Guide to Centerless External Cylindrical Grinding – part II of the reference work

27
After Chapter Fundamentals gave a short overview of centerless grinding, the fol-
lowing explains it in more detail.
The grinding zone is the heart of centerless external cylindrical grinding. To achieve
optimal roundness of the workpiece, specific geometric conditions are necessary in
the grinding zone. If the acting forces and relationships are known, the roundness can
be significantly improved.
Why, for example, does the regulating wheel determine the speed of the workpi-
ece instead of the grinding wheel? Or why does the workpiece not jump out of the
grinding zone? The following chapter will answer these questions. It also includes
formulas for calculating important grinding parameters.
3
The grinding zone
Grinding zone geometry
Center shift
Roundness
Stability
Height position
Polygons
Forces

28
3.1 Center shift
A special feature of centerless grinding is the inclined workpi-
ece support on which the workpiece rotates. During grinding, the
workpiece continuously loses in circumference, so its center incre-
asingly moves into the grinding zone (Fig. 3-1).
This center shift is the reason why diameter-related infeed, not
radius-related infeed (as in grinding between centers) is used in
centerless external cylindrical grinding.
As the grinding zone into which the workpiece sinks becomes
narrower and narrower, normally less infeed is required than the
pure difference between initial diameter and final diameter would
imply. On KRONOS machines with a grinding zone geometry soft-
ware, this effect is taken into account and corrected automatically
in the infeed program.
Fig. 3-1
Center shift of
the workpiece
during the grinding
process
InfeedInfeed
3 The grinding zone | Center shift
… space between
grinding wheel,
regulating wheel,
workpiece, and
workpiece support

29
3.2 Shaping process
Roundness of workpieces has a special significance in centerless
external cylindrical grinding. Though the machined workpieces can
have the same diameter at all points, they may still not be round.
For better understanding, the following provides a definition of
roundness:
3.2.1 Roundness
A workpiece is round if there is a point
in its cross section (e.g. the center) from
which all points on the circumference have
the same distance. Ideally, this is a circle,
but in practice this is never reached (Fig.
3-2).
If the diameter of a workpiece is measured
at various points, this cannot be used to
infer its roundness, because roundness is
not directly related to the diameter (Fig.
3-3).
Misleading:
The workpiece in Fig. 3-3 is definitely not round. Nevertheless, its
diameter is always the same. This is therefore referred to as so-
called curve of constant width.
So, while the diameter can be determined using a two-point
measuring method, the workpiece must also be rotated to measu-
re its roundness (Fig. 3-4).
Fig. 3-2
Roundness of a
circle
Fig. 3-3
Determining the
roundness by the
diameter
… more in the
glossary
Shaping process | 3 The grinding zone

30
3.2.2 Determining the roundness error
Roundness errors arise during the grinding process due to a variety
of influences such as geometric and dynamic factors.
To determine the roundness error (Rd
), one calculates the diffe-
rence between the smallest circumference (da
) and the largest
inner circle (di
) (Fig. 3-4).
This calculation requires that the
inner circle be located centrally to
the circumference (Fig. 3-4).
The workpiece center required for
this can be determined in several
ways.
For correct roundness measure-
ment, a roundness measuring de-
vice should be used. Alternatively,
a dial gauge [1] can be used in
combination with a two-point
support v-block [2] (Fig. 3-5). The
v-block angle of a support v-block
has a crucial significance for the
measurement results.
Fig. 3-5
Determining the
roundness during
rotation
Rd
= da
– di
Fig. 3-4
Workpiece center
with the “Minimum
circular ring zone“
method
↑ Section 3.2.4
Polygons
3 The grinding zone | Shaping process
[2] Two-point
support v-block
[1] Dial gauge

31
Reducing the roundness error
During centerless external cylindrical grinding, the workpiece is both
guided and machined using its outer surface. Therefore, it is not pos-
sible to achieve absolute roundness in principle, resulting in con-
stant width shapes. However, under suitable setting conditions, it is
possible to reduce the remaining roundness error to a large extent.
The results are in ranges that are comparable to those achieved by
processes between centers.
3.2.3 Height position H and mounting angle β
The center line Z is the direct connection between the centers
of the grinding wheel and regulating wheel. If the workpiece axis
during grinding is on this center line Z, this is referred to as a zero
height position of the workpiece (Fig. 3-6). If workpieces are ground
with this setting, they will not be round. If an elevation on the workpi-
ece surface contacts the regulating wheel, a pit is ground exactly at the
opposite side into the workpiece by the grinding wheel. An error (wave
crest) on one side therefore also causes an error (wave trough) on the
other side (Fig. 3-6).
Fig. 3-6
Height position
equals zero
Wave crest Wave trough

32
During further revolutions, these two positions cannot cancel each
other out, because they are diametrically opposite. The workpiece
remains non-round.
To improve the roundness, the workpiece is moved out of the cen-
ter and the support angle β is increased. In the so-called „grin-
ding above center“, the contact points of grinding wheel and
regulating wheel are not directly opposite each other anymore.
The resulting center shift of the workpiece leads to a reduction
of the circularity error, if its amount is smaller than the roundness
error that causes it (x < x1, Fig. 3-7). A limit of increase is achieved
when the forces from the grinding wheel and regulating wheel
start to act upward and the workpiece begins to lift off from the
workpiece support.
However, height position and rotational speed are no guarantee
for success, because other factors play a role as well.
Fig. 3-7
Reducing the
roundness error
by grinding above
center
Wave crest Wave trough
↑ Section 3.2.5
Stability index SI
The height position
H is referred to
as above-center
position H in the
grinding-above-
center process as
shown.

