amp notes unit 4-6 by badebhau

UNIT 4.

ADVANCED MANUFACTURING PROCESS

Micro

Machining

Processes

Semester VII – Mechanical Engineering

SPPU

[email protected]

Mo.9673714743

1.

SND COE & RC . Yeola. [email protected]

Mo.9673714743

Syllabus :

1. Diamond Micro Machining (DMM)

2. Ultrasonic Micro Machining (USMM)

3. Micro Electro Discharge Machining (MEDM)

Higher accuracy and performance requirements, coupled with demands to reduce costs,

has led to significant developments in advanced CNC diamond turning and grinding machines. A

long term manufacturing trend in which tolerances for many strategic products are decreasing by

a factor of 3 every 10 years, on critical dimensions, was highlighted in a USA report .

The use of diamond cutting tools has increased in importance as tighter tolerances and

greater surface integrities are required for high value components. Ultra precision cutting tools

need to be hard and sharp and to have enhanced thermal properties in order to maintain their size

and shape while cutting. Advantages offered by diamond include:

- Crystalline structure, which enables very sharp cutting edges to be produced,

- High thermal conductivity, the highest of any materials at room temperature,

- Ability to retain high strength at high temperatures,

- High elastic and shear moduli, which reduce deformation during machining.

The earliest documented evidence of diamond machining found to date describes the

diamond turning carried out by Jesse Ramsden, FRS in 1779. Ramsden machined a screw harned

and tempered steel , with a diamond pointed tool, for use in his linear dividing engine for precision

scale making. Diamond is, however, chemically attacked by ferrous materials at high temperatures,

and is generally unsuitable for the machining of steels and nickel alloys. This is because of the

very high wear rate of the diamond which results in nonviable tool costs.

More recently diamond machining has been used for the machining of nonferrous metals

such as aluminum and copper, which are difficult materials on which to obtain a mirror surface by

grinding, lapping, or polishing. This is because these metals are relatively soft and the abrasive

processes scratch the finished surface and, furthermore, are unable to produce high levels of

flatness at the edges of the machined surface. However, diamond grinding has become an

1. Diamond Micro Machining (DMM)

EaI svaamaI samaqa-

Micro Machining Processes

AMP

Unit.4


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important process for the machining of brittle materials, for example, glasses and ceramics. The

ability to control precisely the cutting tool position relative to the workpiece is a significant

advantage offered by advanced CNC diamond turning and grinding machines. This enables them

to produce components that are extremely precise and accurate. On the other hand, the relative

position of the tool and workpiece is “force” controlled with lapping and polishing. This makes it

very difficult to obtain precise control of the tool's path for shapes other than simple geometric

forms. Diamond machining is therefore proving to be a cost effective process for the production

of complex shaped components that have high accuracy requirements for form and / or surface

finish.

Diamond micromachining is of particular interest for the optical and electronic industries.

The processes are capable of simultaneously achieving high profile accuracy, good surface finish,

and low sub surface damage in brittle materials needed, for example, for semiconductors, magnetic

read-write heads, and optical components.

Single-point diamond turning and ultra-precision diamond grinding are both capable of

producing extremely fine cuts and small chips.

Fig.1 Scanning electron micrograph of electroplated copper, diamond turned at a depth of around 1 nm.

Figure 1. Shows a scanning electron micrograph of electroplated copper cut by a sharp

diamond on an ultra-precision machine tool. The undeformed chip thickness is approximately

1 nm. Because of the very fine chip thickness produced by the micro-cutting processes, the chip-

1.1 MACHINING PRINCIPLES


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forming model for turning is different from that for grinding , moving from concentrated shear to

micro extrusion as shown in Figure 2.

Fig.2 (a) Cutting concentrated shear model . (b) Fine grinding microextrusion model

Important characteristics of materials considered for diamond micromachining are

impurities (inclusions) in the material, grain boundaries of polycrystalline materials, and in

homogeneities. These can cause small vibrations of the cutting tool, resulting in a deterioration in

surface finish. Another factor affecting the quality of surface finish as well as consistency of form

is the high coefficient of expansion coupled with low thermal conductivity of some plastics which

are diamond turned. These thermal effects are, to some extent, minimized when cutting with a

diamond tool due to its sharp cutting edge, low coefficient of friction, and high thermal

conductivity which conducts the heat away. The theoretical peak-to-valley surface roughness

which can be achieved by diamond turning using a round-nosed cutting tool is limited to :

𝐑𝐭 =𝐟𝟐

𝟖∗𝐓𝐫 (1)

Where ,

Rt - Theoretical peak to surface roughness (mm),

F – Feed-rate per revolution of the work-spindle (mm. rev -1),

Tr -Tool nose radius (mm).

Chip

Vc h

Grit

Plastic h

P Vs


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However, this equation ignores any of the errors inherent in machine, and a more accurate

equation takes account of the asynchronous error motion of the machine in the direction normal to

the component surface. The actual peak-to-valley surface roughness now becomes:

𝐑𝐭 =𝐟𝟐

𝟖∗𝐓𝐫+ 𝑓(𝐸𝑠𝑦𝑛) (2)

where Esyn is the asynchronous error motion (mm) in the direction normal to the machined

surface.

The need for the micromachining of hard and brittle materials has led to significant

improvements in machine tool technology. It is now possible to produce plastically deformed

chips, when machining brittle materials, if the depth of cut is sufficiently small. This process is

known as ductile or shear mode machining. It has been shown that a “brittle-to-ductile” transition

exists when cutting brittle materials at low load and penetration levels. This “ductile” mode

machining is important for the cost-effective production of high-performance optical and advanced

ceramic components, with extremely low levels of subsurface damage (micro-cracking). This

enhances their performance and strength significantly and eliminates, or minimizes, the need for

post polishing.

Figure 3. Cutting model for the brittle/ductile regime diamond turning of brittle materials.

The transition from ductile to brittle fracture has been widely reported and is usually

described as the “critical depth of cut.” This is generally small (i.e. 0.1 to 0.3 μm), as is the

associated feed rate, and this results in relatively slow material removal rates. However it is a cost-

1.2 Brittle Materials


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effective technique for producing high quality spherical and non-spherical optical surfaces,

without the need for polishing.

Figure 3. shows the machining model for turning or fly cutting a brittle material , in which

the depth of cut and critical chip thickness (dc) are shown . The location of the critical chip

thickness is dependent on feed. For example, it is located towards the upper edge of the shoulder

when ne feed-rates (f) are used and the micro-fracture damage zone is removed during machining.

In this case subsurface damage does not extend into the cut surface. However, if the feed-rate

increases, the critical chip thickness moves down towards the cut surface and this results in the

micro-fracture damage penetrating into the final cut surface. In micromachining it is normally

important to ensure that these cracks do not occur by removing the material in a ductile mode.

The process requires careful selection of the machining parameters in order to maximize

the material removal rate while maintaining high surface and subsurface integrity. It also demands

high-precision, high stiffness machine tools with smooth motions.

Ductile mode machining is required when machining mirror like surfaces in hard and brittle

materials. However, in order to achieve this condition the actual depths of cut required, to avoid

crack generation, can be on the order of 0.1 to 0.01 of those used for cutting “mirror” surfaces in

metals.

Gerchman and McLain early work on the machining of germanium in which they diamond-

turned germanium to a surface roughness of 5 to 6 nm These were spherical surfaces, 50 mm in

diameter, for which the removal rate was given in terms of 2.5 μm per revolution of the workpiece

together with a 25-μm depths cut. More recently Shore has reported that removal rates the order

of 2 to 4 mm3 per minute have been obtained when diamond turning germanium optics of 100-mm

diameter. The tool life (expressed as the useful cutting distance of the too) when producing optical

surfaces (<1 nm Ra) at these removal rates was in excess of 12 kilometers. When machining silicon

at similar removal rates, as with germanium, tool life was found to be less than 8 kilometers. The

surface finish quality was also on the order of 1 nm Ra. Tool life was higher, when machining zinc

sulphide, being in excess of 20 kilometers, although the surface quality was lower, with a

roughness value of 3.6 nm Ra.

When diamond micro-turning a large area of brittle material (e.g., optical devices) the

continuous use of a single point tool can result in major problems if it is found necessary to change

the cutting tool when partway through a cut. A grinding wheel, however, has innumerable cutting

1.3 RATES OF MICROMACHINING FOR RELEVANT MATERIALS

1.3.1 Diamond Turning

1.3.2 Diamond Grinding


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points (grits) yield a higher machining rate. Diamond micro-grinding therefore be expected to

improve the commercial viability the ductile mode machining of brittle materials.

While grinding is a multipoint process that relies on mechanical actions, it has been that

chemical effects also Play a significant role in material removal rates when using micron-size

abrasive grits to grind glasses in a ductile mode. A relatively soft hydrated layer is formed on the

glass surface the chemical reaction between the coolant and the glass.

For the ductile mode grinding of optical glasses ,the material removal rates of 0.75 to 1.55

mm3 per minute, when normalized for a 100-mm diameter optical component. This value was

obtained when producing surface rough-nesses of 1 to 3 nm Ra, which are close to what can be

achieved by the polishing process. A possible technique to obtain higher removal rates, when

ductile grinding, is to utilize very high grinding wheel speeds. These should, theoretically, reduce

the un-deformed chip thickness, and thus the cutting force per grit, resulting in more ductile flow

coupled with less strength degradation.

Diamond micromachining is used to produce either:

1. Small workpiece features, by means of tools with cutting features below 100 μm, or

2. Sub-μm or nanometric tolerances and/or surface finishes on macro-components.

Very sharp-edged diamond tools have been used in the production of ultrafine optical gratings

with an accuracy of 1 nm, and gratings with 1-nm resolution can now be obtained for use on ultra-

precision machine tools. The most accurate diamond turning and grinding machines currently

available are capable of achieving geometric accuracies of size and profile on the order of 100 nm

for dimensions of 250 mm. Surfaces of 0.8 nm Ra have also been diamond-machined on several

materials, including germanium. Diamond micromachining demands extremely smooth

movements, particularly between the spindle and the tool. In order to achieve this, hydrostatic oil

and air bearings are generally required for the spindles and guide ways. Other stringent

requirements required from the machine tool are:

1. Extremely high loop stiffness between the tool and workpiece,

2. The ability to apply and maintain very small depths of cut, as low as a few nm in some cases,

3. Low thermal drift, and

4. The ability to operate at uniform feed-rates over a wide range.

Other aspects to be considered during the design stage include:

1. The type of coolant and its application, filtration and temperature control,

2. Work-holding methods.

1.4 ACCURACY AND DIMENSIONAL CONTROL OF DMM


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Over the last 30 years the number of applications for diamond turning of a wide range of

materials, and the diamond grind-in£ of brittle materials (e.g. glasses and ceramics) has in-leased

significantly following the technological breakthrough in direct CNC machining in the so-called

“ductile regime,’ is free from retained brittle fracture damage. Diamonds are used in either single

crystal or compacted polycrystalline form to machine a wide range of essentially nonferrous

materials.

Single crystal diamond turning has been used to machine microgrooves 2.5-μm wide by

1.6-μm deep in copper . The slopes of the grooves were produced with a surface finish to 10 nm

Rmax, and the application was the fabrication of lens master discs for the molding of high efficiency

grating lenses. Diamond turning is being used increasingly as a high-precision, high-production

rate process for a wide range of products including:

1. Spherical and aspherical molds for plastic opthalmic lenses, and for medical

instrumentation and micro-laser optical disc/CD players.

2. A wide range of reflecting optics components. For example, aluminum scanner mirrors,

space communication and high-power machining laser optics, and aluminum substrates for

glancing incidence mirrors for X-ray telescopes.

3. Infrared hybrid lenses for thermal imaging systems. Typical materials include germanium,

zinc sulphide, zinc selenide, and silicon.

4. Aluminum alloy automotive pistons which are machined, in the cold state, to complex

profiles with tolerances on the order of 3 to 10 μm.

5. Aluminum alloy substrate drums for photocopying machines.

There are, however, numerous new applications where components are required to have

lower mass, higher hardness and wear resistance, improved chemical inertness, and higher strength

and fatigue life, often while working at higher temperatures than before.

The worldwide research and development, ceramic and intermetallic materials are ready to

be used in gas turbines, pumps, computer peripherals, piston engines, and many other engineering

products on a much wider scale. For many of these applications grinding, in the ductile mode, is

necessary in order to retain material integrity through the minimization of subsurface damage and

micro-cracking, which reduce the strength and fatigue life of ceramic components.

Diamond micromachining technology for the efficient manufacture of opto-electronics

devices is clearly of critical importance in ensuring its progress covering broadband light-wave

1.5 Application of DMM

1.5.1 Diamond Turning

1.5.2 Diamond Grinding


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communication, high-density optical memories, and optical parallel signal processing. Although

process development has concentrated on VSLI technologies such as lithography, there is clearly

scope for applying ductile mode grinding, slitting, and trenching techniques for the efficient

manufacture of monolithic integrated optics components. These techniques are analogous to the

ultra-precision grinding of magnetic memory disk file sliders (or flying heads), but now in the

ductile regime. The introduction of free abrasives into fixed abrasive processing can be shown to

improve surface finish and productivity. Several hundred components can be machined at one set-

up to submicron tolerances, producing low-energy-loss contacting optical surfaces (<2 nm Ra and

zero surface micro-cracks) lending themselves to kinematic design for submicron assembly, with

large consequent savings in assembly and test labor costs.

2.1 Principle of USMM

Micro ultrasonic machining (micro USM), is one of the efficient material removal

processes especially suitable for the micromachining of hard and brittle materials. The principle

of micro USM is shown in Figure.1. In micro USM workpiece which is placed on the workpiece

table vibrates at ultrasonic frequency (40 KHz). Abrasive slurry is injected on the top of the

workpiece. There is a rotating tool which hits the abrasive particles in the slurry which in turn hit

the workpiece and chip away the material from it. The vibrations given to the workpiece aid in

refreshing the slurry so that fresh abrasive particles are in contact with the workpiece and also in

removing the debris from the tool workpiece gap.

