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Journal of Cleaner Production 18 (2010) 1840e1849

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Journal of Cleaner Production

journal homepage: www.elsevier .com/locate/ jc lepro

Machining evaluation of a hybrid MQL-CO2 grinding technology

J.A. Sanchez a, I. Pombo b,*, R. Alberdi c, B. Izquierdo b, N. Ortega a, S. Plaza a, J. Martinez-Toledano c

a Faculty of Engineering of Bilbao, University of the Basque Country, Alameda de Urkijo s/n, 48013 Bilbao, Spainb Faculty of Technical Engineering of Eibar, University of the Basque Country, Avda Otaola 26, 20600 Eibar, Spainc Ideko Technological Center, Arriaga Kalea 2, 20870 Elgoibar, Spain

a r t i c l e i n f o

Article history:Received 28 January 2010Received in revised form23 June 2010Accepted 4 July 2010Available online 13 July 2010

Keywords:GrindingMinimal quantity lubricationEnvironmentally friendly processes

* Corresponding author. Tel.: þ34 943 030 30 37/6E-mail address: inigo_pombo@ehu.es (I. Pombo).

0959-6526/$ e see front matter � 2010 Elsevier Ltd.doi:10.1016/j.jclepro.2010.07.002

a b s t r a c t

Although there are already successful industrial experiences in other machining industries with theelimination of coolants, this is not so in the grinding industry due to the large amounts of generated heatthat must be evacuated from the contact zone. In this work, a new approach to the elimination of fluidsin grinding is presented. The technology is based on the use of a hybrid Minimum Quantity of Lubricant(MQL)-low temperature CO2 system that reduces lubrication consumption. Abrasive grits are protectedby the layer of frozen oil, resulting in a significant improvement in grinding wheel life and surface qualityof the machined component. Although the cooling action is reduced with respect to the conventionalcoolant, no thermal damage was observed on the workpiece.

� 2010 Elsevier Ltd. All rights reserved.

1. Introduction

New approaches to traditional problems in manufacturing tomeet demands of consumers must contribute to a more sustainableproduction at global level. The new paradigm of sustainableproduction must include social, environmental and economic effi-ciency criteria (Pusavec et al., 2010a,b). It is scientifically acceptedthat production must contribute to sustainable development sinceit is the main enabler thereof (Jovane et al., 2008; Pusavec et al.,2010a,b). In this context, the increasing pressure imposed by lawsand regulations likewise efforts at political levels to support thedevelopment of sustainable technologies within the EU, and also ona global scale, it has become a must-consider input for bothmachine manufacturers and users (Jegatheesan et al., 2009).

High added value sectors such as aeronautics, energy genera-tion, high-speed railways, etc. are customers of high-performancemachining processes. Customers from these sectors demand themanufacturing of parts in hard materials with very tight tolerancesand superior surface finish. Demands come not only from precision.Fatigue life of the component is also a critical criterion in manycases. In this context, the grinding process is an excellent alterna-tive to processes such as turning or milling. Grinding can producevery precise products with enhanced fatigue behaviour in difficult-to-machine materials.

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All rights reserved.

Coolants and lubricants are usually employed in machiningoperations, including turning, milling, drilling and of course,grinding. However, the increased concern for environment andsustainability are pushing industry towards a new paradigmwhereeco-efficiency appears as the new keyword: special attention is notonly focused on energy consumption and waste disposal (see ISO14000), but also the costs generated by non-sustainable industrialpractices must be questioned. No doubt, coolants and lubricantsimprove machining performance in many cases, however, a newpoint of viewarises when considering the costs and risks associatedwith their use. In Fig. 1 the distribution of costs related to the use ofcooling fluids in machining in the automotive industry is shown. Inthis example, it can be noted coolant costs are significatively higherthan tooling costs.

Technologies for reducing the consumption of coolants andlubricants are available in conventional machining processes,involving the use of Minimum Quantity of Lubricant (MQL) or evendry machining. A large number of research works, together withindustrial practice can be found in the fields of drilling, turning, oreven different milling applications (Fratila, 2009). However, this isnot the case of the grinding processes. In grinding, very hightemperatures are reached. To avoid thermal damage, it is essentialthat large amounts of the heat generated be evacuated, otherwisethere may be thermal damage of the workpiece, acceleratedgrinding wheel wear and thermally induced deformations, result-ing in poor quality of the ground components.

