Handbook Rectificado

75

5

The Nature of the Abrasive

5.1 INTRODUCTION

Modern grinding abrasives mainly fall into one of two groups, namely,

• Conventional abrasives based either on silicon carbide (SiC) or aluminum oxide (Alox), and • Superabrasives based either on diamond or cubic boron nitride (CBN).

The division into two groups is based on a dramatic difference in hardness of the grains leadingto very different wheel wear characteristics and grinding strategies. The division is also based oncost; wheels made using superabrasives are typically 10 to 100 times more expensive.

5.2 SILICON CARBIDE

5.2.1 D

EVELOPMENT

OF

S

I

C

SiC was first synthesized in 1891 by Dr. E. G. Acheson, who gave it the trade name “Carborundum.”It was initially produced in only small quantities and sold for $0.40/ct or $880/lb as a substitutefor diamond powder for lapping precious stones. In its time, it might well have been described asthe first synthetic “superabrasive,” certainly compared to the natural emery and corundum mineralsthen otherwise available. However, once a commercially viable process of manufacturing wasdetermined, its price fell precipitously, and by 1938 it sold for $0.10/lb [Heywood 1938]. Todaythe material costs about $0.80/lb.

5.2.2 M

ANUFACTURE

OF

S

I

C

SiC is manufactured in an Acheson resistance heating furnace through the reaction of silica sandand coke at a temperature of around 2,400

°

C. The overall reaction is described by the equation

SiO

2

+

3C

→

SiC

+

2CO

A large carbon resistor rod is placed on a bed of raw materials to which a heavy current is applied.The raw material also includes sawdust to add porosity to help release the CO, and salt to removeiron impurities. The whole process takes about 36 hours and yields 10 to 50 tons of product. Fromthe time it is formed, the SiC remains a solid as no melting occurs (SiC sublimates at 2,700

°

C).After cooling, the SiC is sorted by color; from green SiC, which is 99% pure, to black SiC, whichis 97% pure. It is then crushed and sized as described for alumina below.

5.2.3 H

ARDNESS

OF

S

I

C

SiC has a Knoop hardness of about 2,500 to 2,800 and is very friable. The impurities within the blackgrade increase the toughness somewhat but the resulting grain is still significantly more friable thanalumina. Above 750

°

C, SiC shows a chemical reactivity toward metals with an affinity for carbon,such as iron and nickel. This limits its use to grinding hard, nonferrous metals. SiC also reacts withboron oxide and sodium silicate, common constituents of vitrified wheel bonds [Viernekes 1987].

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76

Handbook of Machining with Grinding Wheels

5.3 ALUMINA (ALOX)-BASED ABRASIVES

Alumina-based abrasives are derived either from a traditional route of electrofusion, or more recentlyby chemical precipitation and/or sintering. Unlike SiC, alumina is available in a large range of gradesbecause it allows substitution of other oxides in a solid solution, and defect content can

be muchmore readily controlled. The following description of alumina-based abrasives is classified intoelectrofused alumina abrasives and chemically precipitated or sintered alumina abrasives.

5.4 ELECTROFUSED ALUMINA ABRASIVES

5.4.1 M

ANUFACTURE

The most common raw material for electrofused alumina is bauxite, which, depending on source,contains 85 to 90% alumina, 2 to 5% TiO

2

, and up to 10% of iron oxide, silica, and basic oxides.The bauxite is fused in an electric-arc furnace at 2,600

°

C using a process demonstrated by CharlesJacobs in 1897 but first brought to commercial viability under the name “alundum” with theintroduction of the Higgins furnace by Aldus C. Higgins of the Norton Company in 1904 (Figure 5.1)[Tymeson 1953].

A Higgins furnace consists of a thin metal shell on a heavy metal hearth. A wall of water runningover the outside of the shell is sufficient to maintain the shell integrity. A bed of crushed and calcinedbauxite (mixed with some coke and iron to remove impurities) is poured into the bottom of the furnaceand a carbon starter rod is laid on it. Two or three large vertical carbon rods are then brought downto touch and a heavy current is applied. The starter rod is rapidly consumed but the heat generatedmelts the bauxite, which then becomes an electrolyte. Bauxite is added continually over the nextseveral hours to build up the volume of melt to as much as 20 tonnes. Current flow is controlled byadjusting the height of the electrodes which are eventually consumed in the process.

Perhaps the most surprising feature of the process is the fact that a thin, water-cooled steelshell is sufficient to contain the process. This is indicative of the low thermal conductivity of

FIGURE 5.1

Higgins-type electric arc furnace for fusion processes of alumina and zirconia. (Courtesy ofSaint-Gobain Abrasives. With permission.)



77

alumina, a factor that is also significant for its grinding performance as will be described in latersections. The alumina forms a solid insulating crust next to the steel. After the fusion is complete,the furnace is either left to cool or, with more modern furnaces typical of those currently in use inthe United States, the melt is poured onto a water-cooled steel hearth to better control microstructure.

Once cooled, the alumina is broken up and passed through a series of hammer, beater, crush,roller, and/or ball mills to reduce it to the required grain size. The type of crush process also controlsthe grain shape, producing either blocky or thin splintered grains. After milling, the product issieved to the appropriate sizes down to about 40 µm (400#).

5.4.2 B

ROWN

A

LUMINA

The resultant abrasive is called brown alumina and contains typically 3% TiO

2

. It has a Knoophardness of 2,090 and a medium friability. Increasing the TiO

2

content increases the toughness butreduces hardness. Although termed brown, the high temperature furnacing in air required insubsequent vitrified wheel manufacture turns the brown alumina grains a gray-blue color due tofurther oxidation of the TiO

2

.

5.4.3 W

HITE

A

LUMINA

Electrofused alumina is also made using low-soda Bayer Process alumina that is >99% pure. Theresulting grain is one of the hardest, but most friable, of the alumina abrasive family providing acool-cutting action especially suitable for precision grinding in vitrified bonds. Also, its low sodiumcontent deters wheel breakdown from coolant attack when used in resin bonds.

White alumina is the most popular grade for micron-sized abrasives in part because the crushingprocess concentrates impurities in the fines when processing other alumina grades. To produce micronsizes, the alumina is further ball-milled or vibro-milled after crushing and then traditionally separatedinto sizes using an elutriation process. This is achieved by passing a slurry of the abrasive and waterthrough a series of vertical columns. The width of the columns is adjusted to produce a progressivelyslower vertical flow velocity from column to column. Heavier abrasive settles out in the faster flowingcolumns while the lighter particles are carried over to the next. The process is effective down to about5 µm and is also used for micron-sizing SiC. More recently, air classification has also been adopted.

