Innovations in Abrasive Products for Precision Grinding - Cool

Innovations in Abrasive Products for Precision Grinding
J. Webster (1), Saint Gobain Abrasives, USA
M. Tricard (3), QED Technologies, USA
Abstract
This paper is a review of recent developments in the design and manufacture of precision, fixed-abrasive tools. The role of
each component within the “engineered composite” is also discussed, with examples showing how the components have
been enhanced to achieve their current high levels of performance. The paper also looks at examples where innovations in
the abrasive tool have enabled the development of innovative abrasive processes. A vision of future abrasive product
developments is also presented by the authors.
Keywords:
grinding, abrasives, machine
Acknowledgements
The authors would like to acknowledge all who have
contributed to this paper through suggestions,
discussions and documents of their work. Special thanks
are given to Prof. E. Brinksmeier and his co-workers at
IWT Bremen, Prof. F. Klocke and his co-workers at WZL
Aachen, Prof. B. Denkena and his co-workers at IFW
Hannover, Prof. D. Stephenson of Cranfield Univ., Prof.
F. Rehsteiner, Prof. J. C. Aurich of Kaiserslautern Univ.,
Prof. K. Weinert of Dortmund Univ., Prof. B. Hon of
Liverpool Univ., Dr. H. W. Hoffmeister of Braunschweig
Univ., Prof. V. Kovalenko of Kiev Polytechnic, Prof. P.
Koshy of McMaster Univ., Dr. R. Stabenow and his coworkers at Hermes Schleifmittel, Dr. T. Tawakoli, and
also Drs. K. Breder and K. Subramanian, Saint Gobain.
1. General Overview of paper
Presenting technical accomplishments from the multibillion dollars abrasive industry necessarily requires
being selective. It was therefore decided to limit the
scope of this paper to precision grinding, a loose
designation merely used to focus on abrasive processes
where forms and/or surface integrity (e.g. roughness, but
also subsurface damage etc.) are the primary figure of
merit. Significant developments have of course also been
demonstrated in abrasive processes where volumetric
removal rates or other goals are the primary drivers, but
these will not be covered here for the sake of brevity.
It was further decided to primarily report on applications
where recent abrasive product developments have
translated into novel abrasive process accomplishments
(e.g. increase in grinding wheel permeability for new
creep feed grinding applications, new grinding wheel
shapes for high speed grinding etc.). It was also the
intention of the authors to exclude the – constant and
obviously important – continual improvements made by
abrasive manufacturer worldwide, to merely improve their
existing product for mere incremental performance
improvement (e.g. new bond formulation to improve
wheel life by 10%).
The fragmented nature of the abrasive industry (in North
America alone, several hundred manufacturers vie for
market share) makes it a competitive one, rife with
proprietary issues. To steer well clear of confidential
developments, and perhaps in a new approach for CIRP,
this paper relies heavily on issued patents, particularly
recent ones, as well as academic publications.
2. Components of fixed abrasive products, design
and recent developments
Fixed abrasive products can be regarded as engineered
composite materials, made of four elements (Fig. 1):
1.
2.
3.
4.
One or several abrasives: either conventional-fused
(e.g. Al2O3, SiC or ZrO2 based); conventionalsintered; or super -abrasives (e.g. cBN or diamond)
A bond to hold or support the abrasive(s): resin or
polymer based; vitrified or ceramic based; or metal
based, sometimes in a single-layer brazed or
electroplated format
Some porosity and/or additives. Porosity is typically
present to provide clearance for the chips created
during the grinding process, for fluid transport, and to
enhance the various interactions taking place in the
grinding zone. The porosity itself can either be
natural or artificially induced. Various grinding aids,
fillers and lubricants can be added
Wheel design, including the composite abrasive
profile, abrasive thickness, hub material (if not a
monolithic design), strength to withstand rotational
stresses, precision and resistance to chemical
attack.
ABRASIVE
BOND
STRUCTURE
ADDITIVES
DESIGN
Fig. 1. Components of a fixed abrasive product.
These four constituents are deployed by the abrasive
product manufacturers to achieve the desired workpiece
requirements (shape, finish, removal rate etc.). Abrasive
manufacturers in turn tend to refer to their products as
either: bonded abrasives (conventional grinding wheels),
superabrasives, or coated abrasives (belts). Coated
abrasives are not covered in this document.
2.1. The abrasive grain component
The next few paragraphs will detail some of the key
innovations in the abrasive grain component of a grinding
wheel. These abrasives are the hard phase components
of a wheel and have the greatest influence on the output
and viability of a grinding process. For example, the size
of the grain has significant influence on the forces,
power, wear rate, surface finish, etc.
2.1.1. Conventional abrasives
th
As mentioned by Cheape [23], until the 18 century,
grinding was generally a manual process employing
sandstone, a naturally formed composite of quartz
th
crystals bonded with silica and iron oxide. The 19
century saw the introduction of “grinding machines” –
initially mere modifications of lathes, from Brown and
Sharpe in 1858, for instance. The first “universal grinder”
followed in 1875, followed by a number of specialized
precision grinders in the 1880’s and 1890’s.
But even as late as in one of it’s 1885 issues, American
Machinist mentioned them “when a machinist has a job of
emery grinding to do, he is pretty apt to sit right down and
hate himself” [23]. In parallel, so called “coated
abrasives”, initially made with a paper or cloth backing
and covered with glued flint, emery or garnet, were first
produced in the USA by the Baeder-Adamson Co. of
th
Philadelphia in 1828 [23]. Throughout the 19 century,
abrasive makers experimented with quartz, flint and
garnet (not to mention crushed milk bottles!) but
preferred emery and its purer form: Alundum.
Corundum was scarce and expensive and was usually
mixed with emery. It was not until 1901 that a new high
quality artificial form of corundum abrasive, called
Alundum, was introduced. The technology was
subsequently refined in 1904 due to the introduction of
the water cooled electric furnace, invented and patented
by Aldus C. Higgins, who later became president of the
Norton Company. This led to a revolution in production of
Alundum abrasives and afterward Higgins was awarded
the John Scott Medal. As early as 1914, manufactured
“fused”
abrasives
(alundum
and carborundum)
production surpassed emery and corundum imports in
the US [23].
oxides, zirconia powder or a zirconia precursor (which
can be added in larger amounts, e.g. 40 wt % or more),
or other compatible additives or precursors thereof.
These additives are often included to modify such
properties as fracture toughness, hardness, friability,
fracture mechanics, or drying behaviour.
Once the gel has formed, it may be shaped by any
convenient method such as pressing, moulding or
extrusion and then carefully dried to produce an uncracked body of the desired shape for firing. The
extrusion is then dried, typically at a temperature below
the frothing temperature of the gel, using any of several
de-watering methods. After drying, it can be cut or
machined to form the desired shape or crushed or broken
to form particles or grains. After shaping, the dried gel
can then be calcined (using an inclined rotary oven) to
remove essentially all volatiles and transform the various
components of the grains into ceramics (metal oxides).
The calcined material is then sintered by heating and is
held within a suitable temperature range (approximately
º
1500 C) until substantially all of the alpha alumina
monohydrate is converted to alpha alumina.
Sol-gel alumina may either be seeded or un-seeded.
With seeded sol-gel aluminas, nucleation sites are
deliberately introduced into, or created in-situ in, the
aluminum oxide monohydrate dispersion. The presence
of the nucleating sites in the dispersion lowers the
temperature at which alpha alumina is formed and
produces an extremely fine crystalline structure.
As illustrated by Stabenow et al [98], the size of the
crystalline structure plays an important role in the
performance of the grinding products. Figs. 2a and 2b
show both a coarser material (representative of Type H1
sintered ceramics) and of a much more fine-crystalline
material (representative of Type K1 sintered ceramics).
Fig. 3 shows the effect of the different grit size on the
tangential grinding force.
Garg [37] demonstrated that nano-sized powders of
alpha alumina can be obtained from a boehmite gel
doped with a barrier-forming material such as silica that
is then dried, fired and comminuted to powder form [84].
2.1.2. Sintered aluminium oxide (seeded-/sol-gel, etc)
Seeded Gel, or Sol-Gel as it is sometimes called, was
originally invented by both 3M and the Norton Company
[27] in the mid-1980’s, and are now manufactured by
these two companies, along with Hermes Schleifmittel.
Sintered Sol-Gel aluminum oxide abrasives present
significant advantages compared to their fused
counterparts – particularly in term of life – and are much
less expensive than superabrasives. When properly
used, sintered abrasives can also result in significantly
increased volumetric removal rates, reduced forces and
lower work surface temperature during grinding. It is
frequently a viable alternative to cBN, particularly in light
of the ease of truing and dressing, and the initial wheel
cost.
Sol-gel aluminous grits are aluminas made by a process
comprising of peptizing a solution of an aluminum oxide
monohydrate so as to form a gel, which is then dried and
fired to form alpha alumina [90]. The initial solution may
further include up to 15 % by weight of spinel, mullite,
manganese dioxide, titania, magnesia, rare earth metal
a)
3µm
b)
3µm
Fig. 2. Sol-gel alumina crystal structure: a) Type H1 b)
Type K1 [98]
Krell and Blank [63] showed that microcrystalline
aluminium oxides of differing crystallite size differ in
terms of hardness. At temperatures of approximately
1000 °C, hardness increases compared to an aluminium
oxide single crystal by almost 100 % as crystallite size
decreases and thus achieves almost the hardness of
silicon carbide (Fig. 4). The authors speculate that the
differing characteristics of the microcrystalline aluminium
oxides described previously are linked to their different
high-temperature hardness.
