Jackson Mark. Machining with Abrasives

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Reduction in the machining forces involved;

Improvement in the supply of coolant into the process zone;

Reduction in friction between tool and chip;

Reduction in tool wear;

Reduction in microscopic and macroscopic material damage; and

Increase in the dimensional accuracy of workpiece proﬁles.

Although ultrasonic assistance has been used successfully in many different

machining processes for some time now, for technical and economic reasons as

yet ultrasonic assisted dressing has not been used to any signiﬁcant extent in

industry. Nevertheless, the latest research reports indicate that with a suitable

ultrasonic system and if the process parameters are set correctly, signiﬁcant

improvements can be achieved with ultrasonic assistance, particularly in the dress-

ing of superabrasive CBN and diamond grinding wheels [11, 53–56].

4.6.1 Ultrasonic Vibration Systems

Elastomechanical longitudinal ultrasonic vibrations are produced by the conversion

of electrical energy to mechanical vibration, for the most part in piez oceramic

but also, in some cases, magnetostrictive altrasonic transducer. The high-

frequency electric alternating voltage that is needed is produced by conver ting

low-frequency mains voltage in a voltage generator. The longitudinal vibrations

that are produced in the transducer are periodic, elastic deformations in the microm-

eter range at frequencies in the ultrasonic range. The ultrasonic amplitude produced

in the ultrasonic transducer is often very small compared with the amplitudes

needed in cutting process. The amplitude of the vibration can be increased consid-

erably by a narrowing of the cross-section of a transition element ﬁxed on the

ultrasonic transducer (what is known as the booster) (Fig. 4.38).

Then, a sonotrode ﬂanged to it transfers the vibration amplitudes through the tool

into the process zone.

Decisive for the effectiveness of the ultrasonic vibration for the process is that all

the elements, through which the vibration passes, exhibit suitable mode shape. In

order to ensure that an increase in amplitude takes place that is largely free of losses,

the system must vibrate at one of its natural frequencies (resonance). In terms of the

design of the system this means that the lengths of the individual elements must

correspond to the half wavelength of the vibrations or a full-number multiple of it, so

that in the entire ultrasonic system a standing wave is generated [48].

4.6.2 Kinematics of Ultrasonic Assisted Dressing

In ultrasonic assisted dressing, ultrasonic vibrations are superimposed on the

conventional acting movement of the process. It is possible to use ultrasonic with

stationary as well as with rotary dressers (Fig. 4.39).

4 Dressing of Grinding Wheels 215

ultrasonic assisted dressing with stationary dressers

grinding wheel

fad

frd

fad

frd

fad

ultrasonic assisted dressing with rotary dressers

single-point

diamond

dressing

plate

fad

frd

form roller

cup

dresser

Fig. 4.39 Some ultrasonic assisted dressing methods

end mass

front mass

booster

sonotrode

tool

transducer

high frequency

generator

piezoceramic discs

vibration systemmechanical oscillation

l /2

longitudinalradial

Fig. 4.38 Vibration chain in the forming of a ﬁxed ultrasonic wave

216 T. Tawakoli and A. Rasifard

4.6.3 Ultrasonic Assisted Dressing with Stationary Dressers

The engagement conditions and the resulting splitting and fracture on the abrasive

grits and the bond are different in conventional and ultrasonic assisted dressing. If

the dressing tool vibrates in the radial direction of the grinding wheel y according to

(4.4), the dresser strikes the wheel with a penetration angle according to (4.5),

which causes the grits and brittle hard bond of the grinding wheel to split more

intensively [57].

yðtÞ¼A

 sinð2 p  f

 tÞ (4.4)

¼ arctgð

2:p:f

: cosð2:p:f

Þ (4.5)

Besides, the sinusoidal active path of the dressing tool in ultrasonic assisted

dressing produces a rougher grinding wheel topography with a smaller number of

kinematic cutting edges. The rougher wheel topography with sharper abrasive grits

leads to lower grinding forces and greater roughness of the ground surface

(Fig. 4.40). However, Jiao et al. have reported that in grinding with a resin bonds

SiC wheel, after the unstable initial grinding phase the wheel dressed with ultra-

sonic assistance has better workpiece roughness. They also observed in their studies

that the inﬂuence of ultrasonic assistance on workpiece roughness can also depend

on the grinding depth of cut a

, so that with depth of cuts of more than 0.03 mm,

after ultrasonic assisted dressing in continuous mode, in which a

 A

, the resin

bonded SiC wheel can produce the lowest surface roughness [58]. Ikuso et al. have

