Industrial challenges in grinding CIRP Annals

Transcription

CIRP Annals - Manufacturing Technology 58 (2009) 663–680
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CIRP Annals - Manufacturing Technology
jou rnal homep age : ht t p: // ees .e lse vi er. com/ci rp/ def a ult . asp
Industrial challenges in grinding
J.F.G. Oliveira (1)a,*, E.J. Silva b, C. Guo (2)c, F. Hashimoto (1)d
a
IPT – Institute for Technological Research of the State of São Paulo, São Paulo, Brazil
Nucleus of Advanced Manufacturing, Department of Production Engineering, University of São Paulo, São Paulo, Brazil
United Technologies Research Center, East Hartford, CT, USA
d
Technology Center, The Timken Company, Canton, OH, USA
b
c
A R T I C L E I N F O
A B S T R A C T
Keywords:
Grinding
Optimization
Industry
This keynote paper aims at analyzing relevant industrial demands for grinding research. The chosen focus
is to understand what are the main research challenges in the extensive industrial use of the process.
Since the automotive applications are the most important driving forces for grinding development, the
paper starts with an analysis on the main trends in more efficient engines and the changes in their
components that will affect the grinding performance. A view from 23 machine tool builders is also
presented based on a survey made in interviews and during the EMO and IMTS machine tool shows. Case
studies received by the STC G members were used to show how research centers and industries are
collaborating. A view from the authors and the final conclusions show hot topics for future grinding
research.
ß 2009 CIRP.
1. Introduction
In 1983 Kegg presented a CIRP keynote paper titled ‘‘Industrial
Problems in Grinding’’ [30]. That survey showed the view from the
industrial grinding users on what would be the issues they would
like to see university research carried out. The main topics
included: lack of process predictability, batch production problems, inventory reduction, reduced skills and need for more
automation.
In the following year of 1984, Peters published another keynote
paper showing the CIRP contribution to industrial problems in
grinding [49]. It was a comprehensive review of the most relevant
papers published in the CIRP annals with focus on the industrial
needs pointed out by Kegg. Peters affirmed that research was
definitely ahead of the industrial needs. This is true, since many
specific problems presented by industry at that time included
today’s well established technologies such as: fast and automatic
wheel balancing systems, flexible and automated dressing devices,
grinding simulation and burn prediction systems, more application
of CNC in grinding/dressing, multiple grinding in one setup and
others.
However some of the industrial needs pointed out by Kegg in
1983 are still not solved in industry due to its high complexity.
Examples are: in process roundness and roughness measurement,
automatic thermal compensation of machine tools (to be able to
work without in process gauging) or better predictability of the
process.
Many grinding developments have been achieved by industry
since the publication of those two papers. One good example is the
development of innovations in abrasive products shown by
Webster and Tricard in 2004 [65]. Regardless the advances in
* Corresponding author.
0007-8506/$ – see front matter ß 2009 CIRP.
doi:10.1016/j.cirp.2009.09.006
grinding research and its industrial applications there is still a gap
between academic and industrial worlds. One good example is the
limited application of higher speeds in grinding with cubic boron
nitride (CBN) wheel in industry. Grinding can perform better at
higher speeds why do most CBN applications run at only 80 m/s?
Therefore, it is relevant to understand the present industrial
situation regarding grinding development. Reducing the gap
between industrial needs and academic research should help in
the orientation for future projects and planning of activities in the
CIRP STC G.
The idea is to focus on the industrial opportunities for grinding
research. These opportunities can be mapped in different ways.
The first is the evaluation of some product trends and related
grinding challenges, or opportunities. Since most industries prefer
to purchase turn-key grinding solutions, the machine tool builder
opinion should also give a good view of the opportunities to fill the
gap. Today, many CIRP members work on research for industry and
the description of these cases is also a source of information on the
tendencies in grinding and industrial opportunities. A last way to
get information is to understand the view from grinding experts in
industry running research projects.
These ideas were used to structure the data collection and the
topics of this paper and are presented in the following pages. This
keynote is not aimed at covering all industrial opportunities in
grinding but to show several aspects of the main trends that can
represent opportunities for grinding research.
The authors would like to acknowledge the members of STC G
for their contributions in the discussions, specially the following
colleagues who sent cases studies and structured information for
the preparation of this paper:
Anil Srivastava – TechSolve, Inc., Cincinatti, USA
Christoph Zeppenfeld – Laboratory for Machine Tools and
Production Engineering, WZL, Aachen, Germany
664
J.F.G. Oliveira et al. / CIRP Annals - Manufacturing Technology 58 (2009) 663–680
Chi-Hung Shen – GM Technical Fellow, GM R&D Center, Michigan,
USA
Christoph Hübert – Institute for Machine Tools and Factory
Management Berlin Technische Universität Berlin
E. Brinksmeier – Foundation Institute for Materials Science (IWT),
Division: manufacturing technologies in Bremen, Germany
John Webster – Cool-Grind Technologies, USA
Juliano Araújo – Nucleus of Advanced Manufacturing, Department
of Production Engineering, University of São Paulo, Brazil
K. Subramanian – Director, Surface Preparation Technology, HPM
Sector, Saint-Gobain, Co, USA
Klaus Weinert – Department of Machining Technology, Technische
Universität Dortmund, Germany
Marc Tricard – QED Technologies, Rochester, USA
Michael Morgan – AMTREL, Liverpool John Moores University, UK
Stephen Malkin – University of Massachusetts, Amherst, USA
2. Grinding as surface generation process
All abrasive finishing systems are intended to ‘‘generate
surfaces.’’ The value/benefit may be described in terms of the
functions served by the surface and how fast that surface can be
generated. The most commonly recognized function served by the
surface is the mechanical aspect or the nature of contact – to
transmit force, motion, selective position, etc. The desired
functions to be served by the surface may not always be unique
or singular in nature. The served functions may also rely on other
engineering properties of the surfaces such as electronic, optical,
magnetic, etc. Not surprisingly, the derived function can also be
purely aesthetic or safety-based, in some cases.
In any such cases, there is an explosive growth in the
exploitation of the ‘‘function served’’ by the surfaces. Furthermore,
every development in thin film technology and most developments in nano-technologies usually require a surface of controlled
function as the starting point. Of course, the rate of generation of
the surface determines the ‘‘productivity,’’ a critical economic
factor for success in any industrial operation. The surface
generation as a value/benefit of abrasive finishing processes can
be mapped as in Fig. 1 [56].
As a result of such value/benefit analysis, one can infer that the
multitude of abrasive finishing processes described thus far can be
divided into three natural segmentations:
Those which strive to achieve the surface at the highest rate of
surface generation or productivity. These are called ‘‘Rough’’
finishing processes.
Those which strive to achieve the highest level of the desired
functionality of the surface, generally requiring very small
amounts of material to be removed. These are called ‘‘Ultra
Precision’’ finishing process.
Those which struggle constantly for a trade-off between the
demands for better functionality and the need for higher
productivity. These are called ‘‘Precision’’ finishing processes.
Selective exploitation of the functionality of surfaces, particularly in the age of ‘‘high technology’’ is creating opportunities for
unique and innovative abrasive finishing processes, particularly
Fig. 1. All applications of abrasive products are intended to generate surfaces [56].
with ultra precision finishing processes. Hence, it may be desirable
to look at a classification such as Micro Finishing Processes, Nano
Finishing Processes and Pico Finishing Processes, which represent
an emerging family of processes within the ‘‘ultra precision’’
surface generation category.
The above discussion will lead one to the conclusion, that the
abrasive products are engineered composites used as cutting tools
to generate surfaces of required value/benefit to the customers.
Every abrasive finishing system in turn is a set point in the value/
benefit map described in Fig. 1. The vectors of progress with
reference to this set point may be to achieve higher productivity at
a given level of function, improved functions of the surface, or both.
Therefore grinding opportunities will be always related to one
of those three segmentations depending on what is the most
critical aspect for an industrial application. Obviously, the
demands for the future will be strongly related to the changes
in product design that may offer new challenges in quality or
process productivity. A good field to exemplify this analysis is the
automotive industry, which is under pressure for better and more
sustainable performance. Some of the main automotive product
changes that affect grinding performance are shortly described in
this paper.
3. Technology trends in grinding machine tool industry
In order to identify the current perception of the machine tool
builders regarding CBN grinding application, a survey was
conducted during two of the main machine tool builder exhibitions
(EMO 2007 and IMTS 2008). A total of 23 original equipment
manufacturers (OEMs) with different nationalities were included
in the survey as presented in Fig. 2. The following minimum
requirements were considered for the OEMs selection:
The OEM must design and manufacture its own grinding
machine and;
The OEM must develop the grinding process to be performed by
its customer.
As presented in Fig. 2 the majority of the OEMs interviewed
were from Germany (35%). Switzerland, US and UK were the other
major nationalities. The others OEM’s include the following
countries with one participant each: Brazil, India, Italy, Spain,
Sweden and Japan. This survey is representative for the technologies developed in Europe as well as in the USA, but does not
represent well the Asian situation.
The questions included in the survey are listed in Table 1. Inputs
from the OEM’s were collected when visiting their booths during
the exhibition and also the survey form was sent by mail for further
response by the authorized OEM personnel.