FIGURE 5.2

Pouring of molten alumina. (From Wellborn 1994. With permission.)


78


Not surprisingly, since electrofused technology has been available for 100 years, many varia-tions of the process exist both in terms of starting compositions and processing routes. Someexamples are illustrated in Figure 5.3.

5.4.4 A

LLOYING

A

DDITIVES

Additives are employed to modify the properties of alumina as described below. Examples ofadditives include chromium oxide, titanium oxide, zirconium oxide, and vanadium oxide.

5.4.5 P

INK

A

LUMINA

The addition of chromium oxide produces pink alumina. White alumina is alloyed with <0.5%chrome oxide to give the distinctive pink hue of pink alumina. The resulting grain is slightly harderthan white alumina, while addition of a small amount of TiO

2

increases its toughness. The resultantproduct is a medium-sized grain available in elongated, or blocky, but sharp, shapes.

FIGURE 5.3

Examples of SiC, fused alumina, and fused alumina-zirconia grain types.

Silicon carbide

White alumina

Alumina-Zirconia



79

5.4.6 R

UBY

A

LUMINA

Ruby alumina has a higher chrome oxide content of 3% and is more friable than pink alumina.The grains are blocky, sharp-edged, and extremely cool cutting making them popular for tool roomand dry grind application on steels (e.g., ice skate sharpening). Vanadium oxide has also been usedas an additive giving a distinctive green hue.

5.4.7 Z

IRCONIA

-A

LUMINA

Zirconia is added to alumina to refine the grain structure and produce a tough abrasive. At leastthree different zirconia-alumina compositions are used in grinding wheels:

• 75% Alox, 25% ZrO

2

• 60% Alox, 40% ZrO

2

• 65% Alox, 30%ZrO

2

, 5%TiO

2

Manufacture usually includes rapid solidification to enhance the nature of the grain structure.The resulting abrasives are fine grain, extremely tough, and give excellent life in medium to heavystock removal applications such as billet grinding in foundries.

5.4.8 S

INGLE

C

RYSTAL

W

HITE

A

LUMINA

Grain growth is closely controlled in a sulphide matrix. The alumina is separated out by acidleaching without crushing. The grain shape is nodular, which aids bond retention, while theelimination of crushing reduces mechanical defects from processing.

5.4.9 P

OSTFUSION

P

ROCESSING

M

ETHODS

As mentioned above, the type of particle reduction method can greatly affect the resulting grainshape. Impact crushers like hammer mills will create a blocky shape, while roll crushers will causemore splintering. It is further possible using electrostatic forces to separate sharp shapes fromblocky grains to provide grades of the same composition but very different cutting action.

5.4.10 P

OSTFUSION

H

EAT

T

REATMENT

The performance of an abrasive can also be altered by heat treatment, particularly for brownalumina. The grit is heated to 1,100

°

C to 1,300

°

C, depending on grit size, in order to anneal cracksand flaws created by the crushing process. This can enhance toughness by 25 to 40%.

5.4.11 P

OSTFUSION

C

OATINGS

Finally, several coating processes exist to improve bonding of the grains in the grinding wheel.Red iron oxide is applied at high temperature to increase surface area for better bonding in resincutoff wheels. Silane is applied for some resin bond wheel applications to repel coolant infiltrationbetween bond and abrasive grit and thus protect the resin bond.

5.5 CHEMICAL PRECIPITATION AND/OR SINTERING OF ALUMINA

5.5.1 I

MPORTANCE

OF

C

RYSTAL

S

IZE

A limitation of the electrofusion route is that the resulting abrasive crystal structure is very large;an abrasive grain may consist of only one to three crystals. Consequently, when grain fracture


80


occurs, the resulting particle loss may be a large proportion of the whole grain. This results ininefficient grit use. One way to avoid this is to dramatically reduce the crystallite size.

5.5.2 M

ICROCRYSTALLINE

G

RITS

The earliest grades of microcrystalline grits were produced in 1963 (U.S. Patent 3,079,243) bycompacting a fine-grain bauxite slurry, granulating to the desired grit size, and sintering at 1,500

°

C.The grain shape and aspect ratio could even be controlled by extruding the slurry.

5.5.3 S

EEDED

G

EL

A

BRASIVE

The most significant development, however, probably since the invention of the Higgins furnace,was the release in 1986 of SG (seeded gel) abrasive by The Norton Company (U.S. Patents 4,312,8271982; 4,623,364 1986). This abrasive was a natural outcome of the wave of technology sweepingthe ceramics industry at that time to develop high strength engineering ceramics using chemicalprecipitation methods. In fact, this class of abrasives is commonly termed “ceramic.” SG is producedby a chemical process whereby MgO is first precipitated to create 50-nm-sized alumina-magnesiaspinel seed crystals in a precursor of boehmite. The resulting gel is dried, granulated to size, andsintered at 1,200

°

C. The grains produced are composed of a single-phase

α

-alumina structure witha crystallite size of about 0.2 µm. Again, defects from crushing are avoided; the resulting abrasiveis unusually tough but self-sharpening because fracture now occurs at the micron level.

5.5.4 A

PPLICATION

OF

SG A

BRASIVES

As with all new technologies, it took significant time and application knowledge to understand howto apply SG. The abrasive was so tough that it had to be blended with regular fused abrasive atlevels as low as 5% to avoid excessive grinding forces. Typical blends are now

• 5SG (50%) • 3SG (30%) • 1SG (10%)

These blended abrasive grades can increase wheel life by up to a factor of 10 over regularfused abrasives although manufacturing costs are also higher.

The grain shape can also be controlled to surprising extremes by the granulation processesadopted. The shape can be varied from the very blocky to the very elongated as illustrated inFigure 5.4.

5.5.5 S

OL

G

EL

A

BRASIVES

In 1981, actually prior to the introduction of SG, 3M Company introduced a sol-gel abrasivematerial they called Cubitron for use in coated abrasive fiber discs. This was again a submicronchemically precipitated and sintered material, but unlike SG, was a multiphase composite structurethat did not use seed grains to control crystallite size. The value of the material for grinding wheelapplications was not recognized until after the introduction of SG. After protracted patent litigationand settlement with Norton, Cubitron is now used by most other wheel makers. In the manufactureof Cubitron, alumina is coprecipitated with various modifiers such as magnesia, yttria, lanthana,and neodymia to control microstructural strength and surface morphology upon subsequent sinter-ing. For example, one of the most popular materials, Cubitron 321, has a microstructure that containssubmicron platelet inclusions, which act as reinforcements somewhat similar to a whisker-reinforcedceramic [Bange and Orf 1998].