By way of example to illustrate the current situation of grindingoperations; in the grinding of turbine blades for the aerospace

Nomenclature

ae depth of cut, mmvw workpiece speed, m/minvs grinding wheel tangential speed, m/svp plunge speed, mm/minbs grinding wheel width, mmlg geometric contact length, mmLc real contact length, mmFt tangential force, NFn normal force, NFt0 specific tangential force, N/mmFn0 specific normal force, N/mmm force ratioVs

0 specific volume of grinding wheel wear, mm3/mmVw

0 specific volume of material removed, mm3/mmG grinding ratioQw material removal rate, mm3/sSME small and medium enterprise Fig. 2. Danobat FG-600-S grinding machine together with the grinding fluid filtering

system.

J.A. Sanchez et al. / Journal of Cleaner Production 18 (2010) 1840e1849 1841

industry a coolant flow of 750 l/min is required to grind witha wheel of 200 mm width. The cooling fluid must be clean and ingood conditions, therefore filtering systems are compulsorynowadays. Those systems are expensive and may occupy consid-erable workshop space, as much as 120% of the machine space.Fig. 2 shows a Danobat FG-600-S together with the grinding fluidfiltering system.

In this paper the existing alternatives to the use of conventionalflooding systems are presented, together with their main advan-tages and limitations. Then, a proposal for a new hybrid MQL-CO2grinding technology is described, and applied to surface grinding.Finally, the machining performance of the new technology isevaluated.

2. Sustainable grinding technologies

From the previous section it is clear that the use of MQL or drycutting grinding techniques is not as extended as in the case ofother metal removal processes. This fact explains that industrialand academic research efforts are currently being directed towardsthe development of grinding techniques enabling elimination, or atleast, minimization of grinding fluid use while maintaining processefficiency. Grinding machine manufacturers are specially con-cerned with this topic, since the entire process cost (grinding wheel

Fig. 1. Distribution of cooling/lubricant costs in machining operations in the automoti

wear, waste disposal, etc.), including the cost and space occupied bythe machine can be drastically reduced. In the following para-graphs, interesting research experiences in the field of grindingfluid minimization are presented.

Dry grinding still presents important limitations to be applied inindustrial practice. Total elimination of the cutting fluid results inhigher temperatures during the process, affecting surface integrityand geometrical precision of the ground part in addition toincreasing grinding wheel wear and clogging (Klocke andEisenblätter, 1997; Malkin and Guo, 2008; Marinescu et al., 2007;Ebbrell et al., 2000). However, in recent years special attentionhas been paid to this technology because of its potential benefitsand the possibilities for direct measurement of process variables itoffers. Thus, Tawakoli and Westkaemper (2007) researched thecapabilities of dry grinding and showed that under special condi-tions heat generation can be largely reduced. They also analysed thepossibility of aiding soft steel grinding with ultrasonic vibrations(Tawakoli and Azarhoushang, 2008). Klocke and Bücker (1996)presented results on external cylindrical dry grinding of 100Cr6V.Brinksmeier et al. (1997) reported research work on surfacegrinding of 16MnCr5 and 42CrMo4. More recently, Aurich et al.(2008) presented a novel approach to the problem based on thedevelopment of a superabrasive electroplated grinding wheel witha defined grain pattern for dry surface grinding. Since complete

ve industry (data from a Danobat-Ideko internal report, updated on March 2010).

J.A. Sanchez et al. / Journal of Cleaner Production 18 (2010) 1840e18491842

elimination of grinding fluids is still limited by important problems,an eye must be kept on the development of alternative coolantswith minimum environmental impact, like those reported byHerrmann et al. (2007).

A promising alternative to dry grinding can be found in MQLtechniques and cryogenic cooling. In the case of MQL grinding,droplets of an air and oil mix are supplied to the working area(Klocke et al., 2000). Oil consumption and therefore lubricant-associated costs are drastically reduced. Literature works reportthat under certain conditions oil droplets reach the abrasive grainsimproving lubrification in the contact zone resulting in a prolon-gation of grinding wheel life and a better surface quality of theworkpiece. da Silva et al. (2007) focused their attention on thesurface integrity of surface ground ABNT 4340 steel parts. Tawakoliet al. (2009) studied the optimal application of the MQL systemtaking into account the workpiece material hardness. Reductions ingrinding power and improvements in wheel wear in MQL-internalcylindrical grinding have also been reported by Hafenbraedl andMalkin (2001).

