Industrial Challenges in Grinding

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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 Sao Paulo, Sao Paulo, Brazilb Nucleus of Advanced Manufacturing, Department of Production Engineering, University of Sao Paulo, Sao Paulo, Brazilc United Technologies Research Center, East Hartford, CT, USAd Technology Center, The Timken Company, Canton, OH, USA

1. Introduction

In 1983 Kegg presented a CIRP keynote paper titled ‘‘IndustrialProblems in Grinding’’ [30]. That survey showed the view from theindustrial grinding users on what would be the issues they wouldlike to see university research carried out. The main topicsincluded: lack of process predictability, batch production pro-blems, inventory reduction, reduced skills and need for moreautomation.

In the following year of 1984, Peters published another keynotepaper showing the CIRP contribution to industrial problems ingrinding [49]. It was a comprehensive review of the most relevantpapers published in the CIRP annals with focus on the industrialneeds pointed out by Kegg. Peters affirmed that research wasdefinitely ahead of the industrial needs. This is true, since manyspecific problems presented by industry at that time includedtoday’s well established technologies such as: fast and automaticwheel balancing systems, flexible and automated dressing devices,grinding simulation and burn prediction systems, more applicationof CNC in grinding/dressing, multiple grinding in one setup andothers.

However some of the industrial needs pointed out by Kegg in1983 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 towork without in process gauging) or better predictability of the

grinding research and its industrial applications there is still abetween academic and industrial worlds. One good example islimited application of higher speeds in grinding with cubic bonitride (CBN) wheel in industry. Grinding can perform bettehigher speeds why do most CBN applications run at only 80 m

Therefore, it is relevant to understand the present industsituation regarding grinding development. Reducing thebetween industrial needs and academic research should helpthe orientation for future projects and planning of activities inCIRP STC G.

The idea is to focus on the industrial opportunities for grindresearch. These opportunities can be mapped in different wThe first is the evaluation of some product trends and relagrinding challenges, or opportunities. Since most industries prto purchase turn-key grinding solutions, the machine tool buiopinion should also give a good view of the opportunities to fillgap. Today, many CIRP members work on research for industrythe description of these cases is also a source of information ontendencies in grinding and industrial opportunities. A last waget information is to understand the view from grinding expertindustry running research projects.

These ideas were used to structure the data collection andtopics of this paper and are presented in the following pages. Tkeynote is not aimed at covering all industrial opportunitiegrinding but to show several aspects of the main trends thatrepresent opportunities for grinding research.

A R T I C L E I N F O

Keywords:

Grinding

Optimization

Industry

A B S T R A C T

This keynote paper aims at analyzing relevant industrial demands for grinding research. The chosen fo

is to understand what are the main research challenges in the extensive industrial use of the proc

Since the automotive applications are the most important driving forces for grinding development

paper starts with an analysis on the main trends in more efficient engines and the changes in t

components that will affect the grinding performance. A view from 23 machine tool builders is

presented based on a survey made in interviews and during the EMO and IMTS machine tool shows. C

studies received by the STC G members were used to show how research centers and industries

collaborating. A view from the authors and the final conclusions show hot topics for future grin

research.

� 2009 C

Contents lists available at ScienceDirect

CIRP Annals - Manufacturing Technology

journal homepage: http: / /ees.elsevier.com/cirp/default .asp

C Gingfor

process.Many grinding developments have been achieved by industry

since the publication of those two papers. One good example is thedevelopment of innovations in abrasive products shown byWebster and Tricard in 2004 [65]. Regardless the advances in

and* Corresponding author.

Please cite this article in press as: Oliveira JFG, et al. Industrial challedoi:10.1016/j.cirp.2009.09.006

0007-8506/$ – see front matter � 2009 CIRP.

doi:10.1016/j.cirp.2009.09.006

The authors would like to acknowledge the members of STfor their contributions in the discussions, specially the followcolleagues who sent cases studies and structured informationthe preparation of this paper:

� Anil Srivastava – TechSolve, Inc., Cincinatti, USA

� Christoph Zeppenfeld – Laboratory for Machine Tools

Production Engineering, WZL, Aachen, Germany

nges in grinding. CIRP Annals - Manufacturing Technology (2009),

http://dx.doi.org/10.1016/j.cirp.2009.09.006

http://www.sciencedirect.com/science/journal/00078506


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i-Hung Shen – GM Technical Fellow, GM R&D Center, Michigan,

A

ristoph Hubert – Institute for Machine Tools and Factory

anagement Berlin Technische Universitat Berlin

Brinksmeier – Foundation Institute for Materials Science (IWT),

vision: manufacturing technologies in Bremen, Germany

hn Webster – Cool-Grind Technologies, USA

liano Araujo – Nucleus of Advanced Manufacturing, Department

Production Engineering, University of Sao Paulo, Brazil

Subramanian – Director, Surface Preparation Technology, HPM

ctor, Saint-Gobain, Co, USA

aus Weinert – Department of Machining Technology, Technische

iversitat Dortmund, Germany

arc Tricard – QED Technologies, Rochester, USA

ichael Morgan – AMTREL, Liverpool John Moores University, UK

ephen Malkin – University of Massachusetts, Amherst, USA

rinding as surface generation process

ll abrasive finishing systems are intended to ‘‘generateaces.’’ The value/benefit may be described in terms of thetions served by the surface and how fast that surface can berated. The most commonly recognized function served by the

ace is the mechanical aspect or the nature of contact – tosmit force, motion, selective position, etc. The desiredtions to be served by the surface may not always be uniquengular in nature. The served functions may also rely on otherneering properties of the surfaces such as electronic, optical,netic, etc. Not surprisingly, the derived function can also bely aesthetic or safety-based, in some cases.

n any such cases, there is an explosive growth in theoitation of the ‘‘function served’’ by the surfaces. Furthermore,y development in thin film technology and most develop-ts in nano-technologies usually require a surface of controlledtion as the starting point. Of course, the rate of generation ofsurface determines the ‘‘productivity,’’ a critical economicr for success in any industrial operation. The surfaceration as a value/benefit of abrasive finishing processes canapped as in Fig. 1 [56].

s a result of such value/benefit analysis, one can infer that thetitude of abrasive finishing processes described thus far can beded into three natural segmentations:

ose which strive to achieve the surface at the highest rate ofrface generation or productivity. These are called ‘‘Rough’’ishing processes.ose which strive to achieve the highest level of the desirednctionality of the surface, generally requiring very small

ounts of material to be removed. These are called ‘‘Ultraecision’’ finishing process.ose which struggle constantly for a trade-off between themands for better functionality and the need for higheroductivity. These are called ‘‘Precision’’ finishing processes.

elective exploitation of the functionality of surfaces, particu-in the age of ‘‘high technology’’ is creating opportunities for

ue and innovative abrasive finishing processes, particularly

with ultra precision finishing processes. Hence, it may be desirableto look at a classification such as Micro Finishing Processes, NanoFinishing Processes and Pico Finishing Processes, which representan emerging family of processes within the ‘‘ultra precision’’surface generation category.

The above discussion will lead one to the conclusion, that theabrasive products are engineered composites used as cutting toolsto 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 withreference to this set point may be to achieve higher productivity ata given level of function, improved functions of the surface, or both.

Therefore grinding opportunities will be always related to oneof those three segmentations depending on what is the mostcritical aspect for an industrial application. Obviously, thedemands for the future will be strongly related to the changesin product design that may offer new challenges in quality orprocess productivity. A good field to exemplify this analysis is theautomotive industry, which is under pressure for better and moresustainable performance. Some of the main automotive productchanges that affect grinding performance are shortly described inthis paper.

3. Technology trends in grinding machine tool industry

In order to identify the current perception of the machine toolbuilders regarding CBN grinding application, a survey wasconducted during two of the main machine tool builder exhibitions(EMO 2007 and IMTS 2008). A total of 23 original equipmentmanufacturers (OEMs) with different nationalities were includedin the survey as presented in Fig. 2. The following minimumrequirements were considered for the OEMs selection:

� The OEM must design and manufacture its own grindingmachine and;� The OEM must develop the grinding process to be performed by

its customer.

As presented in Fig. 2 the majority of the OEMs interviewedwere from Germany (35%). Switzerland, US and UK were the othermajor nationalities. The others OEM’s include the followingcountries with one participant each: Brazil, India, Italy, Spain,Sweden and Japan. This survey is representative for the technol-ogies developed in Europe as well as in the USA, but does notrepresent well the Asian situation.

The questions included in the survey are listed in Table 1. Inputsfrom the OEM’s were collected when visiting their booths duringthe exhibition and also the survey form was sent by mail for furtherresponse by the authorized OEM personnel.