The abrasive slurry acts as lubricating agent as well as coolant in reducing the frictional

heat generated due to the movement of the abrasive particles on the workpiece and heat generated

by the vibrations due to the transducer. The slurry also collects the debris from the machined area.

In general micro USM is carried out with water as the medium due to its properties of excellent

coolant, easy removal of debris from machining zone due to low viscosity, low cost and easy

availability.

Particles affect both part and tool : material removal takes place at the workpiece; wear

occurs on both the tool and particles. The process is therefore characterized by removal rate on the

workpiece, tool wear, and abrasive wear.Ultrasonic machining can deal efficiently with brittle

materials. The tools should be made from materials that can resist wear: they should be either

ductile (aluminum alloys, steel, titanium alloys, nickel alloys) or extremely hard (diamond). The

abrasive particles have to be harder than the workpiece material: aluminum oxide, silicon carbide,

boron carbide, and diamond are used.

2. Ultrasonic Micro Machining (USMM)


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In many conventional machining processes like grinding, milling and broaching processes

oil has been successfully used as cutting fluid. These oils can be used either as straight oils, which

are pure petroleum based oils or emulsifiers which are water based oils. Use of straight oils have

excellent lubricating properties and are used especially for machining process involving low

speeds, low clearance requiring high quality surface finish. These oils have more viscosity and

good lubricating properties than water and causes less tool wear. Hence, it may be prudent to use

oil based slurry in micro USM.

An micro USM system based on the design concept of “vibration on work piece ” is

schematically shown in Figure 2 . A micro tool, attached to a mandrel rested on V- shaped block

is rotated by a DC motor and is free to move in X, Y and Z directions with six degrees of freedom.

Micro tools with different diameters are prepared by Wire Electrical Discharge Grinding (WEDG).

The micro tool is sensitive to elastic bending, vibration, and breakage. Therefore, the contact force

between tool and workpiece needs to be controlled and limited to a certain level during machining.

This is achieved by implementing a close-loop control strategy with force feedback. Key system

2.1.1 Working


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components such as electronic balance and three-axis stage have high resolution (0.1 mg and 25

nm, respectively) to meet the demand of accuracy in micro machining.

Fig.2 Schematic of Micro USM

The USM process is able to machine any material, but is more efficient on brittle materials.

Hard materials like stainless steel, glass, ceramics, carbide, quatz and semi-conductors are

machined by this process. It has been efficiently applied to machine glass, ceramics, precision

minerals stones, tungsten.

The actual cutting tools are the abrasive particles. Their characteristics and dimensions

have to be adapted to the material to be machined and to the specific application intended.

The hardness of the abrasive particles has to be higher than that of the workpiece material.

For example, silicon carbide can machine glass, graphite, silicon, aluminium oxide, or precious

stones; boron carbide has to be used for harder materials such as silicon carbide and silicon nitride.

Diamond is the only abrasive able to machine even harder materials like diamond. For ease of use,

and owing to cost, boron carbide is often chosen for machining every material except diamond,

1. X,Y,Z positions of

the tool

2. Tool holder

3. Tool

4. Workpiece

5. Ultrasonic transducer

6. Ultrasonic vibration

generator

7. Static load sensors

8. Static measurement

unit

9. Computer

2.2. Material of Micro USM

2.2.1 Workpiece Material

2.2.2 Abrasive Materials


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and diamond is used only when boron carbide is insufficiently hard.Wear of particles is a crucial

factor, since removal rate depends on particle diameter. Grains can be worn rapidly.

The abrasive slurry contains fine abrasive grains. The grains are usually boron carbide, aluminum

oxide, or silicon carbide ranging in grain size from 100 for roughing to 1000 for finishing. It is

used to microchip or erode the work piece surface and it is also used to carry debris away from

the cutting area .

Tool material should be tough and ductile. Low carbon steels and stainless steels give good

performance. Tools are usually 25 mm long ; its size is equal to the hole size minus twice the size

of abrasives. Mass of tool should be minimum possible so that it does not absorb the ultrasonic

energy. Two main considerations arise in the selection of the tool material; fabrication and cost.

Tool wear during ultrasonic machining also has to be taken into account.

Piezoelectric transducers utilize crystals like quartz whose dimensions alter when

being subjected to electrostatic fields. The charge is directionally proportional to the applied

voltage. To obtain high amplitude vibrations the length of the crystal must be matched to the

frequency of the generator which produces resonant conditions.

In machining with fine sized grains, the MRR increases with the increase in the abrasive

particles due to increase in the number of particles involved in the machining. MRR is more when

machining with the water as oil is more viscous than water hampers the process of debris removal

in the processing of machining thus accounting for less MRR.

Using medium sized particles, MRR increases with the increase in the concentration as

there are cutting edges for a given volume of abrasive slurry. The MRR is more when machined

with oil compared to water as shows the comparison of MRR between water based and oil based

slurry with respect to the varying abrasive slurry concentrations for the particle size of 1-3.

When machining with coarser grains the MRR is more when machined in aqueous medium

compared to oil. As the grains become coarser the grain boundaries try to interlock reducing the

number if cutting edges. Oil possessing more viscous property interlocks these grains strongly

compared to water thus contributing to less MRR.

In general, needed accuracy entails both roughing and finishing, since quality can seldom

be obtained in a single operation. Roughing is performed with large grains (20 to 120 μm) to give

2.2.3 Tool Materials

2.3.4 Piezoelectric Tranducer

2.4 Material removal rate (MRR)

2.5 ACCURACY AND TOLERANCE IN USMM


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sufficient removal rate; finishing is achieved through grains fine enough (0.2 to 10 μm) to obtain

the desired quality. Drilling of very small holes is performed in a single operation.Tool wear is of

major consideration for accuracy, since it affects both tool geometry and dimension.

Accuracy strongly depends on the machining mode (Fig 4.). In sinking, it is the result of tool initial

accuracy tool wear abrasive dimensions and working parameters. The lateral gap between tool and

part is found bet one and two times more than the abrasive main diameter, frontal gap is a little

larger, due to amplitude of vibrations. Fluctuations of gap are smaller for smaller grains. In general

when drilling, the use of roughing (40 μm) and finishing (5 μm) can provide +/-10-μm accuracy.

When finer grains, about 1 μm or less, are used, accuracy can be better than +/-5 μm. This includes

the tool accuracy. It is difficult to provide estimates of accuracy for very small holes (10 to 200

μm), because of difficulties with tool accuracy. In contouring, accuracy can even be better, since

tool imperfections can be compensated by 3-D movements.

In ultrasonic micromachining, since very fine grains are used, a +/—5 μm (or better) accuracy can

be obtained when tools are conventionally made. A higher accuracy can be achieved by using

specially manufactured tools (e.g„ the tool form is produced on the USM machine, by use of wire

EDM).

When machining with finer grains the surface , finer grains having constant cutting edges

hit the workpiece repeatedly. Further the debris is added in the process of material removal

increasing the frictional heat making the surface rougher with the increase in the concentration.

However oil acting as coolant reduces the surface roughness to some extent. Thus producing good

surface finish compared to machining in water medium.

Fluid +

Abrasive Fluid +

Abrasive

sonotrode

2.5.1 Surface Roughness


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Using medium sized abrasives the surface roughness, with the increase in the concentration

the particle size decreases in the process of machining, indicating more number of particles to

absorb the heat generated during the process which eventually reduces the surface roughness.

However surface roughness for oil is less compared to water since water acting as coolant absorbs

the generated heat.

When machining with coarser size grains, as the concentration increases there are more coarser

particles hitting the work surface thus making it rougher. Since oil acts as better coolant than water

thus giving better surface finish.

1. Machining any materials regardless of their conductivity

2. USM apply to machining semi-conductor such as silicon, germanium etc.

3. USM is suitable to precise machining brittle material.

4. Can drill circular or non-circular holes in very hard materials

5. Less stress because of its non-thermal characteristics .

6. It can be used machine hard, brittle, fragile and non conductive material.

7. It is burr less and distortion less processes.

1. USM has low material removal rate.

2. Tool wears fast in micro USM.

3. Machining area and depth is restraint in micro USM.

4. It is difficult to drill deep holes, as slurry movement is restricted.

It is mainly used for

(1) drilling (2) grinding, (3) Profiling (4) coining (5) piercing of dies

(6) welding operations on all materials which can be treated suitably by abrasives.

7. USM can be used to cut industrial diamonds

8. USM is used for grinding Quartz, Glass, ceramics.

Application of USMM can be found in electronics , aerospace , biomedicine , and surgery.

2.6 Advantages

2.7 Disadvantages

2.9 Applications


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Micro-electro-discharge machining (also known as micro-EDM, μ-EDM, and electro-

discharge micromachining) has been developed in the past 30 years from the nonconventional

manufacturing technique of electro-discharge machining (EDM) commonly known as spark

erosion.

While EDM has been used as a production tool for over 50 years, true μ-EDM only

commenced in 1967 when Kurafuji and Masuzawa succeeded in machining 6-μm diameter circular

holes through GTi 10 cemented carbide 50-μm thick, thus demonstrating the rapid production of

high aspect-ratio holes. Since that time, there has been a concerted effort to improve the micro-

machining rates of various materials, without loss of accuracy, and to improve the excellent surface

finish and dimensional control already associated with the EDM technique.

As might be expected, commercial μ-EDM equipment has been produced by companies

in Switzerland and Japan, acknowledged centers of excellence in micro-technology and precision

engineering , μ-EDM is now being to machine a wide variety of miniature and micro-parts from

electrically conductive materials such as metals, alloys, sintered metal, cemented carbides,

ceramics, and silicon , μ-EDM may also be used to produce molds and dies that can themselves be

utilized to manufacture other micro=parts from both conductive and nonconductive materials such

as plastics.

Micro electro discharge machining (Micro-EDM) is a derived form of EDM, which is

generally used to manufacture micro and miniature parts and components by using the

conventional electro discharge machining principles. Similar to conventional EDM, material is

removed by a series of rapidly recurring electric spark discharges between the tool electrode and

the workpiece in Micro-EDM. Actually main differences of Micro- EDM from conventional

EDM are being in the type of pulse generator, the resolution of the X-, Y- and Z- axes movement,

and the size of the tool used. In Micro-EDM; pulse generator produces very small pulses within

pulse duration of a few micro seconds or nano seconds. Because of this reason, Micro-EDM

utilizes low discharge energies to remove small volumes of material. The most important factor

which makes Micro-EDM very important in micromachining is its machining ability on any type

of conductive and semi-conductive materials with high surface accuracy irrespective of material

hardness. It is preferred especially for the machining of difficult-to-cut material due to its high

efficiency and precision.

Small volumetric material removal of Micro-EDM provides substantial opportunities for

manufacturing of micro-dies and micro-structure such as micro holes, micro slot, and micro gears

etc. The use of Micro-EDM has many advantages in micro-parts, the main advantage is that it

can machine complex shapes into any conductive material with very low forces. The forces are

3. Micro Electro Discharge Machining (MEDM)

3.1 Basic Principles of Micro Electro Discharge Machining


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very small because the tool and the workpiece do not come into contact during the machining

process. This property provides advantages to both the tool and the workpiece. For example, in

EDM, a very thin tool can be used because it will not be bent by the machining force. The other

advantages of Micro-EDM include low set-up cost, high aspect ratio, enhanced precision and

large design freedom. In addition, EDM does not make direct contact between the tool electrode

and workpiece material, hence eliminating mechanical stress, chatter and vibration problems

during machining. Therefore, relying on the above advantages, Micro-EDM is very effective to

machine any kind of holes such as small diameter holes down to 10 µm and blind holes with 20

aspect ratio.

Although Micro-EDM is a very efficient process in micro hole machining and having

many advantages, it has also some disadvantages. One of them is that it is a rather slow machining

process; the other is that while the workpiece electrode is being machined, the tool electrode also

wears at a rather significant rate. This tool-wear leads to shape inaccuracies. Another drawback

is the formation of a heat affected layer on the machined surface. Since it is impossible to remove

all the molten part of the workpiece, a thin layer of molten material remains on the workpiece

surface, which re-solidifies during cooling.

Another significant point of Micro-EDM is the inverted polarity of the tool electrode. Due

to polarity effect in conventional EDM with long pulse duration, the tool electrode is usually

charged as anode to increase material removal rate and to reduce electrode wear. At short pulse

durations as used in Micro-EDM, this effect is reversed. Therefore, in Micro-EDM, the tool

electrode is usually charged as cathode.

Micro-EDM can be used as variant machining processes; they are Micro-ED drilling,

Micro-ED milling, Micro-ED die sinking, Micro-ED contouring, Micro-ED dressing, and Micro

wire electrical discharge grinding (Micro-WEDG). All of these processes are integrated in a

today’s sophisticated micro-EDM machines.

Figure 1. shows the photograph of the Micro-EDM setup. The major components of the

Micro-EDM setup are the machine tool itself, the pulse generator, the CCD camera associated

with the monitor and the microscope for analysis. A more detailed image of the Micro-EDM

machine tool is shown in Figure 2. The Figure 2. shows different parts and components of the

Micro-EDM machine tool.


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Figure 1. Micro EDM Set-up

Figure 2. Detailed image of the Micro- EDM machine tool


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Figure 3. Schematic drawing of Micro-EDM system

A machine made four axis movements Micro-EDM machine as it is shown in Figure 2. is

used for micro hole machining. The machine has a pulse generator. This generator is able to

produce pulses from 50 ns to 2 µs with electrical current peak values up to 50 A. Power supply

can vary voltage levels from 50 V to 250 V, but this is limited up to 100 V for the used Micro-

EDM machine. In addition to the micro erosion generator, the micro-EDM machine is also

composed of the following parts; control unit panel. Micro-electro discharge machining (Micro-

EDM) is a process of removing electrically conductive materials by rapid, repetitive spark

discharges from pulse generator with dielectric flowing between tool and work piece.

Electro-discharge machining (EDM) is well suited for micro machining high-strength

conductive materials since neither mechanical contact nor cutting is necessary. Materials such as

stainless steel and tungsten carbide are machined easily with electro-discharge and negligible

cutting forces are applied to the tool or workpiece. The main disadvantage of electrical discharge

is that during each discharge some material is removed from the tool. The melting temperature,

conductivity, and change of yield stress due to temperature determine the wear rate of the tool and

workpiece.

