36sigma RepoRt 20/2011
The New Hardness for Short Grinding Times
the advantages of generating grinding with vitrified CBN grinding worms are particularly evident in comparison with established procedures. the reliable utilization of the extremely hard cutting material enables, among other things, shorter grinding time and longer tool life.
37sigma RepoRt 20/2011
Generating grinding is an established process
for the hard finishing of gears. The use of
generating grinding is very widespread in the
range of up to Module 5 mm, particularly
in industrial gear manufacturing and in the
service vehicle and automobile sector. For
Module 5 to 8 mm, profile grinding is also
used in addition to generating grinding, and
the former grinding technique is always more
widespread with larger modules. Current
trends in machinery and tool development
are opening up an ever greater potential
here for generating grinding up to Module
14 mm. In the case of generating grinding,
the following process variants are encoun-
tered on the market, each with advantages
and disadvantages with respect to process
technology: ➔
Generating grinding with electroplated CBN grinding worms
Resistant to • hardness fluctuations
High process stability through • long tool life
2-cut process for allowances • of up to 0.18 mm/flank
Machining of collision-critical • gearings
No profile correction • opportunities possible
during ongoing process
Limitations with respect to • profile form and surface quality
Generating grinding with dressable grinding worms made of corundum
Excellent profile and • surface quality
resulting from dressing in the machine
Flexible correction • opportunities for profile angle deviation fhα
Influencing of the surface • roughness through targeted modification of the dressing speed ratio
Low tool costs per workpiece•
A 3-cut process is • necessary, depending on
the allowance situation and pre-machining quality
Proportionate dressing time • per workpiece
Combination grinding (roughing with grinding worm/ finishing with profile grinding disk) with both electroplated and dressable corundum tools
Resistant to • hardness fluctuations
Excellent profile and surface • quality resulting from finishing in the profile grinding process
Machining of collision-critical • gearings with the use of electroplated CBN tools
Longer machining • time during finishing
resulting from the single indexing process
Coordination of the tool life • with CBN or of the dressing cycles with corundum for the roughing worm and the finishing disk
Vitrified CBN
38sigma RepoRt 20/2011
Vitrified CBN
Development of machining times for generating grinding with corundum
Grinding times overall have been considerably reduced with
a variety of further developments in procedures in the recent
past. Numbered among these developments are:
the development of high-performance corundum • grinding worms based on the SG corundum cutting materials or other special corundum
the further developments of ceramic bonding itself •
the optimum integration of these new, more • high-performance and/or of the proven corundum cutting materials in the ceramic matrix
One reason for the shortening of
the grinding time is that the cut-
ting speed has been increased
from 63 m/s to 75 m/s, and even
up to 80 m/s on the LCS machines.
However, the risk of thermal
joint damage also increases with
increasing cutting speed. In addi-
tion to the higher cutting speed,
increased feeds and greater cut-
ting depths per cut are also pos-
sible. This often opens up a 2-cut
process with a total allowance of
0.1 to 0.12 mm/tooth flank. At the
same time, though, the increased
grinding worm wear associated
with this must be compensated
for with greater dressing amounts. This is incorporated into
the proportional dressing time per workpiece and increases
the tool costs per workpiece.
All measures described for increasing performance when gen-
erating grinding with corundum grinding worms push the
grinding process to the limit. A different cutting material
must be selected to tap into further potential. CBN allows the
performance potential to be boosted in a risk-free and safe
way. The safety and stability limit of the grinding process is
increased and fluctuations avoided.
Table: Comparison of properties of different cutting materials important for machining
Corundum SG Corundum CBN
Structure Al2O3 Al2O3 SG-Al2O3 Chip space
Steel core Bonding (nickel)
Bonding (vitrified)
Pores Bonding (vitrified)
Pores
CBNAl2O3 Al2O3 SG-Al2O3 Chip space
Steel core Bonding (nickel)
Bonding (vitrified)
Pores Bonding (vitrified)
Pores
CBNAl2O3 Al2O3 SG-Al2O3 Chip space
Steel core Bonding (nickel)
Bonding (vitrified)
Pores Bonding (vitrified)
Pores
CBN
Bonding characteristics Multi-layer, dressing possible
Multi-layer, dressing possible
Single-layer, not dressable
Hardness [HK01] 1.850 2.150 4.500
Grain size [µm] 100–250 <1µm 100–250
Coefficient of friction[-] (with respect to steel) 0,34 0,19 0,19
Thermal conductivity[W/mK] e e
Temperature resistance[°C] 1.750 1.750 1.200
70–250
200–700
f.l.t.r.: - Tool set for combined generating and profile grinding- Vitrified CBN grinding worm- SG corundum grinding worm
39sigma RepoRt 20/2011
Vitrified CBN
Cutting material with the greatest potential for high machining performance
Cubic boron nitride, better known as CBN, is the second-
hardest cutting material after diamonds – which due to their
chemical properties are either only of limited suitability or un-
suitable for machining hardened steels. In addition to its high
Knoop hardness, CBN is outstanding for its high thermal con-
ductivity and a low coefficient of friction with respect to steel.