An electro-fused, sintered, aluminium-oxy-nitride (AlON
abrasive has recently been introduced into the market. It
is claimed that the high temperature hardness and
friability properties of this material, gives less thermal
damage, better form holding and better surface finish
than conventional, fused-alumina abrasive [81].
Tangential force per unit width tF'
[N/mm]
14
12
10
8
6
4
Type H1 wet; G = 16.8 mm³/mm³
Type H1 dry; G = 71.6 mm³/mm³
Type K1 wet; G = 54,4 mm³/mm³
Type K1 dry; G = 72,4 mm³/mm³
2
0
0
200
400
600
800
1000
Metal removed per unit width V'w [mm³/mm]
1200
Fig. 3. Effect of crystal size on grinding force [98]
2.1.3. Diamond (natural and synthesised)
Even though diamond dust had historically been used to
polish gems, and natural diamond had been used to true
grinding wheels, the first natural diamond grinding wheels
only started to appear in the 1930’s [23].
Raw material >99.99% Al2O3
e
siz
Hardness HV3 (GPa)
e
llit
ta
ys
Cr
0.59µm
0.67µm
0.97µm
3.5µm
10.1µm
SiC single
crystal
Al2O3 single
crystal
Temperature (ºC)
Fig. 4. High temperature hardness variation [63]
Sales of natural diamond wheels increased significantly
during the 1940’s, representing 20% of all sales (by
value) of the Norton Company by 1952 [23].
The first synthesis of artificial diamond was reported by
Bundy and his colleagues at General Electric in 1955
[19], culminating – as they pointed out – “more than a
century of claims and counterclaims for the synthesis of
diamond attest to the fascination of the subject and the
extreme difficulty of the experimental techniques.” In an
interesting historical twist Bovenkerk et al [13]
subsequently declared 34 years later that “the run of
diamond was a small piece of a natural type diamond”.
As subsequently demonstrated in a later publication, [14]
they had nonetheless invented the key process to
synthesise artificial diamond.
2.1.4. Cubic Boron Nitride
In a small “Letter to the Editor” entitled “Cubic Form of
Boron Nitride” received on January 28, 1957, R.H.
Wentorf [119], from General Electric Research
Laboratory, made one of the most significant
announcements in the field of abrasives. Whether called
“Borazon” as Wentorf proposed in this announcement, or
cBN, the second key superabrasive - Cubic Boron Nitride
– had also been invented. Details of this breakthrough
were reported in a subsequent publication [120].
Since Wentorf, additional developments have been made
with a polycrystalline form of cBN, called the ABN series,
as described by Heath [42] and Bohlheim [10]. Spur and
Lachmund [96] also looked at the type-specific
applicability of polycrystalline diamond. In addition to the
mono-crystalline and poly-crystalline form of cBN, Ichida
and Kishi [48] reported on the performance of cBN
wheels made of newly developed nano-crystalline cBN
abrasive (“N-cBN” having a crystal grain size below
1µm).
In 1985, Malkin reported on the current trends in cBN
grinding [68]. Since then, improvements to the grain
structure, shape, toughness, and price, have moved the
abrasive firmly into the market place. With such low
manufacturing costs, cBN is following synthesised
diamond towards being a commodity product.
2.1.5. Abrasive mixtures
Within a wheel bond system, abrasive grains can be
mixed by shape, size, toughness and type [111][29][101].
Mould packing density can be increased by mixing
different sizes of abrasive and bond material. The finer
material will fill the spaces that exist between the larger
sizes. Sintered alumina has been blended with cBN in an
effort to reduce cost but maintain excellent wear
resistance [76]. Another example exists with the
manufacture of the wheel hub with monolithic sintered
abrasive wheels. The higher cost of these abrasives
makes them uneconomical to use for the entire wheel,
giving rise to the practice of filling the centre of the mould
with a lower cost abrasive mixture, and the outer annulus
with the premium abrasive mixture, before pressing.
2.1.6. Strength testing of grains
The performance of any abrasive product depends on the
abrasive properties and grinding conditions (forces, chip
thickness, etc.) to which it is subjected. From the
standpoint of testing conditions, the force per grit and
chip thickness are critical in determining which of the
common wear/fracture mechanisms of a given abrasive
become active. The link between these two areas is
important for predicting the most efficient grinding
regimes to use with a given abrasive; one abrasive that is
an excellent performer in high force per grit applications
may be less than optimal in low force per grit
applications.
Several methods are routinely used for the
characterization of abrasive grains. These include
friability, hardness, toughness, and various abrasion and
single-grit wear tests. None of these tests are application
tests, hence, correlation to the actual behaviour of the
grits in an abrasive product must be sought.
The idea of testing the strength of individual abrasive
grains and relating the results to grinding behaviour has
been proposed by Brecker [15], who developed a
diametral compression test of abrasive grains with the
purpose of relating fracture characteristics to grinding
forces. Both a quasi-static diametral compression and a
roll crushing technique were used. The effective tensile
strength of each grain was calculated assuming the
irregular grain shape is somewhere between a sphere
and a cube. Good correlation between the two methods
was found, and Weibull analysis was performed on the
strength data. It was concluded that the Weibull modulus
was similar for the series of grains tested, and the
median strengths ranked with white aluminas on the
weak end to sintered aluminas on the strong end. It was
further concluded that the method had significant promise
as a grain characterization tool. Recent work by Breder et
al [16] showed the fracture loads were size dependent
and followed a Weibull scaling approach fairly well.
2.2. The bond
The bond used in a grinding wheel has several functions:
• retain the abrasive grain during the process
• wear at a controlled rate with respect to the grain
wear
• resist centrifugal forces, especially in high speed
grinding
• readily exposes the grain to the work, where
possible
B181_V_D_A_V180
K
ev
c.
gra
in
vo
lum
100
80
40
60
60
vP
40
80
20
100
0
me
olu
rev
po
sp
e
0
.
ec
sp
18
20
40
60
80
0
100
spec. bond volume vb
Fig 5. Ternary diagram for abrasive products [55]
Selection of the appropriate bond is not straightforward
and tradition may dominate [80][36]. Also, new bond
development can shift the choice of bond for an
application. For example, polycrystalline diamond
grinding has traditionally been performed using resin
bonded wheels, however, over the last 10 years, metal
bonds have displaced resin in many applications. Even
more recently, the truing of vitrified diamond wheels has
been improved by the introduction of tougher truing tools.
Fig. 5 shows a ternary diagram, with a CBN wheel
nomenclature shown [55]. The small circle represents the
specification of the CBN wheel in the ternary diagram
with specific bond volume, specific grain volume and
specific pore volume. These volume percentages are
manipulated to give the required wheel specification to
suit the application.
2.2.1. Vitreous bond [glass and ceramic]
It is estimated that in 1915 vitrified wheels represented
more than 75% of the grinding wheel business [23]. With
the advent of new vitreous materials and processing
technologies, there is now a spectrum of vitrified bonds
that vary from being rigid (brittle), wear resistant, or soft
(compliant).
Vitrified bonds are composed of glasses that are formed
when clays, ground glass frits, mineral fluxes such as
feldspars, and chemical fluxes such as borax, melt when
the grinding wheel is fired at high temperatures. With
reference to raw material nomenclature, a 'frit' is a preground glass with a predetermined oxide content, a 'flux'
is a low melting point siliceous clay that reduces surface
tension at the bond bridge/abrasive grain interface, a
'pre-fritted' bond is a bond that contains no clay minerals
(i.e. clays and fluxes), and 'firing' refers to vitrification
heat treatment that consolidates the individual bond
constituents together [52]. Considering individual bond
constituents; mineral fluxes and ground glass frits have
little direct effect on the ability to manufacture grinding
wheels. However, most clays develop some plasticity in
the presence of water (from the binder) which improves
the ability to mould the mixture, so that the wheel can be
mechanically handled in its green state.
In some circumstances when siliceous ingredients, such
as clays and mineral feldspars, are heated to high
temperature, bonds consisting of pre-fritted glass powder
can give advantages such as perfectly formed glass
bonds and an absence of adverse reactions. An example
of an adverse reaction is the sudden expansion of quartz
at its inversion temperature (573°C), causing cracks to
form within glass bond bridges and a loss in bond
strength. However, if clays and mineral fluxes are used,
firing procedures may be modified to reduce, or possibly
dissolve, quartz particles. A further benefit of pre-fritting
the bonding materials beforehand is that it avoids the use
of highly active chemical fluxes, such as borax, which
may attack and impair the properties of new abrasive
materials (such as sol-gels). The downside is that none
of the benefits associated with clay materials would be
available, and handling strength in the green state would
have to be developed using organic plasticizing agents
(binders) alone, with a consequently greater risk of
difficulties with binder burnout and black centre. Clearly,
the solution to the correct choice of bonds and their
ingredients lies in a sensible compromise depending on
the type and size of wheel, manufacturing circumstances,
life between wheel dressings, and the higher costs
associated with pre-fritted materials [52].
2.2.2. Organic bond [phenolic, polyimide, etc]
Resinoid-bonded wheels are usually produced by mixing
abrasive grains with phenolic thermosetting resins and
plasticizers, molding to shape, and baking (curing) at
150-200°C. The bond hardness is varied by controlling
the amount of plasticizer and by addition of fillers.
Conventional abrasive resinoid wheels are widely used
for heavy-duty grinding (snagging) operations because of
their high strength and ability to withstand shock loads.
