Uncinventional Machining Process Unit1
Transcript of Uncinventional Machining Process Unit1
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UNIT I - INTRODUCTION
1.1 Introduction
Machining removes certain parts of the workpieces to change them to final parts.
Traditional, also termed conventional, machining requires the presence of a tool that is harder
than the workpiece to be machined. This tool should be penetrated in the workpiece to a certain
depth. Moreover, a relative motion between the tool and workpiece is responsible for forming or
generating the required shape. The absence of any of these elements in any machining process
such as the absence of tool-workpiece contact or relative motion, makes the process a
nontraditional one. Traditional machining can be classified according to the machining action of
cutting (C) and mechanical abrasion (MA) as shown in Fig 1.
Fig.1 Material removal processes
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1.1.1 Machining by cutting
During machining by cutting, the tool is penetrated in the work material to the depth of
the cut. Arelative (main and feed) motion determines the workpiece geometry required. In this
regard, turning produces cylindrical parts, shaping and milling generate flat surfaces, while
drilling produces holes of different diameters. Tools have a specific number of cutting edges of a
known geometry. The cutting action removes the machining allowance in the form of chips,
which are visible to the naked eye. During machining by cutting, the shape of the workpiece may
be produced by forming when the cutting tool possesses the finished contour of the workpiece. A
relative motion is required to produce the chip (main motion) in addition to the tool feed in depth
as shown in Fig. 3.a. The accuracy of the surface profile depends mostly on the accuracy of
the form-cutting tool. Asurface may also be generated by several motions that accomplish the
chip formation process (main motion) and the movement of the point of engagement along the
surface (feed motion). Fig. 3.b provides a typical example of surface generation by cutting. Slot
milling, shown in Fig. 3.c, adopts the combined form and generation cutting principles.
The resistance of the workpiece material to machining by cutting depends on the
temperature generated at the machining zone. Highspeed hot machining is now recognized as
one of the key manufacturing techniques with high productivity. As the temperature rises, the
strength decreases while the ductility increases. It is quite logical to assume that the high
temperature reduces the cutting forces and energy consumption and enhances the machinability
of the cut material. Hot machining has been employed to improve the machinability of glass and
engineering ceramics. El-Kady et al. (1998) claimed that workpiece heating is intended not only
to reduce the hardness of the material but also to change the chip formation mechanism from a
discontinuous chip to a continuous one, which is accompanied by improvement of the surface
finish. Todd and Copley (1997) built a laser-assisted prototype to improve the machinability of
difficult-to-cut materials on traditional turning and milling centers. The laser beam was focused
onto the workpiece material just above the machining zone. The laser-assisted turning reduced
the cutting force and tool wear and improved the geometrical characteristics of the turned parts.
1.1.2 Machining by abrasion
The term abrasion machining usually describes processes whereby the machining
allowance is removed by a multitude of hard, angular abrasive particles or grains (also called
grits), which may or may not be bonded to form a tool of definite geometry. In contrast to metal
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cutting processes, during abrasive machining, the individual cutting edges are randomly oriented
and the depth of engagement (the undeformed chip thickness) is small and not equal for all
abrasive grains that are simultaneously in contact with the workpiece. The cutting edges
(abrasives) are used to remove a small machining allowance by the MA action during the
finishing processes. The material is removed in the form of minute chips, which are invisible in
most cases (Kaczmarek, 1976). The MA action is adopted during grinding, honing, and
superfinishing processes that employ either solid grinding wheels or sticks in the form of bonded
abrasives (Fig. 3.2a). Furthermore, in lapping, polishing, and buffing, loose abrasives are used as
tools in a liquid machining media as shown in Fig. 3.2b.
Fig. 2 Abrasive machining
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Fig.3 Metal cutting processes
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Nontraditional Machining The greatly improved thermal, chemical, and mechanical properties of the new
engineering materials made it impossible to machine them using the traditional machining
processes of cutting and abrasion. This is because traditional machining is most often based on
the removal of material using tools that are harder than the workpiece. For example, the high
ratio of the volume of grinding wheel worn per unit volume of metal removed (50–200) made
classical grinding suitable only to a limited extent for production of polycrystalline diamond
(PCD) profile tools. The high cost of machining ceramics and composites and the damage
generated during machining are major obstacles to the implementation of these materials. In
addition to the advanced materials, more complex shapes, low-rigidity structures, and micro
machined components with tight tolerances and fine surface quality are often needed. Traditional
machining methods are often ineffective in machining these parts. To meet these demands, new
processes are developed. These methods play a considerable role in the aircraft, automobile, tool,
die, and mold making industries. The nontraditional machining methods (Fig.4) are classified
according to the number of machining actions causing the removal of material from the
workpiece.
