Jackson Mark. Machining with Abrasives

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of coolant into the dressing contact zone and reduces the coefﬁcient of friction

between the active dressing partners, as a result of which the heat generation in the

dressing contact zone, and therefore the intensity of graphitisation, decreases.

Therefore, in down dressing ultrasonic assistance leads to an intensity in the

principal mechanism of dressing diamond wear, and in up dressing it leads to a

reduction in this mechanism the principal wear mechanism.

As mentioned above, in industry the conventional dressing of vitriﬁed bonded

CBN wheels is often carried out in down dressing mode, in order to avoid thermal

damage to the workpiece cauesed by a too ﬁne wheel surface. Ultrasonic assisted

dressing tests with form rollers show, however, that with suitable ultrasonic assis-

tance in up dressing (i.e. in the range q

<0) a grinding wheel topography can be

produced that is comparable to the topography produced after conventional down

dressing. From roller wear with ultrasonic assistance in up dressing is, however,

much less than with conventional dressing in down dressing.

4.7 Laser Dressing

With the further development of laser technology the areas of application for

machining with laser beams have been increasing all the time in recent years.

The laser has become established as a fast and versatile tool particularly for the

machining of hard and brittle (difﬁcult-to-machine) materials such as high-

performance ceramics. As well as being extremely versatile, the laser has additional

positive features such as its ability to introduce heat in a controlled condition, the

small zone in which heat exerts an inﬂuence, machining without machining forces

and the fact that it can be controlled easily in terms of space and time. The use of

laser beams for conditioning grinding wheels enables the disadvantages of conven-

tional conditioning methods arising from the tools and methods used to be avoided.

These disadvantages include, for example, dresser wear when dressing with dia-

mond dressers or the presence of process forces in proﬁling. The wear on an

expensive diamond tool results not only in high tool costs but also in an unstable

dressing process in terms of wheel geometry and topography. The process forces

that arise in proﬁling with conventional conditioning methods can lead on the one

hand to dimensional and proﬁle errors on the geomet ry of the wheel, and on the

other hand to the inducing of microcracks in the abrasive grits and the bond of the

grinding wheel.

So far lasers have been used in various ways for conditioning grinding wheels.

With a laser beam it is possible to clean the surface of grinding wheels non-

aggressively. The laser beam can also be used to sharpen superabrasive grinding

wheels, removing only the bond. If sufﬁcient bond material is removed, the

abrasive grits can also fall out. Inﬂuencing of both the abrasive grits and the bond

at the same time with the laser beam and laser-assisted proﬁling are other methods

that have been developed.

4 Dressing of Grinding Wheels 225

4.7.1 Principle of Laser Conditioning

Laser conditioning is a thermal machining method in which the surface of

the grinding wheel is heated in control led conditions by the laser beam. With this

process the components of the abrasive layer and also the chips embedded in

the pores of the abrasive layer can reach melting point. Laser conditioning is

therefore inﬂuenced both by the speciﬁcation of the wheel and also by the condi-

tions of the laser irradiation. La ser conditioning is carried out best with a Nd:YAG

laser because not only is it possible to achieve a high degree of focus with the beam

of the Nd:YAG laser, but also it has the best absorption behaviour and therefore

uses energy to optimum effect on the bond of the grinding wheel. Besides, by

means of optical ﬁbres the beam of the Nd:YAG laser can be directed safely and

ﬂexibly to the machining point, which is of great advantage particularly in indus-

trial applications [13].

The laser irradiation of grinding wheels can be subdivided into three principal

arrangements (Fig. 4.50). With a radial beam relative to the irradiated surface of the

wheel, a Nd:YAG laser beam can produce a uniform irradiated area. This arrange-

ment is therefore best suited for cleaning the wheel and for the controlled cutting

back of bond mater ial, namely for sharpening. For proﬁling with close tolerances on

geometry, the radial irradiation would have to be carried out with a depth cont rol

and also with a cw-Q-switched laser beam (with pulses of a few nanoseconds) [36].

Also, unlike wi th conventional sharpening processes, with radial irradiation no

bond bridges are produced behind the grits that have a supporting effect on the

grit and therefore have a positive effect on wheel wear [59]. Another principal

irradition arrangement is the beam being directed tangentially to the wheel. With

this arrangement it is possible both to produce bond bridges and also to remove

roundness errors. However, with this arrangement the beam power needed to

Fig. 4.50 Possible irradiation arrangements in laser conditioning [13] used with permission

226 T. Tawakoli and A. Rasifard

achieve a process speed comparable to that achieved with radial irrada tion is greater

due to the larger irradiation area and because the focus of the beam is only partly

used. The third arrangement that is possible with laser conditioning is in the axial

direction, but because of the Gaussian beam optics it is not suitable for precision

conditioning. With this arrangement, inevitably the proﬁle of the wheel deteriorates

[13].