Another important application is for cut-off wheels, which
are usually reinforced with fiberglass for added strength
and high-speed operation up to about 100 m/s (20,000
ft/min). For superabrasive wheels, resinoid bonds are the
most popular, the most important applications being with
diamond abrasives for grinding of cemented carbides,
ceramics, PCD, and CBN for cutting tools [40][49][74].
most are active during grinding. The high wheel speed in
HSG also keeps the force per grit low, and a chip size
that can be accommodated between adjacent grains.
cBN grit
nickel layer depths
Polyimide resin bond is a recent development for cutting
tool grinding applications. It is a tougher and more
thermally resistant bond, which can retain diamond
abrasive at higher temperatures than phenolic resin. The
introduction of copper particles into the bond greatly
improves the thermal conductivity. Hollow glass spheres
can also be used to give the wheel a degree of porosity.
Fig. 6. Nickel bond depth adjustability
Since resinoid bonds do not chemically attach to the
abrasive grains, in many cases ‘rough’ or ‘spikey’ metallic
coatings are applied to the grains to increase mechanical
retention in the bond. Rubber, shellac and silicate bonds
are not considered in this paper as they are rarely used
for precision grinding.
2.2.5. Testing bond hardness
The hardness of a grinding wheel is defined as the
resistance against grain pullout. This is directly
influenced by the strength of the bonding at the grain and
the strength of the bonding bridges. Several commercial
testing systems are available to determine the hardness:
2.2.3. Metallic sintered bond
Metal bonds are extensively used with superabrasive
wheels. The most common are from sintered bronze,
which are produced by powder metallurgy methods.
Variation of the wheel grade is controlled by adding
modifiers and altering the bronze composition. Other
powder metal bonds, which are generally stronger,
include iron and nickel. Segmented diamond saws for
cutting stone and granite typically have sintered nickel
bonds. Tungsten powder infiltrated with a low melting
point alloy is used in diamond wheels for grinding
diamond tools. Still stronger bonds consisting of WC-Co
cemented carbide are used in diamond abrasive tools for
geological drilling.
•
2.2.4 Metallic Electroplated bond
Electroplated (galvanically bonded) cBN wheels
represent the largest share of the single-layer market,
especially in automotive and aerospace applications.
Diamond versions are generally used for grinding
ceramics, non-ferrous metals, and construction materials.
They have been effectively utilised on machining centres
with tool change capability, and central to most high
speed grinding (over 120 m/s) applications [57].
The attachment of the abrasive grain to the wheel hub is
primarily by mechanical entrapment [22]. Fig. 6 shows a
schematic of cBN grains held by the nickel matrix. The
abrasive grains that are used, are generally blocky and
tough, in order to give even grit height and long service
life. Chattopadhyay and Hintermann [22][21] experienced
a high level of grit pullout at a wheel speed of 30m/s, and
specific removal rate Q’w of 4 mm3/s.mm, when grinding
an unhardened bearing steel. The relatively low
wheelspeed, possible low nickel bond content, and soft
workpiece material, may have created the high force per
grit that led to grit pullout.
In high speed grinding (HSG), grit pullout does not
appear to be a limitation of the process, despite
extremely high specific material removal rates. Shi and
Malkin [91] showed that radial wear up to 80% of grit
dimension may occur before stripping of the layer occurs.
The depth of nickel can be increased for more strength
(Fig. 6) and the grit height distribution improved to ensure
substrate
•
Zeiss-Mackensen tester, which correlates hardness
to depth of air penetration
Grindo Sonic tester, that estimates E-Moduli from
natural frequency and correlates this to the hardness
The above two methods assess the bond hardness
indirectly, and against known standards. They can also
be used to assess the consistency of a batch of grinding
wheels and, in the case of the Grindo-Sonic, whether
dangerous cracks exist inside the wheel structure.
Bond
breakage
Grain
pullout
Fig. 7. Bond strength testing [55]
Klocke and Merbecks developed a grain pullout system
that can establish the strength of the bond and bond
bridges, using a mechanical probe [55], to address
homogeneity problems, variation of wheel hardness,
variation of concentration, and inconsistent wheel
behaviour. Fig. 7 shows the system that pushes
individual grains to determine the force at which bond
fracture, grain pullout, or grain fracture occurs. The
2.3. Porosity/permeability in the bond
The structure of a grinding wheel is a measure of the
spacing between the abrasive grain. The porosity of a
grinding wheel can be described as a local effect within
the wheel structure, allowing the transport of fluid into the
process [30][39], and giving space for the chips to form.
Permeability is interconnected porosity throughout the
entire structure of the abrasive composite. An analogy of
porosity is closed-cell foam, where there is air trapped
within each cell which can have local influence. An
analogy of permeability is open-cell foam which can
freely pass liquids and gases through the structure [27].
It has been discovered that grinding performance cannot
be predicted only on the basis of porosity as a volume
percentage of the abrasive tool. Instead, the structural
openness (i.e., the pore interconnection) of the wheel,
quantified by its permeability to fluids (air, coolant,
lubricant, etc.), also influences the abrasive tool
performance. Permeability also permits the clearance of
material (e.g., metal chips or swarf) removed from an
object being ground. Debris clearance is essential when
the workpiece material being ground is difficult to
machine or gummy (such as aluminium or some alloys),
producing long metal chips. Loading of the grinding
surface of the wheel occurs readily and the grinding
operation becomes difficult in the absence of wheel
permeability. A method for measuring permeability is
described by Wu [122] and DiCorleto [27], who monitored
the flowrate of air, at a fixed pressure, through the
structure.
Wu [122] notes that there are two major categories of
processes to obtain high porosity abrasive products. The
first category is the burn-out methods, where pore
structure is created by addition of organic pore inducing
media (such as walnut shells) in the wheel mixing stage.
These media thermally decompose upon firing of the
green body of abrasive tool, leaving voids or pores in the
cured abrasive tool. Drawbacks of this method include:
•
moisture absorption during storage of the pore
inducer
• mixing inconsistency and mixing separation, partially
due to moisture, and partially due to the density
difference between the abrasive grain and pore
inducer
• moulding thickness growth or "spring-back" due to
time-dependent strain release on the pore inducer
upon unloading the mould, causing uncontrollable
dimensions of the abrasive tool
• incompleteness of burn-out of pore inducer or
"coring"/"blackening" of a fired abrasive product if
either the heating rate is not slow enough or the
softening point of a vitrified bonding agent is not high
enough
• air-borne emissions and odours when the pore
inducer is thermally decomposed, often causing a
negative environmental impact, i.e. naphthalene
The second category of pore inducement is the closed
cell or bubble method, by introducing materials, such as
bubble alumina (mullite spheres) into an abrasive tool to
induce porosity without a burnout step. However, the
pores created by the bubbles are internal and closed, so
the pore structure is not permeable to the passage of
grinding fluid, and the pore size typically is not large
enough for metal chip clearance.
2.4. Wheel design
The design of the grinding wheel is as critical to the
success of the abrasive product as the other three
components of the composite. Design includes: the
physical dimensions, the form produced on the abrasive
surface, the hub material to withstand rotational and
thermal stresses, the rotational error, dynamic balance
and chemical resistance.
Grinding wheels can range from a thickness of several
microns, for silicon wafer dicing, to a metre wide for wood
pulp grinding. Although the shape of the active abrasive
surface is usually trued into the wheel, there has been
extensive research carried out to produce near-net
shaped, vitrified core, wheels for superabrasive products,
reducing wasted abrasive [103].
Electro-plated bonding
Q´w,max = 1.000 - 10.000 mm³/mms
bonding type
system has also been used in an axial direction to
scratch across the wheel surface. Using an acoustic
emission sensor, the signal produced by the scratch was
used to identify the grain/bond failure mechanisms.
metallic bonding
Q´w,max = 50 - 250 mm³/mms
vitrified bonding
Q´w,max = 50-150 mm³/mms
CBN grinding wheels
resin bonding
Q´w,max = 50-150 mm³/mms
vitrified/bakelite
Q´w,max=10-100mm³/mms
100
conventional grinding wheels
150
200
250
300
circumferential speed of the grinding wheel vs
Fig. 8. cBN bond system speed and Q’w limitations [58]
Rotational stresses during high-speed grinding can lead
to failure if the hub is not correctly designed. The use of
FEM analysis for design of the hub and the number of
segments (in the case of superabrasive wheels) [4], plus
the improved guarding of modern machines, has reduced
the risk of injury due to incorrect wheel design. Fig. 8
shows the speed limitations of different wheel bond
systems, as defined by König et al [58]. Fig. 9 shows an
example of a high speed grinding wheel hub to minimise
radial elongation and reduced effective stress [58].
König’s wheel design was based on high strength steel.
Alternatively, composite wheel hubs, i.e. carbon fibre
based, have been researched and are now commercially
available. As with the wheels and brakes of Formula One
race cars, the orientation of the carbon fibres within the
hub can make a large difference to the ultimate strength
and deformation of the wheel at high speed.
The rotational precision of single-layer superabrasive is
generally a function of the grain size distribution and the
electroplating method. However, micro-truing of these
wheels by 2-4 µm is possible in order to reduce the
surface roughness produced [58].
3. Interactions in the grinding zone
During abrasive finishing processing the abrasive product
(the tool/consumables) contacts with the surface of the
work material. This contact is imposed under a set of
conditions (operation parameters) influenced by the
characteristics of the abrasive tool, work material and the
machine tool involved [104]. Such contact (interactions)
600
600
central bore
µm
effective stress
N/mm
1A1-wheel
400
bs = 5 mm
p = 2,71 kg/dm³
vs = 500 m/s
300
200
400
300
200
100
100
HSG-wheel
0
0
50
100
150
- - - radial elongation
1A1-wheel
0
200
led to significant improvements in grinding quality,
productivity, geometry, precision, stability, control and
economics. This section describes how the manipulation
of the components has lead to the above benefits when
used in typical grinding processes.
