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Effects of cooling lubricant on the thermal regime in the working space of machine tools
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16th Global Conference on Sustainable Manufacturing - Sustainable Manufacturing for Global Circular Economy 16th Global Conference on Sustainable Manufacturing - Sustainable Manufacturing for Global Circular Economy
Effects of cooling lubricant on the thermal regime in the working Effects of cooling lubricant onmachine the thermal space of toolsregime in the working space of machine tools2017, Manufacturing Engineering Conference MESIC 2017,a,b28-30 June a, Society International a b Michael Bräunig *, Joachim Regel , Janine Glänzel 2017, Vigo (Pontevedra), Spain b, Matthias Putza,b a, a Michael Bräunig *, Joachim Regel , Janine Glänzel , Matthias Putz Institute for Machine Tools and Production Processes, Technische Universität Chemnitz, Reichenhainer Str. 70, 09126 Chemnitz, Germany
a
Fraunhofer forProduction Machine Tools and Forming Technology IWU,Chemnitz, Reichenhainer Str. 88, 09126 Chemnitz, GermanyGermany Institute for MachineInstitute Tools and Processes, Technische Universität Str. 70, 09126 Chemnitz, Costing models capacity optimization inReichenhainer Industry 4.0: Trade-off Fraunhofer Institute forfor Machine Tools and Forming Technology IWU, Reichenhainer Str. 88, 09126 Chemnitz, Germany between used capacity and operational efficiency Abstract a
b b
Abstract a many milling and b b to achieve the best possible The use of cooling lubricant isA. indispensable machining primarily Santanafor , P. Afonsoa,*, grinding A. Zanin , R. tasks, Wernke cooling lubricating effectisinindispensable the cutting zone to milling reduce wear. In addition, tempered cooling lubricant carriesthe outbest the function The use and of cooling lubricant for and many and grinding machining tasks, primarily to achieve possible a 4800-058 Portugal of heat removal from the working space.University A reduction due to resource-efficient or ecological efforts has carries a directout impact on the cooling and lubricating effect in the cutting zone andoftoMinho, reduce wear. InGuimarães, addition, tempered cooling lubricant the function b Unochapecó, 89809-000 Chapecó, SC, Brazil temperature regime in the the working working space. space, Awhich affectsdue thetothermal behavior oforthe frame structures anda can cause machining of heat removal from reduction resource-efficient ecological efforts has direct impact on the inaccuracies.regime In thisinarticle, these effects cooling lubricant on thebehavior thermal of behavior of exemplary assemblies aremachining modeled. temperature the working space, of which affects the thermal the frame structures and can cause Experimental In investigations, basedeffects on temperatures displacements, are compared simulation-based calculations. The inaccuracies. this article, these of cooling and lubricant on the thermal behaviorwith of exemplary assemblies are modeled. results includeinvestigations, the description based of the environmental conditions that must beare taken into account modeling frame structures The and Experimental on temperatures and displacements, compared with when simulation-based calculations. Abstract the influencing variables such of as the coolant temperature, suppliedthat volume and extracted air when volume. results include the description environmental conditions mustflow be taken into account modeling frame structures and the influencing variables such as coolant temperature, supplied volume flow and extracted air volume. Under the concept of "Industry 4.0", production processes will be pushed to be increasingly interconnected, © 2018 The Authors. Published by Elsevier Ltd. © 2019 The Authors. Published by Elsevier B.V. necessarily, information based on a realunder time basis and, more efficient. In this context, capacity optimization This is an open access article the CC BY-NC-ND licensemuch (https://creativecommons.org/licenses/by-nc-nd/4.0/) © 2018 The Authors. Published by Elsevier Ltd. This is an openthe access article under thecapacity CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/) goes beyond traditional aim of maximization, contributing for organization’s profitability and value. Peer-review under responsibility of the scientific committee of the 16th Globalalso Conference on Sustainable Manufacturing This is an open access articleunder under CC BY-NC-ND (https://creativecommons.org/licenses/by-nc-nd/4.0/) Selection and peer-review responsibility of thelicense scientific committee ofsuggest the 16thcapacity Global Conference on Sustainable Indeed, lean management and continuous improvement approaches optimization instead of (GCSM) Peer-review under responsibility of the scientific committee of the 16th Global Conference on Sustainable Manufacturing Manufacturing (GCSM). maximization. The study of capacity optimization and costing models is an important research topic that deserves (GCSM)