81

5.5.6 C

OMPARISON

OF

SG

AND

C

UBITRON

A

BRASIVES

Direct comparison of the performance of SG and Cubitron is difficult because the grain is merelyone component of the grinding wheel. SG is harder (21 GPa) than Cubitron (19 GPa). Anecdotalevidence in the field suggests that wheels made from SG give longer life but Cubitron is freer-cutting. This can make Cubitron the preferred grain in some applications but, from a cost/perfor-mance point of view, it is, therefore, also currently more prone to challenge from a well-engineered(i.e., shape-selected) fused grain that is the product of a lower cost, mature technology.

5.5.7 E

XTRUDED

SG A

BRASIVE

SG grain shape can also be controlled by extrusion. Norton has taken this concept to an extremeand in 1999 introduced TG and TG2 (extruded SG) grains in products called Targa and ALTOS.TG grain had an aspect ratio of 4:1, while TG2 had an aspect ratio of 8:1. TG2 grains have theappearance of rods or “worms” due to these high aspect ratios. The resulting natural packing

FIGURE 5.4

Examples of seeded gel abrasive grain shapes. (Courtesy of Saint-Gobain Abrasives. With permission.)


82


characteristics of these shapes in a grinding wheel result in a high-strength, lightweight structurewith porosity levels as high as 70% or even greater. The grains touch each other at only a fewpoints where bond also concentrates like “spot welds.” The product offers potential for both higherstock removal rates and higher wheel speeds due to the strength and density of the resulting wheelbody [Klocke, Mueller, and Englehorn 2000].

5.5.8 F

UTURE

T

RENDS

FOR

C

ONVENTIONAL

A

BRASIVES

With time, it is expected that SG, TG/TG2, Cubitron, and other emerging chemical precipitation/sin-tering processes will increasingly dominate the conventional abrasive market. The production ofelectrofused product is likely to shift more and more from traditional manufacturing sites withgood availability of electricity, such as around the Great Lakes of the United States and Norway,to lower cost, growing economies such as China and Brazil.

5.6 DIAMOND ABRASIVES

5.6.1 N

ATURAL

AND

S

YNTHETIC DIAMONDS

Diamond holds a unique place in the grinding industry. Being the hardest material known it is notonly the abrasive choice for grinding the hardest, most difficult materials, but also it is the onlymaterial that can truly address all abrasive wheels effectively. Diamond is the only wheel abrasivethat is still obtained from natural sources. Although synthetic diamond dominates in wheel manu-facture, natural diamond is preferred for dressing tools and form rolls. Diamond materials are alsoused increasingly as wear surfaces for applications such as end stops and work-rest blades ongrinding machines. In these types of applications, diamond can give 20 to 50 times the life oftungsten carbide.

FIGURE 5.5 Examples of ceramic grain processing microstructures.

C D

BA

A – Unseeded pure alumina-sintered gel with large uncontrolled grain growth

B – Norton SG alumina with controlled microstructure

C – Unseeded sintered alumina gel with magnesia additions

D – 3M cubitron 321 with magnesia and rare earth oxide additions

DK4115_C005.fm 2 Thursday, November 9, 2006 5:17 PM

The Nature of the Abrasive 83

5.6.2 ORIGIN OF DIAMOND

Diamond is created by the application of extreme high temperatures and pressures to graphite.Such conditions occur naturally at depths of 120 miles in the upper mantle or in heavy meteoriteimpacts. Diamond is mined from Kimberlite pipes that are the remnant of small volcanic fissurestypically 2 to 50 m in diameter where magma has welled up in the past. Major producing areasof the world include South Africa, West Africa (Angola, Tanzania, Zaire, Sierra Leone), SouthAmerica (Brazil, Venezuela), India, Russia (Ural Mountains), Western Australia, and mostrecently Canada. Each area, and even each individual pipe, will produce diamonds with distinctcharacteristics.

5.6.3 PRODUCTION COSTS

Production costs are high, with 13 million tons of ore, on average, processed to produce 1 ton ofdiamonds. Much of this cost is supported by the demand for diamonds by the jewelry trade. SinceWorld War II, the output of industrial grade diamond has been far outstripped by demand. Thisspurred the development of synthetic diamond programs initiated in the late 1940s and 1950s[Maillard 1980].

FIGURE 5.6 Norton TG2 abrasive grain and Altos Wheel Structure. (From Norton 1999a, 1999b. Withpermission.)

Loose grain appearance

Wheel structural appearance


84 Handbook of Machining with Grinding Wheels

5.6.4 THREE FORMS OF CARBON

The stable form of carbon at room temperature and pressure is graphite, where the carbon atomsare arranged in a layered structure. Within the layer, atoms are positioned in a hexagonal lattice.Each carbon atom is bonded to three others in the same plane with the strong sp3 covalent bondingrequired for a high hardness material. However, bonding between the layers is weak, being generatedfrom Van de Waals forces only, and results in easy slippage and low friction. (In fact, pure graphiteis highly abrasive because, although there is low friction between the layers, the edges of individualsheets have dangling bonds that are highly reactive. It is only the presence of water vapor in theair of dopants added to the graphite that neutralizes these sites and makes graphite a low-frictionsurface). Diamond, which is meta-stable at room temperature and pressure, has a cubic arrangementof atoms with pure sp3 covalent bonding with each carbon atom bonded to four other carbon atoms.There is also an intermediate material called wurtzite or hexagonal diamond where the hexagonallayer structure of graphite has been distorted above and below the layer planes but not quite to thefull cubic structure. The material is nevertheless almost as hard as the cubic form.

FIGURE 5.7 Common structures of carbon.

FIGURE 5.8 Phase diagram for carbon.

DiamondGraphite Wurtzite

3000250020001500100050000

20

40

60

80

100

Pres

sure

(K b

ar)

Temperature (K)

Melting line formetal solvent

Graphite stablediamond metastable

Diamond growtharea

Diamond stablegraphite metastable

Melting line formetal

solvent/carbon



5.6.5 THE SHAPE AND STRUCTURE OF DIAMOND

The principal crystallographic planes of diamond are the cubic (100), dodecahedron (011), andoctahedron (111). The relative rates of growth on these planes are governed by the temperatureand pressure conditions, together with the chemical environment during both growth and, in thecase of natural diamond, possible dissolution during its travel to the earth’s surface. This, in turn,governs the diamond stone shape and morphology.