1. ABRASIVE/WORK
1.1
Cutting
1.2
Ploughing
1.3
Sliding
wheel radius rs
HSG-wheel
2. CHIP/BOND
Sliding
Fig. 9. Reduced stress design and radial elongation [58]
between the abrasive product and work material can be
characterised as shown in Fig 10:
1. Abrasive / Work interface
1.1 Cutting (chip removal)
1.2 Ploughing (material sideways displacement)
1.3 Sliding (friction)
2. Chip / Bond interface (friction)
3. Chip / Work interface (friction)
4. Bond / Work interface (friction)
These sets of microscopic interactions are influenced by
the input parameters of the abrasive finishing system,
and in-turn influence the output parameters of the
abrasive finishing system. Subramanian [105] has shown
how these microscopic process interactions can be
manipulated to achieve the desired productivity, surface
quality, and production economics. Hence innovations in
abrasive products can be achieved through abrasive,
bond, structure and shape, to influence the physical
events in the grinding zone (mechanical, thermal,
chemical, etc.) in order to:
•
•
•
3. CHIP/WORK
Sliding
4. BOND/WORK
Sliding
Fig. 10. Interactions in the grinding zone [104]
4.1. High permeability wheel structures
The introduction of permeability into an abrasive structure
results in better cooling [43] and the potential for larger
chip thickness. Fig. 11 shows a structure that is produced
by using a high aspect ratio (8:1) abrasive grains
(filaments) within a vitrified structure [122].
Maximize cutting action and minimize friction effects
Utilize the abrasive/work tribology (ploughing and
sliding), as required
Manage all other tribological (sliding) interactions to
generate the surfaces of required value/benefit to the
customers [168].
The interactions listed above are usually experienced by
end-users and abrasive product suppliers, through
controlled tests, but not easily identified from process
outputs such as surface finish, grain wear, power
consumption, etc. Any deviations from the initial
performance over time (due to wear flats on the abrasive
grains, erosion of the bond, loss of chip space, uneven
wear of the abrasive, etc.) are corrected for, by
“dressing” the abrasive product surface [105], without an
understanding of the cause of the deviation.
4. Innovative abrasive products by manipulation of
the components
In section 2, the components that form an abrasive
product were described in detail. Over the last 10 years,
continuous improvements to these components, and the
way they are assembled into the abrasive product, have
Fig. 11. High aspect ratio grain in wheel [122]
The high aspect ratio grains are produced by extruding a
seeded-gel of hydrated alumina, described in section 2.1,
into continuous filaments, drying the filaments, cutting to
the desired length, and then firing the filaments to a
0
temperature of not more than 1500 C [84][83][8]. The
alpha alumina crystallites that make up the abrasive
filaments are less than 1 µm in diameter. When these
abrasive filaments are formed into a grinding wheel, the
bond posts are produced mainly at the interfaces where
the grains touch each other, leading to a high-strength,
very open, structure. During grinding, the microcrystalline abrasive grains remain sharp until fully
consumed by wear, or by truing, giving the economics to
justify the higher initial cost compared to fused alumina.
It is also possible to create organic wheel structures with
up to 80% (by volume) of interconnected porosity
(permeability). The method includes blending a mixture of
abrasive grain, bond material, and evenly dispersed
particles (up to 80% by volume). The powder is then
pressed into an abrasive laden composite and thermally
processed. After cooling, the composite is immersed into
a solvent, which dissolves substantially all of the
dispersed particles, leaving a highly-porous bonded
grinding wheel [79].
Traditional grinding wheels incorporate individual
abrasive grains into the structure, with the intention of
equally separating them in a homogeneous manner.
However, recent developments in the production of
abrasive clusters, has led to highly permeable structures
using primary clusters of grains [53], which are bonded
as secondary clusters to each other to form a wheel.
These structures can either use organic binders [11] or
vitrified binders [12]. Fig. 12 shows the type of structure
that is produced. Grinding tests (described in [11][12])
show that the primary-cluster based wheels exhibit
greater G-ratio, less chatter, and higher material removal
rates, with acceptable surface finish.
•
•
non-constant grinding power, force, profile and
surface finish throughout the life of the wheel
each profile has to be held in stock (unlike dressable
wheels).
4.2.1. Brazed bonds
Much work has been done to produce a brazed singlelayer bond that chemically bonds the abrasive grain to
the hub substrate material [22][21][5][1]. Brazing of
diamond grains is simplified by the spontaneous wetting
of untreated or uncoated synthetic diamond grits at
0
<1000 C, with Ni-Cr alloy or copper/tin bronze alloy.
However, cBN is not so easily wetted, even at higher
temperature, due to its poor reactivity. The CVD coating
of cBN grains with TiC film, allows strong wetting during
the brazing process, leading to a high bonding strength
[22][32] (see Fig. 13).
cBN grit
TiC film
brazed layer
substrate
Fig. 13. TiC film assists brazing process [22]
grain
secondary
cluster
bond
primary cluster
Brazed bonds have several advantages over the galvanic
bond. They hold onto the abrasive more tenaciously;
create more space for the chips to form (with up to 80%
of grit exposure, compared to 60%); do not require many
hours of plating time (less of an issue with large batches);
and are especially suitable for larger grit sizes and for
grinding soft materials.
Disadvantages include: hub distortion from the high
brazing temperature; stripping-off of the worn abrasive
often requires machining of the hub changing overall
size; and some degradation of superabrasive strength
can occur at the elevated firing temperature. A typical
cBN grain distribution, and braze content, is shown in
Fig. 14.
Fig. 12 Clustered grains in wheel structure [11]
4.2. Single-layer abrasive products
Single-layer superabrasive products are becoming more
popular due to their lower initial purchase cost than the
multi-layered vitrified, organic and metal bond products.
Other advantages include:
•
•
•
•
•
metallic cores can be used for high rotational speeds
a truing device is not necessary
a dynamic balancer may not be necessary
no shelf life concerns exist, as compared to organic
bonds
the ability to be stripped and re-coated when worn,
impacts the economics.
133µm
Fig. 14. SEM of cBN in brazed bond
Disadvantages include:
•
a general preference for oil-based fluids due to
longer service life
4.2.2. T-tool and serrated wheels
T-tool is a tool that combines the virtues of both a milling
cutter and a grinding wheel. Tawakoli [109][110] claims
that this type of tool allows for a reduction in fluid flowrate
down to that typically used for milling, with the advantage
of having multiple cutting edges that are active, and
those that will become active as the tool wears. By
contrast, a milling cutter is replaced when the defined
cutting edges are worn. The T-tool, as shown in Fig. 15
can either be produced with a solid fluted core using
electroplated superabrasive grains, or, built with
replaceable superabrasive segments with vitrified, resin
or metallic bonds.
Pellet type wheels consist of large quantities of cylindrical
pellets that are glued onto a flat face wheel. The pellets
can be glued in a variety of patterns to produce even
wear and promote better fluid flow [2][51]. Since the
pellets are all the same size, automatic pressing
techniques can be employed to reduce the manufacturing
costs. They have the additional advantage that damaged
sections can be repaired using a few new pellets. Fig. 17
shows a typical layout of such wheels, which can be used
for both single- and double-sided applications.
source: T. Tawakoli
Fig. 15 Segmented T-tool design [113]
It is also claimed that the interrupted abrasive surface
between successive segments, leads to more
engagement by the following segment, and hence
reduces the power consumption in a way that is similar to
how a reduced abrasive concentration produces larger
chips. It is also claimed that as the abrasive surface
wears there is no profile loss, since the contact point
moves around the periphery of the tool to a position
where the profile is intact.
Suto et al [107] developed a similar serrated grinding
wheel based on electroplated cBN. He also developed a
‘timed’, thru-the-wheel, coolant application method to
take advantage of the design (see Fig. 16).
grinding wheel
nozzle
fluid
chamber
workpiece
perforated
hole
Fig. 16. Timed thru-the-wheel coolant delivery [107]
4.3. Pellet type, flat, superabrasive wheels
Grinding wheels for vertical spindle and double-disc
applications have traditionally been constructed by
bonding large flat segments onto a metal core. These
segments are relatively inexpensive for conventional
abrasives, but require moulds for a large range of
segment shapes. The segments are often grooved to
promote coolant flow and allow the chips to be cleared
away, requiring secondary machining operations. This
large-segment method of manufacture is therefore not
economical for superabrasive wheels.
Fig. 17. Large diameter pellet wheel and parts ground
4.4. Innovative wheel hub design
The grinding wheel hub has also been the subject of
innovation. In the case of superabrasive and other
segmented wheels, the hub provides the strength to
withstand the rotational stress, the stiffness to achieve
the required stock removal, and the precision to produce
a fine surface finish. In addition to the above, the hub can
be tuned to the process dynamics, be fitted with sensors
and can transport grinding fluid into the cutting zone.
4.4.1. Chatter suppression and damping
The replacement of conventional abrasives by
superabrasives has led to an increased tendency for
regenerative chatter. According to Baylis and Stone [7]
and Sexton and Stone [88] this is due to the increased
contact stiffness of thin-rim cBN wheel design over a
monolithic conventional abrasive wheel design. By
observing the harmonic response locus of the wheelwork loop, Baylis and Stone attempted to reduce the
negative real value, at 180 degrees to the grinding force,
of the machine compliance curve. Their attempt was
based on adding compliant material underneath the cBN
layer. It must be emphasized that the decrease in contact
stiffness between the wheel and work was to a level
where the overall static loop stiffness was not
dramatically altered. Follow-up work in this area by
Bzymek et al [20] and Song [94], using the Boundary
Element Method (BEM), showed that strategic machining
of the hub material beneath the abrasive layer provided
an alternative to the addition of compliant material.