Together with the reduction of lubricant, research experienceshave opened the way to the application of cryogenic conditions tothe grinding process. Paul and Chattopadhyay (1996) reported thatcryogenic grinding using a jet of liquid nitrogen could reduce heattransfer into theworkpiece specially at high infeed and with ductilepart materials. Results by Ben Fredj et al. (2006) seem to confirmthis fact, since improvements in surface integrity in terms of bettersurface roughness, higher level of work hardening, lower level oftensile residual stresses and better resistance to corrosion werenoticed in the cryogenic grinding of AISI 304 austenitic stainlesssteel. However, a recent work by Nguyen et al. (2007) on the use ofliquid nitrogen for grind-hardening observes that the penetrationof cold gas into the contact zone is very limited due to a very highevaporation rate, which is also increased by the turbulent air flowproduced bywheel rotational speed. As a consequence, the effect ofheat dissipation is only present in the proximity of the contactzone. Although heat dissipation due to cryogenic temperaturesmaynot be as relevant, cryo-temperatures may favour material removalby shearing and limit the ground surface damage specially inductile materials.

A real industrial application can somehow be useful to evaluatethe economic impact of using conventional grinding fluids on theoverall costs of the grinding operation. Engine valve finishing isa very common application of grinding for the automotive sector. Ofcourse, one can find other industrial case-studies with higherimpact (for instance, the grinding of turbine blades), but also withlower impact (cylindrical grinding of inner parts). Therefore, theselected process can be taken as representative of an averagesituation affecting SMEs facing strong cost and quality competition.

The example refers to valve stem grinding. The preliminary testswere performed on a grindingmachine ESTARTA 327MDA that usesa filtering equipment MONET with a capacity of 50 l/min and anaverage filtration mesh of 0.02 mm. In this example, the expectedreduction in the space occupied by the filtration system is as high as30%. Grinding was carried out under the conditions listed below:

- Valve Material: X53CrMnNi219- Grinding wheel: NORTON SG-120 PVX- Regulating Wheel: MANHATAN 80 ARL- Plunge speed (vp): 25 mm/min- Radius to be machined: 0.5 mm- Coolant type: emulsion at 5%- Coolant flow: 50 l/min

A total of 20 workpieces were machined in the study. The powerconsumed in the grinding process was 6.6 kW for the first

workpiece, increasing to 8.6 kW for the last one due to wheel wear.The power consumed by the cooling pump and cooling equipmentwas 3.3 kW per workpiece. Therefore, in this case as much as38.37% of energy consumption can be attributed to the use ofconventional grinding fluids, with the corresponding impact of theoperation on the total cost and environment.

In view of the above comments, it can be concluded thatpotential economic and environmental benefits could be obtainedfrom the reduction/elimination of conventional fluids in grindingoperations. In order to do so, a new approach to the problem ofrefrigeration and lubrication in the grinding process is presented inthis paper. The objective is to significantly reduce the consumption/flow of oil-based cooling lubricants while maintaining processperformance in surface grinding. As a result, coolant-related costsand space required for the filtering system can be drasticallyreduced. A combined MQL-low temperature CO2 system isproposed as an efficient alternative to traditional cooling/lubrifi-cation systems in grinding. Recent work by Pusavec et al. (2010a,b)suggests that since the cryogenic nitrogen costs are dramaticallyhigher per same amount than conventional emulsion, the use ofthis alternative may lead to much higher costs in comparison toconventional emulsion usage. Even so, factors such as space occu-pied by the filtering system, cost of the filtering system itself,energy consumption per part (see the example above) and grindingwheel consumption must also be considered. This means that withthe new process (which is still under extensive research) a windowof applications in which technical and economical advantages maybe gained is possible. Current research work of our group has setthe objective of achieving 25% reduction in cost associated with thegrinding process of each workpiece, due to respective reductions inenergetic cost, lubricant and coolant cost, cutting fluid recyclingcost, required floor space and grinding wheel consumption.

The proposal of the new system is original in that the coolingaction is not directed towards the contact zone, but to the abrasivegrains themselves. The objective is to protect abrasive grains fromwear, rather than cooling the contact zone. At the same time, toreduce the risk of too a high heat flow into the workpiece due toefficient refrigeration, heat production within the contact zone isminimised by using theMQL oil system. This is due to a reduction offriction. Literature by Malkin and Guo (2008) and other authorsshows that lower temperatures are related to compressive residualstresses on the workpiece surface/sub-surface. The system andexperimental procedure are described in Section 3. Results ofcomparison of performance for a surface grinding applicationbetween a conventional flood cooling with soluble oil system andthe new system in terms of grinding wheel wear, surface finish,grinding forces and force ratio are presented and discussed inSection 4. The cooling action of the new system is expected to bepoor. Therefore, the possibility of thermal damage on the groundpart is discussed in Section 5. Finally conclusions are drawn, con-firming the excellent performance of the new approach.

