Jackson Mark. Machining with Abrasives

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process parameters on the proﬁling and grinding processes will be discussed in

detail later in Sect. 4.4.

4.2.1.2 Rotary Dressers

Compared with stationary dressers, rotary dressers have an additional movement

of rotation. The relative speed in the contact zone is determined by the peripheral

speed of both the dresser and the wheel. There are both diamond-free and diamond-

containing rotary dressers. Nowadays diamond-free rotary dressers are only used in

very few cases.

Rotary diamond dressers have a diamond layer on their periphery or front face.

Therefore, compared with stationary diamond dressers, these dressers contain

much more diamond, as a result of which they are subject to much less wear.

Therefore, with rotary diamond dressers the desired geomet ric and dimensional

accuracy of the wheel can be obtained over a much longer period of time. Further-

more, when proﬁling with rotary dressers, the topography of the grinding wheel,

due to the kinematics of the process can be inﬂuenced in a broader spectrum.

Common rotary diamond dressers are diamond proﬁle rollers, diamond form rollers

and diamond cup wheels (Fig. 4.4). In proﬁling with rotary diamond dressers, the

process results are also strongly inﬂuenced by the process parameter. These para-

meters and their inﬂuence on the dressing and grinding processes are discussed in

detail later in Sect. 4.5.

For the proﬁling of superabrasive grinding wheels with dressable types of

bond, SiC wheels can also be used [2]. Currently, diamond grinding wheels are

proﬁled predominantly with SiC wheels [10, 11]. When proﬁling with a SiC wheel

the bond of the wheel is cut back by the abrasive action of the SiC grits so that

the grits on the wheel that is to be dressed can fall out. The grit size of the SiC wheel

is between 80 and 30 US Mesh; a larger grit size is recommended for dressing

frd

profile roller

form roller cup wheel

fad

frd

fad

Fig. 4.4 Rotary dressers

4 Dressing of Grinding Wheels 185

diamond grinding wheels with a coarser grit [12]. After proﬁling with a SiC whe el

the diamond grinding wheel has a smooth surface, so in many cases additional

sharpening is needed [10, 11]. Due to the continuo us-path controlled movement of

the SiC wheel with an additional slewing motion, to a limited extent also non-

rectilinear wheel proﬁles can be proﬁled. To generate a corresponding relative

speed between the grinding wheel and dressing tool, the SiC wheel is either driven

off course or braked [12].

4.2.2 Sharpening

After proﬁling, because the bond and the grits are cut simultaneously, the grits

on the wheel surface exhibits little or no protrusion, particularly after the proﬁling

of resin bonded and metal bonded superabrasive wheels. In this condition the

wheel does not present a sufﬁcient cutting ability. After proﬁling, the next step is

sharpening. During this step, the grits of the top-most abrasive layer are exposed

by cutting back the bond, whereby the chip pocket needed for the grinding

process is produced (Fig. 4.5).

Sharpening can be carried out immediately after proﬁling or at the same time

as the proﬁling process. Simultaneous sharpening while proﬁling reduces the

wear on the dresser which is caused by the pressure and temperature during

cutting through the grits and the bond [2].

A superabrasive grinding wheel can be sharpened with loose or bonded alumina

or silicon carbide grits. Predominantly the sharpening process is carried out with

a vitriﬁed bonded alumina or silicon carbide block (called a sharpening block). The

Fig. 4.5 Change in topography caused by sharpening

186 T. Tawakoli and A. Rasifard

sharpening process can be carried out either by passing the wheel through

the sharpening block or by plunging the wheel into the sharpening block (Fig. 4.6).

During the jet sharpening, loose abrasive grits are blasted with air and/or

a carrier medium (usually coolant) at high pressure on to the abrasive layer of the

grinding wheel. The sharpening with loose abrasives causes intense contam ination

inside the grinding machine. Hence this process is hardly ever used these days [13].

The topography of the wheel and consequently the grinding process are

inﬂuenced by the sharpening conditions. Figure 4.7 shows the effect of speciﬁc

sharpening material removal rate Q

on the grinding process when grinding with a

resin bonded diamond wheel. Particularly high grinding forces occur with the

unsharpened diamond wheel. Elements that are responsible for this are the

very small chip pockets and the high cutting edge density because of the low

depth of surface smoothness, which lead to increasing grinding force. Furthermore,

the greater grit retention force due to the low grit protrusion leads to the diamond

grits remaining in the bond longer and the grits being ﬂattened as a result, which in

turn causes the grinding forces to increase. The extended period in which the grit

remains in the bond also leads to low radi al wear. The workpiece roughness is also

inﬂuenced strongly by speciﬁc sharpeni ng material removal rate. The smaller chip

thicknesses with greater cutting edge density, the greater covering of the grinding

marks and also the grit ﬂattening and the associated increase in the number of active

cutting edges are responsible for the better quality of the surfaces with a smoother

wheel topography, caused by a lower speciﬁc sharpening material removal rate [14].