Keywords: machine tool;both thermal cooling CFD-simulation; thermal modeling contributions from theanalysis; practical and system; theoretical perspectives. This paper presents and discusses a mathematical Keywords: tool;management thermal analysis; cooling CFD-simulation; thermal (ABC modeling model formachine capacity based onsystem; different costing models and TDABC). A generic model has been developed and it was used to analyze idle capacity and to design strategies towards the maximization of organization’s 1. Introduction value. The trade-off capacity maximization vs operational efficiency is highlighted and it is shown that capacity 1. Introduction optimization might hide operational inefficiency. In machine tool Published operations, thermal © 2017 The Authors. by the Elsevier B.V. error in comparison with the geometric, static and dynamically induced In machine tool operations, the thermal error in comparison withscenarios the Engineering geometric, static and dynamically error can predominate [1]. Consequently, cooling are used in daily operations toinduced reduce Peer-review under responsibility of the scientificenergy-intensive committee of the Manufacturing Society International Conference error can predominate [1]. Consequently, energy-intensive cooling scenarios are of used in daily reduce 2017. thermally induced structural deformations in machine tools. The effects of the use coolants on operations the thermaltobehavior thermally structural deformations in machine tools. The effects of the use of coolants on the thermal behavior are poorlyinduced understood. Keywords: Cost Models; ABC; TDABC; Capacity Management; Idle Capacity; Efficiency [2], [3]. The investigations have areIn poorly understood. other work, the effects of air cooling on the tool structureOperational were investigated In other the effectsfield of air theair tool structure investigated [3]. The investigations have shown that work, the temperature in cooling the tool on with cooling can were be reduced by up [2], to 50%, compared to machining shown the temperature field in the tool with air cooling can be reduced by up to 50%, compared to machining withoutthat coolant. 1.InIntroduction without coolant. addition to a better cooling effect, the use of cooling lubricant brings also a significant change in the air quality. In addition a better of cooling lubricant brings alsoparticles a significant in the air into quality. Depending ontothe main cooling spindleeffect, speed,thea use significant concentration of fog may change be evaporated the The cost of is a fundamental informationconcentration for companiesofandfog their management of extreme importance Depending onidle thecapacity main spindle speed, a significant particles may be evaporated into the in modern production systems. In general, it is defined as unused capacity or production potential and can be measured 2351-9789 © 2018 The Authors. Published by Elsevier Ltd. in several ways: tons of production, available hours of manufacturing, etc. The management of the idle capacity This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/) 2351-9789 © 2018 The Authors. Published by Elsevier Ltd. * Paulo Tel.: +351 253 510 +351 253license 604of741 Peer-review under responsibility of the761; scientific committee the 16th Global Conference on Sustainable Manufacturing (GCSM) This is anAfonso. open access article under CC fax: BY-NC-ND (https://creativecommons.org/licenses/by-nc-nd/4.0/) E-mail address: [email protected] Peer-review under responsibility of the scientific committee of the 16th Global Conference on Sustainable Manufacturing (GCSM)
2351-9789 Published by Elsevier B.V. B.V. 2351-9789©©2017 2019The TheAuthors. Authors. Published by Elsevier Peer-review underaccess responsibility the scientific committee oflicense the Manufacturing Engineering Society International Conference 2017. This is an open article of under the CC BY-NC-ND (https://creativecommons.org/licenses/by-nc-nd/4.0/) Selection and peer-review under responsibility of the scientific committee of the 16th Global Conference on Sustainable Manufacturing (GCSM). 10.1016/j.promfg.2019.04.040
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environment, which can be dangerous for health reasons [4]. The impact of fog particles to the machine will not be considered here. Further work is concerned with the calculation of heat transfer at the workpiece, which is essential, but which was determined for turning in one-dimensional view for different oils [5]. It is previously known by minimum quantity lubrication (MQL) that the emulsion coolant at high speed milling is inefficient because the inner zones of the tool teeth cannot be reached. The flow enters the cutting zone and acts in three different ways. The flow acts on the tool, act on the workpiece and removes chips. The nozzle position with respect to the feed direction is essential to achieve an optimal cooling effect. It is possible that the arrangement of several nozzles makes sense, which are activated and deactivated depending on the feed direction [6]. A holistic investigation has so far been carried out for different cooling scenarios. In particular, the impact on the machine structure is not known. The purpose of this article is to provide important insights in the identification of thermal behavior in machine tools through metrological and numerical investigation. 2. Experimental Investigations The metrological investigations were carried out on a testing stand, which is predestined for thermal analyses in particular. The structure comprises of a machine bed having mounted on a support motorspindle. An enclosure shields the working space from the environment. The tool and the chuck can be heated there in the same way as in the real machining process by a specially designed induction system [7]. A major advantage of this approach is the possibility of separate consideration of thermal and static deformations. The heating of the tool is carried out without contact and in a defined area. The induction system is shown and indicated in Figure 1. The workpiece heating and the heat input into the machine bed by chips are made possible by heating mats and heating rods.