The results for the maximum grinding speed currently used are
presented in Fig. 3. Low cutting speeds are still being used for CBN,
considering that 39% of the OEM indicates maximum values
between 40 and 80 m/s, followed by 26% that indicates speeds
between 100 and 120 m/s as a limit. Three other ranges of speeds
could be identified: 121–140 m/s, 170–180 m/s and 200 m/s.
Fig. 2. OEM’s nationalities.
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Table 1
Questions included in the survey.
Number
Question
1
2
3
What is the maximum grinding speed you are using with CBN?
What are the technical reasons for not using high-speed grinding?
What is the fraction of your customer grinding applications
that use CBN?
What are the current issues that limit your use of CBN?
What breakthrough in CBN wheel technology would increase
the use of it?
4
5
Fig. 5. Fraction of the customers that uses CBN.
Fig. 3. Speeds currently used with CBN wheels.
The reason for not using high-speed grinding according to the
OEM’s is presented in Fig. 4.
The required complexity of the grinding machine with the
necessity of additional systems was the main factor for not using
CBN (33%). Improved dressing, balancing, anti-fire and coolant
systems were listed as required additional systems for enabling the
high-speed CBN application. Also, machine rigidity requirements,
grinding power demands and improved spindle technology were
included under the machine complexity. The second major issue
obtained was the economical aspect. Those two mentioned
answers, which can be obviously merged, represent the majority
of high-speed limitations from the OEM’s point of view. The lack of
interest by costumers and industry in using high-speed was
responsible for only 7% of the answers. Thermal issues and safety
add 22% together. Other reasons listed by the OEM’s were:
workpiece quality issues, vibration, maintenance and a small
percentage who believe that CBN is not feasible with present
technology.
One conclusion from this scenario is that there is still the need
for developing systems and machine features at low cost that
would enable the use of CBN in industrial applications. Industrial
customers of grinding machines are open to CBN application if they
are simple, reliable and at low cost of investment.
When OEMs were questioned about what is the fraction of their
customers grinding applications that use CBN, the results
demonstrated that CBN is still responsible for a narrow market
share when compared to the conventional wheels technology.
According to the OEMs (Fig. 5), 46% of their costumers use CBN
technology in not more than 10% of their grinding applications.
That number reached 63% when considering up to 20% of the
grinding applications. When considering costumers that run
almost only CBN (between 90% and 100%) in their applications,
that number drops to only 8%.
When asked about the limitations for increasing the CBN
market share, the OEM’s response followed the distribution
presented in the Fig. 6.
The cost of the tool was still the main limitation for using CBN
according to the OEM’s, followed by the necessity of a better
understanding of the grinding process with CBN. Other limitation
factors were cited in the same percentage: necessity of better
monitoring systems, more effective dressing operation and lack of
flexibility. Improved wheel porosity, trained operators and process
instability were listed under other factors.
Finally, the survey identified what would be the relevant
breakthroughs in the CBN technology in the OEM’s point of view.
The answers were presented in Fig. 7.
The more relevant breakthroughs can be grouped into CBN
wheel development, including possible use of CBN at higher speeds
with a better performance and allowing easier and less frequent
dressing operation (47% total). If combined with the developments
in the CBN wheel technology (grain, bond and core) those inputs
correspond to 59% of the total. Under the machine improvements
were listed: the combined operations (grinding and turning),
possibility of five axes interpolation along with increased process
control and fast and inexpensive wheel change operations. Other
breakthroughs include: high-precision grinding solutions, partnership with costumers, machine improvements, user friendly, dry
grinding with CBN and ecological aspects.
Fig. 4. Reasons for not using high-speed.
Fig. 7. Relevant breakthroughs in the CBN technology.
Fig. 6. Limitations for increasing the use of vitrified CBN wheels.
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This view from the machine tool builders gives a good picture
on what are the main limitations for expanding the use of CBN in
industry. In most cases, the barriers for a more extensive
application of CBN look quite simple. However, the robustness
of a CBN grinding application should be high to justify the
investment. Process reliability and reduction in risks should be
researched in order to give a more objective scenario for the
decision makers in industry.
Fig. 8. Lightweight valves [48].
4. Grinding technology trends in automotive industry
Automotive industry is one of the main users of ground
components. Many solutions for grinding problems come from
classical operations related to engine or transmission components.
Classical examples are the crankshaft grinding and camshaft
grinding. Since the automotive industry is one of the major drivers
for grinding development, it was chosen to be the focus of this
section.
Automobiles are referred as ‘‘magnets for environmental
criticism’’. This is due to their high visibility, carbon-based fuel
usage, tail-pipe emissions, and hazardous material content [36]. In
response to these factors the auto industry has been undertaking
changes in the selection of materials (more lightweight materials)
and powertrain technologies (e.g., hybrids and fuel cells) of its
vehicles. These changes will surely impact in the manufacturing
process demands for the future.
In order to meet commitments in terms of vehicle CO2 emission
reduction, engine research and development are today exploring
several fields. Reduction of size and weight of gasoline engines is a
strong tendency to improve efficiency [37]. For sure longer term
developments should include fuel cell technologies that may
require a totally different range of need, which are not in the scope
of this paper.
4.1. Light valves
Since the beginning of the 1980s, passenger car engines have,
on average, become about one fourth lighter, while becoming more
powerful and more fuel-efficient at the same time. One important
tendency is the reduction in the weight of engine moving parts,
such as the valves. They are subject to high acceleration levels so,
their weight highly influences the power spent to perform their
action. It is also necessary that valves resist to the wear to maintain
the sealing effect making necessary the use of advanced materials
that are difficult to grind (DTG). In the past, valves made of
ceramics, titanium and titanium aluminide offered a variety of
grinding challenges. Due to very high material and/or production
costs and extremely complex testing methods, these valves never
achieved the economic breakthrough to high volume production
applications [48].
One tendency in lightweight valves is the design using coldformed nickel-based alloy sheet metal. These lightweight valves
are configured as hollow parts, without the need of drilling
processes in the stems. The stem is produced from a precision steel
tube. The materials used are the high-strength steels
X5NiCrAlTi31-20 for intake valves, which are subject to lower
thermal loads, and NiCr23Fe for the exhaust valves. The sheet
metal used has wall thicknesses between 0.8 mm and 2.0 mm for
the formed components [48]. The individual components are
joined by laser beam welding (Fig. 8). The lightweight valve
achieves weight savings of 35–55% compared to a conventional
component of same size.
Compared to valves made of a forging blank, the use of highprecision components reduces the mechanical machining scope by
approximately 25%. However, centerless grinding of these
components can be quite challenging. Due to their small weight,
spinning can easily happen and thermal deflections can lead to
higher cylindricity values. Part flexibility is also an issue since
grinding is highly dependent on the system stiffness. So, the
development of grinding processes that can perform the combina-
tion of DTG materials at low stiffness and lightweight should be an
important industrial challenge in grinding.
One of the main concerns when grinding low stiffness
components is the presence of chatter during the rough phases
of the processes. Vitrified CBN wheels are sensitive to these
dynamic forces and may wear faster [44]. The use of dampening
systems or grinding strategies that allow the use of part support
without loosing the accuracy in concentricity or roundness are the
main research tasks for an optimized grinding of these components.
4.2. Light camshafts
Following the same trend of weight reduction, composite
camshafts are already widely used in automotive engines. One of
the technologies for composite camshafts is based on thermal
shrink fit. In this process the cams (Fig. 9) are heated for a short
period of time and are then joined to the cylinder. This technology
gives flexibility in the modular design and a weight reduction up to
45% in comparison to the solid camshaft [47].
Similarly to the lightweight valves, the composite camshafts
give additional challenges to the grinding process, since the parts
are subjected to assembly stresses and have lower stiffness than
the solid component, so high part deflections during and
deformations after grinding are expected.
4.3. Forged crankshafts
Forged crankshafts are widely used in heavy diesel engines due
to their higher load capacity and ductility. Nowadays their
application is moving to automotive engines due to their lighter
Fig. 9. Components for a composite camshaft [47].
weight and more compact dimensions. However forged steel
crankshafts are sensitive to cracks due to stress concentrations.
Consequently these forged parts are normally designed with a
radius between the diameter of each pin (or main) and the
sidewalls. The grinding of the sidewalls in these components is
critical as it affects the dimension and the surface quality of the
blending radius [53].
Grinding of the sidewalls is done by industry on both sides
simultaneously by plunging a wheel with the same width and
profile of the pin or main to be ground. The appearance of a ridge or
a ‘‘shoulder’’ on the blending radius due to the non-uniform wheel
wear is one of the most frequent causes for part rejection and the
main reason for wheel dressing. These grinding operations are
normally expensive and time consuming. Improvements may be
achieved by combining new grinding strategies and high
performance CBN grinding wheels. The adoption of high-speed
CBN face grinding with axial feed in place of plunge grinding
enabled grinding of different workpiece widths using a single
fixture, thereby increasing the process flexibility [32]. But this
strategy is not used by industry in the grinding of the sidewalls for
forged crankshafts since it frequently leads to grinding burn and
uneven wear on the wheel profile [22].
