. When a mixture of plastic deformation and
fractures appears on the ground surface, the term partial ductile mode grinding is preferably
used instead of semi-ductile, as the exact number of ductile areas is unknown. Miyashita used
the term ‘microcrack machining’ to indicate the transition from brittle to ductile machining as
shown in Figure 3.37. A better term to use may be partial ductile machining as the surface
consists of a mixture of fracture and ductile modes.
To maximize ductile mode machining, the grain size and the material removal rate must be very
small, and the height distribution of the cutting edges also must be extremely tight as compared
to the abrasive distribution height used in conventional grinding, lapping, honing, and
Dr.M.Pradeep , CEG,AU
superfinishing processes.
With certain materials, it is possible to machine almost 100% in the ductile mode condition
using rigid ultra-precision machines, whereas only the partial ductile mode condition is
achievable by conventional grinding due to the random orientation of abrasive grains on the
diamond wheels.
In order to improve the surface integrity of these materials, ductile mode
cutting (DMC), also called ductile regime cutting or ductile cutting, as a
promising technique, has been studied vigorously over the past decades and
it is commonly understood that DMC is to remove work materials by plastic
flow instead of brittle fracture deriving a damage-free surface. As a result,
the subsequent polishing process is no longer necessary or the polishing time
can largely reduced because the crack-free surfaces can be directly produced
by DMC without subsurface damage or the subsurface damage layer
thickness being much smaller, which would significantly reduce the
manufacturing time and cost for brittle materials. This advantage cannot be
under addressed because in machining even a minor improvement in
productivity would lead to a major impact
Dr.M.Pradeep in mass production.
, CEG,AU
Later in 1976, Huerta and Malkin were the first to show reproducible results of diamond
grinding of glass in a ductile mode, which considerably improved the surface quality
and machining accuracy.
Later, precision grinding of brittle materials in a ductile mode had been extended to
others such as silicon and ceramics.
Further improvements in ultra-precision machining technology in the 1990s marked the
progression for DMC to be applied in more advanced brittle materials such as different
types of carbides .
Ductile mode cutting thus became an alternative way for finishing of brittle materials as it
could produce crack-free mirror surface finish at a much higher efficiency and lower cost
than polishing processes owing to its high material removal rate.

Dr.M.Pradeep , CEG,AU
Ductile mode cutting mechanism

The concept of DMC material removal is based on the hypothesis that all
brittle material will experience a brittle-ductile mode transition in cutting with
an UCT below the critical value.

Dr.M.Pradeep , CEG,AU
The material removal happens in four stages.
(a) Material under indenter the indenter.
(b) Radial/median cracks formed on a plane at the elastic-plastic boundary
when further increasing the loading.
(c) Lateral cracks formed in an addition to radial/median cracks, which
spread outward from the deformation z started to subject an elastic
deformation. This creates a small elastic deformation zone due to high
hydrostatic pressure below one, beneath the indentation surface, and may
interact with the radial system.
(d) When severely loaded those cracks turn upward to intersect the free
surface, thereby causing severe disruption of the pattern by chipping.
Residual stresses are the main cause of lateral cracking and eventually
resulted in the material removal by fracturing.

In the nanometer scale cutting of silicon using a diamond tool, this mode of
material removal must be avoided as much
Dr.M.Pradeep , CEG,AU as possible to eliminate brittle
ULTRA PRECISION GRINDING AND SELECTION
OF GRINDING WHEELS

Dr.M.Pradeep , CEG,AU
Dr.M.Pradeep , CEG,AU
The most noteworthy developments in processes capable of providing ultra-
precision are as follows:

(a) Single-point diamond and cubic boron nitride (CBN) cutting

(b) Multi-point
abrasivecutting/burnishing,forexample,indiamondandCBNgrinding,honing,
etc.
(c) Free abrasive (erosion) processes such as lapping, polishing, elastic-
emission machining and
selective chemico-mechanical polishing
(d) Chemical (corrosion) processes such as controlled etch machining