Micro-electro discharge machining process is a widely used micro fabrication technique to

produce micro-parts and components needed in the micro-mechatronic systems and industrial

applications. Micro-hole fabrication is a primary task for this thesis because micro-hole is the most

simple and widely used micro products that can be manufactured by using Micro-EDM.

1. Sarix control unit

2. Wire dressing unit

3. Dielectric liquid

tank

4. GPIB connection

5. Oscilloscope

6. Low resistance3

resistor

7. PC

1.

2.

3.

4.

7.

6.

5.


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The WEDG process, illustrated in Figure 3, is similar to turning on a lathe. A simple RC

circuit generates pulses that produce electrical discharges between the workpiece (anode) and a

φ100 µm brass wire (cathode).

Fig 4.Wire Electro-Discharge Grinding

The discharges occur across a small gap (~ 2µm) filled with dielectric oil. The

workpiece is held vertically in a mandrel that rotates at 3000 RPM, and its position is slowly fed

in the z-direction. The wire is supported on a wire guide, and its position is controlled in the x-

and y-directions. Each electrical discharge erodes material from the workpiece and the brass wire.

To prevent discharges from worn regions of the brass wire, the wire travels at 340 µm/s, and is fed

around a reel and take-up system as illustrated in Figure .5

3.2 Wire Electro-Discharge Grinding (WEDG)


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Fig. 5 Traveling Wire in WEDG

Fig.6.Typical Steps and Conditions for WEDG

A micro shaft is usually produced in three consecutive steps as illustrated in Fig 4. In the

first step, the workpiece is positioned above the traveling wire, and the end of the shaft is machined

by feeding the wire/guide in the x-direction. The second step is to rough cut the shaft and reduce

the diameter of the stock material by feeding the workpiece in the z direction. A high material

removal rate (MRR) is achieved during the rough cut by increasing the energy of each discharge,

which depends upon the energy stored by the capacitor as given in Equation (1). The final step is

to finish cut the shaft. The voltage and capacitance are reduced to achieve improved form and

surface finish. Although the multi-step process is based on the premise that improved precision is

obtained by reducing the capacitance and voltage, a numerical relation for straightness or

roughness is not available.

E=1

2𝐶𝑉2 (1)

Substantial effort has concentrated on the precision of holes or cavities machined by micro

EDM using cylindrical electrodes made by WEDG. Masuzawa et al. [7,8] used a vibroscanning

method to measure holes drilled by micro EDM, and Yu et al. [9,10] developed the uniform wear

method to reduce inaccuracy arising from electrode wear when micromachining cavities. Yu et

al. [11] later studied the influence of current, voltage, layer depth, and feed on the material removal

rate, electrode wear ratio, and gap during contour milling with a cylindrical electrode.

3.3 RATES OF MICROMACHINING FOR RELEVANT MATERIALS

The rate of machining is dependent on the discharge energy, which for the

attainment of fine surface finish is kept low (<10-7 J per pulse) in μ-EDM.


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Various techniques have been utilized to try to increase the material removal rate

such as :

Switching a larger capacitance into the RC circuit, thus producing a large discharge energy,

pielectric flushing,

Jump action and vibrofeeding of the tool, premachining holes to allow debris to flow away

from the electrode , and

Use of controlled pulse generators in WEDM .

The accuracy and dimensional control of parts made by μ -EDM varies with the types of

machines used.

Maximun hole diameter =300 μm

Minimum hole dia. = 10 μm.

Wire material = Tungsten

Wire diameter = 30 μm

Part and part material =injection die and Inbox

Surface finish =0.15 μm

Max. dimensional variations ± 1

Machining time =47 min

The applications of μ-EDM are many and various. :

Micro-shafts and pins

Micropipes

Inkjet nozzles

High aspects ratio holes

High aspects ratio slots

Square cornered cavities

3.4 ACCURACY AND DIMENSIONAL CONTROL

3.5 Application of 𝛍-WEDM


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Dies

Micro-gear wheels

Special orifice

Micro-EDG appears to be the least common form of μ-EDM. However, it has been reported

that 60-mm long channels, 900-μm to deep and 60-μm wide with closed ends have been machined

Fig. Micro-EDG.

into both sides of a stainless steel plate to form part of a microreactor. Such microreactor

structures are also used in mixing chambers, heat exchangers, and pumping systems, and in such

materials as titanium diboride.

The electrode used to form the grooves comprises a round flat disc which is rotated as the

workpiece is moved along its circumference in the opposite direction to the peripheral movement

(see Fig.). A cylindrical electrode has been used to machine microgrippers for high-precision

assembly of RF circuits by pick-and-place units.

**********Thank you ***********

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3.6 Applications of μ-EDM

Unit.5

Additive

Manufacturing

Processes

SPPU Semester VII – Mechanical Engineering

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Bb

*Syllabus :

Introduction and principles, Development of additive manufacturing Technologies,

general additive manufacturing processes, powder based fusion process, extrusion

based system, sheet lamination process, direct write technologies.

Introduction :

Additive Manufacturing (AM) technology came about as a result of developments in a

variety of different technology sectors. Like with many manufacturing technologies,

improvements in computing power and reduction in mass storage costs paved the way for

processing the large amounts of data typical of modern 3D Computer-Aided Design (CAD) models

within reasonable time frames. Nowadays, we have become quite accustomed to having powerful

computers and other complex automated machines around us and sometimes it may be difficult for

us to imagine how the pioneers struggled to develop the first AM machines.

3D printing also known as additive manufacturing is any of various processes used to make

a three-dimensional object. In 3D printing, additive processes are used, in which successive layers

of material are laid down under computer control. These objects can be of almost any shape or

geometry, and are produced from a 3D model or other electronic data source. A 3D printer is a

type of industrial robot.

Additive Manufacturing refers to a process by which digital 3D design data is used to

build up a component in layers by depositing material. The term "3D printing" is increasingly used

as a synonym for Additive Manufacturing. However, the latter is more accurate in that it describes

a professional production technique which is clearly distinguished from conventional methods of

material removal. Instead of milling a work piece from solid block, for example, Additive

Manufacturing builds up components layer by layer using materials which are available in fine

powder form material. A range of different metals, plastics and composite materials may be used.

The technology has especially been applied in conjunction with Rapid Prototyping

(/industries markets /rapid prototyping) - the construction of illustrative and functional prototypes.

Additive Manufacturing is now being used increasingly in Series Production. It gives Original

Equipment Manufacturers (OEMs) in the most varied sectors of industry (/industries markets) the

opportunity to create a distinctive profile for themselves based on new customer benefits, cost-

saving potential and the ability to meet sustainability goals.

Additive Manufacturing Processes Unit-5.

Shri Swami Samarth

AMP

For details Refer CAD-CAM Unit-5 Rapid

Prototyping


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Functional Principle

The system starts by applying a thin layer of the powder material to the building platform. A

powerful laser beam then fuses the powder at exactly the point’s defined by the computer-

generated component design data. The platform is then lowered and another layer of powder is

applied. Once again the material is fused so as to bond with the layer below at the predefined

points. Depending on the material used, components can be manufactured using stereo lithography,

laser sintering or 3D printing.

Development of Additive Manufacturing Technology

Like many other technologies, AM came about as a result of the invention of the computer.

AM takes full advantage of many of the important features of computer technology, both directly

(in the AM machines themselves) and indirectly (within the supporting technology), including:

*Processing power : Part data files can be very large and require a reasonable amount of

processing power to manipulate while setting up the machine and when slicing the data before

building. Earlier machines would have had difficulty handling large CAD data files.

*Graphics capability: AM machine operation does not require a big graphics engine except to

see the file while positioning within the virtual machine space. However, all machines benefit from

a good graphical user interface (GUI) that can make the machine easier to set up, operate, and

maintain.

*Machine control: AM technology requires precise positioning of equipment in a similar way to

a Computer Numerical Controlled (CNC) machining center, or even a high-end photocopy

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machine or laser printer. Such equipment requires controllers that take information from sensors

for determining status and actuators for positioning and other output functions. Computation is

generally required in order to determine the control requirements. Conducting these control tasks

even in real-time does not normally require significant amounts of processing power by today’s

standards. Dedicated functions like positioning of motors, lenses, etc. would normally require

individual controller modules. A computer would be used to oversee the communication to and

from these controllers and pass data related to the part build function.

*Networking: Nearly every computer these days has a method for communicating with other

computers around the world. Files for building would normally be designed on another computer

to that running the AM machine. Earlier systems would have required the files to be loaded from

disk or tape. Nowadays almost all files will be sent using an Ethernet connection, often via the

Internet.

*Integration: As is indicated by the variety of functions, the computer forms a central component

that ties different processes together. The purpose of the computer would be to communicate with

other parts of the system, to process data, and to send that data from one part of the system to the

other. Figure.1 shows how the above mentioned technologies are integrated to form an AM

machine.

Figure.1 General integration of an AM machine

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Without computers there would be no capability to display 3D graphic images. Without 3D

graphics, there would be no Computer-Aided Design. Without this ability to represent objects

digitally in 3D, we would have a limited desire to use machines to fabricate anything but the

simplest shapes. It is safe to say, therefore, that without the computers we have today, we would

not have seen Additive Manufacturing develop. Additive Manufacturing technology primarily

makes use of the output from mechanical engineering, 3D Solid Modeling CAD software. It is

important to understand that this is only a branch of a much larger set of CAD systems and,

therefore, not all CAD systems will produce output suitable for layer-based AM technology.

Currently, AM technology focuses on reproducing geometric form; and so the better CAD systems

to use are those that produce such forms in the most precise and effective way.

NC machining, therefore, only requires surface modeling software. All early CAM systems were

based on surface modeling CAD. AM technology was the first automated computer-aided

manufacturing process that truly required 3D solid modeling CAD. It was necessary to have a fully

enclosed surface to generate the driving coordinates for AM. This can be achieved using surface

modeling systems, but because surfaces are described by boundary curves it is often difficult to

precisely and seamlessly connect these together. Even if the gaps are imperceptible, the resulting

models may be difficult to build using AM. At the very least, any inaccuracies in the 3D model

would be passed on to the AM part that was constructed. Early AM applications often displayed

difficulties because of associated problems with surface modeling software.

Since it is important for AM systems to have accurate models that are fully enclosed, the preference

is for solid modeling CAD. Solid modeling CAD ensures that all models made have a volume and,

therefore, by definition are fully enclosed surfaces. While surface modeling can be used in part

construction, we can not always be sure that the final model is faithfully represented as a solid.

Such models are generally necessary for Computer-Aided Engineering (CAE) tools like Finite

Element Analysis (FEA), but are also very important for other CAM processes.

Additive Manufacturing Processes

The Powder Bed Fusion process includes the following commonly used printing

techniques: Direct metal laser sintering (DMLS), Electron beam melting (EBM), Selective heat

sintering (SHS), Selective laser melting (SLM) and Selective laser sintering (SLS).Powder bed

fusion (PBF) methods use either a laser or electron beam to melt and fuse material powder together.

Electron beam melting (EBM), methods require a vacuum but can be used with metals and alloys

in the creation of functional parts. All PBF processes involve the spreading of the powder material

over previous layers. There are different mechanisms to enable this, including a roller or a blade.

A hopper or a reservoir below of aside the bed provides fresh material supply. Direct metal laser

1. Powder Based Fusion Process

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sintering (DMLS) is the same as SLS, but with the use

of metals and not plastics. The process sinters the

powder, layer by layer. Selective Heat Sintering

differs from other processes by way of using a heated

thermal print head to fuse powder material together.

As before, layers are added with a roller in between

fusion of layers. A platform lowers the model

accordingly.

The technique fuses parts of the layer, and then

moves the working area downwards, adding another

layer of granules and repeating the process until the

piece has built up. This process uses the unfused

media to support overhangs and thin walls in the part

being produced, which reduces the need for temporary auxiliary supports for the piece. A laser is

typically used to sinter the media into a solid.

Fig. 2 Powder bed fusion process

* Powder Bed Fusion – Step by Step

1. A layer, typically 0.1mm thick of material is spread over the build platform.

2. A laser fuses the first layer or first cross section of the model.

part

Energy source (laser)

roller

Build chamber Powder chamber

powder

Inert gas


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3. A new layer of powder is spread across the previous layer using a roller.

4. Further layers or cross sections are fused and added.

5. The process repeats until the entire model is created. Loose, unfused powder is remains in

position but is removed during post processing.

In powder bed fusion, particles of material (e.g., plastic, metal) are selectively fused

together using a thermal energy source such as a laser. Once a layer is fused, a new one is created

by spreading powder over the top of the object and repeating the process. Unfused material is used

to support the object being produced, thus reducing the need for support systems.

Selective laser sintering (SLS) is the first among many similar processes like Direct Metal

Laser Sintering (DMLS), Selective Laser Melting (SLM) and laser cusing. SLS can be defined as

powder bed fusion process used to produce objects from powdered materials using one or more

lasers to selectively fuse or melt the particles at the surface, layer by layer, in an enclosed chamber.

SLM is an advanced form of the SLS process where, full melting of the powder bed particles takes

place by using one or more lasers.

Fig. Laser based powder bed fusion technology

Laser cusing is similar to SLM process where laser is used to fuse each powder bed layer

as per required cross section to build the complete part in the enclosed chamber. The term laser

cusing comes from letter ‘C’ (concept) and the word fusing. The special feature of laser cusing

machine is the stochastic exposure strategy based on the island principle. Each layer of the required

cross section is divided into number of segments called “islands”, which are selected stochastically

during scanning. This strategy ensures thermal equilibrium on the surface and reduces the

component stresses.

1.1 Laser based systems (DMLS/SLM/Laser cusing)

1. Build piston

2. Build platform

3. Powder dispenser

piston

4. Powder dispenser

platform

5. Metal powder supply

6. Recoater arm

7. Laser

8. Lenses

9. Laser beam

10. Sintered part

11. Powder bed

12. XY scanning mirror

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Most of these systems use one fiber laser of 200W to 1 KW capacity to selectively fuse the

powder bed layer. The build chamber is provided with inert atmosphere of argon gas for reactive

materials and nitrogen gas for non-reactive materials. Power of laser source, scan speed, hatch

distance between laser tracks and the thickness of powdered layer are the main processing

parameters of these processes. Layer thickness of 20-100 µm can be used depending on the

material. All of these processes can manufacture fully dense metallic parts from wide range of

metal alloys like titanium alloys, inconel alloys, cobalt chrome, aluminium alloys, stainless steels

and tool steels.