Recently, alternative strategies have been investigated for
crankshaft grinding. Oliveira et al. [46] investigated three grinding
strategies: radial plunge grinding, axial face grinding, and multistep axial face grinding. While all three strategies could produce
acceptable parts, the multi-step face grinding strategy was found
to be particularly advantageous for providing flexibility in the
process control. Comley et al. [10] successfully investigated the use
of high-efficient deep grinding (HEDG) with material removal rate
of 2000 mm3/mm s in a cylindrical plunge-grinding mode for the
machining of automotive crankshafts. Oliveira and Comley’s
research shows some new directions in the solution of the
crankshaft grinding, however industry is not yet using those
concepts and further research needs to be conducted.
These examples of changes in automotive products show a clear
tendency of lighter components manufactured with high performance materials. They are an opportunity to apply CBN grinding at
high speeds. At these conditions it should be possible to grind DTG
steels at lower forces. The main industrial challenge in these cases
will be to achieve reasonable grinding process costs and part
quality at critical product conditions.
Definitely a wider application of CBN should drive the future of
the grinding processes in precision and light automotive components. However the application of CBN in grinding is still much
smaller than expected.
5. Case studies
The STC G participants were invited to send their contributions
on industrial cases. Among 33 received cases there were 9 where
an industrial problem was solved by the research lab. The other
contributions reported industrial demands or problems not yet
fully solved that represent challenges for future research.
The cases presented here are good examples on what kind of
problems that industry is demanding and how they see the
research institutes as potential partners in their solution. The case
studies were organized under major topics and are presented
below.
Fig. 10. SEM pictures of the burr formation.
under severe conditions may lead to part or wheel damage. These
cases are called here grinding of critical materials.
5.1.1. Grinding burn and burr in profiled steel rings using
high-speed grinding
The customer needed high-speed CBN grinding (HSG) processes
for external profiling of C60 (AISI 1060) rings. For that, they
purchased several high-speed grinders. The machine tool manufacturer was asked to prove the damage free high-speed grinding
process in preliminary tests. The material was provided by the
customer. The machine tool manufacturer was successful in
setting up the requested high-speed grinding process without any
thermal damage of the machined parts on their machine/process.
After these preliminary tests, the customer ordered several HSGmachines. When the first machine was delivered the customer
insisted on carrying out the same grinding tests on the new
machine prior to the final acceptance. Again C60 material was
provided by the customer and machined with the verified
parameters. It turned out that now HSG was not possible and
heavy burr and grinding burn occurred (Figs. 10 and 11). Wheel
changes and parameter modifications did not lead to improvements. Both companies asked for scientific support to solve the
problem.
The assumption was that the layout of the high-speed grinding
process was insufficient. As verification procedures, experiments
at the IWT – Bremen detected that a material problem was
apparent. The material composition of the C60 steel used in the
two test series was different. The IWT asked for a piece of material
from the very first preliminary tests and used it for the grinding
tests. Now, grindability was perfect, no burr, no thermal damage
occurred. The grinding result could be related to the material.
Both companies agreed on grinding a C60 steel according the
specifications of DIN 17200. The companies did not know that the
DIN 17200 specifies only a limited range of elements. It turned out
5.1. Grinding of critical materials
The development of materials and the necessity of economic
machining have always driven the development of grinding
processes. In many applications, the use of advanced materials
in industrial scale will be only possible if they can be economically
machined. In that perspective, new grinding processes can be
developed or existing ones can be improved in order to match the
new requirements. Some materials are not difficult to grind but can
be sensitive to the grinding conditions. Grinding these materials
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Fig. 11. Grinding burn on the lateral surface.
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that the material used in the first tests and the one used for the
machine acceptance were from two different batches. One steel
was AL-deoxidized and the other was CA-deoxidized. This leads to
slightly different material structures after heat treatment. The
micrographs showed a structure with the former austenite grains,
hardening has not been successful.
The recommendation to the companies was: specify your
workpiece material very carefully for critical grinding processes.
DIN/ISO standards may not be sufficient by themselves.
This case is quite common in industry. Sensitive materials
under minor composition changes may become very sensitive to
grinding. The solution is always focused in the process and
machine. Sometimes big production delays are caused due to small
materials properties variation in different batches. The development of diagnosis methods for industry is critical today. A
challenge that would allow the development of more robust
grinding solutions.
5.1.2. Machining of carbon fibre-reinforced silicon-carbide
composites (C/C–SiC)
Carbon fibre-reinforced silicon-carbide composites (C/C–SiC)
feature several remarkable mechanical and thermal properties,
like low density, high temperature resistance and noncatastrophic
failure mode. The damage-tolerant, quasi-ductile behavior of this
composite material has facilitated several industrial applications,
most important friction systems [35] and bullet-proof structural
components [51]. The process of liquid silicon infiltration causes a
material expansion, so that for creating contours like drill holes
post-processing is required. For an efficient reduction of manufacturing time it is an option to machine C/C–SiC components after
siliconising. For analyzing hole generation, cutting process has
been compared with a grinding process [67]. The investigations
were carried out using a drill with polycrystalline diamond (PCD)
cutting edges and a mounted point with a sintered grinding head
containing diamond grains. The mounted point features an
abrasive trepanning tool design, which improves the coolant
supply in comparison to the drill [26]. Apart from that, squeeze
processes and friction in the center area can be avoided [67]. The
cutting process evaluation is presented in Fig. 12.
Fig. 12 shows pictures of the unused PCD drill and after a
drilling length of Lf = 90 mm. Here, abrasive wear at the flank face
and flaking at the rake face can be well observed. The process
parameters were recommended by the manufacturer. The
mounted point, in comparison, shows linear wear behavior at a
comparable material removal rate. Drilling length of Lf = 3500 mm
can be achieved. The lower tool wear at a comparable chip removal
rate results in significant higher productivity of the abrasive
Fig. 13. Planetary rolling extruder.
process. Concerning quality aspects, the use of mounted points
yields in fundamental ISO tolerances IT for the drill hole diameters
of down to IT 4 [27]. Producing drill holes with the twist drill leads
to disruptions at the exit of the drill hole even at the very first drill
hole [67]. As can be seen, the cutting process is unsuitable for
machining of C/C–SiC in this case. The development of new
grinding applications in advanced materials should take into
consideration unexpected possibilities where grinding can, as
shown in this example, be applied in unusual situations. This is a
good example on how grinding became a very adequate option due
to its specificities related to the material conditions and process
geometry.
The next case shown below is common in industry. It is an
example on why the grinding process is not always responsible for
part damage.
5.1.3. Deep profile grinding of a planetary rolling extruder
One of the problems detected is the observation of surface
cracks after deep grinding (creep-feed grinding) of main spindle
profile (Fig. 13).
The possible assumption was that the crack formation was
caused by the grinding process due to the use of inadequate
conditions. The analysis of the damages performed at the IWT has
proved that not the grinding process lead to the cracks but rather a
material problem. Metallographic inspection showed big carbide
segregations. The cracks occur along the carbides. The cracks could
be found even in the bulk material (Figs. 14 and 15).
The reason for the cracks was that the steel manufacturer did
not take enough care of a sufficient recrystallization process. Either
the strain rate in metal forming or the annealing temperature was
too low. It was suggested a material inspection before grinding or
demand a material certificate from the steel manufacturer.
In this case, the search for changes in grinding conditions would
never solve the industrial problem.
Fig. 14. Metallographic analysis of the surface layer of the main spindle.
Fig. 12. Rake face and flank wear of a twist drill after a total drilling length of
Lf = 90 mm.
Fig. 15. Structure of the bulk material with cracks and carbine lines.
5.1.4. Detection of optimization potential for the coolant supply when
machining linear guides
The inquiry from industry was the detection of optimization
potential for the coolant supply when machining linear guides. The
assumption was that the isolated grinding burn occurred when
machining linear guides seemed to be related with the orientation
and the type of coolant supply nozzle used.
By the use of a test rig for the measurement of the quantity of
coolant passing the grinding arc, different coolant supply nozzles
with varying alignment were applied. Grinding tests and
measurements of the residual stress of the machined parts were
performed for the assessment of the different coolant supply
strategies. The results showed that by the correct adjustment of the
supply nozzle, the quantity of coolant passing the grinding arc can
be enhanced by approximately 10%. The replacement of the usually
used free jet nozzle by a shoe nozzle shows that a distinct
reduction of the coolant flow rate can be realized.
The recommendation to the companies was the introduction of
a positioning system for the adjustment of the coolant supply
nozzles and verification of the applicability of shoe nozzles for the
grinding process.
5.2. New processes for special requirements
Some classical grinding problems require dedicated or specially
designed processes. Well known examples are the grinding of long
lengths and small diameters requiring the development of the
centerless processes. Other is the low stiffness in (inner-diameter)
ID grinding, requiring the use of a combined process where hard
turning and grinding are performed in the same machine. Another
is the need of grinding both flat surfaces in a spacer or a sealing
component that requires a double disk process. The combination of
critical part geometry and recent new developments in difficult to
grind materials lead to very challenging problems that require
newer processes concepts.
New grinding processes are normally very much related to new
tool or machine concepts. The solutions are normally complex and
a good opportunity for the collaboration between industry and the
research centers. The three cases described below show examples
of new grinding processes based on industrial challenges.