(e) Energy beam processes (removal, deformation and accretion) including

those given below:
(i) Photon (laser) beam for cutting, drilling transformation hardening and hard
coating
(ii) Electron beam for lithography, welding
(iii) Electrolytic jet machining for smoothing and profiling
(iv) Electro-discharge (current) beam (EDM) for profiling
(v) Electrochemical (current) (ECM) for profiling
(vi) Inert ion beam for milling (erosion) microprofiling
(vii) Reactive ion beam (etching)
(viii) Epitaxial crystal growth by molecular-bit accretion for manufacturing
new super-lattice crystals, etc. Dr.M.Pradeep , CEG,AU
In this sense ultra-precision grinding is primarily used to generate high
quality and functional parts usually made from difficult to machine materials.
The aim of ultra-precision grinding processes is to generate parts with

high surface finish,

high form accuracy and
high surface integrity.

Dr.M.Pradeep , CEG,AU
Most of the ultra-precision machines available in the market are equipped with machining
systems that adopt either single-point diamond tools or multi-point abrasive (grinding) wheels.
However, in some cases, both machining systems can be incorporated into one machine on
customer request for enabling both single- and multi-point abrasive machining operations.
An ultra-precision machine is defined as a machine that has machining systems with the
following movement accuracies: (i) slide geometric accuracy of less than 1 μm (ii) spindle error
motions of less than 50 nm (iii) control and feedback resolutions of less than 10 nm With the
aforementioned movement accuracies, it is expected that the ultra-precision machine will be
able to generate the following workpiece accuracies:

(i) a dimensional
accuracy in the range of
some microns
(ii) a surface form
accuracy in the range of
100 nm or better
(iii) a surface texture in
the range of 5 nm or
better

Dr.M.Pradeep , CEG,AU
In order to satisfy the aforementioned requirements, the machine must exhibit a high degree of
thermal stability, stiffness, damping, smoothness of motion and must also be integrated with an
ultra-precision metrology system in the machine tool but isolated from the response of the
machine tool during machining.
These machines are available in two- to five-axes configurations as shown in Figure 4.39. Usually,
the grinding wheel is attached on a vertical spindle, the Y-axis, to perform the grinding operation
on the workpiece where it is vacuum chucked on the main horizontal spindle.

Dr.M.Pradeep , CEG,AU
 Depending on the number of axes, this kind of
machine can produce different types of
surfaces such as cylinders, spheres, aspheres
and conical sections and diffractives, free-form
and microstructures (Figures 4.40, 4.41, and
4.42). Applications of these surfaces include
hard discs, photocopier drums, night vision
devices, lenses (for camera, charged-couple-
device (CCD), CD and DVD pick up), free-form
optics (for laser printers, scanners and
conformal military optics radar systems) and
Dr.M.Pradeepdisplays
, CEG,AU for notebooks/mobile phones, street
sign reflectors.
Precitech’s OPTIMUM 2800 (Figure 4.43) is a high performance, two-axes, computer controlled,
ultra-precision, contouring machine specifically designed for single-point diamond turning and
grinding of ultra-precision optical components. The machine is built on a natural granite base
and uses a pneumatic vibration isolation system. The hydrostatic oil bearing slideways are
constructed in an offset “T” configuration in which the X-axis (spindle) slide represents the cross-
arm of the ‘“T”, and the Z-axis (tool holding) slide represents the stem of the “T”’. Both the X
and Z axes have 200mm of travel length. The workpiece holding spindle is a pneumostatic air
bearing design. The spindle is powered through the use of a brushless type DC motor and will
run up to a maximum speed of 3,000 RPM
Dr.M.Pradeep , CEG,AU
The high-speed aspheric grinding system is designed and manufactured for use on Precitech
OPTIMUM machining systems.

This compact aspheric grinding system uses a high-speed, turbine driven, air bearing spindle.
The air bearing spindle is mounted onto a manually positioned mechanical slide assembly. The
slide is mounted onto a fabricated steel column such that the grinding spindle is positioned in
the vertical direction.