33
Instead of grinding the workpiece as described above the center
line Z, it can also be ground „below center“. Good results are
achieved also with this “grinding below center“. Both grinding
methods have advantages and disadvantages, which are analyzed
more closely in the following comparison:

3.2.4 Polygons
Each grinding zone adjustment has a typical roundness scenario.
This means, the periodic waves on the outer surface always occur
in the same degree of specification.
Grinding “above“ center Grinding “below“ center
In throughfeed grinding, workpieces
with a conical regulating wheel
shape diverge
a frontal surface quality of the
workpieces has no influence
Geometrically more stable areas
Low force action on workpiece
support
Workpieces have a degree of
freedom a less prone to dynamic
influences
Workpiece can jump out of the
grinding zone a workaround in
throughfeed grinding: Application of
an upper guide
Workpieces with a conical regulating
wheel shape stay in one column in
throughfeed grinding
a workpieces do not tilt (important
for rings)
Workpieces cannot jump out
The process is possible even at
greater material removal rates
In throughfeed grinding, workpieces
with a conventional regulating wheel
shape also remain in one column
Few and small stable areas a very
precise machine setting necessary
Workpiece has no degree of freedom
a very susceptible to dynamic
influences
… self-induced
and externally
induced vibrations
of machine and
workpiece
↑ Chapter 6
Influencing factors
Tab. 3-1
Grinding above
and below center
↑ Section 3.3.2
Height position H

34
These factors can be used to classify the roundness error in more
detail. For this purpose, the periodic components of the roundness
profile are eliminated by a so-called FFT analysis. The result of
such an analysis is a roundness spectrum in which the amounts
of the amplitudes of all harmonics are plotted. The harmonics are
also called polygons in this context.
Figure 3-8 shows the superposition of polygons. A possible cause
of the polygon shapes is the grinding zone geometry.
One variable when considering the
polygons is the penetration depth e
(Fig. 3-9). The lower the wave
number of the polygons, the larger
is generally the roundness error (Fig.
3-10).
… Fast Fourier
Transformation
Fig. 3-8
Superposition of
polygons
Fig. 3-9
Penetration
depth e
+ =
Polygon of 3 Polygon of 35
Fig. 3-10
Relationship of
wave number and
roundness error

35
But how are polygons formed? Let us
imagine a grinding zone for plunge grin-
ding (Fig. 3-10a). The workpiece rotates
and the grinding wheel is fed forward.
This creates a new lateral surface (Fig.
3-10b). If it comes in contact with the
workpiece support, the original work-
piece center will shift. The workpiece
moves in the direction of the workpiece
support, whereby the infeed changes
(Fig. 3-10c). A new size of the outer sur-
face is created (Fig. 3-10d). If this then
comes in contact with the workpiece
support, the effect occurs again. Ho-
wever, not only by contact of the new la-
teral surface with the workpiece support
does the center shift occur, but also by
contact with the regulating wheel. This
in turn changes the infeed again, resul-
ting in another roundness error. In total
three roundness errors emerged by a
half workpiece revolution, which in turn
cause further roundness errors.
3.2.5 Stability index SI
The stability index SI provides information on whether a polygon
of a certain order improves (is reduced) or worsens (becomes more
pronounced) during grinding.
Fig. 3-10a
Fig. 3-10b
Fig. 3-10c
Fig. 3-10d

36
SI(z) = positive a polygon is reduced
SI(z) = zero a polygon is not reduced
SI(z) = negative a polygon grows
A geometrically stable setting is reached, if the stability index
of all considered polygons is positive. In practice, usually the
polygons from 2 to 30 are considered.
However, one or more polygons often have negative values. The polygon
with the most negative stability index (hereafter called w) should be re-
flected the most – from a purely geometric point of view. But tests and
energy considerations reveal that the energetically more favorable lower
polygon orders with negative SI are dominating. So, there is no direct
correlation between the degree of stability and the final working result.
For example, a geometrically unstable setting with w > 15 is preferable
over a stable setting with w < 15, because the influence of low polygon
orders on the roundness is considerably higher.
The stability index is calculated according to Reeka as follows:
… polygon with
the most negative
stability index
Formula 3-1
Calculation of
the stability
index according to
Reeka
Fig. 3-11
Stability index SI
vs. polygon order z
z
More on ϕ:
↑ Fig. 3-13
… polygons with
few waves