The results for the maximum grinding speed currently used arepresented in Fig. 3. Low cutting speeds are still being used for CBN,considering that 39% of the OEM indicates maximum valuesbetween 40 and 80 m/s, followed by 26% that indicates speedsbetween 100 and 120 m/s as a limit. Three other ranges of speedscould be identified: 121–140 m/s, 170–180 m/s and 200 m/s.

Fig. 2. OEM’s nationalities.. All applications of abrasive products are intended to generate surfaces [56].

ase cite this article in press as: Oliveira JFG, et al. Industrial challenges in grinding. CIRP Annals - Manufacturing Technology (2009),i:10.1016/j.cirp.2009.09.006


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The reason for not using high-speed grinding according to theOEM’s is presented in Fig. 4.

The required complexity of the grinding machine with thenecessity of additional systems was the main factor for not usingCBN (33%). Improved dressing, balancing, anti-fire and coolantsystems were listed as required additional systems for enabling thehigh-speed CBN application. Also, machine rigidity requirements,grinding power demands and improved spindle technology wereincluded under the machine complexity. The second major issueobtained was the economical aspect. Those two mentionedanswers, which can be obviously merged, represent the majorityof high-speed limitations from the OEM’s point of view. The lack ofinterest by costumers and industry in using high-speed wasresponsible for only 7% of the answers. Thermal issues and safetyadd 22% together. Other reasons listed by the OEM’s were:workpiece quality issues, vibration, maintenance and a smallpercentage who believe that CBN is not feasible with presenttechnology.

One conclusion from this scenario is that there is still the needfor developing systems and machine features at low cost thatwould enable the use of CBN in industrial applications. Industrialcustomers of grinding machines are open to CBN application if theyare simple, reliable and at low cost of investment.

When OEMs were questioned about what is the fraction of theircustomers grinding applications that use CBN, the resultsdemonstrated that CBN is still responsible for a narrow marketshare when compared to the conventional wheels technology.According to the OEMs (Fig. 5), 46% of their costumers use CBNtechnology in not more than 10% of their grinding applications.That number reached 63% when considering up to 20% of thegrinding applications. When considering costumers that run

almost only CBN (between 90% and 100%) in their applicatithat number drops to only 8%.

When asked about the limitations for increasing the Cmarket share, the OEM’s response followed the distribupresented in the Fig. 6.

The cost of the tool was still the main limitation for using Caccording to the OEM’s, followed by the necessity of a beunderstanding of the grinding process with CBN. Other limitafactors were cited in the same percentage: necessity of bemonitoring systems, more effective dressing operation and lacflexibility. Improved wheel porosity, trained operators and procinstability were listed under other factors.

Finally, the survey identified what would be the relevbreakthroughs in the CBN technology in the OEM’s point of viThe answers were presented in Fig. 7.

The more relevant breakthroughs can be grouped into Cwheel development, including possible use of CBN at higher spewith a better performance and allowing easier and less frequdressing operation (47% total). If combined with the developmein the CBN wheel technology (grain, bond and core) those inpcorrespond to 59% of the total. Under the machine improvemewere listed: the combined operations (grinding and turnipossibility of five axes interpolation along with increased proccontrol and fast and inexpensive wheel change operations. Otbreakthroughs include: high-precision grinding solutions, partnship with costumers, machine improvements, user friendly,grinding with CBN and ecological aspects.

Table 1Questions included in the survey.

Number Question

1 What is the maximum grinding speed you are using with CBN?

2 What are the technical reasons for not using high-speed grinding?

3 What is the fraction of your customer grinding applications

that use CBN?

4 What are the current issues that limit your use of CBN?

5 What breakthrough in CBN wheel technology would increase

the use of it?

Fig. 3. Speeds currently used with CBN wheels.

Fig. 5. Fraction of the customers that uses CBN.

Fig. 6. Limitations for increasing the use of vitrified CBN wheels.

Fig. 4. Reasons for not using high-speed. Fig. 7. Relevant breakthroughs in the CBN technology.

Please cite this article in press as: Oliveira JFG, et al. Industrial challenges in grinding. CIRP Annals - Manufacturing Technology (2009),doi:10.1016/j.cirp.2009.09.006


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his view from the machine tool builders gives a good picturehat are the main limitations for expanding the use of CBN instry. In most cases, the barriers for a more extensiveication of CBN look quite simple. However, the robustness

CBN grinding application should be high to justify thestment. Process reliability and reduction in risks should bearched in order to give a more objective scenario for thesion makers in industry.

rinding technology trends in automotive industry

utomotive industry is one of the main users of groundponents. Many solutions for grinding problems come fromsical operations related to engine or transmission components.sical examples are the crankshaft grinding and camshaftding. Since the automotive industry is one of the major driversrinding development, it was chosen to be the focus of this

ion.utomobiles are referred as ‘‘magnets for environmentalcism’’. This is due to their high visibility, carbon-based fuele, tail-pipe emissions, and hazardous material content [36]. Inonse to these factors the auto industry has been undertakingges in the selection of materials (more lightweight materials)powertrain technologies (e.g., hybrids and fuel cells) of itscles. These changes will surely impact in the manufacturingess demands for the future.

n order to meet commitments in terms of vehicle CO2 emissionction, engine research and development are today exploringral fields. Reduction of size and weight of gasoline engines is ang tendency to improve efficiency [37]. For sure longer termlopments should include fuel cell technologies that mayire a totally different range of need, which are not in the scopeis paper.

Light valves

ince the beginning of the 1980s, passenger car engines have,verage, become about one fourth lighter, while becoming moreerful and more fuel-efficient at the same time. One importantency is the reduction in the weight of engine moving parts,as the valves. They are subject to high acceleration levels so,

r weight highly influences the power spent to perform theirn. It is also necessary that valves resist to the wear to maintain

sealing effect making necessary the use of advanced materialsare difficult to grind (DTG). In the past, valves made of

mics, titanium and titanium aluminide offered a variety ofding challenges. Due to very high material and/or productions and extremely complex testing methods, these valves nevereved the economic breakthrough to high volume productionications [48].ne tendency in lightweight valves is the design using cold-ed nickel-based alloy sheet metal. These lightweight valvesconfigured as hollow parts, without the need of drillingesses in the stems. The stem is produced from a precision steel. The materials used are the high-strength steelsiCrAlTi31-20 for intake valves, which are subject to lowermal loads, and NiCr23Fe for the exhaust valves. The sheetal used has wall thicknesses between 0.8 mm and 2.0 mm forformed components [48]. The individual components are

tion of DTG materials at low stiffness and lightweight should be animportant industrial challenge in grinding.

One of the main concerns when grinding low stiffnesscomponents is the presence of chatter during the rough phasesof the processes. Vitrified CBN wheels are sensitive to thesedynamic forces and may wear faster [44]. The use of dampeningsystems or grinding strategies that allow the use of part supportwithout loosing the accuracy in concentricity or roundness are themain research tasks for an optimized grinding of these compo-nents.

4.2. Light camshafts

Following the same trend of weight reduction, compositecamshafts are already widely used in automotive engines. One ofthe technologies for composite camshafts is based on thermalshrink fit. In this process the cams (Fig. 9) are heated for a shortperiod of time and are then joined to the cylinder. This technologygives flexibility in the modular design and a weight reduction up to45% in comparison to the solid camshaft [47].

Similarly to the lightweight valves, the composite camshaftsgive additional challenges to the grinding process, since the partsare subjected to assembly stresses and have lower stiffness thanthe solid component, so high part deflections during anddeformations after grinding are expected.

4.3. Forged crankshafts

Forged crankshafts are widely used in heavy diesel engines dueto their higher load capacity and ductility. Nowadays theirapplication is moving to automotive engines due to their lighter

Fig. 8. Lightweight valves [48].

Fig. 9. Components for a composite camshaft [47].

ed by laser beam welding (Fig. 8). The lightweight valveeves weight savings of 35–55% compared to a conventionalponent of same size.ompared to valves made of a forging blank, the use of high-ision components reduces the mechanical machining scope byoximately 25%. However, centerless grinding of theseponents can be quite challenging. Due to their small weight,ning can easily happen and thermal deflections can lead toer cylindricity values. Part flexibility is also an issue sinceding is highly dependent on the system stiffness. So, thelopment of grinding processes that can perform the combina-



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weight and more compact dimensions. However forged steelcrankshafts are sensitive to cracks due to stress concentrations.Consequently these forged parts are normally designed with aradius between the diameter of each pin (or main) and thesidewalls. The grinding of the sidewalls in these components iscritical as it affects the dimension and the surface quality of theblending radius [53].

