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A STUDY OF LAPPING PARAMETERS TO REDUCE POLISHING TIME OF
OPTICAL GLASS
AMAD ALDDEIN ELSHWAIN
A project report submitted in partial fulfillment
of the requirements for the award of the degree
of Master Engineering
(Mechanical-Advanced Manufacturing Technology)
(Faculty of Mechanical Engineering)
Universiti Teknologi Malaysia
JUNE 2007
iii
In the name of Allah, Most Gracious, Most Merciful
All praise and thanks are due to Allah Almighty and peace and
blessings be upon His Messenger
The results of this effort are truly dedicated to my mother and father whose
example as devoted professionals, as well as, parents taught
me to be perseverant, responsible and loyal
to my belief.
To my father and mother for their support, encouragement, sacrifice, and especially
for their love.
Thank you all and this work is for YOU.
iv
ACKNOWLEDGEMENTS
First and foremost, I thank Allah for giving me the strength to complete my
project. I would like to thank especially supervisor ASSOC. PROF. DR. IZMAN
BIN SUDIN for his constant support and guidance during my graduate studies at
Universiti Teknologi Malaysia. I would also like to convey my deepest gratitude to
Mr. Sazali Ngadiman, Mr. Aidid Hussin, Mr. Maizan Sulaiman and other staff of
production laboratory; Mr. Ayob Abu and other staffs of metallurgy laboratory, Mr
Khalid at metrology laboratory, and Faculty of Mechanical Engineering for their
effort in assisting me in various measurement and laboratory tasks.
Thanks to all my colleagues and friends with whom I had the opportunity to
learn, and share a good time during my stay here. Finally, special and infinite thanks
goes to the most important people in my life; my parents, for their love, prayers,
sacrifice and support.
v
ABSTRACT
Machining of hard and brittle material always pose problems such as rough
surface, cracks, sub-surface damage and residual stress mainly due to its brittle
nature. In recent year researchers and manufactures have put in of lot effort to design
and fabricate highly precise device to achieve low tolerance, better surface finish and
low sub-surface damage at reduced cost. In this study ultrasonic grinding was used to
grind flat surface on the BK7 glass. Only feed rate was varied during grinding, ie.
0.5, 1.5, 2.5, and 3.5mm/min while other parameters such depth of cut (5μm),
frequency (20kHz) and spindle speed (1000rpm) were fixed. The four ground
samples were lapped at various table speeds and followed by polishing operations at
fixed conditions. Surface roughness and surface morphology of the samples were
evaluated after each process. It is found that surface roughness increases when the
feed rate increased. Higher lapping speed (50rpm) remove material faster and fines
better surface finish than lower speeds. Saturation point of surface roughness occurs
at 10 minutes lapping time. The finest polishing surface achievable using less rigid
machine was 38nm. It is expected lower Ra could be obtained after polishing if the
same lapping and polishing machine is used for both processes.
vi
ABSTRAK
Pemensinan bahan keras dan rapuh sentiasa memberi masalah seperti
permukaan kasar, keretakan, kerosakan bawah permukaan dan tegasan tinggal yanh
mana sebahagian besarnya disebabkan oleh sifatnya yang rapuh. Sejak beberapa
tahun kebelakangan ini, penyelidik dan pengeluar telah berusaha keras untuk
merekabentuk dan memasang peranti berketepatan tinggi untuk mencapai tahap had
terima yang rendah, kemasan permukaan yang lebih baik dan keedaan kerosakan
bawah permukaan yang rendah pada kos yang rendah. Dalam kajian ini, pencanaian
ultrasonik telah digunakan untuk mencanai permukaan rata ke atas kaca BK7. Hanya
kadar uluran sahaja yang diubah semasa mencanai, iaitu. 0.5, 1.5, 2.5 dan
3.5mm/min, sementara lain-lain parameter seperti kedalaman pemotongan (5μm),
frekuensi ultrasonik (20kHz) dan kelajuan spindal (1000 rpm) telah ditetapkan.
Keempat-empat sampel yang dicanai talah dipelas pada beberapa kelajuan meja dan
diikuti dengan operasi penggilapan pada keadaan tetap. Kekasaran permukaan dan
morfologi permukaan semua sampel telah dinilai selepas setiap proses. Didapati
bahawa kekasaran permukaan meningkat apabila kadar uluran ditingkatkan.
Kelajuan mempelas yang lebih tinggi (50rpm) dapat membuang bahan dengan lebih
cepat dan memberikan kemasan permukaan yang lebih baik dari kelajuan mempelas
yang rendah. Tetik tepu kekasaran permukaan berlaku pada minit ke 10 masa
mempelas. Kemasan permukaan gilapan yang terhalus yang boleh dicapai
menggunakan mesin yang kurang tegar adalah 38nm. Adalah dijangkakan nilai Ra
yang lebilh rendah boleh diperolehi selepas penggilapan jika mesin mempelas dan
menggilap yang sama digunakan bagi kedua-dua proses.
vii
LIST OF CONTENTNS
CHAPTERES TITLE PAGE
DECLARATION ii
DEDICATION iii
ACKNOWLEDGEMENTS iv
ABSTRACT v
ABSTRAK vi
TABLE OF CONTENTS vii
LIST OF TABLES X
LIST OF FIGURES Xi
LIST OF APPENDICES
Xiii
CHAPTER 1 INTRODUCTION 1
1.1 Overview 1
1.2 Problem statement 2
1.3. Objective of study 2
1.4. Scope of the study 3
1.5. Organization of the thesis
3
CHAPTER 2 LITERATURE REVIEW 4
2.1 Introduction 4
2.2 Back ground on optical glass 4
2.2.1 Composition and properties of BK7
glass
7
2.2.2 Optical flats 7
2.3 Overview on glass grinding process 8
viii
2.3.1 Grinding wheels designation and
selection
9
2.3.2 Over view on rotary ultrasonic
machining
13
2.4 Lapping mechanism 14
2.5 Polishing mechanism 16
2.5.1 Polishing techniques 18
2.6 Components of lapping and polishing processes 20
2.6.1 Work piece 21
2.6.2. Fluid 21
2.6.3 Abrasive 23
2.6.4 Lap 26
2.7 Critical on literatures review
29
CHAPTER 3 RESEARCH METHODOLOGY 33
3.1 Introduction 33
3.2 Overview Work piece preparation 33
3.2.1 Ultrasonic Core Machining 35
3.2.2 Slicing process 36
3.2.3 Flattening Process by RUM Machine 38
3.3 Overall of the methodology 38
3.3.1 Ultrasonic grinding experiment 40
3.3.2 Lapping experiment 41
3.3.3 Polishing experiment 43
3.4 Analytical and measuring instruments 44
3.4.1 Surface roughness measurement 44
3.4.2 Surface morphology analysis
45
CHAPTER 4 EXPERIMENTAL RESULTS AND DEISCUSSION 46
4.1 Introduction 46
4.2 Effect of feed rates in ultrasonic grinding 46
4.3 Effect of table speeds during lapping experiment 49
4.4 Effect of time during polishing 54
ix
CHAPTER 5 CONCLUSIONS AND RECOMMENDATIONS 59
5.1 Introduction 59
5.2 Conclusions 59
5.3 Recommendations for future work 60
REFRENCES 61
APPENDICES 65
x
LIST OF TABLES
TABLE NO. TITLE PAGE
2.1 Physical and chemical properties of some optical glasses
(Izumitani, 1979)
5
2.2 Composition of BK7 optical glass (Izumitani, 1979; Bach and
Neouroth, 1995)
7
2.3 Properties of BK7 glass (Izumitani, 1979; Bach and Neouroth,
1995)
7
2.4 Three major families of manufactures diamond (Krar, 1995) 11
2.5 Preston coefficient of some glass polishing regimen utilizing
cerium oxide slurries (Izumitani, 1979)
19
2.6 List of CIMCOOL Fluids recommended for use in glass grinding
and abrasive machining
22
3.1 Experimental conditions of Ultrasonic Coring Machining 36
3.2 Lapping parameters 42
3.3 Polishing parameters used in the experiment 44
4.1 Summarizes the experimental results of ground surface for BK7
glass
46
4.2 Surface roughness results when measured at different table speed
during lapping
49
4.3 Results of polishing surface roughness for the samples A, B, C
and D.
54
xi
LIST OF FIGURES
FIGURE NO. TITLE PAGE
2.1 Diagram of various types of optical glass produced by Schott
(Anon, 1996)
6
2.2 Classification of optical glass based on chemical composition in
the nd versus νd plot (Clement, 1995)
6
2.3 Optical flat 8
2.4 Material removal mechanism in rotary ultrasonic machining
(Prabhakar el al., 1993)
13
2.5 Four removal hypotheses in glass polishing 16
2.6 Interaction between these base elements of lapping and polishing
process (Belkhir et al, 2007)
20
2.7 An example of cast-iron polisher 27
2.8 An example of soft-metal polisher 27
2.9 An example of spiral-grooved pitch polisher 28
2.10 An example of wax polisher 28
2.11 An example of polyurethane foam 29
3.1 Flow chart for BK glass work piece preparation 34
3.2 Initial state of BK7 Schott glass raw material 34
3.3 Ultrasonic coring tool 35
3.4 Schematic illustration of the experimental set up for rotary
ultrasonic machining (Hu et al., 2002)
36
3.5 BK7 glass work piece complete with holder and stand 37
3.6 Precision cutter used together with a specially designed holder and
stand to hold and prevent glass work piece from chipping
37
3.7 A specially design fixture that is capable of holding 6 work pieces 38
xii
at one time during flattening and grinding
3.8 Schematic diagram summarizes the overall experimental approach 39
3.9 Ultrasonic grinding set-up 40
3.10. Elements of the LP50 auto lapping plate flatness control system 41
3.11 Lapping jig with specimen holder of BK7 glass 42
3.12 Polishing machine and elements of polishing BK7 glass 43
3.13 Mitutoyo Form Tracer C5000 44
3.14 Axio Carl Zeiss high power microscope 45
4.1 Surface roughness increases when feed rate increase during
ultrasonic grinding
47
4.2 Surface morphology on BK7 glass with when grinding at different
feed rate
48
4.3 Effect of table speed of 20rpm on surface roughness during
lapping
50
4.4 Effect of table speed of 30rpm on surface roughness during
lapping
50
4.5 Effect of table speed of 40rpm on surface roughness during
lapping
51
4.6 Effect of table speed of 50rpm on surface roughness during
lapping
51
4.7 The combination effect of lapping speed on surface roughness
against lapping time
52
4.8 Surface morphology of lapped BK7 glass when lapping at
different lapping speeds
53
4.9 Reduction of surface roughness on sample A during polishing 54
4.10 Reduction of surface roughness on sample B during polishing 55
4.11 Reduction of surface roughness on sample C during polishing 56
4.12 Reduction of surface roughness on sample D during polishing 56
4.13 Relation between polishing time and surface roughness with
different initial surface roughness
57
4.14 Optical microscopic images of polished BK7 glass surfaces at
different polishing time
58
xiii
LIST OF APPENDICES
APPENDIX TITLE PAGE
A NG codes coring program of BK7 work piece (φ25 × 25)
mm
65
B NG codes flatting surface program of BK7 work piece
(φ25 × 6)mm
66
C NG codes grinding program of BK7 work piece (φ25 × 6)
mm
69
D Initial plate flatness monitor 70
E Flow chart adjustment of plate flatness monitor 72
F Steps for mounted work piece BK7 glass on stainless steel
holder
73
G Steps for slurry preparation of lapping process 75
I1 Lapping surface profile of Sample B at Lapping speed 50
rpm
76
I2 Lapping surface profile of Sample B at Lapping speed 40
rpm
77
I3 Lapping surface profile of Sample B at Lapping speed 30
rpm
78
I4 Lapping surface profile of Sample B at Lapping speed 30
rpm
79
J1 Polishing Surface profile of Sample A 80
J2 Polishing Surface profile of Sample A 81
J3 Polishing Surface profile of Sample A 82
J4 Polishing Surface profile of Sample A 83
K Machines for work piece preparation 84
CHAPTER 1
INTRODUCTION
1.1 Overview
Optical components can be found in infra-red systems, beam deflectors in
synchrotron radiation facilities and optical lenses. They are either in spherical or flat
surfaces, and require high precision in shape accuracy with low surface roughness
values (Zhong and Venkatesh, 1994).