Because of the porosity of vitriﬁed bonded superarasive wheels, they can also

be efﬁciently used in many cases without sharpening too. Figure 4.8 shows

the effect of sharpening at different dressing conditions. The vitriﬁed bonded

CBN wheel used here was dressed with a diamond form roller. It can be seen that

due to sharpening, the initial relief grinding phase is reduced. Relief grinding

(also called the initial self-sharpening) occurs when grinding with vitriﬁed bonded

superabrasive wheels. During the initial relief grinding phase, the chip pockets

between the grits in the vitriﬁed bond become lar ger due to the removal of bond

material by the abrasive action of the chips. The grinding process is unstable during

sharpening block

grinding wheel

fts

frs

Fig. 4.6 Sharpening with a sharpening block

4 Dressing of Grinding Wheels 187

Fig. 4.8 Inﬂuence of sharpening at different dressing conditions

Fig. 4.7 Inﬂuence of sharpening conditions on the deep grinding of RBSN [14] used with

permission copyright E. Uhlmann

188 T. Tawakoli and A. Rasifard

the relief grinding phase. On the other hand, however, for cost reasons it is desirable

to avoid a separate sharpening process. A stable grinding process after dressing

should therefore be achieved by appropriate selection of the dressing conditions. In

the tests shown in Fig. 4.8, sharpening did not have any signiﬁcant inﬂuence on

workpiece roughness.

4.2.3 Cleaning

The cleaning of a grinding wheel serves mainly to remove the loading chips from

the pores of a grinding whe el, but primarily without any intention to change the

geometry or topography of the wheel. Wheel loading impairs the chip-forming

process due to the reduction of the available chip spaces and the restriction of

coolant lubricant in the grinding contact zone. Hence the wheel loading results in

higher grinding forces and higher workpiece roughness as well as higher wheel

wear [15, 16]. The wear on the diamond dressers is also higher during dressing of a

loaded grinding wheel.

The tendency for the grinding wheel to get loaded depends on almost all the

variables involved in the grinding process and in particular on the workpiece

material. The loading occurs particularly when grinding ductile steels, aluminium

alloys or titanium alloys [2]. During the grinding of tough materials such as nickel-

chrome alloys, the grindin g wheel becomes also very heavily loaded. The coolant

that is used can also strongly affect loading. The use of water-miscible coolants

results in a higher grinding wheel loading compared to when grinding oils are used

instead.

Sharpening and dressing exhibit a cleaning effect, although in practice the

wheels are cleaned rather with a high-pressure/high-speed jet of coolant during

the grinding process itself. High-pressure/high-speed jetting with coolant is carried

out with a special cleaning nozzle. The cleaning effect of jetting with coolant is

with increasing jet pressure, velocity and ﬂow rate higher. However, too high jet

pressure must be avoided, because it can damage the abrasive layer [17, 18].

4.3 Diamond Dressing Tools

Diamond is a crystalline form of carbon, which is formed at high pressures

and temperatures (Fig. 4.9). In a diamond crystal the carbon atoms are connected

to each other by pure energy-rich covalent bonds in a narrow and extremely

strong tetrahedral arrangement. As a result diamond is the hardest known natural

material (Fig. 4.10).

The hardness of diamond depends on properties like purity, crystalline perfec-

tion and crystal direction [19]. Becaus e of the different distances between the

carbon atoms in the different crystal planes, the hardness of diamond varies in

4 Dressing of Grinding Wheels 189

different crystal directions (anisotropy). The hardest is the <110> direction on the

{111} plane [20].

As already mentioned, diamond is the hardest material known in nature.

Therefore grinding whe els are predominantly dressed with diamond tools. As

well as its great hardness, diamond also has good thermal conductivity. However,

in dressing, the good thermal conductivity of diamond presents a danger to the

diamond itself. As the materials that are to be dressed are generally not heat-

sensitive themse lves, a build-up of heat can develop in the diamond holder,

which results in the diamond being destroyed. Hence temperatures above 900



Liquid

carbon

graphite and

metastable

diamond

CVD

high-pressure

high-temperature

synthesis

catalytic

high-pressure

high-temperature

synthesis

diamond

temperature

0°C1000 2000 3000 5000

pressure

GPa

schock

wave

synthesis

diamond and

metastable

graphite

Fig. 4.9 Phase diagram of carbon [21] used with permission copyright American Chemical

Society

Fig. 4.10 Hardness values of

different materials [22] used

with permission copyright V.

