Jackson Mark. Machining with Abrasives

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grinding wheel

vitrified alumina wheel

A60 L – 8V

dressing tool

profile roller (D700/7.5)

dressing parameters

= 29 m/s

= 5000 1/min

initial effective peak-to-valley

roughness of the wheel R

tso

0 80 160 240 320

12.5

µm

10.0

7.5

5.0

number of the roll-out revolutions n

2.5

0.0

= 0.72 µm, up dressing

= 0.72 µm, down dressing

= 0.18 µm, down dressing

= 0.18 µm, up dressing

Fig. 4.25 Inﬂuence of roll-out revolutions on the starting effective peak-to-valley roughness of

the wheel [31] used with permission copyright R. Schmitt

= ±

dressing speed ratio

down dressing

up dressing

−

dressing overlap ratio

fad

frd

+ 0.5=

fad

= f

fad

form roller

Fig. 4.26 Kinematics and parameters in dressing with form rollers

4 Dressing of Grinding Wheels 205

4.5.2 Diamond form Rollers

The pinciple of dressin g with form rollers is shown in Fig 4.26. On the basis of

the process kinematics, the main parameters in dressing with form rollers are:

: dressing speed ratio

: dressing overlap ratio

: the depth of dressing cut.

In the same way as with dressing with a proﬁle roller, the path curve of a point on the

periphery of a form roller can be also described mathematically as an epicycloid in

accordance with (4.1) and (4.2) with f

¼0. Here also the engagement paths of

diamond in practice-relevant range for q

(0.1<|q

|<1) are ﬂatter in up dressing

than in down dressing. Therefore up dressing leads to ﬁner wheel topographies,

resulting in higher grinding forces (Fig. 4.27). In practice, the vitriﬁed bonded CBN

wheels are predominantly dressed in down dressing, in order to avoid thermal damage

to the workpiece.

The overlap ratio U

is other important parameter in dres sing with form rollers.

As with stationary dressing tools, the dressing overlap ratio U

is the value

produced from the quotient of the engagement width of the form roller a

and the axial dressing feed of the roller per grinding wheel revolution f

In practice and in research work, the overlap ratio is often calculated from the

active width b

– referred to here as U

d(bd)

– instead of a

. The U

d(bd)

is greater than

the overlap ratio calculated with the engagement width a

[2, 39]. Increasing the

increases the diffrence beetween the overlap ratios calculated from the active

width and from the engagement width.

A higher dressing overlap ratio produces a smoother wheel topography

and therefore less workpiece roughness, but also higher grinding forces (see

Fig 4.27). This causes an increase in temperatures in the grinding contact zone,

which may lead to thermal damages of the workpiece.

By dressing an inclined contour element of the wheel proﬁle there are different

ways to create the overlap ratio. Also the overlap ratio U

d(c)

along the contour, the

axial overlap ratio U

d(a)

and radial overlap ratio U

d(r)

can be deﬁned (Fig 4.28).

As (Fig 4.29 ) shows, the differences between U

d(c)

and U

d(a)

for dressing para-

meters common used in practice are not remarkable. Fig 4.31 also shows the

inﬂuence of the kinematics of the dressing units on the dressing overlap ratio.

The ﬁgure shows clearly that by CNC continuous-path control at a constant path

speed v

, there is a much smaller deviation in the overlap ratio than by copying the

contour of a master template at a constant axial speed v

fad

by means of a pantograph

system [36]. A constant overlap ratio along the entire grinding wheel proﬁle can

result in a too ﬁne topography at the sleep ﬂanks, which may lea d to thermal

damages of the workpiece in these areas. In these cases, a smaller overlap ratio in

the ﬂank area is more advantageous [32].

When dressing an inclined contour element the process is also inﬂuenced by

the direction of cut (Fig. 4.30). With a pulling dressing cut, the dressing engagement

206 T. Tawakoli and A. Rasifard

Fig. 4.27 Grinding forces and workpiece roughness as a function of dressing conditions when

using form rollers

Fig. 4.28 Engagement conditions on an inclined contour element used with permission copyright

[36] used with permission copyright Vulkan-Verlag

4 Dressing of Grinding Wheels 207

length is more in the ﬂank area and with a pushuing dressing engagement length it is

more in the front face area. Also, at a constant path speed, with the pulling dressing

cut, because of the greater local chip thickness higher process forces occur, which on

the one hand increase dressing roller wear and on the other hand increase the inducing

of cracks in the grits and the bond of the grinding wheel and as a result increase wheel

wear [4, 24].