The phase diagram for diamond/graphite is shown in Figure 5.8.

5.6.6 PRODUCTION OF SYNTHETIC DIAMOND

The direct conversion of graphite to diamond requires temperatures of 2,500 K and pressures of>100 Kbar. Creating these conditions was the first hurdle to producing man-made diamonds. TheGeneral Electric Company (GE) achieved this through the invention of a high-pressure/temperaturegasket called the “belt” and announced the first synthesis of diamond in 1955. Somewhat to theirsurprise, it was then announced that a Swedish company, ASEA, had secretly made diamonds 2years previously using a more complicated six-anvil press. ASEA had not announced the factbecause they were seeking to make gems and did not consider the small brown stones they producedthe culmination of their program! De Beers announced their ability to synthesize diamonds shortlyafter GE in 1958.

The key to manufacture was the discovery that a metal solvent such as nickel or cobalt couldreduce the temperature and pressure requirements to manageable levels. Graphite has a highersolubility in nickel than diamond has; therefore, at the high-process temperatures and pressures thegraphite dissolves in the molten nickel and diamond then precipitates out. The higher the temper-atures, the faster is the precipitation rate and the greater the number of nucleation sites. The earliestdiamonds were grown fast at high temperatures and had weak, angular shapes with a mosaicstructure. This material was released by GE under the trade name RVG, for “Resin VitrifiedGrinding” wheels. Most of the early patents on diamond synthesis have now expired and competitionfrom emerging economies has driven down the price of this type of material to as little as $400/lb,although quality and consistency from these sources are still often sometimes questionable.

5.6.7 CONTROLLING STONE MORPHOLOGY

By controlling the growth conditions, especially time and nucleation density, it is possible to growmuch higher quality stones with well-defined crystal forms: cubic at low temperature, cubo-octahedra at intermediate temperatures, and octahedra at the highest temperatures. The diagram forgrowth morphologies of diamond is shown in Figure 5.9.

The characteristic shape of good-quality natural stones is octahedral, but the toughest stoneshape is cubo-octahedral. Unlike in nature, this can be grown consistently by manipulation of thesynthesis process. This has led to a range of synthetic diamond grades typified by the MBG seriesfrom GE and the PremaDia series from De Beers [1999], which are the abrasives of choice for sawsused in the stone and construction industry and for glass grinding wheels.

FIGURE 5.9 Growth morphologies of diamond.

Octahedral → →Cubo–octahedral Cubic



5.6.8 DIAMOND QUALITY MEASURES

The quality and price of the diamond abrasive grain grade is governed both by the consistency ofshape and the level of entrapped solvent in the stones. Since most of the blockiest abrasive is usedin metal bonds processed at high temperatures, the differential thermal expansion of metal inclusionsin the diamond can lead to reduced strength or even fracture. Other applications require weakerphenolic or polyimide resin bonds processed at much lower temperatures and use more angular,less thermally stable diamonds. Grit manufacturers, therefore, characterize their full range ofdiamond grades by room temperature toughness (TI), thermal toughness after heating at, forexample, 1,000°C (TTI), and shape (blocky, sharp, or mosaic). Included in the midrange, sharpgrades are both crushed natural as well as synthetic materials.

5.6.9 DIAMOND COATINGS

Diamond coatings are common. One range includes thick layers or claddings of electroplated nickel,electroless Ni-P, copper, or silver at up to 60%wt. The coatings behave as heat sinks, while increasingbond strength and keeping abrasive fragments from escaping. Electroplated nickel, for example,produces a spiky surface that provides an excellent anchor for phenolic bonds when grinding wet.Copper and silver bonds are used more for dry grinding, especially with polyimide bonds, wherethe higher thermal conductivity outweighs the lower strength of the coating [Jakobuss 1999].Attention should be paid to wheel Material Data Safety Sheets (MSDS) to confirm chemicalcomposition to ensure any coating used does not present a contaminant problem. For example,silver contamination may be a problem in grinding of titanium alloys.

Coatings can also be applied at the micron level either as a wetting agent or as a passive layerto reduce diamond reactivity with the particular bond. Titanium is coated on diamonds used innickel-, cobalt-, or iron-based bonds to limit graphitization of the diamond while wetting the

FIGURE 5.10 Typical diamond grit shapes, morphologies, and coatings. (From De Beers.)

Blocky cubo-octahedral High strength synthetic Sharp medium-strengthnatural processed

Sharp medium-strengthsynthetic processed

Low-strength synthetic mosaic



diamond surface. Chromium is coated on diamonds used in bronze- or WC-based bonds to enhancechemical bonding and reactivity of the diamond and bond constituents.

Finally, for electroplated bonds, the diamonds are acid etched to remove any surface nodulesof metal solvent that would distort the plating electrical potential on the wheel surface leading touneven nickel plating or even nodule formation. It also creates a slightly rougher surface to aidmechanical bonding.

5.6.10 POLYCRYSTALLINE DIAMOND (PCD)

Since 1960, several other methods of growing diamond have been developed. In 1970, DuPontlaunched a polycrystalline material produced by the sudden heat and pressure of an explosive shock.The material was wurtzitic in nature and produced mainly at micron particle sizes suitable morefor lapping and polishing than grinding or as a precursor for PCD monolithic material.

In 1970, PCD (Poly Crystalline Diamond) blanks were introduced that consisted of a fine grainsintered diamond structure bonded to a tungsten carbide substrate. The material was produced bythe action of high temperatures and pressures on a diamond powder mixed with a metal solvent topromote intergrain growth. Since it contained a high level of metal binder it could be readilyfabricated in various shapes using electrodischarge machining (EDM) technology. Although notused in grinding wheels, it is popular as reinforcement in form dress rolls and for wear surfaceson grinding machines. Its primary use, though, is in cutting tools.

FIGURE 5.11 Effect of coating on surface morphology of diamond grain. (From GE superabrasives. Withpermission.)

FIGURE 5.12 Examples of shock wave–produced diamond grains. (From Saint Gobain Ceramics. Withpermission.)