Being made of advanced ceramics or optical glass, they are very difficult to
machine and shape because of their brittleness, extreme hardness to meet high
requirement on the high shape accuracy and the low surface roughness values in
certain applications (Zhong, 2002). Researchers have made much effort to
manufacture highly precise devices with good surface finish, and low sub-surface
damage (Van Ligten and Venkatesh, 1985; Venkatesh and Zohng, 1995).
In order to reduce the total manufacture time, it is preferable to obtain better
ground/lapped surface, with less fracture mode as possible in order to reduce
polishing time.
2
1.2 Problem statement
Lapping and polishing processes are important steps involved in optical glass
manufacturing activities. Researchers and manufacturers have put lot of efforts for
achieving low tolerance, better surface finish with defect-free in order to reduce
manufacturing cost.
Being made of glass, this material is well known for its difficulty to machine
and shape at higher accuracy because of their brittleness nature and possessive an
extreme hardness.
Optical glass requires these traditional steps of processing, ie. grinding,
lapping and polishing. Among these, polishing process is the most time consuming
process. Polishing time is very much limited dependent on the state of prior two
process, ie. grinding and lapping. Optimization on grinding and lapping will reduce
significantly the polishing time. However, there are many parameters contribute to
the successful of grinding and lapping. Among others, the grit size of abrasive, the
speed (spindle and table), feed etc. These parameters also depend on machine rigidity
which partly contributes to the final finishing of the work piece. To date, there are
very little literature reports on the steps of manufacturing optical flat which can be
considered as confidential to many manufactures.
1.3 Objective of study
The objectives of this project as follows:
i. To evaluate the effect of feed rates on surface roughness of ground BK7
glass.
ii. To evaluate the effect of table speeds on surface surfaces of lapped BK7
glass.
iii. To propose a feasible range of polishing time for BK7 glass.
3
1.4 Scope of the study
The scopes of study are as follows:
i. BK7 optical glass is selected for the study.
ii. Ultrasonic assisted grinding is used for preparing the initial surface before
lapping operation.
iii. Al2O3 abrasive slurry of 9μm is used in the lapping operation.
iv. Colloidal silica of 3μm is used as polishing slurry.
v. Load is fixed during lapping and polishing operations.
1.5 Organization of the thesis
First Chapter describes introduction, followed by the problem statement,
objective of the study and scope of study. The second Chapter prepared the back
ground on optical glass, optical flat and over view the principles of grinding, lapping
and polishing process optical glass. Third Chapter is details out methodology and
experimental works. Results and discussion are discussed in the Chapter four.
Chapter five discuses conclusions and recommendations for future work.
CHAPTER 2
LITERATURE REVIEW
2.1 Introduction
Considerable precision and experience required to work produce optical glass
ie. From milling glass block through grinding then lapping finally polishing to set a
finished optical glass. Grinding process is an important stage, as it prepares the
surface of the glass for subsequence lapping and polishing process. As usually
grinding process is carried out in stages using progressively finer grades.
The initial rough shape of the optics is produced by generating process using
rough grinding wheel. The next step is lapping operation which provides a surface
shape as close as possible to the final geometry. Additionally, the roughness and sub
surface damage left from generating (grinding) have to be reduced. Finally, the
optical surface is obtained by polishing, where finer abrasive lapping is used. Both
lapping and polishing uses loose abrasive as in slurry form to improve the optical
surface (Fielder, 1995; Horne, 1983)
2.2 Back ground on optical glass
Optical glass is defined as group of glass having well defined optical
properties, optical homogeneous performance and absence of bubbles, striae, strain
and inclusion (Clement, 1995). Optical glass is usually described by its refractive
5
index at the helium-d line, nd , (587.6nm) and its νd value (or Abbe number), which
is a measure of the dispersion or variation of index with wavelength. The Abbe
number is given by νd = (nd – 1)/(nf-nc) where (nf-nc) is the principle dispersion. nf is
,the refractive index at the hydrogen F line (486.1nm) and nc is the refractive index at
the hydrogen C line (656.3nm) respectively. The refractive indicates nd vary from
approximately 1.4 to 2.4 while the Abbe number varies from 15 to 100 (Marker and
Neuroth, 1995).
Optical glasses are sometimes classified as crowns, flints, barium crowns, etc.
(Clement, 1995). However, the boundaries of these various classes are not tightly as
shown in Figure 2.1. There are more than 250 different types of optical glasses that
can be classified by their chemical composition as shown in Figure 2.2. Optical
glasses acquire their properties through their chemical composition, melting process
and finishing methods. In order to obtain specific optical properties, chemical
compositions must often be chosen that lead to products with less than optimum
chemical resistance. Some of physical and chemical properties of optical glasses are
given in Table 2.1.
Table 2.1: Physical and chemical properties of some optical glasses (Izumitani, 1979)
Glass type Softening point(C)
Vickers hardness(kg/mm2)
Acid resistance
weight loss (%)
Water resistance
weight loss (%)
Silicate glasses SF6 470 413 1.3 0.03 KF2 490 627 0.07 0.07 FK1 475 666 1.9 - BK7 615 707 0.08 0.13 SK2 700 707 0.7 0.05 SK16 680 689 3.3 0.58
Borate glasses LaK12 670 743 1.7 0.35 LaLK3 650 762 1.9 0.70 LaK10 675 803 1.3 0.25 NbK10 670 803 1.2 0.02 NbF1 650 824 1.0 0.01
NbSF3 650 803 0.76 0.01 TaF2 685 847 0.74 0.01
6
Figure 2.1 Diagram of various types of optical glass produced by Schott (Anon, 1996)
Figure 2.2 Classification of optical glass based on chemical composition in the nd versus νd plot (Clement, 1995)
7
2.2.1 Composition and properties of BK7 glass
BK7 glass most commonly used optical glass materials for manufacture of
optical components such as glass mirror (Bach and Neuroth, 1995; Fang and
Venkatech, 1998). It is relatively hard glass, doesn't scratch easily and can be handled
without special precautions, almost bubble-free, high linear optical transmission in
visible range and chemically stable (Lim et al., 2002). The detailed composition and
properties of BK7 glass are given in Table 2.2 and 2.3 respectively.
Table 2.2: Composition of BK7 optical glass (Izumitani, 1979; Bach and Neouroth,
1995)
composition SiO2 B2O3 Na2O K2O BaO
weight % 70 11.5 9.5 7.5 1.5
Table 2.3: Properties of BK7 glass (Izumitani, 1979; Bach and Neouroth, 1995)
Property Value
Density ρ (g/cm2 ) 2.51
Abbe constant 64.07
Acid resistance weight loss (%) 0.08
Young's modules E, (103 N/mm2) 82
Knoop hardness HK(kg/mm2) 610
Fracture toughness KIC 1.08
Thermal expansion coefficient α, 10-6/K 7.1
2.2.2 Optical flats
Generally optical flats are used for testing and evaluating other optical
elements. An interference pattern is formed in the air between the flat and object
being evaluated, and this pattern is usually seen more through the flat than through
object. The pattern consists of alternating bright and dark bands or fringes which are
8
contour map of the thickness of the air film. If the surface of the optical is
significantly flatter than surface being evaluated, it is correct to interpret the
interference pattern directly as a contour map of the surface being evaluated. If the
flat is used on the top of the object, and interference pattern viewed through the flat,
it is advantageous to have an anti-reflection coating on the top surface of the flat (the
surface which does not touch the object being evaluated) as shown in Figure 2.3.
2.3 Overview on glass grinding process
In comparison to ductile materials, such as copper, the brittleness of glasses is
governed by its minuscule level of plasticity and extremely low fracture energy.
Therefore fracturing occurs before the flow limit is reached, which means that the
critical depth of penetration by an indenter is fairly small (Schinker and Doll, 1984).
Nevertheless, Busch (1968) and Schinker (1987) proved that cracks-free glass
machining (shaving) can be realized under a limited set of condition. Puttick (1989)
observed that after ductile shaving, additional spiral (shaped) swarf is delaminated
from the bottom of the machined grooves. It is assumed that the spiral swarf is due to
longitudinal compressive stress along the tool path
The crack initiation threshold increases with increasing water content, up to
12 wt% in experimental glasses, due to dissolution of large amounts of water in
glass, deformation by plastic flow is promoted. Taka assumed that, due to dissolution
of large amounts of water in glass, deformation by plastic flow is performed. This
finding seems to contradict most crack growth experiments and grinding experience,
Figure 2.3 Optical flat
9
which prove that speed of crack growth is increased in presence of water. (Takata,
1982).
The transition of the material removal regime from viscoplastic to brittle is
extremely crucial respect to the depth of the cut and to the introduced forces
(Schinker, 1984)
2.3.1 Grinding wheels designation and selection
Grinding wheel is made up two materials, the abrasive grains and the bonding
materials. It is produced by mixing the appropriate size of the abrasive with the
required bond and pressed into shape. The abrasive grains do the actual cutting and
the bond holds the grain together (Izman, 2004)
2.3.1.1 Abrasive types
Abrasive grains used for grinding wheels are very hard, highly refractory
materials and randomly oriented. Although brittle, these materials can withstand very
high temperature, there four types of abrasives that commonly used are (Krar, 1995):
i. Aluminum oxide
Aluminum oxide, one of the manufactured abrasive, is made by fusing
bauxite Ore in an arc-type electric furnace, and is used in the manufacture of
about 75% of all grinding wheels, it generally used for grinding high tensile-
strength materials. Aluminum oxide abrasive is manufactures in several
grades, which are identified by the amount of remaining impurities or the
addition of other ingredients. The most common types of aluminum oxide are
regular, semi friable, white, heavy-duty, and extra-heavy-duty.
