Jackson Mark. Machining with Abrasives

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single-layer diamond coatings using the positive method, the diamond grits are ﬁrst

scattered statistically directly on to the base body of the roller and joined then in

an electroplating process with a single nickel layer to the base body [33]. The grit

density is high and the geometric coating shape is determined by the scatter of the

diamond grit size. These diamond rollers can be again re-coated after the end of their

service life [24, 28]. However, because of the way in which they are made and the

scatter of the diamonds in terms of size and shape, the surface of these tools is

relatively rough and irregular. Hence these rollers should be ground before being

used in the dressing process. This serves not only to achieve sufﬁcient geometrical and

dimensional accuracy, but also to generate a high contact area ratio on the roller right

Fig. 4.14 Manufacturing methods for rotary diamond dressers

Fig. 4.15 Schematic illustration of the manufacturing methods for rotary diamond dressers [33]

4 Dressing of Grinding Wheels 195

at the start of dressing, which avoids unstable dressing roller wear at the initial

dressing phase [4].

The manufacturing of the multi-layer diamond coatings through the positive

method is carried out by sintering, whereby a mixture of metal powder and diamond

grits is compressed in a mould at high pressure and then, at high pressure and

high temperature is connected to the tool body. With this method diamond layers

between 2 and 5 mm thick can be produced. So it is possible to renew the proﬁle on

these diamond rollers by regrinding them. Also, the diamond concentration can be

controlled regardless of the diamond grit size and geometry. As the relatively high

sintering temperatures lead to warping and also non-uniform shrinkage of the layer,

with this method often the diamond rotary dresser has to be ground after its

manufacture [33].

In the negative method (also calle d the reverse method), the geometry of the

roller is produced in a high-precision negative lost mould (made of graphite,

aluminium or steel), which is destroyed afterwards. A single layer of diamonds is

ﬁrst applied to the surface of this negative mould and the diamonds are then joined

together with an electroplated nickel bond or by inﬁltration (see Fig. 4.15).

In the manufacture of electroplated diamond dressing rollers by the negative

method, the negative, extremely accurate mould, which is made of metal, is densely

covered with a single layer of statistically distributed diamonds. The diamond grits

are then joined together by electroplating an abrasion-resistant nickel alloy layer.

The electroplating process can last for a number of days or even wee ks. After

the electroplating, a base body is placed inside the negativ e mould. Next, a binder

material is poured into the intermediate space between the base body and the

electroplated layer in order to join these two components. Finally the negative

mould is removed and destroyed. Because the process temperatures can be kept

below 60



C, there is very little warping and change in dimensions. These diamond

roller dressers are char acterised by a high percentage of contact area and better

geometric and roundness accuracy, so they do not need to be ground after manu-

facture [24, 27, 33]. The negative method can also be carried out by inﬁltration. In

this case the negativ e mould is made of graphite. The diamond s are ﬁrst scattered on

to the proﬁle face of the negative mould and/or set in a deﬁned pattern, and then

ﬁxed with an adhesive. During the setting of the diamonds, the position, geometry

and type of the diamonds can be determined speciﬁcally. After laying the base

body, which roughly ﬁts the proﬁle of the roller, the intermediate space between the

base body and the diamond layer is ﬁlled with tungsten powder. After the compac-

tion of the tungsten powder and the subsequent addition of the inﬁltration material

(brazing sold er), the mould is heated in an oven at atmospheric pressure, whereby,

at high temperatures, the inﬁltration and the sintering processes take place. During

the inﬁltration, the inﬁltration material penetrates into the capillaries of the com-

pacted metal powder and joins the diamond layer with the roller body [33]. Due to

the shrinkage of the binder matrix after sintering, the diamond layer becomes very

solidly ﬁxed to the tool body. Furthermore, because of its good thermal conductiv-

ity characteristics inﬁltration binding enables the diamonds to cool well, which

results in longer tool life [28]. While the roller cools down, the bond shrinks in an

196 T. Tawakoli and A. Rasifard

uncontrolled way. In addition, the adhes ive that is used to ﬁx the diamond grits to

the negative mould reduces the accuracy that can be achieved. Therefore, in order to

achieve high tolerances, diamond rollers made by the negative method with inﬁl-

tration have to be ground after man ufacture.