Buck

190 T. Tawakoli and A. Rasifard

should be avoided, as otherwise, even with low pres sures, graphitisation of the

diamond occurs. To prevent graphitisation , there must be sufﬁcient cooling during

the dressing process [23]. The behaviour of a diamond dresser is strongly inﬂuenced

by its diamond coating and its manufacturing method.

4.3.1 Diamond Coating

The concept of diamond coating encompasses the type of diamond used, the

diamond grit size and concentration, as well as the pattern in which the diamonds

are set [2, 23]. The types of diamond used by the manufacturing of diamond

dressing tools can be divided into two main groups: single-crysta l diamond

and polycrystalline diamond (Fig. 4.11). Single-crystal diamond includes natural

diamond and monocrystalline diamond (MCD). Depending on the manufacturing

process, polycrystalline diamond (PCD) can be divided into the groups with

and without binder phase [24].

Natural diamonds are used either in sorted form or as grits that are crushed

mechanically into different grit sizes. In the case of natural diamonds, because

of differences in shape, size and crystal purity as well as internal stresses and

crystallographic irregularities, abrasion resistance or wear rate can vary relatively

greatly [26]. Nevertheless, in these days diamond-dressing tools are made predom-

inantly with single-crystal natural diamonds [7].

MCD is produced by synthesis from graphite in a complex process under high

pressures and high temperatures using metal catalysts, and is characterised

single-crystal diamonds polycrystallline diamond (PCD)

MCDnatural diamond

with binder

phase

without binder

phase

manufacture

hardness

[Knoop]

application

shape

diamond

(2 … 25 µm)

Co matrix

6000 - 9000

8000 - 9000

4000 - 5500

8500 - 9000

structure

CVD process

H2 + 1% CH4

MW plasma

all diamond tools

stationary dressing

tools

edge reinforcement

form rollers

form rollers with

small radii and

angles

1600 °C, 7 GPa

deposits in: Australia,

Zaire, Bostowana,

South Africa

about 4000 years ago

at depth of 150-300 km

Fig. 4.11 Diamond for technical applications [25] used with permission copyright Dr. Kaiser

4 Dressing of Grinding Wheels 191

by extremely great hardness. Monocrystalline diamond logs are made by laser

cutting monocrystalline diamond blanks. The homogenous crystal orientation of

MCD leads to a constant abrasion resistance or wear rate along their length. In

addition, as engineered products, synthetic diamonds have charact eristics of repro-

ducible quality. However, because of their anisotropy, the wear resistance of MCD

logs, as with natural diamond crystals, is orientation-dependent. Therefore the MCD

logs should be orientated in the dresser in their hardest direction [26]. Because of the

high manufacturing costs, MCD is only used in tools where there is the risk of high-

level wear on the tools, such as for example stationary dressers [24, 27].

Polycrystalline diamond with a binder phase consists of synthetic diamond

particles which in a sintering process are bound with a binder matrix

(cobalt or tungsten). The properties of the PCD are determined to a great extent

by the grain size of the diamond particle s. Typical grain sizes lie between 2 and

25 mm[27, 28]. This polycrystalline diamond is predominantly made in the form of

log or disc using cobalt as the binder material [7]. Be cause of the random diamond

orientation, PCD with binder phase forms as a hard, isotropic body, which does not

have a preferred direction of cleavage and hardness. However, because of the

binder phase element, PCD is the “softest” type of diamond. PCD is frequently

used as edge protection in diamond in crushing rollers and in the frequently

re-grindable PCD form rollers [28].

PCD without binder phase is produced by chemical vapour deposition (CVD)

and is often referred to as CVD diamond. The great advantage of CVD diamond lies

in its very high fracture strength because of its homogenous polycrystalline struc-

ture and the dense packing of its structure [28]. The properties of CVD diamonds

are not dependent on their orientation, i.e. their wear resistance is the same in all

directions [29]. Due to the possibility in the CVD process to deposit extremely thin

layers, this method is suitable in particular for coating form rollers with small

angles and radii [27–29].