10
ii. Silicon carbide
Silicon carbide is made by silica sand and carbon in the form of cock react
with one another when subjected to high temperature in electric furnace and
produce hard abrasive crystals. Silicon carbide is generally used for grinding
low-tensile-strength and nonmetallic materials. There are only two generally
recognized types of silicon carbide: regular or block, silicon carbide, and
green silicon carbide. The degree of purity determines the hardness and color.
Silicon carbide abrasive is harder than aluminum oxide, but it splinters more
readily and, therefore, is considered more brittle
iii. Cubic boron nitride (CBN)
CBN is synthesized in crystal form from hexagonal boron nitride (CBN) is
twice as hard as aluminum oxide, and its performance on hardened steels is
far superior. CBN is cool cutting, is chemically resistance to all inorganic
salts and organic compounds, and can with stand grinding temperature
1000°C. There are various types of CBN available to suit a variety of steel
grinding applications. All CBN products are used in the metalworking
industry and do not perform well on nonferrous or nonmetallic materials.
There are two main classes of CBN abrasive, monocrystalline and
microcrystalline.
iv. Diamond
To produce diamond by manufacturing process, the conditions of pressure
and temperature found far below the earth’s surface had to be duplicated.
This involved design and building capable of reproducing the condition
suitable for diamond growth. Manufactured diamond is the hardness
substance known and has a hardness range between 7,000 and 10,000 on
Knoop hardness scale. Manufactured diamond is excellent for machining
non-ferrous metals (like copper, zinc, aluminum and their alloys), plastics,
ceramics, glass, fiberglass bodies, graphite and other abrasive materials, there
are three of families of manufactured diamond as shown in Table 2.4.
11
Table 2.4: Three major families of manufactures diamond (Krar, 1995)
Type Description Product Applications RVG Irregularly shaped,
friable diamond Grinding wheels-
resinoid- and vitreous -bond systems
Grinding tungsten carbide and the metallics
RVG-W Metal-coated, friable Grinding wheels-resinoid-bond systems
Wet grinding tungsten carbide
and other metallics RVG-D Metal-coated, friable
diamond Grinding wheels-
resinoid-bond systems Dry grinding
tungsten carbide MBG-II Medium friability,
blocky diamond crystal, smooth surface
Grinding wheels-metal-bond systems
Grinding glass and ceramics
MBG-T Medium friability, blocky diamond crystal,
smooth surface
Plated tools-metal-bond systems
Grinding glass, ceramics, and composites
MBS Toughest, blocky diamond crystal, smooth
surface
Saw blades-metal-bond systems
Sawing and grinding stone and
concrete FINES Ungraded diamond Compounds, loose
abrasive, polishing wheels
Polishing and lapping metallic and nonmetallics
2.3.1.2 Bonds materials
Grinding wheels are composed of abrasive grains held to gather by bonds.
The amount of bound used determines the hardness (grade) of the wheel. Standard
grinding wheels may use one the following bonds: vitrified, resiniod, rubber, shellac,
silicate, and oxychloride. Vitrified, resiniod, and rubber bonds are the most common
and are used in over 90% the grinding wheels, basically classified as follows (Krar,
1995):
i. Vitrified bond
It is used in about 50% of the wheels produced. It is made of clay or feldspar,
which fuses at high temperature to form a glass like material. Most grinding
wheels with vitrified bonds operate at speed of about 6500 surface feed per
minute.
12
ii. Resinoid bond
These wheels can be operated at much higher speed (9,500 - 16,000 sf/min).
resiniod bond is suitable for all types wheels from hard, dense, coarse wheels
to soft, open, fine. Resinioed wheels are used for rough grinding operation,
such as snagging and cutting off, where rapid stock removal is important.
iii. Rubber bond
It made by vulcanizing a mixture of abrasive grain, crude rubber, and sulphur.
Rubber wheels are used extensively on thin cutoff wheels, where they
produce a relatively burr-free cut. They are also used as regulating wheels on
centerless grinders.
iv. Shellac bonds
Shellac-bonded wheels are used to produce high finishes on cutlery,
camshafts, and mill rolls. This type of wheel may be operated at speed up to
16,000sf/min.
v. Silicate bonds
These bonds release the grains more rapidly, it produces a cooler-cutting
wheel. It is used only to limited extent in wheels for cutlery grinding and in
discs used to grind the ends of springs where the heat generated should be
kept to minimum. Silicate-bonded wheels operate at speed of about
5,500sf/min.
vi. Oxeychloride bond
Abrasive grains are added to a mixture of magnesium chloride, magnesium
oxide, and water to form cold-setting cement. These wheels are used for dry
grinding operations on the ends of compression springs where a cool-cutting
wheel is necessary.
13
2.3.2 Over view on rotary ultrasonic machining
Rotary ultrasonic machining combines the material removal mechanisms of
the ultrasonic machining process and the conventional diamond grinding process.
These Include hammering (indentation and crushing under impact of the
ultrasonic vibrations), abrasion (the rotational motion of the cutting tool can be
modeled as a grinding process) and extraction (produced by the simultaneous action
of ultrasonic vibration and rotational motion of the tool) (Pei and Ferreira, 1998).
This is schematically illustrated in Figure2.4. The combination of these three
material removal mechanisms results in higher material removal rates in rotary
ultrasonic machining than those obtained by either the ultrasonic machining process
or the conventional diamond grinding process(Prabhakar el al., 1993)
Figure 2.4 Material removal mechanism in rotary ultrasonic machining (Prabhakar el al., 1993)
14
2.4 Lapping mechanism
Lapping is defined as a process where two surfaces are worn together
between a free rolling abrasive. This differs from a grinding or turning process,
where the abrasive particle is fixed within the wheel and cuts the material. A
consequence of this is that the shape of the lapping plate is constantly changing as
the plate wears and will greatly influence the flatness of the piece being lapped. This
ability has been exploited in many industries to generate optically flat surfaces on
glass, ceramics and other crystalline materials (Brown, 1979).
In other words, lapping can employed to remove excess material from flatten
chosen surfaces, whether they be held in precise orientation to the lapping plate or
free running within the confines of the plate area. However, the surface of the
lapping plate is continuously fed with free-rolling abrasive particles, in variable drip
rates, using a predetermined carrier fluid whilst the plate is rotating. Therefore hen
the surface is assumed to be lapped then can be applied to lapping plate which reset
on the abrasive grains in the fluid on that plate. A suitable load is applied to the
sample and the plate is set in motion. As the lapping process begins, the material to
be lapped settles on to the lapping plate and should begin to rotate with the plate
(depending on the area, and original flatness, of the material).
The abrasive particles will be forced under and across the material face in all
directions as it rotates. As the lapping process continues the material face being
lapped will assume the matching ‘shape’ to that of the lapping plate.
The main attraction of lapping process of lapping is that it is believed to
produce less stress on the substrate and induce less subsurface damage, particularly
in soft, brittle crystalline materials such as optical glass (Blkhir, 2007).
The objective and capabilities of lapping process, in general lapping
processes have the following characteristics which, however, vary in degree
according to the particular systems and equipment:
15
1. The rate of stock removal is low due to the low cutting speeds and
shallow penetration of the fine abrasive grains into the work surface.
2. Lapping carried out without generating significant heat; it is considered a
“cool” process which does not cause thermal damage.
3. Relatively low force is extending on the work piece that is usually held in
a well-supported manner; consequently lapping is often successfully
applicable to brittle materials and friable parts.
4. The general shape of the surfaces worked by lapping is mostly limited to
basic forms, such as flat, cylindrical, and spherical. It is used only
exceptionally for other shapes, such as screw threads.
5. the accuracy of form produced by lapping is excellent, particularly from
flat surfaces which constitute the major, although not exclusive, field of
application
6. Surface with the lowest degree of roughness can be produced by specific
lapping systems, but even in a general sense, lapped surface are very
smooth with regard to both the measurable surface finish and the visually
discernible reflectivity. This latter property of the worked surface is,
however, also affected by the work material and its hardness. Soft
materials usually have low reflectivity and display a matter surface, even
when lapped to a high degree of physical smoothness.
7. Functionally needed surface characteristics other than finish are also
accomplished by lapping, often degree not attainable by any other
method.
8. Size control can be excellent due to the low and, at the same time,
essentially constant rate of stock. Removed, which condition permits
controlling the amount of size reduction by limiting the time during which
the consistently uniform action of the lapping process is applied.
9. Economic benefits can also be derived when lapping applied to develop
work surfaces which could also be produced by other methods.
16
2.5 Polishing mechanism
In polishing, the material surface to be worked on is pressed against a soft
material (the polishing pad) such as felt, leather or a porous polymer, and there is
relative motion between the two surfaces. This combination of contact pressure and
relative motion alone does not cause material removal since the polishing pad is
usually much softer than the work piece material. So, slurry composed of fine
abrasive particles suspended in chemicals is introduced into the interface between the
work piece and the pad (Huan, 2005)
The material removal process in glass polishing is described either by wear
theory, flow theory, chemical theory or combination of these. Izumitani (1982)
proved by numerous experiments that the polishing rate of glass depends on
chemical durability of glass, and there is no relationship to micro hardness or the
softening point of glasses. That is true because polishing does not remove or bulk
material it self, but just the soft hydrate layer. This is unique property of glass which
is the source of ultra –smooth surfaces (Izumitani, 1979)
A number of other eminent scientists such as Rayleigh nad Beliby (1921),
Bowden and tabor (1950), Kaller, 1960, Bruche and Poope (1960), Izumitani (1980)
and Cook (1990), (1991) have investigated the physical and chemical aspects of
polishing over the years. There are four removal hypotheses for mechanical chemical
glass polishing as shown in Figure 2.5.
Figure 2.5 Four removal hypotheses in glass polishing
17
1. Abrasion
Newton’s abrasion is based on mechanical machining operation similar to
grinding. Material removal is achieved by including very cracks. Generally
hypotheses on abrasion apply only to the very first phase of the polishing
process (Kaller, 1980; Stien et al., 1999; Dunken, 1981).