Most of the laser based PBF systems have low build rates of 5-20 cm 3/hr and maximum

part size that can be produced (build volume) is limited to 250 x 250 x 325 mm 3 which increases

part cost and limits its use only for the small sized parts. So in recent years, the machine

manufactures and the research institutes are focusing on expanding the capabilities of their

machines by increasing the build rates and the build volumes. SLM solution from Germany has

launched SLM500 HL machine in 2012 which uses double beam technology to increase the build

rate up to 35 cm 3/hr and has a build volume of 500 x 350 x 300 mm 3.Two sets of lasers are used

in this machine, each set having two lasers (400W and 1000W). This means four lasers scan the

powder layer simultaneously.

EBM is another PBF based AM process in which electron beam is used to selectively fuse

powder bed layer in vacuum chamber. Electron beam melting (EBM) process is similar to the

SLM with the only difference being its energy source used to fuse powder bed layers: here an

electron beam is used instead of the laser . In EBM, a heated tungsten filament emits electrons at

high speed which are then controlled by two magnetic fields, focus coil and deflection coil as

shown in Fig.4a. Focus coil acts as a magnetic lens and focuses the beam into desired diameter up

to 0.1 mm whereas deflection coil deflects the focused beam at required point to scan the layer of

powder bed. When high speed electrons hit the powder bed, their kinetic energy gets converted

into thermal energy which melts the powder. Each powder bed layer is scanned in two stages, the

preheating stage and the melting stage. In preheating stage, a high current beam with a high

scanning speed is used to preheat the powder layer (up to 0.4 - 0.6 T m) in multiple passes. In

melting stage, a low current beam with a low scanning speed is used to melt the powder . When

scanning of one layer is completed, table is lowered, another powder layer is spread and the process

repeats till required component is formed. The entire EBM process takes place under high vacuum

of 10 -4 to 10 -5 mbar. The helium gas supply during the melting further reduces the vacuum

pressure which allows part cooling and provides beam stability . It also has multi-beam feature

which converts electron beam into several individual beams which can heat, sinter or melt powder

bed layer .

1.2 Electron beam melting (EBM)

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ARCAM EBM system uses high power electron beam of 3000 W capacity to melt powder bed

layers. Electron beam power, current, diameter of focus, powder pre-heat temperature and layer

thickness are main processing parameters of the EBM. Layer thickness of 50-200 µm is typically

used in this process . EBM systems can work with wide range of materials like titanium alloys

(Ti6Al4V, Ti6Al4V EI), cobalt chrome, Titanium aluminide, inconel (625 and 718), stainless

steels, tool steels, copper, aluminium alloys, beryllium etc.

Fig. Schematic of EBM process

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Fig. Steps in EBM process

* Materials Used

The Powder bed fusion process uses any powder based materials, but common metals and

polymers used are:

SHS: Nylon DMLS, SLS,

SLM: Stainless Steel, Titainium, Aluminium, Cobalt Chrome, Steel

EBM: titanum, Cobalt Chrome, ss, al and copper

* Advantages:

1. Relatively inexpensive

2. Suitable for visual models and prototypes

3. (SHS) Ability to integrate technology into small scale, office sized machine.

4. Powder acts as an integrated support structure.

5. Large range of material options.

Build plate heating

Powder spreading

Powder preheating scan

Powder melting scan

Build Table lowering

Repeat process till part completion

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* Disadvantages:

1. Relatively slow speed (SHS).

2. Lack of structural properties in materials.

3. Size limitations.

4. High power usage.

5. Finish is dependent on powder grain size.

COMPARISON BETWEEN SLM AND EBM

As compared to the SLM system, the EBM has higher build rates (upto 80cm 3/hr because

of the high energy density and high scanning speeds) but inferior dimensional and surface finish

qualities.

In both the SLM/EBM process, because of rapid heating and cooling of the powder layer,

residual stresses are developed. In EBM, high build chamber temperature (typically 700- 900 0C)

is maintained by preheating the powder bed layer. This preheating reduces the thermal gradient in

the powder bed and the scanned layer which reduces residual stresses in the part and eliminates

post heat treatment required. Preheating also holds powder particles together which can acts as

supports for overhanging structural members. So, supports required in the EBM are only for heat

conduction and not for structural support. This reduces the number of supports required and allows

manufacturing of more complex geometries. Powder preheating feature is available in very few

laser based systems where it is achieved by platform heating. In addition, entire EBM process

takes place under vacuum since, it is necessary for the quality of the electron beam. Vacuum

environment reduces thermal convection, thermal gradients and contamination and oxidation of

parts like titanium alloys . In SLM, part manufacturing takes place under argon gas environment

for reactive materials to avoid contamination and oxidation whereas non-reactive materials can be

processed under nitrogen environment. So it can be expected that EBM manufactured parts have

lower oxygen content than SLM manufactured parts .

In spite of having these advantages, EBM is not as popular as SLM because of its higher

machine cost, low accuracy and non-availability of large build up volumes. Characteristic features

of SLM and EBM are summarized in Table 1.

TABLE I. CHARACTERISTIC FEATURES OF SLM AND EBM

SLM EBM

Power source One or more fiber lasers of 200

to 1000 W High power Electron beam

of 3000 W

Build chambcr environment Argon or Nitrogen Vacuum / He bleed

Method of powder preheating Platform heating Preheat scanning

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Powder preheating temperature (°

C) [3435] 100-200 700-900

Maximum available build volume

(mm) 500 x 350 x 300 350 x 38O(0xH)

Maximum build rate (cm /hr) 20-35 80

Layer thickness (pm) 20-100 50-200

Melt pool size (mm) 0.1-0.5 0.2-1.2

Surface finish [7] (Ra) 4-11 25-35

Geometric tolerance (mm) [12] ±0.05-0.1 ± 0.2

Minimum feature size(jim) [39] 40-200 100

Fuse deposition modelling (FDM) is a common material extrusion process and is

trademarked by the company Stratasys. Material is drawn through a nozzle, where it is heated and

is then deposited layer by layer. The nozzle can move horizontally and a platform moves up and

down vertically after each new layer is deposited. It is a commonly used technique used on many

inexpensive, domestic and hobby 3D printers.

The process has many factors that influence the final model quality but has great potential

and viability when these factors are controlled successfully. Whilst FDM is similar to all other 3D

printing processes, as it builds layer by layer, it varies in the fact that material is added through a

nozzle under constant pressure and in a continuous stream. This pressure must be kept steady and

at a constant speed to enable accurate results .Material layers can be bonded by temperature control

or through the use of chemical agents. Material is often added to the machine in spool form as

shown in the diagram.

2. Extrusion Based System

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* Material Extrusion – Step by Step

1. First layer is built as nozzle deposits material where required onto the cross sectional area

of first object slice.

2. The following layers are added on top of previous layers.

3. Layers are fused together upon deposition as the material is in a melted state.

Material Extrusion operates in a similar fashion to a hot glue gun; plastic filament is heated

to a malleable state and extruded through a nozzle. In order to create a part, a CAD model is sliced

into layers.

If the part has large overhangs, support material is required to prevent sagging and protect

part integrity. This support material is created either through thin, breakable trusses of the build

material or a second soluble material.

Advantages of the material extrusion process include use of readily available ABS plastic,

which can produce models with good structural properties, close to a final production model. In

low volume cases, this can be a more economical method than using injection moulding. However,

the process requires many factors to control in order to achieve a high quality finish. The nozzle

which deposits material will always have a radius, as it is not possible to make a perfectly square

nozzle and this will affect the final quality of the printed object. Accuracy and speed are low when

compared to other processes and the quality of the final model is limited to material nozzle

Material spool

Object/ model

Support material

Nozzle

Heated Element

Build Platform


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thickness .When using the process for components where a high tolerance must be achieved,

gravity and surface tension must be accounted for. Typical layer thickness varies from 0.178 mm

– 0.356 mm.

* Materials Used

The Material Extrusion process uses polyers and plastics.

Polymers: ABS, Nylon, PC, PC, AB

* Advantages:

1. Widespread and inexpensive process.

2. ABS plastic can be used, which has good structural properties and is easily accessible.

* Disadvantages:

1. The nozzle radius limits and reduces the final quality .

2. Accuracy and speed are low when compared to other processes and accuracy of the final

model is limited to material nozzle thickness.

3. Constant pressure of material is required in order to increase quality of finish.

Sheet lamination processes include ultrasonic additive manufacturing (UAM) and

laminated object manufacturing (LOM). The Ultrasonic Additive Manufacturing process uses

sheets or ribbons of metal, which are bound

together using ultrasonic welding.

The process does require additional CNC

machining and removal of the unbound metal,

often during the welding process. Laminated object

manufacturing (LOM) uses a similar layer by layer

approach but uses paper as material and adhesive

instead of welding. The LOM process uses a cross

hatching method during the printing process to

allow for easy removal post build. Laminated

objects are often used for aesthetic and visual

models and are not suitable for structural use.

UAM uses metals and includes aluminium, copper,

stainless steel and titanium. The process is low

3. Sheet Lamination Process

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temperature and allows for internal geometries to be created. The process can bond different

materials and requires relatively little energy, as the metal is not melted.

* Sheet Lamination – Step by Step

1. The material is positioned in place on the cutting bed.

2. The material is bonded in place, over the previous layer, using the adhesive.

3. The required shape is then cut from the layer, by laser or knife.

4. The next layer is added.

5. Steps two and three can be reversed and alternatively, the material can be cut before being

positioned and bonded.

6.Sheet is adhered to a substrate with a heated roller.

7. Laser traces desired dimensions of prototype.

8. Laser cross hatches non-part area to facilitate waste removal.

9. Platform with completed layer moves down out of the way.

10. Fresh sheet of material is rolled into position.

11. Platform downs into new position to receive next layer.

12. The process is repeated.


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Laminated object manufacturing (LOM) is one of the first additive manufacturing

techniques created and uses a variety of sheet material, namely paper. Benefits include the use of

A4 paper, which is readily available and inexpensive, as well as a relatively simple and inexpensive

setup, when compared to others.

The Ultrasonic Additive Manufacturing (UAM) process uses sheets of metal, which are

bound together using ultrasonic welding. The process does require additional CNC machining of

the unbound metal. Unlike LOM, the metal cannot be easily removed by hand and unwanted

material must be removed by machining. Material saving metallic tape of 0.150mm thick and

25mm wide does however, result in less material to cut off afterwards. Milling can happen after

each layer is added or after the entire process. Metals used include aluminium, copper, stainless

steel and titanium. The process is low temperature and allows for internal geometries to be created.

One key advantage is that the process can bond different materials and requires relatively little

energy as the metal is not melted, instead using a combination of ultrasonic frequency and pressure.

Overhangins can be built and main advantage of embedding electronics and wiring . Materials are

bonded and helped by plastic deformation of the metals. Plastic deformation allows more contact

between surface and backs up existing bonds .

Post processing requires the extraction of the part from the surrounding sheet material.

With LOM, cross hatching is used to make this process easier, but as paper is used, the process

does not require any specialist tools and is time efficient. Whilst the structural quality of parts is

limited, adding adhesive, paint and sanding can improve the appearance, as well as further

machining.

* Materials

Effectively any sheet material capable of being rolled. Paper, plastic and some sheet metals.

The most commonly used material is A4 paper.

* Advantages: 1. Benefits include speed, low cost, ease of material handling, but the strength and integrity

of models is reliant on the adhesive used .

2. Cutting can be very fast due to the cutting route only being that of the shape outline, not

the entire cross sectional area

3. Relatively large parts may be made.

4. Paper models have wood like characteristics, and may be worked and finished accordingly

* Disadvantages: 1. Finishes can vary depending on paper or plastic material but may require post processing

to achieve desired effect

2. Limited material use

3. Fusion processes require more research to further advance the process into a more

mainstream positioning.

4. Dimensional accuracy is slightly less

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Direct-write technologies are the most recent and novel approaches to the fabrication of

electronic and sensor devices, as well as integrated power sources, whose sizes range from the

meso- to the nanoscales. The term direct write refers to any technique or process capable of

depositing, dispensing, or processing different types of materials over various surfaces following

a preset pattern or layout. The ability to accomplish both pattern and material transfer processes

simultaneously represents a paradigm shift away from the traditional approach for device

manufacturing based on lithographic techniques. However, the fundamental concept of direct

writing is not new. Every piece of handwriting, for instance, is the result of a direct-write process

whereby ink or lead is transferred from a pen, or pencil onto paper in a pattern directed by our

hands. The immense power and potential of direct writing lies in its ability to transfer and/or

process any type of material over any surface with extreme precision resulting in a functional

structure or working device.

Direct-write technologies are a subset of the larger area of rapid prototyping and deal with

coatings or structures considered to be two-dimensional in nature. With the tremendous

breakthroughs in materials and the methods used to apply them, many of which are discussed in

this book, direct-write technologies are poised to be far-reaching and influential well into the

future. The industry's push toward these technologies and the pull from applications rapidly

changing circuits, designs, and commercial markets are documented for the first time here.

Although direct-write technologies are serial in nature, they are capable of generating patterns, of

high-quality electronic, sensor, and biological materials among others--at unparalleled speeds,

rendering these technologies capable of satisfying growing commercial demands.

4.1. Laser Direct-Write From the earliest work on laser interactions with materials, direct-write processes have

been important and relevant techniques to modify, add, and subtract materials for a wide variety

of systems and for applications such as metal cutting and welding. In general, direct-write

processing refers to any technique that is able to create a pattern on a surface or volume in a serial

or “spot-by-spot” fashion. This is in contrast to lithography, stamping, directed self-assembly, or

other patterning approaches that require masks or pre-existing patterns. At first glance, one may

think that direct-write processes are slower or less important than these parallelized approaches.