The grinding spindle operates over a speed range of 10,000–70,000 RPM. The turbine drive
provides an extremely smooth friction-free spindle rotation. The grinding system accommodates
grinding wheels from 3 mm– 15 mm in diameter. The system has been designed primarily for
small aspheric components, particularly lenses and lens moulds up to 30 mm in diameter.

Semi-ductile grinding followed by simple mechanical polishing is an economical process for

producing a mirror-like surface for hard and brittle Pyrex. A fine grit resonoid bond grinding
wheel was used to generate a large number of ductile streaks to improve the surface finish and
to reduce the polishing time. The ground samples were polished with different slurries on
Precitech’s OPTIMUM 2800.

Dr.M.Pradeep , CEG,AU
Nanotech 500FG (Figure 4.44) developed by Moore Nanotechnology Systems adopts the
microgrinding technique. The machine is capable of generating arbitrary confocal shapes on
materials ranging from optical glass and infrared materials to non-ferrous metals, crystals,
polymers and ceramics. The microground surface typically requires little or no post-polishing
(Figure 4.45). The machine temperature is maintained stable to less than ―0.5 C. Grinding is
done in a flood-cooled environment.

Dr.M.Pradeep , CEG,AU
The following general guidelines be used for the selection of a grinding wheel:
1. Aluminium oxide for steels and silicon carbide for carbides and non-ferrous metals
2. A hard-grade wheel for soft materials and a soft-grade wheel for hard materials
3. A large grit for soft and ductile materials and a small grit for hard and brittle
materials
4. A small grit for a good finish and a large grit for getting the maximum metal removal
rate
5. A resinoid, rubber or shellac bond for getting a good finish and a vitrified bond for
obtaining the maximum metal removal rate
6. Avoid choosing a vitrified bond for surface speeds greater than 32 m/s

Dr.M.Pradeep , CEG,AU
Abrasive types

• Diamond and cubic boron nitride (CBN) are the two most frequently applied types of
abrasives for ultra-precision grinding . There are two types of diamond abrasives – natural
and synthetic diamond – both sharing high wear resistance, heat conductivity, hardness and a
low coefficient of friction.

• A significant drawback of diamond abrasives is its chemical affinity with some metallic
materials leading to the transformation of the diamonds into graphite in case of high
grinding temperatures . This results in high wear when grinding these metallic materials that
react with carbon to carbides, or grinding at high temperatures. For this reason diamond
abrasives are mainly applied for grinding of brittle non-ferrous materials such as silicon,
glass, ceramics or tungsten carbide.
• Alternatively, diamond grains may be coated with suitable materials for preventing them
from oxidizing or other damage caused by high temperature during bonding . For example,
coated diamonds have been found to improve the grinding ratio and reduce breakout of the
diamond grains during the grinding process. Furthermore, boron doped diamond grits have
an oxidation temperature that is 200 K higher than conventional diamond grits .

Dr.M.Pradeep , CEG,AU
Another approach for extending abrasive tool life is the use of tough and blocky-type, cube-
octahedral coarse-grained diamond grits (Fig. 12) for engineered diamond grinding wheels .
These diamond grits possess high strength, thermal stability and abrasion resistance.
Furthermore, because of the fine screening procedure highly uniform grains are used which
ensures constant cutting process interactions. When combined with a titanium coating, these
grits provide a high mechanical and chemical bonding strength within the electroplating process
which significantly reduces grain pull-out of the grits and, thus a quasiwear- less, semi-
deterministic grinding processes results. Dr.M.Pradeep , CEG,AU
A special type of abrasive is CVD (Chemical Vapor Deposition) poly-crystalline diamond films
with sharp edges of micrometer size diamond crystallites which are used for micro-pencil
grinding tools. These types of tools are also referred to as bondless diamond grinding wheels .
Cubic boron nitride (CBN) has superior thermo-chemical stability compared to diamond.
Ultrafine-crystalline CBN (CBN-U) is a special type of CBN, which has a grinding ratio that is eight
times higher, and a higher wear resistance than conventional CBN (Fig. 13). For grinding of
ferrous components and other materials that react with diamond, cubic boron nitride is the best
choice.