4.4 Dressing with Stationary Diamond Dressing Tools

Dressing with stationary dressing tools is comparable to turning. When dressing

with stationary dressers, the engagement conditions on the grinding wheel depend

on the geometry of the dressing tool as well as on the dressing parameters

(Fig. 4.16). The most important parameters are the depth of dressing cut a

and

overlap ratio U

. The dressing overlap ratio U

is the quotient from the engagement

width a

and the axial dressing feed f

and shows how often a point on the

peripheral face of the wheel is engaged by the dressing tool. The permitted lower

limit value for the overlap ratio is U

¼1.0, with which each peripheral point of

the wheel is engaged once by the dressing tool. The dressing overlap ratio has

a decisive effect on the generation of the topography of the wheel when dressing.

A higher dressing overlap ratio results in a ﬁner wheel topography. Therefore, as the

dressing overlap ratio increases, the grinding forces augment and the workpiece

roughness decreases (Fig. 4.17). Decreasing the U

results in an increase in wheel

wear. This can be due on the one hand to more intensive crack formation in the

abrasive grits and the bond with a higher axial dressing feed, and on the other hand

to the greater engagement forces on the indivi dual gri ts when grinding with a

coarser grindin g wheel. As it can be seen from Fig. 4.17, an increase in U

beyond

a certain overlap ratio does not result in any signiﬁcant change in the grinding

process but in an unnecessary increase in dressing time. This maximum useful

overlap ratio depends on the speciﬁcation of the wheel that is being used [2]. The

fad

diamond

fad

Fig. 4.16 Engagement conditions and parameters when dressing with stationary dressing tools

4 Dressing of Grinding Wheels 197

grinding process is also inﬂuenced by depth of dressing cut. The inﬂuence of depth

of dressing cut on the formation of the wheel topography is, however, less than that

of the overlap ratio. A slight increase in the number of cutting edges with increasing

depth of dressing cut when dressin g with a blade dresser has been reported [33].

The dressing forces and the development of heat occurring in the dressing

contact zone, and therefore dresser wear, are also inﬂuenced by the dressing

parameters. As shown in Fig. 4.18, the dressing ratio G

, which is calculated

from the quotient of the volume of grinding wheel that has been dressed and the

volume of wear on the dressing tool, decreases with increasing the depth of dressing

cut a

and axial dressing feed f

. On the other hand, however, an increase in wheel

peripheral speed v

at a constant U

leads to an increase in the dresser wea r ratio.

An increase in the dressing overlap ratio U

leads also to an increase in the wear of

the dresser.

The generation of the grinding wheel topography can be inﬂuenced by the wear on

the dressing tool. The wear of a diamond with the originally circular-arc-shaped tip

leads to the engagement width becoming greater, and as a result to a reduction in the

effective peak-to-valley roughness of the wheel at a constant axial dressing feed

rate. The gradu ally increasing ﬂattening wear causes irregularities in the results of a

recurrently performed dressing operation. This type of wear behaviour cannot be

avoided in the case of single point dressers with natural diamond. A solution to this

problem is the use of synthetic diamond dresser logs with a constant cross-section,

which despite wear have an engagement width that remains the same [26].

workpiece

roughness R

specific grinding

forces F`

, F`t

overlap ratio U

02 46 810

F´

grinding wheel

vitrified alumina wheel

A60 K–8 V

dressing tool

dressing plate

dressing parameters

= 45 m/s

= 30 µm

workpiece

Ck 45 N

grinding parameters

= 45 m/s

q = 60

Q´W = 1 mm

/mm.s

V´W = 400 mm

/mm

= 10 s

radial wheel

wear Δr

overlap ratio U

µm

0246810

105 µm

25 µm

effective profile of the

dressing plate

overlap ratio U

Fig. 4.17 Inﬂuence of dressing overlap ratio on the grinding process [2] used with permission