Warnecke and Barth [117] used the Finite Element
Method (FEM) to optimise the dynamic behaviour of a
diamond wheel for grinding ceramics. Using FEM
analysis, they compared the synthetic resin aluminium
composite to a solid aluminium disk. Using this data, they
predicted the dynamic behaviour of both grinding wheel
designs, and their influence on the material removal
mechanism. Fig. 18 shows the FEM analysis of the
contact stiffness. Fischbacher [33] took a similar
approach.
b
temperature sensor
c
contact elements simulating
sliding and penetration
synthetic resin aluminum composite hub
telemetry stator
grinding wheel deformation relative to
the circumference of the nonrotating
grinding wheel
a
9.4 µm
circumference of the
rotating grinding wheel
d
aluminum hub
F’t =
18 N/mm
F’n = F’resultant= 110 N/mm
3
2 1
µm
nodal displacement
0 (vector sum,
aluminum hub wheel)
workpiece material
depth of cut
cutting speed
feed rate
4 µm
circumference of the
rotating grinding wheel
telemetry ring
dimensions : displacement
1 : 400
: hot-pressed silicon nitride
: a e = 1.5 mm
: v c = 50 m/s
: v w = 160 mm/min
source: IWT, Bremen
Fig. 18. FEM analysis of contact stiffness [117]
Fig. 19. Sensor integrated wheel [9]
4.4.2. Sensor-integrated wheels
In the quest for lower process cost and improved quality,
machine tool companies and end users are increasingly
integrating sensors into grinding machine systems for inprocess and post-process control. Typically, these
sensors measure displacement, motor power, grinding
force, vibration and acoustic emission (AE). In some
cases, the outputs from these sensors are used to
indirectly predict the conditions occurring within the
grinding arc. To improve the measurement accuracy, it is
advantageous to make these measurements within the
grinding zone and transmit them to a signal processing
system. Three University research teams have
developed innovative cBN wheel hubs with integral
sensors to monitor the grinding process.
Boehm et al [9] integrated temperature, vibration and
force sensors into their wheel. The temperature sensor
was proved to have sufficient response time (20-50 ns) to
measure the temperature close to the grinding zone, and
can be compensated for changing abrasive layer
thickness with time. The piezoelectric force and vibration,
thin-film, sensors are still being perfected for this
application. Fig. 19 shows a schematic of the wheel.
Varghese and Malkin [115] integrated an AE sensor into
the aluminium hub of the wheel, and fixed a force
transducer underneath one of the cBN segments. Using
Digital Signal Processing (DSP) and Radio Frequency
Transmission (RFT), both signals were received by a
host computer. Force monitoring was successfully
applied to identify wheel rounding during truing. The AE
signal was found to be sensitive to grinding and truing
parameters, and could identify initial wheel-work contact
to help minimize air grinding time. The technology behind
this wheel has been patented [67].
Karpuschewski et al [54] based their grinding monitoring
system on an AE sensor-integrated wheel by Wakuda et
al [116]. They showed that the wheel and signal
processing system could be used to reliably detect
events, such as: wheel-work contact, for reduced cycle
time; and wheel-dresser contact, to ensure the minimum
number of truing passes are given. The ground part
surface finish was also monitored by the integral AE
sensor. With the help of a fuzzy neural system, based on
parameters calculated from sensor data, a roughness
prediction can be achieved.
The ‘Sensor Integrated’ wheel concept needs to be
embraced by machine tool manufactures, in order to
become seamlessly embedded into the process control,
and not considered as a retrofit. Future designs must
also consider the easy removal and replacement of glued
vitrified cBN segments, without causing damage to the
hub each time.
4.4.3. Through-wheel coolant application
Instead of relying on the abrasive structure to transport
the grinding fluid into the grinding arc, several
researchers have enhanced the fluid path by using radial
internal tubes [107][34][47][38][5] or machining the
sidewall with flutes [77][107]. The radial tube approach
can be wasteful of fluid, since the flow exits the entire
3600 periphery of the grinding wheel despite the grinding
arc being a considerably smaller angle.
Sidewall friction can be reduced using external grooves
in this area. The grooves help to get the grinding fluid into
the sidewall area, but the process suffers from increased
noise and reduced abrasive content.
5. Innovative abrasive finishing processes due to
advanced abrasive product design
The development of new abrasive finishing processes
depends, in many cases, on the development of new
abrasive products. These products may have some of the
following attributes as compared to conventional
products: high peripheral speed ability, electrical
º
Workpiece surface temperature ( C)
5.1. High efficiency deep grinding (HEDG).
The introduction of high performance grinding machines,
in combination with the latest superabrasive technology,
has lead to the development of HEDG [121][100][109].
The HEDG process is characterised by extremely high
specific removal rates, using high wheel speed, high
workspeed and high depth of cut. In conventional
grinding, as the removal rate is increased the surface
temperature in the grinding zone increases, and burn
often results. However, if the wheel speed and table
speed are increased further, the surface contact
temperature reaches a peak value and then decreases,
due to the greater amount of grinding energy going into
the chip instead of the work. The greater thermal
conductivity of superabrasives also aids the heat removal
from the workpiece. Fig. 20 shows that with HEDG [109]
the workpiece surface temperature first increases, then
decreases with increased wheel speed.
.
conventional
wheel
ds = 400mm; a = 6mm
Material: 16MnCr5
Q’w = 100 mm3/s.mm
Fluid: mineral oil
cBN
wheel
Wheelspeed(m/s)
Fig. 20. The HEDG effect [109]
The use of electroplated cBN grinding wheel speeds up
to 250 m/s, work speeds in excess of 100mm/s (surface
grinding), depths of cut up to 30mm, and mineral oil fluid
has enabled HEDG to compete with conventional cutting
processes, with the advantage of better surface finish,
improved surface integrity, improved form accuracy, and
the possibility of using fully hardened workpieces [26].
Comley et al. [26], have demonstrated crankshaft webgrinding removal rates of up to Q’w = 2000 mm3/s.mm,
and journal grinding rates of 250 mm3/s.mm on low-alloy
automotive steel. The specific grinding energy can be as
low as 9 J/mm3 at such high removal rates. To achieve
this level of performance, modern single-layer
superabrasive wheels are 2-plane balanced, have
rotational error less than 3µm, are able to withstand high
periphery speeds (up to 250m/s), and have an even grit
distribution.
can be easily trued and dressed by a rotary device and it
can be used in water-based fluid, whilst not suffering the
attritious wear associated with cBN in water. Fig. 21
shows the grain wear for this wheel at a specific removal
3
rates of 375 mm /s.mm, with inconel 718 workpieces.
work speed: vft = 15 m/min,
work material: Inconel 718,
cutting speed: vc = 140 m/s,
depth of cut: ae = 1.5 mm,
3
spec. rem. rate:Qw =' 375 mm /(mm.s)
coolant: Hysol X,. 7%
flow rate: 130 l/min, 7 bar
wheel cleaning: 30 l/min,17 bar
Radial wheel wear, ∆rs
conductivity, tolerant of high surface temperatures, high
wear resistance, high permeability, etc. This section
describes these processes and why the abrasive product
is unique to the application.
1400
µm
cBN
1000
800
600
G = 2.6 mm3 /mm3
alumina
fibres
3
3
G = 19.0 mm /mm
400
200
0
0
500
1000
1500
2000
2500
mm3/mm 3500
spec. stock removal V'w
Fig. 21. Radial wear for cBN versus alumina fibres [69]
The result showed that after low initial wear the
comparison B126 vitrified cBN wheel gradually
developed greater grain wear than the 80 mesh sintered
alumina fibre wheels, giving a G-ratio of 2.6 mm³/mm³ for
the cBN-wheel, and a G-ratio of 19.0 mm³/mm³ for the
alumina fibre wheel. It must be noted that this result was
obtained in water-based fluid.
5.2. High speed traverse/contour/peel grinding
Traverse, contour or peel grinding, as the process is
known, is analogous to turning of a cylindrical part, with a
grinding wheel instead of a single-point cutting tool. The
advantage of this approach, as compared to plunge
grinding, is that a single wheel can form a multi-diameter,
fully hardened, shaft in only a few passes. These single
pass traverse approaches are only possible using high
wear resistant
superabrasive wheels,
typically,
electroplated, vitrified, or metal-bond types. The wheels
primarily cut the workpiece material on their sides and
therefore require the abrasive to wrap around the edges.
Narrow, but sufficiently stiff, wheels help reduce the
contact width to reduce the thermal loading on the
workpiece surface [100][113]. Fig 22 shows the principle
of the process.
vf
bc
Fig. 22. High speed traverse grinding of shaft [100]
There is also a role for conventional abrasives in HEDG
[69], providing that they can reach a sufficiently high
peripheral wheel speed to be classified as working in the
HEDG domain. For example, the 8:1 aspect ratio,
sintered, seeded-gel alumina finres, described in Section
4.1, can be formed into segments and bonded to a steel
core to allow grinding speeds up to 140 m/s. This grain
Another approach to reducing the contact area is by
tilting the grinding wheel by a few degrees to create an
almost point contact with the workpiece [56].