Dr.M.Pradeep , CEG,AU
Bonding of abrasives tools

The major bond systems are

• metal,
• resin, and
• vitrified bonding .

Metal bonding can be separated into two different types:

• sintered metal bonding and

• electroplating.

In ultra-precision grinding the sintered metal bond system is used for thin wheels which cut brittle
materials, for example silicon wafers (slicing/dicing) or for micro-pencil grinding tools . Often
electroplated metal bonding is applied to single-layered grinding wheels with stochastically or well-
defined positioning of the grains. High heat conductivity and good wear resistance are the major
benefits of metal bonded grinding tools. Resin bond is made of synthetic resin or synthetic resin
combinations.
Today, the most common bond components for synthetic resin bonded grinding wheels are phenol
plasters or phenol resins. This bonding type allows high chipping volumes and rotational speeds and
is insensitive to shock or impact as well as lateral pressure. The main applications for resin bonded
grinding wheels are rough grinding and abrasive machining. For ultraprecision grinding, epoxy or
polyester resins are used to generate high surface qualities by soft, smooth grinding or polishing, for
example in silicon wafer grinding. Polyurethane resin, CEG,AU
Dr.M.Pradeep is applied when a high elasticity of the abrasive
is needed.
Vitrified bonds have a glass-like structure and are fabricated at high temperatures from mineral
fluxes such as feldspars, firing clays, ground glass frits and chemical fluxes. Resin and vitrified
bonds are used for surface grinding, as for example silicon wafer grinding. Vitrified bonds have a
higher strength to hold the abrasive grains and are easier to dress. The elastic modulus is nearly
four times higher than that of resin bonding.

Special cases of ultra-precision grinding bonding types are rubber bonded grains and ceramic-
forming polymer bonding. In rubber bonding the abrasive grains are granulated with resin to
avoid burying small grains in the rubber bond. These granules with diameters of approx. 100
mm are dispersed in the rubber. The major reason for the application of grinding tools with
rubber and other elastic bonded abrasive layers is the excellent damping and therefore the
capability to generate surface roughness in the nanometer range. A ceramic-forming polymer
was developed by Sherwood to avoid the oxidation or other damage to the diamond grains
during the baking process of vitrified bonding systems. For preventing graphitization of diamond
grains it is possible to melt ceramic-forming polymers by heating at low temperatures .

Dr.M.Pradeep , CEG,AU
Dr.M.Pradeep , CEG,AU
ELECTROLYTIC IN-PROCESS DRESSING

Dr.M.Pradeep , CEG,AU
 Schematic illustration of ELID (Murata et al 1985).

When the grits are worn out, a new layer in the outer surface is electrolysed and necessary
bonding material is removed from the grinding wheel surface in order to realise grit protrusion
The power supply for ELID is used to control the dressing current, voltage and pulse width of the
dressing process. The basic ELID system consists of a metal-bonded diamond grinding wheel, an
electrode, a power supply and an electrolyte. The metal-bonded grinding wheel is made the
positive pole through the application of a brush smoothly contacting the wheel shaft and the
electrode is made negative pole. In the small clearance between the positive and negative
poles, electrolysis occurs through the supply of the grinding fluid and an electrical current.
Dr.M.Pradeep , CEG,AU
The electrochemical material removal (ECM) can be efficiently used especially for continuous in-process
dressing of grinding wheels. The metal bond is continuously removed during the grinding process by
electrolysis. Thus, the constant usage of sharp grits can be guaranteed. In addition, the electrochemical
material removal will not damage the diamond grits. The coolant used in the grinding process also serves as
an electrolyte for the ECM process.
Rapid wear of the bond material can be avoided by a defined electrochemical process in the passive area of
the electrolysis system. An oxide layer formation at the surface of the anode prevents excessive grinding
wheel wear. Only due to the mechanical removal of the oxide layer during grinding the electrolysis will
continue. With suitable process parameters a dynamic equilibrium of oxide layer growth and removal will be
formed. This will result in stable dressing conditions, and therefore in a stable finish grinding process.

