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Innovations in Abrasive Products for Precision GrindingJ. Webster (1), Saint Gobain Abrasives, USA

M. Tricard (3), QED Technologies, USA

AbstractThis paper is a review of recent developments in the design and manufacture of precision, fixed-abrasive tools. The role ofeach component within the engineered composite is also discussed, with examples showing how the components havebeen enhanced to achieve their current high levels of performance. The paper also looks at examples where innovations inthe abrasive tool have enabled the development of innovative abrasive processes. A vision of future abrasive productdevelopments is also presented by the authors.

Keywords:grinding, abrasives, machine

AcknowledgementsThe authors would like to acknowledge all who have

contributed to this paper through suggestions,discussions and documents of their work. Special thanksare given to Prof. E. Brinksmeier and his co-workers atIWT Bremen, Prof. F. Klocke and his co-workers at WZLAachen, Prof. B. Denkena and his co-workers at IFWHannover, 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 ofLiverpool Univ., Dr. H. W. Hoffmeister of BraunschweigUniv., Prof. V. Kovalenko of Kiev Polytechnic, Prof. P.Koshy of McMaster Univ., Dr. R. Stabenow and his co-workers at Hermes Schleifmittel, Dr. T. Tawakoli, andalso Drs. K. Breder and K. Subramanian, Saint Gobain.

1. General Overview of paperPresenting technical accomplishments from the multi-

billion dollars abrasive industry necessarily requiresbeing selective. It was therefore decided to limit thescope of this paper to precision grinding, a loosedesignation merely used to focus on abrasive processeswhere forms and/or surface integrity (e.g. roughness, butalso subsurface damage etc.) are the primary figure ofmerit. Significant developments have of course also beendemonstrated in abrasive processes where volumetricremoval rates or other goals are the primary drivers, butthese will not be covered here for the sake of brevity.

It was further decided to primarily report on applicationswhere recent abrasive product developments havetranslated into novel abrasive process accomplishments(e.g. increase in grinding wheel permeability for newcreep feed grinding applications, new grinding wheel

shapes for high speed grinding etc.). It was also theintention of the authors to exclude the constant andobviously important continual improvements made byabrasive manufacturer worldwide, to merely improve theirexisting product for mere incremental performanceimprovement (e.g. new bond formulation to improvewheel life by 10%).

The fragmented nature of the abrasive industry (in NorthAmerica alone, several hundred manufacturers vie formarket share) makes it a competitive one, rife withproprietary issues. To steer well clear of confidentialdevelopments, and perhaps in a new approach for CIRP,this paper relies heavily on issued patents, particularlyrecent ones, as well as academic publications.

2. Components of fixed abrasive products, design

and recent developmentsFixed abrasive products can be regarded as engineeredcomposite materials, made of four elements (Fig. 1):

1. One or several abrasives: either conventional-fused(e.g. Al2O3, SiC or ZrO2 based); conventional-sintered; or super -abrasives (e.g. cBN or diamond)

2. A bond to hold or support the abrasive(s): resin orpolymer based; vitrified or ceramic based; or metalbased, sometimes in a single-layer brazed orelectroplated format

3. Some porosity and/or additives. Porosity is typicallypresent to provide clearance for the chips createdduring the grinding process, for fluid transport, and toenhance the various interactions taking place in thegrinding zone. The porosity itself can either be

natural or artificially induced. Various grinding aids,fillers and lubricants can be added

4. Wheel design, including the composite abrasiveprofile, abrasive thickness, hub material (if not amonolithic design), strength to withstand rotationalstresses, precision and resistance to chemicalattack.

Fig. 1. Components of a fixed abrasive product.

These four constituents are deployed by the abrasiveproduct manufacturers to achieve the desired workpiecerequirements (shape, finish, removal rate etc.). Abrasivemanufacturers in turn tend to refer to their products aseither: bonded abrasives (conventional grinding wheels),superabrasives, or coated abrasives (belts). Coatedabrasives are not covered in this document.

DESIGN

BOND

ABRASIVE

STRUCTUREADDITIVES

DESIGN

BOND

ABRASIVE

STRUCTUREADDITIVES

DESIGN

BOND

ABRASIVE

STRUCTUREADDITIVES

DESIGN

BOND

ABRASIVE

STRUCTUREADDITIVES

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2.1. The abrasive grain componentThe next few paragraphs will detail some of the keyinnovations in the abrasive grain component of a grindingwheel. These abrasives are the hard phase componentsof a wheel and have the greatest influence on the outputand viability of a grinding process. For example, the sizeof the grain has significant influence on the forces,

power, wear rate, surface finish, etc.

2.1.1. Conventional abrasivesAs mentioned by Cheape [23], until the 18

th century,

grinding was generally a manual process employingsandstone, a naturally formed composite of quartzcrystals bonded with silica and iron oxide. The 19

th

century saw the introduction of grinding machines initially mere modifications of lathes, from Brown andSharpe in 1858, for instance. The first universal grinderfollowed in 1875, followed by a number of specializedprecision grinders in the 1880s and 1890s.

But even as late as in one of its 1885 issues, AmericanMachinist mentioned them when a machinist has a job ofemery grinding to do, he is pretty apt to sit right down andhate himself [23]. In parallel, so called coatedabrasives, initially made with a paper or cloth backingand covered with glued flint, emery or garnet, were firstproduced in the USA by the Baeder-Adamson Co. ofPhiladelphia in 1828 [23]. Throughout the 19

th century,

abrasive makers experimented with quartz, flint andgarnet (not to mention crushed milk bottles!) butpreferred emery and its purer form: Alundum.

Corundum was scarce and expensive and was usuallymixed with emery. It was not until 1901 that a new highquality artificial form of corundum abrasive, calledAlundum, was introduced. The technology wassubsequently refined in 1904 due to the introduction ofthe water cooled electric furnace, invented and patented

by Aldus C. Higgins, who later became president of theNorton Company. This led to a revolution in production ofAlundum abrasives and afterward Higgins was awardedthe John Scott Medal. As early as 1914, manufacturedfused abrasives (alundum and carborundum)production surpassed emery and corundum imports inthe US [23].

2.1.2. Sintered aluminium oxide (seeded-/sol-gel, etc)Seeded Gel, or Sol-Gel as it is sometimes called, wasoriginally invented by both 3M and the Norton Company[27] in the mid-1980s, and are now manufactured bythese two companies, along with Hermes Schleifmittel.

Sintered Sol-Gel aluminum oxide abrasives presentsignificant advantages compared to their fused

counterparts particularly in term of life and are muchless expensive than superabrasives. When properlyused, sintered abrasives can also result in significantlyincreased volumetric removal rates, reduced forces andlower work surface temperature during grinding. It isfrequently a viable alternative to cBN, particularly in lightof the ease of truing and dressing, and the initial wheelcost.

Sol-gel aluminous grits are aluminas made by a processcomprising of peptizing a solution of an aluminum oxidemonohydrate so as to form a gel, which is then dried andfired to form alpha alumina [90]. The initial solution mayfurther include up to 15 % by weight of spinel, mullite,manganese dioxide, titania, magnesia, rare earth metal

oxides, zirconia powder or a zirconia precursor (whichcan 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 suchproperties as fracture toughness, hardness, friability,fracture mechanics, or drying behaviour.

Once the gel has formed, it may be shaped by anyconvenient method such as pressing, moulding orextrusion and then carefully dried to produce an un-cracked body of the desired shape for firing. Theextrusion is then dried, typically at a temperature belowthe frothing temperature of the gel, using any of severalde-watering methods. After drying, it can be cut ormachined to form the desired shape or crushed or brokento form particles or grains. After shaping, the dried gelcan then be calcined (using an inclined rotary oven) toremove essentially all volatiles and transform the variouscomponents of the grains into ceramics (metal oxides).The calcined material is then sintered by heating and isheld within a suitable temperature range (approximately1500

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 aredeliberately introduced into, or created in-situ in, thealuminum oxide monohydrate dispersion. The presenceof the nucleating sites in the dispersion lowers thetemperature at which alpha alumina is formed andproduces an extremely fine crystalline structure.

As illustrated by Stabenow et al [98], the size of thecrystalline structure plays an important role in theperformance of the grinding products. Figs. 2a and 2bshow both a coarser material (representative of Type H1sintered ceramics) and of a much more fine-crystallinematerial (representative of Type K1 sintered ceramics).

Fig. 3 shows the effect of the different grit size on thetangential grinding force.

