Jackson M.J. Micro and Nanomanufacturing

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Microgrinding 261

6.2 Grinding Wheels

Grinding wheels are made up of two materials, the abrasive grains

and the bonding material (Fig. 6.3). They are produced by mixing

the appropriate grain size of the abrasive with the required bond and

pressed into shape. The abrasive grains do the actual cutting and the

bond holds the grain together and supports them while they cut. The

cutting action of a grinding wheel is dependent on bonding materi-

als,

the abrasive type, grain size (grit size), wheel grades, and wheel

structure. Selection of the right combination of these features is es-

sential for optimum solution of different grinding tasks.

6.2.1 Bond Materials

Bonds are usually made of different types of raw materials and are

classified as follows [21,22]:

i. Vitrified materials (ceramics consist of glass,

feldspar and clay),

ii.

Resinoid materials (thermoset plastics -

phenolformaldehyde resin),

iii.

Rubber (both natural and synthetic),

iv. Shellac,

v. Metal (sintered powdered metals and electroplated -

bronze, nickel aluminum alloys, zinc, etc.)

vi.

Oxychloride (chemical action of magnesium

chloride and manganese).

vii.

Silicate (sodium silicate NaSiOs or water glass)

viii.

No bond

Vitrified bonds are also called ceramic bonds [23,24], which al-

low porosity of up to 55%, and the rigidity of this bond makes it

262 Micro- and Nanomanufacturing

possible to obtain high stock removal rates. Some cBN wheels are

made from this kind of bond for grinding steel. Compared to vitri-

fied and metal bonds, resinoid bonds furnish more flexibility while

grinding and thus produce a finer surface than the two. Both resin

and metal bonds are commonly used in the manufacture of diamond

and cBN wheels. Because of the resiliency of rubber, it makes them

excellent bond materials for polishing wheels and is used where burr

and burn must be held to a minimum. Rubber bond is also used for

thin flexible cut-off wheel application [21,23,25] and regulating

wheel in centerless grinding [21]. Shellac-bonded wheels are good

for producing high finish required in roll, camshaft, and cutlery

grinding [22]. Oxychloride bonds are considered to be the weakest

bond among the grinding wheel and used particularly for disc grind-

ers.

The wheels are cool cutting and seldom produce burn. Silicate

bond wheel can be used in operations that generate less heat. It is not

as strong as vitrified. Among all, silicate, shellac, and oxychloride

have limited uses [23].

6.2,2. Abrasive Types

Abrasive grains used for grinding wheels are very hard, highly re-

fractory materials and randomly oriented. Although brittle, these

materials can withstand very high temperatures. They have the abil-

ity to fracture into smaller pieces when the cutting force increases.

This phenomenon gives the abrasive its self-sharpening effect. Dur-

ing grinding, whenever dulling begins, the abrasive fractures thus

create new cutting points. The following four types of abrasives are

commonly used:

i. Aluminum oxide, or alumina (AI2O3)

ii.

Silicon carbide (SiC)

iii.

Cubic boron nitride (cBN)

iv. Diamond

Microgrinding 263

Aluminum oxide and silicon carbides are known as conventional

abrasives while cBN and diamond are known as superabrasives.

Aluminum oxide wheel is usually used for grinding metals such as

carbon steel, alloy steel, high-speed steel, annealed malleable iron,

wrought iron and bronzes, and similar metals. On the other hand,

silicon carbide wheel is harder but more brittle than alumina and

commonly used to grind low tensile strength materials such as gray

iron, chilled iron, brass, soft bronze and aluminum, as well as

stone/marble, rubber, leather and other non-ferrous metals [26,27].

Diamond wheels are suitable for machining non-ferrous metal while

cBN is normally good for grinding ferrous metal. However, the lat-

ter also used for grinding titanium alloys and the performance is bet-

ter than SiC and AI2O3 wheels. Aluminum oxide wheel is often re-

placed by cBN for hardened steel (>45HRc), superalloys (nickel,

cobalt, or iron base with hardness greater than 35HRc), high-speed

steels and cast iron. cBN has four times the abrasion resistance of

aluminum oxide. The high thermal conductivity of cBN prevents

heat buildup and associated problems such as wheel glazing and

workpiece metallurgical damage [27]. Some properties of these

abrasives compared to hardened steel and glass are shown in Table

6.2

Table 6.2 shows that diamond has promising properties compared

to the three other abrasives. One of the unique properties of diamond

that stands out is its extreme hardness. The hardness of this material

gave it the greatest electrical resistance and thermal conductivity of

all known substances. Also, it is chemically inert. Chemical

inertness normally prevents the diamond from bonding to or reacting

with other substances [32]. For these reasons it is the most desirable

abrasive for many applications, but there are limitations to its

usefulness other than cost. The surface chemistry of diamond limits

it usefulness in certain conditions. Diamond is carbon and at high

enough temperatures will burn, or will react with carbide-forming

metals. If either event occurs to any significant extent, the diamond

is lost. The service conditions that are required to avoid such losses

are low temperatures and avoidance of carbide-forming metals

except at temperatures close to room temperature such as in lapping

and polishing operations. The high thermal conductivity of diamond

helps to relieve the problem by conducting heat away [33].