1.2 Need for Non Traditional Machining
Conventional machining sufficed the requirement of the industries over the decades. But new
exotic work materials as well as innovative geometric design of products and components were
putting lot of pressure on capabilities of conventional machining processes to manufacture the
components with desired tolerances economically. This led to the development and
establishment of NTM processes in the industry as efficient and economic alternatives to
conventional ones. With development in the NTM processes, currently there are often the first
choice and not an alternative to conventional processes for certain technical requirements. The
following examples are provided where NTM processes are preferred over the conventional
machining process:
Intricate shaped blind hole – e.g. square hole of 15 mmx15 mm with a depth of 30
mm
• Difficult to machine material – e.g. same example as above in Inconel, Ti-alloys or
carbides.
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• Low Stress Grinding – Electrochemical Grinding is preferred as compared to
conventional grinding
• Deep hole with small hole diameter – e.g. φ 1.5 mm hole with l/d = 20
• Machining of composites.
1.3 Classification of Nontraditional machining processes
Fig.4 Classification of Nontraditional machining processes
Single-action nontraditional machining
For these processes only one machining action is used for material removal. These can be
classified according to the source of energy used to generate such a machining action:
mechanical, thermal, chemical, and electrochemical.
Mechanical machining.
Ultrasonic machining (USM) and water jet machining (WJM) are typical examples of single-
action, mechanical, nontraditional machining processes. Machining occurs by MA in USM while
cutting is adopted using a fluid jet in case of WJM. The machining medium is solid grains
suspended in the abrasive slurry in the former, while a fluid is employed in the WJM process.
The introduction of abrasives to the fluid jet enhances the cutting in case of abrasive water jet
machining (AWJM) or ice particles during ice jet machining (IJM).
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Thermal machining.
Thermal machining removes the machining allowance by melting or vaporizing the
workpiece material. Many secondary phenomena relating to surface quality occur during
machining such as microcracking, formation of heat-affected zones, and striations. The source of
heat required for material removal can be the plasma during electro discharge machining (EDM)
and plasma beam machining (PBM), photons during laser beam machining (LBM), electrons in
case of electron beam machining (EBM), or ions for ion beam machining (IBM). For each of
these processes, the machining medium is different. While electrodischarge occurs in a dielectric
liquid for EDM, ion and laser beams are achieved in a vacuum during IBM and LBM as shown
in Fig.5
Fig.5 Mechanical machining processes
While electrodischarge occurs in a dielectric liquid for EDM, ion and laser beams are
achieved in a vacuum during IBM and LBM.
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Chemical and electrochemical machining.
Chemical milling(CHM) and photochemical machining (PCM), also called chemical
blanking (PCB), use a chemical dissolution (CD) action to remove the machining allowance
through ions in an etchant. Electrochemical machining (ECM) uses the electrochemical
dissolution (ECD) phase to remove the machining allowance using ion transfer in an electrolytic
cell (Fig.7).
Hybrid machining
Technological improvement of machining processes can be achieved by combining different
machining actions or phases to be used on the material being removed. A mechanical
conventional single cutting or MA action process can be combined with the respective
machining phases of electrodischarge (ED) in electrodischarge machining (EDM) or ECD in
ECM. The reason for such a combination and the development of a hybrid machining process is
mainly to make use of the combined advantages and to avoid or reduce some adverse effects the
constituent processes produce when they are individually applied. The performance
characteristics of a hybrid process are considerably different from those of the single-phase
processes in terms of productivity, accuracy, and surface quality. Depending on the major
machining phase involved in the material removal, hybrid machining can be classified into
hybrid chemical and electrochemical processes and hybrid thermal machining.
Hybrid chemical and electrochemical processes.