RVG uncoated RVG with 60 wt% electroplated nickel

Shock wave diamond grain

Acc.V Spot Magn30.0 kV 2.0 39800x SE 6.4 0.11 SPD-RF

500 nmWDDet Acc.V Spot Magn30.0 kV 1.8 51200x SE 4.9 926U-10

500 nmWDDet0

Exp

76.8 nm

34.5 nm

Ultra detonated diamond UDD



5.6.11 DIAMOND PRODUCED BY CHEMICAL VAPOR DEPOSITION (CVD)

In 1976, reports began to come out of Russia of diamond crystals being produced at low pressuresthrough Chemical Vapor Deposition. This was treated with some skepticism in the West even thoughRussia had a long history of solid research on diamond. However, within 5 years, Japan was alsoreporting rapid growth of diamond by CVD at low pressures and the product finally became availablein commercial quantities by about 1992. The process involves reacting a carbonaceous gas in thepresence of hydrogen atoms in near vacuum to form the diamond phase on an appropriate substrate.Energy is provided in the form of hot filaments or plasmas at >800°C to dissociate the carbon andhydrogen into atoms. The hydrogen interacts with the carbon and prevents any possibility of graphiteforming while promoting diamond growth on the substrate. The resulting layer can form to athickness of >1 mm.

5.6.12 STRUCTURE OF CVD DIAMOND

CVD diamond forms as a fine crystalline columnar structure. There is a certain amount of preferredcrystallographic orientation exhibited; more so than, for example, PCD, but far less than in singlecrystal diamond. Wear characteristics are therefore much less sensitive to orientation in a tool. Again,the CVD diamond is not used as an abrasive but is proving very promising when fabricated in theform of needle-shaped rods for use in dressing tools and rolls. Fabrication with CVD is slightly moredifficult as it contains no metal solvents to aid EDM wire cutting and diamond wetting also appearsmore difficult and must be compensated for by the use of an appropriate coating.

5.6.13 DEVELOPMENT OF LARGE SYNTHETIC DIAMOND CRYSTALS

In the last 10 years, increasing effort has been placed on growing large synthetic diamond crystalsat high temperatures and pressures. The big limitation has always been that press time and hence

FIGURE 5.13 Chemical vapor deposition, diamond samples, and microstructure. (From Gigel 1994. Withpermission.)

Cross section As-deposited surface



cost goes up exponentially with diamond size. The largest saw grade diamonds are typically 30to 40#. The production of larger stones in high volume, suitable for tool and form-roll dressingapplications, is not yet cost-competitive with natural diamond. However, there has recently provedto be an exception to this, namely, the introduction, first by Sumitomo, of needle diamond rodsproduced by the slicing up of large synthetic diamonds. The rods are typically less than 1 mmin cross section by 2 to 5 mm long (similar in dimensions to the CVD diamond rods discussedabove) but orientated along the principal crystallographic planes to allow optimized wear andfracture characteristics when orientated in a dressing tool. Several companies now supply asimilar product.

5.6.14 DEMAND FOR NATURAL DIAMOND

Even with the dramatic growth in synthetic diamond, the demand by industry for natural diamondhas not declined. If anything, the real cost of natural diamond has actually increased especially forhigher quality stones. The demand for diamonds for jewelry is such that premium stones used in the1950s for single-point diamonds are now more likely to be used in engagement rings; while verysmall gem quality stones once considered too small for jewelry and used in profiling dressing discs,are now being cut and lapped in countries such as India. With this type of economic pressure it isnot surprising that the diamonds used by industry are those rejected by the gem trade because ofcolor, shape, size, crystal defects such as twins or naats, or excessive inclusion levels; or are theprocessed fragments from, for example, cleaving gems. Although significant quantities of processedmaterial are still used in grinding wheel applications, it is the larger stones used in single-point andform-roll dressing tools that are of most significance. Here the quality of the end product depends onthe reliability of the diamond source and of the ability of the tool maker to sort diamonds accordingto requirements. The highest quality stones will be virgin as-mined material. Lower quality stonesmay have been processed by crushing and/or ball-milling, or even reclaimed from old form dressingrolls or drill bits where they had previously been subjected to high temperatures or severe conditions.

5.6.15 FORMS OF NATURAL DIAMOND

Natural diamond grows predominantly as the octahedral form that provides several sharp pointsoptimal for single-point diamond tools. It also occurs in a long-stone form, created by the partialdissolution of the octahedral form as it ascended to the Earth’s surface. These are used in dressingtools such as the Fliesen blade developed by Ernst Winter & Son. It should be noted, though, thatlong-stone shapes are also produced by crushing and ball-milling of diamond fragments; these willhave introduced flaws which significantly reduce strength and life. The old adage of “you get whatyou pay for” is very pertinent in the diamond tool business!

Twinned diamond stones called maacles also occur regularly in nature. These are typicallytriangular in shape. The twinned zone down the center of the triangle is the most wear-resistantsurface known and maacles are used both in dressing chisels as well as reinforcements in the mostdemanding form-roll applications.

5.6.16 HARDNESS OF DIAMOND

The hardness of diamond is a difficult property to define for two reasons. First, hardness is ameasure of plastic deformation but diamond does not plastically deform at room temperature.Second, hardness is measured using a diamond indenter. Measuring hardness in this case is,therefore, akin to measuring the hardness of soft butter with an indenter made of hard butter!Fortunately, the hardness of diamond is quite sensitive to orientation and using a Knoop indenter;a distorted pyramid with a long diagonal seven times the short diagonal, orientated in the hardest



direction, gives somewhat repeatable results. The following hardness values have been obtained[Field 1983]:

(001) plane. [110] direction. 10,400 kg/mm2



5.6.17 WEAR RESISTANCE OF DIAMOND

More important than hardness is mechanical wear resistance. This is also a difficult property to pindown because it is so dependent on load, material, hardness, speed, and so on. Wilks and Wilks[1972] showed that when abrading diamond with diamond abrasive, wear resistance increases withhardness but the differences between orientations are far more extreme. For example, on the cubeplane, the wear resistance between the [100] and the [110] directions varies by a factor of 7.5,giving good correlation with wear data of needle diamonds reported in Figure 5.14. In other planes,the differences were as great as a factor 40, sometimes with only relatively small changes in angle.Not surprisingly, diamond gem lappers often speak of diamond having “grain”-like wood. Factorsregarding the wear resistance of diamond on other materials in a machining process such as grinding,however, must include all possible attritious wear processes including thermal and chemical.