37
Here, z is the number of corners of the polygon to be calculated. So
the formula has to be calculated individually for each polygon. The
results can then be represented, for example, in a column chart.
3.2.6 Stability cards
With the formula for the stability index and the subsequent eva-
luation in a column chart (Fig. 3-11), only one geometric configura-
tion can be investigated at a time.
The so-called stability cards graphically depict multiple results
regarding the specificity of polygons at specified grinding settings.
They allow a quick view of the optimal grinding settings in each
case.
This requires that the information content of the stability cards be
reduced compared to the column charts shown in Fig. 3-11. Gene-
rally only the polygon with the most negative SI is considered. In
doing so, it is often tried to represent both its stability index and
the number of corners in the same chart.
Advances in computer performance have made it possible to cal-
culate cards for the current case at hand. This allows optimising
grinding zone and grinding process even further. In the machines
by Schaudt Mikrosa GmbH, a software is used that calculates and
outputs a current stability card to the operator already when ente-
ring data into the machine control.
Over time, several types of stability cards have been developed,
with usage and evaluation options that partly greatly differ from
each other. The following table provides a small overview of some
stability cards:
↑ Formula 3-1

38
Universal stability cards
• Universally usable
a therefore kept generic and not very meaningful
Reeka type
provides information about:
• Stable and unstable areas
a bright areas are geometrically
stable areas
• Number of corners with the worst SI
(w) a indicated as a number in the
unstable areas
as a function of:
• Tangent angle γ
• Support angle βrel Note: Graphic has been calculated for
dR
/dS
=0.8
Applies only to a specific size ratio
of grinding wheel to regulating wheel
Meis type
• Stable and unstable areas
a dark areas are geometrically
stable areas
as a function of::
• Tangent angle γS
• Tangent angle γR
Suitable for examining the stability
during throughfeed grinding
Well suited for examining the wear
of the grinding wheel and regulating
wheel
Note: Graphic has been calculated for β=30°
Applies only to a specific support angle β
Tab. 3-2
Universal
stability
cards

39
Specific stability cards
• Are calculated specifically for the case at hand
a therefore very accurate
Grindaix type (Cegris)
• Stable areas (blue) and unstable
areas (grey)
• Large number of corners (bright)
and low number of corners (dark)
as a function of:
• ϕ1 (also α)
• ϕ2 (also βG
)
Mikrosa type (Heureeka)
• Stability index a color
as a function of:
• ϕ1
• ϕ2
• Optionally height position H and
support angle β or grinding wheel
diameter ds
and regulating wheel
diameter dr
Evaluation:
• Yellow, green and blue areas are
stable
-10 0 10 H 30
10
20
30
βrel
50
• Red areas are instable and should be avoided
Tab. 3-3
Specific
stability
cards
More on ϕ:
↑Fig.3-13

40
3.3 Grinding zone geometry
An accurate understanding of the grinding zone geometry is ne-
cessary to optimally control the grinding process and counteract
roundness errors. The following chapter therefore focuses on the
angular relationships in the grinding zone and their calculation.
3.3.1 Tangent angle γ
The center line Z is the direct connection between the centers
of the grinding wheel and regulating wheel. The two wheel ra-
dii form the two angles γS and γR towards the workpiece center
above this center line. From these two angles results the tangent
angle γ (Fig. 3-12).
The tangent angle γ depends on the:
• Diameter of the grinding wheel dS
• Diameter of the regulating wheel dR
• Diameter of the workpiece dW
• Value of the height position H
↑ Section 3.2.2
Determining the
roudness error
Fig. 3-12
Angle relationship
in the grinding
zone 1:
Tangent angle
3 The grinding zone | Grinding zone geometry

41
3.3.2 Height position H
The contact points of the workpiece on the grinding wheel and
regulating wheel are decisively determined by the height position
H of the workpiece. If the height position is changed, γS and γR
as well as γ itself change also. This means there is a direct rela-
tionship between the tangent angle γ and the height position H
(Fig. 3-12).
3.3.3 Angle at the contact points
For efficient calculation of the grinding zone, the four parameters
γS, γR, β and H are reduced to the two variables ϕ1 and ϕ2:
The angle ϕ1 captures the contact points of the workpiece with
the grinding wheel [ 1 ] and the workpiece support [ 2 ].
In contrast, the angle ϕ2 represents the relationship between the
workpiece and the grinding wheel [ 1 ] and between the workpiece
and the regulating wheel [ 3 ] (Fig. 3-13).
↑ Section 3.2.3
Height position
H and support
angle β
Fig. 3-13
Angle relationship
in the grinding
zone 2:
Contact points
Grinding zone geometry | 3 The grinding zone

42
3.3.4 Relative support angle βrel
The angle βrel
is the difference bet-
ween the support angle β and the
inclination angle of the center line
Z. βrel
is relevant during throughfeed
grinding and/or grinding with lowered
regulating wheel.
3.3.5 Grinding zone geometry during infeed grinding
To grind the workpiece “above center“, there are two ways to po-
sition the workpiece above the center line Z: by raising the work-
piece or lowering the regulating wheel.
Height position of the workpiece
=
Simple setup
Tangent angle changes due to grinding wheel wear and regulating wheel wear
Fig. 3-14
Angle βrel
Tab. 3-4
Raising the work-
piece support
Also in plunge grin-
ding, the regulating
wheel is slightly
inclined. However,
since the inclina-
tion is very small,
it is neglected in
the figures, and
the lines of contact
on the workpiece
are considered as
points of contact.
In Tab. 3-4, the
height position H
corresponds to the
height position of
the workpiece Hw
.
↑ Chapter 4
Infeed grinding