3. Experimental procedure

The combined MQL-low temperature CO2 system was installedin a Danobat surface grinding machine, together with theconventional flooding system to carry out the comparison tests.The objective of theMQL-CO2 system is to fix the oil droplets on thesurface of the abrasive grains. In other words, to create a durabletribofilm around the grits that improves sliding and lubrificationunder extreme pressure conditions. The oil used for such purpose isBiocut 3000, with a biodegradability index over 90% in the test ofZahn-Wellens andmelting temperature�253 K. As shown in Fig. 3,first the MQL is applied directly onto the grinding wheel surface,

Fig. 3. Schematic of the proposed system.

J.A. Sanchez et al. / Journal of Cleaner Production 18 (2010) 1840e1849 1843

and immediately afterwards, a CO2 jet at 238 K is responsible forfreezing the oil on the abrasive grains.

The MQL system is a LubriLean Basic with oil consumption of0.06 l/min, using two coaxial nozzles that spray the oil in aerosolform with droplets between 15 and 35 mm diameter. At the sametime, the CO2 jet is controlled by a Hoke 2315 F4Y regulating valve,and applied to the grinding wheel surface using a 2 mm diametercircular cross section nozzle. A photograph of the surface of thewheel at 200� (see Fig. 4) shows that the layer of frozen oil hasbeen effectively achieved.

During the set-up tests it was observed that the frozen oillayer thickness is a parameter that must be controlled, since toothick a layer is responsible for the occurrence of a suddenincrease in the normal force at the entrance of the wheel in theworkpiece. Grinding forces were measured under industrialconditions using a Kistler 9257B force measuring device (piezo-electric technology). Fig. 5 (left) shows a peak followed bya reduction in the normal grinding force. Grinding parameters forthis test are listed in Table 1. Together with this reduction, anincrease in the tangential component is observed. This effect is

Fig. 4. Grinding wheel surface at 200�. Left: abrasive grains before application of th

attributed to a minimization of the abrasive action at thebeginning of the operation due to the excessive presence offrozen oil in the contact. Sliding between grinding wheel andworkpiece occurs until the thickness of the frozen oil layer is lowenough to permit the abrasive action of the grits (moment whenthe normal component decreases and the tangential increases)but at the same time, oil is still present to reduce friction in theinteraction grit-part. This latter statement is confirmed by theresults presented in Section 4.

To control layer thickness, additional tests were carried outvarying the flow of CO2 on the grinding wheel surface fromamaximumvalue of 430 l/min down to aminimumof 40 l/min. Theminimum value is the one which ensures that the frozen layer iseffectively built on the wheel surface. Through testing it was foundthat stable operation without increase of the normal component atthe entrance was obtained for the minimum value of CO2 flow (thisis, 40 l/min), as shown in Fig. 5 (up-right).

Together with the new MQL-CO2 system, a conventional flood-ing system using a water-based emulsion (Rhenus TY100 ata concentration of 5% in water) was installed, so comparison

e MQL-CO2 system. Right: the layer of frozen oil on the grits can be observed.

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Fig. 5. Normal and tangential grinding force components. Up-left: too thick a layer of frozen oil is present on the grinding wheel surface (CO2 flow 430 l/min). Up-right: record afteroptimization of the layer thickness (CO2 flow 40 l/min). Down: the process is carried out with coolant.

J.A. Sanchez et al. / Journal of Cleaner Production 18 (2010) 1840e18491844

between the new proposal and the conventional system can bedirectly carried out. The fluid is delivered by a high pressure pump(20 bar) at 40 l/min, and uses a filtering deposit of 300 l of capacity.

4. Analysis of the process performance

Surface grinding experiments in an industrial environmentwere carried out, aiming at performance comparison of the newsystem with the traditional water-based emulsion in terms ofgrinding wheel wear, grinding forces, surface roughness andtemperatures in the workpiece. The workpiece material in theindustrial tests was AISI D2 tool steel which is commonly used forthemanufacture ofmetal forming tools. An alumina grindingwheelof 400 mm diameter with specification CBL46I6V489 was used inall the grinding experiments. Grinding wheel speed was 35 m/s. Inthese experiments the depth of cut was varied from 0.015 mm to0.03 mm and the workpiece speed was varied from 1 m/min to30 m/min.