5.2.1. High-speed double face grinding with planetary kinematics for
the machining of coplanar surfaces
Technical parts with coplanar functional surfaces of various
materials and shapes are used for different purposes such as
regulator discs, bearing races or seal discs as depicted in Fig. 16.
A production process is very appropriate to machine these parts if
both functional surfaces of the workpiece are engaged simultaneously, thus mistakes caused by positioning or clamping may not
occur. Double face grinding machines with planetary kinematics can
be used for this kind of processing. High quality surfaces with an
accurate shape and geometry can be realized. However the material
removal rate is limited because of the low cutting speed and grinding
pressure. Another difficulty is the highly dynamic load state of the
grinding wheels causing an uneven wear profile. Referring to the
particular kinematics the careful choice of process parameters plays
an important role when machining workpieces that way. Currently
production technology and experience with the process only exist
Fig. 16. Ceramic seal and regulator discs (CeramTec AG) being machined with
double face grinding with planetary kinematics.
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Fig. 17. Influence of the process environment on the microscopic grain wear
mechanism.
for low cutting speed and material removal rates. To generate a
permanent progress in process efficiency being essential for the
production in European high wage countries new measures had to
be implemented in terms of machine tools and manufacturing
technology. It is basic knowledge about grinding that a very high
material removal rate at low grinding wheel wear can be realized
when the shivering of the abrasive grain is mainly microcrystalline.
The nature of this wear is determined by the conditions in the
contact zone shown in Fig. 17. To reach the area of microcrystalline
shivering a minimum cutting velocity is necessary.
In close cooperation with the Institute for Machine Tools and
Factory Management of the Technical University Berlin a prototypic
machine tool system has been developed by Stähli AG, PieterlenBiehl, Switzerland. In order to qualify this new machine tool and to
develop a suitable process technology for the machining of the
aforementioned ceramic seal discs a number of technological
investigations have been carried out. Each test series included five
tests at a low cutting speed vc = 1.65 m/s and immediately afterwards another one at a high cutting speed vc = 16.5 m/s. Finally a
further test at a low cutting speed was done. In each test there was a
stock removal volume of Vw = 1850 mm3 up to 18050 mm3 taken
from the workpiece. This happened at a low pressure of p(1.65 m/
s) = 0.051 N/cm2 and p(16.5 m/s) = 1.54 N/cm2. The grinding wheel
had not been sharpened between the single tests only before the first
one. The specific workpiece height reduction was detected
continuously during the process. The quality of the workpiece
surface and the grinding wheel wear were measured with a laser
triangulation system.
After the test running at high and low cutting speed the surface
roughness was almost the same at Ra = 0.4 mm and Rz = 4 mm.
Fig. 18 shows that the specific material removal rate, as expected,
Fig. 18. Influence of the cutting speed on the height reduction.
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is low during the tests with low cutting speed (test number I–V). It
is also obvious that the sharpness of the grinding wheels decreases
almost straight proportionally. At high cutting speed (test number
VI), however, the specific material removal rate Q0 w is very high
and measures more than 40 mm/s in contrast to the values around
2 mm/s at low grinding pressure. If the grinding conditions are set
back to the original conditions of low cutting velocities and
grinding pressure (test number VII) a substantial higher material
removal rate can be observed which can be attributed to a selfsharpening regime during high-speed grinding operation.
Increasing the cutting speed raises the possible material removal
rate significantly whereas the quality of the workpiece remains on a
high level. It is clearly evident that by inducing a self-sharpening
regime due to high cutting speed and grinding pressure a stable
process behavior and a reduction in down-time consuming
sharpening and profiling operations can be achieved. A process
combination of roughing at a high cutting speed and finishing by
adjusting the speed ratio between grinding wheels and pin circle
over a wide range represents a further solution concerning rising
economic and technological demands toward the discussed
machining process.
5.2.2. Grinding of camlobes and crankpin using abrasive belts
Camshaft lobe grinding is one of the most difficult tasks in all
grinding applications due to its peculiar geometry and orientation. Traditionally, the camlobes on a camshaft have been
grounded by a bonded abrasive wheel in sequential grinding
operations requiring very long cycle times [38]. A new grinding
machine using coated abrasives belts has been developed to grind
all the camlobes (6–8) simultaneously (Fig. 19). The cost of the
machine was about the same as a conventional grinder but the
total capital investment was only 1/4 since the belt grinder can
produce 4 times or more camshafts per shift. It also has the unique
capability to grind re-entrant camlobe profiles at typical mass
production cycle time because of the small radius of the PCD
(polycrystalline diamond) backup shoe. The current status is that
the machine was patented and licensed to machine tool builder
and 40 grinders were installed at production plants. The potential
enhancements are:
Spliceless, reliable, and longer CBN abrasive belt life.
Abrasive wear and belt slippage/breakage monitoring.
Increase grinding speed to 3 or more (right now the belt speed
is 25–50 m/s).
Additionally, several key areas of development for the camlobe
grinder can be identified as: jointless belts, precision dressing of
the belts, thin film chemical vapor deposition (CVD) diamond
coated backup shoes, and more efficient servo drives and CNC
programming.
In the case of the abrasive belt crankpin grinder, using the
prototype machine, it was not able to meet the pin roundness
specification consistently at that time because of its one order of
magnitude tighter tolerance requirements than the camlobes and
Fig. 19. Eight-belt camshaft grinder [38].
so the project was discontinued after a few years. Additional
technical challenges for the development are:
Higher resolutions in motion control and synchronization.
Optimize infeed and spark-out grinding algorithms.
Improve belt tensioning and slippage controls.
More precise and robust diamond (PCD) back-up shoe.
Increase grinding speed to 3 or more.
5.2.3. Productivity enhancement in internal traverse grinding with
electroplated (EP) CBN wheels
The machining of bores after the hardening process is
frequently a quite challenging task. Gears, hubs or bearing rings
have small dimensional, form and positional tolerance values as
well as high demands concerning the surface integrity. Also, as the
batch sizes are decreasing it is necessary to have highly predictable
and reliable machining processes. For this reason, processes with
geometrically defined and geometrically undefined cutting edge
have been combined with the aim to use the advantage of high
material removal rates in hard turning and the high achievable
quality in grinding.
To this extent, the potentials of electroplated CBN grinding
wheels in combination with high circumferential wheel speeds in
order to maximize the material removal rate, could be used to top
the performance of processes with defined cutting edge. Factors
which make the use of the grinding technology in the case of
internal operations difficult and so limit the process are the
complex interaction between grinding wheel and workpiece
because of the relatively big arc of contact length and the small
size of the grinding wheel which is bound to the diameter of the
workpiece [11,21].
Investigations in the use of electroplated grinding wheels in
internal traverse grinding lead to the result that with this grinding
technology the high potentials of such wheels can be utilized to
achieve an enormous enhancement of the grinding performance,
resulting in even higher removal rates than those in hard turning.
On the other hand, the internal traverse grinding process makes
use of axial feed kinematics similar to hard turning and is
characterized by a functional partition of the grinding wheel in a
conical roughing zone and a cylindrical finishing zone. This
circumstance not only allows high removal rates but also the
production of smooth surfaces and narrow tolerances in one stroke
only. This is achieved through the specific modification of the
finishing zone by touch dressing. Acoustic emission monitoring is
used to carry out the necessary small dressing infeeds. In addition,
only a narrow grinding wheel is needed what results in a small area
of contact between wheel and workpiece thus leading to lower
forces and temperatures as well [68,69,3] (Fig. 20).
The results are very encouraging since the obtained quality is as
good as a conventional plunge-grinding process but at lower cycle
times. The key advantage of this work is the gain in productivity in
Fig. 20. Experimental setup for internal traverse grinding with electroplated
grinding wheels.
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gear wheel production. Here the dressing of EP becomes an
important feature of the process.
6. Industrial opportunities
Several cases received from the STC G members are not fully
solved. They represent demands brought by industries to the
research centers. These cases are useful for understanding research
demands in grinding for the future. These cases are presented in
the following section. They were classified with regard to the main
technical challenge in: critical tolerances, machine developments,
topographic control of grinding wheels, information systems, cost
analysis and human factors.
6.1. Critical tolerances
6.1.1. Finishing of steel rollers for printing machines
Finishing of steel rollers and cylinders for modern offset
printing machines requires a very high process stability of
cylindrical grinding. Smallest surface defects as a result of
unintended changes of the grinding conditions may have a severe
impact on the functionality that can only be detected by analyzing
the print-outs. In many cases a cause for the incidence of such
defects is a small variation of the grinding tool properties in
consequence of the variance of influencing factors in the tool
fabrication. These grinding processes use conventional wheels
consisting of vitrified bond systems, aluminum-oxide grains and
pores. The high number of influencing factors in the fabrication of
grinding wheels to their characteristics can be roughly related to
following categories: composition of grain, bonding material,
additives and fillers, grain sizes/geometries, mixing quality and
temperature time characteristic at sintering. It is a common
experience in the industrial application that grinding wheels with
same specification in terms of values for grain size, hardness and
structure may give different grinding results for the same machine/
part/condition. In this case study a grinding wheel test method has
been developed that enables a characterization of grinding wheels
aside from their physical properties as well as generates
characteristic indices for comparing new grinding wheels prior
to a long term industrial application. The potential use of grinding
wheels under test is evaluated in relation to a reference grinding
wheel with well known grinding properties in the respective
grinding process as shown in Fig. 21.