However, direct-write allows for precise control of material properties with high resolution and

enables structures that are either impossible or impractical to make with traditional parallel

techniques. Furthermore, with continuing developments in laser technology providing a decrease

in cost and an increase in repetition rates, there is a plethora of applications for which laser direct-

write (LDW) methods are a fast and competitive way to produce novel structures and devices.

This issue of MRS Bulletin seeks to assess the current status and future opportunities of LDW

processes in the context of emerging applications.

4. Direct Write Technologies

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In LDW, the beam is typically focused or collimated to a small spot (in industrial

processes, this “small” spot can be several millimeters in diameter). Patterning is achieved by

either rastering the beam above a fixed surface or by moving the substrate or part within a fixed

beam. An important feature of LDW is that the desired patterns can be constructed in both two

and three dimensions on arbitrarily shaped surfaces, limited only by the degrees of freedom and

resolution of the motion-control apparatus. In this manner, LDW can be considered a “rapid

prototyping” tool, because designs and patterns can be changed and immediately applied without

the need to fabricate new masks or molds.

The key elements of any LDW system can be divided into three subsystems: (1) laser

source, (2) beam delivery system, and (3) substrate/target mounting system (Shown in Figure

1). At the heart of any LDW process is the laser source. Typical experiments and applications use

anywhere from ultrafast femtosecond-pulsed systems to continuous-wave systems employing

solid-state, gas, fiber, semiconductor, or other lasing media. In choosing an appropriate source,

one must consider the fundamental interactions of lasers with the material of interest. This requires

knowledge of the pulse duration, wavelength, divergence, and other spatial and temporal

characteristics that determine the energy absorption and the material response. In beam delivery,

there are a variety of ways to generate a laser spot, including fixed focusing objectives and mirrors,

galvanometric scanners, optical fibers, or even fluidic methods such as liquid-core wave- guides

or water jets. The choice depends on the application demands, for instance, the required working

distances, the focus spot size, or the energy required. The ultimate beam properties will be

determined by the combination of laser and beam delivery optics. Finally, the substrate mounting

is done in accordance with experimental or industrial requirements and can be manipulated in

multiple directions to achieve a desired result. Robotics and active feedback control, on either

the substrate or beam delivery optics, can add further design flexibility to the technique.

There is a vast range of LDW processes. For the purposes of this issue, we categorize them

into three main classes: 1.laser direct- write subtraction (LDW-), where material is removed by

ablation; 2.laser direct-write modification (LDWM), where material is modified to produce a

desired effect; and 3.laser direct-write addition (LDW+), where material is added by the laser.


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Figure 1. Schematic illustration of a laser direct-write system.

The basic components of an LDW system are (left to right) a substrate mounting system, a

beam delivery system, and a laser source. Motion control of either the beam delivery system or the

substrate mounting system is typically accomplished using computer-assisted design and

manufacturing (CAD/CAM) integrated with the laser source.

4.1.1. Laser Direct-Write Subtraction (LDW–)

LDW (-) is the most common type of laser direct-write. In general, this entails processes

that result in photochemical, photo thermal, or photo physical ablation on a substrate or target

surface, directly leading to the features of interest. Common processes include laser scribing, cut-

ting, drilling, or etching to produce relief structures or holes in materials in ambient or controlled

atmospheres. Industrial applications using this technique range from high-throughput steel

fabrication, to inkjet and fuel-injection nozzle fabrication, to high-resolution manufacturing and

texturing of stents or other implantable biomaterials. At a smaller scale, inexpensive bench top

laser cutting and en- graving systems can be purchased by the hobbyist or small company for

artistic and architectural renderings. More recent developments in LDW- include chemically

assisted techniques such as laser-drilling ceramics or biomaterials and laser-induced backside wet

etching (LIBWE) of glass. In fact, one may also consider laser cleaning to be a controlled LDW-

process. The fundamental interactions leading to material removal can be thermal or a thermal,

depending primarily on the material/environment characteristics and the pulse duration of the laser.

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These interactions have a direct effect on the quality of the resulting features. For instance, a heat-

affected zone (HAZ) tends to occur in the vicinity of thermally removed material. This region has

structures and properties that can differ from the bulk material and can exhibit additional surface

relief. Either of these effects may be beneficial or detrimental, depending on the application. In

contrast, a thermal and multiphoton absorption processes caused by ultrafast lasers can reduce the

formation of a HAZ and enable features smaller than the diffraction limit.

4.1.2. Laser Direct-Write Modification (LDWM)

In LDWM, the incident laser energy is usually not sufficient to cause ablative effects but

is sufficient to cause a permanent change in the material properties. Typically, these processes rely

on thermal modifications that cause a structural or chemical change in the material. A common

example of such processes is the rewritable compact disc, in which a diode laser induces a phase

transition between crystalline and amorphous material. In industrial applications, one may consider

laser cladding, where a surface layer different from the bulk material is produced through melting

and resolidification, or solid free-form fabrication (SFF) approaches such as selective laser

sintering (SLS), as important modifying processes that would fall under the umbrella of LDWM.

Many LDWM applications require a specific optical response in the material of interest beyond

simple thermal effects. Optically induced defects or changes in mechanical properties can lead to

many non-ablative material modifications. For instance, photoresists respond to light by breaking

or reforming bonds, leading to pattern formation in the material. Alternatively, LDW can cause

defects in photo- etchable glass ceramics or other optical materials through single- and multiphoton

mechanisms, enabling novel applications in optical storage, photonic devices, and microfluidics.

4.1.3. Laser Direct-Write Addition (LDW+)

LDW+ is perhaps the most recent of the laser direct-write processes. In this technique,

material is added to a substrate using various laser-induced processes. Many techniques are derived

from laser- induced forward transfer (LIFT), where a sacrificial substrate of solid metal is

positioned in close proximity to a second substrate to receive the removed material. The incident

laser is absorbed by the material of interest, causing local evaporation. This vapor is propelled

toward the waiting substrate, where it recondenses as an individual three-dimensional pixel, or

voxel, of solid material. Such an approach has found important use in circuit and mask repair and

other small-scale applications where one needs to deposit material locally to add value to an

existing structure. This general technique has significant advantages over other additive direct-

write processes, in that these laser approaches do not require contact between the depositing

material and a nozzle, and can enable a broad range of materials to be transferred. Variations on

the general LIFT principle allow liquids, inks, and multi- phase solutions to be patterned with

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computer-controlled accuracy for use in a variety of applications such as passive electronics or

sensors.

Alternatively, LDW+ techniques can rely on optical forces to push particles or clusters into

precise positions, or on chemical changes in liquids and gases to produce patterns. For instance,

laser-induced chemical vapor deposition, or multiphoton polymerization schemes of liquid

photoresists, can be used to fabricate three-dimensional stereographic patterns. Examples of this

have been demonstrated and show promise for many applications such as fabricating photonic

structures or biological scaffolding.

*Applications

In many cases, applications tend to drive the development of new technologies, and direct

writing is one such technology. The need for direct writing electronic and sensor materials is

founded in exciting and often revolutionary applications, numerous examples of which will be

given here. The specific applications presented individually in each chapter are representative of

some areas where direct-write technologies could have an impact. As successful applications are

commercialized demonstrating the inherent flexibility of direct-write techniques the potential for

using direct-write products in other areas grows. Part I is devoted to applications of direct-write

material deposition, in particular, applications to defense electronics, chemical and biological

sensors, industrial applications, and small-scale power-management applications. Other exciting

applications are on the horizon for use in medicine, tissue engineering, wireless and other

communications, optoelectronics, and semiconductors.

Directed Energy Deposition (DED) covers a range of terminology: ‘Laser engineered net

shaping, directed light fabrication, direct metal deposition, 3D laser cladding’ It is a more complex

printing process commonly used to repair or add additional material to existing components.

A typical DED machine consists of a nozzle mounted on a multi axis arm, which deposits

melted material onto the specified surface, where it solidifies. The process is similar in principle

to material extrusion, but the nozzle can move in multiple directions and is not fixed to a specific

axis. The material, which can be deposited from any angle due to 4 and 5 axis machines, is melted

upon deposition with a laser or electron beam. The process can be used with polymers, ceramics

but is typically used with metals, in the form of either powder or wire.

Typical applications include repairing and maintaining structural parts.

5. Directed Energy Deposition

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* Direct Energy Deposition – Step by Step

1. A4 or 5 axis arm with nozzle moves around a fixed object.

2. Material is deposited from the nozzle onto existing surfaces of the object.

3. Material is either provided in wire or powder form.

4. Material is melted using a laser, electron beam or plasma arc upon deposition.

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5. Further material is added layer by layer and solidifies, creating or repairing new material features

on the existing object.

The DED process uses material in wire or powder form. Wire is less accurate due to the

nature of a pre- formed shape but is more material efficient when compared to powder (Gibson et

al., 2010), as only required material is used. The method of material melting varies between a laser,

an electron beam or plasma arc, all within a controlled chamber where the atmosphere has reduced

oxygen levels. With 4 or 5 axis machines, the movement of the feed head will not change the flow

rate of material, compared to fixed, vertical deposition.

* Materials

The Electron Beam Melting process uses metals and not polymers or ceramics.

Metals: Cobalt Chrome, Titanium

* Advantages: 1. Ability to control the grain structure to a high degree, which lends the process to repair work of

high quality, functional parts.

2. A balance is needed between surface quality and speed, although with repair applications, speed

can often be sacrificed for a high accuracy and a pre- determined microstructure.

* Disadvantages:

1. Finishes can vary depending on paper or plastic material but may require post processing to

achieve desired effect.

2. Limited material use

3. Fusion processes require more research to further advance the process into a more mainstream

positioning

6. Material Jetting (Not in Syllabus)

Material jetting creates objects in a

similar method to a two dimensional ink

jet printer. Material is jetted onto a build

platform using either a continuous or Drop

on Demand (DOD) approach. Material is

jetted onto the build surface or platform,

where it solidifies and the model is built

layer by layer. Material is deposited from

a nozzle which moves horizontally across

the build platform. Machines vary in

complexity and in their methods of

controlling the deposition of material. The

material layers are then cured or hardened

using ultraviolet (UV) light. As material must be deposited in drops, the number of materials

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available to use is limited. Polymers and waxes are suitable and commonly used materials, due to

their viscous nature and ability to form drops.

* Material Jetting – Step by Step

1. The print head is positioned above build platform.

2. Droplets of material are deposited from the print head onto surface where required, using either

thermal or piezoelectric method.

3. Droplets of material solidify and make up the first layer.

4. Further layers are built up as before on top of the previous.

5. Layers are allowed to cool and harden or are cured by UV light. Post processing includes

removal of support material.

Drop on Demand (DOD) is used to dispense material onto the required surface. Droplets

are formed and positioned into the build surface, in order to build the object being printed, with

further droplets added in new layers until the entire object has been made. The nature of using

droplets, limits the number of materials available to use. Polymers and waxes are often used and

are suitable due to their viscous nature and ability to form drops. Viscosity is the main determinant

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in the process; there is a need to re-fill the reservoir quickly and this in turn affects print speed.

Unlike a continuous stream of material, droplets are dispensed only when needed, released by a

pressure change in the nozzle from thermal or piezoelectric actuators. Thermal actuators deposit

droplets at a very fast rate and use a thin film resistor to form the droplet. The piezoelectric method

is often considered better as it allows a wider range of materials to be used. The designs of a typical

DOD print head changes from one machine to another but according to Ottnad, typically include

a reservoir, sealing ring, Piezo elements and silicon plate with nozzle, held together with high

temperature glue.

* Materials The material jetting process uses polymers and plastics.

Polymers: Polypropylene, HDPE, PS, PMMA, PC, ABS, HIPS, EDP

* Advantages: 1. The process benefits from a high accuracy of deposition of droplets and therefore low waste.

2. The process allows for multiple material parts and colours under one process.

* Disadvantages: 1.Support material is often required.

2. A high accuracy can be achieved but materials are limited and only polymers and waxes can be

used.

7. Binder Jetting (Not in syllabus)

The binder jetting process uses two

materials; a powder based material and a binder.

The binder acts as an adhesive between powder

layers. The binder is usually in liquid form and

the build material in powder form. A print head

moves horizontally along the x and y axes of the

machine and deposits alternating layers of the

build material and the binding material. After

each layer, the object being printed is lowered

on its build platform.

Due to the method of binding, the material

characteristics are not always suitable for

structural parts and despite the relative speed of

printing, additional post processing (see below)

can add significant time to the overall process.

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* Binder Jetting – Step by Step

1. Powder material is spread over the build platform using a roller.

2. The print head deposits the binder adhesive on top of the powder where required.

3. The build platform is lowered by the model’s layer thickness.

4. Another layer of powder is spread over the previous layer. The object is formed where the

powder is bound to the liquid.

5. Unbound powder remains in position surrounding the object.

6. The process is repeated until the entire object has been made.

The binder jetting process allows for colour printing and uses metal, polymers and

ceramic materials. The process is generally faster than others and can be further quickened by

increasing the number of print head holes that deposit material. The two material approach allows

for a large number of different binder-powder combinations and various mechanical properties of

the final model to be achieved by changing the ratio and individual properties of the two materials.

The process is therefore well suited for when the internal material structure needs to be of a specific

quality.

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* Materials

1. Metals: Stainless steel

2. Polymers: ABS, PA, PC

3. Ceramics: Glass

All three types of materials can be used with the binder jetting process.

* Advantages: 1. Parts can be made with a range of different colours.

2. Uses a range of materials: metal, polymers and ceramics.

3. The process is generally faster than others.

4. The two material method allows for a large number of different binder-powder combinations

and various mechanical properties.

* Disadvantages: 1. Not always suitable for structural parts, due to the use of binder material.

2. Additional post processing can add significant time to the overall process.

# Use and Benefits of AMP Additive manufacturing offers consumers and professionals alike the ability to create,

customize and/or repair products, and in the process, redefine current production technology. It is

a means to create highly customized products, as well as produce large amounts of production

parts. Products are brought to market in days rather than months and designers save money by

using additive manufacturing instead of traditional manufacturing methods. In addition, the risk

factor is much lower and those involved can receive near-immediate feedback because prototypes

take less time to produce.

