Innovations in Abrasive Products for Precision Grinding

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 As mentioned by Cheape [23], until the 18th 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 l ta ys Cr

0.59µm 0.67µm 0.97µm 3.5µm 10.1µm

si z e

Hardness HV3 (GPa)

lite

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 superabrasive 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 v

100

gra in vo lu

40

60

60

40

80

20

100 0

vP

sp ec .

80

me olu rev po

me

K

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 3 specific removal rate Q’w of 4 mm /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

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 0

µm

HSG-wheel 0

50

100

150

wheel radius rs

- - - radial elongation

1A1-wheel

0 200

HSG-wheel

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 1.2

Ploughing

1.3

Sliding

2. CHIP/BOND

Fig. 9. Reduced stress design and radial elongation [58]

3. CHIP/WORK

between the abrasive product and work material can be characterised as shown in Fig 10:

4. BOND/WORK

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: • • •

Cutting

Sliding Sliding 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 agglomerates, has led to highly permeable structures using these pre-formed clusters of grains [53], which are bonded 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 agglomerate-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 grain Wheel bond

Agglomerate bond

Fig. 13. TiC film assists brazing process [22] 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 Agglomerated 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.

Disadvantages include:

•

a general preference for oil-based fluids due to longer service life

133µm

Fig. 14. SEM of cBN in brazed bond 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

contact elements simulating sliding and penetration

c

temperature sensor 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)

4 µm circumference of the rotating grinding wheel

dimensions : displacement 1 : 400 workpiece material : hot-pressed silicon nitride : a e = 1.5 mm depth of cut cutting speed : v c = 50 m/s : v w = 160 mm/min feed rate

telemetry ring

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 0 360 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 web3 grinding removal rates of up to Q’w = 2000 mm /s.mm, 3 and journal grinding rates of 250 mm /s.mm on low-alloy automotive steel. The specific grinding energy can be as 3 low as 9 J/mm 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

3

200 0

3

G = 2.6 mm /mm alumina fibres 3 3 G = 19.0 mm /mm

400

0

500

1000

1500

2000

2500

spec. stock removal V'w

mm3/mm 3500

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

Wafer

Air Bearing Spindle

Side Vi

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 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.

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 Vi

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 B4C 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. 8. References [1] Andrews, R., 1996, Wear Resistant Bond for Brazing and Abrasive Tool, Norton Company, US Patent 5,846,269. [2] Ardelt, T., 2000, Einfluss der Relativbewegung auf den Prozess und das Arbeitsergebnis beim Planschleifen mit Planetenkinematik, PhD Dissertation, Technical Univ. of Berlin. [3] Aurich, J., Braun, O., and Warnecke, G., 2003, Development of a Superabrasive Grinding Wheel with Defined Grain Structure Using Kinematic Simulation, Annals of the CIRP, 52/1:275-280. [4] Barlow, N., Jackson, M., and Hitchiner, M., 1996, Mechanical Design of High-Speed Vitrified cBN Grinding Wheels, Proc. of IMEC, Univ. of Connecticut, USA , p. 568-570. [5] Barnard, J., 1990, Grinding Tools at High Speeds Using MSL Wheels, SME Int. Grinding Conf. [6] Barry, J., Burrows, J., Aspinwall, D., Dewes, R., 2003, The use of Diamond and cBN Mounted Grinding Points for the Machining of Nickel-based Superalloys, Intertech, Vancouver. [7] Baylis, R., and Stone, B., 1989, The Effect of grinding Wheel Flexibility on Chatter, Annals of the CIRP, 38/1:307-310. [8] Berg; T., 1993, Shaped abrasive particles and method of making same, assigned to Minnesota Mining and Manufacturing Company, United States Patent 5,201,916. [9] Boehm, C., Seedorf, T., Meyer, L., Brinksmeier, E., and Binder, J., 2001, Temperature and Force Sensor Integrated Grinding Wheel for Process Monitoring, Abrasives Magazine, October/December. [10] W. Bohlheim, 1994, Performance of ABN260 and ABN360 in Surface Grinding HSS, IDR 4/94:187191. [11] Bonner, A., Filed October 2003, Abrasive Articles with Novel Structures and Methods for Grinding, World Patent 03/086703 A1. [12] Bonner, A., Filed October 2003, Method of Roll Grinding, World Patent 03/086702 A1. [13] Bovenkerk, H. P.; Bundy, F. P.; Hall, H. T.; Strong, H. M., and Wentorf, R. H. Jr., Oct. 10 1959, Preparation of Diamond, Nature, 184 (4693):10941098.