In this family of hybrid machining processes, the major material removal phase is either CD
or ECD. Such a machining action can be combined with the thermal assistance by local heating
in case of laser-assisted electrochemical machining (ECML). In other words, the introduction of
the mechanical abrasion action assists the ECD machining phase during electrochemical grinding
(ECG) and electrochemical super finishing (ECS). Ultrasonic-assisted electrochemical
machining (USMEC) employs an USM component with ECM. The mechanical action of the
fluid jet assists the process of chemical dissolution in electrochemical buffing (ECB). Kozak and
Rajurkar (2000) reported that the mechanical interaction with workpiece material changes the
conditions for a better anodic dissolution process through mechanical depassivation of the
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surface. Under such conditions, removing thin layers of oxides and other compounds from the
anode surface makes the dissolution and smoothing processes more intensive.
Fig.6 Thermal nonconventional processes
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Fig.7 Electrochemical and chemical machining processes
Significant effects of the mechanical machining action have been observed with ultrasonic
waves. The cavitations generated by such vibrations enhance the ECM by improving electrolyte
flushing and hence the material removal from the machined surface.
Hybrid thermal machining.
In this case the main material removal mechanism is a thermal one. The combination of this
phase with the ECD phase, MA action, and ultrasonic (US) vibration generates a family of
double action processes. The triplex hybrid machining is also achievable by combining the
electrodischarge erosion (EDE) phase, the ECD action, and the MA in grinding (G). Such a
combination enhance the rate of material removal and surface quality in electrochemical
discharge grinding (ECDG) and the other hybrid processes shown in Fig.8
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Fig.8 Hybrid machining processes
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1.4 Considerations in Process & Material Selection
1.4.1 Process Selection
Machinability
Relative ease with which a material (usually a metal) can be machined using
appropriate tooling and cutting conditions
• Depends not only on work material
• Type of machining operation, tooling, and cutting conditions are also important
factors
• Machinability Criteria in Production
1. Tool life – how long the tool lasts for the given work material
2. Forces and power – greater forces and power mean lower machinability
3. Surface finish – better finish means better machinability
4. Ease of chip disposal – easier chip disposal means better machinability
Tolerances and Surface Finish
1. Tolerances
o Machining provides high accuracy relative to most other shape-making
processes
o Closer tolerances usually mean higher costs
2. Surface roughness in machining determined by:
o Geometric factors of the operation
o Work material factors
o Vibration and machine tool factors
Selection of Cutting Conditions
• One of the tasks in process planning
• For each operation, decisions must be made about machine tool, cutting
tool(s), and cutting conditions
• Cutting conditions: depth of cut, feed, speed, and cutting fluid
• These decisions must give due consideration to workpart machinability, part
geometry, surface finish, and so forth
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Design of an engineering component involves three interrelated problems:
(i) Selecting a material,
(ii) Specifying a shape, and
(iii) Choosing a manufacturing process.
Getting this selection right the first time by selecting the optimal combination your
design has enormous benefits to any engineering-based business. It leads to lower product costs,
faster time-to-market, a reduction in the number of in-service failures and, sometimes, significant
advantages relative to your competition.
But to realize these benefits, engineers have to deal with an extremely complex problem.
There are literally tens of thousands of materials and hundreds of manufacturing processes. No
engineer can expect to know more than a small subset of this ever-growing body of information.
Furthermore, there are demanding and shifting design requirements such as cost, performance,
safety, risk and aesthetics, as well as environmental impact and recycle-ability. This document is
meant to provide an introduction to the material selection process.
1.4.2 Material Selection
The basic question is how do we go about selecting a material for a given part? This may
seem like a very complicated process until we realize than we are often restrained by choices we
have already made. For example, if different parts have to interact then material choice becomes
limited.
When we talk about choosing materials for a component, we take into account many
different factors. These factors can be broken down into the following areas.
o Material Properties
o The expected level of performance from the material
o Material Cost and Availability
Material must be priced appropriately (not cheap but right)
Material must be available (better to have multiple sources)
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Material Properties
As mechanical engineers we are most concerned with characteristics such as:
1. Mechanical Properties
2. Strength
3. Yield Strength
4. Ultimate Tensile Strength
5. Shear Strength
Ductility
Young’s Modulus
Poisson’s ratio
Hardness
Creep
High or low temperature behavior
Density
Anisotropy
6. Fatigue strength
7. Fracture Toughness
8. Thermal Properties
Thermal expansion coefficient Thermal conductivity
Specific heat capacity
9. Magnetic Properties
10. Fabrication Properties
Ease of machining
Ease of welding, casting, etc
Hardening ability
11. Formability
12. Availability
13. Joining techniques
14. Environmental Properties
Corrosion properties
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Toxic effects
Out-gassing properties
15. Gas and Liquids
16. Viscosity
1.5 Ultrasonic Machining (USM)
Ultrasonic machining is a non-traditional machining process. USM is grouped under the
mechanical group NTM processes. Fig. briefly depicts the USM process.