Full depth, traverse grinding can also be used to
generate internal forms in a component, as a
replacement for plunge or reciprocating grinding, which
require much wider wheels. One of the biggest
advantages with this approach by Weinert and Finke
[118] is a reduction in the total grinding force, and
therefore a reduction in the tendency to produce a
tapered hole. Fig. 23 shows a schematic of the contact
zone for this mode of grinding.
Grinding
wheel
profile after grind hardening a 42CrMo4 steel. WEA is the
white etching area.
The grinding wheel for grind-hardening, requires the
following attributes:
•
low thermal conductivity abrasive
•
tough bond material to retain the grain
•
low tendency to load with the work material
•
low-porosity, if minimum coolant applied to cool
wheel
•
closed structure
•
monolithic design, or thick segments that protect
adhesive from heat
•
low speed rating of <40m/s
surface grinding
material: heat treatable steel (tempered)
grinding wheel: Al2O3
Q'w = 2,5 mm3/(mm.s), up grinding
residual stress σ⊥
300
work
MPa
white etching
area (WEA)
0
- 150
structure of
etchable
martensite
- 300
- 450
- 500
Fig. 23. Contact zone in deep traverse grinding [118]
Internal, deep traverse grinding uses a narrow grinding
wheel with a tapered roughing zone and a cylindrical
finishing zone. The radial infeed motion takes place
outside of the workpiece. During the axial feed motion,
the tapered roughing zone removes the material and the
cylindrical finishing zone generates a good surface
quality. Due to the tapered geometry, high material
removal rates can be achieved, because the consumed
power of the grinding process is distributed over a large
surface area. The specific material removal rate
determines the grinding wheel power. When using a
tapered dressed grinding wheel the largest specific
material removal rate occurs in zone II (Fig. 23). The
profile angle Χ , and the feed engagement af , determine
the effective engagement ae,eff. By using this relation the
specific material removal rate for internal traverse
grinding can be described in equation (1).
Q / w = π ⋅ d w ⋅ nw ⋅ a f ⋅ tan χ .
(1)
5.3. Grind-hardening
It has been shown that the heat flux generated in grinding
can be used to induce martensitic phase transformations
into the surface layers of annealed or tempered steel,
creating a hardened surface, with pre-dominantly
compressive surface residual stresses. This technique
can replace both the rough grinding in ‘soft state’ and
heat treatment operations, that are traditionally used in a
production sequence [17][18]. Grind-hardening is not to
be confused with grinding burn, where a hardened
workpiece adopts a martensitic white layer, containing
tensile surface residual stresses.
The maximum depth of hardness penetration, and
hardness profile, obtained by grind-hardening, to date,
ranges from 2mm for flat surfaces and 1.6mm for round
surfaces. Fig. 24 shows the surface residual stress
0
0,05
0,1
mm
depth beneath surface
0,2
Fig. 24. Residual stresses after grind-hardening [18]
A suitable wheel for this process would therefore be
aluminium oxide abrasive in a resin bond. Whilst the
above attributes appear to give the best results, it is
possible to grind harden with cBN wheels, but with less
favourable economics at this time [102].
5.4. Fixed abrasive wire sawing
The traditional method of slicing silicon wafers off an
ingot, is the ID saw. In recent years this has been
replaced by loose abrasive wire sawing and fixed
abrasive wire sawing [86]. Even though the loose
abrasive method is very efficient (up to a thousand
wafers can be cut in a few hours), it is dirty, not
environmentally friendly, and consumes significant
quantities of loose SiC. This technique may also preclude
the cutting of hard ceramics (such as sapphire or SiC)
since the initial loading of a wire saw machine with loose
diamond abrasive would be cost prohibitive.
Fixed abrasive steel-core wires, namely electroplated
diamond and resinoid diamond wires, have been
developed to overcome these issues. The productivity of
this wire saw technology is high since multiple wires are
threaded through the machine to simultaneously cut the
ingot with a single stroke, like a bread-slicing machine.
Wire lengths greater than 1 kilometre are sometimes
used to slice an ingot into many wafers (see Fig. 25).
Resinoid diamond wires are produced at lower cost than
electroplated ones, but they have a lower breaking twist
strength. To overcome this, metal powder is added to the
resin to strengthen it [31][71]. The researchers also found
that the heat curing process caused brittleness to
develop in the 0.2 mm diameter piano wire.
To decrease the wear associated with the resinoid
diamond wire, the surface was modified by adding a 20
nm SiO2 film to it using radio-frequency magnetron
sputtering. This technique increases the gripping strength
of the diamond grains to the resin matrix.
Since electroplated diamond wires are not subjected to
the damaging curing temperatures that resinoid wires
experience, they are therefore less prone to breaking.
Although the manufacturing process for this type of wire
is slower than for resinoid, their increased cost is offset
by higher wear resistance. These electroplated diamond
wires have proven to be very effective in cutting hard
ceramics, such as sapphire in the LED industry.
coolant
nozzle
work
coolant
nozzle
The technique has also been applied to truing and
dressing of grinding wheels [87], which are then used for
high precision grinding of cylindrical workpieces. Grinding
wheel surface quality and roundness error are claimed to
be superior to those obtained by conventional grinding.
5.5.2. Electrolytic in-process dressing
A variation of the ECDM technique is ELID, where the
grinding wheel is the anode and the electrolyte is
supplied into the inter-electrode gap [51]. The aim is to
eliminate wheel loading and ensure permanent dressing.
Using this technique on an ultra-stiff precision grinding
machine, has achieved surface finish values of less than
10 nm Ra [99].
spacer rollers
feed roller
preferably bronze or copper. The ECM and EDM time
periods need to be balanced. In the first stage ECM
occurs and anodic dissolution of the outer layer of the
workpiece takes place. In parallel, mechanical grinding
also occurs. On increasing the voltage in the second
stage of the process, the concentration of the ions
increases until electrical discharges occur. A plasma
channel is created and material removed by evaporation.
take-up roller
ELID usually employs a cast iron fibre bonded wheel. An
oxide layer (rust) is produced on the wheel surface during
the electro-chemical reaction between bond and fluid,
shedding worn diamond grains (see Fig. 26). ELID
enables the use of nano-order diamond grains to be
used, and can be applied to all modes of grinding.
outer diameter (OD) grinding
Fig. 25. Fixed abrasive wire saw machine [71]
Cast iron fiber bonded
(CIFB) grinding wheel
Chiba et al [24] developed an ultra high-speed method of
producing electroplated wire using nickel-coated, 10-20
µm diamond grains.
5.5. Grinding, truing and dressing by electrical and
electro-chemical methods
Several electrical and electro-chemical grinding, truing
and dressing systems have been developed in the last
decade. The systems have been developed for diamond
tool grinding, diamond wheel preparation, ceramic
grinding, and others. The following acronyms describe a
few of the developed systems:
•
Electro-Chemical Discharge Machining (ECDM)[87]
•
Contact Discharge Truing and Dressing (CDTD)[70]
•
Electro-Contact Discharge Dressing (ECDD)[112]
•
Electro-Discharge
Machining
and
Grinding
(EDMG)[93]
•
Electrochemical in-process Controlled Dressing
(ECD)[62]
•
Electro-Discharge Diamond Grinding (EDDG)[60]
•
Rotary Electro-Discharge Machining by grinding
wheel (REDM)[78]
•
Abrasive Electro-Discharge Grinding (AEDG)[97]
•
Electrolytic In-process Dressing (ELID)[51]
Some of the above systems will work with standard
metal-bond superabrasive wheels, although in some
cases, specially developed electrically conductive bonds
and coatings have been developed to enhance the
process. A selection of these systems follows:
5.5.1. Electrochemical discharge machining
ECDM combines ECM and EDM. The ECM action is
assisted by the thermal erosive effects of discharges.
The grinding wheel must have a conducting metal bond,
rotation grinding
coolant
(electrolyte)
grinding wheel (CIFB)
= anode
brush
+
+
workpiece
+
-
U
U
workpiece
electrode
= cathode (copper, graphite)
electrode
= cathode
coolant
(electrolyte)
source : Ohmori/Nakagawa, Japan
Fig. 26. Principle of ELID [51]
5.5.3. Electro-discharge diamond grinding
In EDDG, a bronze bonded diamond wheel is used in a
kerosene dielectric fluid. The process integrates electrical
discharge machining with diamond grinding for
electrically conductive, hard, materials. The role of the
spark is to thermally soften the work material, in an effort
to reduce normal and tangential forces [60], as shown in
Fig. 27.
5.5.4. Laser-assisted dressing of superabrasive
wheels
The precision of a formed grinding wheel is often
dependent on the geometry of the truing tool. With
superabrasive grinding wheels, especially diamond, wear
of the truer can be significant and ever changing. Several
researchers have investigated laser-assisted truing and
dressing, but with limited success. Such systems are
expensive and must be protected from the hostile
environment in a grinding machine.
Shin [92] developed a system that used a laser to soften
the vitrified bond prior to contact with a single-point
diamond truer. The laser was applied in an axial direction
just ahead of the diamond truer. The effect was a very
open wheel structure and some grain pullout.
metal bond
abrasive
workpiece
Fig. 27. Principle of EDDG [60]
The cBN that Shin used was typical of that used in a
standard vitrified product. However, Hoffmeister and
Timmer [44] found that the greater transparency of
natural diamonds allowed them to withstand the energy
of the laser far better than the yellow synthetic diamonds.
They also determined that larger grit sizes are more
tolerant of the laser energy, with regards to grit fracturing
due to a reduction in fracture toughness.