[14] Bovenkerk, H. P.; Bundy, F. P.; Chrenko, R. M.; Codella, P. J.; Strong, H. M., and Wentorf, R. H. Jr., Sept. 1993, Errors in Diamond Synthesis, Nature, 365:19. [15] Brecker, J. N., 1974, The Fracture Strength of Abrasive Grains, Trans. Of ASME J. Eng. For Industry, 96:253-57. [16] Breder, K., Lara-Curzio, E., and Riester, L., 2004, Strength Testing System for Ceramic Grains, Ceramic Engineering and Science Proceedings, Published by the American Ceramic Society, 25. [17] Brinksmeier, E., and Brockhoff, T., 1996, Utilisation of Grinding Heat as a New Heat Treatment Process, Annals of the CIRP, 45/1:283-286. [18] Brockhoff, T., and Brinksmeier, E., 1999, GrindHardening: A Comprehensive View, Annals of the CIRP 48/1:255-260. [19] Bundy, F. P., Hall, H. T.; Strong, H. M., and Wentorf, R. H, Jul 9 1955, Man-Made Diamonds, Nature, 176:51-55. [20] Bzymek, Z., Song, G., Howes, T., and Garrett, R., 1994, Design of Flexible Grinding Wheel with Variable Hub Thickness, Trans. Of ASME, J. of Engineering for Industry, 250/116:260-262. [21] Chattopadhyay, A., and Hintermann, H., 1994, On Performance of Brazed Single-Layer CBN Wheel, Annals of the CIRP, 43/1:313-316. [22] Chattopadhyay, A., and Hintermann, H., 1993, On Improved Bonding of TiC-Coated cBN Grits in Nickel-Based Matrix, Annals of the CIRP, 42/1:413416. [23] Cheape, Charles W., 1985, Family Firm to Modern Multinational - Norton Company, a New England Entreprise, Cambridge, MA, Harvard University Press. [24] Chiba, Y., Tani, Y., Enomoto, T., and Sato, H., 2003, Development of a High Speed Manufacturing Method for Electroplated Diamond Wire Tool, Annals of the CIRP, 52/1:281-284. [25] Comley, P., Smith, N., Pearce, T., Smith, D., and Ashfold, M., 2003, Performance and Wear Behaviour of Diamond Fibre Grinding Wheels when Grinding Glass, Advances in Abrasive Technology VI, ISAAT Conf., Bristol, UK, :245-250. [26] Comley, P., Stephenson, D., and Corbett, J., 2003, High Efficiency Deep Grinding and the Effect on Surface Integrity, Advances in Abrasive Technology VI, ISAAT Conf., Bristol, UK, :207-212. [27] DiCorleto, J. 2001, Innovations in Abrasive Products for Precision Grinding, Precision Grinding and Finishing Conference, Gorham. [28] DMG Sauer, 2004, Product information, Seebach Germany, www.gildemeister.com. [29] Einset, E., 2003, Mixture of Abrasive Particles with Different Toughness, GE Company, World Patent 03043784. [30] Engineer, F., Guo, C., and Malkin, S., 1992, Experimental Measurement of Fluid Flow Through the Grinding Zone, J. of Engineering for Industry 114:61-66. [31] Enomoto, T., Shimazaki, Y., Tani, Y., Suzuki, M., and Kanda, Y., 1999, Development of a Resinoid Diamond Wire Containing Metal Powder for Slicing Silicon Ingot, Annals of the CIRP, 48/1:273-276. [32] d’Evelyn, M., 2003, Surface Functionalized Diamond Crystals and Methods for Producing Same, GE Company, European Patent 01008553. [33] Fisbacher, M., 2000, External Cylindrical Grinding with CBN – an Innovative Solution, Industrial

[34]

[35]

[36] [37] [38]

[39] [40] [41] [42]

[43] [44]

[45] [46] [47]

[48]

[49] [50] [51]

[52]