Grinding wheel life is commonly evaluated in terms of the so-called grinding ratio G, which is the ratio between the volume ofpart material removed (ground) and the volume of grinding wheellost (worn). Radial wear of the wheel complements the grindingratio to evaluate wheel wear. Tests were carried out with differentgrinding condition severities, i.e., varying the part material removalrate (Qw, measured in mm3/s) while keeping the geometric contact

Table 1Parameters of the test which results are shown in Fig. 5.

ae [mm] 0.02vw [m/min] 15vs [m/s] 35bs [mm] 10CO2 flow [l/min] 40e430

length (lg, measured in mm) constant. For a given grinding wheelspeed, a higher contact length involves a longer duration of thecontact griteworkpiece. In other words, if contact length is keptconstant, variations in grinding wheel wear can only be attributedto different friction conditions between wheel and workpiece.

Fig. 6 shows the evolution of radial wear as a function of volumeof part material ground for different values of the material removalrate. A first look at the results reveals an impressive increase ingrinding wheel life in the case of the new ecological system withrespect to the conventional flooding alternative. Not only is radialwear lower in every single case for the new system, but also, it iseven lower in the case of theMQL-CO2 systemwhen using themostaggressive grinding parameters (Qw¼ 100 mm3/s) than in the caseof the conventional system using the less efficient (from a produc-tive point of view) grinding variables (Qw¼ 6.7 mm3/s).

Results in terms of grinding ratio are also favorable to the newsystem. As expected, G decreases with an increase in the severity ofthe operation (when the volume of part material removed persecond increases, more intense grinding wheel wear is expected).However, it can be noticed that the grinding ratio in the case of theMQL-CO2 system with the most aggressive conditions (G¼ 4.5) isstill higher than that of the conventional flooding system (G¼ 4.22)with the less aggressive conditions. Therefore, all the results ofgrinding wheel wear confirm the hypothesis that the abrasive gritsare effectively protected by the layer of frozen oil.

The surface finish of the parts ground using both systems wasalso researched. It is well known that when using oil instead ofwater emulsions in conventional flooding systems the surfacequality of the parts is clearly better since friction conditionsbetween grinding wheel and workpiece are improved. In the caseof the MQL-CO2 system, it has been shown that frozen oil protectsthe integrity of the abrasive grits, and therefore it may be expectedthat oil is also present in the interface between grit and workpiecematerial.

Fig. 6. Grinding wheel wear vs total volume of part material removed with theconventional and new ecological systems. Up: Qw¼ 6.7 mm3/s (ae¼ 0.04 mm;vw¼ 1 m/min; vs¼ 35 m/s); center: Qw¼ 50 mm3/s (ae¼ 0.015 mm; vw¼ 20 m/min;vs¼ 35 m/s); down: Qw¼ 100 mm3/s (ae¼ 0.02 mm; vw¼ 30 m/min; vs¼ 35 m/s). Datarelated to grinding ratio G in all the tests have also been included.

J.A. Sanchez et al. / Journal of Cleaner Production 18 (2010) 1840e1849 1845

Surface finish was measured using a Mitutoyo SJ-301 roughnessmeasuring instrument, and both Ra and Rz parameters were quan-tified (see Fig. 7). Results confirm that surface qualitywhenusing thenew MQL-CO2 system is slightly better than that obtained whenusing the conventional emulsion. The tests were carried out witha similar depth of cut (0.015e0.02 mm) but varying the workpiecespeed from 20 m/min (left) to 30 m/min (right). With this approachthe contact length is kept constant in both cases, although thematerial removal rate is increased from 50 mm3/s up to 100 mm3/s.

The general trend of an increase in surface roughness whenincreasing workpiece speed can be noticed. In terms of average

roughness Ra, there does not seem to be a large difference betweenboth systems. However, the definition of Ra involves some filteringof the surface peaks and valleys. When taking a look at Rz, thedifference becomes more apparent between the new and conven-tional cooling methods. Again, the results confirm that the oileffectively reaches the contact zone. Both Ra and Rz show lowervalues at the beginning of the tests, the trend again being moremarked in the case of Rz. Probably, as the experiment progresses thetemperature on the contact zone increases slightly and part ofthe frozen oil is lost. In other words, the amount of oil present at thebeginning of the tests is higher resulting in an improved surfacefinish, and although frozen oil is partially lost, there is still enoughlubricating action to protect the grits and produce better surfacequality. A similar trend is observed in the case of the conventionalcooling system, although the effect is less noticeable. The decreasein surface roughness at the end of the tests can be attributed to theappearance of wear flats on the surface of the abrasive grits as thetest progresses, which is accompanied by a loss of cutting actionand an increase in sliding between grinding wheel and workpiece.

Finally the evolution of grinding forces and force ratio duringthe tests with the new system was studied. During the grindingtests force signals are acquired and collected using a NI USB-6259signal acquisition card and a LabView software. The force ratio m isa commonly used variable in grinding. It provides a measure of thelubrication conditions between wheel and workpiece, and it isobtained as the ratio Ft/Fn.