5.6.18 STRENGTH OF DIAMOND

Diamond is very hard and brittle. It can be readily cleaved along its four (111) planes. Its measuredstrength varies widely due in part to the nature of the tests, but also because it is heavily dependenton the level of defects, inclusions, and impurities present. Not surprisingly, small diamonds (withsmaller defects) give higher values for strength than larger diamonds. The compressive strength oftop-quality synthetic diamond (100#) grit has been measured at 1,000 kg.mm−2.

5.6.19 CHEMICAL PROPERTIES OF DIAMOND

The diamond lattice is surprisingly pure, as the only other elements known to be incorporated arenitrogen and boron. Nitrogen is present in synthetic diamonds at up to 500 parts per million in

FIGURE 5.14 Monocrystal diamond needles cut with controlled crystallographic orientation for enhancedrepeatability and life in dressing tools. (From De Beers 1993. With permission.)

(100)

(100)(100)

(100)

(100)

(100) W

TL

(De Beers)

50

50

28

28

12

12

7

Relative wear rates as functionof needle orientation for rotary

diamond truers made with orientateddiamond needles



single substitutional sites and gives the stones their characteristic yellow/green color. Over anextended time at high temperature and pressure, the nitrogen migrates and forms aggregates, andthe diamond becomes the colorless stone found in nature. Synthetic diamond contains up to 10%included metal solvents, while natural diamond usually contains inclusions of the minerals in whichit was grown (e.g., olivine, garnet, and spinels).

5.6.20 THERMAL STABILITY OF DIAMOND

Diamond is metastable at room temperatures and pressures and it will convert to graphite given asuitable catalyst or sufficient energy. In a vacuum or in inert gas, diamond remains unchanged up

FIGURE 5.15 Natural industrial Diamonds. (From Henri Polak Diamond Corp. 1979. With permission.)

Premium dressing stones Lower quality/processed dressing stones

Premium long stones Lower quality/processed long stones

Maacles Ballas



to 1,500°C; in the presence of oxygen it will begin to degrade at 650°C. This factor plays asignificant role in how wheels and tools are processed in manufacturing.

5.6.21 CHEMICAL AFFINITY OF DIAMOND

Diamond is readily susceptible to chemical degradation from carbide formers, such as tungsten,tantalum, titanium, and zirconium, and true solvents of carbon, which include iron, cobalt, man-ganese, nickel, chromium, and the Group VIII platinum and palladium metals.

5.6.22 EFFECTS OF CHEMICAL AFFINITY IN MANUFACTURE

This chemical affinity can be both a benefit and a curse. It is a benefit in the manufacture of wheelsand tools where the reactivity can lead to increased wetting and, therefore, higher bond strengthsin metal bonds. For diamond tool manufacture, the reactant is often part of a more complex eutecticalloy (e.g., copper-silver, copper-silver-indium, or copper-tin) in order to minimize processingtemperature, disperse and control the active metal reactivity, and/or allow simplified processing inair. Alternatively, tools are vacuum brazed. For metal-bonded wheels, higher temperatures and morewear-resistant alloy bonds are used but fired in inert atmospheres.

5.6.23 EFFECTS OF CHEMICAL AFFINITY IN GRINDING

The reactivity of diamond with transition metals such as nickel and iron is a major limitation tothe use of diamond as an abrasive for machining and grinding these materials. Thornton and Wilks[1978, 1979] showed that certainly in single-point turning of mild steel with diamond, chemicalwear was excessive and exceeded abrasive mechanical wear by a factor of 104. Hitchiner and Wilks[1987] showed that difference when turning nickel was >105. Turning pearlitic cast iron, however,the wear rate was only 102 greater. Furthermore, the wear on pearlitic cast iron was actually 20times less than that measured using CBN tools. Much less effect was seen on ferritic cast iron,which unlike the former material contained little free carbon; in this case, diamond wear increasedby a factor of 10 when turning workpieces of comparable hardness.

5.6.24 GRINDING STEELS AND CAST IRONS WITH DIAMOND

It is generally considered, as the before-mentioned results imply, that chemical-thermal degradationof the diamond prevents it being used as an abrasive for steels and nickel-based alloys, but thatunder certain circumstances free graphite in some cast irons can reduce the reaction betweendiamond and iron to an acceptable level. For example, in honing of automotive cast iron cylinderbores, which is performed at very similar speeds (2 m/s) and cut rates to that used in the turningexperiments mentioned above, diamond is still the abrasive of choice outperforming CBN by afactor of 10. However, at the higher speeds (80 m/s typical) and temperatures of cylindrical grindingof cast iron camshafts, the reverse is the case.

5.6.25 THERMAL PROPERTIES

Diamond has the highest thermal conductivity of any material with a value of 600 to 2,000 W/mKat room temperature, falling to 70 W/mK at 700°C. These values are 40 times greater than the thermalconductivity of alumina. Much is written in the literature of the high thermal conductivity of bothdiamond and CBN, and the resulting benefits of lower grinding temperatures and reduced thermalstresses. Despite an extremely high thermal conductivity, if the heat capacity of the material is lowit will simply get hot quickly! Thermal models for moving heat sources, as shown by Jaeger [1942],employ a composite transient thermal property. The transient thermal property is , where kis the thermal conductivity, ρ is the density, and c is the thermal heat capacity.

β ρ= k c. .



The value of for diamond is 6 × 104 W/m K compared to 0.3 to 1.5 × 104 W/m K for mostceramics, including alumina and SiC, and for steels. Copper has a value of 3.7 × 104 W/m K duein part to a much higher heat capacity than that of diamond. This may explain its benefit as acladding material and wheel filler material.

Steady-state conditions are quickly established during the grain contact time in grinding. Thisis because the heat source does not move relative to the grain. The situation is similar to rubbinga finger across a carpet. It is the carpet that sees the moving heat source and stays cool, rather thanthe finger that sees a constant heat source and gets hot! In grinding, the abrasive grain is like thefinger and the workpiece is like the carpet. In this case, it is the thermal conductivity of the grainthat governs the heat conducted by the grain rather than the transient thermal property [Rowe et al.1996]. For nonsteady conduction, a time-constant correction is given by Rowe and Black [Marinescuet al. 2004, Chapter 6]. The application of thermal properties to calculation of temperatures isdiscussed in more detail in Chapter 17 on external cylindrical grinding.

The coefficient of linear thermal expansion of diamond is 1.5 × 10−6/K at 100°C increasing to4.8 × 10−6/K at 900°C. The values are significant for bonded wheel manufacturers who must tryto match thermal expansion characteristics of bond and grit throughout the firing cycle.