Fig.9 Ultrasonic machining
In ultrasonic machining, a tool of desired shape vibrates at an ultrasonic frequency (19 ~
25 kHz) with an amplitude of around 15 – 50 μm over the workpiece. Generally the tool is
pressed downward with a feed force, F. Between the tool and workpiece, the machining zone is
flooded with hard abrasive particles generally in the form of water based slurry. 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. Hence, USM is mainly used for machining brittle materials {which are
poor conductors of electricity and thus cannot be processed by Electrochemical and Electro-
discharge machining (ECM and ED)}.
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1.5.1 Elements of the Process
Abrasive Slurry
Abrasive slurry consists of dust of very hard particles. It is filled into the machining zone.
Abrasive slurry can be recycled with the help of pump.
Workpiece
Workpiece of hard and brittle material can be machined by USM. Workpiece is clamped
on the fixture I the setup.
Cutting Tool
Tool of USM does not do the cutting directly but it vibrates with small amplitude and
high frequency. So it is suitable to name the tool as vibrating tool rather than cutting tool. The
tool is made of relatively soft material and used to vibrate abrasive slurry to cut the workpiece
material. The tool is attached to the arbor (tool holder) by brazing or mechanical means.
Sometimes hollow tools are also used which feed the slurry focusing machining zone.
Ultrasonic Oscillator
This operation uses high frequency electric current which passes to an ultrasonic
oscillator and ultrasonic transducer. The function of the transducer is to convert electric energy
into mechanical energy developing vibrations into the tool.
Feed Mechanism
Tool is fed to the machining zone of workpiece. The tool is shaped as same to the cavity
of be produced into the workpiece. The tool is fed to the machining area. The feed rate is
maintained equal to the rate of enlargement of the cavity to be produced.
1.5.2 Mechanisms of Material Removal & MRR in USM
USM can be applied to machine nearly all materials; however it is not economical to use
USM for materials of hardness less than 50 HRC. Generally the workpiece materials are of
stainless steel, cobalt-base heat-resistant steels, germanium, glass, ceramic, carbide, quartz and
semiconductors. It is highly useful in the machining of materials that cannot be machined by any
conventional machining process that are ceramic and glass.
Material removal rate is inversely proportional to the cutting area of the tool. Tool
vibrations also affect the removal rate. The type of abrasive, its size and concentration also
directly affect the MRR Material removal in USM appears to proceed by a complex mechanism
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involving both fracture and plastic deformation to varying degrees, depending on several process
variables.
As has been mentioned earlier, USM is generally used for machining brittle work
material. Material removal primarily occurs due to the indentation of the hard abrasive grits on
the brittle work material. As the tool vibrates, it leads to indentation of the abrasive grits. During
indentation, due to Hertzian contact stresses, cracks would develop just below the contact site,
then as indentation progresses the cracks would propagate due to increase in stress and ultimately
lead to brittle fracture of the work material under each individual interaction site between the
abrasive grits and the workpiece. The tool material should be such that indentation by the
abrasive grits does not lead to brittle failure. Thus the tools are made of tough, strong and ductile
materials like steel, stainless steel and other ductile metallic alloys.
Other than this brittle failure of the work material due to indentation some material
removal may occur due to free flowing impact of the abrasives against the work material and
related solid-solid impact erosion, but it is estimated to be rather insignificant. Thus, in the
current model, material removal would be assumed to take place only due to impact of abrasives
between tool and workpiece, followed by indentation and brittle fracture of the workpiece.
Fig.10 Schematic representation of abrasive grit
During indentation by the abrasive grit onto the workpiece and the tool, the local
spherical bulges contact the surfaces and the indentation process is characterised by db
rather
than by dg. Fig.11 shows the interaction between the abrasive grit and the workpiece and tool.