Grinding of the sidewalls is done by industry on both sidessimultaneously by plunging a wheel with the same width andprofile of the pin or main to be ground. The appearance of a ridge ora ‘‘shoulder’’ on the blending radius due to the non-uniform wheelwear is one of the most frequent causes for part rejection and themain reason for wheel dressing. These grinding operations arenormally expensive and time consuming. Improvements may beachieved by combining new grinding strategies and highperformance CBN grinding wheels. The adoption of high-speedCBN face grinding with axial feed in place of plunge grindingenabled grinding of different workpiece widths using a singlefixture, thereby increasing the process flexibility [32]. But thisstrategy is not used by industry in the grinding of the sidewalls forforged crankshafts since it frequently leads to grinding burn anduneven wear on the wheel profile [22].

Recently, alternative strategies have been investigated forcrankshaft grinding. Oliveira et al. [46] investigated three grindingstrategies: radial plunge grinding, axial face grinding, and multi-step axial face grinding. While all three strategies could produceacceptable parts, the multi-step face grinding strategy was foundto be particularly advantageous for providing flexibility in theprocess control. Comley et al. [10] successfully investigated the useof high-efficient deep grinding (HEDG) with material removal rateof 2000 mm3/mm s in a cylindrical plunge-grinding mode for themachining of automotive crankshafts. Oliveira and Comley’sresearch shows some new directions in the solution of thecrankshaft grinding, however industry is not yet using thoseconcepts and further research needs to be conducted.

These examples of changes in automotive products show a cleartendency of lighter components manufactured with high perfor-mance materials. They are an opportunity to apply CBN grinding athigh speeds. At these conditions it should be possible to grind DTGsteels at lower forces. The main industrial challenge in these caseswill be to achieve reasonable grinding process costs and partquality at critical product conditions.

Definitely a wider application of CBN should drive the future ofthe grinding processes in precision and light automotive compo-nents. However the application of CBN in grinding is still muchsmaller than expected.

5. Case studies

The STC G participants were invited to send their contributionson industrial cases. Among 33 received cases there were 9 wherean industrial problem was solved by the research lab. The othercontributions reported industrial demands or problems not yetfully solved that represent challenges for future research.

The cases presented here are good examples on what kind ofproblems that industry is demanding and how they see theresearch institutes as potential partners in their solution. The casestudies were organized under major topics and are presentedbelow.

under severe conditions may lead to part or wheel damage. Thcases are called here grinding of critical materials.

5.1.1. Grinding burn and burr in profiled steel rings using high-sp

grinding

The customer needed high-speed CBN grinding (HSG) procefor external profiling of C60 (AISI 1060) rings. For that, tpurchased several high-speed grinders. The machine tool mafacturer was asked to prove the damage free high-speed grindprocess in preliminary tests. The material was provided bycustomer. The machine tool manufacturer was successfusetting up the requested high-speed grinding process withoutthermal damage of the machined parts on their machine/procAfter these preliminary tests, the customer ordered several Hmachines. When the first machine was delivered the custoinsisted on carrying out the same grinding tests on the nmachine prior to the final acceptance. Again C60 materialprovided by the customer and machined with the veriparameters. It turned out that now HSG was not possibleheavy burr and grinding burn occurred (Figs. 10 and 11). Whchanges and parameter modifications did not lead to improments. Both companies asked for scientific support to solveproblem.

The assumption was that the layout of the high-speed grindprocess was insufficient. As verification procedures, experimeat the IWT – Bremen detected that a material problemapparent. The material composition of the C60 steel used intwo test series was different. The IWT asked for a piece of matefrom the very first preliminary tests and used it for the grindtests. Now, grindability was perfect, no burr, no thermal damoccurred. The grinding result could be related to the material

Both companies agreed on grinding a C60 steel accordingspecifications of DIN 17200. The companies did not know thatDIN 17200 specifies only a limited range of elements. It turned

Fig. 10. SEM pictures of the burr formation.

Fig. 11. Grinding burn on the lateral surface.

5.1. Grinding of critical materials

The development of materials and the necessity of economicmachining have always driven the development of grindingprocesses. In many applications, the use of advanced materialsin industrial scale will be only possible if they can be economicallymachined. In that perspective, new grinding processes can bedeveloped or existing ones can be improved in order to match thenew requirements. Some materials are not difficult to grind but canbe sensitive to the grinding conditions. Grinding these materials



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the material used in the first tests and the one used for thehine acceptance were from two different batches. One steelAL-deoxidized and the other was CA-deoxidized. This leads totly different material structures after heat treatment. Theographs showed a structure with the former austenite grains,ening has not been successful.he recommendation to the companies was: specify yourkpiece material very carefully for critical grinding processes./ISO standards may not be sufficient by themselves.his case is quite common in industry. Sensitive materialser minor composition changes may become very sensitive toding. The solution is always focused in the process andhine. Sometimes big production delays are caused due to smallerials properties variation in different batches. The develop-t of diagnosis methods for industry is critical today. Alenge that would allow the development of more robustding solutions.

. Machining of carbon fibre-reinforced silicon-carbide

posites (C/C–SiC)

arbon fibre-reinforced silicon-carbide composites (C/C–SiC)ure several remarkable mechanical and thermal properties,low density, high temperature resistance and noncatastrophicre mode. The damage-tolerant, quasi-ductile behavior of thisposite material has facilitated several industrial applications,t important friction systems [35] and bullet-proof structuralponents [51]. The process of liquid silicon infiltration causes aerial expansion, so that for creating contours like drill holes-processing is required. For an efficient reduction of manu-ring time it is an option to machine C/C–SiC components after

onising. For analyzing hole generation, cutting process hascompared with a grinding process [67]. The investigations

e carried out using a drill with polycrystalline diamond (PCD)ing edges and a mounted point with a sintered grinding headaining diamond grains. The mounted point features ansive trepanning tool design, which improves the coolantly in comparison to the drill [26]. Apart from that, squeezeesses and friction in the center area can be avoided [67]. Theing process evaluation is presented in Fig. 12.ig. 12 shows pictures of the unused PCD drill and after aing length of Lf = 90 mm. Here, abrasive wear at the flank faceflaking at the rake face can be well observed. The processmeters were recommended by the manufacturer. Thented point, in comparison, shows linear wear behavior at aparable material removal rate. Drilling length of Lf = 3500 mmbe achieved. The lower tool wear at a comparable chip removal

results in significant higher productivity of the abrasive

process. Concerning quality aspects, the use of mounted pointsyields in fundamental ISO tolerances IT for the drill hole diametersof down to IT 4 [27]. Producing drill holes with the twist drill leadsto disruptions at the exit of the drill hole even at the very first drillhole [67]. As can be seen, the cutting process is unsuitable formachining of C/C–SiC in this case. The development of newgrinding applications in advanced materials should take intoconsideration unexpected possibilities where grinding can, asshown in this example, be applied in unusual situations. This is agood example on how grinding became a very adequate option dueto its specificities related to the material conditions and processgeometry.

The next case shown below is common in industry. It is anexample on why the grinding process is not always responsible forpart damage.

5.1.3. Deep profile grinding of a planetary rolling extruder

One of the problems detected is the observation of surfacecracks after deep grinding (creep-feed grinding) of main spindleprofile (Fig. 13).

The possible assumption was that the crack formation wascaused by the grinding process due to the use of inadequateconditions. The analysis of the damages performed at the IWT hasproved that not the grinding process lead to the cracks but rather amaterial problem. Metallographic inspection showed big carbidesegregations. The cracks occur along the carbides. The cracks couldbe found even in the bulk material (Figs. 14 and 15).

The reason for the cracks was that the steel manufacturer didnot take enough care of a sufficient recrystallization process. Eitherthe strain rate in metal forming or the annealing temperature wastoo low. It was suggested a material inspection before grinding ordemand a material certificate from the steel manufacturer.

In this case, the search for changes in grinding conditions wouldnever solve the industrial problem.

Fig. 14. Metallographic analysis of the surface layer of the main spindle.

Fig. 13. Planetary rolling extruder.

Fig. 15. Structure of the bulk material with cracks and carbine lines.

2. Rake face and flank wear of a twist drill after a total drilling length of

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5.1.4. Detection of optimization potential for the coolant supply when

machining linear guides

The inquiry from industry was the detection of optimizationpotential for the coolant supply when machining linear guides. Theassumption was that the isolated grinding burn occurred whenmachining linear guides seemed to be related with the orientationand the type of coolant supply nozzle used.

By the use of a test rig for the measurement of the quantity ofcoolant passing the grinding arc, different coolant supply nozzleswith varying alignment were applied. Grinding tests andmeasurements of the residual stress of the machined parts wereperformed for the assessment of the different coolant supplystrategies. The results showed that by the correct adjustment of thesupply nozzle, the quantity of coolant passing the grinding arc canbe enhanced by approximately 10%. The replacement of the usuallyused free jet nozzle by a shoe nozzle shows that a distinctreduction of the coolant flow rate can be realized.