2. Flow Hypothesis
The flow hypothesis links plastic materials displacement with local material
softening due to the frictional heating. These effects can be observed, taking
account of the viscoelastic characteristics of the chemically modified glass
substrate. The fundamental concept of the flow hypotheses was developed by
Rayleigh and Beilby (March, 1964; Bruch and Poppa, 1955; Poppa, 1957).
3. Chemical hypothesis
In the Chemical hypothesis, material removal is attributes to the formation
and removal of layer of gel. This layer is produced by incorporation of water
into substrate area of the glass. Various interaction between the constituents
of the glass and slurry have been observed, these depend on the composition
and properties of the polishing liquid and on the characteristic of the glass
materials. In addition to the dissolutions of the glass due to water, certain
elements may also affected by elution from the subsurface (Events et al.,
2001).
4. Friction wear hypothesis
The friction wear hypothesis was developed in response to the lack of
information provided by the chemical hypothesis regarding the influence
extorted by the polishing medium. The interaction between the polishing
medium and the glass material is not disputed. Instead, the mechanical
material removal has been replaced by a chemical reaction between the
polishing grain and constituent parts of the glass”chemical tooth”. The solid
body reaction tends to take place at defects in crystals lattice. This bond is
stronger than the bond within the glass matrix, causing removal of glass.
(Keller, 1957)
18
2.5.1 Polishing techniques
2.5.1.1 Pitch polishing
The final shape and finish were obtained by pitch polishing. There were many
different types of "rouge", and it was the secret of the skilled optician to choose the
proper combination of pitch and rouge to make miracle happen: production of
surface-unique properties of glass –with sub-nm smoothness and sub- μm shape
accuracy. At the same time, these small numbers expressed surface qualities that
unattainable in other production areas. The tangential components of any force put
on the work piece will force it to side along the lap surface, which is wetted with
slurry containing abrasive. Beside the speed of the motion, the normal force is
responsible for the amount of material removed from the work piece surface as well
as the amount of pitch displaced. At the same time, high spots of the lap and the
workpiece surface undergo higher pressures than the average parts, and, therefore,
they are removed preferentially. The proper choice of the pitch viscosity and
temperature control of the environment are decisive for the shape accuracy and the
obtainable smoothness (Brown, 1977).
2.5.1.2 Polyurethane polishing
This is polishing technique especially for time consuming processing steps. In
order to reach lower polishing cycle times, the polishing speed had been increased by
one or two orders of magnitude. As a consequence increased polishing pressures
were also necessary so that the workpiece did not merely float hydrodynamically on
the polishing slurry film-without any material removal. The Preston coefficient for
all kinds of standard glass polishing technique is almost the same as long as similar
chemistry is applied, see Table 2.5. The differences of material removal rate are
merely due to the different speed and pressures (Izumitani, 1979).
19
Table 2.5: Preston coefficient of some glass polishing regimen utilizing cerium oxide
slurries (Izumitani, 1979)
Polishing technique Glass type lap
Preston
coefficient c
10-13 P-1
High speed polishing of
eyeglass
Crown glass Polyurethane
pad
10
High speed polishing of
spherical precision optics
BK7 Polyurethane 6-10
Pitch polishing of spherical
precession optics
BK7 pitch 8
Pitch polishing of spherical
precision optics
Fused silica pitch 2
Full aperture pitch polishing
of spherical optics
ZERODUR Pitch with
cloth
2-8
Sub-aperture pitch polishing
of aspherical optics
ZERODUR 6
0.013
Elastic emission machining
Internal result
BK7 Polyurethane
ZrO2 grains
CeO2 grains 0.3
Float polishing Sapphire Tin/SiO2 0.02
2.5.1.3 Teflon polishing
Teflon polishing system is for the production of ultra precise flats. Using
Teflon polisher, the friction forces between lap and workpiece were reduce
dramatically in comparison to pitch polishing slow machine polishing process is able
to produce surfaces repeatable, with flatness of λ/200, which needed for Fabry-
Perot interfometers (Leistner, 1976).
20
2.5.1.4 Float polishing
Float polishing technique which was later applied to the gentle polishing
ceramics, cermets and glasses (Bennet et al., 1977). The technological and
commercial importance of the process gained because of its superiority heads. Float
polishing uses a rabidly rotating tin lap with an aqueous polishing slurry of colloidal
silicon oxide. The size of these polishing particles is in the range of just 4-7 nm,
where the particles for conventional polishing are 1 μm (Namba, 1977)
2.6 Components of lapping and polishing processes
All lapping and polishing process can be described as four component
system, that the mechanisms involved can be grasped understanding the interactions
among those components as shows in Figure 2.6.
Lapping and polishing are differentiated technology, but not is this
mechanistic view: the relative size of the (grit) and the surface layer removed may
change dramatically, but the processes as rely on interaction between these base
elements.
Figure 2.6 Interaction between these base elements of lapping andpolishing process (Belkhir et al, 2007)
21
2.6.1 Work piece
The object of polishing is to modify the work piece. Work piece vary in bulk
composition and may have probably variation as functions of both lateral dimension
and depth. Chemical compositions of material to be polished fall into specific
categories, based on the type of bonding involved. Materials to be polished may be
pure or may be mixtures with characteristics components sizes. Lateral or depth
dimension variation and may be discrete or continuous. The importance of the
chemical composition is that characteristically different materials are susceptible to
different polishing regimes (Evans et al., 1994).
2.6.2. Fluid
The fluid phase of the slurry may be characterized by its chemical
composition and by its physical properties. Chemical compositions of fluids include
water and non aqueous fluid like hydrocarbons and alcohols. Physical properties of
the fluid affect both fluid dynamics and material transport in polishing. These
properties include viscosity, density and thermal conductivity, all of which are
pressure and temperature dependent. These properties can also be varied by change
in the chemical composition of the fluid (Evans et al., 1994)
Fluids for Glass Processes, semisynthetic and synthetic fluids have both been
used for glass processes Table 2.6 provides a list of CIMCOOL Fluids recommended
for use in glass grinding and abrasive machining. In the industry today, the product
type of choice when working with glass typically is a synthetic water based fluid for
the following reasons: Clear product providing excellent visibility, Clean, Improved
settling of glass fines and Low foaming. Additives for Glass Processing Fluids may
be required as makeup to extend the life and performance of the fluid used in the
glass processing system. Examples of additives that may be required are: Settling
Aids, Corrosion Inhibitors and Antifoams, Fluid Requirements for Glass Processes:
A glass processing fluid should provide the following:
22
Table 2.6: List of CIMCOOL Fluids recommended for use in glass grinding and
abrasive machining
i. Optimum settling of glass fines
If settling is too rapid, fines will plug the lines. If it is too slow, re-circulation
may cause a rapid increase in the alkalinity of the mix, resulting in etching of
polished faces. In addition, fluids with optimum settling characteristics, keep
the glass fines from hard packing in the filtration system.
ii. Adequate lubrication
Lubrication is required for various operations, tool life, and shear blade life
and to improve diamond wheel life. If a product provides too much
lubrication it will cause the glass to slip in the machine.
iii. Good washing action
A grinding fluid with good washing action removes fines from polished faces
of the plate and any oil left on the glass from previous processing.
iv. Corrosion protection for the machine
The wet environment of glass processing has the potential to cause corrosion
to the machine and its tools. Glass processing fluids are formulated with
materials that provide protection for the tools and machine.
23
v. Foam Control
Low foaming products are important since the presence of foam may impede
the grinding process causing quality issues such as chipped edges
2.6.3 Abrasive
The function of abrasive in slurry is to mechanically remove material from
the surface of the work piece. The abrasives themselves can be distinguished by
number of factors, including chemical composition, size, shape and concentration,
while the effect of each of there factors may be important. Abrasives Requirements
are: The abrasive must have a shape that presents several sharp cutting surfaces,
High hardness at room and elevated temperatures, Controlled toughness or, rather,
ease of fracture allows fracture to occur under imposed mechanical stresses, Low
adhesion to the work piece, and Chemical stability (Evans, 1994).
Chemical composition of abrasive very to include material like diamond, and
CBN (which have network covalent bonds and are relatively inert), or alumina ,
silica ,etc (which are ionic network materials whose surface properties change with
Ph) chemical effect may be present of absent in given system, and may be permanent
or become activated(light or other means) in situ. Designer abrasives, with hard
oxide cores and soft organic shells, or with organic cores and hard oxide shells, may
also have special mechanism removal properties.
Size parameters involve both average size and size distribution, abrasives
used in polishing in range size three orders of magnitude from colloidal silica or
alumina particles which tens of nanometers in diameter in diameter to diamond and
silicon carbide which are tens of microns in size. Under some condition,
agglomeration of smaller particles become important, affecting the average size and
the size distribution. In some cases, the abrasives fracture, changing the average
particles size and shape during the lapping and polishing processes.
24
Shapes of different abrasives may be significant. Some granules are
characteristically spherical, but elliptical, blocky and shaper fractured are also used,
in the case fracturing of larger abrasives, the smaller ones will tends to present
sharper under polishing conditions.
Concentration effect may be negligible (in some polishing operations)
concentration may be given as weight or particles per volume.
Types of abrasives which are using in lapping and polishing processes as
follows (Hans and Neuroth, 1995):
i. Iron oxide (Fe2O3)
Only few are suitable for use as polishing media. Depending on the base
material and process use some ferric oxides have particularly good specific
polishing properties and combining different oxide it is possible to produce
exceptionally useful polishing media capable of imposing high degree of
luster to the surface of soft metals and alloys, particularly precious metals, in
addition to polishing metals iron oxide is used in fairly wide scale for
polishing glass (mirror glass, spectacle lens, and glass for precision optics) . It
has hardness 6-7 mohs scale.
ii. Chromium trioxide (Cr2O3)
More limited to use, probably because of it is high price it's known as" green
compo" is extra hard and finely crystalline. It is suitable to polishing hard
metal such as chromium and its alloys and steel, it has hardness 8-9 on mohs
scale.
iii. Vienna lime
It produced mostly from dolomite, and is an exceptionally fine powder with
particles size below 1 μm consists of mixture of calcium and magnesium
oxides. Emulsions
iv. Polishing chalk
Consists in the main (80-85%) of silica (SiO2) and (10-16%) of alumina
Al2O3, it is not carbonate. Polishing medium which has been in use for a long
25
time is polishing chalk. Chalk is very fine powder of laminar structure
although it is not as fine as Vienna lime.
v. Alumina (Al2O3)
polishing alumina to be suitable for polishing the alumina chosen must be
remain stable under the conditions of temperature and pressure which exit
during polishing and it my be remove executive material from the surface
being polishing. The aluminas can be divided in to group γ (gamma) and α
(alpha) alumina, α -alumina that is the real row material for polishing
alumina. γ –alumina is soft and does not posses any pronounced polishing
properties. α – alumina is hardened and depending on its crystals size can be
used for a wide variety of polishing process. Aluminas can be formulated
ranging from soft to very hard and from very fine to very coarse, resulting in
many different compositions, pastes, emulsions.
vi. Alundum also called "diamantine"
Which hardening oxides are incorporated into crystal structure by
incorporating e.g. chromium, Alumdum becomes ruby in color. It is an
exceptionally fine powder and it used particularly for polishing of precious
metals.
vii. Cerium oxide Formula (Ce2O3)
This material to great extent has replaced iron oxide in the glass – making
industry with good results. It is vet fine powder with no sharp edges, small
edition of another rate earth oxide, praseodymium oxide. Minor impurities of
this type do not influence the materials polishing efficiency.
viii. Beryllium oxide
Another rare earth plashing oxide is (Be O), it is very similar to cerium oxide,
it is white, very light, amorphous powder which is hard but also brittle i.e. in
some ways it is similar to aluminum oxide, and its high price. But very good
results are obtained with this product when it is used for polishing hard
metals
26
ix. Diamond
It is little used for polishing metals and plastics, its main applications being
for polishing precious stones, principally diamonds. Hard metals, glass,
quartz, porcelain, natural and artificial gem stones, precious stones, plastics,
hardened steel, etc can all be worked with diamond-impregnated wheels or by
the use of diamond –powder coatings on bobs, mops and belts.