Dr.M.Pradeep , CEG,AU
Grinding with superabrasive wheels is an excellent way to produce a precision surface
finish on hard and brittle materials. To achieve this, superabrasive diamond grits need
a higher bonding strength during grinding, which can be offered by metal-bonded and
resinoid-bonded wheels.
However, truing and dressing of the wheels are major problems, as they tend to glaze
because of wheel loading. These problems can be avoided by dressing periodically, but
this interrupted action makes the grinding process very tedious and time consuming. A
Japanese research group has introduced an effective technique to overcome the poor
self-dressing properties of metal bonds, especially cast iron bonds, in the presence of
aqueous lubricants.

Dr.M.Pradeep , CEG,AU
Ohmori and Nakagawa have referred to the method as electrolytic in-process dressing (ELID). It
uses an electro-chemical method to remove the metal bonds and properly expose the diamond
particles, thereby maintaining the high efficiency of the grinding operation. The basic ELID
system consists of a metal or cast-iron-bonded diamond grinding wheel, an electrode (copper or
graphite), a power supply and an electrolyte as shown in Figure 4.23.

Dr.M.Pradeep , CEG,AU
The power supply for ELID is used to control the dressing current, voltage and
pulse width of the dressing process. The metal-bonded wheel is made into the
positive pole through the application of a brush smoothly contacting the wheel
shaft, and the electrode is made into the negative pole. In the small clearance
between the positive and negative poles (0.1-0.3 mm), electrolysis occurs
through the supply of the grinding fluid and an electrical current.
It is to be noted that cast iron is a recommended bond for use in an ELID
grinding wheel. An important feature to note on ELID grinding is that an oxide
hydroxide (insulation) layer is formed on the surface of the ELID wheel by
electrolysis. The oxide hydroxide layer has a lower electrolytic conductivity, and
it stops undergoing excessive electrolysis on the grinding wheels.

Dr.M.Pradeep , CEG,AU
Figure 4.24 describes the mechanism of the ELID grinding of a metal-bonded diamond wheel. After
truing (a), the grains and the bonding material of the wheel surface are flattened. The trued wheel
needs to be electrically pre-dressed so that the grains on the wheel surface protrude. When pre-
dressing is started (b), the bonding material flows out from the grinding wheel, and an insulating layer
composed of the oxidized bonding material is formed on the wheel surface (c). This insulating layer
reduces the electrical conductivity of the wheel surface and prevents an excessive flow of the
bonding material from the wheel. As grinding begins, (d), the diamond grains wear out, and the layer
also becomes worn out (e). As a result, the electrical conductivity of the wheel surface increases and
the electrolytic dressing restarts with the flow of the bonding material from the grinding wheel. This
cycle is repeated during the grinding process to achieve a stable grinding. ELID has now become the
most efficient method for dressing metal-bonded grinding wheels continuously, which eliminates the
wheel loading and glazing problems encountered during the grinding process. It has been reported
that surface roughness (Ra) achieved with the ELID process can be as low as 0.33 nm on BK7 glass and
Dr.M.Pradeep , CEG,AU
silicon when using an ultra-fine #3000000 grit metallic bond wheel.
There are numerous applications of ELID, which have been successfully used for processes such
as surface grinding, cylindrical grinding, internal grinding and centreless grinding. Some other
applications are in abrasive cut-off of ceramics , mirror surface grinding of silicon wafers , small-
hole machining of ceramic materials , sawing of steel, polymer, sapphire and glass,precision
machining of CVC-SiC reflection mirrors and mirror internal cylindrical grinding on steels and
alumina components.

Precision internal grinding using ELID (Ohmori

et al 1999).

Dr.M.Pradeep , CEG,AU
Component containing different materials finished
using ELID-lap grinding (Itoh 1998).

Dr.M.Pradeep , CEG,AU
Diamond turning

Dr.M.Pradeep , CEG,AU
Introduction

Diamond turn machines (DTMs) are ultra-precision machines having the

capability to generate surfaces with nonmetric levels of accuracy and

precision.