Garg [37] demonstrated that nano-sized powders ofalpha alumina can be obtained from a boehmite geldoped with a barrier-forming material such as silica thatis then dried, fired and comminuted to powder form [84].

Fig. 2. Sol-gel alumina crystal structure: a) Type H1 b)Type K1 [98]

Krell and Blank [63] showed that microcrystallinealuminium oxides of differing crystallite size differ interms of hardness. At temperatures of approximately1000 C, hardness increases compared to an aluminiumoxide single crystal by almost 100 % as crystallite sizedecreases and thus achieves almost the hardness ofsilicon carbide (Fig. 4). The authors speculate that thediffering characteristics of the microcrystalline aluminiumoxides described previously are linked to their differenthigh-temperature hardness.

3m

a) b)

3m

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An electro-fused, sintered, aluminium-oxy-nitride (AlONabrasive has recently been introduced into the market. Itis claimed that the high temperature hardness andfriability properties of this material, gives less thermaldamage, better form holding and better surface finishthan conventional, fused-alumina abrasive [81].

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 topolish gems, and natural diamond had been used to truegrinding wheels, the first natural diamond grinding wheelsonly started to appear in the 1930s [23].

Fig. 4. High temperature hardness variation [63]

Sales of natural diamond wheels increased significantlyduring the 1940s, representing 20% of all sales (byvalue) of the Norton Company by 1952 [23].

The first synthesis of artificial diamond was reported byBundy and his colleagues at General Electric in 1955[19], culminating as they pointed out more than acentury of claims and counterclaims for the synthesis ofdiamond attest to the fascination of the subject and theextreme difficulty of the experimental techniques. In an

interesting historical twist Bovenkerk et al [13]subsequently declared 34 years later that the run ofdiamond was a small piece of a natural type diamond.As subsequently demonstrated in a later publication, [14]they had nonetheless invented the key process tosynthesise artificial diamond.

2.1.4. Cubic Boron NitrideIn a small Letter to the Editor entitled Cubic Form ofBoron Nitride received on January 28, 1957, R.H.Wentorf [119], from General Electric ResearchLaboratory, made one of the most significantannouncements in the field of abrasives. Whether calledBorazon as Wentorf proposed in this announcement, orcBN, the second key superabrasive - Cubic Boron Nitride had also been invented. Details of this breakthroughwere reported in a subsequent publication [120].

Since Wentorf, additional developments have been madewith a polycrystalline form of cBN, called the ABN series,as described by Heath [42] and Bohlheim [10]. Spur andLachmund [96] also looked at the type-specificapplicability of polycrystalline diamond. In addition to themono-crystalline and poly-crystalline form of cBN, Ichidaand Kishi [48] reported on the performance of cBNwheels made of newly developed nano-crystalline cBNabrasive (N-cBN having a crystal grain size below1m).

In 1985, Malkin reported on the current trends in cBNgrinding [68]. Since then, improvements to the grainstructure, shape, toughness, and price, have moved theabrasive firmly into the market place. With such lowmanufacturing costs, cBN is following synthesiseddiamond towards being a commodity product.

2.1.5. Abrasive mixturesWithin 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 mixingdifferent sizes of abrasive and bond material. The finermaterial will fill the spaces that exist between the largersizes. Sintered alumina has been blended with cBN in aneffort to reduce cost but maintain excellent wearresistance [76]. Another example exists with themanufacture of the wheel hub with monolithic sinteredabrasive wheels. The higher cost of these abrasivesmakes them uneconomical to use for the entire wheel,giving rise to the practice of filling the centre of the mouldwith a lower cost abrasive mixture, and the outer annuluswith the premium superabrasive mixture, before pressing.

2.1.6. Strength testing of grainsThe performance of any abrasive product depends on the

abrasive properties and grinding conditions (forces, chipthickness, etc.) to which it is subjected. From thestandpoint of testing conditions, the force per grit andchip thickness are critical in determining which of thecommon wear/fracture mechanisms of a given abrasivebecome active. The link between these two areas isimportant for predicting the most efficient grindingregimes to use with a given abrasive; one abrasive that isan excellent performer in high force per grit applicationsmay be less than optimal in low force per gritapplications.

Several methods are routinely used for thecharacterization of abrasive grains. These includefriability, hardness, toughness, and various abrasion and

0

2

4

6

8

10

12

14

0 200 400 600 800 1000 1200

Metal removed per unit width V'w [mm/mm]

TangentialforceperunitwidthF't

[N/mm]

Type H1 wet; G = 16.8 mm/mmType H1 dry; G = 71.6 mm/mmType K1 wet; G = 54,4 mm/mmType K1 dry; G = 72,4 mm/mm

0

2

4

6

8

10

12

14

0 200 400 600 800 1000 1200

Metal removed per unit width V'w [mm/mm]

TangentialforceperunitwidthF't

[N/mm]

Type H1 wet; G = 16.8 mm/mmType H1 dry; G = 71.6 mm/mmType K1 wet; G = 54,4 mm/mmType K1 dry; G = 72,4 mm/mm

Raw material >99.99% Al2O3

0.59m

0.67m

0.97m

3.5m

10.1m

Al2O3 single

crystal

SiC single

crystal

Temperature (C)

HardnessHV3(GPa)

Crystallite

size

Raw material >99.99% Al2O3

0.59m

0.67m

0.97m

3.5m

10.1m

Al2O3 single

crystal

SiC single

crystal

Temperature (C)

HardnessHV3(GPa)

Crystallite

size

Raw material >99.99% Al2O3

0.59m

0.67m

0.97m

3.5m

10.1m

Al2O3 single

crystal

SiC single

crystal

Temperature (C)

HardnessHV3(GPa)

Crystallite

size

Raw material >99.99% Al2O3

0.59m

0.67m

0.97m

3.5m

10.1m

0.59m

0.67m

0.97m

3.5m

10.1m

Al2O3 single

crystal

SiC single

crystal

Temperature (C)

HardnessHV3(GPa)

Crystallite

size

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Another important application is for cut-off wheels, whichare usually reinforced with fiberglass for added strengthand high-speed operation up to about 100 m/s (20,000ft/min). For superabrasive wheels, resinoid bonds are themost popular, the most important applications being withdiamond abrasives for grinding of cemented carbides,ceramics, PCD, and CBN for cutting tools [40][49][74].

Polyimide resin bond is a recent development for cuttingtool grinding applications. It is a tougher and morethermally resistant bond, which can retain diamondabrasive at higher temperatures than phenolic resin. Theintroduction of copper particles into the bond greatlyimproves the thermal conductivity. Hollow glass spherescan also be used to give the wheel a degree of porosity.

Since resinoid bonds do not chemically attach to theabrasive grains, in many cases rough or spikey metalliccoatings are applied to the grains to increase mechanicalretention in the bond. Rubber, shellac and silicate bondsare not considered in this paper as they are rarely usedfor precision grinding.

2.2.3. Metallic sintered bondMetal bonds are extensively used with superabrasivewheels. The most common are from sintered bronze,which are produced by powder metallurgy methods.Variation of the wheel grade is controlled by addingmodifiers and altering the bronze composition. Otherpowder metal bonds, which are generally stronger,include iron and nickel. Segmented diamond saws forcutting stone and granite typically have sintered nickelbonds. Tungsten powder infiltrated with a low meltingpoint alloy is used in diamond wheels for grindingdiamond tools. Still stronger bonds consisting of WC-Cocemented carbide are used in diamond abrasive tools forgeological drilling.

2.2.4 Metallic Electroplated bondElectroplated (galvanically bonded) cBN wheelsrepresent the largest share of the single-layer market,especially in automotive and aerospace applications.Diamond versions are generally used for grindingceramics, non-ferrous metals, and construction materials.They have been effectively utilised on machining centreswith tool change capability, and central to most highspeed grinding (over 120 m/s) applications [57].

The attachment of the abrasive grain to the wheel hub isprimarily by mechanical entrapment [22]. Fig. 6 shows aschematic of cBN grains held by the nickel matrix. Theabrasive grains that are used, are generally blocky andtough, in order to give even grit height and long servicelife. Chattopadhyay and Hintermann [22][21] experienced

a high level of grit pullout at a wheel speed of 30m/s, andspecific removal rate Qwof 4 mm

3/s.mm, when grinding

an unhardened bearing steel. The relatively lowwheelspeed, possible low nickel bond content, and softworkpiece material, may have created the high force pergrit that led to grit pullout.