43
Tab. 3-4 Continued
Raising the work-
piece support
Height position of the regulating wheel
Ideal engagement conditions during angular infeed grinding
Tangent angle remains constant despite grinding wheel wear (the angle changes
by regulating wheel wear)
Center height remains constant during dual grinding.
Correction for H are made by changing HR
All formulas apply
also to grinding
“below center“.
Set of formulas:
Tab. 3-5
Lowering the
regulating wheel

44
3.3.6 Grinding zone geometry during throughfeed grinding
The inclination of the regulating wheel by the angle α and the
hyperbolic shape of the regulating wheel generated by dressing
results in continuously changing geometric conditions during
throughfeed grinding. Therefore, the grinding zone geometry du-
ring throughfeed grinding is considered separately.
γS
, γR
, βrel
, dR
and dW
change along the grinding zone width de-
pending on the dressing model used. The result is that “stability
zones“ are passed along the grinding zone. Software tools should
therefore be used to design the geometry during throughfeed grin-
ding.
It is important to ensure that geometrically stable areas (positive
SI) are present at least in the outlet area. In the inlet area, geome-
trically unstable areas (negative SI) can be accepted, since they
can be ground out again in the further course of grinding.
↑ Chapter 5
Throughfeed
grinding
Set of formulas:
Tab. 3-5 Continued
Lowering the
regulating wheel
When lowering the
regulating wheel,
the value of HR
is
negative.
Observe the sign!

45
Fig. 3-15
Inclination of the
regulating wheel
during through-
feed grinding
Fig. 3-16
Relationship
between degree
of stability, wave
number and grin-
ding wheel width
applies to an
inclination angle of
αR
= 4° in the exa-
mined workpiece
diameter range0 60 120 180 240 300mm
5 36 32 30 26 24 18
-0.06
0.00
0.06
StabilityindexSI
Wave number
Width of grinding wheel bs
stable
instable
Workpiece diameter dw = 30 mm
dR
Exit
dR
Entry
γ0
γ1
β0
β1
H1
H0

46
3.4 Forces in the grinding zone
As already described in Section 3.2.3, the height position H and
the support angle β influence the roundness. To optimize it speci-
fically, precise calculation of forces is useful.
As with other machining methods, the forces depend to a large
extent on the selected cutting conditions. The decisive forces act
on the three contact points of the workpiece:
1. Workpiece–Grinding wheel
2. Workpiece–Workpiece support
3. Workpiece–Regulating wheel
Tangent forces: FtS
, FtR
, FtA
Normal forces: FnS
, FnR
, FnA
Horizontal forces: FxS
, FxR
, FxA
Vertical forces: FyS
, FyR
, FyA
Resultant forces: FS
, FR
, FA
↑ Section 3.2.3
Height position
H and support
angle β
Fig. 3-18
Forces in the
grinding zone
Fig. 3-17
Contact points
3 The grinding zone | Forces in the grinding zone

47
The friction force relationships at the individual contact points
provide the information on the composition of the forces in the
normal and tangent directions. As can be seen in Fig. 3-18, the re-
sultant forces can also be transformed into vertical and horizontal
components. The following rules apply:
The magnitude of these forces is influenced by the grinding zone
geometry and the machining conditions:
3.4.1 Forces on the grinding wheel
The tangential force FtS
on the grinding wheel during grinding
is the desired force. This is why it is called the main cutting force.
Influences by grinding zone
geometry
Influences by machining condi-
tions
• Diameter of grinding wheel, workpi-
ece, and regulating wheel
• Tangent angle γS
, γR
and
support angle β
• Inclination of the machine bed (θ)
• Power applied to the system (PS
, PR
,
infeed amount ae
)
• Deadweight G of the workpiece
• Contact conditions at the three
contact points
… also referred to
as friction coeffici-
ent µS ,
µA
andµR
Tab. 3-6
Influences on
the forces in the
grinding zone
Forces in the grinding zone | 3 The grinding zone

48
FtS
... Tangential force on the grinding wheel [N]
PS
... Drive power on the grinding wheel [W]
vS
... Circumferential speed of the grinding wheel [m/s]
The friction force ratio at the grinding wheel, also called cutting
force ratio µS
, provides information about the friction conditions
in the contact zone between the grinding wheel and workpiece. It
mainly depends on the type of cooling lubricant used. The table
below summarizes common values for the cutting force ratio:
3.4.2 Forces on the workpiece support
The forces on the workpiece support cannot be determined solely
by power measurement. Accurate determination is possible only
by force sensors that are integrated into the workpiece support.
Cooling lubricant class µS
according to Ott
CL 1 Water without lubricant 0.6
CL 2 Emulsion with 10% mineral oil in the concentrate 0.5
CL 2.5 Emulsion with 20% mineral oil in the concentrate 0.47
CL 5 Pure grinding oil with additives 0.3
Tab. 3-7
Cooling lubricant
classes and
associated cutting
force ratio µS