Diamond Association, Intertech Conf., Vancouver, Canada. Fischer, D., Ulmer, Filed 2001, Grindstone Cooled by Coolant has Coolant Supplied to Contact Surface Through Recess in Grindstone into which Coolant is Sprayed from Outside, German Patent 10103435. Gabler, J., Hoffmeister, H-W., Menze, B., and Schafer, L., 2003, Micro-Abrasive Pencils with CVD Diamond Coating, Diamond Relat. Mat., 12/3-7:707710. Galen, E., 2001, Superabrasive Grinding, Why Bond Selection Matters, Manufacturing Engineering Magazine, SME, 2. Garg, Ajay K., April 11, 2000, Nano-sized Alpha Alumina Particles having a Silica Coating Thereon, assigned to Norton Company, US Patent 6,048,577. Graham, W. and Whiston, M., 1978, Some Observations of Through-wheel Coolant Application in Grinding, Int. J. Machine Tool Design and Research, No. 18:9-18. Guo, C., and Malkin, S., 1992, Analysis of Fluid Flow through the Grinding Zone, Trans. of the ASME, J. of Eng. for Industry, 114:427-434. Harris, G., 1989, Long Life Resin Bond Wheels, Industrial Diamond Review, 3/89:123-125. He, D., Akaishi, M., Scott, B., and Zhao, Y., 2002, J. Material Research, 17/284. Heath, P. 1985, Structure, Properties and Applications of Polycrystalline Cubic Boron Nitride, Technical Report MR85-02, Soc. of Manufacturing Engineers, Apr 22-25. Hill, P., et al, Filed December 1998, Method and Apparatus for Grinding, Rolls Royce, GB Patent 2332634A. Hoffmeister, H-W, and Timmer, J.-H., 2000, Laser Conditioning of Superabrasive Grinding Wheels, Industrial Diamond Review, No. 60, Booklet 586, :209-218. Hon, K. B. and Gill, T., 2003, Selective Laser Sintering of SiC/Polyimide Composites, Annals of the CIRP, 52/1:173-176. Hubert, H., Devouard, B., Garvie, L., O’Keeffe, M., Buseck, P., Petuskey, W., and McMillan, P., 1998, Nature 391/ 376. Hyatt, G., Filed 2001, Grinding Wheel Comprising of Porous Material and a Device for Delivering Fluid Through the Wheel from a Location Inboard from the Outer Peripheral Surface, World Patent 200258886. Ichida, Y., and Kishi, K., 1995, The Development of Nano-crystalline cBN for Enhanced Superalloy Grinding Performance, J. Manufacturing Science and Engineering, ASME, 1:349-361. Inasaki, I., 1990, A New Preparation Method for Resinoid Bonded cBN Wheels, Annals of the CIRP, 39/1:317-320. Ito, K., Noritake, Filed August 3 2001, Vitrified Abrasive Solid Mass Having Pores Filled with Resin and Solid Lubricant Agent, US Patent 6428587. Itoh, N., et al., 1998, Characteristics of ELID Surface Grinding by Fine Abrasive Metal-Resin Bonded Wheel, Int. J. Japan Soc. Prec. Eng, 32:273-274. Jackson, M., Wakefield, R., Jones, S., Mills, B., and Rowe, B., 2001, Materials Selection Applied to Vitrified Corundum Grinding Wheels, British Ceramic Trans., 100/5:229-236.