In high speed grinding (HSG), grit pullout does notappear to be a limitation of the process, despiteextremely high specific material removal rates. Shi andMalkin [91] showed that radial wear up to 80% of gritdimension 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

most are active during grinding. The high wheel speed inHSG also keeps the force per grit low, and a chip sizethat can be accommodated between adjacent grains.

Fig. 6. Nickel bond depth adjustability

2.2.5. Testing bond hardnessThe hardness of a grinding wheel is defined as theresistance against grain pullout. This is directlyinfluenced by the strength of the bonding at the grain andthe strength of the bonding bridges. Several commercialtesting systems are available to determine the hardness:

Zeiss-Mackensen tester, which correlates hardnessto depth of air penetration

Grindo Sonic tester, that estimates E-Moduli fromnatural frequency and correlates this to the hardness

The above two methods assess the bond hardnessindirectly, and against known standards. They can alsobe used to assess the consistency of a batch of grindingwheels and, in the case of the Grindo-Sonic, whetherdangerous cracks exist inside the wheel structure.

Fig. 7. Bond strength testing [55]

Klocke and Merbecks developed a grain pullout systemthat can establish the strength of the bond and bondbridges, using a mechanical probe [55], to addresshomogeneity problems, variation of wheel hardness,variation of concentration, and inconsistent wheelbehaviour. Fig. 7 shows the system that pushesindividual grains to determine the force at which bondfracture, grain pullout, or grain fracture occurs. The

substrate

cBN grit nickel layer depths

substrate

cBN grit nickel layer depths

substrate

cBN grit nickel layer depths

substrate

cBN grit nickel layer depths

Bond

breakage

Grain

pullout

Bond

breakage

Grain

pullout

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system has also been used in an axial direction toscratch across the wheel surface. Using an acousticemission sensor, the signal produced by the scratch wasused to identify the grain/bond failure mechanisms.

2.3. Porosity/permeability in the bondThe structure of a grinding wheel is a measure of the

spacing between the abrasive grain. The porosity of agrinding wheel can be described as a local effect withinthe wheel structure, allowing the transport of fluid into theprocess [30][39], and giving space for the chips to form.

Permeability is interconnected porosity throughout theentire structure of the abrasive composite. An analogy ofporosity is closed-cell foam, where there is air trappedwithin each cell which can have local influence. Ananalogy of permeability is open-cell foam which canfreely pass liquids and gases through the structure [27].It has been discovered that grinding performance cannotbe predicted only on the basis of porosity as a volumepercentage of the abrasive tool. Instead, the structuralopenness (i.e., the pore interconnection) of the wheel,quantified by its permeability to fluids (air, coolant,lubricant, etc.), also influences the abrasive toolperformance. Permeability also permits the clearance ofmaterial (e.g., metal chips or swarf) removed from anobject being ground. Debris clearance is essential whenthe workpiece material being ground is difficult tomachine or gummy (such as aluminium or some alloys),producing long metal chips. Loading of the grindingsurface of the wheel occurs readily and the grindingoperation becomes difficult in the absence of wheelpermeability. A method for measuring permeability isdescribed by Wu [122] and DiCorleto [27], who monitoredthe flowrate of air, at a fixed pressure, through thestructure.

Wu [122] notes that there are two major categories of

processes to obtain high porosity abrasive products. Thefirst category is the burn-out methods, where porestructure is created by addition of organic pore inducingmedia (such as walnut shells) in the wheel mixing stage.These media thermally decompose upon firing of thegreen body of abrasive tool, leaving voids or pores in thecured abrasive tool. Drawbacks of this method include:

moisture absorption during storage of the poreinducer

mixing inconsistency and mixing separation, partiallydue to moisture, and partially due to the densitydifference between the abrasive grain and poreinducer

moulding thickness growth or "spring-back" due totime-dependent strain release on the pore inducerupon unloading the mould, causing uncontrollabledimensions of the abrasive tool

incompleteness of burn-out of pore inducer or"coring"/"blackening" of a fired abrasive product ifeither the heating rate is not slow enough or thesoftening point of a vitrified bonding agent is not highenough

air-borne emissions and odours when the poreinducer is thermally decomposed, often causing anegative environmental impact, i.e. naphthalene

The second category of pore inducement is the closedcell or bubble method, by introducing materials, such asbubble alumina (mullite spheres) into an abrasive tool toinduce porosity without a burnout step. However, thepores created by the bubbles are internal and closed, so

the pore structure is not permeable to the passage ofgrinding fluid, and the pore size typically is not largeenough for metal chip clearance.

2.4. Wheel designThe design of the grinding wheel is as critical to thesuccess of the abrasive product as the other three

components of the composite. Design includes: thephysical dimensions, the form produced on the abrasivesurface, the hub material to withstand rotational andthermal stresses, the rotational error, dynamic balanceand chemical resistance.

Grinding wheels can range from a thickness of severalmicrons, for silicon wafer dicing, to a metre wide for woodpulp grinding. Although the shape of the active abrasivesurface is usually trued into the wheel, there has beenextensive research carried out to produce near-netshaped, vitrified core, wheels for superabrasive products,reducing wasted abrasive [103].

Electro-plated bondingQw,max= 1.000 - 10.000 mm/mms

metallic bonding

Qw,max= 50 - 250 mm/mms

vitrified bonding

Qw,max= 50-150 mm/mms

resin bonding

Qw,max= 50-150 mm/mms

vitrified/bakelite

Qw,max=10-100mm/mms

CBN grinding wheels

conventional grinding wheels

bonding

type

circumferential speed of the grinding wheel vs

100 150 200 250 300

Fig. 8. cBN bond system speed and Qwlimitations [58]

Rotational stresses during high-speed grinding can leadto failure if the hub is not correctly designed. The use ofFEM analysis for design of the hub and the number ofsegments (in the case of superabrasive wheels) [4], plusthe improved guarding of modern machines, has reducedthe risk of injury due to incorrect wheel design. Fig. 8shows the speed limitations of different wheel bondsystems, as defined by Knig et al [58]. Fig. 9 shows anexample of a high speed grinding wheel hub to minimiseradial elongation and reduced effective stress [58].Knigs wheel design was based on high strength steel.Alternatively, composite wheel hubs, i.e. carbon fibrebased, have been researched and are now commerciallyavailable. As with the wheels and brakes of Formula Onerace cars, the orientation of the carbon fibres within the

hub can make a large difference to the ultimate strengthand deformation of the wheel at high speed.

The rotational precision of single-layer superabrasive isgenerally a function of the grain size distribution and theelectroplating method. However, micro-truing of thesewheels by 2-4 m is possible in order to reduce thesurface roughness produced [58].

3. Interactions in the grinding zoneDuring abrasive finishing processing the abrasive product(the tool/consumables) contacts with the surface of thework material. This contact is imposed under a set ofconditions (operation parameters) influenced by thecharacteristics of the abrasive tool, work material and themachine tool involved [104]. Such contact (interactions)

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HSG-wheel

1A1-wheel

600

400

300

200

100

N/mm

effective

stress

wheel radius rs

00 50 100 150 200

600

400

300

200

100

m

0-

-

-

radialelongationcentral bore

1A1-wheel

bs = 5 mm

p = 2,71 kg/dmvs = 500 m/s

HSG-wheel

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

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

1. Abrasive / Work interface1.1 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 bythe input parameters of the abrasive finishing system,and in-turn influence the output parameters of theabrasive finishing system. Subramanian [105] has shownhow these microscopic process interactions can be

manipulated to achieve the desired productivity, surfacequality, and production economics. Hence innovations inabrasive products can be achieved through abrasive,bond, structure and shape, to influence the physicalevents in the grinding zone (mechanical, thermal,chemical, etc.) in order to:

Maximize cutting action and minimize friction effects

Utilize the abrasive/work tribology (ploughing andsliding), as required

Manage all other tribological (sliding) interactions togenerate the surfaces of required value/benefit to thecustomers [168].

The interactions listed above are usually experienced by

end-users and abrasive product suppliers, throughcontrolled tests, but not easily identified from processoutputs such as surface finish, grain wear, powerconsumption, etc. Any deviations from the initialperformance over time (due to wear flats on the abrasivegrains, erosion of the bond, loss of chip space, unevenwear of the abrasive, etc.) are corrected for, bydressing the abrasive product surface [105], without anunderstanding of the cause of the deviation.