During a series of short-time technological tests the workpiece
diameter and roughness, representing the work result as well as
radial wear, and grinding forces representing the process parameters are measured at different specific material removal rates.
Fig. 22. Variation of process forces during staged grinding test runs for different
wheel specifications.
Based on the measurement values of the grinding wheels under
test in relation to the values of the reference wheel at the process
working point a comparative benchmarking can be carried out.
Fig. 22 depicts an exemplarily result of one of the proposed staged
tests schemes within the developed test sequence applied to
conventional grinding wheels of very similar specification. The
main task during this stage of the study was to identify an
appropriate test sequence, sets of parameters, the necessary extent
and volume of the test runs as well approaches to analyze the
output data from multiple sensor systems applied in the test runs.
As a result of the presented case study a number of different
tests schemes and parameters could be found which allow a
confirmed proposition concerning the use potential of conventional grinding wheels for the considered application. Within the
given scope of these investigations a practicable solution was
established, whereas the formulation of an all-embracing correlation between the physical properties and the grinding behavior
proved to be highly difficult. Taking into account the considerable
demand for other industrial needs in this area shows that further
research on this topic seems indispensable.
6.1.2. Strut rod or valve stems centerless grinding
The main improvements required for the actual Strut rod
through feed centerless grinding are the reduction of the
magnitude and variation in surface finishing in order to replace
the super-finishing operation. Similar problem happen in chrome
plated engine valves, where surface finishing is frequently below
0.1 mm, Ra and roundness should be below 5 mm.
6.2. Machine developments
Machine tool development is an important topic considering
the nowadays requirements of cost reduction with high efficiency.
Additionally, new grinding processes and new grinding tools are
increasing the requirements for power, stiffness, stability and
spindle rpm.
Many present industrial challenges and new product demands
are driving the development of machines for grinding. Here are
some demands.
Fig. 21. Schematic representation of the proposed test scheme.
6.2.1. Machine tool stiffness for combined machining
Over a research period of four years, the combined simultaneous machining by hard turning, grinding and hard roller
burnishing was investigated in a public founded project. Therefore,
industry partners developed a machine tool and adapted tools. The
machine setup was designed for the production of hardened shaft
components between centers. The machining tests were carried
out at WZL, Aachen.
It was found out that one of the main influence factors in
simultaneous machining is the machine tool itself. The prototype
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in the project had several disadvantages and defects for the use in
industrial applications. The low stiffness of the workpiece clamping
leads to chattering in hard turning and grinding. Different stiffness of
the system at the spindle and the tailstock side leads to a loss of
cylindricity when using hard turning or external peel grinding. In
simultaneous machining operations, the normal cutting forces of
one process can lead to a dislocation of the workpiece rotational axis.
This is responsible for a further loss of dimension and form accuracy
because the stock removal of the other process is changed with the
position of the rotational axis. The machining tests at WZL showed
up that the prototype, developed by industry partners was
inadequate for the machining of components under industrial
aspects and need to be completely overworked.
6.2.2. Machine requirements for the hydraulic lash adjuster internal
grinding
The hydraulic lash adjuster (HLA) is one of the most precise
components in an engine. The bottleneck when grinding the HLA is
to keep the wheel profile on the corners. This is normally done by
optimizing the wheel and coolant selections. The main challenge is
to increase dressing cycles from currently 15–20 parts to more
than 30 parts and reduce cycle time. An integrated solution for
grinding this component is highly desirable by industry.
With the objectives of reducing scrap, minimizing cycle time and
obtaining standardized settings across all production plants, the ID
grinding of HLA will require machines that are capable of generating
micron-level tight tolerances. This is an important industrial
demand. Such precision grinding requires optimized selection and
setup for grinding wheel, dressing tool, and coolant along with
robust combination of grinding and dressing parameters.
6.2.3. Machine requirements for tripot spider infeed centerless
application
Tripod spiders are used in automotive transmissions in
substitution to universal or constant speed joints. Machines for
tripot spider infeed centerless grinding application (Fig. 23) will
require improved roundness control according to the wheel wear.
Usually, roundness values must the reduced in 50%. These
components are of high precision since their three outer-diameter
(OD) cylindrical surfaces are used as roller bearings. The geometry
is an additional challenge for the grinding of these components.
6.3. Wheel topography: characterization and control
The efficiency of the grinding operation is highly dependent on
the grinding wheel surface topography [41]. Some main parameters are grain distribution, wear flat areas, grain protrusion and
grit geometry. These properties can highly change the result of a
grinding process. A classical industrial challenge is how to control
these parameters in an industrial environment. There is still no
system available for industry able to control the topographic
properties of a grinding wheel while grinding. Following are some
of the presented case studies by CIRP members.
Fig. 24. Methodology for mapping a grain wear flat area.
wheel contour accuracy and surface roughness. A basic understanding of the generation and formation of the dressing tool wear
is necessary to compensate the dressing tool wear actively or to
optimize the dressing tool design.
At WZL, Aachen, research has been conducted to understand the
wear mechanisms of single diamond grains. Detailed electron-beam
microscope analyses enable the definition of temperature and
pressure conditions during the dressing process. For macroscopic
wear compensation an optical form roller wear measurement
system was constructed. It can be used flexibly for different form
rollers and enables measurement on the grinding machine. The
dressing roller geometry is recorded as contour with transmitted
light. The exact detection of the angle position at the standing
dressing roller with an encoder gives new opportunities to measure
the change of effective dressing width. Also the formation of the
dresser circumference depending on the angular position is possible.
6.3.2. Wheel topography characterization via light reflection
Several researchers [23,70] analyzed the use of surface texture
measurement systems on machining processes. Some others
[6,40,45,58,60,62] specifically looked at the relation between the
topographic characteristics of the wheel surface and its behavior
on the grinding operation. In this case study, Oliveira et al. [41]
proposed a monitoring system for the wheel sharpness measurement based on the characteristics of a light beam reflected from the
grinding wheel surface (Fig. 24) on the abrasive grains wear flat
areas over the whole wheel peripheral surface. In the proposed
systems, the light reflected from the top surfaces of the abrasive
grains is converted to an electrical signal by a charge coupled
device (CCD) sensor and processed by a microcomputer.
The proposed mapping system for grinding wheels (MSGW)
was able to acquire data with the wheel running at the cutting
speed (30 m/s) and the measurement carried out on the grinding
machine without stopping. The system was applied on a surface
grinding operation where an Al2O3 grinding wheel was used. The
results showed that the system could be used to map the wear flats
at the grinding wheel surface and also to analyze the wear
phenomena efficiently (Fig. 25). A good correlation between the
6.3.1. Dressing tool wear: electron-beam microscope to define
pressure and temperature conditions
The performance of grinding processes is defined significantly
by preparation of the grinding tools. The dressing tool wear
influences the effective dressing width and therefore the grinding
Fig. 23. Tripod spider, a challenge for the centerless grinding concept.
Fig. 25. Surface maps obtained according to the wheel wear [41].
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Fig. 27. Image construction procedure for fast RMS analysis [42].
Fig. 26. Results of the normal grinding force and percentage of flat areas according
to the wear on the grinding wheel peripheral surface [41].
average reflected light level and the normal force was also found
(Fig. 26).
The vision systems are becoming more versatile and less
expensive. Researches are being carried out in order to analyze the
wheel texture using vision systems [1]. This is a relevant challenge
for industrial application where the percentage of wear flats can be
a good parameter to identify the need for wheel dressing or to
prevent from grinding burn.
6.3.3. Wheel topography characterization via AE
The application of acoustic emission (AE) in grinding monitoring has been researched since 1984 [12]. One of the first
conclusions of this research was the high sensitivity of the AE
(root mean square) RMS level in the detection of contact between
the grinding wheel and the workpiece. This was confirmed by Refs.
[24,63,4,34,33]. The CIRP Tool Condition Monitoring keynote by
Byrne et al. 1995 [7] also confirms that and presents several
features that can be extracted from the AE RMS signal for process
monitoring and diagnosis. The contact detection capabilities of AE
information were investigated relative to the topographic characteristics of both contacting surfaces in 1994 [43,64].
The two main limitations of the AE monitoring solutions are the
random oscillation of its RMS level and signal saturation. However
AE can be very effective and fast for the contact detection of
moving surfaces [42].
In this method the acoustic emission obtained from the contact
between diamond tool and grinding wheel (or grinding wheel and
workpiece in grinding monitoring) is converted to RMS level and
acquired by a computer. The sampling rate is very fast, reaching
almost 1 mega samples per second. An AE signal processing unit
has been developed for the system. The data acquisition is made in
data arrays corresponding to a full rotation of the grinding wheel
and its triggered by a sensor positioned in the spindle [42].
The image is built up representing the level of each acquired
sample with a color scale in a three-dimensional graph. During the
dressing operation the image is constructed in real time by adding
columns in the array as the dresser travels along the wheel surface
(Fig. 27).
Fig. 28 shows an output from the acoustic mapping system
when used during a dressing operation. The vertical and horizontal
directions are the wheel circumference and width respectively.