[53] Joseph, W., 3M Company, Filed 2002, Agglomerate Production by Firing Spray Dried Mixture of Abrasive Particles and a Solution Formed of Oxide and Water, World Patent 200238338. [54] Karpuschewski, B., Wehmeier, M., and Inasaki, I., 2000, Grinding Monitoring System Based on Power and Acoustic Emission Sensors, Annals of the CIRP 49/1:235-240. [55] Klocke, F., and Merbecks, T., 2001, Characterization of Vitrified cBN Grinding Wheels, 4th Int. Machining and Grinding Conf., SME, Michigan. [56] Klocke, F., Brinksmeier, E., Evans, C., Inasaki, I., Minke, E., Tonshoff, H-K., Webster, J., and Stuff, D., 1997, High Speed Grinding - Fundamentals and State of the Art in Europe, Japan and USA, Annals of the CIRP, 46/2:715-724. [57] König, W., Schroder, B., and Treffert, C., 1993, High Speed Grinding of Any Contour Using cBN Grinding Wheels, SME 5th Int. Grinding Conf. [58] König, W., Klocke, F., and Stuff, D., 1997, High Speed Grinding with cBN Wheels - Boundary Conditions, Applications and Prospects of a Future Oriented Technology, 1st French and German Conf. on High Speed Machining, Metz, June, 207-218. [59] Koshy, P., Iwasaki, A., and Elbestawi, M., 2003, Surface Generation with Engineered Diamond Wheels: Insights from Simulation, Annals of the CIRP, 52/1:271-274. [60] Koshy, P., Jain, V., and Lal, G., 1997, Grinding of Cemented Carbide with Electrical Spark Assistance, J. of Materials Processing Technology, 72:61-68. [61] Kovalenko, V., Golovko, L., Novikov, N., Shepelev, A., Sorochenko, V., 2003, Laser Technology Application For Diamond Tool Manufacturing, Proc. Int. Conf. Laser Technologies in Welding and Material Processing, Kiev, E.O. Paton Electric Welding Institute, NASU, 159-164. [62] Kramer, D., Rehsteiner, F., and Schumacher, B., 1999, ECD, a New Method for Grinding of Modern High Performance Cutting Materials to Highest Quality, Annals of theCIRP, 48/1:265-268 [63] Krell, A., and Blank, P., 1995, Grain Size Dependence of Hardness in Dense Submicrometer Alumina, J. Am. Ceram. Soc. 78/4:1118-1120. [64] Krueger, M., Filed December 312002, Sulfur Impregnated Grinding Wheel, Cimcool, US Patent 6500220. [65] Kruth, J-P., 1998, Progress in Additive Manufacturing and Rapid Prototyping, Annals of the CIRP, 47/2:525-540. [66] Maekawa, K., Yokoyama, Y., and Ohshima, I., 1999, Fabrication of Metal-Bonded Grinding/Polishing Tools by the Greentape Laser Sintering Method, 3rd Int. Conf. On Abrasive Technology, Brisbane, Australia. [67] Malkin, S., August 5 2003, Grinding Wheel System, US Patent 6,602,109. [68] Malkin, S., 1985, Current Trends in cBN Grinding Technology. Annals of the CIRP 34/2:557-563 [69] Minke, E., 1999, Investigation on Grinding Potential and Wear Resistance of TG-Grinding Wheels, Internal Report for Saint-Gobain by IWT Bremen. [70] Mizuno, M., 2003, Contact-Discharge Truing/Dressing Method and Device Therefore, Japan Science Technology Corporation, European Patent 1306164A1. [71] Nakai, M., Ogawa, H., Yamanaka, M., Urakawa, N., and Miyake, M., 2000, Development of a Fixed

[72] [73]

[74]

[75] [76]

[77] [78]

[79] [80] [81]

[82] [83] [84] [85] [86]

[87] [88]

[89] [90]