4. Innovative abrasive products by manipulation ofthe componentsIn section 2, the components that form an abrasiveproduct were described in detail. Over the last 10 years,continuous improvements to these components, and theway they are assembled into the abrasive product, have

led to significant improvements in grinding quality,productivity, geometry, precision, stability, control andeconomics. This section describes how the manipulationof the components has lead to the above benefits whenused in typical grinding processes.

Fig. 10. Interactions in the grinding zone [104]

4.1. High permeability wheel structuresThe introduction of permeability into an abrasive structureresults in better cooling [43] and the potential for largerchip thickness. Fig. 11 shows a structure that is producedby using a high aspect ratio (8:1) abrasive grains(filaments) within a vitrified structure [122].

Fig. 11. High aspect ratio grain in wheel [122]

The high aspect ratio grains are produced by extruding aseeded-gel of hydrated alumina, described in section 2.1,into continuous filaments, drying the filaments, cutting tothe desired length, and then firing the filaments to atemperature of not more than 1500

0C [84][83][8]. The

alpha alumina crystallites that make up the abrasivefilaments are less than 1 m in diameter. When theseabrasive filaments are formed into a grinding wheel, thebond posts are produced mainly at the interfaces wherethe grains touch each other, leading to a high-strength,very open, structure. During grinding, the micro-crystalline abrasive grains remain sharp until fullyconsumed by wear, or by truing, giving the economics tojustify the higher initial cost compared to fused alumina.

1.1

1.2

1.3

Cutting

Sliding

Sliding

Sliding

Sliding

1. ABRASIVE/WORK

3. CHIP/WORK4. BOND/WORK

2. CHIP/BOND

Ploughing

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It is also possible to create organic wheel structures withup to 80% (by volume) of interconnected porosity(permeability). The method includes blending a mixture ofabrasive grain, bond material, and evenly dispersedparticles (up to 80% by volume). The powder is thenpressed into an abrasive laden composite and thermallyprocessed. After cooling, the composite is immersed into

a solvent, which dissolves substantially all of thedispersed particles, leaving a highly-porous bondedgrinding wheel [79].

Traditional grinding wheels incorporate individualabrasive grains into the structure, with the intention ofequally separating them in a homogeneous manner.However, recent developments in the production ofabrasive agglomerates, has led to highly permeablestructures using these pre-formed clusters of grains [53],which are bonded to each other to form a wheel. Thesestructures can either use organic binders [11] or vitrifiedbinders [12]. Fig. 12 shows the type of structure that isproduced. Grinding tests (described in [11][12]) show thatthe agglomerate-based wheels exhibit greater G-ratio,less chatter, and higher material removal rates, withacceptable surface finish.

Fig. 12 Agglomerated grains in wheel structure [11]

4.2. Single-layer abrasive productsSingle-layer superabrasive products are becoming morepopular due to their lower initial purchase cost than themulti-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 tolonger service life

non-constant grinding power, force, profile andsurface finish throughout the life of the wheel

each profile has to be held in stock (unlike dressablewheels).

4.2.1. Brazed bondsMuch work has been done to produce a brazed single-layer bond that chemically bonds the abrasive grain tothe hub substrate material [22][21][5][1]. Brazing ofdiamond grains is simplified by the spontaneous wettingof untreated or uncoated synthetic diamond grits at

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down to that typically used for milling, with the advantageof having multiple cutting edges that are active, andthose that will become active as the tool wears. Bycontrast, a milling cutter is replaced when the definedcutting edges are worn. The T-tool, as shown in Fig. 15can either be produced with a solid fluted core usingelectroplated superabrasive grains, or, built with

replaceable superabrasive segments with vitrified, resinor metallic bonds.

source: T. Tawakoli Fig. 15 Segmented T-tool design [113]

It is also claimed that the interrupted abrasive surfacebetween successive segments, leads to moreengagement by the following segment, and hencereduces the power consumption in a way that is similar tohow a reduced abrasive concentration produces largerchips. It is also claimed that as the abrasive surfacewears there is no profile loss, since the contact pointmoves around the periphery of the tool to a positionwhere the profile is intact.

Suto et al [107] developed a similar serrated grindingwheel based on electroplated cBN. He also developed atimed, thru-the-wheel, coolant application method to

take advantage of the design (see Fig. 16).

workpiece

grinding wheelnozzle

fluidchamber

perforatedhole

Fig. 16. Timed thru-the-wheel coolant delivery [107]

4.3. Pellet type, flat, superabrasive wheelsGrinding wheels for vertical spindle and double-discapplications have traditionally been constructed bybonding large flat segments onto a metal core. Thesesegments are relatively inexpensive for conventionalabrasives, but require moulds for a large range ofsegment shapes. The segments are often grooved topromote coolant flow and allow the chips to be clearedaway, requiring secondary machining operations. Thislarge-segment method of manufacture is therefore noteconomical for superabrasive wheels.

Pellet type wheels consist of large quantities of cylindricalpellets that are glued onto a flat face wheel. The pelletscan be glued in a variety of patterns to produce evenwear and promote better fluid flow [2][51]. Since thepellets are all the same size, automatic pressingtechniques can be employed to reduce the manufacturingcosts. They have the additional advantage that damaged

sections can be repaired using a few new pellets. Fig. 17shows a typical layout of such wheels, which can be usedfor both single- and double-sided applications.

Fig. 17. Large diameter pellet wheel and parts ground

4.4. Innovative wheel hub designThe grinding wheel hub has also been the subject ofinnovation. In the case of superabrasive and othersegmented wheels, the hub provides the strength towithstand the rotational stress, the stiffness to achievethe required stock removal, and the precision to producea fine surface finish. In addition to the above, the hub canbe tuned to the process dynamics, be fitted with sensorsand can transport grinding fluid into the cutting zone.

4.4.1. Chatter suppression and dampingThe replacement of conventional abrasives bysuperabrasives has led to an increased tendency forregenerative chatter. According to Baylis and Stone [7]and Sexton and Stone [88] this is due to the increasedcontact stiffness of thin-rim cBN wheel design over amonolithic conventional abrasive wheel design. Byobserving the harmonic response locus of the wheel-work loop, Baylis and Stone attempted to reduce thenegative real value, at 180 degrees to the grinding force,of the machine compliance curve. Their attempt wasbased on adding compliant material underneath the cBNlayer. It must be emphasized that the decrease in contactstiffness between the wheel and work was to a levelwhere the overall static loop stiffness was not

dramatically altered. Follow-up work in this area byBzymek et al [20] and Song [94], using the BoundaryElement Method (BEM), showed that strategic machiningof the hub material beneath the abrasive layer providedan alternative to the addition of compliant material.

Warnecke and Barth [117] used the Finite ElementMethod (FEM) to optimise the dynamic behaviour of adiamond wheel for grinding ceramics. Using FEManalysis, they compared the synthetic resin aluminiumcomposite to a solid aluminium disk. Using this data, theypredicted the dynamic behaviour of both grinding wheeldesigns, and their influence on the material removalmechanism. Fig. 18 shows the FEM analysis of thecontact stiffness. Fischbacher [33] took a similarapproach.

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Fig. 18. FEM analysis of contact stiffness [117]

4.4.2. Sensor-integrated wheelsIn the quest for lower process cost and improved quality,machine tool companies and end users are increasinglyintegrating sensors into grinding machine systems for in-process and post-process control. Typically, thesesensors measure displacement, motor power, grindingforce, vibration and acoustic emission (AE). In somecases, the outputs from these sensors are used to

indirectly predict the conditions occurring within thegrinding arc. To improve the measurement accuracy, it isadvantageous to make these measurements within thegrinding zone and transmit them to a signal processingsystem. Three University research teams havedeveloped innovative cBN wheel hubs with integralsensors to monitor the grinding process.

Varghese and Malkin [115] integrated an AE sensor intothe aluminium hub of the wheel, and fixed a forcetransducer underneath one of the cBN segments. UsingDigital Signal Processing (DSP) and Radio FrequencyTransmission (RFT), both signals were received by ahost computer. Force monitoring was successfullyapplied to identify wheel rounding during truing. The AEsignal was found to be sensitive to grinding and truing

parameters, and could identify initial wheel-work contactto help minimize air grinding time. The technology behindthis wheel has been patented [67].