198 T. Tawakoli and A. Rasifard

4.5 Dressing with Rotary Diamond Dressing Tools

Rotary diamond dressing tools are extremely important in grinding, particularly

when dressing of superabrasive CBN grinding wheels. Rotary diamond dressers

contain much more diamond than stationary diamond dressers, which enables

the wheel to be proﬁled with the required geometric and dimensional accuracies

over a longer period. As Fig. 4.4 shows, depending on the way in which the proﬁle

is produced, there are essentially three types of rot ary diamond dresser, namely:

Diamond proﬁle rollers

Diamond form rollers

Diamond cup wheels.

The choice of which dressing tool should be used depends, apart from what is

technically possible with the machine being used, on the criteria of dressing time

and tool costs as well as on the wheel proﬁle that is to be dressed. While the area

of application of the diamond cup wheel is limited to the dressing of rectilinear

wheel proﬁles, complicated grinding wheel contours can also be dressed with

diamond proﬁle and form rollers. Dressing with proﬁle rollers is a proﬁle copying

method, in which the proﬁle roller has the negative form of the proﬁle of the

grinding wheel. The whole of the proﬁle of the wheel is produced by plunging

the rotating proﬁle roller radially into the grinding wheel, which is also rotating.

Diamond proﬁle rollers are characterised by very short dressing times and also great

proﬁle accuracy. However, the use of a diamond proﬁle roller is economically

convenient only from a certain minimum batch size. For this reason diamond proﬁle

rollers are used mainly in mass production.

Dressing with form rollers is a proﬁling method in which the wheel proﬁle

is produced by the continuous-path controlled movement of a form roller with a

small engagement width. Diamond form rollers are characterised by great ﬂexibility.

What is characteristic of a form roller is the tip radius of the roller, which determines,

what shapes can be produced on the grinding wheel. The applications in which a

diamond form roller can be used can be increased by an additional slewing move-

ment [27, 33]. A problem with an additional slewing axis is, however, the fact that

usually it involves greater costs and time in terms of programming and control, and

dressing ratio G

dressing axial

feed f

wheel peripheral

speed v

depth of dressing

cut a

overlap ratio U

Fig. 4.18 Trend in the relationship between dressing parameters and dresser wear ratio [33] used

with permission copyright Riegger Diamantwerkzeuge

4 Dressing of Grinding Wheels 199

also the fact that it reduces the stiffness of the dressing system. When dressing with

form rollers, the dressing forces are lower than with proﬁle rollers, because of the

smaller engagement width. This can have a positive effect on the achievable dressing

accuracy [4]. Furthermore, the grinding wheel topography can be inﬂuenced in the

dressing with form rollers, because of their additional axial movement compared to

proﬁle rollers, over the overlap ratio in a wider range.

Due to the small engagement width, unlike with the proﬁle roller, this results

in longer dressing times. On the other hand, the cost of diamond form rollers,

in contrast to diamond proﬁle rollers are independent of the width of the grinding

wheel. This can bring about cost advantages for form rollers even in mass

production particularly in grinding with wide grinding wheels [4].

The dressing process is inﬂuenced by the speciﬁcations of the grinding wheel

and the dresser, and also by the dressing parameters. The section below explains the

effect of these inﬂuencing factors on the dres sing and grinding results when

dressing with rotary diamond dressing tools.

4.5.1 Diamond Proﬁle Rollers

The kinematics of dressing with proﬁle rollers are shown in Fig. 4.19. The most

important parameters when dressing with proﬁle rollers are:

: radial dressing feed

: dressing speed ratio

: number of roll-out revolutions.

The dressing speed ratio q

is the ratio of the peripheral speed of the roller dresser

to the peripheral speed of the grinding wheel when dressing v

. The sign of the

dressing speed ratio is deﬁned by the speed directions of the active partners in the

dressing contact zone. If the speed directions of the active partners in the contact

zone are the same, the dressing speed ratio is called down dressing and is designated

as positive. If the speed directions of the active partners in the contact zone are

contrary to each other, the dressing speed ratio is called up dressing and is

designated as negative.