Diamond Wire Saw for Electronics Applications, Advances in Abrasive Technology III, Soc. of Grinding Engineers, Japan, 233-240. Nakano, S., Akaishi, M., Sasaki, T., and Yamaoka, S. 1994, Chem. Materials 6/2246. Okumiya, M., Tsunekawa, Y., Saida, T., and Ichino, R.l, 2001, Creation of High Strength Bonded Abrasive Wheel with Ultrasonic Aided Composite Plating, Surface and Coatings Technology, Frontiers of Surface Engineering, 169-170, 1/3:112115. Oliveira, J. F., Souza, G., and Bianchi, E., 1995, Resin Bond cBN Wheels Wear Analysis Through the Chip Geometry, 1st Int. Machining and Grinding Conf., SME, Michigan. Pritchard, P., Filed February 21 2001, Abrasive Articles with Optimally Oriented Abrasive Particles, 3M Company, US Patent 200300009949. Prusak, Z., and Webster, J., 1995, Preparation of Hybrid cBN/Seeded Gel Wheels for Precision Grinding. 1st Int. Machining and Grinding Conf.. SME, Michigan. Raghunath, V., and Radhakrishnan, V., 1993, Performance Evaluation of a New Crankshaft Grinding Wheel, SME 5th Int. Grinding Conf., Ohio. Rajurkar, K. P., Wei, B., Kozak, J., and Nooka, S., 1995, Abrasive Electrodischarge Grinding of Advanced Materials, Proc. XI Int. Symposium for Electromachining. Ramanath, S., 2003, Porous Abrasive Tool and Method for Making the Same, Saint Gobain, US Patent 20030097800 A1. Renaud, W., Hitchiner, M., 1995, The Development of Camshaft Lobe Grinding with Vitrified cBN. 1st Int. Machining and Grinding Conf., SME, Michigan. Roquefeuil, F., 2001, A New Electrofused Aluminum Oxynitride Abrasive Grain for Precision Grinding Operations, Gorham Conf. on Precision Grinding and Finishing in the Global Economy, Chicago. Rossetti, G., Fox, S., and Tricard, M., 2000, Bonded Abrasive Articles Filled with Oil/Wax Mixture, Norton Company, US Patent 6086648. Rue, C., 1993, Sintered Sol Gel Alumina Abrasive Filaments, Norton Company, US Patent 5,194,072. Rue, C., 1993, Sintered Sol Gel Alumina Abrasive Filaments, Norton Company, US Patent 5,244,477. Salmon, S., 2003, The Effects of Hard Lubricant Coatings on the Performance of Electroplated Superabrasive Wheels, Int. Grinding Conf., SME. Schmid, F., Khatak, C., Smith, M., and Lynch, L., 1982, Recent Developments in Multi-Wire Fixed Abrasive Slicing Technique (FAST), 4th E.C. Photovoltaic Solar Energy Conf., 980-984, D. Reidel Publishing Company. Schopf, M., 2001, ECDM Conditioning of Metal Bonded Diamond Grinding Wheels, Annals of the CIRP, 50/1:125-128 Sexton, J., and Stone, B., 1981, The Development of an Ultrahard Abrasive Grinding Wheel which Suppresses Chatter, Annals of the CIRP, 30/1:215218. Shaji, 2003, Solid Lubricant Impregnated Wheel, Int. Journal of Machine Tool Manufacture, 43:965972. Sheldon, D., 1996, Sol-Gel Alumina Abrasive Grinding Wheel with Improved Corner Holding, Norton Company, US Patent 5573561.

[91] Shi, Z., and Malkin, S., 2003, An Investigation of Grinding with Electroplated CBN Wheels, Annals of the CIRP, 52/1:267-270. [92] Shin, Y., 2003, A Novel Laser-Assisted Dressing Technique for Vitrified Superabrasive Wheels, Int. Grinding Conf., SME, Itasca. [93] Shu, K. M., 2003, Study of Electrical Discharge Grinding using Metal Matrix Composite Electrodes, Int. Journal of Machine Tools and Manufacture, 43:845-854. [94] Song, G., 1994, A Study of the Grinding Wheel Structure Improvement to Suppress Chatter, PhD thesis, Univ. of Connecticut. [95] Song, Y. A. and König, W., 1997, Experimental Study of the Basic Process Mechanism for Direct Selective Laser Sintering of Low Melting Point Metallic Powder, Annals of the CIRP, 46/1:127-130. [96] Spur, G., and Lachmund, U., 1993, Type-Specific Applicability of Polycrystalline Diamond. Research and Development in Germany, Hanser, Annals of the German Academic Society for Production Engineering. [97] Shrivastava, S., Rajurkar, K., and Zhao, W., 1999, Rotary Electrodischarge Machining of Polycrystalline Diamond, Trans. of NAMRI, 27:141146. [98] Stabenow, R., Bruhn, J. and Golla, B., 2003, Microcrystalline Sintered Ceramics in Bonded Abrasives - Exploiting New Potential. [99] Stephenson, D., Hedge, J., and Corbett, J., 2002, Surface Finishing of Ni-Cr-B-Si Composite Coatings by Precision Grinding, Int. Journal of Machine Tools and Manufacture, 42:357-363. [100] Stephenson, D., Jin, T., and Corbett, J., 2002, High Efficiency Deep Grinding of a Low Alloy Steel with Plated cBN Wheels, Annals of the CIRP, 51/1:241244. [101] Stoetzel, W., and Provow, R., 2003, Abrasive Articles Having Bond Systems Containing Abrasive Particles, 3M Company, European Patent 01159109. [102] Stöhr, R., and Heinzel, C., 2002, Grind-hardening with CBN, Abrasives Magazine, June-July, 22-30. [103] Subramanian, K., and Lindsay, R., 1992, A Systems Approach for the Use of Vitrified Bonded Superabrasive Wheels for Precision Production Grinding, ASME J. of Engineering for Industry, 114:41-52. [104] Subramanian, K., Ramanath, S., and Tricard, M., 1997, Mechanisms of Material Removal in the Precision Production Grinding of Ceramics, J. of Manufacturing Science and Engineering, 119:509514. [105] Subramanian, K., 1999, Measurement and Analysis of Grinding Processes, Abrasives Magazine, April/May. [106] Sunarto, and Ichida, Y., 1999, Grinding Performance of Newly Developed Polycrystalline cBN Abrasives having an Ultrafine Crystal Structure, 3rd Int. Conf. On Abrasive Technology, Brisbane, Australia. [107] Suto, T., Waida, T., Noguchi, H., and Inoue, H., 1990, High Performance Creep Feed Grinding of Difficult to Grind Materials with New Slotted Wheel, JSPE, 24/1:39-44. [108] Tang, J., Pu, X., Xu, H., and Zhang, Y., 1990, Studies on Mechanisms and Improvement of Workpiece Burn During Grinding of Titanium Alloys, Annals of the CIRP, 39/1:353-356.