Karpuschewski et al [54] based their grinding monitoringsystem on an AE sensor-integrated wheel by Wakuda etal [116]. They showed that the wheel and signalprocessing system could be used to reliably detectevents, such as: wheel-work contact, for reduced cycletime; and wheel-dresser contact, to ensure the minimumnumber of truing passes are given. The ground partsurface finish was also monitored by the integral AEsensor. With the help of a fuzzy neural system, based onparameters calculated from sensor data, a roughnessprediction can be achieved.

Fig. 19. Sensor integrated wheel [9]

Boehm et al [9] integrated temperature, vibration andforce sensors into their wheel. The temperature sensorwas proved to have sufficient response time (20-50 ns) tomeasure the temperature close to the grinding zone, andcan be compensated for changing abrasive layerthickness with time. The piezoelectric force and vibration,thin-film, sensors are still being perfected for thisapplication. Fig. 19 shows a schematic of the wheel.

The Sensor Integrated wheel concept needs to beembraced by machine tool manufactures, in order tobecome seamlessly embedded into the process control,and not considered as a retrofit. Future designs mustalso consider the easy removal and replacement of gluedvitrified cBN segments, without causing damage to thehub each time.

4.4.3. Through-wheel coolant applicationInstead of relying on the abrasive structure to transportthe grinding fluid into the grinding arc, severalresearchers have enhanced the fluid path by using radialinternal tubes [107][34][47][38][5] or machining thesidewall with flutes [77][107]. The radial tube approachcan be wasteful of fluid, since the flow exits the entire

3600

periphery of the grinding wheel despite the grindingarc being a considerably smaller angle.

Sidewall friction can be reduced using external groovesin this area. The grooves help to get the grinding fluid intothe sidewall area, but the process suffers from increasednoise and reduced abrasive content.

5. Innovative abrasive finishing processes due toadvanced abrasive product designThe development of new abrasive finishing processesdepends, in many cases, on the development of newabrasive products. These products may have some of thefollowing attributes as compared to conventionalproducts: high peripheral speed ability, electrical

synthetic resin alu-minum composite hub

aluminum hub

grinding wheel deformation relative tothe circumference of the rotating

grinding wheelnon

contact elements simulatingsliding and penetration

nodal displacement(vector sum,

aluminum hub wheel)workpiece materialdepth of cutcutting speedfeed rate

::::

hot-pressed silicon nitridea = 1.5 mmv = 50 m/sv = 160 mm/min

e

c

w

dimensions : displacement

1 : 400m

0123

circumference of thegrinding wheelrotating

circumference of thegrinding wheelrotating

4 m

9.4 m

F = F = 110 N/mmresultantn

F =

18 N/mmt

b

a d

c

source: IWT, Bremen

telemetry ring

telemetry stator

temperature sensor

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conductivity, tolerant of high surface temperatures, highwear resistance, high permeability, etc. This sectiondescribes these processes and why the abrasive productis unique to the application.

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 highspecific removal rates, using high wheel speed, highworkspeed and high depth of cut. In conventionalgrinding, as the removal rate is increased the surfacetemperature in the grinding zone increases, and burnoften results. However, if the wheel speed and tablespeed are increased further, the surface contacttemperature reaches a peak value and then decreases,due to the greater amount of grinding energy going intothe chip instead of the work. The greater thermalconductivity of superabrasives also aids the heat removalfrom the workpiece. Fig. 20 shows that with HEDG [109]the workpiece surface temperature first increases, thendecreases with increased wheel speed.

.

Fig. 20. The HEDG effect [109]

The use of electroplated cBN grinding wheel speeds upto 250 m/s, work speeds in excess of 100mm/s (surfacegrinding), depths of cut up to 30mm, and mineral oil fluidhas enabled HEDG to compete with conventional cuttingprocesses, with the advantage of better surface finish,improved surface integrity, improvedform accuracy, andthe possibility of using fully hardened workpieces [26].

Comley et al. [26], have demonstrated crankshaft web-grinding removal rates of up to Qw = 2000 mm

3/s.mm,

and journal grinding rates of 250 mm3/s.mm on low-alloy

automotive steel. The specific grinding energy can be aslow as 9 J/mm

3at such high removal rates. To achieve

this level of performance, modern single-layersuperabrasive wheels are 2-plane balanced, haverotational error less than 3m, are able to withstand highperiphery speeds (up to 250m/s), and have an even gritdistribution.

There is also a role for conventional abrasives in HEDG[69], providing that they can reach a sufficiently highperipheral wheel speed to be classified as working in theHEDG domain. For example, the 8:1 aspect ratio,sintered, seeded-gel alumina finres, described in Section4.1, can be formed into segments and bonded to a steelcore to allow grinding speeds up to 140 m/s. This grain

can be easily trued and dressed by a rotary device and itcan be used in water-based fluid, whilst not suffering theattritious wear associated with cBN in water. Fig. 21shows the grain wear for this wheel at a specific removalrates of 375 mm

3/s.mm, with inconel 718 workpieces.

Fig. 21. Radial wear for cBN versus alumina fibres [69]

The result showed that after low initial wear thecomparison B126 vitrified cBN wheel graduallydeveloped greater grain wear than the 80 mesh sinteredalumina fibre wheels, giving a G-ratio of 2.6 mm/mm forthe cBN-wheel, and a G-ratio of 19.0 mm/mm for thealumina fibre wheel. It must be noted that this result wasobtained in water-based fluid.

5.2. High speed traverse/contour/peel grindingTraverse, contour or peel grinding, as the process isknown, is analogous to turning of a cylindrical part, with agrinding wheel instead of a single-point cutting tool. Theadvantage of this approach, as compared to plungegrinding, is that a single wheel can form a multi-diameter,

fully hardened, shaft in only a few passes. These singlepass traverse approaches are only possible using highwear resistant superabrasive wheels, typically,electroplated, vitrified, or metal-bond types. The wheelsprimarily cut the workpiece material on their sides andtherefore require the abrasive to wrap around the edges.Narrow, but sufficiently stiff, wheels help reduce thecontact width to reduce the thermal loading on theworkpiece surface [100][113]. Fig 22 shows the principleof the process.

Fig. 22. High speed traverse grinding of shaft [100]

Another approach to reducing the contact area is bytilting the grinding wheel by a few degrees to create analmost point contact with the workpiece [56].

Full depth, traverse grinding can also be used togenerate internal forms in a component, as a

conventionalwheel

cBNwheel

ds = 400mm; a = 6mmMaterial: 16MnCr5Qw = 100 mm

3/s.mmFluid: mineral oil

Wheelspeed(m/s)

W

orkpiecesurfacetemperature(C)

conventionalwheel

cBNwheel

ds = 400mm; a = 6mmMaterial: 16MnCr5Qw = 100 mm

3/s.mmFluid: mineral oil

Wheelspeed(m/s)

conventionalwheel

cBNwheel

ds = 400mm; a = 6mmMaterial: 16MnCr5Qw = 100 mm

3/s.mmFluid: mineral oil

Wheelspeed(m/s)

conventionalwheel

cBNwheel

ds = 400mm; a = 6mmMaterial: 16MnCr5Qw = 100 mm

3/s.mmFluid: mineral oil

Wheelspeed(m/s)

conventionalwheel

cBNwheel

ds = 400mm; a = 6mmMaterial: 16MnCr5Qw = 100 mm

3/s.mmFluid: mineral oil

Wheelspeed(m/s)

W

orkpiecesurfacetemperature(C)

W

orkpiecesurfacetemperature(C)

W

orkpiecesurfacetemperature(C)

conventionalwheel

cBNwheel

ds = 400mm; a = 6mmMaterial: 16MnCr5Qw = 100 mm

3/s.mmFluid: mineral oil

Wheelspeed(m/s)

conventionalwheel

cBNwheel

ds = 400mm; a = 6mmMaterial: 16MnCr5Qw = 100 mm

3/s.mmFluid: mineral oil

Wheelspeed(m/s)

conventionalwheel

cBNwheel

ds = 400mm; a = 6mmMaterial: 16MnCr5Qw = 100 mm

3/s.mmFluid: mineral oil

Wheelspeed(m/s)

G = 2.6 mm /mm

work material: Inconel 718,

cutting speed: v = 140 m/s,depth of cut: a = 1.5 mm,

work speed: v = 15 m/min,

c

e

ft spec. rem. rate:Q = 375 mm /(mm.