For further details on the properties of diamond, see Field [1979, 1983].

5.7 CBN

5.7.1 DEVELOPMENT OF CBN

CBN is the final and most recent of the four major abrasive types, and the second hardest super-abrasive after diamond. Trade names include Borazon (from GE who first synthesized it commer-cially), Amborite and Amber Boron Nitride (after De Beers), or in Russian literature as Elbor,Cubonite, or β-BN.

Boron nitride at room temperatures and pressures is made using the reaction:

BCl3 + NH3 → BN + 3HCl

The resulting product is a white slippery substance with a hexagonal layered atomic structure calledHBN (or α-BN) similar to graphite but with alternating nitrogen and boron atoms. Nitrogen andboron lie on either side of carbon in the periodic table, and it was postulated that high temperaturesand pressures could convert HBN to a cubic structure similar to diamond. This was first shown tobe the case by a group of scientists under Wentdorf at GE in 1957. The first commercial productwas released 12 years later in 1969.

Both the cubic (CBN) and wurtzitic (WBN or γ-BN) forms are created at comparable pressuresand temperatures to those for carbon. Again, the key to successful synthesis was the selection ofa suitable solvent to reduce conditions to a more manageable level. The chemistry of BN was quitedifferent to carbon; for example, bonding was not pure sp3 but 25% ionic, and BN did not showthe same affinity for transition metals. The successful solvent/catalyst turned out to be any one ofa large number of metal nitrides, borides, or oxide compounds of which the earliest commercialone used (probably with some additional doping) was Li3N. This allowed economic yields at60 kbar, 1,600°C, and <15-min cycle times.

5.7.2 SHAPE AND STRUCTURE OF CBN

As with diamond crystal growth, CBN grain shape is governed by the relative growth rates on theoctahedral (111) and cubic planes. However, the (111) planes dominate and, because of the presenceof both B and N in the lattice, some (111) planes are positive terminated by B atoms and some arenegative terminated by N atoms. In general, B (111) plane growth dominates and the resulting

β



crystal morphology is a truncated tetrahedron. Twinned plates and octahedra are also common. Themorphology can be driven toward the octahedral or cubo-octahedral morphologies by further dopingand/or careful control of the pressure-temperature conditions.

5.7.3 TYPES OF CBN GRAINS

As with diamond, CBN grain grades are most commonly characterized by toughness and by shape.Toughness is measured both at room temperatures and at temperatures up to >1,000°C comparableto those used in wheel manufacture, the values being expressed in terms of a toughness index (TI)and thermal toughness index (TTI). The details of the measurement methods are normally propri-etary but, in general, grains of a known screened-size distribution are treated to a series of impactsand then rescreened. The fraction of grain remaining on the screen is a measure of the toughness.For TTI measurements, the grains may be heated in a vacuum or a controlled atmosphere or evenmixed with the wheel bond material, which is subsequently leached out. TI and TTI are bothstrongly influenced by doping and impurity levels. Additional degradation of the grain within thewheel bond during manufacture can also occur due to the presence of surface flaws that may beopened up by penetration of bond.

The surface roughness of CBN is a more pronounced and critical factor than for diamond interms of factors influencing grinding wheel performance. A rough angular morphology provides abetter, mechanical anchor. Of the examples illustrated in Figure 5.17, GE Type 1 abrasive is arelatively weak irregular crystal. The coated version GE Type II abrasive used in resin bonds hasa simple nickel-plated cladding. However, GE 400 abrasive is a tougher grain with a similar shape

FIGURE 5.16 Phase diagram for cubic boron nitride.

140

120

100

80

60

40

20

0 1000 2000 3000 4000 5000Temperature K

Pres

sure

kilo

bars

HBN stableCBN, WBN metastable

CBN, WBN stableHBN metastable

WBN growthpromoting

CBN growthpromoting

Liquid



but with much smoother, flaw-free faces. The coated version GE 420 is, therefore, first coated witha thin layer of titanium to create a chemically bonded roughened surface to which the nickelcladding can be better anchored.

Only a relatively few grades of CBN are tough and blocky with crystal morphologies shiftedaway from tetrahedral growth. The standard example is GE 500 used primarily in electroplatedwheels. De Beers also has material, ABN 600, where the morphology has been driven toward thecubo-octahedral.

5.7.4 MICROCRYSTALLINE CBN

Interestingly, GE also developed a grit-type GE 550 that is a microcrystalline product; this couldbe considered the “SG” of CBN grains. It is extremely tough and blocky and wears by microfrac-turing. However, just like SG grains, it also generates high grinding forces and is, therefore, limitedto use in the strongest bonds, such as bronze metal, for high force/grit applications, especiallyhoning. It has also been used in limited quantities in plated applications. One problem with itsmicrocrystalline nature is that the surface of GE 550 is much more chemically reactive with vitrifiedbonds.

5.7.5 SOURCES AND COSTS OF CBN

The manufacture of CBN has been dominated by GE in the United States, by De Beers fromlocations in Europe and South Africa, and by Showa Denko, Iljin, and Tomei from the Far East.Russia and Romania have also been producing CBN for over 30 years, and, more recently, Chinahas rapidly become an extremely important player. Historically, consistency has been in questionwith materials from some of these latter sources but with intermediate companies such as ABCAbrasives (Saint-Gobain Ceramics) controlling the QC aspects of the materials to the end user,they are becoming a very real low-cost alternative to traditional suppliers. It is, therefore, expectedthat CBN prices will be driven down over the next decade offering major new opportunities andapplications for CBN technology. Currently, CBN costs are of the order of $1,500 to $5,000/lb orat least three to four times that of the cheapest synthetic diamond.

5.7.6 WURTZITIC BORON NITRIDE

As with carbon, wurtzitic boron nitride (WBN) has also been produced by explosive shock methods.Reports of commercial quantities of the material began appearing about 1970 [Nippon Oil and Fats 1981],

FIGURE 5.17 Cubic boron nitride crystal growth planes and morphology. (After Bailey and Juchem, DeBeers 1998. With permission.)

(100) FaceB and N

(111) FaceB or N

alternating

Cleavageplane(110)

Cubooctahedral

Octahedral Tetrahedral



but its use has again been focused more on cutting tool inserts with partial conversion of the WBNto CBN, and this does not appear to have impacted the abrasive market.