Fig. 4.40 Comparison of conventional and ultrasonic assisted dressing

4 Dressing of Grinding Wheels 217

also reported less workpiece roughness after ultrasonic assisted dressin g when

grinding with a vitriﬁed bonded CBN wheel [53]. Therefore, the effect of ultrasonic

assistance on the generation of the wheel topography during the dressing with

stationary dressers and the resulting workpiece roughness clearly depends on the

wheel speciﬁcation and the grinding conditions. Ultrasonic assistance can also

improve dressing accuracy (Fig. 4.41). This may be due to the non-sensitive

vibration cutting mechanisms and the increase in the stiffness of the dressing unit

due to the ultrasonic vibration [53]. According to Jiao et al., in discontinues

dressing mode, in which a

< A

, the process forces are on average smaller and

more uniform, which results in a more even height of the micro cutting edges

produced during dressing [58].

Diamond dresser wear can also be reduced by ultrasonic assistance (Fig. 4.42).

The reduction in wear comes back to improved cool ant supply and the reduction in

= 1.56 µm

= 2.51 µm

after ultrasonic assisted dressing

2.0 µm

−2.0 µm

after conventional dressing

0 15 mm 0 15 mm

= 0.96 µm

= 1.58 µm

Fig. 4.41 Comparison of cross-sectional shapes of ground surface

grinding wheel

vitrified bonded

CBN grinding wheel

B126 C100

= 300 mm

= 15 mm

dressing tool

dressing blade with two

CVD diamond sticks

dressing parameters

= 50 m/s

= 12

= 20 x 3 µm

ultrasonic parameters

= 19 kHz

= 3 µm

coolant lubricant

solution (5%)

wear length of

diamond sticks Δl

µm

conventional dressing

ultrasonic assisted

dressing

Fig. 4.42 Reduction in the wear lengths of the diamond logs of a CVD tile dresser due to

ultrasonic assistance

218 T. Tawakoli and A. Rasifard

friction coefﬁcient with ultrasonic assistance, which reduces the heat generation in

the dressing contact zone. As a result, the risk of graphitisation of the dressing

diamond, which is directly related to the forming of wear on the dressing diamond,

is reduced [23]. This explanation can be conﬁrmed on the basis of SEMs of the

CVD diamond logs of a blade dresser after conventional dressing and ultrasonic

assisted dressing. As can be seen in Fig. 4.43, the surface of the diamond log after

ultrasonic assisted dressing displays much more uniform wear with few abrasive

traces and hardly any grit edge splitting compared with after conventional dressing.

Furthermore, after ultrasonic assisted dressing the dressing tool body displays

around the diamond log only slight damage. This comes back also to the less heat

generation by applying ultrasonic vibration. Also, after conventional dressing there

are a large number of white marks, which are clearly adhesive layers of the grit and

binder elements. The abrasion and adhesion wear marks on the surface of the

diamond after conventional dressing are due to the high development of tempera-

ture, caused by the continuous contact and the high friction between the grinding

wheel and the stationary dresser.

4.6.4 Ultrasonic Assisted Dressing with Rotary Dressers

Superabrasive CBN and diamond grinding wheels are dressed almost exclusively

with rotary diamond dressers, so ultrasonic assisted dressing with rotary dressers

has attracted researchers’ attention in the last decade as a new conditioning method.

Liebe used the longitudinal ultrasonic vibrations in the proﬁling of vitriﬁed and

resin bonded grinding wheels with a diamond cup wheel, in which the angle

between the axes of rotation of the grinding wheel and the cup wheel was 75



With the ultrasonic assist ance Liebe was able to achieve higher removal rates

compared with conventional dressing (Fig. 4.44). During ultrasonic assisted

Fig. 4.43 SEMs of the diamond logs

4 Dressing of Grinding Wheels 219

dressing of vitriﬁed bonded wheel, however, cracks were induced in the bond by the

vibration, which in subsequent grinding operetion led to a reduction in the grinding

ratio. In subsequent grinding operation with resin bonded wheels, the increasing

in grindin g forces at the initial grinding phase was higher after ultrasonic assisted

dressing, although the grinding forces were approximately at the same level over

the course of the grinding operation. This is due to the sharp cutting edges

becoming blunt relatively more quickly [11].