49
The factors that influence the friction coefficient at the workpi-
ece support µA
include:
• Material pairing of workpiece support and workpiece
• Lubrication conditions (depending on cooling lubricant, nozzle
and workpiece width)
• Roughness of the friction partners
• Circumferential speed of the workpiece
• Infeed rate
The friction coefficient µA
varies within wide ranges of about
µA
= 0.12–0.4
3.4.3 Forces on the regulating wheel
These forces determine the circumferential speed of the workpi-
ece. The value of the friction coefficient of the regulating wheel
µR
, depends on the material and the binding of the regulating
wheel, among other factors.
FtR
... Tangential force on the regulating wheel [N]
PR
... Drive power on the regulating wheel [W]
vR
... Circumferential speed of the regulating wheel [m/s]
The sign of this friction coefficient depends on the machining si-
tuation, depending on whether the workpiece is driven or slowed
down by the regulating wheel.
µR
= positive a decelerating
µR
= negative a accelerating
FtA
...tangential
force on the work-
piece support [N]
Type of regulating wheel µR
Rubber-bound regulating wheel (grain 150) 0.34
Cast steel regulating wheel 0.17
Tab. 3-8
Typical values
for µR

50
3.4.4 Consideration of the forces
Consideration of the forces may be useful for the following
reasons:
Loss of contact to the workpiece support
The angle of the absolute forces between workpiece–grinding
wheel (FS
) and workpiece–regulating wheel (FR
) must not be grea-
ter than 180° (Fig. 3-18). Upward forces can be prevented by redu-
cing the height H or by pressure rollers.
Avoid transverse forces on the workpiece support
The horizontal forces (FxA
) acting on the workpiece support should
be low or ideally be even zero. This prevents bending of the work-
piece support in the direction of the grinding or regulating wheel.
If the friction coefficient µA
is known, the support angle β is calcu-
lated using the following formula:
Reducing signs of wear on the workpiece support
Reducing the friction force acting on the workpiece support
will extend its service life.

51
Rotation of the workpiece during idling
To prevent initial grind issues of workpieces, they may be rotated
by the regulating wheel before grinding (idling). This requires that
the friction force on the workpiece support (µ0A
·FnA
) be lower than
the friction force on the regulating wheel (µR
·FnR
). This means:
Here, µ0A
is the static friction of the workpiece support. For heavy
workpieces, a start-rotation device may be used.
Slippage between the regulating wheel and the workpiece
The radial contact pressure of the regulating wheel, FnR
, is normal-
ly 1.5 to 4 times greater than the tangential cutting force FtS
of the
grinding wheel. This is why only the regulating wheel determines
the rotational speed of the workpiece.
To minimize the risk of slipping, the friction coefficient must be
high enough for continuous workpiece rotation on the regulating
wheel. Measures: Increase infeed ae
and decrease H.

53
In external cylindrical plunge grinding, the workpiece is ground by advancing the
grinding wheel. For this purpose, the grinding wheel and regulating wheel have a
negative profile of the workpiece, which is generated by means of dressing tools.
A workpiece can be ground in a single plunge in spite of different diameters, cham-
fers and fillets. If the grinding zone is wide enough, multiple workpieces can be
machined simultaneously. This is called double or multi-production.
By tilting the workpiece or the grinding spindle axis, machining an angle to the work-
piece axis is possible. This allows the machining of end or lateral surfaces. Find out
more on the following pages.
4
Infeed grinding
Infeed grinding
Profiling
Roughing and finishing
Stop
Tilting the grinding wheel
End surfaces

54
4.1 General principle
In external cylindrical plunge grinding, also called crossgrinding,
the regulating wheel is inclined only minimally (e.g. 0.1 … 0.2°).
The rotation of the regulating wheel creates an axial force acting
on the workpiece. This moves the workpiece against an axial stop
where it is accurately positioned.
The workpieces are loaded into the grinding zone from the side,
with integrated gantry, or from the top with an external gantry
while the grinding wheel is in the base position. Then the grinding
zone closes by the infeed slide and the workpiece is ground. At
the end of the process the grinding wheel retracts back to the base
position and the workpiece can be changed.
4.2 Process control
The grinding cycle consists of several consecutive steps, which
is why it is also called a multi-stage grinding process. In multi-
stage grinding, the task is to remove the grinding allowance of a
workpiece in the shortest time possible, i.e. cost-efficiently, whi-
le maintaining the required roughness of the workpiece. This can
be achieved if the grinding process is divided into several stages
4 Infeed grinding | General principle
... that can also
be the regulating
wheel slide (X4-
axis)
Fig. 4-1
Grinding cycle
during infeed
grinding
X 1
X 1
X 1
X 1
1. Base position 2. Grinding 3. Base position