In dressin g with rotary diamond dressing tools the wheel topography is

produced by engagements of the roller dresser diamonds with the surface of the

wheel. When dressing with proﬁle rollers the paths of the diamonds are determined

by f

and q

. Assuming a roller running in an ideal round movement, the path curve

of a peripheral point of the rotating proﬁle roller in the ﬁxed-wheel coordinate

system (x,y) can be described in accordanc e with (4.1) and (4.2). The angle ’

describes the starting angle of the point to be looked at in the polar coordinate

system of the roller [5, 36].

x ¼ððr

þ r

Þf



2  p

Þcos f

 r

 cosðf

ð1 þ q



Þf

Þ (4.1)

200 T. Tawakoli and A. Rasifard

y ¼ððr

þ r

Þf



2  p

Þsin f

 r

 sinðf

ð1 þ q



Þf

Þ (4.2)

Figure 4.20 shows some examples for path curves at different values of q

and f

As it can be seen from this ﬁgure, the dressing speed ratio q

has a dominant

inﬂuence not only on the relative speed between grinding wheel and dressing tool,

but also on the shape of the engagement path curves, under which the diamond grits

of the roller strike the surface of the grinding wheel. For values q

that are common

in practice (0.1<|q

|<1), the engagement paths in up dressing are ﬂatter than in

down dressing.

The ﬂatter diamond engagement paths and the resulting greater overlapping of

the diamond grit engagements following each other lead in up dressing generally to

a ﬁner wheel topography, which results in a smaller initial effective peak-to-valley

roughness of the grinding wheel (Fig. 4.21). A reduction in the radial dres sing feed

also results in a reduction in the starting effective peak-to-valley roughness

[5, 30, 37].

As expected, the grinding process is inﬂuenced by the wheel topography gener-

ated during dressing operation (Fig. 4.22). After up dressing, because of its ﬁner

topography, the grinding wheel has a higher number of kinematic cutting edges.

Consequently, the grinding forces are higher after up dressing and the workpiece

roughness is lower. On the other hand, the topography of the wheel is itself

inﬂuenced by the grinding conditions; therefore, after dressing an unstable initial

grinding phase can occur. During this unstable initial grinding phase the effective

peak-to-valley roughness of the grinding wheel seeks to achieve a quasi-stable

value which particularly with conventional wheels depends only to a lesser

extent on the initial effective peak-to-valley roughness (Fig. 4.23).

When down dressing, it is possible to vary the starting effective peak-to-valley

roughness of the wheel in a greater range (see Fig. 4.21). However, the dressing

forces in down dressing are higher than in up dressing (Fig. 4.24). Hence the wear

of the diamond roller is in down dressing higher [33].

frd

±=

frd

dressing speed ratio

down dressing

60000

dressing radial feed

wheel

roller

: positive q

: negative

up dressing

Fig. 4.19 Kinematics of dressing with proﬁle rollers

4 Dressing of Grinding Wheels 201

When dressing with proﬁle rollers, due to the radial feed, an Archimedes’

spiral is produced on the periphery of the wheel. Therefore, if the proﬁle roller

is lifted after proﬁling immediately, a circular defect about the size of the

feed amount remains on the grinding wheel. To avoid this error and to relieve

the system, before lifting there should be a short roll-out, which corresponds

to spark-out in grinding process. Also, by rolling-out, the true-running error

−1.5

1.0 0.5 0

−0.5 −1.5−1.0

µm

initial effective peak-to-valley roughness

of the wheel surface R

tso

grinding wheel

vitrified alumina wheel

A60 L–8 V

dressing tool

profile roller (D700/7.5)