Fig. 8 shows that the level of the grinding forces is similar inboth cases. Since power consumption is directly related to thetangential force Ft, it can be stated that the new system does notintroduce new power requirements in the machine spindle.However, the normal force Fn tends to be higher in the new system,and as a result, the force ratio is 28% lower when using the MQL-CO2 lubricating technology. Again, this is confirmation of the factthat oil is present and friction conditions are improved in thegriteworkpiece interface.

In view of the above results it can be observed that the newsystem is competitive in terms of process performance, since itensures the presence of lubricating oil in the interface wheel-eworkpiece, resulting in longer wheel life (reduced wear andincreased grinding ratio), better surface finish and similar powerconsumption than in the case of the conventional flooding system.All these advantages are obtained with minimum oil consumption,which was the primary requirement of the development. However,the cooling action of the MQL-CO2 technology has yet to be ana-lysed. It has been shown that the lubricating action is optimal,however oil-based systems are traditionally poorer than water-based fluids in the cooling of the ground surface. Unacceptablethermal damage may occur on the workpiece as a consequence. Inthe next Section, an analysis of the extent of thermal damageproduced on the component when applying the MQL-CO2 tech-nology is presented.

5. Thermal analysis of the process

The quality of a ground component is not only related to itssurface finish or its dimensional accuracy, but also to its surfaceintegrity. It must be considered that, should the heat producedduring grinding not be effectively removed from the ground part,surface integrity may be negatively affected in the form of metal-lurgical transformations, oxidation, etc. leading to the appearanceof tensile residual stresses and reduced fatigue life. Therefore,measures directed towards the reduction and effective evacuationof the heat generated during grinding are very commonly taken. Nodoubt, the delivery of efficient cooling fluids at very high pressuresand flowrates is the most popular technique in industry.

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Fig. 7. Surface roughness (Ra and Rz) with the conventional and the MQL-CO2 systems. Left: Qw¼ 50 mm3/s (ae¼ 0.015 mm; vw¼ 20 m/min; vs¼ 35 m/s); right: Qw¼ 100 mm3/s(ae¼ 0.02 mm; vw¼ 30 m/min; vs¼ 35 m/s).

J.A. Sanchez et al. / Journal of Cleaner Production 18 (2010) 1840e18491846

In the newMQL-CO2 system proposed here the cooling action ofthe water-based emulsion is not available. As a consequence itcould be expected that, thermal damage of the ground componentmay be so high as to invalidate the new technology. In this Sectionan analysis of the thermal damage resulting from the application of

Fig. 8. Evolution of normal (Fn) and tangential (Ft) grinding forces (up) and force ratio(m) (down) during the tests with the new and the conventional cooling systems.Workpiece speed vw¼ 2 m/min, depth of cut ae¼ 0.02 mm, grinding wheel tangentialspeed vs¼ 35 m/s.

the MQL-CO2 system is presented. To do so, sub-surface tempera-tures are measured in the workpiece during grinding. Metallurgicalanalysis of the workmaterial is also presented, so that in the case ofthermal damage appearing its extent can be determined.

Measuring temperatures during the grinding process isa difficult task. A look at previously published work givesdifferent solutions to the problem. A complete revision of thedifferent experimental techniques available for temperaturemeasurement in grinding can be found in the works of Davieset al. (2007) and Batako et al. 2005. The first experimentalmeasurement of grinding zone temperature using thermocoupleswas introduced by Peklenik (1958). Workpieceewheel thermo-couples have been used, but the high impedance of the grindingwheel introduces important distortions in the measurement.Infra-red thermography is also commonly used. Infra-red ther-mography shows important limitations such as the difficulties inestimating good values of material emissivity (dependent onaspects such as surface roughness, temperature itself, etc.) andthe fact the grinding tests must be carried out without fluid,which is far from actual industrial conditions.

The alternative is the use of contact thermocouples, whichprovide a simple robust economical method for temperaturemeasurement. In this work commercial K-type thermocouples oflow diameter (0.5 mm) were used. Thanks to their low diameter,thermal inertia is very low which enables a high samplingfrequency as well as minimising the effect of temperature aver-aging. One of the most important problems when using thermo-couples is the uncertainty in the positioning with respect to theground surface. The distance between the contact work-pieceethermocouple and surface of the workpiece must be accu-rately known. Also, the sensor must be firmly held to theworkpiece, otherwise contact may get lost and therefore, notemperature would be registered.