These surface quality parameters are affected by the machine quality, which

is controllable at the machine building stage, as well as by the inaccuracies

arising out of process variables. Hence, building machines with very high

levels of accuracy becomes the first step in the journey towards achieving

ultra-precision machined surfaces.

Dr.M.Pradeep , CEG,AU
omponents of diamond turn machine.

Dr.M.Pradeep , CEG,AU
Classification of Diamond Turn Machines
Like most of the machines, diamond turn machines are also classified based
on their number of axes and their configurations. Following is one of the
popular ways of classifying diamond turn machines.
Type A: X, Z Lathe machines
Type B: X, Z, C Lathe machines
Type C: X, Z, C, B Lathe machines
Type D: X, Y, Z, A, B Milling machines

Dr.M.Pradeep , CEG,AU
Accuracy of diamond turn machines depends on the following factors in a

significant way:

• Positional accuracy and repeatability of moving elements

• Balanced loop stiffness

• Thermal effects

• Vibration effects

Dr.M.Pradeep , CEG,AU
Characteristics and capabilities of diamond turn machines.

Dr.M.Pradeep , CEG,AU
Tool Geometry
Diamond tools are commercially
available in a variety of macro-
geometrical shapes – both standard
shapes and customised shapes..

The most common cutting edge

shape is triangular with the nose
radius at one apex of the triangle.
Other edge shapes include:

• Flat rectangular shape used in

grooving applications with
customized groove widths down to a
few micrometres
• Two straight cutting edges meeting
at an obtuse angle – used for nano-
milling
• Single arc cutting edges – used for milling
• Multiple discontinuous connected arcs –
used for nano-milling applications
• Elliptical arc shape
Dr.M.Pradeep , CEG,AU
Materials Machinable by DTM

Metals
Metals that can be diamond turn machined are
Copper, brass, aluminum alloys, electroless nickel, bronze, copper beryllium,
tin, antimony, silver, gold, zinc, magnesium, lead and platinum.

Polymers
Polymers that can be diamond turn machined are PMMA (Chemical & Scratch resistance),
Polycarbonate (Impact strength/temperature resistance), Polystyrene (Low cost/highly
transparent), PolyEthirmide (High Thermal/chemical/Impact resistance/high index), Cyclic Olefin
Copolymer (High modulus/low moisture absorption), Cyclic Olefin Polymer (Completely
Amorphous/high chemical resistance), Nylon, Acrylonitrile Butadiene Styrene.

Crystals
Crystals that can be diamond turn machined are Barium Fluoride (BaF2), Cadmium Telluride
(CdTe), Cesium Bromide (CsBr), Cesium Iodide (CsI), Chalcogenide Glass, Gallium Arsenide
(GaAs), Germanium (Ge), Lithium Fluoride (LiF), Magnesium Fluoride (MgF2), Potassium
Bromide (KBr), Potassium Chloride (KCl), Silicon (Si), Sodium Chloride (NaCl), Thallium
Bromoiodide (KRS-5), Zinc Selenide (ZnSe), Zinc Sulfide.

Dr.M.Pradeep , CEG,AU
Dr.M.Pradeep , CEG,AU
Dr.M.Pradeep , CEG,AU
Magnetorheological finishing(MRF)
In MRF, the MR polishing fluid is deposited by a
nozzle on the rim of a rotating wheel, which
transports the fluid to the workpiece surface
(fig.1a). The wheel rim and the surface to be
polished form a converging gap exposed to a
magnetic field. The moving wall, which is in the
rim surface, generates a flow magnetically
stiffened MR polishing fluid through converging
gap. The magnetically stiffened MR fluid
generates a unique pressure distribution in the gap
that is associated with an unsheared fluid, which is
attached to the moving wall (fig.1b). A quasi –
solid moving boundary is effectively formed
very close to the surface of the workpiece
resulting in the high shear stress in the contact
zone and material removal over a portion of the
workpiece surface. This area is designated as
polishing spot. The material removal is enhanced
by nonmagnetic abrasive particles, which are
constitutes of the slurry and forced out to the
polishing interface by a magnetic field gradient.
When the MR fluid mixed with abrasives flows
over specimen surface, the shear stress of the fluid
generates a drag force to move the abrasives,
which results in material removal.
• In MRF, the magnetic-field-dependent yield stress and viscosity of magnetorheological
polishing (MRP) fluid are controlled by controlling magnetizing current in the
electromagnet coils producing magnetic field across the finishing zone.