dressing parameters

= 29 m/s

= 0

= 0.72 µm

= 0.18 µm

up dressingdown dressing

dressing speed ratio q

Fig. 4.21 Inﬂuence of the parameters q

and f

on the initial effective peak-to-valley roughness

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

= −2.5 q

= −0.8

= +1.0

= 0 f

= 0

= +2.5 q

= +0.8 q

= +1.0

= 0 f

= 0

≠

Fig. 4.20 Example of engagement paths of a peripheral point of a proﬁle roller [36] used with

permission copyright Vulkan-Verlag

202 T. Tawakoli and A. Rasifard

specific normal

grinding force F´

workpiece

roughness R

grinding wheel

vitrified alumina wheel

A60 N9 V

dressing tool

cylindrical profile roller

dressing parameters

= 35 m/s

= ± 0.3, ± 0.5, ± 0.7

= 0.1 and 1 µm

workpiece

100Cr6 (60HRC)

grinding parameters

= 35 m/s

= 500 mm/min

= 0.2 mm

coolant lubricant

grinding oil

= 0.1 µm

= 1 µm

µm

−0.7−0.5−0.3

0.30.50.7

−0.1

0.1

dressing speed ratio q

Fig. 4.22 Inﬂuence of dressing conditions on the grinding process

µm

0 200 400 800

/mm.s

grinding wheel

vitrified alumina wheel

A60 K – 8V

dressing tool

profile roller (D700)

dressing parameters

= 45 m/s

= -0.7, + 0.9

= 0.7 µm

workpiece

Ck 45 N

grinding parameters

= 45 m/s

q = 60 mm/min

Q´W = 0.5, 1, 3 mm

/mm.s

specific material removal V´

Q´

= 3 mm

/mm.s, q

= 0.9

Q´

= 3 mm

/mm.s, q

= -0.7

Q´

= 1 mm

/mm.s, q

= 0.9

Q´

= 1 mm

/mm.s, q

= -0.7

Q´

= 0.5 mm

/mm.s, q

= 0.9

Q´

= 0.5 mm

/mm.s, q

=-0.7

effective surface roughness of

the grinding wheel R

Fig. 4.23 Change over time of the effective peak-to-valley roughness of the grinding wheel as a

function of dressing and grinding conditions [38]

4 Dressing of Grinding Wheels 203

of the grinding wheel that is caused by the true-running error of the proﬁle roller can

be reduced. However, the rolling-out decreases the initial peak-to-valley roughness

of the wheel (Fig. 4.25). The change in the topography of the grinding wheel

depends on the number of roll-out revolutions n

. As shown in Fig. 4.25, the initial

peak-to-valley roughness decreases strongly with the increasing the n

, especially

in down dressing and asymptotically approaches a limit value. There is no beneﬁt in

there being more than 100 roll-out revolutions because after this point no signiﬁcant

change in the initial peak-to-valley roughness of the wheel is achieved [31].

The generation of the grinding wheel topog raphy is also inﬂuenced by the

diamond grit size and concentration of the proﬁle roller. Reducing the diamond grit

size or increasing the concentration leads to a reduction in the initial peak-to-valley

roughness of the wheel. After Schmitt [31], the theoretical proﬁle height of the wheel

can be calculated as a function of the diamond grit size D and grit concentration K

and also the dressing speed ratio q

with (4.3).

H ¼

k K

 8  r

 q

 1ðÞ

(4.3)

In dressing with rotary dressing tools, the wheel peripheral speed v

is a readily

adjustable process parameter. If possible, the grinding process is carried out at the

peripheral speed of the grinding wheel when grinding, which is, with the grindin g

wheel speed v

of the following grinding process. Thus the wheel deformations that

occur in dressing and in grinding due to the centrifugal forces remain equal.

When dressing of conventional grinding wheels at a constant dressing speed

ratio, v

does not have any signiﬁcant effect on grinding forces and workpiece

roughness [31, 37].

1.25 1 0.5 0

−0.5 −1 −1.25

7.5

2.5

specific radial dressing force F´

dressing speed ratio q

Fig. 4.24 Speciﬁc radial dressing force as a function of dressing conditions when using proﬁle

rollers [37] used with permission copyright H. Scheidermann

204 T. Tawakoli and A. Rasifard