The recommendation to the companies was the introduction ofa positioning system for the adjustment of the coolant supplynozzles and verification of the applicability of shoe nozzles for thegrinding process.

5.2. New processes for special requirements

Some classical grinding problems require dedicated or speciallydesigned processes. Well known examples are the grinding of longlengths and small diameters requiring the development of thecenterless processes. Other is the low stiffness in (inner-diameter)ID grinding, requiring the use of a combined process where hardturning and grinding are performed in the same machine. Anotheris the need of grinding both flat surfaces in a spacer or a sealingcomponent that requires a double disk process. The combination ofcritical part geometry and recent new developments in difficult togrind materials lead to very challenging problems that requirenewer processes concepts.

New grinding processes are normally very much related to newtool or machine concepts. The solutions are normally complex anda good opportunity for the collaboration between industry and theresearch centers. The three cases described below show examplesof 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 variousmaterials and shapes are used for different purposes such asregulator discs, bearing races or seal discs as depicted in Fig. 16.

A production process is very appropriate to machine these parts ifboth functional surfaces of the workpiece are engaged simulta-neously, thus mistakes caused by positioning or clamping may notoccur. Double face grinding machines with planetary kinematics canbe used for this kind of processing. High quality surfaces with anaccurate shape and geometry can be realized. However the materialremoval rate is limited because of the low cutting speed and grindingpressure. Another difficulty is the highly dynamic load state of thegrinding wheels causing an uneven wear profile. Referring to theparticular kinematics the careful choice of process parameters playsan important role when machining workpieces that way. Currentlyproduction technology and experience with the process only exist

for low cutting speed and material removal rates. To generapermanent progress in process efficiency being essential forproduction in European high wage countries new measures habe implemented in terms of machine tools and manufactutechnology. It is basic knowledge about grinding that a very hmaterial removal rate at low grinding wheel wear can be realiwhen the shivering of the abrasive grain is mainly microcrystallThe nature of this wear is determined by the conditions incontact zone shown in Fig. 17. To reach the area of microcrystalshivering a minimum cutting velocity is necessary.

In close cooperation with the Institute for Machine ToolsFactory Management of the Technical University Berlin a prototymachine tool system has been developed by Stahli AG, PieterBiehl, Switzerland. In order to qualify this new machine tool andevelop a suitable process technology for the machining ofaforementioned ceramic seal discs a number of technologinvestigations have been carried out. Each test series includedtests at a low cutting speed vc = 1.65 m/s and immediately afwards another one at a high cutting speed vc = 16.5 m/s. Finalfurther test at a low cutting speed was done. In each test there wstock removal volume of Vw = 1850 mm3 up to 18050 mm3 tafrom the workpiece. This happened at a low pressure of p(1.65s) = 0.051 N/cm2 and p(16.5 m/s) = 1.54 N/cm2. The grinding whhad not been sharpened between the single tests only before theone. The specific workpiece height reduction was deteccontinuously during the process. The quality of the workpsurface and the grinding wheel wear were measured with a ltriangulation system.

After the test running at high and low cutting speed the surroughness was almost the same at Ra = 0.4 mm and Rz = 4 mFig. 18 shows that the specific material removal rate, as expec

Fig. 17. Influence of the process environment on the microscopic grain

mechanism.

Fig. 18. Influence of the cutting speed on the height reduction.

Fig. 16. Ceramic seal and regulator discs (CeramTec AG) being machined with

double face grinding with planetary kinematics.



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w during the tests with low cutting speed (test number I–V). Itso obvious that the sharpness of the grinding wheels decreasesost straight proportionally. At high cutting speed (test numberhowever, the specific material removal rate Q0w is very highmeasures more than 40 mm/s in contrast to the values around

/s at low grinding pressure. If the grinding conditions are setto the original conditions of low cutting velocities and

ding pressure (test number VII) a substantial higher materialoval rate can be observed which can be attributed to a self-pening regime during high-speed grinding operation.ncreasing the cutting speed raises the possible material removalsignificantly whereas the quality of the workpiece remains on alevel. It is clearly evident that by inducing a self-sharpening

me due to high cutting speed and grinding pressure a stableess behavior and a reduction in down-time consumingpening and profiling operations can be achieved. A processbination of roughing at a high cutting speed and finishing bysting the speed ratio between grinding wheels and pin circlea wide range represents a further solution concerning risingomic and technological demands toward the discussedhining process.

. Grinding of camlobes and crankpin using abrasive belts

amshaft lobe grinding is one of the most difficult tasks in allding applications due to its peculiar geometry and orienta-. Traditionally, the camlobes on a camshaft have beennded by a bonded abrasive wheel in sequential grindingations requiring very long cycle times [38]. A new grindinghine using coated abrasives belts has been developed to grindhe camlobes (6–8) simultaneously (Fig. 19). The cost of thehine was about the same as a conventional grinder but thel capital investment was only 1/4 since the belt grinder canuce 4 times or more camshafts per shift. It also has the uniquebility to grind re-entrant camlobe profiles at typical massuction cycle time because of the small radius of the PCD

ycrystalline diamond) backup shoe. The current status is thatmachine was patented and licensed to machine tool builder40 grinders were installed at production plants. The potentialancements are:

liceless, reliable, and longer CBN abrasive belt life.rasive wear and belt slippage/breakage monitoring.

crease grinding speed to 3� or more (right now the belt speed25–50 m/s).

dditionally, several key areas of development for the camlobeder can be identified as: jointless belts, precision dressing ofbelts, thin film chemical vapor deposition (CVD) diamonded backup shoes, and more efficient servo drives and CNCramming.

n the case of the abrasive belt crankpin grinder, using theotype machine, it was not able to meet the pin roundnessification consistently at that time because of its one order ofnitude tighter tolerance requirements than the camlobes and

so the project was discontinued after a few years. Additionaltechnical 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 isfrequently a quite challenging task. Gears, hubs or bearing ringshave small dimensional, form and positional tolerance values aswell as high demands concerning the surface integrity. Also, as thebatch sizes are decreasing it is necessary to have highly predictableand reliable machining processes. For this reason, processes withgeometrically defined and geometrically undefined cutting edgehave been combined with the aim to use the advantage of highmaterial removal rates in hard turning and the high achievablequality in grinding.

To this extent, the potentials of electroplated CBN grindingwheels in combination with high circumferential wheel speeds inorder to maximize the material removal rate, could be used to topthe performance of processes with defined cutting edge. Factorswhich make the use of the grinding technology in the case ofinternal operations difficult and so limit the process are thecomplex interaction between grinding wheel and workpiecebecause of the relatively big arc of contact length and the smallsize of the grinding wheel which is bound to the diameter of theworkpiece [11,21].

Investigations in the use of electroplated grinding wheels ininternal traverse grinding lead to the result that with this grindingtechnology the high potentials of such wheels can be utilized toachieve 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 makesuse of axial feed kinematics similar to hard turning and ischaracterized by a functional partition of the grinding wheel in aconical roughing zone and a cylindrical finishing zone. Thiscircumstance not only allows high removal rates but also theproduction of smooth surfaces and narrow tolerances in one strokeonly. This is achieved through the specific modification of thefinishing zone by touch dressing. Acoustic emission monitoring isused to carry out the necessary small dressing infeeds. In addition,only a narrow grinding wheel is needed what results in a small areaof contact between wheel and workpiece thus leading to lowerforces and temperatures as well [68,69,3] (Fig. 20).

The results are very encouraging since the obtained quality is asgood as a conventional plunge-grinding process but at lower cycletimes. The key advantage of this work is the gain in productivity in

Fig. 20. Experimental setup for internal traverse grinding with electroplated

grinding wheels.Fig. 19. Eight-belt camshaft grinder [38].



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gear wheel production. Here the dressing of EP becomes animportant feature of the process.