264 Micro- and Nanomanufacturing

Table 6.2. Properties of some hard and brittle materials [23,28-31]

Material Melting Thermal Hardness Density

point (°C) conductivity Knoop (kg/m

)

(W/m.K) (kg/mm

)

Hardened

steel

Glass

Aluminum

oxide

Silicon

carbide

Cubic boron

nitride

Diamond

1371-

1532

350 - 750

2040

2050

2830

2500

3200

3700

15-52

0.6-1.7

63 -155

1300

2000

700-

300-

2000

2100-

4000-

7000 -

1300

-810

-3000

-5000

•8000

6920-

9130

2270-

6260

4000-

4500

3100

3480

3500

Diamond is excellent for machining non-ferrous metal (such as

copper, zinc, aluminum and their alloys), plastics, ceramics, glass,

fiberglass bodies, graphite, and other highly abrasive materials [34].

Though diamonds are very hard they wear out when machining

steel, titanium alloys, and stainless steel because they consist of pure

carbon. Carbon in diamond dissolves in y-Fe at a high rate at a tem-

perature higher than 900°C [3]. Diamond also is not particularly ef-

fective for machining superalloys that contain cobalt or nickel,

probably for the same reason stated earlier

[3,35].

A recent study on

grinding wear mechanisms showed that cBN wheel is superior to

AI2O3 and SiC wheel due to greater chemical stability of cBN at

higher temperatures when grinding titanium alloy (Ti6A14V) and

nickel-based alloy (K417) [36].

Microgrinding 265

6.2.3 Abrasive Grain Size

The size of an abrasive grain is identified by a number, which nor-

mally is a function of mesh width of the sieve size either in micron

or mesh openings per inch. Table 6.3 shows the equivalent grain

size used by FEPA (micron), ASTM 11 (inches), ISO, and DIN (mi-

cron) standard both for diamond and cBN wheels. In the metric sys-

tem (microgram size), the smaller the number the smaller the grain

size.

However, the coding is reversed in the imperial system in

which smaller number represents coarser grain size.

6.2.4 Mechanical Grade

The grade of a grinding wheel refers to its strength in holding the

abrasive grains in the wheel. This is largely dependent on the

amount of bonding material used. As the amount of bonding mate-

rial is increased, the linking structure between grains becomes lar-

ger, which makes the wheel act harder. A hard wheel has a stronger

bond than a soft wheel. The type and the amount of bond in the

wheel also influence the overall strength. In standard marking sys-

tems,

the grade of the grinding wheel is labeled by A-Z (soft to

hard).

6.2.5 Abrasive Structure

The structure of a grinding wheel represents the grain spacing and is

a measure of the porosity of a bonded abrasive wheel. Figure 6.3 il-

lustrates the structure of a grinding wheel, showing bigger pore areas

(voids) in open structure than medium and dense structure. Porosity

allows clearance space for the grinding chips to be removed for

proper cutting action during grinding operations. This clearance

space must not be too small or the chip will stay in the wheel, caus-

ing what is called wheel loading. A loaded cutting wheel heats up

and is not efficient in its cutting action. When this happens, frequent

dressing is needed to remove loaded workpiece particles off the

266 Micro- and Nanomanufacturing

wheel. On the other hand, too large a space is also inefficient, as

there will be too few cutting edges. A dense structure has stronger

grit holding power than an open structure. Some porosity is essential

in bonded wheels to provide not only clearance for the minute chips

being produced but also cooling; otherwise they would interfere with

the grinding process. In standard marking systems, the structure of

the wheel is labeled by numbers. Smaller numbers denote open

structure while dense structures are represented by higher numbers.

" bond material

(a) dense structure (b) medium structure (c) open structure

Fig. 6.3. Grinding wheel structure [38]

International efforts have been carried out to minimize variability of

grit spacing and projection height of the grain in order to make the

grinding process more predictable [39, 40]. As illustrated in Fig. 6.4,

grain depths of cut and space between grains are higher in (a) than

(b) and these are distinct advantages for effective grinding,

involving less loading and heat generation.

Microgrinding 267

Table 6.3. Equivalent international standard of grit sizes for diamond and cubic

boron nitride used by FEPA, US, DIN, and ISO standards compared to an indus-

trial designation [37]

International Standardization of Grit Sizes for Diamond and Cubic Boron Nitride

Sieve Grit Designations

Diamo

FEPA-Sta

WINTER des

narrow

D1161

D1001

D 851

D 711

D 601

D 501

D 426

D 356

D 301

D 251

D 213

D 181

f D 151

f D 128

Y D 107

TD 91

ID 78

TO 64

| D 54

[D 46

ridard

ignation

wide

D1182

D 852

D 602

D 427

D 252

FEPA-Sta

WINTER des

narrow

1181

81001

B 851

B 711

B 601

B 501

B 426

B 356

B 301

B 251

B 213

^B 181

^B 151

^B 126

B 107

^B 91

B 76

^B 64

B 54

^B 46

ndard

ignation

wide

1182

B 852

B 602

B 427

B 252

Diamond

US-Stan

ASTM-E-

narrow

16/

18/

20/

25/

30/35

35/

40/

45/

50/

60/

70/

80/100

100/120

120/140

140/170

170/200

200/230

230/270

270/325

325/400

+ CBN

dsrd

11-70

wide

16/20

20/30

30/40

40/50

Nominal mesh

size

ISO 6106 DIN 848

Part

1,1980

1180/1000

1000/

850

850/

710

710/

600

60a'