The dept h of dressing cut a

also affects the forming of the wheel topography

and dressing rol ler wear. When the dressing of superabrasive CBN wheels, for

technical and economic reasons, the depth of dressing cut is between 2 and 6 mm.

Tests show that when dressing with a depth of dressing cut of less than 2 mm the

CBN grits do not split completely, which results in the cutting ability of the gri ts

Fig. 4.30 Kinematics of engagement in the pressing and drawing dressing cut [5 ]

contour inclination an

le β

axial overlap ratio U

d(a)

axial overlap ratio U

d(c)

−60 −40 −20 +20 +20

degree degree +60 +60

−60 −40 −20

d(a)

d(c)

d(a)

d(c)

contour inclination an

le β

fad

= 20 mm/min = const.

= 40°

= 5 µm

= 0.3 mm

= 60 m/s, d

= 400 mm

fdress

= 20 mm/min = const.

= 40°

= 5 µm

= 0.3 mm

= 60 m/s, d

= 400 mm

Fig. 4.29 Overlap ratio as a function of the contour inclination angle b at a constant axial feed

speed (left) and at a constant path speed (right)[36] used with permission copyright Vulkan-

Verlag

208 T. Tawakoli and A. Rasifard

being reduced [40]. While reducing the dept h of dres sing cut leads to an improve-

ment in workpiece surface quality, becau se the wheel topography is levelled

considerably, it does result in an increase in grinding forces and theref ore in a

reduction in the material removal rates that can be achieved [4]. Increasing depth of

dressing cut leads to an increas e in dressing roller wear [41].

The long-term behaviour of the wheel is also inﬂuenced by the dressing

parameters. In the grinding tests illustrated in Fig. 4.31 workpiece roughness decreases

at the beginning phase of the grinding operation. This is due to grit ﬂatting or

grit edge rounding. But on the other hand the grinding forces also fall at the start of

the grinding process, which is due to an initial relief grinding phase. It can be

established that after the unstable initial grinding phase the grinding process is only

inﬂuenced to a lesser extent by the dressing parameters. The changes in grinding

forces and workpiece roughness during the unstable initial grinding phase depend not

only on the dressing parameters but also on the speciﬁcations of the grinding wheel

and the dressing roller, so at the beginning of the grinding operation an increase in

workpiece roughness or an increase in grinding forces can also occur [4, 42].

The dressing conditions can also have a signiﬁcant inﬂuence on wheel wear, particu-

larly when grinding with water-miscible cooling lubricants (Fig. 4.32).

The wear of a form roller is inﬂuenced by the dressing parameters.

Generally, dressing roller wear is greater in down dressing than in up dressin g.

This may be due to the higher process forces and the resulting micro-splitting

Fig. 4.31 Long-term wheel behaviour under different dressing conditions

4 Dressing of Grinding Wheels 209

of the diamond grits in down dressing [4, 23]. A reduction in U

and also an

increase in a

can also lead to increased dressing roller wear. Form roller wear

can also be inﬂuenced by the size of the grits in the grinding wheel. When

dressing superabrasive CBN wheels, with a coarser grit wheel there is greater

wear on the form roller [4].

4.5.3 Diamond Cup Wheels

Diamond cup wheels can be used economically for the dressing of rectilinear

small grinding wheels, which is for example nearly always the case when

internal cylindrical grinding [43]. Diamond cup wheels are usually driven with an

electric spindle motor, but for simple dressing tasks diamond cup with a rotary

wing wheels can also be used (Fig. 4.33). Cup wheels with a rotary wing are driven by

the grinding wheel through frictional contact or by the coolant or compressed air and

braked by the wings at higher rotational speeds [44]. When dressing, the spindle

motors are loaded axially and radially. To avoid high axial tilting moments on the