Hoffmeister and Timmer [44] compared traditional
phenolic resin bond with high-temperature, copper filled,
polyimide resin bond, showing the former took longer to
profile, using a tangential laser. Their tests on resin bond
wheels concluded that careful control of the bond
material, and use of more transparent diamonds, will lead
to a product that is tuned to the laser-truing process.
5.6. Fixed abrasive grinding, with a vertical spindle
Manufacturing of silicon wafers consists of a succession
of abrasive processes: saw, edge, lap, etch and polish.
As the requirements for flatness and wafer size
tolerances increase, new process were needed to
replace some of the loose abrasive (lapping and
polishing) steps by “fixed abrasive”. In addition, it was felt
the throughput of loose abrasive process in the finishing
of a wide variety of ceramic materials could be improved
by grinding processes, provided of course that the
necessary low roughness (typically sub-100 Angstroms
Rms) could be achieved. This led to the development of
a grinding process [114] with a narrow abrasive rim
(approx. 6 mm) and special kinematics (e.g. centre of
wafer directly under abrasive rim) to generate mirror
finishes on a variety of advanced ceramic components.
Fine
Infeed Grinding Wheel
Air
Bearing
Spindle
6.1. Self-lubricating grinding wheels
The push towards minimum fluid application and a
reduction in fluid disposal costs, has encouraged
researchers to examine the integration of lubricants into
the structure of the grinding wheel. This has resulted in
several patented designs, including:
•
•
•
Superabrasive segments impregnated with resin and
proprietary solid lubricant [50]
Sol-gel alumina grain in vitrified bond, filled with
oil/wax mixture [82]
Vitrified aluminium oxide wheels impregnated with
water-insoluble, sulphur bearing, organic substance
[64]
Fig. 29. Graphite impregnated wheel [89]
Salmon [85] used cutting tool coating technology on cBN
grains to counter the boric oxide formation when waterbased fluids are used. His two-step approach was to use
a hard titanium aluminium nitride (TiAlN) coating on an
electroplated grinding wheel, followed by a layer of
molybdenum disulphide (MoS2) hard lubricant. Salmon’s
tests on MAR nickel-based alloy showed that the
coatings gave longer life, lower power, and no capping of
the grains, despite the absence of cleaning jets.
Air
Bearing
Spindle
Side
View
6. Future trends in abrasive products
This section attempts to predict the future developments
in abrasive products that will give the following benefits:
•
require reduced fluid flowrate, etc.
•
reduced time to manufacture
•
produce better surfaces
•
are less expensive to manufacture
•
more of a consumable, not niche, product
•
easier to prepare surface for grinding
•
greater wear resistance
Wafer
Wafer
Vacuum
Chuck
types of infeed grinding processes, have been made
possible by the development of a new generation of selfdressing diamond products [114]. Impressive results
were presented by combining some of these newly
developed diamond wheels, with the grinding kinematics
described in Fig. 28. Tricard et al [114] reported an
impressive sub-20 Angstroms (Rms) surface roughness
(measured with a 10 x 10 µm AFM scan) achieved on
aluminium titanium carbide (AlTiC) wafers in a production
environment.
Top
View
Fig. 28. Fixed abrasive grinding kinematics [114]
Although small compared to vertical axis segmented and
pellet wheels, the still broad area of contact between the
grinding wheel and the workpieces, present in these
Tang et al [108] also explored the use of MoS2 lubricant,
but with titanium alloy workpieces and silicon carbide
wheels. Tang found a reduction in grinding forces, lower
specific grinding energy due to reduced ploughing and
sliding, and less adhering of titanium on the SiC grains.
Unlike Salmon, or Tang et al, Shaji’s approach [89] was
based on a unique wheel design that incorporated a solid
lubricant. The vitrified aluminium oxide wheel, shown in
Fig. 29, has dovetail slots on the periphery, filled with a
phenolic resin-alumina-graphite mixture. Shaji’s dry
grinding tests on Rc60, EN31 bearing steel showed:
better surface finish, lower spindle power, and higher
wheel wear, as compared to wet grinding without the
solid lubricant. Shaji attributed this to the interrupted
cutting action promoting high forces on the leading
edges, resulting in breakdown of the vitreous structure.
6.2. Rapid prototyping of vitreous, metallic and
organic grinding wheels
Industrial application of Rapid Prototyping as a material
additive manufacturing process started fifteen years ago
[65]. Since then, several methods of building grinding
wheels have been researched, based on ceramic,
metallic and polymer bonds. The most popular method
explored is Selective Laser Sintering (SLS).
graphitisation, or reduction of toughness, was detected
by Kovalenko et al. Using a special 90% cobalt
composite and diamond, clusters of the abrasive were
produced. This work is highly relevant to future singlelayer diamond wheel production (see Fig. 32).
Maekawa et al [66] also developed metal bonded
abrasive tools by the ‘greentape’ laser sintering method.
‘Greentape’ refers to the tape of abrasive and binders
prior to sintering. The tapes are formed in advance with
the required density of abrasive grains and copper-based
bond content. This method is potentially more consistent
than the traditional re-coating method. The ‘Greentape’
can also be pressed before each pass of the laser, to
give greater packing density. Abrasive products using
diamond, cBN and aluminium oxide have been produced
by this method.
scanner
laser
recoater
part
powder
platform
laser
beam
sintered
layer
substrate
plate
Fig. 31. SEM micrograph of surface of SLS parts [45]
powder
133µm
Fig. 30. Principle of SLS [95]
In the SLS process, shown in Fig. 30, a focussed laser
beam locally sinters/melts a heat fusible powder [95].
Infrared CO2 or Nd:YAG lasers, with a maximum beam
energy of 50-100W are used as energy sources. After
one layer has been formed, a new powder layer is
applied and the laser beam solidifies it. The laser beam
intensity is modulated so as to melt the new powder and
bond it to the layer. This procedure is repeated until the
whole part is completed.
The idea of using selective laser sintering for making
grinding wheels is especially attractive for low volume
customized production of special profiles. The viability of
this approach was reported by Hon and Gill [45], who
produced polyamide/SiC matrix composites . Samples
were produced using FEPA standard SiC F240 grit
blended with polyamide to produce a 50/50 mix. The
sample, shown in Fig. 31, reached a UTS value of 31
MPa and a Young’s Modulus of 2100 MPa.
Kovalenko et al [61] demonstrated laser sintering of a
composite 80%Co-20%Sn and diamond abrasive, using
2
up to 2 kW/cm power density. No evidence of
Fig. 32. Laser sintering of single-layer diamond [61]
6.3. Engineered grinding wheels
Discussions on the “deterministic” performance of
machining versus the “black art” of grinding, are often
based on the fact that cutting tools have evenly spaced
cutting edges of defined geometry (unless worn). In North
America, HEDG is sometimes referred to as “micromilling” due to the large ductile chips that are removed
from the workpiece by coarse grit electroplated wheels.
In an attempt to take some of the randomness out of
grinding,
several
researchers
have
developed
engineered grinding wheels of defined distribution and, in
some cases, defined orientation.
Aurich et al [3] built and tested a wheel with defined grain
structure, using kinematic simulation to develop the
pattern. The cBN grains were glued onto the hub in the
required pattern, followed by electroplating over the top
with nickel. The aim of the work was to improve the
process stability, minimise heat generation and achieve
better surface quality, all without compromising the
material removal rate. Fig. 33 shows one pattern
investigated by the simulation, and the possible defect
that may occur if a single peripheral line around the
wheel has no abrasive.
Unfortunately, the engineered wheel by Aurich et al [3]
suffered stripping of the abrasive during grinding tests.
This was attribiuted to contamination of the electroplating
by the glue. These sorts of issues are dealt with regularly
within the grinding wheel industry, during the
manufacture of reverse-plated truing rolls. In fact, the
reverse-plated approach to engineered wheels would
ensure that all the abrasive grits have similar protrusion
from the axis of rotation prior to plating.
multiple cutting edges on each wire. Tests on a single
diamond wire, bonded radially into a narrow disk, showed
that once the leading edge diamond crystals were
chamfered by initial wear, and many more of them
became active, the wear rate reduced dramatically.
Subsequent tests on multiple fibres, randomly positioned
into a metal bond matrix, produced optical quality
surfaces on BK7 glass, with a surface finish value of
70nm Ra, and less than 2µm sub-surface damage.
Furthermore these tests suggested that diamond fibres
could lead to longer wheel life, when grinding in the
ductile region, compared with existing resin bond wheels.
resulting workpiece surfaces
∆zV= 300 µm
∆zV= 200 µm
possible defect:
gap in grit pattern
axial profile
Roughness Ra (µm)
3.5
3.0
57.6 µm
52.6 µm
axial pitch 47.6 µm
2.5
Axial offset
2.0
vs
1.5
1.0
Axial pitch
0.5
0.0
0
10
20
30
40
50
60
Axial Offset (µm)
Fig. 33. Influence of grit displacement on work surface [3]
Fig. 34. Effect of axial pitch and offset on roughness [59]
Another possible reason for the stripping may be due to
the far lower abrasive concentration of the engineered
wheel, as compared to a standard electroplated wheel. At
similar material removal rates, the engineered wheel
would remove a larger chip with a correspondingly higher
force per grit. Also, in patterns such as in Fig. 33, even
spacing of the grains around the wheel periphery may
also lead to cyclic fluctuations in the grinding force, and
develop into a forced chatter vibration. To overcome this,
small variations in the pitch of the pattern around the
wheel periphery may give less opportunity for instability
to arise.