• The MRP fluid comprises of carbonyl iron particles (CIPs) and very fine abrasives
dispersed in the carrier fluid, which exhibits unique reversible change in its rheological
properties on the application and removal of external magnetic field.

• The carbonyl iron particles acquire magnetic dipole moment proportional to field
strength and aggregate into interconnected chain-like columnar structure aligned in the
field direction, embedding non-magnetic abrasive particles in between or within.

• The rheological characteristics and bonding strength gained by abrasive particles in

presence of CIPs and magnetic field play an important role in MRF action.
High-speed Machining

Dr.M.Pradeep , CEG,AU
Machining with high speeds (HSM) is one of the modern technologies, which in comparison
with conventional cutting enables to increase efficiency, accuracy and quality of workpieces and
at the some time to decrease costs and machining time.

The first definition of HSM was proposed by Carl Salomon in 1931. He has assumed that at a
certain cutting speed which is 5 –10 times higher then in conventional machining, the chip tool
interface temperature will start to decrease

Dr.M.Pradeep , CEG,AU
Dr.M.Pradeep , CEG,AU
With continuing demands for higher productivity and lower production costs,
investigations have been carried out since the late 195Os to increase the cutting speed
and the material-removal rate in machining, particularly with applications in the
aerospace and automotive industries.
The term “high” in high-speed machining (HSM) is somewhat relative; as a general
guide, an approximate range of cutting speeds may be defined as follows:

° High speed: 600 to 1,800 m/min

° Very high speed: 1,800 to 18,000 m/min
° Ultrahigh speed: >18,000 m/min.

Spindle rotational speeds in machine tools now range up to 50,000 rpm, although the
automotive industry generally has limited them to 15,000 rpm for better reliability and
less downtime should a failure occur. The spindle power required in high-speed
machining is generally on the order of 0.004 W/rpm -much less than in traditional
machining, which is typically in the range from 0.2 to 0.4 W/rpm.

Feed rates in high-speed machining are now up to 1 m/s, and the acceleration rates of
machine-tool components are very high.

Dr.M.Pradeep , CEG,AU
Spindle designs for high speeds require high stiffness and accuracy and generally
involve an integral electric motor. The armature is built onto the shaft, and the stator
is placed in the wall of the spindle housing.
The bearings may be rolling elements or hydrostatic; the latter is more desirable
because it requires less space than the former. Because of inertia effects during the
acceleration and decelaration of machine-tool components, the use of lightweight
materials (including ceramics and composite materials) is an important consideration.

The selection of appropriate cutting-tool materials is, of course, a major

consideration.
it is apparent that (depending on the workpiece material)

• multiphase coated carbides,

• ceramics, cubic-boron nitride, and
• diamond
are all candidate tool materials for this operation.

It also is important to note that high-speed machining should be considered primarily

for operations in which cutting time is a significant portion of the time in the overall
machining operation
Dr.M.Pradeep , CEG,AU
These studies have indicated that high-speed machining is economical for
certain specific applications.

(a) Aluminum structural components for aircraft;

(b) submarine propellers 6 m in diameter, made of a nickel-aluminum-bronze
alloy, and weighing 55,000 kg; and
(c) automotive engines, with 5 to 10 times the productivity of traditional
machining.

High-speed machining of complex three- and five-axis contours has been

made possible by advances in CNC control technology.

Another major factor in the adoption of high-speed machining has been the
requirement to further improve dimensional tolerances in cutting operations

as the cutting speed increases, more and more of the heat generated is
removed by the chip; thus, the tool and (more importantly) the workpiece
remain close to ambient temperature. This is beneficial, because there is no
thermal expansion or warping of the workpiece during machining.