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Fig.11 Interaction between grit and workpiece and tool
In the current model, all the abrasives are considered to be identical in shape and size. An
abrasive particle is considered to be spherical but with local spherical bulges as shown in Fig.
9.2.2. The abrasive particles are characterized by the average grit diameter, dg. It is further
assumed that the local spherical bulges have a uniform diameter, db
and which is related to the
grit diameter by db
= μdg
2
. Thus an abrasive is characterised by μ and dg.
1.5.3 Process Parameters and their Effects
During discussion and analysis as presented in the previous section, the process
parameters which govern the ultrasonic machining process have been identified and the same are
listed below along with material parameters
• Amplitude of vibration (ao) – 15 – 50 μm
• Frequency of vibration (f) – 19 – 25 kHz
• Feed force (F) – related to tool dimensions
• Feed pressure (p)
• Abrasive size – 15 μm – 150 μm
• Abrasive material – Al2O
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- SiC
- B4C
- Boronsilicarbide
- Diamond
• Flow strength of work material
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• Flow strength of the tool material
• Contact area of the tool – A
• Volume concentration of abrasive in water slurry – C
Fig.12 Depicts the effect of parameters on MRR
1.5.4 Machine
The basic mechanical structure of an USM is very similar to a drill press. However, it
has additional features to carry out USM of brittle work material. The workpiece is mounted on a
vice, which can be located at the desired position under the tool using a 2 axis table. The table
can further be lowered or raised to accommodate work of different thickness. The typical
elements of an USM are (Fig.13)
• Slurry delivery and return system
• Feed mechanism to provide a downward feed force on the tool during machining
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• The transducer, which generates the ultrasonic vibration
• The horn or concentrator, which mechanically amplifies the vibration to the required
amplitude of 15 – 50 μm and accommodates the tool at its tip.
Fig.13 Schematic view of an Ultrasonic Machine
The ultrasonic vibrations are produced by the transducer. The transducer is driven by suitable
signal generator followed by power amplifier. The transducer for USM works on the following
principle
• Piezoelectric effect
• Magnetostrictive effect
• Electrostrictive effect
Magnetostrictive transducers are most popular and robust amongst all. Fig. 9.2.8 shows a
typical magnetostrictive transducer along with horn. The horn or concentrator is a wave-guide,
which amplifies and concentrates the vibration to the tool from the transducer.
1.5.5 Advantages
engineered ceramics CVD ,SiC- Chemical Vapor Deposition Silicon Carbide quartz ,single
crystal materials PCD - Polycrystalline diamond, ferrite graphite ,glassy carbon composites
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,piezoceramics Square cavities, round through holes and crossing beams in a 4-in. borosilicate
wafer.
—including round, square and odd-shaped
thru-holes and cavities of varying depths, as well as OD-ID features—can be machined with high
quality and consistency.
-to-1 are possible, depending on the material type and
feature size.
existing machined features or metallization is possible
without affecting the integrity of the preexisting features or surface finish of the workpiece.
induced in the top layer enhances the fatigue strength of the workpiece.
fractures that might lead to device or application failure over the life of the product.
-traditional processes such as laser beam, and electrical discharge
machining, etc., ultrasonic machining does not thermally damage the workpiece or appear to
introduce significant levels of residual stress, which is important for the survival of brittle
materials in service.
sub-surface damage and no heat-affected zone.
change the metallurgical, chemical or physical properties of the workpiece.
1.5.6 DISADVANTAGES
low mrr. Material removal rates are quite low,
usually less than 50 mm3/min.
abrasive slurry also "machines" the tool itself, thus causing high rate of tool
wear, which in turn makes it very difficult to hold close tolerances.
surface, which limits the accuracy, particularly for small holes.
.
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1.5.7 Economic Considerations
Apart from tool lifetime, the replacement cost of a worn tool (consumable cost) and the
time to replace a worn-out tool are important in machining economics. Machining economics
will be considered in Section. Some different forms of cutting tool have already been illustrated
in Figure 1.12. High speed steel (HSS) tools were traditionally ground from solid blocks. Some
cemented carbide tools are also ground from solid, but the cost of cemented carbide often makes
inserts brazed to tool steel a cheaper alternative. Most recently, disposable, indexable, insert
tooling has been introduced, replacing the cost and time of brazing by the cheaper and quicker
mechanical fixing of a cutting edge in a holder. Disposable inserts are the only form in which
ceramic tools are used, are the dominant form for cemented carbides and are also becoming more
common for high speed steel tools. Typical costs associated with different sizes of these tools, in
forms used for turning, milling and drilling.