The advantages of CBN in comparison with other corundum
cutting materials becomes clear in light of this criterion (see
table on the previous page). This makes CBN the ideal cutting
material for the machining of hardened toothed gears – with
the potential of reducing the grinding time even further and of
increasing tool life thanks to its high wear resistance. Further-
more, CBN offers the possibility of machining materials with
a high surface hardness in excess of 64 HRC.
Development from electroplated to vitrified CBN
30 years ago, Liebherr developed generating and combi-
nation grinding with electroplated grinding tools. With the
development of a dressing unit integrated in the machine,
the possibility was opened up of profiling and sharpening
grinding worms and profile grinding disks made of corundum
on the LCS. This LCS grinding machine type has been estab-
lished on the market since 1998.
As a result of the many years of experience and the know-
ledge of the performance potential of the CBN cutting ma-
terial with the grinding of gears, and in view of the dressing
unit available in the LCS, Liebherr developed together with
two tool suppliers the idea of bonding the cutting material
CBN in a dressable ceramic matrix for grinding worms for
generating grinding. The objective here is to achieve the
performance advantages of the CBN cutting material with the
positive characteristics of the chemical bond such as:
higher flexibility for profile angle corrections • by dressing
attaining the highest profile form and surface qualities•
combining multi-layer and dressable CBN cutting • material structure for a long tool life utilization.
This basic principle was already known on the market, but
vitrified CBN grinding worms have only been used only in a
few special applications to date. A cost-effective and efficient
utilization of these tools was still not yet successfully imple-
mented in large-series production. The reason for this was that
the manufacture of vitrified CBN grinding worms and their pro-
cess-reliable dressing involved a few technical difficulties. Two
different tool systems manufactured by the Wendt and Lapport
companies were introduced at EMO 2007 in Hanover. Together
with the dresser manufacturer Dr. Kaiser and two users, the
process was then developed in a joint project ranging from
basic research to the start of production. ➔
Fig. 1: Vitrified CBN grinding worm (Wendt Co.)
REM detail exposure of the bonding structure
40sigma RepoRt 20/2011
Vitrified CBN
Challenges from the point of view of machinery and technology
For Liebherr as a manufacturer of gearing machinery, it was
important to develop the dressing and grinding process in
such a way that it would be employable on an already existing
LCS grinding machine with an integrated dressing unit. The
necessary additions with respect to machine technology
should be simple to retrofit as well.
One of the greatest challenges is represented by the dress-
ing of the vitrified CBN structure. Due to economic consid-
erations, it is on the one hand a prerequisite that the entire
dressing amount per dressing cycle is kept as low as possible.
On the other hand, the wear caused by the use of grinding
must be rectified by sufficient profiling. In order to ensure
this, the initial contact between dresser and grinding worm
must be detected down to a micrometer. In the case of the
individual infeeds for the entire dressing amount as well, only
up to 4 µm should be dressed per tooth flank in order to
ensure that the wear on the dressing tool does not become
too great. All this is made possible by the utilization of an
Acoustic Emission System (AE System). In addition to the AE
System, additional prerequisites are a very rigid dressing spin-
dle and the high-precision controllable tool spindle.
For the development of the dress-
ing technology, i.e. the dressing
speed ratio, a different strategy of
synchronus and asynchronus dress-
ing, and also the traversing speed
along the shift axis, it was advanta-
geous to carry out the single-flank
dressing usually used by Liebherr.
As a result, the processing forces
which arise when dressing the
CBN structure were considerably
reduced. It is only with the single-
flank dressing with the swiveling
dressing axis offered by the LCS
that precise adjustment or dressing,
respectively, of the pressure angle
at a point became possible.