55
(e.g. roughing, finishing and sparking-out) A detailed division of
these stages can be found in the following figure:
A short grinding time can be realized only by an increased stock
removal rate QW
or a higher plunging rate in the roughing pha-
se. The quality is achieved in the finishing and sparking-out pha-
ses. No further infeed occurs during sparking-out. The workpiece
is then ground only by releasing the elastic deformation of the
Process control | 4 Infeed grinding
Fig. 4-2
Positions of the
grinding cycle
↑ Chapter 6
Influencing factors
Fig. 4-3
Forces occuring
during the infeed
process
t
X1.W
X1.0
X1.1
X1.2
X1.3
X1.4
Airgrinding
Roughing2
Stock + safety
Part load position
Roughing1
Final dimension
Semiﬁnishing
Finishing
Sparking-out
FX1.0 FX1.1 FX1.2 FX1.3 FX1.4 FX1.5
t
Fts vfa
Ps
Q’w

56
grinding wheel and the machine. This improves the roundness and
surface.
The infeed amounts and infeed rates can be defined for each of
these individual processes in the control of the machine. But it
should be noted that much heat can be generated at a high cutting
rate, resulting in expansion of the workpiece. After cooling, its
diameter may be less than desired.
4.2.1 Grinding from solid stock
The so-called grinding from solid stock is used for cutting large
allowances. Reduction of the diameter moves the center of the
workpiece towards the regulating wheel. In the process, the pro-
jection of the workpiece over the workpiece support usually redu-
ces and a risk is incurred of grinding into the workpiece support. In
addition, the height position changes. To counter the shifts, infeed
is applied by both the grinding wheel axis X1 and the regulating
wheel axis X4. The switching points of the axis X4 are reached at
the same time as those of the axis X1.
4 Infeed grinding | Process control
... a large amount
of stock is remo-
ved
Fig. 4-4
Compensating
the center shift
by advancing the
X4-axis
X1
X4
... hence good
cooling should be
ensured

57
So it is possible to influence the angle of the center shift.
Opening the grinding zone on both sides also provides a favora-
ble loading and unloading situation, allowing the workpiece to be
changed on the regulating wheel side without any contact. To do
this v-nests have to be applied on the workpiece support to hold
the workpiece in position while loading and unloading the part.
During grinding the regulation wheel is pushing the workpiece
away from the v-block.
4.2.2 Grinding with additional functions
The additional functions provide ways to influence the operation
in more detail:
On the one hand, it is possible to store different workpiece speeds
for each switching point. This provides a way to influence the chip-
forming process.
On the other hand, there is a retraction function (temporary ope-
ning of the grinding zone) and/or an intermediate sparking-out
function. The aim of this is to take the load of the system, which
can lead to a reduction of grinding time in particular for flexible
shafts. However, this process control is rarely used in practice,
because the support by the regulating wheel of this kind of work-
pieces is stiff enough.
Process control | 4 Infeed grinding
↑ Section 3.2.3
Height position
H and support
angle β

58
4.2.3 Oscillation
Oscillating plunge grinding is possible also on centerless grinding
machines. Prerequisite is axial relative movement between the work-
piece and the grinding wheel besides radial movement. In the KRO-
NOS S series, this is realized by the Z-axis of the grinding wheel‘s
cross slide system. The oscillation rate and the oscillation path can
be variably programmed depending on the workpiece length and
workpiece geometry. In most cases, the path lies between 0.2 mm
and 5.0 mm, while considerably higher values are possible.
The advantages of oscillating plunge grinding are:
• Improvement of surface quality
• Increasing the specific stock removal rate and reducing the grin-
ding wheel wear when grinding extra-hard materials
4.2.4 Influence of the workpiece geometry
If several diameters are to be ground on a workpiece simultaneously,
the regulating wheel requires different diameters as well. This re-
sults in different circumferential speeds of the regulating wheel.
Theoretically, the result would also be different workpiece speeds.
But obviously this is not possible in practice. The result is a single
speed. Which one this is, depends mainly on the diameter/width
ratio and the allowance of each workpiece diameter. At the diame-
ters that do not determine the speed, differential speeds result by
the slippage occurring there, which cause increased abrasion of the
regulating wheel. These differential speeds can also cause irregula-
rities in the rotation of the workpiece, resulting in roundness errors.
On the grinding wheel side, the same effect causes, on the one
hand, cutting speed differences and, on the other hand, different
specific stock removal rates. Points where the cutting performance
is too high may be prone to thermal damage to the workpiece. Using
4 Infeed grinding | Process control
… friction between
the regulating
wheel and workpi-
ece is lost and the
workpiece slips
through

59
the example of grinding of engine valves, the following demons-
trates in principle how widely the value of Q’w
varies with the work-
piece geometry. To prevent thermal damage to the border zones, it
is important to ensure good cooling lubrication or to reduce the feed
rate.
4.3 Grinding zone layout
4.3.1 Undercuts and spacers
In plunge grinding, the grinding wheel moves radially into the work-
piece. Grinding is possible simultaneously with straight or shaped
grinding wheels or with a grinding wheel set. (see Fig. 4-6)
Grinding zone layout | 4 Infeed grinding
Fig. 4-5
Variation of Q’w
by
the workpiece
geometry
© WZL Aachen
z
1 2 3 4 5 z position [mm]
Q‘W
[mm³/mms]
0
2
4
6
8
10
7,20
1,34
20 30 40