Fig.14 Unit Cost Vs Cutting Speed
There are three sorts of information. The second column gives purchase prices. It is the
third column, of more importance to the economics of machining, that gives the tool consumable
costs. A tool may be reconditioned several times before it is thrown away. The consumable cost
Ct is the initial price of the tool, plus all the reconditioning costs, divided by the number of times
it is reconditioned. It is less than the purchase price (if it were more, reconditioning would be
pointless). For example, if a solid or brazed tool can be reground ten times during its life, the
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consumable cost is one tenth the purchase price plus the cost of regrinding. If an indexable
turning insert has four cutting edges (for example, if it is a square insert), the consumable cost is
one quarter the purchase price plus the cost of resetting the insert in its holder (assumed to be
done with the holder removed from the machine tool). If a milling tool is of the insert type, say
with ten inserts in a holder, its consumable cost will be ten times that of a single insert.
1.5.8 Applications
• Used for machining hard and brittle metallic alloys, semiconductors, glass, ceramics,
carbides etc.
• Used for machining round, square, irregular shaped holes and surface impressions.
• Machining, wire drawing, punching or small blanking dies.
1.5.9 Limitations
• Low MRR
• Rather high tool wear • Low depth of hole
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1.6 Recent Developments in Ultrasonic Machining
The recent development of modern hi-tech industries has given rise to the creation of a
whole range of new materials. These include high strength, stainless and heat resistant steels and
alloys, titanium, ceramics, composites, and other nonmetallic materials. These materials may not
be suitable for traditional methods of machining due to the chipping or fracturing of the surface
layer, or even the whole component, and results in a poor product quality. Similarly, the creation
of new materials often highlights some problems unsolvable in a framework of traditional
technologies. In certain cases these problems are caused by the construction of the object and the
requirements particular to it. As an example, in microelectronics, its often necessary to connect
some components without heating them or adding any intermediate layers. This forbids the use
of traditional methods such as soldering or welding. Many of these and similar problems can be
successfully solved using ultrasonic technologies. The USD (Ultrasonic Drilling Machine) uses a
novel drive mechanism to transform the ultrasonic or vibrations of the tip of a horn into a sonic
hammering of a drill bit through an intermediate free-flying mass.
Micro ultrasonic machining (MUSM) is a method derived from conventional ultrasonic
machining, in which a tool and free abrasives are used. The tool that is vibrated at ultrasonic
frequency drives the abrasive to create a brittle breakage on the workpiece surface. The shape
and the dimensions of the workpiece depend on those of the tool. Since the material removal is
based on brittle breakage, this method is suitable for machining brittle materials such as a glass,
ceramics, silicon and graphit. The chip size required for micromachining can be realized when
submicron particles are available for the use as an abrasive. Microtools can be supplied in the
same way as that in micro-EDM, because the same types of tools are used to specify the
corresponding shapes of products, although the microscopic removal phenomenon is completely
different. The major problems are the accuracy of the setup and the dynamics of the equipment.
Ultrasonic vibration of the machining head makes accurate tool holding difficult. The
implementation of a transducer and rotation mechanism is too difficult to maintain high
equipment precision. However, recent development has overcome most of these difficulties. In
the earliest works, the vibrations were applied to the tool, resulting in problems in tool holding
and in machining accuracy. In order to overcome tool holding problems, the on-the machine tool
preparation was introduced. In this approach the tool was soldered to the machine head prior to
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its preparation then machined by wire electro discharge grinding (WEDG) to the desired
dimensions and the subsequent machining of workpiece took place on the same machine tool.
However this method prevents measurement of the size and the shape of the fabricated tool. By
means of the on-the-machine fabrication of the tool, holes in silicon with 20 µm in diameter have
been produced.
The main problem in tool holding accuracy arises from the soldering process that has
been applied to solve the problem of loosening caused by ultrasonic vibration. If the tool material
is soldered before it is fabricated into a micro tool, the completed tool setting is free from the low
accuracy of soldering.