2.6.4 Lap
The lap imposes relative motion between the granules and the work and
affects slurry and swarf transport through the contact. For example the cast iron
plates commonly used with alumina or silicon carbide to generate he base radii on
optical elements. A variety of other metals are used. Metal laps may be faced with
pitch or variety of different “cloth” (pellon, flet) or pads (e.g. polyurethane foams)
laps are also made using a thin film to face a substrate with a specific texture and by
stacking pads with different properties. Pads may contain a variety of groove
patterns. In addition to their role in imposing motion, laps can be important in
panelizing or otherwise shaping the work piece. Lap characteristics which affect the
material process depend on the specific lap condition. Bulk modulus (or in the case
of pitch, viscosity) affect the penetration of loaded graduals into the lap. Surface pads
may have different horizontal and vertical elastic and plastic behaviors which could
also affect polishing. Laps types which are using in lapping and polishing processes
as follows (Flynn and Powell, 1989):
i. Cast iron
This material can be used for polishers typical with a scratch hardness of nine
of greater on mohs scale as shown in Figure 2.7. When one becomes worn
and loses its figure, it is resurfaced and used again by completely machining
away the old surface by turning off at least 0.02 mm with one cut on center
lath. For a new, scrolled surface a practical initial loading is 0.0015 g for each
square centimeter-(e.g. 0.25 of 3-W-45 Hyprez diamond compound is spread
on 15 cm polisher with 3 cm central recess.
27
ii. Soft metal
The use of a soft-metal matrix in conjunction with diamond abrasives has
long been a conventional metal polishing combination and very good finishes
with the minimal edge turndown that speedily result can be obtained, too, on
a range of crystals.
Thus copper polishers are used when working crystals circa 9 mohs, indium
for those as soft as 2-4 mohs and tin or solder are generally useful at
intermediate values as shown in Figure 2.8.
Flatness better than λ/10(He) is readily obtained solder, tin and indium are easy
cast and since tin similar to solder in performance, one or other is
superfluous. Copper is machine able from solid but is slightly prone to
accidental contamination.
iii. Pitch
The tools that are made range from pure pitch to blends of pitch, wax, resin
and wood-flour. The Harding of pure it for a period of up to 24 hour. The
Figure 2.7 An example of cast-iron polisher
Figure 2.8 An example of soft-metal polisher
28
degree of hardness traditionally undergoes a qualitative test such as biting
thumbnail indenting. When flat surfaces together with an abrasive compound
and a lubricating film between them, a non –polishing state is gradually
achieved because the film become greater in thickness than the embedded
particle protrusion, as shown in Figure 2.9.
iv. Wax
Wax polishers as shown in Figure 2.10 can provide surfaces on glass and
crystals freer from sleeks than would be possible normally with pitch.
Moreover, edge definition is largely retained and this is often of paramount
importance when allied to surface finish. They are frequently in use and the
fact that is difficult to form and flatten them is circumvented now by facing
and scrolling technique. Wax polishers show a tendency to pick-up under the
damp-dry conditions considered ideal for pitch. Because of this inability to
withstand much drag even at 20 º C the polishing action slow, but finishes are
usually excellent. They are much less susceptible to accidental contamination
from coarser abrasive than pitch is, probably because particles are more really
embedded.
Figure 2.9 An example of spiral-grooved pitch polisher
Figure 2.10 An example of wax polisher
29
v. Polyurethane foam
This material, in sheet form, can be used in substitute for pitch when
polishing glass. It capable of conforming to a moderate radius when warmed
as shown in Figure 2.11. Two stainless steel plates are prepared by
machining, surface grinding and lapping to one or two fringes in 150-200
mm. these are warmed to 35 ºC and quantity of epoxy resin. The thin,
uniform layer thus obtained has space for lateral expansion under pressure.
The second plate is pressed upon it and the whole assembly kept for six hours
at 40-60 ºC on a hot plate. Polisher prepared in this way gives work flatness
to better than three bands in 80 mm without further running in.
2.7 Critical on literatures review
The finishing of glass objects by grinding, lapping and polishing has a long-
standing history. The use of glass lenses for optical experiments led scientists such as
(Feyman, 1985; Rayleigh, 1983) to ponder over the question as to what happens on a
microscopic scale during these abrasive processes. Significant progress on this
subject was made in past decades It was found that, for brittle materials like glass, a
threshold exists for the normal load on the abrasive particle, below which material is
removed from the work piece surface via plastic deformation.
The largest effort went into exploration of the microscopic mechanisms
underlying grinding of brittle materials. These are now well described by (Groenou,
1978/79; Molloy et al., 1987) Polishing, especially of glass, has also received a fair
amount of attention, very often with regard to the chemical aspects of the process
Figure 2.11 An example of polyurethane foam polishers
30
(Cook, 1990). Although lapping is an important abrasive finishing process,
comparable to grinding and polishing of brittle materials, very little is known about it
from a fundamental view point (Buijs and Van Houten, 1993).
Moreover, the aim of the lapping process is to generate a surface as smooth as
possible in order to minimize the subsequent polishing time. The material removal
rate is high as compared to polishing, because fracture is allowed to occur (Belhir et
al., 2007).
The lapping is a load-controlled process, which means that the load on the lap
is the parameter that is controlled, and not the in-feed rate. Because the load is only
transferred to the glass where the abrasives make contact between the tool and the
glass, and because the tool is a rigid one, high stresses are applied to the glass
causing fracturing to occur (Kirk and Wood, 1994).
The most important work on lapping of glass has been carried out by
Izumitani and Suzuki (Izumitani and Suzuki, 1973) who found the "lapping
hardness" to be a measure of resistivity to fracture by abrasive grains. They related
this behavior to indentation hardness and yield stress through Hill's theory on plastic-
elastic materials.
Buijs et al., (1993) studied the influence of glass composition, as reflected in
the material parameters Young's modulus, hardness and fracture toughness, on the
removal rate and surface roughness. It appears that the influence can be well
described with the concept of the lateral crack. The results obtained base on those
theory are in good agreement with the experimental results the model enables the
average load per particle to be calculated from Preston's coefficient, the material
parameters of the work piece and the shape and size of the abrasive.
Belkhira et al., (2007) studied the relation between the optical glass surface
quality and the wear of abrasive grains used in finishing process. The glass surface
quality was characterized by the roughness (rms, CLA and peak to valley). Alumina
abrasive grains (Al2O3) are used with average sizes (80, 40, 20, 7 mm) respectively.
The results obtained from this experimnatal indicate the abrasive grains size
31
reduction is proportional to the improvement of the surface quality during the
lapping. In the other hand, the abrasive powder undergoes some modifications during
its use in lapping. This modification appears under at least two aspects. The first
consists of the edges rounding caused by the grains wear and the second is the edges
angles change due to the grains fracture. Then, the granulometric distribution id
modified. In the end the participation rate of abrasives grains in every stage of
lapping process is between 0.19% and 15.63%, implying a slow wear of grains. The
marks the increase of the use time of lapping grains.
Zhong, (2000) studied the surface finish and integrity of glass, silicon, some
advanced ceramics and aluminum-based metal matrix composites (MMCs)
reinforced with ceramic particles, precision machined by various machining process.
The studies revealed that grinding/lapping operations using inexpensive machine
tools can produce ductile streaks on glass and silicon surface under good
grinding/lapping conditions. This resulted in significantly shortened polishing time to
secure an acceptable surface finish. If there several manufacture a lens, each
preceding process is very important for the successive processes. In order to reduce
the total manufacturing time, it is preferable to obtain better ground/lapped surface
with as many ductile streaks as possible in order to reduce polishing time.
On other word the forming and polishing processes of high precision optics
aspheric lens are currently being widely studied, because of the growing need in the
electronics industries for miniaturization and high performance, along with the
advent of opto-electrical technology (Kim et al., 2003). Aspheric lenses are known
for their superior optical characteristics over spherical lenses. They can improve
system performance and reduce the number of optical components required.
However, the aspheric form needs higher geometric accuracy than those which
traditional machining can provide (Lin, 2000; Kuriyagawa et al., 2001; Chiu and
Lee, 1997), especially as the wavelength of the light used in modern optics is getting
progressively smaller (Lin et al., 2000).
An experimental work described by Chuang and Tso, (2006) studied the
effect of lapping characteristics for improving the form error of an aspheric lens by
the experiment of this study was in three stages: the lens generation, lapping and
32
polishing. The abrasive grain size, lapping pads and the uniformity of the lapping
pressure distribution were selected as the major factors that affect the lapping process
the results obtained from the paper show that the methods proposed are effective for
improving the form accuracy and for reducing surface roughness with proper lapping
parameters.
CHAPTER 3
RESEARCH METHODOLOGY
3.1 Introduction
This chapter highlights the research methodology used to carry out this
project. This research work was divided into two main activities: (1) work piece
preparation, (2) experimental trials. Results from experimental trail provide
information on: (1) the effect ultrasonic feed rate on the ground surface of BK7 glass,
(2) the effect plate speed on the surface roughness (Ra), and surface morphology of
BK7 glass. Variations of surface roughness due to differences in table speed affect
polishing time.
3.2 Overview Work piece preparation
First step of sample preparation was coring where cylindrical glass was
obtained from glass block, the size was about φ25 ×25 thick. Precision cutter was
used to slice the cylinder to optical flat size φ25 ×6 thick. Ultrasonic assisted
grinding was used to flatten the surface which was slightly tapered after cutting with
precision cutter. Figure 3.1 shows the flow chart work piece preparation.