In designing laser conditioning the inﬂuencing factors such as beam power,

irradiation time and defocus distance should be set so that by heating the surface of

the wheel, the desired melting and eventually the required removal of the elements

of the abrasive layer and also of the chips embedded in the pores of the abrasive

layer can be achieved . Therefore the relationship between inﬂuencing factors and

the development of heat in laser conditioning has to be analyzed.

4.7.2 Thermal Consideration of Laser Conditioning

The heat induced in the surface of the wheel by laser irradiation leads to effects such

as melting, evaporation and the forming of plasma, which enable the components of

the abrasive layer and/or the chips embedded in it to be removed [60].

The development of heat in a surface irradiated with the laser depends on various

inﬂuencing factors. These include in particular the power of the beam and the area

that is to be machined by the laser. With laser conditioning the temperature

distribution and eventually the depth to which material is melted can be described

by (4.7) as the basic equation gover ning one-dimensional heat transfer [61].

(4.7)

T is the temperature in a gri nding wheel depth of z and t is the time after starting

laser irradiation. a is the thermal diffusivity of the grinding wheel. If there is no loss

of heat from the radiation and no melting, then (4.8) can be derived from (4.7) [60].

Tðz; tÞ¼

2AP

ierfcð

ﬃﬃﬃﬃﬃﬃﬃ

4at

Þ (4.8)

With pulsed laser irradiation, the laser beam intensity ( P )is

PðtÞ¼

P 0<t<t

0 t<0; t>t



(4.9)

In (4.8) A is the material’s absorption coefﬁcient, and K is the thermal conductivity.

As can be seen, at a constant P, incr easing the irradiation time t

leads to an increase

in the temperature in the lower layer of the grinding wheel. But this can affect the

4 Dressing of Grinding Wheels 227

topography of the wheel. Therefore, in order to clean the wheel, a high laser power

ﬂux and short irradiation laser puls es should be set [61, 62]. The laser beam

intensity depends on the irradiation arrangement, the average power P

, the focus

beam diameter d

, the defocus distance z and the full divergent angle y (Fig. 4.51).

With radial irradiat ion, the laser power ﬂux input can be calculated by (4.10) [60].

Pðx; y; zÞ¼

p  dðz Þ

 exp 

8ðx

þ y

ðdðzÞÞ

(4.10)

d(z) is the beam diameter at z from the focus plane along the propagating beam and

is calculated according to (4.11). z is positive if the focus plane is outside the wheel.

Conversely, it is negative if the focus plane is inside the wheel.

dðzÞ¼d

þ 2z  tanðy=2Þ (4.11)

As shown from (4.10) and (4.11), the laser power ﬂux input into the wheel and

therefore the development of heat in the surface of the wheel depends among other

things on the defocus distance. The angle of inciden ce of the laser beam is another

parameter that can inﬂuence laser conditioning. As the angle of incidence increases,

the absorbed energy and conse quently the process speed decrease [63].

4.7.3 Laser Conditioning of Conventional Grinding Wheels

With proper laser irradiation, conventional grinding wheels can be both cleaned and

dressed. Fusion and the evaporation of chips are the main mechanisms in the laser

cleaning of a wheel [61, 62]. The wheel can only be cleaned effectively and reliably

by laser irradiation if all the process parameters are selected appropriately. The

factors that lead to different inﬂuences on the components of the wheel and

loosened chips by the laser irradiation have to be taken into consideration. These

factors include, for example, melting point, optical reﬂectivity, thermal and optical

conductivity, and speciﬁc heat capacities [61].

grinding wheel

lens

Laser beam

Fig. 4.51 Schematic of the laser conditioning arrangement

228 T. Tawakoli and A. Rasifard

With laser dressing, a higher laser power ﬂux input is generated compared with

laser cleaning. This leads to melting (followed by re-solidiﬁcation) and/or evapora-

tion of the components of the grindin g wheel, as a result of which the topography of

the wheel changes in terms of its morphology and composition. Depending on the

cooling speed, re-solidiﬁcation produces new microstructures. As Fig. 4.52a shows,

after laser dressing the alumina wheel has a compacted surface with reduced

porosity [64]. In their studies, Jackson et al. also reported a reduction in porosity

but without a full compaction of the surface of the wheel [61]. Figure 4.52b shows

the microstructure of the grinding wheel surface before and after laser dressing.