A further development is the introduction of vibrations applied to the workpiece instead
of to the tool. This enables a better tool holding and the use of a high-precision spindle
mechanism. A high- precision rotation/feed mechanism is essential for most machine tools. The
introduction of a vibration mechanism such as that in USM is, therefore, an idea contrary to the
concept of the micromachining. One of the solutions to this problem is to attach the vibration
mechanism to the workpiece side. This will cause little problem in terms of accuracy because a
transducer can simply be inserted between the worktable and the workpiece. Such a setup
enables the use of a universal, highprecision machine head and its influence on workpiece
holding accuracy is negligible.
With MUSM, micro holes of 5 µm in diameter were machined in quartz glass and silicon,
using a tool with diameter of 4 µm and an abrasive with average grain size of 0.2 µm.
Furthermore a microhole with diameter 9 µm and an aspect ratio of 4 was realized in quartz
glass. A major problem in MUSM is the high tool wear ratio. Tungsten carbide is used for tool
fabrication because tools of approximately Ø 5 µm can be machined and it is tough enough to
withstand machining load. However a wear ratio always higher than 0.5 leads to limitations of
the machining efficiency with the impossibility of machining deep holes or multiple holes with
the same tool. The introduction of sintered diamond tools has enabled to overcome the problem,
giving a wear ratio of 0.01 when machining soda glass. However, tools in sintered diamond are
limited to a minimum diameter of 15 µm since they are fragile and tend to break during
machining.
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Rotary ultrasonic machining (RUM) is a hybrid machining process that combines the
material removal mechanisms of diamond grinding with ultrasonic machining (USM), resulting
in higher removal rates than those obtained by either diamond machining or USM alone. As
main tool is made of diamond, steel is not machinable, and its main advantage is obtained
machining fragile materials (ceramics, glass), so its application to metals is quite marginal.
Expected accuracy is in the order of few microns and roughness of 0.2 microns Ra.
In pure ultrasonic machining (USM), the tool, shaped conversely to the desired hole or
cavity, oscillates at high frequency, typically 20 kHz, and is fed into the workpiece by a constant
force. Abrasive slurry composed of water and small abrasive particles is supplied between the
tool tip and the workpiece. Material removal occurs when the abrasive particles impact the
workpiece due to the down stroke of the vibrating tool.
In rotary ultrasonic machining (RUM), a rotating core drill with metal bonded diamond
abrasives is ultrasonically vibrated in the axial direction while the spindle is fed toward the
workpiece at a constant pressure. Coolant pumped through the core of the drill washes away the
swarf, prevents jamming of the drill and keeps it cool. By using abrasives bonded directly on the
tools and combining simultaneous rotation and vibration, RUM provides a fast, high-quality
machining method for a variety of materials.
Material removal in micro USM is by the mechanical action of abrasives as well as by the
cavitation erosion due to rapid pressure changes caused by the ultrasonic vibration of fluid in
working zone. This non-thermal, nonchemical and non-electrical process is especially suitable
for the micro machining of hard brittle and inert insulators such as glass, ceramics, composites,
quartz, precious stones and for the machining of fragile and porous materials such as graphite.
Irregular shaped hard abrasive particles are dispersed in a liquid medium (called abrasive slurry)
and fed into the gap between tool and workpiece. The tool is vibrating with an ultrasonic
frequency (usually 20~40 kHz) with an amplitude of several to tens micrometers. When static
load is applied between tool and workpiece, abrasive particles impact and chip away material
from both workpiece and to a lesser extent from the tool .
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Condition of the abrasive and its grain size affect the machining rate. A continuous flow
of abrasive slurry flushes away the debris from the working zone. Since actual machining is
carried out by abrasive particles, the tool can be softer than the workpiece.
A vibration of the workpiece improves the machining accuracy. It not only simplifies the
structural design of the tool system, but also stirs the abrasive slurry during the machining to
improve renewing particles in the working zone and facilitates debris removal. Precise
measurement of the vibration amplitude at micron level is a challenging task. An online
measurement method proposed in drives the tool tip to touch the workpiece surface and captures
two vertical positions of the surface with respect to turning on and turning off of the vibration.