For those looking to do rapid prototyping, additive manufacturing is extremely beneficial.

The technology lends itself to efficiently create quick prototypes, allowing designers and

businesses to get their products more quickly. When done in a large printer, multiple parts can be

done at once in less time.

A variety of industries use additive manufacturing to fabricate end-use product, consumer

and otherwise, including aerospace, architecture, automotive, education, game and medical

industries. The technology is popular among design and architecture firms as well. Industries and

businesses that build products and prototypes, as well as short run and on demand manufacturing

of components benefit from the use of additive manufacturing.


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ADDITIVE MANUFACTURING : THE OPPORTUNITIES AND CHALLENGES

The main AM opportunities lie in the design flexibility and in mass customization,

industrial secrecy protection, process sustainability and rapid product development, while the

challenges are related to Intellectual Property protection, standards certification, mass production

applications, regulatory issues and - at the moment - limited scalability. Hybrid machine tools that

incorporate CNC and AM could represent the next step for the development of the industry.

# Materials Used in AM Three types of materials can be used in additive manufacturing: polymers, ceramics and

metals. All seven individual AM processes, cover the use of these materials, although polymers

are most commonly used and some additive techniques lend themselves towards the use of certain

materials over others. Materials are often produced in powder form or in wire feedstock.

Other materials used include adhesive papers, paper, chocolate, and polymer/adhesive

sheets for LOM. It is essentially feasible to print any material in this layer by layer method, but

the final quality will be largely determined by the material. The processes above can also change

the microstructure of a material due to high temperatures and pressures, therefore material

characteristics may not always be completely similar post manufacture, when compared to other

manufacturing processes.

1. Polymers Common plastics can be used in 3D printing, including ABS and PC. The common

structural polymers can also be used, as well as a number of waxes and epoxy based resins. Mixing

different polymer powders can create a wide range of structural and aesthetic materials. The

following polymers can be used:

1. ABS (Acrylonitrile butadiene styrene)

2. PLA (polylactide), including soft PLA

3. PC (polycarbonate) Polyamide (Nylon)

4. Nylon 12 (Tensile strength 45 Mpa)

5. Glass filled nylon (12.48 Mpa)

6. Epoxy resin

7. Wax

8. Photopolymer resins

2. Ceramics Ceramic powders can be printed, including:

1. Silica/Glass

2. Porcelain

3. Silicon-Carbide

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3. Metals Metals: A range of metals can be used, including a number of options suitable for structural and

integral component parts. Common metals used:

1.Steel,

2.TItanium,

3. Aluminium,

4. Cobalt Chrome Alloy.

# Advantages of AM

Greater design ability. The technology allows assemblies to be printed in one process and

organic shapes to be easily produced. Traditional constraints of manufacture are reduced or

eliminated.

Unlike many widely used manufacturing techniques such as injection moulding, no tooling is

required, which can be a barrier to production due to the high cost.

Anywhere manufacture. Parts can be sent digitally and printed in homes or locations near to

consumers, reducing the requirement and dependence on transport.

Compared to conventional techniques with more geometric limitations, additive

manufacturing can produce models quickly, in hours, not weeks.

Fewer resources for machines and little skilled labour when compared to conventional model

making craftsmanship.

Customisation - Particularly within the medical sector, where parts can be fully customised to

the patient and their individual requirements.

Efficient material use due to the exact production of parts and no overproduction based on

estimated demand.

Commercial advantage and increased competitiveness, in the form of reduced costs and risk,

as the development time from concept to manufacture is minimised.

Material efficiency. Material required matches material used. Support material and powder can

often be recycled at source, back into the system.

Environmental benefits. The emissions from transport are reduced because of the ability to

manufacture anywhere.

With increasing numbers of machines, 3D printing is becoming more affordable, whereas

injection moulding machines remain relatively expensive and inaccessible.

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# Applications for Additive Manufacturing

technology Initially seen as a process for concept modelling and rapid prototyping, AM has expanded

over the last five years or so to include applications in many areas of our lives. From prototyping

and tooling to direct part manufacturing in industrial sectors such as architectural, medical, dental,

aerospace, automotive, furniture and jewellery, new and innovative applications are constantly

being developed.

It can be said that AM belongs to the class of disruptive technologies, revolutionising the

way we think about design and manufacturing. From consumer goods produced in small batches

to large scale manufacture, the applications of AM are vast.The number of users of these

technologies has been growing constantly, from artists, designers and individuals to large

companies and enterprises using AM to manufacture a wide range of final products.

INDUSTRIES CURRENT APPLICATIONS POTENTIAL FUTURE APPLICATIONS

COMMERCIAL AEROSPACE

AND DEFENSE

Concept modeling and prototyping Structural and non-structurat production

parts Low-volume replacement parts

Embedding additwely manufactured

electronics directly on parts

Complex engine parts

Aircraft vring components

Other structural aircraft components

SPACE

Specialized parts for space exploration

Structures using Ight-weight, Ngh-

strength materials

On-demand parts/spares in space

large structures directly created in

space, thus circumventing launch

vehicle size Imitations

Automotive

Rapid prototyping and manufacturing of

end-use auto parts

Parts and assemblies for antique cars

andracecars Quick production of parts or entire

Sophisticated auto components

Auto components designed through

crowdsourcing

Health Care

Prostheses and implants

Medical instruments and models

Hearing aids and dental implants

Developing organs for transplants Large-scale pharmaceutical

production

Developing human tissues for

regenerative therapies

CONSUMER PRODUCTS/

RETAIL

Rapid prototyping

Oeatmg and testing design iterations

Customized jewelry and watches

Umited product customization

Co-designing and creating with

customers Customized living spaces Growing mass customization of

consumer products

Currently, metal AM is not a process suitable for the mass production of millions of

identical simple parts. However, as systems and technologies advance, and processing time is

reduced, the use of AM for producing large quantities of parts will become a viable option.

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The advantages of AM derive from its high flexibility due to the product being produced

directly from a CAD model without the need for tooling. This also allows the AM process to

produce almost any geometry that can be designed.

There are some applications, for example dental restorations, that really tap the full

potential of AM. In this highly individualized production process it is economically viable to use

AM technologies, speeding up the production time without inflating the costs per part.

Applications in aerospace, for example the fuel nozzles for the GE LEAP engine, highlight

the possibilities of AM in this demanding sector. Additive Manufacturing allowed engineers to

design a fuel nozzle which is 25% lighter and five times more durable than the previous part.

********** Thank You *************


For details Refer CAD-CAM Unit-5 Rapid Prototyping

Unit.6

Measurement

Techniques in

Micro Machining

Semester VII – Mechanical Engineering SPPU

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Syllabus :

Introduction, Classification of measuring System, Microscopes : Optical Microscope, Electron

Microscopes, Laser based System, Interference Microscopes and comparators, Surface

profiler, Scanning Tunneling Microscope, Atomic force micro scope, Applications.

The last two decades have shown an ever-increasing interest in higher precision and

miniaturization in a wide range of manufacturing activities. These growing trends have led to new

requirements in machining, positioning control, and metrology down to nanometer tolerances.

Recent developments in silicon micromachining have made possible the fabrication of

micromechanical elements of sizes typically ranging from 0.1 to 100 μm . Slots and apertures for

some applications such as color TV, electron gun masks, and. jet-engine turbines are made as small

as 5 um.-Microcircuit, elements of ,0.5.to 1 𝛍m are commonly manufactured using X-ray or

electron-beam lithography . In order to assess and control the quality of micromachined parts it

has been necessary to develop new measuring techniques, capable of effectively and accurately

measuring the dimensions, geometry, profile, and surface roughness of microholes, slots, very thin

films, microspheres, steps, and grooves of different configurations in micromachined parts. These

parts and features can be either checked for configuration and completeness, or measured to

determine actual sizes. Inspection and measurement of these features raise the demand for special

equipment some of which depends on entirely new principles.

In addition to high-resolution calipers and coordinate measuring machines, equipment used

for measurement of micro-machined parts includes high resolution microscopes, laser-based

surface followers, scanning electron microscopes (SEM), interferometers, profilometers and

scanning probes (e.g., scanning tunneling microscopes STM), and scanning force micro-scopes

(SFM). The practical use of almost all these methods depends on the development of high

precision scanning tables as well as high resolution linear transducers. Measurements are carried

out offline, in a metrology laboratory, as well as online, or in process while the parts are being

fabricated. In most measuring applications, noncontact methods are eventually used. Measuring

systems rely, in their function, upon different principals and apply several technological methods.

The systems used for dimensional measurement and topographic inspection can ,however be

classified into two categories :

Shri Swami Samarth

Measurement Techniques in Micro machining AMP

Unit-6

Introduction

6.1 CLASSIFICATION OF MEASURING SYSTEMS


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In this category the size of an inspected features is determined by measuring the distance

between its edges Fig.a .Accordingly , the system consists of three main parts : a precision table

and a displacement transducer. By such an arrangement, the sensor determines the exact position

of the feature edges while the precision table moves the object or the sensor for edge location. The

displacement transducer can then measure the distance moved between edges and indicate the size

of the feature in the specific direction .

Figure 1. Different configurations of

measuring instruments.

Sensors can be mechanical, magnetic, capacitive, and in many instances optical. Tables of

stable and precise movement have recently been developed. For short travels of sub micrometer

and nanometer resolution levels, piezoelectric driven stages are recommended.

In optical sensors, the position of the measured edge is realized by the change in the

reflected pattern as a light beam crosses an edge. In optical microscopes, the edge position is

determined by a stationary index line placed inside the eye piece unit. For capacitive and magnetic

sensors the position of the edge is determined by the change in output signal noted as the sensor

crosses the edge. Mechanical sensors (e.g., in coordinate measuring machines (CMM)) touch the

inside (or outside) walls of the part with a preset pressure. The translation of the sensor is

determined with account being taken of the site of the sensor tip. When mechanical sensors are

6.1.1 Category 1

Sensor

Object

Table

Linear

Tanducer


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used, which are not stationary in the case of CMM, the minimum internal dimension to be

measured is limited by the size of the sensor (stylus) tip.

Displacement transducers of different resolutions can be used depending on the accuracy

required. Linear variable differential transducers (LVDT), optical grating encoders , and

displacement interferometers are widely used in many applications. These transducers have a

major advantage of being readily integrated in a computer controlled measurement system.

Consequently, electric output signals from these transducers are fed to the computer for direct

measurement and/or control. The above-mentioned category of measuring instruments is used to

measure sizes of object features other than the height except for the case of CMM.

In order to measure height, profile, or surface topography another category is used. This

can be classified into two main types, whole field contouring and single profile methods. Whole

field contouring includes interferometric and holographic techniques. Single profile (SP) methods

include mechanical stylus instruments, optical profile followers (OPF), scanning tunneling

microscopy, scanning electron microscopy, and atomic force microscopy (AFM).

Single Profile And Height Measuring Methods Here the sensor is forced to follow the profile of the inspected surface based on specific

criteria (Fig. 1.b). Height variations can be recorded provided they are small enough to maintain

the validity of the working principle. The sensor can be of the contact type as in most CMM

machines and stylus type roughness meters or it may be noncontact utilizing several principles.

CMM machines can accurately measure step height provided there is sufficient room for the

insertion of the sensor tip (Fig. 1 c). The resolution depends on the operating principle, and values

as small as 1A are characteristic of some of these instruments (e.g., SEM, STM, AFM), The

working principles of some of those systems that have potential use in the measurement of micro

machined parts are illustrated in the following sections. The range of their application, accuracy,

and resolution limits are also examined.

Whole Field Contouring In whole field contouring (WFC), a contour image (inter ferogram) of the inspected object

surface is recorded by use of several inter ferometric or holographic arrangements. The contour

images are analyzed by means of appropriate computer algorithms, and surface height ordinates

are accordingly determined. In this case, the resolution depends on both the arrangement used as

well as the algorithm adopted. Some interference microscopes have vertical resolution on the order

of 0.1 nm, with maximum vertical step height of 100 μm. Besides roughness such methods can

also measure and evaluate height of microsteps and grooves.

6.1.2 Category 2


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Microscopes are widely used for the inspection and measurement of tiny object features.

Basically, a microscope produces a magnified virtual image of the inspected object, Three types

of microscopes, namely, optical, electron, and interference are used. The principle and application

of the first two types are discussed in the following :

6.2.1 Optical Microscopes :

An optical microscope consists basically of two lenses (Fig. 3) high-power short focal

length objective, and a low-power, longer focal length, eyepiece. In practice the objective and the

eyepiece are not single lenses. To reduce the effects of aberrations, each is assembled from two or

more lenses. Generally a microscope is equipped with several objectives to provide different

magnifications, which can be as high as 1000x . Optical microscopes can be used as stand-alone

inspection instruments that are commonly employed for the visual inspection of printed circuit

boards. Modem microscopes are equipped with video or CCD cameras where the field of view is

observed on a cathode ray tube (CRT) monitor. Such advanced systems are presently fitted on

production lines to monitor the quality of microfeatures of manufactured parts.

Figure 3. The principle of the optical microscope

6.2 MICROSCOPES

Eyepiece

Image

object

objectives


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The resolving power of an optical microscope a is given by the Abbe equation,

𝒂 =𝟏.𝟐𝟐𝝀

𝟐𝒏𝒔𝒊𝒏(𝒊) (1)

Where

a : distance between two points on the surface of an object that can be seen separated in

the image plane

λ : effective wavelength of illumination used

n: refractive index of objective medium λ

i: suspended angle of lens which depends on its diameter and focal length

(n sin(i)) is the numerical aperture (NA) of the objective lens which is higher for a high-power

lens having short focal length. For white light illumination, λ = 5.6 X 10-4 mm and, assuming NA

= 1, then, a = 0.27 μm. An optical microscope with NA = 0.6 can effectively detect two points less

than 0.5 μm apart.

Many commercially available measuring machines integrate the high resolving power of

an optical microscope with a high resolution x-y stage to measure different dimensional features

of a product. Examples of these machines are the tool maker’s microscopes (TMM), and the

universal measuring machines (UMM) (Fig. 4). The microscope helps to form a magnified image

of the inspected workpiece. An appropriate reticle with cross-lines fixed inside the eyepiece is

used to mark the ends of the dimensions to be measured.