34
BK7 glass material were supplied by Schott glass in a rectangular block
having dimension of 100 mm (length) × 70mm (width) × 25mm (thick) as shown in
Figure 3.2.
BK7 glass was chosen in this study because it is one of the most commonly
used optical glass materials for the manufacture of optical components such optical
glass mirror (Bach and Neuroth, 1995; Fang and Venkatesh, 1998). It has good
scratch resistance properties, almost bubble-free, high linear optical transmission in
the visible range and chemically stable (Lim et al. 2002).
Figure 3.2 Initial state of BK7 Schott glass raw material
Initial block (100*70*25) BK7 glass
Ultrasonic core machining (φ25mm x 25mm)
Flattening process by Ultrasonic grinding φ 25 × 6mm
Slicing process by Precision cutter (φ25mm x 6mm)
Figure 3.1 Flow chart for BK glass work piece preparation
35
3.2.1 Ultrasonic Core Machining
Rotary ultrasonic machining is regarded as one of the cost-effective
machining methods for optical glass. It is a hybrid machining process that combines
the material removal mechanisms of diamond grinding and ultrasonic machining
(Jahanmir, et al, 1993; Prabhkar, 1992). The ultrasonic system comprises of an
ultrasonic spindle, a power supply, and a motor speed controller.
The power supply converts 50 Hz electrical supply to high frequency (20
kHz) AC output. This is fed to the piezoelectric transducer located in the ultrasonic
spindle. The ultrasonic transducer converts electrical input into mechanical
vibrations. The motor attached at top the ultrasonic spindle supplies the rotational
motion of the tool and different speeds can be obtained by adjusting the motor speed
controller. RUM experiments were performed on a Sonic-mill 10 series.
The cutting tool was metal-bonded diamond core drill (N.B.R. Diamond Tool
Corp., outer diameter = 25.24 mm, inner diameter = 27.68 mm, grit mesh size =
120~140). This shown is in Figure 3.3.
Figure 3.4 is schematic illustration of RUM. In RUM, a rotary core drill with
metal-bonded diamond abrasives is ultrasonically vibrated and fed toward the work
piece at a constant feed rate or a constant force (pressure). Coolant pumped through
the core of the drill washes away the swarf, prevents jamming of the drill, and keeps
it cool. Experimental conditions are listed in Table 3.1.
Figure 3.3 Ultrasonic coring tool
36
Table 3.1 Experimental conditions of Ultrasonic Coring Machining
Parameter Unit Value
Spindle speed RPM 1000
Frequency kHz 20
Vibration power supply a % 25
Coolant pressure MPa 200
Vary feed rate mm/min 2.4 5.4 8.4 a
Vibration power supply controls the amplitude of ultrasonic vibration.
3.2.2 Slicing process
Precision cutter was used to cut the core drilled BK7 glass into the required
thickness, ie. φ25 × 6mm.
Before slicing the core drilled BK7 glass work piece (φ25 × 25), a specially
designed holder and stand was developed to facilitate the slicing on the precision
cutter. The holder so assists two small bolts to prevent chipping of glass during
Figure 3.4 Schematic illustration of the experimental set up for rotary ultrasonic machining (Hu et al., 2002)
37
cutting off. The glass piece was fixed into by melting at 70 C° to melt wax after
cooled. As shown Figure 3.5.
Figure 3.5 BK7 glass work piece complete with holder and stand
The bronze bonded diamond wheel of 100/120 US mesh with 35%
concentration was used for slicing the glass piece (1.54 carats/cm3) as shown in
Figure 3.6. Parameter for cut off BK7 as follow:
• Speed: 300 RPM.
• Feed rate: 1.2 mm/min.
Bk7 glass
Holder
Stand
Figure 3.6: Precision cutter used together with a specially designed holder and stand to hold and prevent glass work piece from chipping
BK7 glass (φ25 × 25) mm
Fixture
Stand
Diamond wheel
38
3.2.3 Flattening Process by RUM Machine
Flattening glass A specially designed fixture that capable to hold 6 work
pieces simultaneously was used during flattening and grinding the specimen. This
fixture is shown in Figure 3.7.
Figure 3.7 A specially design fixture that is capable of holding 6 work pieces at
one time during flattening and grinding
The parameters for flattening condition as follows:
• Work piece materials: BK7 optical glass (φ 25 × 6) mm
• Coolant pressure: 120-150 MPa.
• Vibration frequency: 20 kHz.
• Speed: 1000 rpm
• Vary depth of cut: (50, 40, 30, 15, and 10) μm.
• Feed rate: 2.5 mm/min
• Grinding tool: diamond tool with mesh size =100-120, inside diameter (15.7
mm) and outside diameter (25.38 mm).
3.3 Overall of the methodology
The over all methodology used for experimentation to achieve objectives as
listed below:
39
1. Identify and select suitable ultrasonic feed rate parameter to be investigated
that affect surface roughness and surface morphology.
2. Identify lapping speeds and rage of lapping time to be investigated that
affect surface roughness and surface morphology.
3. Explore the feasible range of polishing time for BK7 glass. Figure 3.8
shows the summary of the overall experimental approach adopted in this
study.
Figure 3.8 Schematic diagram summarizes the overall experimental approach
Input
Experiments
Response
Output Feasible range of lapping speeds and polishing time for BK7 glass Effect of feed rate on surface roughness
Quantitative & qualitative analysis Surface roughness = Ra and surface morphology
Lapping Vary table speeds
Polishing Fixed all polishing parameters
Work material BK7 glass
Ultrasonic grinding at various feed rates
40
3.3.1 Ultrasonic grinding experiment
The set-up of Rotary Ultrasonic grinding process is shown in Figure 3.9. The
purpose of this experiment is to study the effect of process parameters (feed rate) on
the surface roughness (Ra) and surface morphology.
Grinding with ultrasonic conditions is as follows:
• Work piece materials: BK7 optical glass.
• Coolant pressure: 120-150 MPa.
• Vibration frequency: 20 kHz.
• Speed: 1000 rpm.
• Depth of cut: 5μm.
• Vary feed rate (mm/min): 0.5, 1.5, 2.5, and 3.5.
• Specification of tool: diamond, mesh size = 325-400, inside diameter
(15.7mm), and outside diameter (25.38mm).
fixture
Diamond tool
Work piece
Ultrasonic head
Diamond tool
Figure 3.9 Ultrasonic grinding set-up
41
3.3.2 Lapping experiment
3.3.2.1 Setting-up the flatness of lapping plate
Lapping plate flatness control system is controlled from the "Process Screen"
section of the machine control panel. The system allows the operator to set a target
value for the shape of the lapping plate, which is then constantly checked by the plate
flatness monitor and displayed on the "Process Screen". Any variations from this
plate shape are automatically corrected by the machine’s control system while
processing continues. Figure 3.10 shows the elements of lapping plate flatness
control system.
The aim of the lapping process is to generate a surface as smooth as possible
in order to minimize the subsequent polishing time. The material removal rate is high
as compared to polishing, because fracture is allowed to occur. In lapping a rigid iron
is moved under load over a glass surface, or vice versa, with abrasive particles
suspended in water between them. Abrasive used during lapping operation is alumina
particles (loose abrasive lapping).
Typical material removal rates are 3 mm/min for 5 mm particles to 90
mm/min for 100 mm particles. (Lambropoulos et al., 1999).
The lapping process is carried out by setting in contact a BK7 glass work
piece (φ25 × 6) mm, in rotation with a cast iron lap (diameter 400mm) under constant
lapping load.
Figure 3.10 Elements of the LP50 auto lapping plate flatness control system
42
BK7 glass specimen was glued with wax on stainless steel holder as shown in
Figure 3.11(a). After that the holder was screwed on the lapping jig as shown in
Figure 3.11(b).
The gap between the work piece and the lapping plate is fed continuously by
alumina abrasive slurry, during lapping operation. The experiment was conducted at
four different table speeds: 20, 30, 40, and 50 rpm. Table 3.2 shows lapping
parameters used in the experiment.
The pressure applied on the work piece was set constant and lapping time
varied from 2 to 12 minute. The lapping operation was stopped every two minutes to
check the surface roughness and to evaluate the changes in surface morphology on
the glass specimen.
Table 3. 2: Lapping parameters
Parameter Value
Abrasive slurry(alumina) 9μm
Oscillation( cycle/min) 18
Vary plate speed(rpm) 20 30 40 50
Lapping time(minutes) 2 to 12
Figure 3.11 Lapping jig with specimen holder of BK7 glass
(a) (b)
43
3.3.3 Polishing experiment
The polishing process is used to get rid of damage layer introduced from
earlier processes, and to provide high quality surface. The polishing is a load-
controlled process, which means that the load on the polishing pad is the parameter
that is controlled, and not the in-feed rate. Because the load is only transferred to the
glass where the abrasives make contact between the pad and the glass.
The polishing process is carried out by setting in contact a BK7 glass work
piece (φ25 × 6) mm, in rotation with a chemomnt pad fixed on stainless steel plate
(diameter 20 cm) in rotation, under a lapping load.
BK7 glass was waxed on aluminum holder. After that the holder was placed
on driving plate as shown Figure 3.12. Colloidal silica was controlled to drip on the
polishing plate continuously.
The gap between the work piece and the polishing plate is fed continuously
by the colloidal silica abrasive slurry. The polishing experiment was conducted using
the following fixed parameters: table speed 100 rpm, pressure 5N, and slurry with
3μm colloidal silica. Table 3.3 shows polishing parameters used in this experiment.
Polishing time was varied from 1 to 15 minutes. The polishing operation was stopped
BK7 glass
Colloidal silica
Chemomnt pad
Spindle
Driving Plate
Figure 3.12 Polishing machine and elements of polishing BK7 glass
Holder
44
every two minutes for checking surface roughness (Ra) and evaluates surface
morphology.
Table 3.3: Polishing parameters used in the experiment
Parameter unit
Abrasive slurry 3 μm Colloidal silica
Vary plate speed 100 rpm
Lapping time 1 to 15 minutes
3.4 Analytical and measuring instruments
3.4.1 Surface roughness measurement
Figure 3.13 shows the mitutoyo form Trace C5000 which was used to
measure surface roughness. Among the parameters set during measurement were
Measured length: 10.00mm, Measured pitch 0.0100mm, Cut off 0.8 mm ,Roughness
pitch 0.005 m/sec, and Measuring speed: 0.1000m.
Figure 3.13 Mitutoyo Form Tracer C5000
45
3.4.2 Surface morphology analysis
Figure 3.14 shows the Axio Carl Zeiss high power microscope. This
equipment was used to capture the image of ground, lapped and polished BK7 glass
surfaces. All the measurements were done at 20× magnification using dark and bright
field objectives.