spindle bearing, the external diameters of diamond cup wheels are kept small. As a

result, only relatively low tool peripheral speeds can be achieved [33]. The dressing

operation with diamond cup wheels is carried out by the axial feed of the cup wheel,

whereby the cup wheel is tilted at an angle of 3–10



to the rotation axis of the grinding

wheel [4]. The process parameters that can be set freely when dressing with diamond

cup wheels are the dressing speed ratio q

, the dressing overlap ratio U

, the depth of

dressing cut a

and the wheel peripheral speed v

.Likeindressingwithdiamond

rollers, carrying out the dressing process in up dressing results in a ﬁner wheel

topograpy, which leads to higher grinding forces and a ﬁner workpiece surface.

When dressing with diamond cup wheels the depth of dressing cut a

also plays

a very important role. As Fig. 4.34 shows, with increasing a

the dressing normal

forces incr ease more by superabrasive CBN wheels than by conventional alumina

Fig. 4.32 Radial grinding wheel wear when grinding with different coolant lubricants and

dressing parameters

210 T. Tawakoli and A. Rasifard

or SG grinding wheels. This is due to the greater compressive strength and greater

hardness of the CBN grits, and also the greater strength of the vitriﬁed bond used in

CBN wheels. Higher dressing forces can lead to shape deformation particularly if

the grinding systems are not sufﬁciently stiff, as for example by internal cylindrical

grinding. Therefore CBN wheels are dressed with depth of dressing cuts that are as

small as possible [43].

4.5.4 Crushing

Crushing is a special case of dressing with rollers, in which the dressing speed

ratio q

is equal to one. In this process, the dressing diamonds impact on the surface

dressing tool

dressing cup wheel

D426 M2720 C150

dressing parameters

= 60 m/s (for CBN wheel)

= 40 m/s (for SG wheel)

= 40 m/s (for alumina wheel)

= 45

= 47 m/s

dressing method : uni-directional

coolant lubricunt

3% emulsion

0 5 10 15 µm 25

normal dressing force F

depth of dressing cut a

CBN wheel

B91 V47-2 C175

SG wheel 5SGG60 L7 V

alumina wheel A60 L5 V

Fig. 4.34 Dressing normal forces with alumina, SG and vitriﬁedbonded CBN grinding wheels [9]

used with permission copyright M. C¸ inar

air pressure: 6 bar

depth of dresing cut a

: 3-5 µm

feed rate v

fad

:120-200 mm/min

fad

G 1/8

connection

aluminum

housing

air

7°

Fig. 4.33 Cup wheel with a rotary wing [9] used with permission copyright Winterthur Technology

Group

4 Dressing of Grinding Wheels 211

of the wheel at a penetration angle of 90



. In the absence of the relative

speed between grinding wheel and dressing roll, the removal mechanism in

crushing is based on the fracture of bond posts in the abrasive layer by applied

normal forces. There fore, crushing is only used for wheels with brittle bonds such

as vitriﬁed or brittle bronze bonds. When a diamond of the dressing roller engages

the wheel surface, a normal force acts on the abrasive layer. When this normal force

has exceeded a certain level, it causes the bond posts to break, thereby generating

the wheel proﬁle.

Depending on the process kinematics a distinction is made between full form

crushing and point crushing. In full form crushing, a proﬁle roller is u sed as dressing

tool. In the point crushing, a diamond form roller is used as dressing tool, which

produces the necessary wheel proﬁle through the CNC control of the roller move-

ment. Crushing can be carried out with both diamond dressing rollers and diamond-

free rollers made of steel or tungsten carbide. Compared with point crushing, full

form crushing is characterised by little being needed in terms of control and

relatively short proﬁling times. However, in full form crushing, there is a relatively

large contact cross-section, which results in high radial forces. Besides, in full

crushing of a non-cylindrical grinding wheel the dressing speed ratio q

can be

equal to one only at one contact point. The relative speeds which are present over

the rest of the proﬁle lead to wear on the crushing roller [24, 36 ]. In full form

crushing, the radial dressing feeds f

are less than 5 mm per wheel rotation and the

peripheral speeds are less than 5 m/s, which are considerably less than the cutting

speed in the grinding operation [24].