6.4. Single-layer superabrasive wheel developments
The future can expect to see greater applications of
single-layer superabrasive wheels, especially on
machining centers. As electroplating and brazing singlelayer manufacturing techniques become faster, and their
geometry becomes more accurate, they will replace
some milling tools in tool changing cabinets. The
increased integration of HSK tapers and other similar
spindle mounts, into the wheel body, will reduce the
radial and axial error associated with collets and
adapters.
Koshy et al [59] modelled both vitrified and electroplated
wheels with defined grain structure, and modelled the
effect of axial pitch and axial offset of adjacent rows, on
the surface finish of the finished workpiece. For the best
surface finish, he showed that the axial offset should be
greater than zero but less than 25-40% of the average
grain diameter (42 µm). Fig. 34 shows surface finish as a
function of axial pitch and axial offset.
Pritchard [75] developed a method of optimally orienting
and spacing the abrasive grain, for coated abrasive belts.
His technique relies on a perforated polymeric sheet that
passes under falling abrasive grains and traps one grain
in each perforation. The shape of the perforation
preferentially traps the point of the grain rather than a flat,
hence orienting it. Excess grains are blown away using
compressed air. The grains are then sprayed with a
solvent to soften the polymer and bond the abrasive to it.
The last steps involve sintering the grains to a metal tape
using a special brazing powder. The method claims to
work with all conventional and superabrasive grains.
Engineered abrasive grains may also encompass
diamond fibres produced by depositing diamond on to
tungsten wire, using hot filament CVD [25]. The diamond
that is produced has a polycrystalline structure giving
Advances in the friability, structure and shape of
synthesised superabrasives will also improve the
performance of these wheels in terms of life and formholding. Recent tests grinding nickel alloy, polycrystalline
cBN abrasives with ultrafine crystal structure, have
recently shown G-ratio improvements up to 15 times
greater than conventional polycrystalline cBN [106] with
lower specific grinding energy. It is also expected that the
nano-crystalline cBN will constantly regenerate micro
cutting edges as radial wear progresses, much in the
same way as a sol-gel sintered alumina.
6.4.1. Ultrasonic aided, electro-less nickel plating
The application of ultrasonic vibration during electroless
nickel plating has been shown to improve the wetting
between an abrasive grain and the nickel matrix. This
grain exposure is comparable to electroplated nickel
wheels [73]. The plating rate is claimed to increase with
vibration amplitude, up to a maximum value of 11 µm at a
frequency of 15.5 kHz. The process is experimental at
this time but has potential for complex forms due to
reduced tendency to build up at sharp corners, as
compared to the electroplated process.
6.4.2. Direct deposition of abrasive layers
Much work has been done developing CVD coatings, and
their application for wear resistance, improved thermal
properties, etc. However some work has also been done
using the coating as a single-, or multi-layer, abrasive
tool. Although this work by Gabler et al [35] involved the
manufacture of small abrasive burrs, there may be
potential for creating larger wheels if the process cost
can be reduced. Using a hot-filament CVD reactor, 240
tools were coated with crystal sizes up to 50 µm.
Although the coating time was 90 hours, economies-ofscale can make the process viable. A close up of the
CVD abrasive burr is shown in Fig. 35.
The innovation in this broaching replacement process, is
in the HEDG pre-forming of the disk, the precision and
stiffness of the mounted points, and negligible form error
st
th
from the 1 to 70 slot. The process uses wheel speeds
between 50-100,000 rpm, on difficult-to-grind nickelbased alloys [6]. Carbide shanks are designed to
fracture, not bend, to ensure that the spindle nose and
workpiece are not damaged during an unexpected
collision.
6.4.4. Tools for ultra-sonic assisted machining
The possibility to apply abrasive grains to almost any
given geometry has also led to the development of ultrasonic assisted material removal processes, sometimes
referred to as “ultra-sonic milling” [28]. With a new
generation of machine tools dedicated to apply this
technology complex geometrical 3D-features can be
machined in mainly brittle materials such as glass or
ceramics, as shown in Fig. 37.
booster
(for amplitude
amplifying)
US spindle (axial
movement initiated by
an ultrasonic generator)
f = 20 kHz
xspi = 1 ÷ 3 µm
single layer diamond tools
vc
ae ± adyn
oscillating tool
xtoo = 3 ÷ 50 µm
diamond layer
workpiece
(e.g. glass, ceramics,
CFRP, ...)
source: DMG Sauer
vft
Fig. 37. Ultra-sonic assisted machining [28]
C (diamond)
Fig. 35. CVD coated abrasive burr [35]
6.4.3. Profile grinding to replace machining
Re-entrant internal slots, such as dovetails and turbine
disk rootforms, are traditionally produced using a large
broaching machine. Turbine disk broaching machines are
extremely large and contain hundreds of cutting edges to
produce the slots. The sheer cost, space allowance and
maintenance of such machines, makes the process
expensive and destined to remain inside the turbine
manufacturer plants. Recent developments in mounted
point grinding have showed the viability of producing
entire disks with just a few small, profiled wheels, on a
modified machining centre, making future production
possible at subcontractor facilities. Fig. 36 shows an
example of both pre-formed and finished slots in a disk,
using rough and finish electroplated cBN mounted points.
Fig. 36. Internal rootforms produced by point grinding [6]
BC4N
BC2N
C3N4
B 4C
cBN
N
B
Fig. 38. Superhard Materials in B-C-N System [123]
6.5. Superhard material development
The search for novel superhard materials continues
following the successful synthesis of man-made diamond
and cubic boron nitride at high temperatures and
pressures. “Designing” new superhard materials with
novel properties, and developing practical methods of
production, are the goals of several research teams
[123][41][72][46]. Potential candidates are from the
systems of carbon nitrides (C3N4), boron-carbon-nitrides
(BC2N), boron carbides (B4C), and boron nitrides (cBN),
as illustrated in Fig. 38.
In one exciting example, a theoretical calculation from
first principles predicted that certain carbon nitrides have
bulk moduli comparable to or even greater than that of
diamond. Based on the assumption that hardness
correlates with the bulk modulus, cubic C3N4 (with a
calculated bulk modulus of 496Gpa) will likely be harder
than diamond. Fig. 39 shows measured and calculated
properties of materials based on boron, carbon and
nitrogen. However, after 10 years of extensive research,
attempts to make this material have not been successful.
The major difficulties are the loss of nitrogen, and the
strong N-N bond that favours the formation of N2.
The possibility of synthesizing α, β-C3N4 phases using Ni
as the catalyst, at 7 GPa and 1400°C in a large-volume
press was shown to be feasible [41]. However, the crystal
obtained was too small to provide conclusive structure
and compositional measurement. In the B-C-N system,
synthesis of a nano-sized powder mixture of diamond,
cBN and the cubic phase of BC2N has been reported at
7.7 GPa and 2000°C without the use of a catalyst [72].
Phase
Hardness
Bulk Mod.
Shear Mod.
(GPa)
(GPa)
(GPa)
Diamond
70-90
443
535
Cubic BN
48
400
409
Cubic C3N4
??
496
332*
β - C3N4
??
437
320*
BC2N
60
408
445*
B4C
30
247
171
Fig. 39. Measured and Calculated Modulus Values [123]
Low oxygen content cBN and its production, has also
been reported [124]. Higher pressures are needed to
obtain single-phase material and to search for suitable
catalysts to lower the pressure and temperature
conditions. In the boron-oxide system, single phase of
B6O has been produced at conditions of 5-7.5 GPa and
1700°C [46], while cubic B6O was reported to be
synthesized at much lower pressure and temperature
ranges of 3.5-5.5 GPa, 1000°C-1200°C [123]. However,
further work is needed to characterize their structure and
properties.
Zhao et al [123] carried out high-pressure synthesis of
well-sintered millimetre-sized bulks of superhard BC2N
and BC4N materials in the form of a nano-crystalline
composite with diamond-like amorphous carbon grain
boundaries. These new high-pressure phases of B-C-N
compound have extreme hardnesses, second only to
diamond. The final products are well-sintered millimetre
size chunks which are translucent and yellowish in
colour. The synthesized BC2N and BC4N materials have
a zinc-blend structure and a face-centred cubic unit cell.
The hardness measurements show that the BC2N and
BC4N samples synthesized under high pressure and
temperature have nominal hardnesses of 62 GPa and 68
GPa respectively, which is very close to diamond and far
higher than cBN.
Zhao et al [123] states that reactive sintering of diamondSiC nano-composites, based on thorough mixing of
diamond and silicon nano-size powder, can be applied to
produce large specimens. It is expected that by better
sample preparation, carefully designed mixing protocols,
and by using silicon powder of smaller grain size, it will
be possible to eliminate graphitization, reduce porosity
and decrease SiC content, and thus further improve
properties of diamond-SiC nano-composites.
7. Concluding remarks
This paper does not reflect the views of Saint Gobain or
QED Technologies, but is the result of extensive
literature and patent searches by the authors. The
authors extend their gratitude to Saint Gobain and QED
Technologies for use of library facilities and some
preparation time.
In 95% of cases the text is referenced, with some
industrial viewpoint statements added by the authors
where their experience warrants it. It has been
impossible to include examples of innovation and future
trends where there are no public domain documents
available, and where it is secret within companies.
Although many abrasive producers were contacted early
on in the writing of the paper, it was not an effective
means of obtaining information. In many cases the
information could not be referenced to a public domain
document, other than a patent. Special thanks go to
Hermes Schleifmittel for their input in the sol-gel abrasive
section.
Finally, the authors would like to thank STC
chairpersons, Profs. S. Malkin and B. Karpuschewski, for
their input and review of the document.
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