Figure 3.14 Axio Carl Zeiss high power microscope
CHAPTER 4
EXPERIMENTAL RESULTS AND DEISCUSSION
4.1 Introduction
This chapter presents the experimental results and discussion. The results are
divided into three, ie. grinding results, lapping results and polishing results. The
discussion focuses on the effect of feed rate, table speed and time on the surface
roughness and surface morphology. At the end of this chapter, feasible range of
grinding, lapping and polishing parameters are concluded.
4.2 Effect of feed rates in ultrasonic grinding
Table 4.1 summarizes the experimental results for the four BK7 work piece
when ultrasonic ground at different feed rates. These results are plotted as shown in
Figure 4.1.
Table 4.1: Summarizes the experimental results of ground surface for BK7 glass
Specimens Parameters
A B C D
Feed rate (mm/min) 0.5 1.5 2.5 3.5
Surface roughness (μm) 0.721 0.774 0.829 0.847
47
0.847
0.7213
0.774
0.8297
0.7
0.72
0.74
0.76
0.78
0.8
0.82
0.84
0.86
0 0.5 1 1.5 2 2.5 3 3.5 4
Feed rate( mm/min)
sur
face
roug
hnes
s R
a(um
)
Figure 4.1 Surface roughness increases when feed rate increase during ultrasonic grinding
Based on Figure 4.1, it is clearly shown that as the feed rate increases from
0.5 to 3.5 mm/min, the surface roughness also increased. This phenomenon is similar
to other machining processes without ultrasonic features.
Figure 4.2 illustrates view graphs of the surface morphology at these above
grinding conditions. The micro fractures on the glass surface become deeper and
larger as the feed rate increased from 0.5 mm/min to 3.5 mm/min. The introduction
of ultrasonic amplitude and frequency in these experiments do not facilitate ductile
mode material removal. Instead, all the results obtained from these trails are 100%
fracture surface. The micro fractures on the ground surfaces show clearly the semi-
circular cracks which are typical crack found on hard and brittle materials like glass
as conducted by many other researcher (Lawn, 1993).
The Rougher surface occurs at higher feed rate which due to higher resistance
for chip removal to escape from cavity. The rubbing action between chip and rod’s
periphery worsen the rod surface.
48
Figure 4.2 Surface morphology on BK7 glass when grinding at different feedrate
Ra=0.849μmF/R= 3.5mm
Ra=0.829μm F/R= 2.5mm
Ra=0.774μmF/R= 1.5mm
Ra=0.721μm F/R= 0.5mm
Surface roughness (Ra)
Feed rate (mm/min)
49
4.3 Effect of table speeds during lapping experiment
Table 4.2 shows the experiments date and surface roughness results when
table speeds were increased from 20 to 50 rpm. The surface roughness on the
specimen was measured at interval of 2 minutes to twelve minutes.
Table 4.2: Surface roughness results when measured at different table speed during
lapping
Surface roughness Ra (μm) after lapping time Lapping
speed
(rpm)
Specimens 2min. 4 min. 6min. 8min. 10.min. 12min.
20 A 0.716 0.677 0.613 0.507 0.454 0.366
30 B 0.737 0.682 0.572 0.401 0.354 0.305
40 C 0.718 0.665 0.531 0.354 0.294 0.227
50 D 0.767 0.594 0.477 0.302 0.200 0.221
The lapping parameters (lapping speed and lapping time) affect the material
removal BK7 glass. As the table speed increases, the material removal rate is faster.
The progresses of in surface roughness over charges in lapping speeds are shown in
Figures 4.3 – 4.6. Figure 4.7 summarizes the results. As can bee seen in Figure 4.7,
the roughest surface was lapped faster than the rest samples. This indicates that table
speed influence the material removal rate of on ground BK7 glass.
It is noticed also at the highest table speed of 50rpm, the surface roughness
reached at saturation point within 10 minutes. After 10 minutes, surface finish
becomes roughest. This phenomenon is called saturation lapping point. Beyond this
point, the surface roughness will no longer improved regardless of extended lapping
time.
50
0.721 0.7160.667
0.613
0.507
0.454
0.366
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0 2 4 6 8 10 12 14
Lapping time (Min)
Surf
ace
roug
hnes
s R
a(um
)
Figure 4.3 Effect of table speed of 20rpm on surface roughness during lapping
0.7740.737
0.682
0.572
0.401
0.3050.354
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
0 2 4 6 8 10 12 14
Lapping time (Min)
Surf
ace
roug
hnes
s R
a(um
)
Figure 4.4 Effect of table speed of 30rpm on surface roughness during lapping
51
0.8297
0.718
0.665
0.531
0.354
0.2270.296
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
0 2 4 6 8 10 12 14
Lapping time (Min)
surf
ace
roug
hnes
s R
a(um
)
Figure 4.5 Effect of table speed of 40rpm on surface roughness during lapping
0.847
0.767
0.594
0.477
0.302
0.2 0.221
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
0 2 4 6 8 10 12 14
Lapping time (Min)
Surf
ace
roug
hnes
s Ra(
um)
Figure 4.6 Effect of table speed of 50rpm on surface roughness during lapping
Saturated surface roughness
52
In this experiment, the highest surface roughness was purposely chosen to be
lapped using speed 50rpm. This is to evaluate the effect of table speed. The
smoothest surface roughness 0.721 was lapped using table speed of 20 rpm. The
order uses followed accordingly. At the end of 12 minutes lapping operation, the
results become reversed to initial surface roughness condition.
It is concluded that table speed during lapping directly influence the material
removal rate. Slow table speed removes material much slower that faster table speed.
0.366
0.305
0.227
0.221
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
0 2 4 6 8 10 12 14
Lapping time (Min)
Surf
ace
roug
hnes
s R
a(um
)
RPM20RPM30RPM40RPM50
Figure 4.7 The combination effect of lapping speed on surface roughness against lapping time
Figure 4.8 is shows a series of selected microscopic images which were taken
lapping times (2, 6, 12 minutes). Fracture density of the ground work piece at initial
surface roughness decrease with gradually increase lapping time. The size of
microfractures also reduced as lapping time increase.
Figure 4.8 Surface morphology of lapped BK7 glass when lapping at different lapping speeds
Lapping time (min)
12 6 2
30
40
50
0
20
30
40
50
Lapping Speed (rpm)
53
54
4.4 Effect of time during polishing
The four lapped specimens were polished at the same condition and the
progress of surface finish reduction was observed at two minutes interval fifteen
minutes. The surface roughness results of each samples are shown in Table 4.3 and
plotted in Figure 4.9- 4.12. Figure 4.13 summarizes all the plots into one graph.
Table 4.3: Results of polishing surface roughness for the samples A, B, C, and D.
Surface roughness Ra (μm) after polishing time
specimens 1min. 3min. 5min. 7min. 9min. 11min. 13min. 15min.
A 0.264 0.189 0.162 0.143 0.133 0.107 0.103 0.087
B 0.214 0.175 0.169 0.134 0.118 0.107 0.090 0.083
C 0.208 0.128 0.124 0.103 0.084 0.076 0.075 0.063
D 0.199 0.122 0.078 0.057 0.054 0.044 0.040 0.038
0.305
0.264
0.189
0.1620.143
0.133
0.107 0.1030.087
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0 2 4 6 8 10 12 14 16
Polishing time (min)
Surf
ace
roug
hnes
s R
a(um
)
Figure 4.9 Reduction of surface roughness on sample A during polishing
55
A similar trend of reduction in surface roughness over polishing time is seen
in the plotted graphs. Sample D with lowest initial surface finish during lapping
exhibits faster material removal rate as compared to other sample A, B, C as
expected. As the polishing time increase, the dark area On the initial lapped dark area
on the initial lapped surface becomes brighter (see Figure 4.14) the surface finish
improves when dark area diminish from the sample surface.
0.305
0.214
0.175 0.169
0.134
0.1180.107
0.090.083
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0 2 4 6 8 10 12 14 16
Polishing time (min)
Surf
ace
roug
hnes
s(um
)
Figure 4.10 Reduction of surface roughness on sample B during polishing
These dark spots represent microfracture on the sample surface. As for
compression purposes, at the end of 15 minutes, polishing, sample D shows very
minimum number of dark spots than sample A, B and C. The surface roughness of
trained on these samples correspond to these conditions.
56
0.227
0.208
0.128 0.124
0.103
0.0840.076 0.075
0.063
0
0.05
0.1
0.15
0.2
0.25
0 2 4 6 8 10 12 14 16Polishing time (Min)
Sura
fce
roug
hnes
s R
a(um
)
Figure 4.11 Reduction of surface roughness on sample C during polishing
0.221
0.078
0.199
0.122
0.0570.054
0.0440.04 0.038
0
0.05
0.1
0.15
0.2
0.25
0 2 4 6 8 10 12 14 16
Polishing time(min.)
surf
ace
roug
hnes
s R
a(um
)
Figure 4.12 Reduction of surface roughness on sample D during polishing
57
0.087
0.083
0.063
0.038
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0 2 4 6 8 10 12 14 16
polishing time (min)
surf
ace
roug
hnes
s R
a (u
m)
20RPM
30 RPM
40 RPM
50 RPM
Figure 4.13 Relation between polishing time and surface roughness with different initial surface roughness
It is obvious that when initial surface roughness is good, the end result after
polishing will be good, and vice versa.
The minimum Ra obtained on sample D was 38nm. This result would much
lower if rigid polishing machine is used.
The current polishing results were obtained using Phoenix Beta machine
which is far less rigid as compared to Logitch lapping and polishing machine. It is
expected a surface finish of less than 10nm will be obtained with the Logitech
machine.
CHAPTER 5
CONCLUSIONS AND RECOMMENDATIONS
5.1 Introduction
This chapter describes conclusions and recommendations based on the
current research work. The main objectives of this research are to study the lapping
parameters on the surface roughness and to propose a feasible range of polishing
time for BK7 glass.
5.2 Conclusions
The following conclusions can be drawn from the project:
1. Higher feed rates produce rougher surface than lower feeds on ground BK7
glass during ultrasonic grinding. The reason behind this phenomenon is
similar to other conventional material removal processes without ultrasonic
features.
2. Table speeds of lower than 50 rpm are less effective for lapping BK7 glass
using 9μm Aluminum Oxide. Saturation point for lapping at this condition is
10 minutes beyond which it increases the Ra value.
60
3. Polishing time of less than 15 minutes is feasible to polish BK7 glass on a
more rigid machine.
5.3 Recommendations for future work
Grinding, lapping and polishing are important processes for shaping brittle
materials like glass, silicon, and some advanced ceramics.
These three step operations are commonly being used in optical glass
manufacturing where polishing is the most time consuming time among the three
processes
Good surface finish obtained from grinding operation facilitates lapping and
polishing processes.