The problems that arise when trying to correctly place ther-mocouples inside a very hard material, such as tool steel withhardness of approx. 64HRc, are therefore evident. In this worka new method for thermocouple location is presented. The work-piece is first cut into 2 halves using wire electrical dischargemachining (WEDM), and then precision beds are machined on eachof the parts using sinking electrical discharge machining. At thisstage thermocouples can be placed on the locations, and then thetwo halves are glued together using an industrial product. Usingthis method the uncertainty in the contact location is kept below5 mm which is a very good value. It can also be ensured that thesensor does not lose contact with theworkpiece. Fig. 9 shows one ofthe halves of the workpiece, and a detail of the contactworkpieceethermocouple.

Fig. 9. Left: one half (cut by WEDM) of the workpiece for the grinding tests. Locations for thermocouples at different depths from the surface can be observed. Right: detail of thecontact that can be achieved with a repeatability of 5 mm.

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Fig. 10. Temperatures inside the workpiece as a function of depth from the ground surface. Left: water-based conventional cooling. Right: new MQL-CO2 system. Tests carried outwith workpiece speed vw¼ 2 m/min, depth of cut ae¼ 0.03 mm, grinding wheel tangential speed vs¼ 35 m/s.

Fig. 11. Simulation of temperatures reached at a given point of the workpiece in twocases: results with a value of geometric contact length of lg¼ 2.64 mm and a value ofactual contact length of Lc¼ 3.96 mm (ae¼ 0.03 mm; vw¼ 2 m/min; vs¼ 35 m/s).

J.A. Sanchez et al. / Journal of Cleaner Production 18 (2010) 1840e1849 1847

Again, grinding tests with different parameters were carried outto compare the temperatures reached inside the workpiece duringthe process using the new MQL-CO2 and the conventional water-based coolant. Aggressive grinding conditions enabling the possi-bility of thermal damage were set for the tests, involving the use oflow workpiece speeds. In the grinding charts the limitation ofgrinding burn is related to low workpiece speed. If no thermaldamage is observed when using vw¼ 2 m/min, it can be expectedthat when using higher values of vw the part will be also free ofburn. The geometrical contact length, that defines the theoreticalarea of contact through which the heat is dissipated towards theworkpiecewas kept constant, so that the results fromdifferent testscan be properly compared.

Taking into account the dispersion accepted in the measure-ments provided by thermocouples, it can be seen that the surfacetemperature in the case of the water-based fluid is within the673e723 K range, whereas in the case of the new system themaximum workpiece surface temperature ranges from 723 to773 K. At a depth of 100 mm below the surface temperaturedecreases to 503e603 K in the first case, and 543e643 K with thenew technology. These results show that even though the water-cooling action is not available in the case of the MQL-CO2 system,surface and sub-surface workpiece temperatures are only slightlyhigher than those measured in the case of the conventionalflooding system.

It has been largely advocated by Malkin and Guo (2007) thatconventional cooling is largely inefficient in shallow-cut grinding,characterized by small contact lengths. This is probably the primaryreason for the similarity in thermal properties of conventional andMQL-CO2 cooling lubrication. A further effect can be observed.Normal forces are higher in the MQL-CO2 system (see Section 3,Fig. 8). Consequently, deformations in the contactwheeleworkpiece

will also behigher, leading to an increase in the actual contact lengthwith respect to the conventional cooling system. A longer contactlength means a larger contact surface, and therefore, for the sameamount of power consumption the heat density towards thecomponent is lower.

Numerical simulation of the heat flow into the workpieceillustrates the above-explained fact. Fig. 11 represents the temper-atures (obtained by numerical simulation) reached at a given point

Fig. 12. Microphotograph (200�) of the steel structure after grinding with the newMQL-CO2 technology (left) and after conventional grinding (right). Experiments carried out withworkpiece speed vw¼ 2 m/min, depth of cut ae¼ 30 mm.

J.A. Sanchez et al. / Journal of Cleaner Production 18 (2010) 1840e18491848

of the workpiece, as the grinding wheel is moved from left to right.In the two curves represented, the grinding parameters (type anddiameter of wheel, workpiece material, depth of cut, wheel speedand workpiece speed, coolant delivery system) have been keptconstant, although the actual contact length has been varied. Thesymbol Lc was used to make it clear the actual contact length isdifferent (in fact, higher) from the geometric contact length lg, dueto the deformations introduced by normal forces in the contactwheeleworkpiece. Simulation shows that a higher contact lengthalso results in lower temperatures in the ground component, con-firming thus the benefits in the cooling action provided by anincrease in Fn as that produced by the MQL-CO2 system.