The behaviour of a diamond dresser is also inﬂuenced by the grit size of the

diamonds. In general, smaller diamonds are stronger than larger diamonds, because

they have fewer and smaller crystal defects [8]. A further inﬂuencing factor is the

concentration of diamond in the dresser. Increasing the diamond concentration of a

rotary dresser with a constant diamond grit size leads to an increase in the life of the

dresser, but also at the same time, leads to a reduction in the surface roughness of

the dressed wheel [30, 31].

The pattern in which the diamonds are set is another important factor

that inﬂuences the behaviour of diamond dresser. The diamond layer of a rotary

diamond dresser can be either scattered or set (Fig. 4.12). With a scattered diamond

coating the diamonds are distributed stochastically, whereby the average space

between the grains depends on the diamond grit size [2]. Most form rollers with

this type of diamond material have a radius in the range R ¼ 1.5–5 mm. The

diamond dressing tools with scattered pattern exhibit a good tool life [28].

The manufacturing of diamond dressers with scattered pattern is fast and cost-

effective. However, form rollers with a scattered pattern cannot be re-ground.

Scattered diamond coating s are made using natural diamonds [24].

192 T. Tawakoli and A. Rasifard

In set-diamond coatings, diamonds are set – commonly by hand – in a speciﬁc

pattern. In this system, by appropriate choosing the position and the geometry of the

diamonds, as well as their surface concentration on individual elements of the contour

of the dresser, the topography of the dresser can be tailored to the proﬁling task

[33, 34]. With this type of diamond coating it is possible to manufacture form rollers

with a very small tip radius. Needle-shaped natural diamond and also monocrystalline

or polycrystalline synthetic diamond logs can be used for set-diamond coatings. Form

rollers with set-diamonds can be re-ground [24].

A combination of both types of patterns can also be useful for some proﬁle

rollers. For example, when grinding steep ﬂa nks, the small effectiv e peak-to-valley

roughness of the wheel causes thermal damage to the workpiece. A solution to this

problem can be setting the diamonds by hand accordin g to speciﬁc patterns on

critical areas of the proﬁle, as a result of which the effective peak-to-valley

roughness of the wheel in these areas is increased [32]. A speciﬁc selection of the

shape and the arrangement of diamond grits can be beneﬁcial for stationary

diamond tools as well. A deﬁned contact width and a uniform wear characteristic,

for example, can be achieved in blade dressers with ne edle diamonds throu gh the

array arrangement of the diamonds.

4.3.2 Manufacture of Diamond Dressing Tools

The Performance of a dressing tool can be inﬂuenced signiﬁcantly by its

manufacturing process. There are several manufacturing processes for both stationary

and rotary diamond dressers. In stationary diamond dressers, the bonding of diamonds

can be done mainly through an electroplated nickel layer or in a sintering process with

a hard and wear-resistant binder matrix. Electroplaed bonds have little hardness and

Fig. 4.12 Different types of diamond pattern [32] used with permission copyright Tyrolit

4 Dressing of Grinding Wheels 193

low heat conductivity, as well as a weak diamond binding; nevertheless, there is no

thermal load on the diamond during manufacture. In sintered stationary diamond

dressers, the sintered body is brazed to the steel base body. Brazing can, however,

cause thermal damage to the diamond. Stationary diamond dressers with a brazed

sintered body also have little bond hardness, but they can be produced readily. The

whole dressing tool can also be made by sintering. These stationary dressers are

characterised by their good bond hardness and high heat conductivity, as well as

because of their strong diamond binding [25].

In the selection of stationary dressers, the speciﬁcations of the wheel that is to be

dressed also have to be taken into account. For example, if a blade dresser with a

soft bond is employed to dress sintered alumina or silicon carbide wheels, the bond

matrix of the dres ses is soon cut back. As a result, after dressing for a short time, the

diamonds protrude from the tool and break out due to vibration or dressing forces.

On the other hand, if a dresser with a highly hard bond is used to dress white-fused

alumina wheels, the bond matrix does not get cut back enough. Here, the wheel is

actually dressed also with the hard bond matrix, which resu lts in a smooth grinding

wheel [35].

The required manufacturing accuracy for rotary diamond dresser is very high

(Fig. 4.13). The manufacturing accuracy of a rotary diamond dresser is inﬂuenced

to a great extent by its manufacturing method. Diamond roller dressers are

produced either in the positive method or the negative method (Fig. 4.14).

In the positive method (also called the direct method) both single-layer and also

multi-layer diamond coatings can be produced (Fig. 4.15). In the manufacturing of

Fig. 4.13 Typical forms and tolerances of diamond form rollers used with permission copyright

Dr. Kaiser

194 T. Tawakoli and A. Rasifard