6. Industrial opportunities

Several cases received from the STC G members are not fullysolved. They represent demands brought by industries to theresearch centers. These cases are useful for understanding researchdemands in grinding for the future. These cases are presented inthe following section. They were classified with regard to the maintechnical challenge in: critical tolerances, machine developments,topographic control of grinding wheels, information systems, costanalysis 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 offsetprinting machines requires a very high process stability ofcylindrical grinding. Smallest surface defects as a result ofunintended changes of the grinding conditions may have a severeimpact on the functionality that can only be detected by analyzingthe print-outs. In many cases a cause for the incidence of suchdefects is a small variation of the grinding tool properties inconsequence of the variance of influencing factors in the toolfabrication. These grinding processes use conventional wheelsconsisting of vitrified bond systems, aluminum-oxide grains andpores. The high number of influencing factors in the fabrication ofgrinding wheels to their characteristics can be roughly related tofollowing categories: composition of grain, bonding material,additives and fillers, grain sizes/geometries, mixing quality andtemperature time characteristic at sintering. It is a commonexperience in the industrial application that grinding wheels withsame specification in terms of values for grain size, hardness andstructure may give different grinding results for the same machine/part/condition. In this case study a grinding wheel test method hasbeen developed that enables a characterization of grinding wheelsaside from their physical properties as well as generatescharacteristic indices for comparing new grinding wheels priorto a long term industrial application. The potential use of grindingwheels under test is evaluated in relation to a reference grindingwheel with well known grinding properties in the respectivegrinding process as shown in Fig. 21.

During a series of short-time technological tests the workpiecediameter and roughness, representing the work result as well asradial wear, and grinding forces representing the process para-meters are measured at different specific material removal rates.

Based on the measurement values of the grinding wheels untest in relation to the values of the reference wheel at the procworking point a comparative benchmarking can be carriedFig. 22 depicts an exemplarily result of one of the proposed statests schemes within the developed test sequence appliedconventional grinding wheels of very similar specification.main task during this stage of the study was to identifyappropriate test sequence, sets of parameters, the necessary exand volume of the test runs as well approaches to analyzeoutput data from multiple sensor systems applied in the test ru

As a result of the presented case study a number of differtests schemes and parameters could be found which allowconfirmed proposition concerning the use potential of convtional grinding wheels for the considered application. Withingiven scope of these investigations a practicable solutionestablished, whereas the formulation of an all-embracing corrtion between the physical properties and the grinding behaproved to be highly difficult. Taking into account the considerademand for other industrial needs in this area shows that furtresearch on this topic seems indispensable.

6.1.2. Strut rod or valve stems centerless grinding

The main improvements required for the actual Strutthrough feed centerless grinding are the reduction ofmagnitude and variation in surface finishing in order to repthe super-finishing operation. Similar problem happen in chroplated engine valves, where surface finishing is frequently be0.1 mm, Ra and roundness should be below 5 mm.

6.2. Machine developments

Machine tool development is an important topic considethe nowadays requirements of cost reduction with high efficieAdditionally, new grinding processes and new grinding toolsincreasing the requirements for power, stiffness, stabilityspindle rpm.

Many present industrial challenges and new product demaare driving the development of machines for grinding. Heresome demands.

Fig. 22. Variation of process forces during staged grinding test runs for diffe

wheel specifications.

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s inypeFig. 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 simuneous machining by hard turning, grinding and hard roburnishing was investigated in a public founded project. Therefindustry partners developed a machine tool and adapted tools.machine setup was designed for the production of hardened scomponents between centers. The machining tests were carout at WZL, Aachen.

It was found out that one of the main influence factorsimultaneous machining is the machine tool itself. The protot



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e project had several disadvantages and defects for the use instrial applications. The low stiffness of the workpiece clampings to chattering in hard turning and grinding. Different stiffness ofsystem at the spindle and the tailstock side leads to a loss ofdricity when using hard turning or external peel grinding. Inltaneous machining operations, the normal cutting forces of

process can lead to a dislocation of the workpiece rotational axis.is responsible for a further loss of dimension and form accuracyuse the stock removal of the other process is changed with thetion of the rotational axis. The machining tests at WZL showedthat the prototype, developed by industry partners wasequate for the machining of components under industrialcts and need to be completely overworked.

. Machine requirements for the hydraulic lash adjuster internal

ding

he hydraulic lash adjuster (HLA) is one of the most preciseponents in an engine. The bottleneck when grinding the HLA iseep the wheel profile on the corners. This is normally done bymizing the wheel and coolant selections. The main challenge iscrease dressing cycles from currently 15–20 parts to more30 parts and reduce cycle time. An integrated solution for

ding this component is highly desirable by industry.ith the objectives of reducing scrap, minimizing cycle time and

ining standardized settings across all production plants, the IDding of HLA will require machines that are capable of generatingon-level tight tolerances. This is an important industrialand. Such precision grinding requires optimized selection andp for grinding wheel, dressing tool, and coolant along withst combination of grinding and dressing parameters.

. Machine requirements for tripot spider infeed centerless

ication

ripod spiders are used in automotive transmissions intitution to universal or constant speed joints. Machines fort spider infeed centerless grinding application (Fig. 23) willire improved roundness control according to the wheel wear.

ally, roundness values must the reduced in 50%. Theseponents are of high precision since their three outer-diameter) cylindrical surfaces are used as roller bearings. The geometry

additional challenge for the grinding of these components.

Wheel topography: characterization and control

he efficiency of the grinding operation is highly dependent ongrinding wheel surface topography [41]. Some main para-ers are grain distribution, wear flat areas, grain protrusion andgeometry. These properties can highly change the result of ading process. A classical industrial challenge is how to controle parameters in an industrial environment. There is still noem available for industry able to control the topographicerties of a grinding wheel while grinding. Following are somee presented case studies by CIRP members.

. Dressing tool wear: electron-beam microscope to define

sure and temperature conditions

he performance of grinding processes is defined significantlypreparation of the grinding tools. The dressing tool wearences the effective dressing width and therefore the grinding

wheel contour accuracy and surface roughness. A basic under-standing of the generation and formation of the dressing tool wearis necessary to compensate the dressing tool wear actively or tooptimize the dressing tool design.

At WZL, Aachen, research has been conducted to understand thewear mechanisms of single diamond grains. Detailed electron-beammicroscope analyses enable the definition of temperature andpressure conditions during the dressing process. For macroscopicwear compensation an optical form roller wear measurementsystem was constructed. It can be used flexibly for different formrollers and enables measurement on the grinding machine. Thedressing roller geometry is recorded as contour with transmittedlight. The exact detection of the angle position at the standingdressing roller with an encoder gives new opportunities to measurethe change of effective dressing width. Also the formation of thedresser 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 texturemeasurement systems on machining processes. Some others[6,40,45,58,60,62] specifically looked at the relation between thetopographic characteristics of the wheel surface and its behavioron the grinding operation. In this case study, Oliveira et al. [41]proposed a monitoring system for the wheel sharpness measure-ment based on the characteristics of a light beam reflected from thegrinding wheel surface (Fig. 24) on the abrasive grains wear flatareas over the whole wheel peripheral surface. In the proposedsystems, the light reflected from the top surfaces of the abrasivegrains is converted to an electrical signal by a charge coupleddevice (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 cuttingspeed (30 m/s) and the measurement carried out on the grindingmachine without stopping. The system was applied on a surfacegrinding operation where an Al2O3 grinding wheel was used. Theresults showed that the system could be used to map the wear flatsat the grinding wheel surface and also to analyze the wearphenomena efficiently (Fig. 25). A good correlation between the

Fig. 24. Methodology for mapping a grain wear flat area.

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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average reflected light level and the normal force was also found(Fig. 26).

The vision systems are becoming more versatile and lessexpensive. Researches are being carried out in order to analyze thewheel texture using vision systems [1]. This is a relevant challengefor industrial application where the percentage of wear flats can bea good parameter to identify the need for wheel dressing or toprevent from grinding burn.

6.3.3. Wheel topography characterization via AE

The application of acoustic emission (AE) in grinding monitor-ing has been researched since 1984 [12]. One of the firstconclusions of this research was the high sensitivity of the AE(root mean square) RMS level in the detection of contact betweenthe grinding wheel and the workpiece. This was confirmed by Refs.[24,63,4,34,33]. The CIRP Tool Condition Monitoring keynote byByrne et al. 1995 [7] also confirms that and presents severalfeatures that can be extracted from the AE RMS signal for processmonitoring and diagnosis. The contact detection capabilities of AEinformation were investigated relative to the topographic char-acteristics of both contacting surfaces in 1994 [43,64].

The two main limitations of the AE monitoring solutions are therandom oscillation of its RMS level and signal saturation. HoweverAE can be very effective and fast for the contact detection ofmoving surfaces [42].

In this method the acoustic emission obtained from the contactbetween diamond tool and grinding wheel (or grinding wheel andworkpiece in grinding monitoring) is converted to RMS level andacquired by a computer. The sampling rate is very fast, reachingalmost 1 mega samples per second. An AE signal processing unithas been developed for the system. The data acquisition is made indata arrays corresponding to a full rotation of the grinding wheeland its triggered by a sensor positioned in the spindle [42].