Fig. 4 The principle of the tool makers microscope (TMM).

microscope

Precision

table

Sample

Light

source

condenser


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The dimension size is determined by the distance moved with the x-y stage between the

two ends. An accuracy level of 1 μm is commonly available with Abbe’s metroscopes. Better

resolutions are achieved from interferometric displacement measurement. Indeed, dimensions of

holes, slots, and other features of an object less than 200 μm can easily be measured with accuracy

better than 1 μm. TMMs are used for dimensional measurement of both internal and external part

features.

6.2.2 Electron Microscopes :

In electron microscopes the inspected surface is interrogated by a focused beam of

electrons. The beam is collimated and then focused by means of coils that generate a radial

magnetic field to control the shape of the electron beam. Focusing is achieved by varying the focal

length of the objective lens coil through the control of the coil current. The effective resolution is

almost 105 that of an optical microscope . In this case the wave length λ is given by the De Broglie

formula ,

𝛌 =𝒉

𝒎𝒗 (2)

where

h: Plank’s constant

m: mass of electron

v: electron velocity

As an electron of mass m and charge e pass through a potential difference V, its kinetic energy is,

𝟏

𝟐𝒎𝒗𝟐 = 𝒆𝑽 (3)

And

𝛌 = 𝒉

(𝟐𝒎𝒆𝑽)𝟏𝟐

(4)

It is obvious that the wavelength λ depends on the potential difference V. For V = 60 kV, λ is about

0.05 A. From Abbe’s equation (1), the resolving power a for the electron microscope can be on

the order of 2.4 A for λ = 0.05 A.

Two main types of electron microscopes are available namely, transmission electron

(TEM) and scanning electron (SEM) types. In most engineering applications SEM is used Figure

5(a) shows a schematic diagram of the main components of an SEM microscope. The electron gun

generates a stream of electrons (electron beam) that is collimated by a coil (lens). The objective

(lens) focuses the electron beam onto the surface of the specimen. In order to scan the specimen


Mo.9673714743

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surface with the focused electron beam (electron probe), a beam deflecting unit is used. When an

electron beam impinges on the surface of the specimen different phenomena are observed.

Figure. 5. Schematic of the electron microscope (a), emissions resulting from electron

bombardment (b).

Some electrons are absorbed (losing their energy on collision). Others are back scattered

(reflected) either elastically or inelastically and these are called primary electrons. In elastic

reflection, electrons do not lose any of their energy but change their direction. For inelastic

reflection, electrons interact with specimen atoms and lose some of their energy before deflecting

back out of the specimen surface. Electrons that penetrate inside the specimen interact with the

material atoms resulting in the ejection of secondary electrons. Secondary electrons formed near

the surface may escape producing secondary electron emits. In addition to primary scattered

electrons and secondary emitted ones, X-ray and even light photons are also produced as a result

of electron bombardment (Fig. 5 b). The amount and ratio of back-scattered electrons, secondary

emission, and other radiation depend on the beam energy, specimen geometry, and substrate

atomic number. Since the atomic number and beam energy are practically constant, specimen

geometry is therefore the main controlling factor.

A detector is used to collect the emission from the specimen surface (Fig. 5 a). Several

types of detectors are available for the different phenomena resulting from electron bombardment.

The detector signal is amplified by the electronic unit and used to modulate the brightness of a

1.elctron gun

2.collimating coil

3. focusing coil

4.deflecting coil

5. Sample

6. Detector

7.signal processing

unit

8.display unit


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cathode ray tube. The CRT is adjusted to scan synchronously with the electron probe. Variations

in the recorded brightness produce the highly magnified image of the scanned object.

SEM are used in two modes. In the conventional raster scanning mode the deflection coils

(Fig. 5.a) move the electron beam across the stationary object to produce a two -dimensional image

of the surface. In the second mode the specimen is fixed to a precision table which scans the

specimen under a stationary focused electron beam to produce a trace of the surface relief. This

intensity profile mode provides quantitative linear measurement of an object feature. Swyte and

Jensen used SEM for the calibration of linear dimensions in the range 0.1 to 100 μm. The table

translation is measured by a laser interferometer with measurement precision of 0.01 μm. Electron

detector and interferometer signals are fed to a computer that analyzes the electron intensity with

respect to position profile to obtain the linear dimension for a specific feature. A typical example

of inspected objects is a microscopic chromium metal line deposited on a glass substrate by means

of a photolithography technique. In such an application, line is 0.5μm wide, and l-um thick, with

an edge slope of 700.

A newly developed SEM featuring two secondary electron detectors was used to measure

the cutting edge radius which is on the order of 45 nm. The image created by the difference signal

of the two detectors emphasizes the convexity and concavity of the sample surface.

As well as profile recording, SEM is used for roughness measurement. Based on the

principle that the back-scattered electron signal is proportional to the slope of the surface in the

direction of scanning, the surface profile can be obtained. By integrating the signal, Sato and

Ohmori detected a roughness profile of surfaces having slope in a specific direction. Three-

dimensional surface topography of the specimen can be obtained by integrating scans covering the

entire image, at resolutions on the order of 0.001 μm. To measure the roughness of surfaces having

slope in an arbitrary direction, Sato and Ohmori proposed a method to detect the normal of the

object surface by comparing the intensity of the back- scattered electron signal of the specimen

with that of a standard ball. The surface topography is processed from the measured data of the

normal.

In laser-based systems optical phenomena, observed as laser light scans across an

engineering surface, are applied for online and in-process inspection of surface features. These

phenomena include diffraction, reflection, refraction, scattering, and others. Some methods that

depend on such phenomena are explained below.

6.3 LASER - BASED SYSTEMS


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6.3.1 Diffraction Method

In micromachining, slots of dimensions on the order of few nanometers up to 200 μm are

produced. In the range of 50 μm or higher, these dimensions can readily be measured, in the

laboratory with high magnification microscopes; however, for online measurement other

techniques are adopted.

A focused laser beam for online measurement of fine surface grooves having different

cross-sections (Fig. 6). The principle of this technique depends on the condition that the reflected

pattern from a flat surface will be modulated by the presence of a micro surface groove. A flat

smooth surface reflects the beam into a single spot (Fig. 6 b); however, as the focused beam crosses

the edge of a groove, the reflected pattern is divided into two parts (Fig. 6 c). Scanning of this

field, therefore, produces a double-peaked signal (Fig. 6 d), the relative amplitudes of which

change as the surface moves against the focused spot. The slot width can be evaluated from the

recorded signal of a photosensitive device (PSD) as the sample scans beneath the stationary laser

spot. Moreover, the recorded signal also represents an approximation of the slot profile (cross-

section). Fig. 6(a) shows the arrangement used, while the PSD signals recorded as the focused

spot scanned three different grooves are shown in Fig. 6(e). In-spected surfaces should be smooth,

since a speckle pattern limits the measurement accuracy.

The double-peaked pattern can be explained by the Pekrinck and Kennedy model . Levy

observed a similar pattern when inspecting the height of a submicrometer step. The method

described above is noncontact and can be used online provided the specimen travel is precisely

controlled. The proposed system is sensitive to changes in groove width; however, variations in

depth are less detectable especially for high depth-to-width ratios, or steep sides. Although

different incidence angles can be used, normal incidence re- suits in more accurate slot description.

The resolution of measurement is directly proportional to the sampling rate of the data acquisition

unit R and inversely proportional to the table speed u. For v =300 mm/min, and R = 10 kHz, the

resolution can be on the order of 0.5 μm. A linear photodetector array can be used to indicate the

start and finish of the double-peaked pattern, while a linear transducer measures the distance

covered during this event.


Mo.9673714743

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Figure 6. The principle of online dimensional measurement using diffraction method: (a) setup;

(b) reflection from a flat surface; (c) diffraction by a microgroove; (d) intensity distribution of the

diffrac-tion pattern in (c); (e) PSD signal when scanning typical grooves.

1.PSD , 2.amplifier , 3.ADC , 4.PC-computer . 5.Sample

PS

D o

utp

ut

arbit

rary

Angular Position

Table Travel mm table travel mm table travel mm

e

PS

D o

utp

ut

arbit

rary

PS

D o

utp

ut

arbit

rary


Mo.9673714743

11

6.3.2 Optical Triangulation Method :

In this method (Fig. 7), the geometric principle of triangulation is applied to perform

distance measurements. The sample surface is illuminated with a laser beam, through a projection

lens. The spot image formed by the imaging lens is received on the position sensor. Variations in

surface height ∆Z cause the image on the PSD to be displaced from its reference position by a

distance S which is directly proportional to ∆Z. A position sensor produces an electrical signal

proportional to the distance S that can be calibrated to give the height variation. For the

configuration shown in Figure (7), the displacement S, corresponding to a height variation ∆Z is:

𝑺 =𝑴∆𝐙 𝐬𝐢𝐧(𝛂+𝛃)

𝐜𝐨𝐬(𝛂). (5)

where

M - magnification in the image receiver system

α - illumination angle

β- imaging angle

∆Z- height deviation from a reference level

Fig .7 The principal of the triangular method

Some applications use normal illumination (α = 0), for which Equation (5) reduces to

S = M .∆Z • sin(θ) (6)

1. Projection lens

2. Imaging lens

3. Position sensor

4. Samlpe surface


Mo.9673714743

12

Where ,

θ is the angle between the illumination and imaging directions.

The height resolution and range of this method depend on the wavelength of the

illumination γ, the numerical aperture (NA) of the imaging lens, and the geometrical configuration,

Using inclined illumination and normal imaging, Costa achieved height resolution of 0.49 μm (λ

= 0.6328 μm, NA = 0.6, inclination angle = 65°); however, it can be as high as a few microns. The

range of height variation detected can be as small as few micrometers or as large as several

millimeters. The application ranges from the measurement of surface topography, in-process

measurement and control , range-finding, and as noncontact probes on coordinate measuring

machines. A critical analysis of errors evolved in triangulation-based systems is given by Kilgus

and Svetkoff.

6.3.3 Optical Followers :

The optical follower is a measuring instrument that scans an object surface with a focused

laser beam. If the beam is initially focused on a specific surface point, height variations as the

object moves set the beam out of focus. The out-of-focus condition is detected by a sensory unit

that activates a servomechanism to bring the beam back in focus. The vertical displacement

required to bring the beam back in focus is recorded against the linear displacement of the object.

This record represents the profile of the surface along the scanned line. Figure 8. shows the main

components of an optical follower.

Figure 8. The main components of an optical follower.

1. Laser light

2. Spatial filter

3. Collimating lens

4. Focusing lens

5. Sample

6. Beam splitter

7. Imaging lens

8. PSD

9. PC-computer

10. Table

11. Linear transducer

12. Servo mechanism

13. Linear transducer


Mo.9673714743

13

The laser beam is filtered using the lens-pinhole spatial filter, then collimated and focused

on the sample surface. The focused spot is imaged on the surface of the position sensor by use of

the beam splitter and the imaging lens. The detector signal is fed to the personal computer, where

the focusing condition is deter-mined. As the table moves, height variations in the sample surface

set the beam out of focus. Accordingly, the servomechanism is activated to bring the spot back in

focus by moving the focusing lens vertically. Two linear transducers are used: the first measures

the horizontal displacement of the tabtei while the other measures the vertical displacement of the

lens. A record of the vertical lens displacement against the horizontal table displacement produces

a profile trace of the sample surface along the scanned line. By use of an x-y table multiple traces,

and consequently, three-dimensional maps of surfece profiles can be constructed. The accuracy of

height measure ment depends on two main factors, the resolution of the displacement transducer,

and the precision of the sensory On the other hand, spatial resolution depends on the precision of

the stage moving the object. Several techniques are used to detect the focusing condition in optical

followers . They include defect-of-focus and astigmatic methods.

Interference microscopes provide higher resolution than their optical counterparts. They

form magnified images of the inspected part surface modulated by interference contour ; fringes.

These represent the micro topography of the inspected part surface and provide invaluable

information about micro surface features such as form, profile, and roughness as well as

dimensions of grooves, slots, and scratches. In many applications, calibrated reticles can be placed

inside the eyepiece unit to measure dimensions of surface features. Several arrangements are

adopted to produce the interferograms of examined parts. In all cases light is split into two wave

fronts, one going to a reference plane and the other to the inspected surface. After reflection,

wavefronts recombine undergoing constructive and destructive interference, producing the inter-

ferograms with dark and bright fringe pattern.

Figure9.(a) shows Tolansky’s arrangement for interference microscopy. By this

arrangement, it is possible to I measure microsteps of 500-μm height with an accuracy on the order

of ±3 μm. In the Tolansky interferometer the wavefront is divided into two types, the reference

and the object waves. In other types of interferometers, the two waves are formed by separating

the electric and the magnetic components of the electromagnetic wave of light. In the interference

microscope (Fig. 9.b) a Wollastone birefringence prism 5 is placed in the space between the

objective 2 and eyepiece 7. A polarizer plate 4 is placed in front of the Wollastone prism while an

analyzer 6 is in the back. The semireflecting plate 1 directs the light onto the surface of the

inspected object 3. With this arrangement the reflected wavefront imaged by the objective 2 is

divided into two wavefronts representing the electric and the magnetic fields. These wavefronts

interfere with each other after passing through the analyzer 6.

6.4 Interference Microscopes


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Figure 9. The principle of interference microscopy

The resulting interferogram shows minute variations in the inspected surface. This

procedure is useful for examining fine surface structures (e.g., grooves, slots, and scratches) as

well as surface roughness. Commercially available interference microscopes can measure height

variations as small as 0.1 nm. Moreover, roughness values in the sub-Angstrom range can also be

evaluated. Like optical microscopes, interference microscopes are now equipped with CCD

cameras that are interfaced to PCs (Fig. 9.c).

A major limitation of interferometry is the need for fairly reflective surfaces. Apart from

this limitation, it provides a powerful tool that produces a whole field image revealing

1. Beam spliter 2. Focusing lens 3.

Sample surface 4. Polarizer 5. Wollastone

prism 6. analyzer 7. Imaging lens

8.interferogram


Mo.9673714743

15

microfeatures of inspected surfaces. The resulting interferogram can be used for direct visual

inspection, or numerical evaluation of dimensions and surface roughness.