The resolution is of two samples/mm. The depth interaction
between the diamond and the grinding wheel was 1 mm (in the
range of the elastic contact). The color intensity shows the RMS
value measured from the interaction between the dressing tool and
the abrasive grains. Darker areas mean less acoustic energy
detected by the sensor. The L shaped mark was created in the
wheel surface in order to check the system functionality. The
darker band on the left side was caused by a grinding operation
using that area of the wheel.
The system is being used by industry in two different ways
(Fig. 29):
(a) Dressing evaluation: the interaction between the dresser and
the grinding wheel can be monitored. Lack of contact between
dresser and grinding wheel will appear as dark areas in the
map.
(b) Grinding evaluation: during a plunge grinding the interaction
between the grinding wheel and the workpiece can be
evaluated.
This AE tool has already been used in industry as a prototype,
but needs further development in order to be robust in industrial
applications. The main challenges are still: AE signal detection
when using roller or ball bearings in the grinding wheel spindle,
automatic gain control of AE signal, image recognition systems for
process diagnosis and feed-back.
6.4. Information and database systems
Nowadays, the design of grinding processes is mostly based on
individual experiences of the process planner. Decreasing batch
sizes and increasing product variety result in an increasing
frequency of process designs. Hence, an effective process planning,
which is based on companywide process knowledge, becomes
more important.
The development of a grinding process planning system can be
a very complex challenge if it intends to be generic or universal.
Fig. 28. Output from the acoustic mapping system [42].
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respectively. A new approach of improving the possibilities of
process control in grinding is to integrate different methods of
process analysis and a knowledge management tool like the one
mentioned above.
Additionally, the data management in grinding (means how to
get the process monitoring data, how to store it efficiently and
effectively and how to use it) is still an issue. Companies could
know much more about the processes and the problems (and how
to solve them) if they handle their data in a more efficient or
transparent way. An effective data management could reduce lead
times considerably, but a generic solution is difficult since grinding
is a complex process.
6.5. Education concepts and human resources
Fig. 29. Examples of acoustic mapping applications.
Grinding is very sensitive to the product properties and requirements, so the development of a process planning system is a hard
task.
Relevant work has been presented on grinding modelling and
simulation. The keynote paper 2006 [5] shows a comprehensive
analysis of grinding modelling and simulation possibilities. Some
advanced software solutions are already in the market, such as the
GrindSim developed by Malkin and Guo [39] or the software for
centerless simulation developed by Gallego in 2007 [15]. These
systems can predict the grinding results regarding the cycle time,
part form error, burn occurrence and size variation for a given
grinding condition. However for specific grinding problems the
process planning is done in industry by trial and error on the
machine.
The development of database and knowledge management
systems for process control is another task depending on database
and process information. This is reported by a STC G member in the
following case.
Manufacturing as an industrial discipline is undergoing rapid
changes thanks to the access of resources from worldwide sources
as well as the supply of manufactured components to users
worldwide. As a result, research and development in manufacturing needs to be decoupled between advances in process technology
(to conceive and foster new capabilities to transform the objects)
and the enabling means to replicate known processes using
information technologies. While substantial progress has been
made in the past two to three decades in terms of skills to replicate
what is already known, there are major unfilled opportunities in
terms of conception and realization of ‘‘new processes’’. Beyond
the conception and proof of concept, it will be necessary to develop
these new systems into new applications or solutions of industrial
use. Such effort will bring benefit not only for the researchers but
also for all industrial participants in the value chain. We see this as
a major need and challenge for research and education in all
industrial processes, which include grinding [56].
Research in grinding technology needs to move beyond isolated
pockets of efforts in science (exploratory efforts to probe the
phenomena), engineering (efforts to integrate components, with or
without explicit knowledge of the underlying science) and
management (the skills that foster the what, why, how, when
and where) initiatives. Instead, there is a need for an integrated
approach for all these three disciplines and such integration needs
to move beyond limited research groups [57].
Finally, the future will belong to those with unique skills to
integrate systems and solutions. Hence education will be a key
need for all industrial processes. Grinding technology offers these
possibilities and hence provides leadership in this direction for
future education.
Therefore the main industrial challenge for industry these days
seems to be finding and keeping talented engineering staff that can
comprehend the fundamentals and can apply them in practice. The
problem is especially critical these days because young talented
people are attracted elsewhere.
6.6. Cost analysis
6.4.1. Integrated process analysis and knowledge
management in grinding
Grinding process database software has been developed and
successfully tested in industry. Furthermore the demand for highly
optimized and a more flexible grinding process at the same time
results in high challenges concerning its stability. Hereby unstable
grinding processes are characterized by the appearance of
undesirable grinding results such as white layers, scattering,
micro cracks in the workpiece surface or the non-compliance of
general geometrical tolerances. Process control aims to ensure
high process stability by early identifying desirable grinding
results. In the past, numerous promising approaches have been
developed to improve the possibilities of process control in
grinding by means of different sensor signals (e.g. acoustic
emission, spindle power, forces). But most approaches are
successfully applied only for simple geometries. Some enable
the control of more complex grinding processes but just by means
of a long ‘‘learning period’’ and only for the same grinding process,
New process development in industry always requires detailed
cost analysis. Grinding costs per part can vary depending on many
different influences, such as parts per dressing, wheel life,
production rate, need for maintenance or adjustments, dressing
time and others. It is common in industry to compare grinding
costs by simply comparing the tool costs when the production rate
is similar. This happens due to the high process instability that may
require high variable human assistance or correction, factors
which are not often measured or evaluated. It is a mistake not to
consider the more indirect grinding costs. Machine investment,
dressing costs, fluid costs, labor needs and many other parameters
can be more expensive than the direct tool costs. A cost comparison
between two grinding tools seems to be very obvious, however it
can be a complex task. For example, when testing CBN wheels, the
number of parts produced between two dressing operations can be
larger than the production batch size and the real wheel life may
not be accessed. The performance in the production of different
part materials in the same machine may also be of difficult
evaluation. An analysis where a CBN is compared to conventional
for two different materials is shown in the following case study.
6.6.1. Advantages of vitrified CBN wheels
The aim of this study was to reduce costs of precision grinding.
It is known that increasing wheel speed reduces chip thickness
[16,2]. This can result in improved workpiece quality or may allow
increased removal rates. However, high grinding speeds tend to:
Increase grinding temperature [52].
Make fluid delivery more difficult [14,28]
Require high-speed bearings [66].
Increase risk of resonance [59].
To avoid problems at high speeds, a special machine, 150% more
costly, was used. Features included high dynamic stiffness,
hydrostatic bearings for high speeds, temperature-controlled
bearing oil and high-pressure temperature-controlled coolant
delivery. A high-speed rotary dresser was used, for high-speed CBN
grinding. Touch dressing was used, taking few, very fine, dressing
cuts <5 mm to maintain quality and reduce wheel consumption.
The contact detection required an acoustic emission sensor.
Vitrified CBN and conventional abrasives were compared at
conventional speeds. CBN wheels were used at high speeds. CBN
has advantages of being hard wearing, lower grinding temperatures and high-speed capability [25].
Costs in precision cylindrical grinding are compared for
different abrasives, machines and grinding conditions. The analysis
is for repeated batch production. Account is taken of machine cost
(M/C) and abrasive (wheel) cost. Cost comparisons were based on
extensive trials to assess re-dress life against workpiece quality
requirements (Figs. 30 and 31).
Experiments show that different workpiece materials require
different strategies to reduce costs. Easy-to-grind AISI 52100
(Fig. 30) and difficult to grind Inconel 718 materials (Fig. 31) were
ground at conventional speeds and at high speeds (left and right in
each figure, respectively). It is shown that wheel speed affects
production rate through acceptable values of re-dress life, removal
Fig. 30. Costs per part when grinding AISI 52100. From left to right, the values of redress life used were, 35, 75, 25, 75 and 330 parts/dress respectively.
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rate and dwell time. Advantages were gained using vitrified CBN at
conventional speed and at high speed. For both materials, vitrified
CBN wheels used at high speed, gave better quality at lower cost
than conventional abrasives. Wheel costs became negligible and
labor costs greatly reduced. Re-dress life trials, usually neglected,
are shown to be essential to reduce costs and maintain quality [13].
The conclusion is that machine and labor cost are the most
advantages of CBN application, since higher production rates are
possible and less human assistance is necessary.
7. Selected industrial challenges
The issues listed in this section were selected based on the
authors industrial experience in grinding and from the evaluation
of all information gathered to prepare this keynote paper. The
following topics represent the view of the authors on what are the
breakthrough advances that will lead grinding to a highly adequate
process for the finishing of components.
7.1. Predictive surface integrity
Since, in nearly all fatigue loading or stress corrosion
environments, failure of a component initiates at or very near
the surface, the nature of the surface that results from manufacturing processes such as machining and grinding has long been
recognized as having a significant impact on the product
performance, longevity and reliability. The combination of stress
and elevated temperatures generated during grinding can lead to
an altered material zone (AMZ) near the surface. The AMZ can be
mechanical (plastic deformation, hardness alternation, cracks,
residual stresses, etc.), metallurgical (phase transformation, grain
size and distribution, precipitate size and distribution, recrystallization, un-tempered martensite or over-tempered martensite,
white etch layer, etc.), chemical (inter-granular attacks, embitterment, corrosion, etc.), thermal (heat affected zone, recast layer,
etc.), and electrical (conductivity, magnetic) in nature. Surface
integrity is a term that is broadly used to describe technical
conditions of machined surfaces.