Results for the temperatures in the workpiece have been given,nevertheless the possibility of thermal damage produced by thosetemperatures in the integrity of the component must be discussed.Based on the work from Marinescu et al. (2004), the grindingtemperatures should be maintained below the softening temper-ature, above which the onset of tensile residual stresses tends tooccur. For the work material used in the tests (AISI D2 tool steel)over 823 K only a minimal loss in hardness is observed (Dumitrescuet al., 2006). Therefore, after these considerations and in viewof theresults plotted in Fig. 10, the ground specimens should not beaffected by the amount of heat generated.

Metallurgical analysis of the ground part confirms thishypothesis. Samples were polished and prepared for study in themicroscope and for measurement of microhardness on a crosssection with respect to the ground surface. Fig. 12 shows thestructure of the tool steel at 200� after having been ground usingthe MQL-CO2 technology (left) and after conventional grinding(right). Visual observation reveals a similar structure in the basematerial and sub-surface of the sample. Table 2 collects the valuesobtained for themicrohardness at different depths from the groundsurface. It can be seen that, with respect to the unaffected material,a slight increase in the hardness can be noticed. From the data given

Table 2Results of microhardness corresponding to the sample shown in Fig. 12.

MQL-CO2 Coolant

Distance to the surface[mm]

Hardness[HRC]

Distance to the surface[mm]

Hardness[HRC]

0.05 62.1 0.045 61.90.095 60.85 0.100 59.70.130 58.6 0.140 57.40.192 54.8 0.180 55.20.238 54.1 0.242 54.30.293 57.2 0.295 57.50.408 57 0.350 58

by the steelmanufacturer, a slight increase in hardness with respectto the base material occurs until a temperature of 790 K is reached,temperature above which a dramatic reduction in hardness isobserved. Therefore, from the results of microhardness gathered inTable 2 it can be stated that the MQL-CO2 technology does notinduce thermal damage in the material.

6. Conclusions

From the work carried out the following conclusions can bedrawn:

- A new grinding technology has been presented. The systeminvolves the use of two nozzles: first, oil is supplied as MQL,and then a flow of CO2 at 238 K is in charge of fixing the frozenoil on the surface of the abrasive grits, protecting them fromwear and improving sliding conditions, resulting in a bettersurface quality of the component.

- It is of primary importance to control the thickness of thefrozen oil layer If too thick a frozen layer is generated, animpact at the entrance of the grinding wheel in the operationmay occur, increasing grinding forces and leading to problemsin the process. Supply parameters have been optimized, and ithas been found that the problem can be suppressed by settingthe flow of CO2 at 40 l/min.

- The performance of the new system was evaluated in surfacegrinding on AISI D2 tool steel. Experiments show that grindingwheel wear with the MQL-CO2 system, even under the mostaggressive conditions (G¼ 4.5), is lower than with conven-tional coolant under the lightest grinding conditions (G¼ 4.22).

- Sliding conditions are also improved as shown by surface finishand force ratio, which is 28% lower in the case of the MQL-CO2system.

- Results show that even though the water-cooling action is notavailable in the case of the MQL-CO2 system, workpiecetemperatures are only slightly higher than those in theconventional system. In both cases the temperatures arebelow that responsible for thermal damage of the material(833 K).

- The primary reason for the similarity in thermal propertiesof conventional and MQL-CO2 cooling lubrication is the fact(already explained by previous authors) that conventionalcooling is largely inefficient in shallow-cut grinding, char-acterized by small contact lengths. Moreover, a high normalforce Fn is related to high deformations in the contactwheeleworkpiece, which in turn are responsible for an

J.A. Sanchez et al. / Journal of Cleaner Production 18 (2010) 1840e1849 1849

increase in the actual contact length. Finite Element simu-lation confirms that for the same conditions, a highercontact length produces lower temperatures.

- Finally, metallurgical analysis of the components ground withthe new technology was carried out to confirm the absence ofthermal damage. Microphotographs showa similar structure inthe base material and in the sub-surface of the sample. Thevalues obtained for the microhardness at different depths fromthe ground surface reveal a slight increase in the hardness fromwhich it can be stated MQL-CO2 technology does not inducethermal damage in the material.

- Further research work is currently being carried out with theobjective of reducing operating costs associated with CO2consumption. Optimum nozzle design using CFD, use of a mixof CO2 and other gases, as well as other alternatives are beinganalysed. Optimization of the application of the technology tocylindrical grinding and to other workpiece materials is alsobeing studied.

Acknowledgements

The authors wish to thank the Department of Industry of theBasque Government for their support for the Research Project‘Ecological Grinding EKOGRIND’.

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