.s)coolant: Hysol X, 7%

flow rate: 130 l/min, 7 barwheel cleaning: 30 l/min,17 bar

'w

3

spec. stock removal V'w0 500 1000 1500 2000 2500 mm3/mm 3500

0

m

200

400

600

1000

1400

800

G = 19.0 mm /mm3 3

3 3

cBN

alumina

fibres

Radialwheelwear,

r s

G = 2.6 mm /mm

work material: Inconel 718,

cutting speed: v = 140 m/s,depth of cut: a = 1.5 mm,

work speed: v = 15 m/min,

c

e

ft spec. rem. rate:Q = 375 mm /(mm.

.s)coolant: Hysol X, 7%

flow rate: 130 l/min, 7 barwheel cleaning: 30 l/min,17 bar

'w

3

spec. stock removal V'w0 500 1000 1500 2000 2500 mm3/mm 3500

0

m

200

400

600

1000

1400

800

G = 19.0 mm /mm3 3

3 3

cBN

alumina

fibres

Radialwheelwear,

r s

bc

vf

bc

vf

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replacement for plunge or reciprocating grinding, whichrequire much wider wheels. One of the biggestadvantages with this approach by Weinert and Finke[118] is a reduction in the total grinding force, andtherefore a reduction in the tendency to produce atapered hole. Fig. 23 shows a schematic of the contactzone for this mode of grinding.

Fig. 23. Contact zone in deep traverse grinding [118]

Internal, deep traverse grinding uses a narrow grindingwheel with a tapered roughing zone and a cylindricalfinishing zone. The radial infeed motion takes placeoutside of the workpiece. During the axial feed motion,the tapered roughing zone removes the material and thecylindrical finishing zone generates a good surfacequality. Due to the tapered geometry, high material

removal rates can be achieved, because the consumedpower of the grinding process is distributed over a largesurface area. The specific material removal ratedetermines the grinding wheel power. When using atapered dressed grinding wheel the largest specificmaterial removal rate occurs in zone II (Fig. 23). Theprofile angle , and the feed engagement af , determinethe effective engagement ae,eff. By using this relation thespecific material removal rate for internal traversegrinding can be described in equation (1).

tan/ = fwww andQ . (1)

5.3. Grind-hardeningIt has been shown that the heat flux generated in grinding

can be used to induce martensitic phase transformationsinto the surface layers of annealed or tempered steel,creating a hardened surface, with pre-dominantlycompressive surface residual stresses. This techniquecan replace both the rough grinding in soft state andheat treatment operations, that are traditionally used in aproduction sequence [17][18]. Grind-hardening is not tobe confused with grinding burn, where a hardenedworkpiece adopts a martensitic white layer, containingtensile surface residual stresses.

The maximum depth of hardness penetration, andhardness profile, obtained by grind-hardening, to date,ranges from 2mm for flat surfaces and 1.6mm for roundsurfaces. Fig. 24 shows the surface residual stress

profile after grind hardening a 42CrMo4 steel. WEA is thewhite etching area.The grinding wheel for grind-hardening, requires thefollowing 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 coolwheel

closed structure

monolithic design, or thick segments that protectadhesive from heat

low speed rating of

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nm SiO2 film to it using radio-frequency magnetronsputtering. This technique increases the gripping strengthof the diamond grains to the resin matrix.

Since electroplated diamond wires are not subjected tothe damaging curing temperatures that resinoid wiresexperience, they are therefore less prone to breaking.

Although the manufacturing process for this type of wireis slower than for resinoid, their increased cost is offsetby higher wear resistance. These electroplated diamondwires have proven to be very effective in cutting hardceramics, such as sapphire in the LED industry.

Fig. 25. Fixed abrasive wire saw machine [71]

Chiba et al [24] developed an ultra high-speed method ofproducing electroplated wire using nickel-coated, 10-20m diamond grains.

5.5. Grinding, truing and dressing by electrical andelectro-chemical methodsSeveral electrical and electro-chemical grinding, truingand dressing systems have been developed in the lastdecade. The systems have been developed for diamondtool grinding, diamond wheel preparation, ceramicgrinding, and others. The following acronyms describe afew 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 grindingwheel (REDM)[78]

Abrasive Electro-Discharge Grinding (AEDG)[97]

Electrolytic In-process Dressing (ELID)[51]

Some of the above systems will work with standardmetal-bond superabrasive wheels, although in somecases, specially developed electrically conductive bondsand coatings have been developed to enhance theprocess. A selection of these systems follows:

5.5.1. Electrochemical discharge machiningECDM combines ECM and EDM. The ECM action isassisted by the thermal erosive effects of discharges.The grinding wheel must have a conducting metal bond,

preferably bronze or copper. The ECM and EDM timeperiods need to be balanced. In the first stage ECMoccurs and anodic dissolution of the outer layer of theworkpiece takes place. In parallel, mechanical grindingalso occurs. On increasing the voltage in the secondstage of the process, the concentration of the ionsincreases until electrical discharges occur. A plasma

channel is created and material removed by evaporation.

The technique has also been applied to truing anddressing of grinding wheels [87], which are then used forhigh precision grinding of cylindrical workpieces. Grindingwheel surface quality and roundness error are claimed tobe superior to those obtained by conventional grinding.

5.5.2. Electrolytic in-process dressingA variation of the ECDM technique is ELID, where thegrinding wheel is the anode and the electrolyte issupplied into the inter-electrode gap [51]. The aim is toeliminate wheel loading and ensure permanent dressing.Using this technique on an ultra-stiff precision grindingmachine, has achieved surface finish values of less than10 nm Ra [99].

ELID usually employs a cast iron fibre bonded wheel. Anoxide layer (rust) is produced on the wheel surface duringthe electro-chemical reaction between bond and fluid,shedding worn diamond grains (see Fig. 26). ELIDenables the use of nano-order diamond grains to beused, and can be applied to all modes of grinding.

Fig. 26. Principle of ELID [51]

5.5.3. Electro-discharge diamond grindingIn EDDG, a bronze bonded diamond wheel is used in akerosene dielectric fluid. The process integrates electricaldischarge machining with diamond grinding forelectrically conductive, hard, materials. The role of thespark is to thermally soften the work material, in an effortto reduce normal and tangential forces [60], as shown inFig. 27.

5.5.4. Laser-assisted dressing of superabrasivewheelsThe precision of a formed grinding wheel is oftendependent on the geometry of the truing tool. Withsuperabrasive grinding wheels, especially diamond, wearof the truer can be significant and ever changing. Severalresearchers have investigated laser-assisted truing anddressing, but with limited success. Such systems areexpensive and must be protected from the hostileenvironment in a grinding machine.

Shin [92] developed a system that used a laser to softenthe vitrified bond prior to contact with a single-pointdiamond truer. The laser was applied in an axial direction

coolant

nozzle

coolant

nozzlework

feed roller take-up roller

spacer rollers

coolant

nozzle

coolant

nozzlework

feed roller take-up roller

spacer rollers

+

Cast iron fiber bonded(CIFB) grinding wheel

coolant

(electrolyte)

workpiece

source : Ohmori/Nakagawa, Japan

+

-

electrode

= cathode (copper, graphite)

grinding wheel (CIFB)= anode

brush

workpiece

coolant(electrolyte)

U

U

electrode= cathode

+-

outer diameter (OD) grinding rotation grinding

++

Cast iron fiber bonded(CIFB) grinding wheel

coolant

(electrolyte)

workpiece

source : Ohmori/Nakagawa, Japan

+

-

electrode

= cathode (copper, graphite)

grinding wheel (CIFB)= anode

brush

workpiece

coolant(electrolyte)

U

U

electrode= cathode

+-

outer diameter (OD) grinding rotation grinding

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just ahead of the diamond truer. The effect was a veryopen 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 astandard vitrified product. However, Hoffmeister andTimmer [44] found that the greater transparency ofnatural diamonds allowed them to withstand the energyof the laser far better than the yellow synthetic diamonds.They also determined that larger grit sizes are moretolerant of the laser energy, with regards to grit fracturing

due to a reduction in fracture toughness.

Hoffmeister and Timmer [44] compared traditionalphenolic resin bond with high-temperature, copper filled,polyimide resin bond, showing the former took longer toprofile, using a tangential laser. Their tests on resin bondwheels concluded that careful control of the bondmaterial, and use of more transparent diamonds, will leadto a product that is tuned to the laser-truing process.

