Uncinventional Machining Process Unit1

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1 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

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

3

- 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.