A further focal point of develop-
ment from the point of view of
machine technology was to develop
a collision protection for the consid-
erably more expensive CBN grind-
ing worms which makes it possible
to protect the tool against either partial or entire break-outs of
worm segments or complete destruction. These kinds of colli-
sions could negate at one blow the very high tool life and the
economic efficiency associated with it. This collision protec-
tion developed by Liebherr on its own (see interview with Mr.
Florian Schuon on page 42) can be retrofitted on all LCS gear
grinding machines that have Siemens control units.
With respect to grinding technology, a new shift strategy
enabled an optimization of the tool life of the grinding worm.
For this new shift strategy, machining is not carried out in
segment shifts, as is usual on the LCS for dressable corun-
dum grinding worms, but instead is based on the example of
electroplated grinding tools with a roughing and a finishing
area. This does not involve two separate grinding worms, but
rather a mono-grinding worm which is divided into a finish-
ing area (main bearing side) and a roughing area (side oppo-
site the bearing) (Fig. 2). In each area, a prescribed number
of workpieces is ground in either axial or diagonal proce-
dure at one shift position, after which it is shifted further by a
defined offset. The following sequence is thereby maintained:
The first workpieces are each ground at the start positions at
Fig. 2: Function principle of the dynamic shift strategy
Finishing areaCounter bearing
Roughing area
Shift-Start Shift-Start
Direction Direction
Main bearing
Offset
30 P
ositi
on o
f w
orkp
iece
s 1
30 P
ositi
on o
f w
orkp
iece
s 2
diag
onal
, con
vent
iona
l
axia
l, cl
imb
50 P
ositi
on o
f w
orkp
iece
s 1
50 P
ositi
on o
f w
orkp
iece
s 2
Offset… etc. … etc.
Dyn
amic
ally
dis
plac
eabl
e SH
IFT
CEN
TER
Dep
endi
ng o
n th
e nu
mbe
r of
wor
kpie
ces
with
rou
ghin
g an
d fin
ishi
ng
Vitrified CBN
the ends of the grinding worm and then moved in the direc-
tion of the middle of the wheel by means of onward shifting.
The automatic dressing is triggered when the finishing and the
roughing areas meet in the middle of the worm.
The quantity of workpieces which can be ground for each
shift position is dependent on the grinding worm wear and
thus dependent on the allowance and the established grind-
ing parameters. The amount of the shift offset is dependent
on the respective gear geometry (module, pressure angle, ad-
dendum modification, etc.). Depending on the workpiece and
wear behavior, the length and the position of the roughing
and finishing area on the worm is managed dynamically by
the machine control unit and adjusted automatically.
This dynamic shift traversing can naturally only show its
strength if the grinding worm has a certain length and thus
makes sufficient shift positions available. In order to keep the
procurement costs of the grinding worm as low as possible,
the outside diameter can be reduced with corresponding worm
facewidth without any reduction in tool life. This is based on
the assumption of a tool spindle with high rotation speed and
one main bearing and one counter bearing. Both of these are
fulfilled by the LCS. ➔
Technology Comparison with a Machining ExampleWorkpieceModule 1.75 mm, z2 = 81
Worm facewidth 150 mm
Shift positions 24 per dressing cycle
Number of workpieces per shift position 40
24 x 40 = 960 workpieces per dressing cycle
Tim
e/Pa
rt [
min
.]
Process variation
With vitrified CBN, up to 40 times as many workpieces can be machined in a single shift position as with SG corunded worms – with a constantly high quality, as the comparison (on the right) between component no. 1 and no. 40 shows: The quality within one shift position is consistently high with all parts.
electroplated CBN: 2st, 66 m/s
SG corundum: 4st, 59 m/s
SG corundum: 4st, 75 m/s
vitrified CBN: 4st, 59 m/s
vitrified CBN: 4st, 75 m/s
41sigma RepoRt 20/2011
1st Cut 2nd Cut Dressing Idle Time Total Time
1,60
1,40
1,20
1,00
0,80
0,60
0,40
0,20
0
42sigma RepoRt 20/2011
Vitrified CBN
Collisions can result in high costs on gearing machines. Thus Liebherr offers a software solution which reduc-es the damages and their subsequent costs to a min-imum. This is particularly expedient with expensive tools and tool clamping fixtures. Florian Schuon, Con-trol Unit Development/CNC Development at Liebherr-Verzahntechnik GmbH, explains the principle of opera-tion and advantages of the unique collision monitoring in an interview.
sigma REPORT: Mr. Schuon, how do collisions happen
on gearing machines?