Sroka also conducted research concerning the dressing of resin bonded diamond

grinding wheels with an ultrasonically vibrated diamond cup wheel. With the use of

ultrasonic assistance he was able to increase the dressing ratio by a factor of 10 and

to reduce dressing costs by 59% [55].

Nomura et al. investigated the effect of ultrasonic vibrations on the dressing of

small CBN wheels in the intern al cylindrical grinding of small bores. In their

investigations, the longitudinal ultrasonic vibrations were exerted on the grinding

tool. A diamond cup wheel was used for dressing of the grinding tool. As results, it

was found that applying ultrasonic vibration decreased the dressing forces by more

than 22%, and improved the run-out of the grinding wheel by 30% compared to the

conventional dressing [54].

The use of ultrasonic assistance in dressing with diamond dressing rollers is of

great interest in view of the widespread use of these dressers. But there are

difﬁculties because of the dimensions and weight of the dressing rollers and

spindles that are usual in industrial practice. An effective way of carrying out

ultrasonic assisted dressing with a form roller is to generate the radial vibrations

in the form roller. Here, the form roller is alternatingly expanded and compressed

radially in the micrometer range at a ultrasonic frequency. This can be achieved by

positioning the dressing roller on a nodal point of the sonotrode, whereby the

sonotrode functions as the shaft of the dresser spindle. The change in cross-section

of the sonotrode is greatest in the nodal points of the standing longitudinal vibration

(see Fig. 4.38). This change in cross-section is ampliﬁed by the special shape of the

Fig. 4.44 Ultrasonic assisted dressing of diamond grinding wheels with a diamond cup wheel [11]

220 T. Tawakoli and A. Rasifard

dressing roller [57]. The radius of the dresser, which changes continuously due to

the ultrasonic vibration, can be calculated with (4.6).

R; US

¼ r

þ A

: sinð2  p f

:tÞ (4.6)

It is clear that the engagement path of the diamonds in the dressing roller are

different in conventional and ultrasonic assisted dressing. This leads to the forma-

tion of different wheel topographies with conventional and ultrasonic assisted

dressing, which in turn leads to different grinding results. As Fig. 4.45 shows,

with ultrasonic assisted dressing the grinding forces are lower and the workpiece

roughness is greater than with conventional dressing. This indicates that a rougher

wheel topography is formed with ultrasonic assisted dressing.

With ultrasonic assistance, the lengths of the diamond engagements are shorter

(Fig. 4.46). This leads to a greater theoretical effective peak-to-valley roughness on

the grinding wheel. Besides, the penetration angles are greater with ultrasonic

assisted dressing, resulting in greater normal loads and therefore more intensive

splitting of the CBN grits when they impact with the diamond grits. As a result, the

CBN wheels dressed with ultrasonic assistance have a sharper abrasive layer with a

specific normal

grinding force F´

workpiece

roughness R

grinding wheel

vitrified bonded

CBN grinding wheel

B151 C150

dressing tool

dressing form roller

D356

dressing parameters

= 50 m/s

= 4

= ± 0.1, ± 0.4, ± 0.7

= 2 x 3 µm

ultrasonic parameters

= 36 kHz

= 0, 6 µm

grinding parameters

= 50 m/s

= 3000 mm/min

= 0.2 mm

workpiece

100Cr6 (60HRC)

coolant lubricant

grinding oil

conventional dressing (A

= 0) ultrasonic assisted dressing (A

= 6 µm)

dressing speed ratio q

−0.7−0.4−0.1

0.10.40.7

µm

Fig. 4.45 Grinding forces and workpiece roughness after conventional and ultrasonic assisted

dressing with a form roller

4 Dressing of Grinding Wheels 221

smaller number of kine matic cutting edges, which causes lower grinding forces and

greater workpiece roughness.