The image is built up representing the level of each acquired

the abrasive grains. Darker areas mean less acoustic enedetected by the sensor. The L shaped mark was created inwheel surface in order to check the system functionality.darker band on the left side was caused by a grinding operausing that area of the wheel.

The system is being used by industry in two different w(Fig. 29):

(a) Dressing evaluation: the interaction between the dresserthe grinding wheel can be monitored. Lack of contact betwdresser and grinding wheel will appear as dark areas inmap.

(b) Grinding evaluation: during a plunge grinding the interacbetween the grinding wheel and the workpiece canevaluated.

This AE tool has already been used in industry as a prototybut needs further development in order to be robust in industapplications. The main challenges are still: AE signal detecwhen using roller or ball bearings in the grinding wheel spinautomatic gain control of AE signal, image recognition systemsprocess diagnosis and feed-back.

6.4. Information and database systems

Nowadays, the design of grinding processes is mostly basedindividual experiences of the process planner. Decreasing basizes and increasing product variety result in an increasfrequency of process designs. Hence, an effective process plannwhich is based on companywide process knowledge, becommore important.

The development of a grinding process planning system cana very complex challenge if it intends to be generic or univer

Fig. 26. Results of the normal grinding force and percentage of flat areas according

to the wear on the grinding wheel peripheral surface [41].

Fig. 27. Image construction procedure for fast RMS analysis [42].

Fig. 28. Output from the acoustic mapping system [42].

sample with a color scale in a three-dimensional graph. During thedressing operation the image is constructed in real time by addingcolumns in the array as the dresser travels along the wheel surface(Fig. 27).

Fig. 28 shows an output from the acoustic mapping systemwhen used during a dressing operation. The vertical and horizontaldirections are the wheel circumference and width respectively.The resolution is of two samples/mm. The depth interactionbetween the diamond and the grinding wheel was 1 mm (in therange of the elastic contact). The color intensity shows the RMSvalue measured from the interaction between the dressing tool and



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ding is very sensitive to the product properties and require-ts, so the development of a process planning system is a hard.elevant work has been presented on grinding modelling andlation. The keynote paper 2006 [5] shows a comprehensive

ysis of grinding modelling and simulation possibilities. Somenced software solutions are already in the market, such as thedSim developed by Malkin and Guo [39] or the software forerless simulation developed by Gallego in 2007 [15]. Theseems can predict the grinding results regarding the cycle time,

form error, burn occurrence and size variation for a givending condition. However for specific grinding problems theess planning is done in industry by trial and error on thehine.he development of database and knowledge managementems for process control is another task depending on databaseprocess information. This is reported by a STC G member in thewing case.

. Integrated process analysis and knowledge management in

ding

rinding process database software has been developed andessfully tested in industry. Furthermore the demand for highlymized and a more flexible grinding process at the same timelts in high challenges concerning its stability. Hereby unstableding processes are characterized by the appearance of

respectively. A new approach of improving the possibilities ofprocess control in grinding is to integrate different methods ofprocess analysis and a knowledge management tool like the onementioned above.

Additionally, the data management in grinding (means how toget the process monitoring data, how to store it efficiently andeffectively and how to use it) is still an issue. Companies couldknow much more about the processes and the problems (and howto solve them) if they handle their data in a more efficient ortransparent way. An effective data management could reduce leadtimes considerably, but a generic solution is difficult since grindingis a complex process.

6.5. Education concepts and human resources

Manufacturing as an industrial discipline is undergoing rapidchanges thanks to the access of resources from worldwide sourcesas well as the supply of manufactured components to usersworldwide. As a result, research and development in manufactur-ing 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 usinginformation technologies. While substantial progress has beenmade in the past two to three decades in terms of skills to replicatewhat is already known, there are major unfilled opportunities interms of conception and realization of ‘‘new processes’’. Beyondthe conception and proof of concept, it will be necessary to developthese new systems into new applications or solutions of industrialuse. Such effort will bring benefit not only for the researchers butalso for all industrial participants in the value chain. We see this asa major need and challenge for research and education in allindustrial processes, which include grinding [56].

Research in grinding technology needs to move beyond isolatedpockets of efforts in science (exploratory efforts to probe thephenomena), engineering (efforts to integrate components, with orwithout explicit knowledge of the underlying science) andmanagement (the skills that foster the what, why, how, whenand where) initiatives. Instead, there is a need for an integratedapproach for all these three disciplines and such integration needsto move beyond limited research groups [57].

Finally, the future will belong to those with unique skills tointegrate systems and solutions. Hence education will be a keyneed for all industrial processes. Grinding technology offers thesepossibilities and hence provides leadership in this direction forfuture education.

Therefore the main industrial challenge for industry these daysseems to be finding and keeping talented engineering staff that cancomprehend the fundamentals and can apply them in practice. Theproblem is especially critical these days because young talentedpeople are attracted elsewhere.

6.6. Cost analysis

New process development in industry always requires detailedcost analysis. Grinding costs per part can vary depending on manydifferent influences, such as parts per dressing, wheel life,production rate, need for maintenance or adjustments, dressingtime and others. It is common in industry to compare grindingcosts by simply comparing the tool costs when the production rate

Fig. 29. Examples of acoustic mapping applications.

esirable grinding results such as white layers, scattering,o cracks in the workpiece surface or the non-compliance ofral geometrical tolerances. Process control aims to ensure

process stability by early identifying desirable grindinglts. In the past, numerous promising approaches have beenloped to improve the possibilities of process control in

ding by means of different sensor signals (e.g. acousticssion, spindle power, forces). But most approaches areessfully applied only for simple geometries. Some enable

control of more complex grinding processes but just by meanslong ‘‘learning period’’ and only for the same grinding process,

ase cite this article in press as: Oliveira JFG, et al. Industrial challi:10.1016/j.cirp.2009.09.006

is similar. This happens due to the high process instability that mayrequire high variable human assistance or correction, factorswhich are not often measured or evaluated. It is a mistake not toconsider the more indirect grinding costs. Machine investment,dressing costs, fluid costs, labor needs and many other parameterscan be more expensive than the direct tool costs. A cost comparisonbetween two grinding tools seems to be very obvious, however itcan be a complex task. For example, when testing CBN wheels, thenumber of parts produced between two dressing operations can belarger than the production batch size and the real wheel life maynot be accessed. The performance in the production of different

enges in grinding. CIRP Annals - Manufacturing Technology (2009),


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part materials in the same machine may also be of difficultevaluation. An analysis where a CBN is compared to conventionalfor 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 allowincreased 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% morecostly, was used. Features included high dynamic stiffness,hydrostatic bearings for high speeds, temperature-controlledbearing oil and high-pressure temperature-controlled coolantdelivery. A high-speed rotary dresser was used, for high-speed CBNgrinding. Touch dressing was used, taking few, very fine, dressingcuts <5 mm to maintain quality and reduce wheel consumption.The contact detection required an acoustic emission sensor.Vitrified CBN and conventional abrasives were compared atconventional speeds. CBN wheels were used at high speeds. CBNhas advantages of being hard wearing, lower grinding tempera-tures and high-speed capability [25].

Costs in precision cylindrical grinding are compared fordifferent abrasives, machines and grinding conditions. The analysisis for repeated batch production. Account is taken of machine cost(M/C) and abrasive (wheel) cost. Cost comparisons were based onextensive trials to assess re-dress life against workpiece qualityrequirements (Figs. 30 and 31).

Experiments show that different workpiece materials requiredifferent strategies to reduce costs. Easy-to-grind AISI 52100(Fig. 30) and difficult to grind Inconel 718 materials (Fig. 31) wereground at conventional speeds and at high speeds (left and right ineach figure, respectively). It is shown that wheel speed affectsproduction rate through acceptable values of re-dress life, removal

rate and dwell time. Advantages were gained using vitrified CBconventional speed and at high speed. For both materials, vitriCBN wheels used at high speed, gave better quality at lowerthan conventional abrasives. Wheel costs became negligiblelabor costs greatly reduced. Re-dress life trials, usually neglecare shown to be essential to reduce costs and maintain quality [

The conclusion is that machine and labor cost are the madvantages of CBN application, since higher production ratespossible and less human assistance is necessary.

7. Selected industrial challenges

The issues listed in this section were selected based onauthors industrial experience in grinding and from the evaluaof all information gathered to prepare this keynote paper.following topics represent the view of the authors on what arebreakthrough advances that will lead grinding to a highly adequprocess for the finishing of components.