5.7.7 HARDNESS OF CBN

The hardness of CBN at room temperature is approximately 4,500 kg/mm2. This is about half ashard as diamond and twice as hard as conventional abrasives.

5.7.8 WEAR RESISTANCE OF CBN

The differences in abrasion resistance are much more extreme. A hardness factor of 2 can translateinto a factor of 100 > 1,000 in abrasion resistance depending on the abrading material. The author

FIGURE 5.18 Examples of cubic boron nitride grain types and morphologies. (From General Electric 1998.With permission.)

GE type 1 weak, sharp, monocrystal

GE type II (Ge type 1 60% Ni coated) GE 420 (GE 400 Ti bonded, 60% Ni coated)

GE 500 very tough, microcrystallineGE 500 tough, blocky, monocrystal

GE 400 tough, sharp, monocrystal



remembers, as a research student under Wilks, when the first CBN samples were supplied forabrasion-resistance measurements using the same technique used for measuring the wear resistanceof diamond. The CBN was so soft in comparison to diamond that it was impossible to obtain avalue on the same wear scale. As with diamond, the key is the total wear resistance to all attritious-wear processes.

Like diamond, CBN is brittle, but it differs in having six (110) rather than four (111) cleavageplanes. This gives a more controlled breakdown of the grit especially for the truncated tetrahedralshape of typical CBN grains. The grain toughness is generally much less than that of blocky cubo-octahedral diamonds. This, combined with its lower hardness, provides the very useful advantage thatCBN wheels can be dressed successfully by diamond (rotary) tools.

5.7.9 THERMAL AND CHEMICAL STABILITY OF CBN

CBN is thermally stable in nitrogen or vacuum to at least 1,500°C. In air or oxygen, CBN formsa passive layer of B2O3 on the surface, which prevents further oxidation up to 1,300°C. However,this layer is reactive with water, or more accurately high temperature steam at 900°C, and willallow further oxidation of the CBN grains following the reactions [Carius 1989, Yang, Kim, andKim 1993]

2BN + 3H2O → B2O3 + 2NH3

BN + 3H2O → H3BO3 + N2 > 900°C4BN + 3O2 → 2B2O3 + 2N2 > 980°CB2O3. + 3H2O → 2H2BO3 > 950°C

5.7.10 EFFECT OF COOLANT ON CBN

Reactivity has been associated with reduced wheel life when grinding in water-based coolantscompared with straight-oil coolants. However, the importance of this reaction is not clear-cut aswater also inflicts a much higher thermal shock to the crystal as it is heat cycled through thegrinding zone. Regardless of root cause, the effect is dramatic as illustrated in Table 5.2, whichgives comparative life values for surface grinding with CBN wheels [Carius 2001].

CBN is also reactive toward alkali oxides—not surprising in light of their use as solvents andcatalysts in CBN synthesis! The B2O3 layer is particularly prone to attack or dissolution by basicoxides such as Na2O by the reaction

B2O3 + Na2O →. Na2B2O4 . . . . [GE Superabrasives 1988]

TABLE 5.1Mechanical Properties of Typical Alumina and SiC Abrasives

AbrasiveHardness

KnoopRelative

Toughness Shape/Morphology Applications

Green SiC 2840 1.60 Sharp/angular/glassy Carbide/ceramics/precisionBlack SiC 2680 1.75 Sharp/angular/glassy Cast iron/ceramics/ductile

nonferrous metalsRuby Alox 2260 1.55 Blocky/sharp-edged Hss and high-alloy steelWhite Alox 2120 1.75 Fractured facets/sharp Precision ferrousBrown Alox 2040 2.80 Blocky/faceted General purposeAlox/10% ZrO 1960 9.15 Blocky/rounded Heavy-duty grindingAlox/40% ZrO 1460 12.65 Blocky/rounded Heavy-duty snaggingSintered Alox 1370 15.40 Blocky/rounded/smooth Foundry billets/ingots



Such oxides are common constituents of vitrified bonds and the reactivity can become extreme attemperatures above 900°C affecting processing temperatures for wheels [Yang, Kim, and Kim 1993].

5.7.11 EFFECT OF REACTIVITY WITH WORKPIECE CONSTITUENTS

CBN does not show any significant reactivity or wetting by transition metals such as iron, nickel,cobalt, or molybdenum until temperatures reach in excess of 1,300°C. This is reflected in a lowrate of wear when grinding these materials with CBN abrasive in comparison with wear of diamondabrasive. CBN does show marked wetting by aluminum at only 1050°C and also with titanium. Asdemonstrated in wetting studies of low temperature silver–titanium eutectics, CBN reacts readilyat 1,000°C to form TiB2 and TiN [Benko 1995]. This provides an explanation of why in grindingaerospace titanium alloys such as Ti-6Al-4V, CBN wheels wear typically five times faster thandiamond wheels [Kumar 1990]. By comparison, the wear rate using the alternative of SiC abrasiveis 40 times greater than CBN. This is a further example of the need to consider the combinedeffects of the mechanical, chemical, and thermal wear processes as much as abrasive cost.

Pure, stoichiometrically balanced CBN material is colorless, although commercial grades areeither a black or an amber color depending on the level and type of dopants present. The blackcolor is believed to be due to an excess (doping) of boron.

5.7.12 THERMAL PROPERTIES OF CBN

The thermal conductivity of CBN is almost as high as that of diamond. At room temperature, thermalconductivity is 200 to 1,300 W/mK, and the transient thermal property = 2.0 × 104 to 4.8 × 104 J/m2sK.The thermal expansion of CBN is about 20% higher than diamond.

5.8 GRAIN SIZE DISTRIBUTIONS

Several national and international standards define particle size distributions of abrasive grains. Allare based on sizing by sieving in the sizes typical of most regular grinding applications.

5.8.1 THE ANSI STANDARD

In the case of the ANSI standard [B74.16 1995], mesh size is defined by a pair of numbers thatcorresponds to sieves with particular mesh sizes. The lower number gives the number of meshesper linear inch through which the grain can only just fall, while staying on the surface of the sievewith the next highest number of meshes which is the higher number.

TABLE 5.2 Effect of Coolant Type on CBN Wheel Performance

Workpiece

SyntheticLight Duty

2%

SolubleHeavy Duty

10%Straight

Oil

M2 X 1.7X 5XM50 X 4X 16XT15 X 1.7X 3XD2 X 1.3X 11X

52–100 X 10X 14X410 SS X 25X 44XIN 718 X 8X 50X

β


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