Although the grits after laser dressing are of very different sizes, from 5 to 100 mm,

they are more regular and symmetrical in shape with well-deﬁned edges and sides

[61, 64]. The morphological features evolved during laser dressing reduce the

surface roughness of the dressed wheel, which in turn affects the grinding perfor-

mance [64]. Jackson et al. have found that there is little difference between grinding

ratios for diamond-dressed and laser-dressed vitriﬁed alumina grinding wheels,

although the mechanisms of grinding wheel wear after dressing with a diamond

tool and with the laser are different [61].

Fig. 4.52 Vitriﬁed alumina grinding wheel surface before laser dressing (undressed) and after

laser dressing with 750 W: (a) Optical micrographs of the grinding wheel surface, (b) Microstruc-

ture of the grinding wheel surface [64] used with permission copyright Elsevier

4 Dressing of Grinding Wheels 229

4.7.4 Laser Conditioning of Superabrasive Grinding Wheels

Laser conditioning is of great interest particularly for preparing superabrasive CBN

and diamond grinding wheels. The laser can be used successfully for cleaning

superabrasive wheels. For example, Chen et al. were able to clean a vitriﬁed bonded

CBN wheel successfully with laser irradiation. They found that the laser beam has

to have greater laser energy for cleaning CBN wheels than alumina wheels due to

the greater thermal conductivity of CBN [62].

Laser can also be used for dressing superabrasive grinding wheels with different

types of bond. The inﬂue nce on wheel topography depends not only on the irradia-

tion parameters but also on the wheel speciﬁcations such as grit size and bond

material. With the laser dressing of superabrasive grinding wheels the aim is to

remove bond material in such a way that a new wheel topography is produced with

the desired grit protrusion. The bond material should be removed without any

thermal damage being caused to the abrasive grits, because this can increase the

wear on the wheel. By irradiating the grits with individual laser pulses with

different energy densities and then analysing the SEMs, the maximum permissible

energy density for the laser irradiation can be determined. Any inﬂuence on

or damage to the unclad grits can be seen in the form of discolouration and spalling.

In the case of nickel-clad grits, if the energy density is too high, laser irradiation

causes the nickel coating to melt (Fig. 4.53). Here, the maximum permissible

energy density is deﬁned as the energy density at which while most of the top

side of the grit is exposed as far as the grit, the areas of grit to the side and

below remain unaffected. With natural diamond as an example, Fig. 4.54 shows

the boundary between damaged and undamaged grits calculated as a function

of energy density and grit size. It can be seen that with increasing grit siz e, also,

higher energy densities can be used for laser conditioning without damaging the grit

[13]. The removal behaviour of the bond is another important inﬂuencing factor

in the laser dressing of superabrasive grinding wheels. It could be established in

Fig. 4.53 Inﬂuence of laser beam energy density with nickel-clad diamond grits as an example

[13] used with permission copyright institute of machine tools and production technology (IWF),

technical university Braunschweig

230 T. Tawakoli and A. Rasifard

the laser irradiation of the ﬂat sample of pure binder material that both with

the vitriﬁed and resin bond essentially there is the same removal behaviour and a

linear relationship between energy density and the depth of material removal per

laser pulse. This is why by setting the laser energy density adequately, a targeted

removal of the bond and eventually a suitable grit protrusion (la ser sharpening) or

a desired grit removal (laser dressing) can be achieved. In general, resin bonds can

be removed successfully with much lower energy densities than very strong vitri-

ﬁed bonds [59].

Similarly, an almost linear relationship between energy density and depth of

removal per pulse can be established with the bronze bonded grinding wheel.

However, greater laser energy density is needed for the removal of metal bonds,

which in the case of diamond grinding wheels increases the risk of grit failure or

graphitisation of the diamond particles. Hosokawa et al. have used an air jet

directed on the laser irradiating spot in the laser dressing of a bronze bonded

diamond wheel. The air jet blows away the molten binder before it solidiﬁes

again and thereby increases the efﬁciency of the process [66]. The colour of the

bond also plays an important part for the quality of material removal. For example,

by a change in colour, which does not inﬂuence the grinding process, a change in

the absorption behaviour for the laser irradiation and therefore an improvement in

the bond can be obtained [59].