Florian Schuon: Collisions on gearing machines usually arise
from faulty operation or incorrect programming. Most collisions
occur as early as the setup stage when the attendant inadver-
tently selects an incorrect workpiece file that does not fit the
loaded workpiece or if the wrong direction key is pressed.
sigma REPORT: What are the difficulties in connection
with collision monitoring and how does your system
react to possible collisions?
Florian Schuon: The demand for ever-shorter machin-
ing times requires highly dynamic drives. Therefore a reduc-
tion of the torque is naturally unthinkable as a general solu-
tion. Furthermore, the monitoring should be active in manual
and automatic mode as well as in every phase of the axis
movement (rapid traverse, feed, running empty and machin-
ing). Naturally, we still continue to have a deliberate collision
during the gearing of the workpieces (hobbing, shaping,
grinding) to which the monitoring simply must not respond.
Our collision monitoring is purely a software solution which
can detect inadvertent collisions and which reacts to these
by quickly stopping the drive. Damages can be reduced to a
minimum or even prevented completely as a result.
sigma REPORT: How does your monitor function and how
does your system differ from systems of external suppliers?
Florian Schuon: The openness of new control unit genera-
tions permits the evaluation of drive signals in real time and
the initiation of reactions directly in the core of the control
unit. Rapid algorithms observe the current torque output of
the drives and link these to the process conditions. As a re-
sult of the direct assessment in the core of the control unit,
we have on the one hand the shortest reaction time, while on
the other hand we adjust our limit values dynamically to the
respective drive condition. In the event of an error, we effect
“ Reducing Damages to a Minimum or even Preventing them Completely”
Dr.-Ing. Andreas Mehr
Applications technology for grinding and shapingLiebherr-Verzahntechnik GmbH
Further development
Once the dressing and grinding technology and the grinding
machine technique were developed up to the start of produc-
tion, the next objective now is to optimize the tool life of the
grinding and dressing tools further in the series testing stage.
Initial reports from Production indicate that generating grinding
with vitrified CBN grinding worms on Liebherr gear grinding
machines can be implemented economically and with stable
reproducibility.
Currently, however, there is still no generally valid formula
which makes it possible to apply the experience gathered to
date with respect to wearing behavior and tool life to
other modules or applications. It is therefore in the interest
of Liebherr to expand its empirical knowledge in the future
in close collaboration with the users – through experiments
on other gears. n
43sigma RepoRt 20/2011
Vitrified CBN
Dipl.-Ing. (FH) Florian Schuon
Control Unit Development/CNC DevelopmentLiebherr-Verzahntechnik GmbH
a standstill as rapidly as possible. Several axes are monitored
at the same time with this system.
External systems work with sensors. They thus have no access
to information in the control unit core. For me, the greatest
disadvantage is in the reaction speed of these monitoring
systems. Due to the scanning and evaluation cycles of this
kind of sensor, the reaction speed to a collision is much too
slow and it is difficult to prevent a larger-sized damage with
this. Furthermore there is also the problem that there is no
optimum position at which, for example, a 3-D acceleration
sensor of this sort could be fitted.
sigma REPORT: On which Liebherr machines do you
install the collision monitoring?
Florian Schuon: Fundamentally speaking, monitoring is
expedient on all grinding, shaping and hobbing machines.
Particularly useful however is monitoring on machines with
relatively expensive tools. This means that collision monitor-
ing is recommended in any event for grinding machines with
tools made of dressable CBN.
sigma REPORT: Mr. Schuon, thank you
for this discussion.� n
with monitoring without monitoring
Reaction path/time 0.04 mm / ≤1 ms 18.45 mm / 448 ms
Brake path/time 0.36 mm / 20 ms 01
Deformation path 0.4 mm / 21 ms 18.45 mm / 448 ms
max. torque 4.39 Nm 37.13 Nm
max. force at collision point
4.81 kN 93.83 kN
1) 0 because reaction (contour monitoring) takes place only with movement against an absolute fixed stop
Axis speed vX1 = 2,5 m/min
without collision monitoring
with collision monitoring
Consequences of a collision of the corundum worm (head against head), at a feed rate of 4 m/min.The activated collision monitoring detects the collision immediately, stops the drive as fast as possible and minimizes the damage.