60
Undercuts
Since length tolerances may occur on workpieces, the grinding
wheel is slightly wider than the corresponding workpiece seat. On
the regulating wheel, it is the opposite: It is narrower than the
corresponding workpiece seat.
The regulating wheel and workpiece support are undercut at tran-
sitions from one diameter to another. This ensures a stable and
process-reliable position of the workpiece in the grinding zone.
Spacers
Open spaces are important for better drain of cooling lubricant
– the regulating wheel supports the workpiece only at essenti-
al points. Rings between the individual regulating wheels create
spaces through which coolant and chips can be carried away.
Polypenco rings
So-called polypenco rings are used to compensate for any une-
venness between the wheels and the spacers. Compared to rings
made of cardboard, they have much better resistance to cooling
lubricant and do not change their size. But their width of about 0.5
mm must be taken into account in setting up the grinding zone.
4 Infeed grinding | Grinding zone layout
↑ Fig. 4-6
Undercuts and
spacers
↑ Fig. 4-6 Under-
cuts and spacers
Polypenco
ring
Undercut
Spacer
Fig. 4-6
Undercuts and
spacers
… plastic rings

61
4.3.2 Stop
The stop‘s function is to fix the workpiece in the axial direction
in the grinding zone. The handling system is usually programmed
to insert the workpiece at about 0.2 mm distance to the stop. Dif-
ferent stops are necessary, depending on the workpiece shape;
essentially three types can be distinguished:
1. Stop at the workpiece support
This type of stop is used if the zero
point for the longitudinal dimension
of the workpieces is not on the
outside, but in the middle of the
workpiece. Depending on the run-
out of the face touching the stop,
a longitudinal movement of the
workpiece occurs in the grinding
zone. This causes errors on contour
elements (chamfers, fillets, cham-
fers, cones, etc.) depending on the
size of the longitudinal movement.
2. Point stop
A point stop is preferably used
for workpieces with planar faces.
It should be positioned exactly in
the center of the workpiece. This
will ensure that there is no axial
movement of the workpiece and
the axial run-out during grinding is
not copied from the stop surface to
the face.
Tab. 4-1
Stops

62
3. Surface stops
a) Surface on surface
On this mechanically very sturdy stop,
the workpiece rests with its face. It is
used mainly when only cylindrical parts
of a workpiece are to be ground or the
workpiece‘s longitudinal dimensions
are relative to the face. If end surfaces
or slants are to be ground, it must
be ensured that the stop is 100%
perpendicular in two planes to the
workpiece support. As in general the
stop is adjustable, this requirement is
very difficult to realize. For face grinding
operations, this possibility is therefore
used only if the workpieces have no
fixed center (such as rings, tubes or
workpieces with a hole in the center)
or to machine very large end surfaces.
Because the stop surface and the work-
piece surface contact each other, even
small angle errors at the stop result in
significant longitudinal movements of
the workpiece in the grinding zone.
b) Point on surface
This type of stop is only used when the
workpieces have ball-like or conical
frontal surfaces or centers. As the work-
piece only has a small point of contact
with the stop, hardly any lengthways
movement is caused during grinding.
This means that the quality of the
frontal surface even with the existence
of small angle errors at the stop is very
good.
Tab. 4-1
Continued
Stops

63
4.3.3 Initial grind
In centerless grinding, stable positioning of the workpiece is par-
ticularly important. The workpiece support and the grinding and
regulating wheel include a negative profile of the workpiece that
is being ground.
The initial grind situation must be considered especially for pro-
filed workpieces with different allowances on the diameters. It
must be tested where the grinding wheel comes in first contact
with the workpiece. To ensure it is stable in the grinding zone, it
should be ground at several points simultaneously if possible.
The regulating wheel system has to be assessed as well. If, for
example, only a small point of the regulating wheel has contact
at the beginning of the grinding process, the regulating wheel will
not be able to stop the workpiece.
It must absolutely be avoided to start grinding on chamfers or end
surfaces. Some parts require special measures due to their shape
in order to lie stable in the grinding zone when grinding starts:
Pressure roller and hold-down device
In the event that only certain areas of a workpiece are ground,
pressure rollers are used. They push the workpiece against the
workpiece support and regulating wheel. This increases the fric-
tional force between the workpiece and the regulating wheel.
The workpiece diameter not to be ground forms the guide basis,
and any roundness errors here are transferred to the areas to be
ground. Therefore it must be ensured that the roundness of the
guide basis is better than the roundness that is to be achieved
with the grinding process.
Pressure rollers are used also for workpieces with large end sur-
faces or for workpieces with large unbalance. They are often pus-
… the instance
of first grinding
contact