The following are the recommendation for the future work:
(1) A rigorous study should be carried out to determine the effect of more than
one permeates of lapping process the affect on surface roughness and surface
integrity. A suitable design of the work piece under of experiment method is
recommended to be used for a achieving this purpose.
(2) The effect of ultrasonic features on the ground surface should be explored
further on the surface finish and ductile streaks formation. A suitable range of
amplitude must be studied to avoid massive chipping on the ground surface.
61
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65
APPENDIX A
NG codes coring program of BK7 work piece (φ25 × 25) mm
O0307 (CORRING)
N1 G40 G49 G80 G17;
N2 G90 G54 G00;
N3 G54 G90 G00 X0 Y0;
N4 X82.0 Y52.0;
N5 G43 Z5.0 H04 m03 S1000;
N6 m08;
N7 m52;
N8 Z-26.0 F5.4;
N9 G80 Z10.0;
N10 m05;
N11 m09;
N12 G91 G28 Z0;
N13 m30 G53 G00 Z0;
N14 G28 X0 Y0;
N15 m30;
66
APPENDIX B
NG codes flattening surface program of BK7 work piece (φ25 × 6)mm
O0407 (FLATINING)
N1 G45 G17 G40 G49 G80 G91 G21;
N2 G0 G90 X0.0 Y0.0;
N3 m08;
N4 S1000 m03;
N5 m52;
N6 G43 H05 Z50.0;
N7 G80 X-28.0 Y0.0 Z10.0;
N8 G0 Z1.5;
N9 G1 Z-0.050 F2.5
N10 G1 X27.0;
N11 G0 Z20.0;
N12 G0 X-28.0 Y0.0;
N13 G1 Z1.5;
N14 G1 Z-0.100;
N15 G1 X27.0
N16 G0 Z20.0;
N17 G0 X-28.0 Y0.0;
N18 G1 Z1.5;
N19 G1 Z-0.150;
N20 G1 X27.0
N21 G0 Z20.0;
N22 G0 X-28.0 Y0.0;
N23 G1 Z1.5;
N24 G1 Z-0.190.
N25 G1 X27.0
67
N26 G0 Z20.0;
N27 G0 X-28.0 Y0.0;
N28 G1 Z1.5;
N29 G1 Z-0.230;
N30 G1 X27.0
N31 G0 Z20.0; N12 G0 X-28.0 Y0.0;
N32 G1 Z1.5;
N33 G1 Z-0.260;
N34 G1 X27.0
N35 G0 Z20.0; N12 G0 X-28.0 Y0.0;
N36 G1 Z1.5;
N37 G1 Z-0.290.
N38 G1 X27.0
N39 G0 Z20.0;
N40 G0 X-28.0 Y0.0;
N41 G1 Z1.5;
N42 G1 Z-0.310.
N43 G1 X27.0
N44 G0 Z20.0;
N45 G0 X-28.0 Y0.0;
N46 G1 Z1.5;
N47 G1 Z-0.330;
N48 G1 X27.0
N49 G0 Z20.0;
N50 G0 X-28.0 Y0.0;
N51 G1 Z1.5;
N52 G1 Z-0.340.
N53 G1 X27.0
N54 G0 Z20.0;
N55 G0 X-28.0 Y0.0;
N56 G1 Z1.5;
N57 G1 Z-0.350.
N58 G1 X27.0
N59 G0 Z20.0;
68
N60 m53;
N61 m05;
N62 m09;
N63 G91 G28 Z0;
N64 G53 G0 Z0;
N65 G91 G28 X0.0 Y0.0;
N66 m30;
69
APPENDIX C
NG codes grinding program of BK7 work piece (φ25 × 6) mm
O0407 (GRINDING)
N1 G45 G17 G40 G49 G80 G91 G21;
N2 G0 G90 X0.0 Y0.0;
N3 m08;
N4 S1000 m03;
N5 m52;
N6 G43 H05 Z50.0;
N7 G80 X-28.0 Y0.0 Z10.0;
N8 G0 Z1.5;
N9 G1 Z-0.05 F3.5
N10 G1 X27.0;
N11 G0 Z20.0;
N12 G0 X-28.0 Y0.0;
N13 G1 Z1.5;
N14 G1 Z-0.100
N15 G1 X27.0
N16 G0 Z20.0;
N17 m53;
N18 m05;
N19 m09;
N20 G91 G28 Z0;
N21 G53 G0 Z0;
N22 G91 G28 X0.0 Y0.0;
N23 m30;
70
APPENDIX D
Initial plate flatness monitor
The plate flatness monitor is calibrated at Logitech prior to dispatch of the
machine. However, as it is a precision instrument, it is necessary for the operator to
check this calibration prior to the first use of the monitor in his or her own
laboratory. If correctly calibrated, it should always be possible to obtain a monitor
reading of between +/- 10 microns, which can then be offset by zeroing the monitor.
After selecting the "Machine Setup" screen has been pressed, the screen will
change to show a screen similar to "A" below. To setup your LP50 for Automatic
Lapping Plate Flatness Control, ensure that the "Process Type" display reads
"Lapping", that the "Jig Type" display reads as the correct jig being used and that the
"Arm Control" display is changed to show "Flatness". At this point the screen ("B")
will display an option for zeroing the monitor and a warning message telling the
operator to remove the roller arm assemblies from each workstation.
Thoroughly clean the bearing surfaces of the Plate Flatness Monitor; clean
the Granite Master Flat Block supplied equally thoroughly. Attach the long monitor
cable to the sockets on the monitor and the machine. To zero the Plate Flatness
Monitor, select "Monitor Set-up" using the joystick. The screen should change to
display the "Raw Plate Shape" reading ("C").
71
The "Raw Plate Shape" represents a snapshot reading taken from the Monitor
which is then averaged against the "Actual Plate Shape" reading which is read from
the monitor over a 30 second interval. During this time the operator is not able to
navigate away from this screen as the machine requires to collect data from across
the monitor. This information is then assimilated and processed before the reading is
shown as in screen "D". Once the "Raw Plate Shape" and the "Actual Plate Shape"
information have been displayed, the operator should navigate to the "Auto Zero"
option shown in screen "E".
NB If the reading is not between +/-10 microns it will be necessary to
recalibrate the monitor as described in the next section of this manual. Select the
"Auto Zero" option by pressing the button on top of the joystick and the machine will
"Zero" the monitor, as shown in screen "F". This operation involves the machine
taking the averaged reading from the "Actual Plate Shape" and recognizing it as
being the official zero reading, i.e. a completely flat reading, which will be used
throughout the lapping process.
At this point the monitor has been zeroed and the operator can exit this screen,
returning to the Machine Setup screen.
72
APPENDIX E
Flow chart adjustment of plate flatness monitor
73
APPENDIX F
Steps for mounted work piece BK7 glass on stainless steel holder
Sample Mounting Before lapping, samples must be mounted on stainless steel holder
.Samples must be mounted on the flat side of stainless steel holder
1 Clean the stainless steel holder with acetone and rinse by DI water. Blow dry
or wipe dry with clean room wipes.
2 Place sample on the holder facing down. Measure the thickness of the
sample by the dial micrometer on the table next to the machine.
3 Place the stainless steel holder on the hotplate and turn the temperature to
100 set point (150 for black wax). After ~10 min or when the temperature reading is
70 to 80°C, proceed to the next step.
4 Place a very small amount of wax in the center of the glass substrate with the
metal spatula (stored in the tool box). For most applications the white wax, (ocon-
195 in a can at the station), should be used. For very small samples or when thinning
down to < 20um, the black stick wax may be used.
5 Spread the wax uniformly on an area slightly larger than the sample to be
mounted. Place your sample face down on the wax and slide the sample a little bit to
get uniform wax thickness with the sample ultimately centered on the stainless steel
holder.
6 Apply pressure and/or weight on the sample if necessary. Weight should not
be used on soft materials. Place a piece of filter paper on the sample and gently place
the weight on the paper. Turn thehot plate off and wait for at least 20 minutes.
74
7 Take stainless steel holder off the hotplate. Put it on the table and let it cool
down. Use wiper with acetone to clear the wax around the mounted sample.
8 Measure the thickness of the sample after mounting. Compared with the
thickness obtained in step 2, the wax thickness should be noted.
75
APPENDIX G
Steps for slurry preparation of lapping process
Slurry Preparation In order to obtain a reasonable lapping rate and acceptable surface
roughness, a proper abrasive slurry sequence should be determined in advance.
1 Check the abrasive size left in the cylinder. If the slurry in the cylinder is the
size you want to use, shake it well before you use. Add more slurry if needed. The
total amount of slurry should never exceed. the grooved line on the cylinder.
2 To make a new slurry, open the cap carefully with the screwdriver in the tool
box. Untighten the four screws for the flux controller. To take the valve out of the
cylinder, turn the valve head counterclockwise. Dump the old slurry into the slurry
waste bottle.. Clean the cylinder, cap, and valve thoroughly. Put the valve back when
done.
3 Make up Al2O3 abrasive slurry of desired particle size by mixing – ml DI
water and 1.5 cups of powders in a beaker. Stir the solution entirely with the Teflon
boat holder in the fume hood. Pour the slurry into the cylinder with a funnel. Add
more water into cylinder until the level reaches the grooved line. However, keep
slurry volume more than half of the maximum to obtain constant feeding rate.
4 Tighten the cap and close the header completely by turning header clockwise.
Then open it by turning it counterclockwise for about one turn.
76
APPENDIX I1
Lapping surface profile of Sample D at Lapping speed 50rpm
Surface profile after 2min
Surface profile after 12min
77
APPENDIX I2
Lapping surface profile of Sample C at Lapping speed 40rpm
Surface profile after 2min
Surface profile after 12min
78
APPENDIX I3
Lapping surface profile of Sample B at Lapping speed 30rpm
Surface profile after 12min
Surface profile after 12min
79
APPENDIX I4
Lapping surface profile of Sample A at Lapping speed 20rpm
Surface profile after 2min
Surface profile after 12min
80
APPENDIX J1
Polishing Surface profile of Sample D
Surface profile after 1min
Surface profile after 15min
81
APPENDIX J2
Polishing Surface profile of Sample C
Surface profile after 15min
Surface profile after 1min
82
APPENDIX J3
Polishing Surface profile of Sample B
Surface profile after 1min
Surface profile after 15min
83
APPENDIX J4
Polishing Surface profile of Sample A
Surface profile after 1min
Surface profile after 15min
84
APPENDIX K
Machines for work piece preparation
Rotary Ultrasonic machine (Sonic-Mill 10 series) for coring, flattening , and grinding
and flattening BK7 glass
Precision cuter (ISOMET 5000) for slicing BK7 glass
Work piece BK7 glass after slicingWork piece BK7 glass after coring