7.1. Predictive surface integrity

Since, in nearly all fatigue loading or stress corrosenvironments, failure of a component initiates at or very nthe surface, the nature of the surface that results from mafacturing processes such as machining and grinding has long brecognized as having a significant impact on the prodperformance, longevity and reliability. The combination of stand elevated temperatures generated during grinding can leaan altered material zone (AMZ) near the surface. The AMZ canmechanical (plastic deformation, hardness alternation, craresidual stresses, etc.), metallurgical (phase transformation, gsize and distribution, precipitate size and distribution, recrystzation, un-tempered martensite or over-tempered martenwhite etch layer, etc.), chemical (inter-granular attacks, embitment, corrosion, etc.), thermal (heat affected zone, recast laetc.), and electrical (conductivity, magnetic) in nature. Surintegrity is a term that is broadly used to describe technconditions of machined surfaces.

In critical applications such as components for aerospindustry, grinding processes are required to produce surfacesatisfy the very stringent surface integrity requirements to satthe increasing demands of component performance and reliabiThese components made of nickel and titanium alloys are requto have compressive residual stress, no deformation layer, andwhite layer. To guarantee the required surface integrity, prodtion-grinding processes were developed and certified usexperimental trial-and-error approach and extensive surevaluations. These production-grinding processes are often vconservative in process parameters and the removal rates are lFurther more, post-processes such as shot peening and polishwith abrasive slurry are often used to treat the ground surfaccreate compressive residual stress or remove other posssurface defects, which add additional cost to the products.

In principle, it is understood that the extent of these defdepends on the interaction of the mechanical and thermal eneproduced and the workpiece’s material properties duringgrinding process. We still do not have a thorough understandingthe amount of mechanical and thermal energy acting onworkpiece. We have even less understanding about the sensiti

Fig. 30. Costs per part when grinding AISI 52100. From left to right, the values of re-

dress life used were, 35, 75, 25, 75 and 330 parts/dress respectively.

ies,tingted.ize

and

ls issin-

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.


of the material to the energy induced. It is critical for industrlike aerospace, to understand the effects of changing operaparameters before new grinding strategies can be implemenIndustry needs predictive models for surface integrity to optimexisting grinding processes, develop new grinding processes,adopt new grinding technologies.

7.2. Predictive wheel life for grinding with plated CBN wheels

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ded or vitrified CBN wheels where the grinding performance isored by periodic dressing [31], single-layer plated CBN wheelsot be dressed. Therefore, the grinding performance of plated

wheels varies significantly as the wheel wears down54,55,8,9,20] which imposes great challenges in cost-effec-ly utilizing the wheel and process control. The workpieceerial is found to be one of the dominant factors affecting theel wear. For example, it was found that the grindingormance of plated CBN wheels deteriorates much faster whending a 100Cr6 steel than a GG40 cast iron [8] and nickel alloy00 than an AISI 52100 bearing steel [54]. An example of theer and roughness variation with accumulated materialoval is shown in Fig. 32. When wheel is worn, surface integrityes such as white layer may be produced on the machinedace [20].or cost-effective application of plated CBN wheels andding process control, it is necessary to know when to replacewheel. First, we need to establish criteria parameters that cansed to judge wheel life in production. Secondly, we need tolop practical in-process measuring or tracking tools for wheelFor large volume production of relative simple surfaces,

ding power or number of parts ground may be used. For lowme production of geometrically complex parts, it becomesh more challenging to use grinding power or number of partsmeasure of wheel life. Wheel wear flat area should be a goodsure of the wheel life. However, it is not easy to measure andk wear flat area even under well-controlled laboratoryitions, as show in Section 6.3.

Grinding to achieve microstructure and metallurgical properties

urrently, grinding is mainly used to produce smooth surfacesprecise tolerances without causing metallurgical changes toground surface. The surface hardness, micro-structural andallurgical requirements are achieved by heat treatment orr post grinding processes such as shot peening. Great cost

ngs can be achieved if the grinding processes can be tailored touce surfaces with both the required geometrical features suchoughness and tolerance, the mechanical property such asness, and the correct metallurgical requirements such as no

te layer. To achieve this goal, the grinding process needs to putppropriate amount of thermal and mechanical energy into the

kpiece by the grinding action. It also needs to dissipate the

emission and grinding power. For production use, the most reliablemeans so far seems to be monitoring the grinding power. Theacoustic emission is a good tool for grinding gap elimination andwheel 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 effectivelycontrol the grinding process, which requires a threshold for thepower. For simple surface grinding or cylindrical ID/OD grinding,the theoretical threshold power to avoid burn can be readilyobtained using the thermal models. However, the establishment ofthe threshold power becomes much more challenging for grindingcomplex parts such as serrations of turbine blades [18]. Whencontinuous dress creep-feed grinding is used, the situationbecomes more complicated and the power threshold should bechanging with the wheel diameter caused by continuous dressingas shown in Fig. 33 [17].

The most challenging situation arises when grinding parts suchas airfoils under multi-axis machine motion [19]. The contactbetween the grinding wheel and the workpiece changes along thegrinding path. This requires sophisticated grinding models toestablish the threshold for process monitoring. It is obvious thatthe threshold will not be a constant rather a variable.

7.5. Grinding process energy efficiency improvement

Up to now, energy consumption by machining and grindingprocesses has not been a concern for industry because the energycost is much lower than the other costs such as material, labor andtooling. Grinding process improvements mostly focus on cycletime reduction, wheel savings, and quality improvements. Thesituation appears to be changing due to recent increase in energydemand world wide, the significant energy price fluctuations, andthe concern over global warming.

The energy intensity or efficiency of all material removalprocesses such as machining, grinding, and electrical dischargemachining (EDM) can be measured using the concept of specificenergy defined as the energy required for remove unit volume ofmaterial. Grinding is one of the most energy-intensive among allmachining processes. The grinding specific energy is typicallyhigher than the energy required for melting the material. Forexample, 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

Fig. 33. Power variations due to variation of wheel diameter and workpiece

property [17].Fig. 32. Grinding performance variation with the use of wheel [20].

gy at the appropriate rate by cooling and other means. Thisalso require laser-assisted grinding or a controlled cooling

em. It may also call for cryogenic cooling technology. Clearly,grinding process is required to be more precisely controlled.

Robust grinding process monitoring and control

ffectively monitoring the production-grinding process is still at challenge especially when grinding of components withplex geometries and plated CBN wheels are used. A number ofitoring technologies have been studied such as acoustic


abrasive wheels. Considering the large amount of grindingoperations used by industry worldwide, the impact can besignificant if we can improve the energy efficiency of the grindingprocess. Furthermore, the high energy intensity of the grindingprocess is also the root causes of the workpiece surface andsubsurface damages caused by the grinding operation such asburn, white layer, and residual stresses.

It is understood that the grinding energy is used for forminggrinding chips, plowing some material on the ground surface, andovercoming frictions between the grinding wheel and the work-piece [39]. Depending on the grinding conditions, the relative



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percentage of the three components can vary significantly. Thehighest percentage of the energy seems to be used for overcomingthe friction, especially for creep-feed grinding and grinding at lowremoval rate. It is logical to think that research in new grindingwheel technology, advanced coolants, and coolant deliver tech-nology will make the biggest impact in reducing friction at thegrinding zone. It is known that oil-based coolant provides betterlubrication than water soluble coolant. However, grinding processalso requires coolant to provide effective cooling at the grindingzone, which the water soluble coolant is much better. Thechallenge is to develop a technology that can provide effectivecooling and lubrication at the grinding zone which is typicallyunder high pressure and temperature.

The chip formation energy alone is close to the melting energyof the material for grinding steels with conventional aluminum-oxide wheels as shown in Fig. 34.

This is the amount of energy to adiabatically shear the materialto melting. In order to reduce chip formation energy, grindingtechnology is needed to remove material without inducing thelarge amount of deformation or strain. The main reason of largestrain by grinding is due to the dull cutting edges of the abrasivegrits and their inability to align the cutting edges properly in thegrinding wheel. Grinding wheels similar to a milling cutter withlarge number of micro inserts should provide significant advantageover the traditional grinding wheels in terms of energy consump-tion. Grinding with CBN wheels typically consumes less energythan with conventional aluminum-oxide wheels but the specificenergy level is still much higher than the melting energy.

7.6. Selection of cost-effective grinding technology

In academic research of advanced grinding technology, superiorgrinding performances with super-abrasive wheels over conven-tional grinding wheels have been reported many times for the past

reconditioning of grinding wheel. These reports from acadegrinding research excited engineers in industry. This is becauseEP-CBN grinding technology gives industry an opportunityimmediately improve grinding performances without any retrof existing machines, as most of manufacturing companies alreown many old grinding machines without precision dressing un

In spite of these benefits of super-abrasive grinding technolreported in academic grinding research, however, the majoritgrinding technology applied in industry is still conventiogrinding with aluminum-oxide (Al2O3) wheels, as shown in Sec3. The obstruction of implementation of CBN grinding technolin industry comes from the cost performance of grinding procesSo far, grinding academy has not provided the full technassessment of the CBN grinding technologies in terms ofpractical productivity giving a big impact on manufacturing cof products. Without good understanding in the impact of Cgrinding processes on the cost performance, industry cannot ma fair decision whether or not the advanced CBN grinding shouldemployed in their production systems.