Interference comparators are used for two major applications: to produce contour

interferograms of inspected surfaces or as displacement transducers. Evaluation of contour

interference patterns renders valuable information regarding surface topography, surface

roughness, as well as dimensions of micro surface features. Interferometric transducers are also

used to measure the translation of highly precise tables down to the nanometer resolution .

Figure 10. The principle of the Michelson interferometer

Many widely used interferometers are based on the Mi-chelson principle (Fig. 10.).

Coherent light from the monochromatic source 1, is collimated by lens 2, then divided into two

waves by the beam splitter 3. The reference wave is reflected back by the reference plane 4, while

the object type is reflected after being modulated by the object surface 5. The two waves recombine

to form a fringe pattern by using imaging unit 6. The fringe pattern is determined according to the

6.5 Interference Comparators


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phase relationship between the object and the reference waves, which in turn depends on the shape

and orientation of the object surface with respect to that of the reference.

If the object and reference surfaces are normal to each other, the pattern produced

represents contour fringes; however, for inclined surfaces, the fringes are rows of profiles. Parallel

equidistant straight fringes indicate a plane surface indined to the reference plane, Any out-of-

flatnen in the sample surface alters the obtained fringe pattern, In effect, the fringe pattern

represents contour lines of equidistant points from the reference plane. The contour step in this

case equal λ/2, where λ is the wavelength of the light used,

Personal computers (PCs) have recently been used for the analysis of interferograms, CCD

and video cameras are used to capture the fringe pattern, which is then analyzed, by means of

dedicated software. Different surface topography parameters are evaluated from the interference

pattern. The image of the fringe pattern is transferred electronically to digital values representing

the grey levels or intensity that are treated mathematically to produce contour maps and surface

profiles as well as surface parameters. Dimensions of microsurface grooves or steps can also be

evaluated from the interferograms.

Surface profilers are instruments that use a fine stylus or tip to trace the fine details of an

engineering surface. Height variations along the traced line modulate the force interacting between

tip and surface or the tunnel current passing between them when they are very close to each other.

By monitoring the deflection of the tip caused by the tip-surface force or the tunnel current, it is

possible to produce a profile record of the traced line. Multiple line traces generate three-

dimensional records of the inspect surfaces.

Mechanical stylus instruments are most widely used for surface topography

assessment. They have a wide range of height resolutions; however, their spatial resolution is

limited by the size of the stylus tip used. As shown in Figure 11, an arm (lever) carrying a microtip

(stylus) represents the sensor that scans across the inspected surface. Height variations along the

scanned line change the force between the stylus tip and the surface points, and consequently lever

deflection. An appropriate transducer transforms the lever deflection into an electrical signal

proportional to height variations. The signal is then amplified, digitized, and analyzed to evaluate

surface parameters.

6.6 Surface Profiler

6.6.1 Stylus Instruments


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Fig.11. The principal of the stylus instrument

The stylus movement is measured relative to a datum that should conform with the

nominal shape of the measured surface . While a straight-line datum has been used, a simple skid

arrangement (Fig. 11) is most common. Generally the stylus is a conical or pyramid diamond with

a flat or rounded tip. The pyramid is normally 90° with a 2 to 2.5-μm flat, while the cone is 60°

with 12.5-μm radius. Tips of 2.5 μm are also in use.

The tip radius should be smaller than the radius of curvature of the bottom of surface

valleys, otherwise the profile is commonly modulated by the stylus tip. A measuring force must

be applied to ensure contact between stylus tip and surface points. Typical force levels are about

70 mg giving a pressure of 2500 Nmm-2. Such pressure is less than the yield strength of most

materials. However, for soft materials or coatings this pressure may exceed the yield strength and

cause surface damage.

Stylus instruments provide profile records of traced lines.Three-dimensional surface

profile maps can also be plotted by Use of scanning tables. Since profile signals can be digitized,

8urface geometry is analyzed and different surface parameters valuated using PCs interfaced with

the measuring instrument. In addition to the assessment of surface roughness, sty. lus instruments

can be used to measure microsurface grooves with depth values on the order of several microns.

A major disadvantage of stylus instruments is that they rely on contact- type methods. They are

relatively slow and the measuring pressure may, in some cases, damage the inspected surface.

For Full PDF Go to www.slideshare.com Search by My Gmail ID

1.stylus 2.sensors 3.skid

4.data acquisition unit 5.

Analysis and display unit


Mo.9673714743

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Scanning tunnelling microscopes (STM) are another family of stylus-type

instruments where the stylus does not contact the inspected surface. STM instruments are capable

of lateral resolution sufficient to resolve protruding atoms on reconstructed surfaces. The vertical

resolution is as small as 0.02 μm.

These instruments operate in vacuum, air, oil, liquid nitrogen, and water, giving

images which are direct topo graphs of inspected surfaces . In addition to the promising advantages

of STMs in micro topographic mapping of highly finished surfaces, other applications such as

microlithography, micromachining, polymer science, and biotechnology are also being studied.

The usefulness of STMs for the analysis of diamond-turned surfaces, ruled grating replicas, X-ray

reflecting optics, and optical discs has recently been demonstrated .

Although the concept of tunnelling in solid-state physics first appeared in the late

1920s , the first successful tunnelling experiment with an externally and reproducibly adjustable

vacuum gap was reported by G. Binnig et al. of the IBM Zurich Research Laboratory in 1982. The

principle of scanning tunnelling microscopy is demonstrated in Figure 12 (a), in which a stylus of

very sharp tip (ultimately one atom) is brought very close to the inspected surface (<1 nm apart)-

At such closely adjacent distances, free electrons from the conductive sample surface atoms tunnel

to the conductive stylus tip, producing a very weak current. In the one-dimension^ case, the tunnel

resistance and, consequently, the tunnelling current at low voltage and temperatures is

exponetially dependent on the tip-sample seperation ‘d’ :

𝑰 ∝ 𝐞𝐱𝐩(−𝟐𝒌𝒅) (7)

Where ,

I- tunnelling current

d: distance between tip and surface

k: constant.

6.6.2 Scanning Tunneling Microscope



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Figure 12. Scanning tunneling microscopy basic principle (a), constant current mode (b), and

constant height mode (c).

For vacuum tunnelling, k = h-1 (2m𝛟)1/2 where h is Planck's Constant, m is electron

mass, and φ is effective local work function, for a work, function of 4 eV, k = 1.0 A-1. The current

decreases by an order of distance d is increased by 1.0 μm. If the current is kept constant within

±2% the gap remains unchanged to within 0.01μm. This condition represents the basis for

interpreting the image as simply a contour of constant height above the measured surface.

To record a topographic map of a surface, the tip scans in a raster pattern. It is stepped

in the positive X-direction; at each step the tunnel current is read, and tip height adjusted to get the

desired value. When the first scan is finished, the tip is returned to the starting position of this scan,

then moved one step in the y-direction until it covers the required area. Surface roughness

complicates the scanning process. In this regard, the rougher the surface, the more difficult it is to

obtain a proper image. Therefore, STMs are limited to conductive materials having fine surface

structures.

Two modes of operation can be used with STM, constant current and constant height

modes. In the first case, Figure 12(b), the tunnel current is kept constant by adjusting the tip height

to keep a constant separation between the tip and the surface. The displacement of the

servomechanism required to bring the tip to the constant current (separation) position is then

recorded as the z-ordinate. In this case, the scanning speed is determined by the response of the

(C)

1. stylus

2. sample

3. power

supply

4. meter


Mo.9673714743

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feedback circuit, which maintains the average current constant. The constant current mode can be

used for surfaces that are not atomically flat (i.e.≥ 10-nm peak-to-valley height).

In the constant height mode (Fig. 12.c) the tip scans the surface while its vertical

position, relative to a mean reference plane, and the current are kept constant, the voltage variation

being monitored. Under such circumstances, the scanning speed depends on how fast the feedback

circuit responds to achieve constant current by adjusting the voltage. Scanning is faster; however,

the tip may be damaged unless the surface is tolerably smooth.

Special attention should be paid to vibration conditions, since the performance of an

STM microscope can be enhanced by use of proper isolation. Fine resolutions, required for

scanning the tip and measuring height variations, are realized by using piezoelectric elements for

x, y, and z translations Controlled voltage is applied to the piezoelectric crystal in specified

directions. Consequently, the crystal contracts or elongates according to the sign of the electric

field. This effect is linear and precisely controlled . Separate piezoelectric elements for each axis

translation have been used . On the other hand, a single piezoelectric tube that provides translation

in the three axes has been described by Binnig and Smith .

STM microscopes have many potential applications in measurement and fabrication

of micromachined parts. They are successfully used for the measurement of surface topography as

low as the atomic scale. Yang and Talke used STM to investigate surface roughness of magnetic

recording disks, using line graphs and aerial images at sampling intervals of 125 and 5 nm. Fine

diffraction gratings, 2000 lines/mm, were also examined using STM to reveal detailed surface

topography. Besides measurement of surface topography STMs were used to modify surface

structure and manufacture parts bits on highly oriented pyrolytic graphite.

Atomic force microscopes (AFM) are noncontact profilometers which use a very fine

stylus fixed to the end of a thin cantilever. They trace actual profiles of highly smooth and flat

surfaces (Fig. 13.a). The profile is recorded by making the stylus follow the profile of a constant

force between stylus tip and surface points. As the fine stylus tip is brought close to the surface

(30 to 150 nm) attraction forces between atoms are generated. As the tip comes closer to the surface

the atomic force increases. Such a force is balanced by the plastic force generated by bending the

cantilever. Therefore the stationary cantilever bend is a direct measure of the atomic force . Most

probes use capacitive sensors to determine cantilever deletion, and hence the atomic force . A

feedback loop 18 usually used to keep the force at a specified value by mainlining a constant tip-

sample spacing. The signal, from the capacitive sensor, is fed to a servomechanism that controls

the tip-sample spacing.

6.6.3 Atomic Force Microscope



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Figure 13. The principle of atomic force microscopy with capacitive sensor (a), and with

vibrating tip (b).

Another arrangement (Fig. 13.b) uses a fine tip fixed to the end of a vibrating

lever. Changing the tip-sample spacing leads to proportional change in the attraction force, which

modifies the compliance of the lever. The vibration amplitude is also affected by the shift in the

lever resonance. The tip vibration amplitude as a function of the frequency ω is given by ,

𝐴 = 𝐴𝑜 (𝜔

𝜔𝑜) /[1 + 𝑄2 {(

𝜔

𝜔𝑜) − (

𝜔𝑜

𝜔)}2]

1

2 (8)

Where,

A: amplitude of vibration

A0: amplitude at resonance

ω: tip frequency

ωo: tip resonance frequency, ωo = C1√k ,C1 is a function of lever mass, and

k is the spring constant

Q: quality factor (Q >>1), Q is a measure of system damping, and

Q = (l/c)√km

Where , c : damping factor

m: Lever mass

Sensor lever

Stylus

tip

sample

Laser light

(b)

Vibrator

Vibrating

lever

Sample

Table

Beam splitter

Interferometer


Mo.9673714743

22

The larger the value of Q, the smaller is the damping. As the tip approaches

the sample surface, interatomic attraction forces reduce the spring constant of the lever by the

atomic force derivative f '. The resonance frequency then becomes 𝜔𝑜 = √𝑘 − √𝑓′.Thus by

measurement of the shift in the lever resonance ∆ω= (ωo - ω), the force derivative can be found.

The change in the resonance frequency changes the amplitude of vibration. This means that, as the

tip approaches the surface, the attraction force rises leading to a proportional change in both lever

resonance frequency and vibration amplitude. By monitoring these values as the tip scans across

the surface, it is possible to trace the surface profile.

The amplitude of vibration can be measured optically, or by use of an

interferometer . A signal representing the amplitude of vibration is usually used in a feedback loop

to maintain the tip at a specific distance from the surface as the tip scans across a determined path.

This technique has been applied to measure surface roughness of ultrafine surfaces. Micro- and

sub-micrometer surface features can also be measured. Line profiles and 3-D maps of surfaces can

be recorded and presented. V-shaped grooves on silicon wafer and steps of height 50 μm have

been mapped and measured . Profiles of a photoresist grating (0.1 μm line width and 0.09 μm

thickness) have been recorded by the same technique.

New trends in manufacturing industries including miniaturization and large-scale

integration have enhanced the development of measurement science and technology.

Miniaturization calls for new measuring techniques to evaluate micro features of tiny parts at high

resolution. In addition, computer-integrated, high-speed, noncontact techniques are required for

applications including in-process and online inspection. In the future, accurate and intelligent

measurement systems will therefore be an integral part of industrial manufacturing lines.

Advances in the field of measurement are expected to be integrated in manufacturing

systems along four main lines: probe and sensor performance, precision and resolution of

translation mechanisms, accuracy and response of linear and angular transducers, together with

computer interfaces and software.

Probes and sensors are being greatly improved to achieve higher resolutions and better

accuracy. Fine details of micro machined parts, including features that are difficult to reach, need

to be inspected at an appropriate resolution. AFM, STM, and electron microscopes provide high

resolutions; however, the measuring time is relatively long, which limits the application of such

systems for online inspection especially for large-scale production.

Precision tables that can provide very accurate fine motion for both workpiece and sensor

at reasonable speed are another challenge facing the development of measuring instruments.

Piezoelectric actuators are highly precise; however, their speed and range of travel are limited.

6.7 Applications



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Linear transducers are used in measuring systems to determine the displacement of axes.

High-response long-range transducers are necessary for the realization of precise measurement.

Research is being undertaken to improve transducer response, resolution, and range of

measurement and to reduce environmental impacts on accuracy.

Integration of accurate sensors, precision tables, and high-resolution transducers in a

measuring system is only possible through computer control. Table movement, displacement

measurement, analysis of sensor signals, and activation of feedback systems will all be controlled

by computers. Great improvements are therefore expected in computer hardware and software, as

well as interfacing technology.

***********Thank You ************