64
hed by means of spring force, and they are mostly driven passively
by the connection between regulating wheel and workpiece.
In the case of so-called top-heavy workpieces, hold-down de-
vices are used (Fig. Function of pressure roller and hold-down de-
vice). They are usually operated by a spring and hold the top-heavy
workpiece on the workpiece support. They also improve the initial
rotation of the workpiece because the hold-down device increases
the frictional force between the workpiece and the regulating
wheel.
Grinding of end surfaces
Besides the lateral surfaces of a
workpiece, its end surfaces can be
ground at the same time. There are
three different methods, which are
considered in more detail in the fol-
lowing sections.
Fig. 4-7
Function of
pressure roller and
hold-down
Hold-down devicePressure roller
Lateral surface
End surface
… also called
circumferential
… also called
shoulder surfaces
Fig. 4-8
Lateral surfaces
and end surfaces
4.4

65
In end surface grinding, it is important to take care of the allo-
wance distribution, besides the initial grind situation. Grinding
starts on the lateral surface of the workpiece. Only when the work-
piece is stable in the grinding zone, contact with the end surface is
allowed. It should be ensured that the radial allowance is at least
1.2 times as large as the axial allowance.
4.4.1 Inclination of the grinding wheel
• Feasible on KRONOS S
• Lowering the regulating wheel required
• Requires Z-axis for grinding wheel
No differential speeds of the workpiece at the regulating
wheel (as under 4.4.2)
Differences in cutting speed
Grinding of end surfaces | 4 Infeed grinding
Fig. 4-9
End surface
grinding by tilting
the grinding wheel
by 15°15°
X + ZX + Z
The grinding wheel axis is located at
an angle to the regulating wheel axis.
In the axial plunging process depicted
here, the X- and Z-axes are advanced
diagonally interpolating. This allows
finish-machining the lateral and end
faces in one plunge, resulting in time
and precision benefits. The grinding
contact length on the shoulder is less
than in straight plunge grinding so
that the risk of thermal damage is re-
duced.

66
4.4.2 Inclination of the workpiece support
The axes of grinding wheel and regulating wheel are aligned par-
allel to each other. However, the workpiece support is manufac-
tured at an angle:
The grinding wheel and re-
gulating wheel are dressed
according to the angle of this
inclined position. As a result
the diameter of the regulating
wheel changes along the work-
piece axis and different cir-
cumferential speeds occur. As
this should theoretically lead
to different workpiece speeds,
which is not possible, this me-
thod may cause an unstable po-
sition and slippage, especially
for long workpieces.
Use: Possible on all KRONOS machines, because infeed move-
ment is necessary only in the X direction
No Z-axis required
Can be realized on all KRONOS machines
Special workpiece supports necessary
Differential speeds of grinding wheel and regulating wheel
Dressing and correction programs usually complex
4 Infeed grinding | Grinding of end surfaces
Fig. 4-10
End surface grin-
ding by tilting the
workpiece support
by 6°
6°
XX

67
4.4.3 Axial plunging in straight plunge with Z-axis
The axes of grinding wheel and regulating wheel are aligned par-
allel to the workpiece axis. Axial plunging machines the shoulder
surfaces here as well by moving the grinding wheel along its Z1-
axis in addition to the radial infeed movement.
In this grinding process, it
must be observed that, due
to dressing and wear, the
grinding wheel contour moves
in the -Z and -X directions.
This means that the size bb
is
getting smaller and smaller.
When the value falls below a
minimum, the grinding wheel
must be newly profiled.
Use: On all machines with X- and Z-axis:
KRONOS S; KRONOS M 250 with Z1-axis
• Requires Z-axis for grinding wheel
• Contours to be dressed require inclination of the dressing tool
No differential speeds
Profile shift during shoulder dressing
Risk of overheating when grinding
Grinding of end surfaces | 4 Infeed grinding
Fig. 4-11
End surface
grinding by axial
plunge of the grin-
ding wheel
bb
2.
1.
XX
ZZ

68
4.5 Removal of workpieces
There are three ways to unload workpieces from the grinding
zone:
4 Infeed grinding | Removal of workpieces
Clearing movement Pusher Grippers
The regulating wheel is
moved away from the work-
piece support, the workpiece
falls into the resulting zone
and is carried away by a
conveyor belt.
X4X4
An axial ejector pushes
against the workpiece, the
workpiece slides across the
workpiece support and exits
the grinding zone.
The grippers remove the
workpiece. Usually these are
double grippers that remove
the finished workpiece while
inserting a new blank at the
same time.
Very fast
Check required that the
workpiece has been
carried away properly
or whether it is still in
the machine (requires
additional time)
Risk of a crash by dropped
workpieces
Workpieces can be
damaged by falling down
Position of workpieces
has to be checked for
subsequent reworking
Large parts possible,
which are impossible
with clearing movement
Workpiece may be
damaged when ejected
Check required that the
workpiece has been
carried away properly
or whether it is still in
the machine (requires
additional time)
Little/no damage to the
workpiece
Position of workpieces
need not be checked for
subsequent reworking
Not necessary to check
that the workpiece
has been carried away
properly
Somewhat more time-
consuming
Tab 4-2
Removal of
workpieces

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