In critical applications such as components for aerospace
industry, grinding processes are required to produce surfaces to
satisfy the very stringent surface integrity requirements to satisfy
the increasing demands of component performance and reliability.
These components made of nickel and titanium alloys are required
to have compressive residual stress, no deformation layer, and no
white layer. To guarantee the required surface integrity, production-grinding processes were developed and certified using
experimental trial-and-error approach and extensive surface
evaluations. These production-grinding processes are often very
conservative in process parameters and the removal rates are low.
Further more, post-processes such as shot peening and polishing
with abrasive slurry are often used to treat the ground surface to
create compressive residual stress or remove other possible
surface defects, which add additional cost to the products.
In principle, it is understood that the extent of these defects
depends on the interaction of the mechanical and thermal energy
produced and the workpiece’s material properties during the
grinding process. We still do not have a thorough understanding on
the amount of mechanical and thermal energy acting on the
workpiece. We have even less understanding about the sensitivity
of the material to the energy induced. It is critical for industries,
like aerospace, to understand the effects of changing operating
parameters before new grinding strategies can be implemented.
Industry needs predictive models for surface integrity to optimize
existing grinding processes, develop new grinding processes, and
adopt new grinding technologies.
7.2. Predictive wheel life for grinding with plated CBN wheels
Fig. 31. Comparison of costs per part when grinding Inconel 718. From left to right,
the values of re-dress life used were 1, 25 and 30 parts/dress respectively.
The main shortcoming of grinding with plated CBN wheels is
the transient behavior of its grinding performance. Unlike resin-
676
Fig. 32. Grinding performance variation with the use of wheel [20].
bonded or vitrified CBN wheels where the grinding performance is
restored by periodic dressing [31], single-layer plated CBN wheels
cannot be dressed. Therefore, the grinding performance of plated
CBN wheels varies significantly as the wheel wears down
[61,54,55,8,9,20] which imposes great challenges in cost-effectively utilizing the wheel and process control. The workpiece
material is found to be one of the dominant factors affecting the
wheel wear. For example, it was found that the grinding
performance of plated CBN wheels deteriorates much faster when
grinding a 100Cr6 steel than a GG40 cast iron [8] and nickel alloy
B1900 than an AISI 52100 bearing steel [54]. An example of the
power and roughness variation with accumulated material
removal is shown in Fig. 32. When wheel is worn, surface integrity
issues such as white layer may be produced on the machined
surface [20].
For cost-effective application of plated CBN wheels and
grinding process control, it is necessary to know when to replace
the wheel. First, we need to establish criteria parameters that can
be used to judge wheel life in production. Secondly, we need to
develop practical in-process measuring or tracking tools for wheel
life. For large volume production of relative simple surfaces,
grinding power or number of parts ground may be used. For low
volume production of geometrically complex parts, it becomes
much more challenging to use grinding power or number of parts
as a measure of wheel life. Wheel wear flat area should be a good
measure of the wheel life. However, it is not easy to measure and
track wear flat area even under well-controlled laboratory
conditions, as show in Section 6.3.
7.3. Grinding to achieve microstructure and metallurgical properties
Currently, grinding is mainly used to produce smooth surfaces
and precise tolerances without causing metallurgical changes to
the ground surface. The surface hardness, micro-structural and
metallurgical requirements are achieved by heat treatment or
other post grinding processes such as shot peening. Great cost
savings can be achieved if the grinding processes can be tailored to
produce surfaces with both the required geometrical features such
as roughness and tolerance, the mechanical property such as
hardness, and the correct metallurgical requirements such as no
white layer. To achieve this goal, the grinding process needs to put
the appropriate amount of thermal and mechanical energy into the
workpiece by the grinding action. It also needs to dissipate the
energy at the appropriate rate by cooling and other means. This
may also require laser-assisted grinding or a controlled cooling
system. It may also call for cryogenic cooling technology. Clearly,
the grinding process is required to be more precisely controlled.
7.4. Robust grinding process monitoring and control
Effectively monitoring the production-grinding process is still a
great challenge especially when grinding of components with
complex geometries and plated CBN wheels are used. A number of
monitoring technologies have been studied such as acoustic
Fig. 33. Power variations due to variation of wheel diameter and workpiece
property [17].
emission and grinding power. For production use, the most reliable
means so far seems to be monitoring the grinding power. The
acoustic emission is a good tool for grinding gap elimination and
wheel dressing especially for vitrified CBN wheels.
Grinding power can be easily measured using a power monitor.
The challenge has been to use the power signal to effectively
control the grinding process, which requires a threshold for the
power. For simple surface grinding or cylindrical ID/OD grinding,
the theoretical threshold power to avoid burn can be readily
obtained using the thermal models. However, the establishment of
the threshold power becomes much more challenging for grinding
complex parts such as serrations of turbine blades [18]. When
continuous dress creep-feed grinding is used, the situation
becomes more complicated and the power threshold should be
changing with the wheel diameter caused by continuous dressing
as shown in Fig. 33 [17].
The most challenging situation arises when grinding parts such
as airfoils under multi-axis machine motion [19]. The contact
between the grinding wheel and the workpiece changes along the
grinding path. This requires sophisticated grinding models to
establish the threshold for process monitoring. It is obvious that
the threshold will not be a constant rather a variable.
7.5. Grinding process energy efficiency improvement
Up to now, energy consumption by machining and grinding
processes has not been a concern for industry because the energy
cost is much lower than the other costs such as material, labor and
tooling. Grinding process improvements mostly focus on cycle
time reduction, wheel savings, and quality improvements. The
situation appears to be changing due to recent increase in energy
demand world wide, the significant energy price fluctuations, and
the concern over global warming.
The energy intensity or efficiency of all material removal
processes such as machining, grinding, and electrical discharge
machining (EDM) can be measured using the concept of specific
energy defined as the energy required for remove unit volume of
material. Grinding is one of the most energy-intensive among all
machining processes. The grinding specific energy is typically
higher than the energy required for melting the material. For
example, melting iron or nickel consumes roughly less than 10 J/
mm3 energy while the specific grinding energy is typically 30–
50 J//mm3 for grinding steels with conventional aluminum-oxide
abrasive wheels. Considering the large amount of grinding
operations used by industry worldwide, the impact can be
significant if we can improve the energy efficiency of the grinding
process. Furthermore, the high energy intensity of the grinding
process is also the root causes of the workpiece surface and
subsurface damages caused by the grinding operation such as
burn, white layer, and residual stresses.
It is understood that the grinding energy is used for forming
grinding chips, plowing some material on the ground surface, and
overcoming frictions between the grinding wheel and the workpiece [39]. Depending on the grinding conditions, the relative
Fig. 34. Specific cutting energy vs. specific volumetric removal rate [39].
percentage of the three components can vary significantly. The
highest percentage of the energy seems to be used for overcoming
the friction, especially for creep-feed grinding and grinding at low
removal rate. It is logical to think that research in new grinding
wheel technology, advanced coolants, and coolant deliver technology will make the biggest impact in reducing friction at the
grinding zone. It is known that oil-based coolant provides better
lubrication than water soluble coolant. However, grinding process
also requires coolant to provide effective cooling at the grinding
zone, which the water soluble coolant is much better. The
challenge is to develop a technology that can provide effective
cooling and lubrication at the grinding zone which is typically
under high pressure and temperature.
The chip formation energy alone is close to the melting energy
of the material for grinding steels with conventional aluminumoxide wheels as shown in Fig. 34.
This is the amount of energy to adiabatically shear the material
to melting. In order to reduce chip formation energy, grinding
technology is needed to remove material without inducing the
large amount of deformation or strain. The main reason of large
strain by grinding is due to the dull cutting edges of the abrasive
grits and their inability to align the cutting edges properly in the
grinding wheel. Grinding wheels similar to a milling cutter with
large number of micro inserts should provide significant advantage
over the traditional grinding wheels in terms of energy consumption. Grinding with CBN wheels typically consumes less energy
than with conventional aluminum-oxide wheels but the specific
energy level is still much higher than the melting energy.
reconditioning of grinding wheel. These reports from academic
grinding research excited engineers in industry. This is because the
EP-CBN grinding technology gives industry an opportunity to
immediately improve grinding performances without any retrofit
of existing machines, as most of manufacturing companies already
own many old grinding machines without precision dressing units.
In spite of these benefits of super-abrasive grinding technology
reported in academic grinding research, however, the majority of
grinding technology applied in industry is still conventional
grinding with aluminum-oxide (Al2O3) wheels, as shown in Section
3. The obstruction of implementation of CBN grinding technology
in industry comes from the cost performance of grinding processes.
So far, grinding academy has not provided the full technical
assessment of the CBN grinding technologies in terms of the
practical productivity giving a big impact on manufacturing costs
of products. Without good understanding in the impact of CBN
grinding processes on the cost performance, industry cannot make
a fair decision whether or not the advanced CBN grinding should be
employed in their production systems.