[109] Tawakoli, T., 1993, High Efficiency Deep Grinding, VDI-Verlag GmbH - Mechanical Engineering Publications Limited. [110] Tawakoli, T., 1993, T-Tool - The New Superabrasives Tool System for Grinding and Dressing, SME, 5th International Grinding Conf. [111] Tonshoff, H. K., and Heuer, W., Evaluation of the Workpiece Surface Integrity After Grinding with SG and Conventional Abrasives. Abschlussbericht. [112] Tonshoff, H., and Friemuth, T., 1999, The Cutting Tool Performance depends on the Tool Grinding Process, SME 3rd Int. Machining and Grinding Conf. [113] Tonshoff, H. K., and Friemuth, T., and Becker, J-C, 2000, Continuous Path Controlled Grinding of Crankshafts, Abrasives Magazine, Dec./Jan. [114] Tricard, M., Kassir, S., Herron, P., and Pei, Z., 1998, New Abrasives Trends in Manufacturing of Silicon Wafers, 13th Annual Meeting of the ASPE, 631-634, October. [115] Varghese, B., and Malkin, S., 2000, Development of a Sensor Integrated ‘Intelligent’ Grinding Wheel for In-Process Monitoring, Annals of the CIRP, 49/1:231-234. [116] Wakuda, M, Inasaki, I., Ogawa, K., and Takahara, M., 1993, Monitoring of the grinding Process with an AE Sensor Integrated cBN Wheel, Journal of Advanced Automation Technology, 5/4:179-184. [117] Warnecke, G., and Barth, C., 1999, Optimization of the Dynamic Behavior of Grinding Wheels for Grinding of Hard and Brittle Materials Using the Finite Element Method, Annals of the CIRP, 48/1:261-264. [118] Weinert, K., and Finke, M., 2002, Bohrungsbearbeitung in einem Uberschliff durch Innenrund-Langsschleifen, 60. Jahrbuch Schleifen, Honen, Lappen und Polieren, Vulkan-Verlag, Essen, :129-141. [119] Wentorf, R. H. Jr., 1957, Cubic Form of Boron Nitride, The Journal of Chemical Physics, 26/4:956. [120] Wentorf, R. H., 1961, The Synthesis of the Cubic Form of Boron Nitride?, The Journal of Chemical Physics, 34/3:809-812. [121] Werner, G., and Tawakoli, T., 1988, Advances in High Efficiency Deep Grinding, SME 3rd Int. Grinding Conf., MR88-588. [122] Wu, M., 1998, Method for Making High Permeability Grinding Wheels, Norton Company, US Patent 5,738,696. [123] Zhao, Y., He, D., Qian, J., Daemen, L., Huang, J., Pantea, C., Zhang, J., Voronin, G., Zerda, T., 2003, Novel Superhard Materials to Rival Diamond, Intertech, Vancouver. [124] Zimmermann, and M., Einset, E., 2003, Low Oxygen Cubic Boron Nitride and Its Production, GE Company, World Patent 03040062.