Figure 4.47 shows wheel behaviour after conventional and ultrasonic assisted

dressing up to a speciﬁc material removal of V

¼12,000 mm

/mm. In all tests the

grinding forces decrease during the initial unstable relief grinding phase. The relief

grinding phase lasts longer after conventional dressing, particularly when dressing in

up dressing. A shorter relief grinding phase after ultrasonic assisted dressing comes

back to higher grit protrusions caused by a greater splitting of bond material with

ultrasonic assisted dressing. With increasing material removal the grinding forces

gradually become similar, so that at a speciﬁc material removal of V

¼12,000

/mm there are hardly any differences in the grinding forces. In Fig. 4.47 too a

reduction in workpiece roughess can be seen at the start of the grinding process in all

the tests; this is due to ﬂattening and/or edge rounding of the grits and the resulting

increase in the number of kinematic cutting edges. It is also striking in Fig. 4.47 that,

interestingly, the changes in the grinding forces and in the workpiece roughness are

very similar after conventional down dressing and ultrasonic assisted up dressing,

with q

¼+0.4 and q

¼0.4 respectively.

Figure 4.48 shows the grinding ratio G after a speciﬁc material removal

of V

¼12,000 mm

/mm. It is obvious that the wheel wear is greater after ultra-

sonic assisted dressing. This comes back to the greater load on the CBN layer

simulation parameters

= 400 mm a

= 3 µm f

= 36 kHz

−4 −3 −2 −1012mm

path in direction of

rindin

wheel periphery

path in direction of grinding wheel radius

= 3

grinding wheel

surface

= 6

−5

µm

−5

µm

= + 0.4

= − 0.4

= 110 mm

= 50 m/s

= 0

= 6

= 0

= 3

Fig. 4.46 Simulation of the engagement paths in ultrasonic assisted dressing with different

dressing conditions

222 T. Tawakoli and A. Rasifard

during the ultrasonic assisted dressing, as a result of which more intensive cracking

occurs in the CBN grits and in the bond, which as weak points intensify the wheel

wear. In addition, a grea ter effective peak-to-val ley-roughness on the grinding

wheel after ultrasonic assisted dressing leads to a reduction in the number of

kinematic cutting edges and therefore to an increase in the individual chip thick-

nesses in the grinding process. Increase in the individual chip thicknesses leads to

an increase in the cutting loads acted on the CBN grits in the grinding operation. As

a result, in particular those CBN grits that are exposed and therfore are not as solidly

ﬁxed in the bond break out right at the start of the grinding process. Interestingly,

the grinding ratios after conventional do wn dressing and ultrasonic assisted up

dressing, with q

¼+0.4 and q

¼0.4 respectively, are the same.

Figure 4.49 shows the radial form roller wear Dr

in conventional and ultrasonic

assisted dressing with q

¼0.4. With up dressing, ultrasonic assistance leads to a

reduction in dresser wear, and with down dressing it leads to an increase.

The different effects of ultrasonic assistance on the wear behaviour of the roller

dresser in up dressing and down dressing are due to the different principal wear

mechanisms that are dominant in the two dressing methods. While in down dressing

the micro-splitting that results from the high mechanical loading is the principal

wear mechanism in the development of wear on the dressing diamond s, in up

grinding wheel

vitrified bonded

CBN grinding wheel

B151 C150

dressing tool

dressing form roller

D356

dressing parameters

= 50 m/s

= 4

= ± 0.4

= 2 x 3 µm

ultrasonic parameters

= 36 kHz

= 0, 6 µm

grinding parameters

= 3000 mm/min

= 0.2 mm

workpiece

100Cr6 (60HRC)

coolant lubricant

solution (5%)

specific normal grinding

force F´

(N/mm)

workpiece roughness

(µm)

12000

specific material removal V´

( mm

/mm)

0 3000 6000 9000

= -0.4, A

= 0

(conventional dressing)

= + 0.4, A

= 0

(conventional dressing)

= -0.4, A

= 6 µm

(ultrasonic assisted dressing)

= + 0.4, A

= 6 µm

(ultrasonic assisted dressing)

Fig. 4.47 Grinding wheel behaviour after conventional and ultrasonic assisted dressing

4 Dressing of Grinding Wheels 223

dressing the graphitisation of the dressing diamonds resulting from the abrasive

friction is the principal mechani sm of wear on the dressing diamonds [4, 23].

As already shown above, ultrasonic assistance leads to an increase in the kinematic

penetration angle, as a result of which the mechanical load on the diamonds of the

dressing tool increases. On the other hand, ultrasonic assistance improves the ﬂow

Fig. 4.48 Wear behaviour of the CBN wheel after conventional dressing and ultrasonic assisted

dressing with a form roller

Fig. 4.49 Inﬂuence of ultrasonic assistance on the wear behaviour of the roller dresser

224 T. Tawakoli and A. Rasifard