In order to understand the impact of CBN grinding technoloon the grinding costs, an example is following presented whinternal grinding tests were performed with various bore sizering workpieces (hardness: HRC 60) made of case carburized stThree kinds of grinding wheels were tested. These are Ni pla200/220 grit CBN wheels (EP-CBN), vitrified bonded 200/220CBN wheels (Vit-CBN) and 80 grit aluminum-oxide (Al2O3) wheThe wheel life and dress interval were determined fromgrinding power monitoring. The G ratios and the amoundressing stocks were measured, and the proper cycle times for ewheel were determined by the geometrical grinding accurchecks and metallurgical checks. These grinding test results wused for the cost analysis with various bore sizes of workpieceseach wheel. The cost analysis includes wheel cost, wheel chacost, dressing cost including time and diamond consumptionthe labor cost in the US. The grinding costs with one yoperations were analyzed. It is assumed that the widthworkpieces is 90%, and the new wheel diameter is 70% ofwork bore diameter. It is also assumed that the worn whdiameter replaced is 80% of new wheel diameter.

Fig. 35 shows the cost index normalizing the grinding coststhree wheel types in internal grinding operations of workpiewith bore size ranging from 1 mm to 100 mm in diameter.

In case of EP-CBN grinding, the grinding cost is exponentiincreased with increased bore diameter. The lowest grinding codemonstrated when the bore size is less than 7 mm over Vit-Cand Al2O3 wheels (Fig. 35). This advantage comes from lonwheel life with no dressing requirement in the EP-CBN grindingcase of Vit-CBN grinding, the lowest cost is obtained when the bsize is between 7 mm and 75 mm. The cost is rapidly increasethe bore size of less than 7 mm and greater than 75 mm.conventional grinding with Al2O3 wheel gives the lowest grindcost in case of the bore size of greater than 75 mm. The anal

Fig. 34. Specific cutting energy vs. specific volumetric removal rate [39].

Fig. 35. Cost comparisons in internal grinding.

three decades. For instance, CBN grinding technology demon-strates very long-term stable grinding performance with high Gratio and long dressing interval compared with conventionalgrinding technology. So, CBN grinding can be expected tosignificantly improve the productivity of grinding operationsand drastically reduce the grinding cost in industry. In addition tothese advantages, the superior surface integrity with compressiveresidual stresses can be generated with CBN grinding technology.With electroplated CBN (EP-CBN) wheels, very high stock removalgrinding can be achieved with no dressing requirement and thegrinding performance lasts for many days and months without any



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rly reveals why CBN grinding technology has not been appliedse of large bore grinding in industry and why EP-CBN grindingnology has been employed in the grinding of small boreponents. It is also shown that the applications of Vit-CBNding has been widely employed in internal grinding forhing small-medium size components, such as 7–75 mm boreworkpieces in this study. By this example it is possible to learn

the limitation of cost-effective CBN applications can bended if the grinding performance, such as shorter cycle time,er G ratio, longer wheel life, higher stock removal ratebility, is improved. Industry has been looking for opportu-s for further improving grinding cost performances withessive advanced grinding technology.

Key grinding technology for ultra large components

ind energy as a power resource is favorable as an alternativeossil fuels, as it is renewable, clean and produces lowernhouse gas emissions. Wind energy business has beening rapidly and the push comes from a renewed concern

he environment and an economical power resource in manys of the world. New technologies have decreased the cost ofucing electricity from wind, and the business growth in winder has been encouraged by tax breaks for renewable energygreen pricing programs.he windmills for power generation of over 2 MW require ultrae components greater than 1m in diameter as shown in Fig. 36.weight of these components sometimes reaches over 5 tons.grinding technology for critical components of windmills suchars [29] and bearings plays very important roles for improving

efficiency of power generation from wind and for ensuring thebility of components under high stress working conditions.37 shows internal grinding operations for ultra large bearingponents.here are many technical challenges for grinding these ultrae components due to the heavy weight and the great amount ofding stocks. As a typical example, Fig. 38 shows the relation-

between the work diameter and the stocks removed inding for case carburized rings.he grinding stock is rapidly increased with increased ringeter. For instance, the grinding stock removed on rings ofdiameter reaches over 4 mm [50]. The great amount of stock

oval requirement mainly comes from the distortion in heattment processes. In order to reduce the grinding time and the, the low distortion heat treatment process will be the key.ig. 39 shows the total grinding time under the condition of theific stock removal rate Q0w of 1, 3 and 10 mm3/mm s in OD andrinding of rings with various sizes (size: OD = D, ID = 0.75D,

Width = 0.2D). In OD and ID grinding of 2m ring with Q0w = 1 mm3/mm s, the grinding takes about 5 h. The time does not include theother process time, such as the work loading/unloading time,centering time, final finishing time and gauging time. So, the cycletime 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 increasedstock removal rate Q0w = 3 mm3/mm s which is typical for smallcomponents. The grinding time is further reduced to 0.5 h withQ0w = 10 mm3/mm s. In this case, the total cycle time of a few hourswill be possible. For the grinding of ultra large components, thedevelopment of high stock removal grinding process is indis-pensable. It is required for grinding academy to provide thesolutions for reducing grinding cycle time of ultra large compo-nents by developing new grinding technologies, such as newgrinding wheel with high stock removal capability, on-machinegauging as the workpieces are extraordinarily heavy, anddeveloping multi-surface grinding processes in one chucking onhighly flexible grinding machine.

8. Summary

This paper explored different sources of information onindustrial challenges in grinding. From the gathered information,the main conclusions that can be drawn, which would represent asummary of relevant grinding hot research topics for industrial

Fig. 39. Ring diameter vs. grinding time.

Fig. 36. Ultra large components for windmill.

Fig. 38. Ring diameter vs. grinding stock.

Fig. 37. Internal grinding for ultra large components.


applications, are:

� Use of EP wheels: there is an important opportunity for theimplementation of electroplated wheels in situation wheregrinding competes with hard turning or even in more precisegrinding applications. In any case, the topographic control bydressing 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 isan important issue. Future research should be conducted in orderto find a structured way to control and monitor the most relevant



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variables, including wheel topography and work material, toachieve 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 opera-tions. The main challenges are related to the better under-standing of cost and process reliability. Further analysis as theone showed in the Section 7.6 needs to be done for otherprocesses. A broader guidance on the advantages of CBN useneeds to be cleared up for industry. Cheaper vitrified CBN wheelsmay 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 isnecessary to research how to better control and even to designthe thermal cycle in grinding by using precisely controlledcooling or additional heating systems.� Sustainable processes: the development of a more sustainable

grinding, from the industry prospective, seems presently to bemore related to make it adequate to the production ofsustainable components than to focus on more environmentallyefficient process. However, decreasing grinding energy andimproving the sustainability of cooling systems is clearly atendency.� 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, lashadjusters, fuel injectors, tripod spiders or camshafts requireresearch for the development of dedicated solutions capable ofdealing with their very particular issues: part geometry, typicaltolerances, typical materials, and possible fixtures.� Process combinations: many research opportunities are related to

process combinations such as grinding and turning in the sameequipment. This should require machine tools with specialfeatures regarding: chip removal, cooling systems, stiffness andcontrols.� Large components: new energy systems will require large

components that surely need specific grinding solutions.Grinding of large components is a challenge regarding therealization of tests and research costs, which could only beachieved in cooperation with industry.� Lightweight parts: in the automotive segment there will be an

increasing demand for the grinding of lightweight parts made ofDTG materials. This gives additional challenges due to the lowerpart stiffness and lower blade friction forces in the case ofcenterless grinding application.� CBN belt grinding: one of the case studies showed that develop-

ments in CBN belts can bring new applications of belt grindingcombining good stock removal with higher quality surfaces.� Grinding database: the research on grinding models combined

with actual factory parameters database can help industry in thedesign of new grinding operations. The comparison between themodel results and factory parameter can also help in thedetermination of anomalies in the actual grinding operations in aplant.� Cost analysis: cost analysis of any new grinding solution can lead

research to be better accepted by industry reducing the gapbetween academia and application.� Dressing decision: the development of systems and solutions that

allow the machine to decide for the start of a dressing cycle canimprove the process performance. Today’s method is based on a

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7.gine:ogy–

ated

chin-

ding–60.er toe and

counter of number of parts. This is not precise since the grindingstock per part can be very variable.

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