500

500/

425

42S'

355

im 300

30a'

250

250/

212

Wt 180

180/

150

150/

125

125'

106

106/

90/

75'

63/

53/

45/

38 |

Micron Grit Sizes']

Diamond

WINTHR

desig-

nation

D25

^D20B

^D20A

D15

D15C

^D15B

D15A

?*!

D 3

D 0,7

CBN

WINTER

desig-

nation

B30

B15

B 9

B 6

B 3

B 1

00,25 i

For

comparison

grit size

32-52

30-40 I

25-30

10-25

20-25

15-20 |

10-15

5-10

2-5 |

1-2 |

0,5-

<0,5 |

f -

Grits

recommended by '

WINTER |

*) Similar

FEPA Standard

exists with

designations

63...

M 1.0

FEPA

Federation

Europeenne

des

Fabncants

ProduitsAbrasifs.

268 Micro- and Nanomanufacturing

(a)

(b)

Fig. 6.4. Schematic diagrams show (a) ideal grain structure with controlled grain

spacing and projection height, (b) typical single-layer grinding wheel with random

grain spacing and projection height [41]

6.2.6 Abrasive Grain Concentration

While the percentages of grain, bond, and their spacing in the wheel

determine the wheel's structure, the concentration indicates the vol-

ume of diamond or cBN in the grinding layer. It is defined by the

percentage weight of the abrasive grit per cubic unit of the grinding

layer. For diamond, the basic value of CI00 means that every cm

layer volume contains 4.4 carats of diamond (1 ct = 0.2 g, diamond

density = 3.53 g/cm

), which is equivalent to 25% by volume of

diamond content in the grinding layer. As a general rule for selection

of the desired concentration, high concentrations are suitable for

small contact areas and low concentrations for large contact areas to

minimize wheel breakdown [37].

6.2.7 Grinding Wheel Design and Selection

Wheels used for grinding operations can range widely in their shape,

size and configuration. The successful application of grinding

wheels will depend both on a thorough understanding of the grind-

ing process and the wheel configurations.

Microgrinding 269

Figure 6.5 shows some of available shapes for conventional (left

side) and superabrasive (right side) wheels, respectively. It is clearly

seen that in this figure the entire shape of conventional wheels is of-

ten made of abrasives, whereas in contrast, only small sections of the

superabrasive wheel contain abrasives.

Since superabrasive wheels are very expensive compared to

conventional abrasive grains, it is not economical to make entire

wheels with abrasives, as is common for conventional wheels.

Hence, superabrasive wheels are often designed with a core section

that does not carry the abrasive. The abrasive forms a layer of a few

millimeters thick, and the outer section of

the

core and their shape is

partly regarded as a standard wheel configuration. Extra care should

be taken when deciding the grinding face during grinding operations

for different wheel configurations as it may damage/break the wheel

if the wrong side is engaged to the workpiece and thus subject the

operator to unsafe conditions. Apart from the wheel shape and con-

figurations, users must also take account of outside diameter, height,

width of

abrasive,

bore size, and other dimensions as necessary.

Bonded abrasives are marked with a standardized system of let-

ters and numbers, indicating the type of abrasive, grain size, grade,

structure, concentration, bond type and layer thickness of abrasive.

Table 6.4. shows one type of marking order in a block diagram that

is generally used for wheel marking system.

Table 6.4. Standard method of wheel marking order [42]

Marking

order

Grain/

abrasive

type

Grain

size

Grade

Structure

Bond

type

Manufacturer's

no.

(optional)

270 Micro- and Nanomanufacturing

Grinding

:kc

Type 1—«trai(jh*

ici p<Jriiidir((

bet

Type ^8—depressed cenler

LEFT

RIGHT

Fig.

6.5.

Pictures

on the

left show some common types

grinding wheels made

with conventional abrasives

[23]

while

on the

right

are

typical shapes

standard

metallic rim configurations for superabrasive wheels [37]

This coding system simplifies

all

technical specifications

the

bonded abrasive required

for the

wheel

and

normally

it is

designed

slightly different

for

conventional

and

superabrasive wheels

indi-

cated

Figs.

6.6

and 6.7.

the superabrasive wheel coding system,

the diamond/cBN concentrations replace structure

and

the thickness

of the abrasive layer

added

the end. Usually wheel coding sys-

tems

use the

letters

A, C, B, and D to

represent abrasive types

for

aluminum oxide, silicon carbide, cubic boron nitride,

and

diamond,

respectively. However, manufacturers also often use their own acro-

nym

distinguish variety

specific abrasive types available within

the same group. Table 6.5 lists some examples of these.