Point crushing is much more ﬂexible than full form crushing. With this method,

using a dressing spindle with control of speed the whole of the proﬁle of a non-

cylindrical gri nding wheel can also be dressed with q

¼1, thereby minimising the

wear of the roller and as a result maximising the accuracy of the proﬁle of the wheel.

In addition, compared with full crushing, because of the additional parameter of

overlap ratio U

, in the point crushing the wheel topography can be created over a

wider range. Also, point crushing can be carried out at higher peripheral speeds than

form full crushing. Point crushing with diamond form rollers has signiﬁcant eco-

nomic advantages particularly in the proﬁling of ﬁne-grained diamond wheels for

proﬁle or tool grinding [24]. The effects of system variables and process parameters

on the process rasults by point crushing of diamond grinding wheels are summarised

in Fig 4.35.

4.5.5 Touch Dressing

Touch dressing is used for conditioning superabrasive CBN grinding wheels,

whereby the wheel is dressed with a form roller with very small depth of dressing

cuts (in the micrometer range). In touch dressing, the exposed peaks of the grits are

only splintered in order to increase the number of dynamic cutting edges on the

surface of the wheel and thereby reduce the roughness of the workpi ece. On the other

212 T. Tawakoli and A. Rasifard

hand, however, increasing the number of dynamic cutting edges and reducing the

grit protrusions and chip spaces by touch dressing leads to an increase in grinding

forces, which must be taken into consideration in the design of the process. Touch

dressing is mainly used for new electroplated CBN wheels, because in these wheels

due to the different grit geometries and the scatter of the grit size the CBN grits

protrude to a varying extent, as a result of which the workpiece roughness that is

generated can be much higher than expected (Fig . 4.36).

When dressing multi-layer vitriﬁed and metal bonded CBN wheels the attempt is

also made to carry out the process with depth of dressing cuts that are as low as

possible in order to avoid a serious reduction in the chip spaces between the

abrasive grits.

workpiece

max

workpiece

max

wheel body wheel body

CBN grits

before touch dressin

after touch dressin

= 10–15% of the grit size

Fig. 4.36 Schematic of touch dressing an electroplated CBN grinding wheel

system parameters

increase the grain size of the

grinding wheel

increase the hardness of the

grinding wheel bond

process parameters

increase the overlap ratio U

increase the depth of cut a

increase the speed of the

grinding wheel v

dressing ratio G

specific normal

dressing force F´

roughness of the

grinding wheel

Fig. 4.35 Effect of system and tool setting parameters with the point crushing of diamond

grinding wheels [25] used with permission copyright Dr. Kaiser Co

4 Dressing of Grinding Wheels 213

4.5.6 Continous Dressing

Continuous dressing is a special type of conditioning, in which a conventional wheel

is dressed continuously during the grinding process with a diamond proﬁle roller.

As a result, the wheel always has an accurate proﬁle during grinding and a sharp

cutting face. Figure 4.37 shows the principle of grinding with continuous dressing.

In this process the amount of material removed by dressing must always be greater

than the wear on the wheel. When continuous dressing the reduction in the diameter

of the wheel during grinding has to be compensated by the machine’s control system.

Continuous dressing is generally used for the grinding of hard-to-cut materials.

A typical application is the grinding of turbine blades made of super alloys such as

high nickel base alloys [45]. The main advantage of continuous dressing is the greatly

reduced manufacturing time due to the greater speciﬁc material removal rates that can

be achieved with high grinding quality and stability [46].

4.6 Ultrasonic Assisted Dressing

In ultrasonic assisted machining, the kinematics of the conventional machining

process are superimposed with an additional vibration in the micrometer range at

ultrasonic frequencies, namely higher than 16 kHz [47]. With the ultrasonic assis-

tance, considerable process improvements can be achieved in machining processes

with geometrically deﬁned cutting edges as well with geometrically undeﬁned

cutting edges. The most important advantages of using ultrasonic assistance in

traditional machining processes are [48–52]:

workpiece

profile roller

grinding wheel

2.v

frd

Fig. 4.37 Principle of continuous dressing

214 T. Tawakoli and A. Rasifard