Dr.M.Pradeep , CEG,AU
The important machine-tool characteristics in high-speed machining may be

summarized as follows:

I. Spindle design for stiffness, accuracy, and balance at very high rotational speeds.

2. Bearing characteristics.

3. Inertia of the machine-tool components.

4. Fast feed drives.

5. Selection of appropriate cutting tools.

6. Processing parameters and their computer control.

7. Work-holding devices that can withstand high centrifugal forces.

8. Chip-removal systems that are effective at very high rates of material removal.

Dr.M.Pradeep , CEG,AU
Dr.M.Pradeep , CEG,AU
Dr.M.Pradeep , CEG,AU
Dr.M.Pradeep , CEG,AU
Dr.M.Pradeep , CEG,AU
Dr.M.Pradeep , CEG,AU
Dr.M.Pradeep , CEG,AU
Dr.M.Pradeep , CEG,AU
Applications:
When milling cavities in such hard materials, it is crucial to select adequate cutting and holding
tools for each specific operation; roughing, semi-finishing and finishing. To have success, it is also
very important to use optimised tool paths, cutting data and cutting strategies.

Die casting dies. This is an area where HSM can be utilised in a productive way as most die
casting dies are made of demanding tool steels and have a moderate or small size.

Forging dies. Most forging dies are suitable for HSM due to the shallow geometry that many of
them have. Short tools always results in higher productivity due to less bending (better stability).
Maintenance of forging dies (sinking of the geometry) is a very demanding operation as the
surface is very hard and often also has cracks.

Injection moulds and blow moulds are also suitable for HSM, especially because of their (most
often) small size. Which makes it economical to perform all operations (from roughing to
finishing) in one set up. Many of these mould have relatively deep cavities. Which calls for a very
good planning of approach, retract and overall tool paths. Often long and slender
shanks/extensions in combination with light cutting tools are used.

Milling of electrodes in graphite and copper. An excellent area for HSM. Graphite can be
machined in a productive way with TiCN-, or diamond coated solid carbide endmills. The trend is
that the manufacturing of electrodes and employment
Dr.M.Pradeep , CEG,AU of EDM is steadily decreasing while
HSM is also very often used in direct production of

• Small batch components

• Prototypes and pre-series in Al, Ti, Cu for the Aerospace industry Electric/Electronic
industry Medical industry Defence industry
• Aircraft components, especially frame sections but also engine parts
• Automotive components, GCI and Al
• Cutting and holding tools (through hardened cutter bodies)

Dr.M.Pradeep , CEG,AU
 High speed machining applications
in die and mould manufacturing with a
view to produce surfaces which are very
close to the required final shape
accuracy and the surface quality,
aerospace engineering, production of
critical thin walled components prone
to heat distortion as well as production
of precision parts ...

Dr.M.Pradeep , CEG,AU
Chemical-Mechanical polishing

Dr.M.Pradeep , CEG,AU
With the development of ultra large scale IC (ULSI), chemical mechanical polishing (CMP) has
already become a practical and major planarization technology because of its global and local
planarization ability.
It is not only the most effective method of achieving a nano-scale and ultra-smooth surface
without damage in monocrystalline silicon wafer manufacturing, but also the irreplaceable
planarization method for multi-level wiring on the chip interconnects in ULSI manufacturing.

Dr.M.Pradeep , CEG,AU
A schematic of the CMP system is shown in Fig. 1.1. The system consists of a rotational wafer
carrier, a table supporting the polishing pad, and a slurry feedway. In the CMP process, a rotating
wafer is pressed facedown onto a rotating The polishing slurry, containing submicron or
nanometer abrasive particles and chemical reagents such as the oxidant, activator,
etc.,polishing pad at a proper pressure. flows between the wafer and the pad. A chemical
reaction between chemical reagents in the slurry and the material of the wafer surface is
conducted. A chemical reactant film is formed on the wafer surface and then is removed from
the wafer surface by the abrasive particles in the slurry. The new surface will emerge and the
next CMP cycle begins. In this way, the surface material will be removed from the wafer surface
Dr.M.Pradeep , CEG,AU
by the combined action of chemical reagents and abrasive particles.