The difference between two positions is treated as the vibration amplitude. The accuracy of this
measurement method is highly affected by the precision and responding time of driving
components and force sensor. A force sensor with short responding time and high resolution is
required for monitoring and controlling the static load to avoid tool breakage during machining.
In some studies, the static loads are recorded under constant tool feed rate. The static load under
constant tool feed rate usually shows cyclic fluctuate pattern. A closed loop control can be
employed for the better evaluation of the effect of static load as a parameter on machining
characteristics. In this case, the tool feed rate is adjusted to obtain a stable static load.
The mechanism and modeling in macro USM are not yet fully understood. Several
models have been proposed to predict the material removal rate, and most of them are with rough
accuracy in prediction. Material removal mechanism in micro USM is believed to be similar to
conventional USM. However theoretical work in micro USM has rarely been reported and there
is a lack of knowledge about the process behaviours of micro USM under various conditions.
Theoretical models of macro USM may not exactly be applicable to micro USM due to effects
such as difficulty of refreshing of abrasive particle and debris removal caused by the
downscaling of tool and abrasive particle. Existing knowledge is far from sufficient to provide a
complete understanding and instructive rules for industrial users. A tentative mechanistic
modeling of material removal in micro USM was proposed in. The basic assumptions in this
modeling are similar to those in Rotary Ultrasonic Machining (RUSM).
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All materials tending to a brittle fracture behavior can be machined by micro USM.
Examples include high performance ceramics, glass, graphite and a part of the fiber-reinforced
plastics. Geometrical capabilities of micro USM have been testified by drilling, slot machining
and 3D machining. Micro holes with a diameter less than 10 μm were successfully drilled on
silicon, quartz glass and alumina. Slotting on low melting glass has been reported. The tool wear
compensation strategy “Uniform Wear Method” originally developed for micro EDM has been
applied in micro USM to successfully generate 3D micro cavities.
Micro USM has not yet been commercialized with a functional machine tool similar to
micro EDM. However, it is believed that this process could provide solutions to easily and
quickly achieve the larger MEMS structures as well as packaging for both prototype and
production in silicon, glass and ceramic.
Proper selection of micro USM process parameters at present is not well understood due
to lack of related experimental results. Abrasive particle size, vibration amplitude, static load and
tool rotation are the main parameters influencing the micro USM machining speed for the given
workpiece material. It was found that a slight tool rotation drastically improves the drilling
speed. However, there was no significant improvement for speed higher than 50 rpm. The debris
accumulation affects the machining speed in micro USM due to the poor fluidic circulation
around the machining zone. The dependence of dimensional accuracy on tool diameter, vibration
amplitude, and abrasive size needs further research. By means of calculating the maximum
impact force, combined with microcrack models obtained from the research on indentation, it is
possible to correlate the depth of microcracks with process parameters. A predictive model for
the microcrack depth can be employed to optimally select the process parameters.
High tool wear is an intrinsic drawback of micro USM. It is difficult to get a constant
depth of cut due to longitudinal tool wear. Tool wear is affected by parameters such as vibration
amplitude, static load and tends to increase when harder and coarser abrasives are used. As a
consequence, harder abrasives, like diamond, cause higher tool wear than softer abrasives such
as silicon carbide. Therefore, it is necessary to account for and to compensate the tool wear
during machining. The feasibility of applying the “Uniform Wear Method” for generating
accurate 3D microcavities by micro USM has been tested and found that the tool shape remains
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unchanged and the tool wear has been compensated. Due to inadequate research done on tool
wear in micro USM, the selection of tool materials is not well supported by the experimental
data and related analysis. Also, the tool wear mechanisms, the wear rate dependence upon tool
hardness, toughness, abrasive type and size, abrasive hardness and material toughness need to be
studied to reduce and control the tool wear in micro USM.
The micro tools used in micro USM can be prepared by a WEDG unit. A micro tool with
multi tips was made using batch mode micro EDM. Tool material with enough abrasive wear
resistance and small deflection under mechanical load is preferable in micro USM. A PCD tool is
helpful in reducing tool wear.
Some of the important micro USM issues requiring systematical research include the
study of material removal mechanism, innovative tooling, tool wear mechanism and reduction,
on-line sensing, subsurface damage control, and surface roughness improvement. In addition, in-
process monitoring and model-based self tuning strategies are needed for improving the process
stability and performance.