5.6. Fixed abrasive grinding, with a vertical spindleManufacturing of silicon wafers consists of a successionof abrasive processes: saw, edge, lap, etch and polish.As the requirements for flatness and wafer sizetolerances increase, new process were needed toreplace some of the loose abrasive (lapping andpolishing) steps by fixed abrasive. In addition, it was feltthe throughput of loose abrasive process in the finishingof a wide variety of ceramic materials could be improvedby grinding processes, provided of course that thenecessary low roughness (typically sub-100 AngstromsRms) could be achieved. This led to the development ofa grinding process [114] with a narrow abrasive rim(approx. 6 mm) and special kinematics (e.g. centre ofwafer directly under abrasive rim) to generate mirrorfinishes on a variety of advanced ceramic components.

Fig. 28. Fixed abrasive grindingkinematics [114]

Although small compared to vertical axis segmented andpellet wheels, the still broad area of contact between thegrinding wheel and the workpieces, present in these

types of infeed grinding processes, have been madepossible by the development of a new generation of self-dressing diamond products [114]. Impressive resultswere presented by combining some of these newlydeveloped diamond wheels, with the grinding kinematicsdescribed in Fig. 28. Tricard et al [114] reported animpressive sub-20 Angstroms (Rms) surface roughness

(measured with a 10 x 10 m AFM scan) achieved onaluminium titanium carbide (AlTiC) wafers in a productionenvironment.

6. Future trends in abrasive productsThis section attempts to predict the future developmentsin 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 wheelsThe push towards minimum fluid application and areduction in fluid disposal costs, has encouragedresearchers to examine the integration of lubricants intothe structure of the grinding wheel. This has resulted inseveral patented designs, including:

Superabrasive segments impregnated with resin andproprietary solid lubricant [50]

Sol-gel alumina grain in vitrified bond, filled withoil/wax mixture [82]

Vitrified aluminium oxide wheels impregnated withwater-insoluble, sulphur bearing, organic substance[64]

Fig. 29. Graphite impregnated wheel [89]

Salmon [85] used cutting tool coating technology on cBNgrains to counter the boric oxide formation when water-based fluids are used. His two-step approach was to usea hard titanium aluminium nitride (TiAlN) coating on anelectroplated grinding wheel, followed by a layer ofmolybdenum disulphide (MoS2) hard lubricant. Salmonstests on MAR nickel-based alloy showed that thecoatings gave longer life, lower power, and no capping ofthe grains, despite the absence of cleaning jets.

Tang et al [108] also explored the use of MoS 2 lubricant,but with titanium alloy workpieces and silicon carbidewheels. Tang found a reduction in grinding forces, lowerspecific grinding energy due to reduced ploughing andsliding, and less adhering of titanium on the SiC grains.

Wafer

Air

Bearing

SpindleVacuum

Chuck

AirBearing

SpindleFine

Infeed WaferGrinding Wheel

Side Top

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Unlike Salmon, or Tang et al, Shajis approach [89] wasbased on a unique wheel design that incorporated a solidlubricant. The vitrified aluminium oxide wheel, shown inFig. 29, has dovetail slots on the periphery, filled with aphenolic resin-alumina-graphite mixture. Shajis drygrinding tests on Rc60, EN31 bearing steel showed:better surface finish, lower spindle power, and higher

wheel wear, as compared to wet grinding without thesolid lubricant. Shaji attributed this to the interruptedcutting action promoting high forces on the leadingedges, resulting in breakdown of the vitreous structure.

6.2. Rapid prototyping of vitreous, metallic andorganic grinding wheelsIndustrial application of Rapid Prototyping as a materialadditive manufacturing process started fifteen years ago[65]. Since then, several methods of building grindingwheels have been researched, based on ceramic,metallic and polymer bonds. The most popular methodexplored is Selective Laser Sintering (SLS).

In the SLS process, shown in Fig. 30, a focussed laserbeam locally sinters/melts a heat fusible powder [95].Infrared CO2 or Nd:YAG lasers, with a maximum beamenergy of 50-100W are used as energy sources. Afterone layer has been formed, a new powder layer isapplied and the laser beam solidifies it. The laser beamintensity is modulated so as to melt the new powder andbond it to the layer. This procedure is repeated until the

whole part is completed.

The idea of using selective laser sintering for makinggrinding wheels is especially attractive for low volumecustomized production of special profiles. The viability ofthis approach was reported by Hon and Gill [45], whoproduced polyamide/SiC matrix composites . Sampleswere produced using FEPA standard SiC F240 gritblended with polyamide to produce a 50/50 mix. Thesample, shown in Fig. 31, reached a UTS value of 31MPa and a Youngs Modulus of 2100 MPa.

Kovalenko et al [61] demonstrated laser sintering of acomposite 80%Co-20%Sn and diamond abrasive, usingup to 2 kW/cm

2 power density. No evidence of

graphitisation, or reduction of toughness, was detectedby Kovalenko et al. Using a special 90% cobaltcomposite and diamond, clusters of the abrasive wereproduced. This work is highly relevant to future single-layer 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 bindersprior to sintering. The tapes are formed in advance withthe required density of abrasive grains and copper-basedbond content. This method is potentially more consistentthan the traditional re-coating method. The Greentapecan also be pressed before each pass of the laser, togive greater packing density. Abrasive products usingdiamond, cBN and aluminium oxide have been producedby this method.

Fig. 31. SEM micrograph of surface of SLS parts [45]

Fig. 32. Laser sintering of single-layer diamond [61]

6.3. Engineered grinding wheelsDiscussions on the deterministic performance ofmachining versus the black art of grinding, are oftenbased on the fact that cutting tools have evenly spacedcutting edges of defined geometry (unless worn). In North

America, HEDG is sometimes referred to as micro-milling due to the large ductile chips that are removedfrom the workpiece by coarse grit electroplated wheels.In an attempt to take some of the randomness out ofgrinding, several researchers have developedengineered grinding wheels of defined distribution and, insome cases, defined orientation.

Aurich et al [3] built and tested a wheel with defined grainstructure, using kinematic simulation to develop thepattern. The cBN grains were glued onto the hub in therequired pattern, followed by electroplating over the topwith nickel. The aim of the work was to improve theprocess stability, minimise heat generation and achievebetter surface quality, all without compromising thematerial removal rate. Fig. 33 shows one pattern

133m133m133m

scanner

recoater

part

powder

platform

laser

sintered

layer

laser

beam

substrate

plate

powder

Fig. 30. Principle of SLS [95]

scanner

recoater

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powder

platform

laser

sintered

layer

laser

beam

substrate

plate

powder

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recoater

part

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beam

substrate

plate

powder

scanner

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part

powder

platform

laser

sintered

layer

laser

beam

substrate

plate

powder

Fig. 30. Principle of SLS [95]

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investigated by the simulation, and the possible defectthat may occur if a single peripheral line around thewheel 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 regularlywithin the grinding wheel industry, during themanufacture of reverse-plated truing rolls. In fact, thereverse-plated approach to engineered wheels wouldensure that all the abrasive grits have similar protrusionfrom the axis of rotation prior to plating.

Fig. 33. Influence of grit displacement on work surface [3]

Another possible reason for the stripping may be due tothe far lower abrasive concentration of the engineeredwheel, as compared to a standard electroplated wheel. Atsimilar material removal rates, the engineered wheelwould remove a larger chip with a correspondingly higherforce per grit. Also, in patterns such as in Fig. 33, evenspacing of the grains around the wheel periphery mayalso 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 thewheel periphery may give less opportunity for instabilityto arise.

Koshy et al [59] modelled both vitrified and electroplatedwheels with defined grain structure, and modelled theeffect of axial pitch and axial offset of adjacent rows, onthe surface finish of the finished workpiece. For the bestsurface finish, he showed that the axial offset should begreater than zero but less than 25-40% of the averagegrain diameter (42 m). Fig. 34 shows surface finish as afunction of axial pitch and axial offset.

Pritchard [75] developed a method of optimally orientingand spacing the abrasive grain, for coated abrasive belts.

His technique relies on a perforated polymeric sheet thatpasses under falling abrasive grains and traps one grainin each perforation. The shape of the perforationpreferentially traps the point of the grain rather than a flat,hence orienting it. Excess grains are blown away usingcompressed air. The grains are then sprayed with asolvent to soften the polymer and bond the abrasive to it.The last steps involve sintering the grains to a metal tapeusing a special brazing powder. The method claims towork with all conventional and superabrasive grains.