A comparison between laser conditioning and conventional conditioning can be

carried out on the basis of the grinding force ratio m ¼F

. The higher m is, the

more favourable are the machining characteristics of the grinding wheel. Fig-

ure 4.55 shows the change in grinding force ratio in internal cylindrical grinding

with a resin bonded CBN wheel that has been laser-conditioned and a wheel that has

Fig. 4.54 Inﬂuence of grit size in the laser irradiation of natural diamond PDA 665 [13] used with

permission copyright institute of machine tools and production technology (IWF), technical

university Braunschweig

4 Dressing of Grinding Wheels 231

been conditioned through the conventional method. With the wheel conditioned

conventionally, the wheel was ﬁrst dressed with a diamond cup wheel and then

sharpened with an alumina sharpening block. At the start of the grinding tests, the

wheels conditioned conventionally and by radial laser irradiation display higher

grinding force ratios. However, with increasing special material removal the grind-

ing force ratio for these two processes decreases drastically, so that at approx.

2,500 mm

/mm the wheel conditioned with tangential laser irradiation has the

highest grinding force ratio. This shows that while grinding after conventional and

radial laser-irradiated conditioning, the topography of the wheel is changed consid-

erably.

The efﬁciency of laser conditioning was also investigated on the basis of the wear

behaviour of the wheel (Fig. 4.55). At the start of grinding, the radially irradiated wheel

displays the least wear, but from a spesiﬁc material removal V

of approx. 1,500 mm

mm a clear and constant increase could be seen. At the end of the series of tests the

lowest wheel wear was on the tangentially laser-sharpened CBN wheel [66].

The inﬂuence of the conditioning method on the change in wheel topography was

also investigated on the basis of the effective peak-to-valley roughness of the wheel.

As Fig. 4.56 shows, radial laser conditioning generates a high effective peak-to-valley

Fig. 4.55 Dependence of the grinding process on the conditioning method and material removal

rate [66] used with permission copyright Vulkan-Verlag

232 T. Tawakoli and A. Rasifard

roughness on the wheel, while on the other hand the roughness of a wheel produced

by tangential laser conditioning lies in the range of that produced by conventional

conditioning. The effective peak-to-valley roughness of all the wheels becomes

smaller with increasing material removal. However, this change is greatest with the

radially irradiated wheel, so that after a spesiﬁc material removal of 3,100 mm

/mm

the effective peak-to-valley roughness is almost the same on all three wheels.

Lasers can also be used to assist in conventional dressing with diamond

dressers; here, the surface of the rotating grinding wheel is heated by a focused

laser beam at a short distance in front of the dressing tool. This method, known as

laser-assisted dressing, has been developed by Zhang and Shin for the conditioning

of vitriﬁed bonded CBN wheels with a single-point diamond dresser (Fig. 4.57).

With the appropriate selection of laser parameters such as laser energy density

and heating time, the brittle-hard vitriﬁed bond of the wheel is softened and

even melted, as a result of which the mechanism of materia l removal of the

vitriﬁed bond is changed from brittle fracture to ductile ﬂow. This makes it

Fig. 4.56 Effective peak-to-valley roughness of a resin-bonded CBN grinding wheel as a function

of the conditioning method [66] used with permission copyright Vulkan-Verlag

Fig. 4.57 Schematic of laser-assisted dressing [67] used with permission copyright Elsevier

4 Dressing of Grinding Wheels 233

easier for the bond material to be removed and as a result leads to a reduction

in dressing forces and dresser wear (Fig. 4.58). The reduction in dresser wear

brought about by laser assistance results in the production of wheel topographies

that are more uniform with dressing operations carried out successively than those

produced with conventional dressing [ 67].

4.8 Electro-Assisted Conditioning Methods

The electro-assisted conditioning methods are used for preparing metal bonded

superabrasive grinding wheels, whereby the bond is removed by an electrical

process. These conditioning methods can be controlled read ily and are very efﬁ-

cient. So far various electro-assisted conditioning methods have been developed,

but only the most important of these methods, namely electrolytic in-process

dressing (ELID), electro-discharge dressing (EDD) and electrocontact discharge

dressing (ECDD) are discussed in brief below.

Fig. 4.58 Dressing forces and dresser wear in conventional and laser-assisted dressing [67] used

with permission copyright Elsevier

234 T. Tawakoli and A. Rasifard