In order to understand the impact of CBN grinding technologies
on the grinding costs, an example is following presented where
internal grinding tests were performed with various bore sizes of
ring workpieces (hardness: HRC 60) made of case carburized steel.
Three kinds of grinding wheels were tested. These are Ni plated
200/220 grit CBN wheels (EP-CBN), vitrified bonded 200/220 grit
CBN wheels (Vit-CBN) and 80 grit aluminum-oxide (Al2O3) wheels.
The wheel life and dress interval were determined from the
grinding power monitoring. The G ratios and the amount of
dressing stocks were measured, and the proper cycle times for each
wheel were determined by the geometrical grinding accuracy
checks and metallurgical checks. These grinding test results were
used for the cost analysis with various bore sizes of workpieces for
each wheel. The cost analysis includes wheel cost, wheel change
cost, dressing cost including time and diamond consumption and
the labor cost in the US. The grinding costs with one year
operations were analyzed. It is assumed that the width of
workpieces is 90%, and the new wheel diameter is 70% of the
work bore diameter. It is also assumed that the worn wheel
diameter replaced is 80% of new wheel diameter.
Fig. 35 shows the cost index normalizing the grinding costs for
three wheel types in internal grinding operations of workpieces
with bore size ranging from 1 mm to 100 mm in diameter.
In case of EP-CBN grinding, the grinding cost is exponentially
increased with increased bore diameter. The lowest grinding cost is
demonstrated when the bore size is less than 7 mm over Vit-CBN
and Al2O3 wheels (Fig. 35). This advantage comes from longer
wheel life with no dressing requirement in the EP-CBN grinding. In
case of Vit-CBN grinding, the lowest cost is obtained when the bore
size is between 7 mm and 75 mm. The cost is rapidly increased in
the bore size of less than 7 mm and greater than 75 mm. The
conventional grinding with Al2O3 wheel gives the lowest grinding
cost in case of the bore size of greater than 75 mm. The analysis
7.6. Selection of cost-effective grinding technology
In academic research of advanced grinding technology, superior
grinding performances with super-abrasive wheels over conventional grinding wheels have been reported many times for the past
three decades. For instance, CBN grinding technology demonstrates very long-term stable grinding performance with high G
ratio and long dressing interval compared with conventional
grinding technology. So, CBN grinding can be expected to
significantly improve the productivity of grinding operations
and drastically reduce the grinding cost in industry. In addition to
these advantages, the superior surface integrity with compressive
residual stresses can be generated with CBN grinding technology.
With electroplated CBN (EP-CBN) wheels, very high stock removal
grinding can be achieved with no dressing requirement and the
grinding performance lasts for many days and months without any
677
Fig. 35. Cost comparisons in internal grinding.
678
Fig. 36. Ultra large components for windmill.
clearly reveals why CBN grinding technology has not been applied
in case of large bore grinding in industry and why EP-CBN grinding
technology has been employed in the grinding of small bore
components. It is also shown that the applications of Vit-CBN
grinding has been widely employed in internal grinding for
finishing small-medium size components, such as 7–75 mm bore
size workpieces in this study. By this example it is possible to learn
that the limitation of cost-effective CBN applications can be
extended if the grinding performance, such as shorter cycle time,
higher G ratio, longer wheel life, higher stock removal rate
capability, is improved. Industry has been looking for opportunities for further improving grinding cost performances with
aggressive advanced grinding technology.
Fig. 38. Ring diameter vs. grinding stock.
7.7. Key grinding technology for ultra large components
Wind energy as a power resource is favorable as an alternative
to fossil fuels, as it is renewable, clean and produces lower
greenhouse gas emissions. Wind energy business has been
growing rapidly and the push comes from a renewed concern
for the environment and an economical power resource in many
areas of the world. New technologies have decreased the cost of
producing electricity from wind, and the business growth in wind
power has been encouraged by tax breaks for renewable energy
and green pricing programs.
The windmills for power generation of over 2 MW require ultra
large components greater than 1m in diameter as shown in Fig. 36.
The weight of these components sometimes reaches over 5 tons.
The grinding technology for critical components of windmills such
as gears [29] and bearings plays very important roles for improving
the efficiency of power generation from wind and for ensuring the
reliability of components under high stress working conditions.
Fig. 37 shows internal grinding operations for ultra large bearing
components.
There are many technical challenges for grinding these ultra
large components due to the heavy weight and the great amount of
grinding stocks. As a typical example, Fig. 38 shows the relationship between the work diameter and the stocks removed in
grinding for case carburized rings.
The grinding stock is rapidly increased with increased ring
diameter. For instance, the grinding stock removed on rings of
2.5 m diameter reaches over 4 mm [50]. The great amount of stock
removal requirement mainly comes from the distortion in heat
treatment processes. In order to reduce the grinding time and the
cost, the low distortion heat treatment process will be the key.
Fig. 39 shows the total grinding time under the condition of the
specific stock removal rate Q0 w of 1, 3 and 10 mm3/mm s in OD and
ID grinding of rings with various sizes (size: OD = D, ID = 0.75D,
Fig. 37. Internal grinding for ultra large components.
Fig. 39. Ring diameter vs. grinding time.
Width = 0.2D). In OD and ID grinding of 2m ring with Q0 w = 1 mm3/
mm s, the grinding takes about 5 h. The time does not include the
other process time, such as the work loading/unloading time,
centering time, final finishing time and gauging time. So, the cycle
time for finishing processes of the 2 m ring can be more than 10 h.
The grinding time can be reduced from 5 h to 1.6 h with increased
stock removal rate Q0 w = 3 mm3/mm s which is typical for small
components. The grinding time is further reduced to 0.5 h with
Q0 w = 10 mm3/mm s. In this case, the total cycle time of a few hours
will be possible. For the grinding of ultra large components, the
development of high stock removal grinding process is indispensable. It is required for grinding academy to provide the
solutions for reducing grinding cycle time of ultra large components by developing new grinding technologies, such as new
grinding wheel with high stock removal capability, on-machine
gauging as the workpieces are extraordinarily heavy, and
developing multi-surface grinding processes in one chucking on
highly flexible grinding machine.
8. Summary
This paper explored different sources of information on
industrial challenges in grinding. From the gathered information,
the main conclusions that can be drawn, which would represent a
summary of relevant grinding hot research topics for industrial
applications, are:
Use of EP wheels: there is an important opportunity for the
implementation of electroplated wheels in situation where
grinding competes with hard turning or even in more precise
grinding applications. In any case, the topographic control by
dressing of EP wheels is still a hot topic for future research.
Process reliability: several cases presented here on the diagnosis
of grinding problems in industry show that process reliability is
an important issue. Future research should be conducted in order
to find a structured way to control and monitor the most relevant
variables, including wheel topography and work material, to
achieve a predictable and reliable process.
More CBN applications: it is clear that there is still a big
opportunity for the implementation of CBN in grinding operations. The main challenges are related to the better understanding of cost and process reliability. Further analysis as the
one showed in the Section 7.6 needs to be done for other
processes. A broader guidance on the advantages of CBN use
needs to be cleared up for industry. Cheaper vitrified CBN wheels
may help in understanding the advantages of their application.
Energy control: the use of grinding as a way to control the
material properties is a quite promising approach. Therefore it is
necessary to research how to better control and even to design
the thermal cycle in grinding by using precisely controlled
cooling or additional heating systems.
Sustainable processes: the development of a more sustainable
grinding, from the industry prospective, seems presently to be
more related to make it adequate to the production of
sustainable components than to focus on more environmentally
efficient process. However, decreasing grinding energy and
improving the sustainability of cooling systems is clearly a
tendency.
Dedicated solutions: from the presented cases, it was clear that
grinding innovations are connected to the classical operation/
part. Therefore many parts with specificities such as valves, lash
adjusters, fuel injectors, tripod spiders or camshafts require
research for the development of dedicated solutions capable of
dealing with their very particular issues: part geometry, typical
tolerances, typical materials, and possible fixtures.
Process combinations: many research opportunities are related to
process combinations such as grinding and turning in the same
equipment. This should require machine tools with special
features regarding: chip removal, cooling systems, stiffness and
controls.
Large components: new energy systems will require large
components that surely need specific grinding solutions.
Grinding of large components is a challenge regarding the
realization of tests and research costs, which could only be
achieved in cooperation with industry.
Lightweight parts: in the automotive segment there will be an
increasing demand for the grinding of lightweight parts made of
DTG materials. This gives additional challenges due to the lower
part stiffness and lower blade friction forces in the case of
centerless grinding application.
CBN belt grinding: one of the case studies showed that developments in CBN belts can bring new applications of belt grinding
combining good stock removal with higher quality surfaces.
Grinding database: the research on grinding models combined
with actual factory parameters database can help industry in the
design of new grinding operations. The comparison between the
model results and factory parameter can also help in the
determination of anomalies in the actual grinding operations in a
plant.
Cost analysis: cost analysis of any new grinding solution can lead
research to be better accepted by industry reducing the gap
between academia and application.
Dressing decision: the development of systems and solutions that
allow the machine to decide for the start of a dressing cycle can
improve the process performance. Today’s method is based on a
counter of number of parts. This is not precise since the grinding
stock per part can be very variable.
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Industrial challenges in grinding CIRP Annals

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