Engineered abrasive grains may also encompassdiamond fibres produced by depositing diamond on totungsten wire, using hot filament CVD [25]. The diamondthat is produced has a polycrystalline structure giving

multiple cutting edges on each wire. Tests on a singlediamond wire, bonded radially into a narrow disk, showedthat once the leading edge diamond crystals werechamfered by initial wear, and many more of thembecame active, the wear rate reduced dramatically.Subsequent tests on multiple fibres, randomly positionedinto a metal bond matrix, produced optical quality

surfaces on BK7 glass, with a surface finish value of70nm Ra, and less than 2m sub-surface damage.Furthermore these tests suggested that diamond fibrescould lead to longer wheel life, when grinding in theductile region, compared with existing resin bond wheels.

Fig. 34. Effect of axial pitch and offset on roughness [59]

6.4. Single-layer superabrasive wheel developmentsThe future can expect to see greater applications ofsingle-layer superabrasive wheels, especially onmachining centers. As electroplating and brazing single-layer manufacturing techniques become faster, and theirgeometry becomes more accurate, they will replacesome milling tools in tool changing cabinets. Theincreased integration of HSK tapers and other similar

spindle mounts, into the wheel body, will reduce theradial and axial error associated with collets andadapters.

Advances in the friability, structure and shape ofsynthesised superabrasives will also improve theperformance of these wheels in terms of life and form-holding. Recent tests grinding nickel alloy, polycrystallinecBN abrasives with ultrafine crystal structure, haverecently shown G-ratio improvements up to 15 timesgreater than conventional polycrystalline cBN [106] withlower specific grinding energy. It is also expected that thenano-crystalline cBN will constantly regenerate microcutting edges as radial wear progresses, much in thesame way as a sol-gel sintered alumina.

6.4.1. Ultrasonic aided, electro-less nickel platingThe application of ultrasonic vibration during electrolessnickel plating has been shown to improve the wettingbetween an abrasive grain and the nickel matrix. Thisgrain exposure is comparable to electroplated nickelwheels [73]. The plating rate is claimed to increase withvibration amplitude, up to a maximum value of 11 m at afrequency of 15.5 kHz. The process is experimental atthis time but has potential for complex forms due toreduced tendency to build up at sharp corners, ascompared to the electroplated process.

6.4.2. Direct deposition of abrasive layersMuch work has been done developing CVD coatings, andtheir application for wear resistance, improved thermal

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resulting workpiece surfaces

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resulting workpiece surfaces

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properties, etc. However some work has also been doneusing the coating as a single-, or multi-layer, abrasivetool. Although this work by Gabler et al [35] involved themanufacture of small abrasive burrs, there may bepotential for creating larger wheels if the process costcan be reduced. Using a hot-filament CVD reactor, 240tools were coated with crystal sizes up to 50 m.

Although the coating time was 90 hours, economies-of-scale can make the process viable. A close up of theCVD abrasive burr is shown in Fig. 35.

Fig. 35. CVD coated abrasive burr [35]

6.4.3. Profile grinding to replace machiningRe-entrant internal slots, such as dovetails and turbinedisk rootforms, are traditionally produced using a largebroaching machine. Turbine disk broaching machines areextremely large and contain hundreds of cutting edges toproduce the slots. The sheer cost, space allowance andmaintenance of such machines, makes the processexpensive and destined to remain inside the turbinemanufacturer plants. Recent developments in mountedpoint grinding have showed the viability of producingentire disks with just a few small, profiled wheels, on amodified machining centre, making future productionpossible at subcontractor facilities. Fig. 36 shows anexample 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]

The innovation in this broaching replacement process, isin the HEDG pre-forming of the disk, the precision andstiffness of the mounted points, and negligible form errorfrom the 1

stto 70

thslot. The process uses wheel speeds

between 50-100,000 rpm, on difficult-to-grind nickel-based alloys [6]. Carbide shanks are designed tofracture, not bend, to ensure that the spindle nose and

workpiece are not damaged during an unexpectedcollision.

6.4.4. Tools for ultra-sonic assisted machiningThe possibility to apply abrasive grains to almost anygiven geometry has also led to the development of ultra-sonic assisted material removal processes, sometimesreferred to as ultra-sonic milling [28]. With a newgeneration of machine tools dedicated to apply thistechnology complex geometrical 3D-features can bemachined in mainly brittle materials such as glass orceramics, as shown in Fig. 37.

oscillating toolvc

vft

ae

adyn

workpiece(e.g. glass, ceramics,CFRP, ...)

booster(for amplitudeamplifying)

US spindle (axialmovement initiated byan ultrasonic generator)

f = 20 kHzx = 1 3 mspi

x = 3 50 mtoo

diamond layersource: DMG Sauer

single layer diamond tools

Fig. 37. Ultra-sonic assisted machining [28]

Fig. 38. Superhard Materials in B-C-N System [123]

6.5. Superhard material developmentThe search for novel superhard materials continuesfollowing the successful synthesis of man-made diamond

and cubic boron nitride at high temperatures andpressures. Designing new superhard materials withnovel properties, and developing practical methods ofproduction, are the goals of several research teams[123][41][72][46]. Potential candidates are from thesystems 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 fromfirst principles predicted that certain carbon nitrides havebulk moduli comparable to or even greater than that ofdiamond. Based on the assumption that hardnesscorrelates with the bulk modulus, cubic C3N4 (with acalculated bulk modulus of 496Gpa) will likely be harder

C (diamond)

B NcBN

B4C

C3N4

BC4N

BC2N

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than diamond. Fig. 39 shows measured and calculatedproperties of materials based on boron, carbon andnitrogen. 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 thestrong N-N bond that favours the formation of N2.

The possibility of synthesizing , -C3N4phases using Nias the catalyst, at 7 GPa and 1400C in a large-volumepress was shown to be feasible [41]. However, the crystalobtained was too small to provide conclusive structureand 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 at7.7 GPa and 2000C without the use of a catalyst [72].

Phase Hardness(GPa)

Bulk Mod.(GPa)

Shear Mod.(GPa)

Diamond 70-90 443 535

Cubic BN 48 400 409

Cubic C3N4 ?? 496 332*

- 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 alsobeen reported [124]. Higher pressures are needed toobtain single-phase material and to search for suitablecatalysts to lower the pressure and temperatureconditions. In the boron-oxide system, single phase ofB6O has been produced at conditions of 5-7.5 GPa and1700C [46], while cubic B6O was reported to besynthesized at much lower pressure and temperatureranges of 3.5-5.5 GPa, 1000C-1200C [123]. However,further work is needed to characterize their structure andproperties.

Zhao et al [123] carried out high-pressure synthesis ofwell-sintered millimetre-sized bulks of superhard BC2Nand BC4N materials in the form of a nano-crystallinecomposite with diamond-like amorphous carbon grainboundaries. These new high-pressure phases of B-C-Ncompound have extreme hardnesses, second only todiamond. The final products are well-sintered millimetresize chunks which are translucent and yellowish incolour. The synthesized BC2N and BC4N materials havea zinc-blend structure and a face-centred cubic unit cell.The hardness measurements show that the BC2N andBC4N samples synthesized under high pressure andtemperature have nominal hardnesses of 62 GPa and 68GPa respectively, which is very close to diamond and farhigher than cBN.

Zhao et al [123] states that reactive sintering of diamond-SiC nano-composites, based on thorough mixing ofdiamond and silicon nano-size powder, can be applied toproduce large specimens. It is expected that by bettersample preparation, carefully designed mixing protocols,and by using silicon powder of smaller grain size, it willbe possible to eliminate graphitization, reduce porosityand decrease SiC content, and thus further improveproperties of diamond-SiC nano-composites.

7. Concluding remarksThis paper does not reflect the views of Saint Gobain orQED Technologies, but is the result of extensiveliterature and patent searches by the authors. The

authors extend their gratitude to Saint Gobain and QEDTechnologies for use of library facilities and somepreparation time.

In 95% of cases the text is referenced, with someindustrial viewpoint statements added by the authorswhere their experience warrants it. It has been

impossible to include examples of innovation and futuretrends where there are no public domain documentsavailable, and where it is secret within companies.

Although many abrasive producers were contacted earlyon in the writing of the paper, it was not an effectivemeans of obtaining information. In many cases theinformation could not be referenced to a public domaindocument, other than a patent. Special thanks go toHermes Schleifmittel for their input in the sol-gel abrasivesection.

Finally, the authors would like to thank STCchairpersons, Profs. S. Malkin and B. Karpuschewski, fortheir input and review of the document.

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