Fig. 1. Experimental setup and testing stand
The temperature measurement takes place with resistance thermometers in the form of thin-film sensors and larger platinum resistances. These are positioned on freely accessible areas and also glued on the tool. The signal transmission takes place there with slip rings. The arrangement of the sensors builds on previous investigations [8], [9]. In addition, free fluid flows are detected with an impeller anemometer and guided air flows with a compressed air meter. The setup is shown in Figure 1. The experiments were varied with the following conditions. The heating of the tool and workpiece structure as well as the mapping of chips by heating mats took place. The experiments were carried out: • without cooling and suction,
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• with suction, • with cooling lubricant, • with cooling lubricant and suction, • with MQL and suction, • with air cooling and suction conditions. Table 1 lists the main parameters for each cooling method. The cooling lubricant is water-soluble and intended for milling. The oil of the minimum quantity lubrication system is not soluble in water and is also used for milling, whereby it can only be used with a suction. Cutting experiments with MQL have shown that with metered oil supply, the main part of the oil quantity is a lubricating film on the tool and workpiece and this can be declared as dry machining [11]. Over a longer period, however, oil accumulates, especially in the air, which must be extracted. Table 1. Technical details of the cooling methods Coolant Details Cooling Lubricant
DAW AEROLAN 2200, 1.25 L/min coolant amount, 22°C coolant temperature
MQL
ITEC-M 4300, 1.2 bar over pressure, 50 ml/h oil amount, 23°C aerosol temperature
Air Cooling
10 Nm³/h supplied air volume, 21.5°C air temperature
Suction
9 m³/min exhausted air volume
So far, a regression analysis and a corresponding mathematical basic function can be used to evaluate the experimental results very well [10]. For these experiments, however, this is not possible with the proven models (Pt1, Pt2, tanh). This likely occurs because the inclusion of multiple heat sources results in an increased complexity. This is evident, for example, in the temperature behavior near the tool center point (Figure 2).
Fig. 2. Measured and calculated temperatures on the tool
For example, the Pearson correlation coefficient is 0.92 and could be significantly higher. Normally values of 0.99 are often possible. Above all, it is noticeable that the temperature increase of the experimental results deviates strongly from the calculations at the beginning of the measurement. This makes it clear that new mathematical models have to be found for such thermal differences, which can represent the experimental results better. In the following, this is limited to the evaluation of absolute temperatures. These results already show significant differences in the thermal microclimate of the working space for different cooling scenarios. The use of cooling lubricant has a direct effect on the tool and workpiece temperature (Figure 3a and 3b). The temperatures are much lower than with dry processing, and constancy is reached quickly. The heating time of the machine bed takes much longer. Cooling lubricants cause less severe effects on the tool and workpiece. Particularly extreme is the influence of the suction system (Figure 3d). The temperature in the working space can be significantly reduced by the suction. The influence of the cooling lubricant is less strong here.
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Fig. 3. Experimental results for different cooling scenarios (a)
Tool temperature; (b) Workpiece temperature; (c) Temperatures of the machine bed; (d) Temperatures in the working space.
This is also evident in Figure 4. Here, the resulting frame deformation was determined. Without cooling and suction, the thermal drift increases considerably. Only with the use of cooling lubricants, especially in the case of air extraction, are thermal displacements significantly lower than in dry machining.
Fig. 4. Spindle Support Deformation
In addition, individual experiments with MQL could be evaluated. This shows that minimally lubricated processes can be declared as dry processing. Air cooling and MQL show only the slightest differences. In the machine bed, the temperatures with air cooling are still significantly lower than with MQL. The addition of oil in the air stream does
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not cause a noticeable cooling effect. In the machine bed even significantly lower temperatures were measured by air cooling. While MQL can cause 5 K colder temperatures in the machine bed than air cooling, it is of particular importance that exactly the opposite effect can be observed on the workpiece. This likely occurs because the geometry of the proportion of turbulence flows on the workpieces is significantly higher than on the surface of the machine bed. This may encourage convective heat transfer and increase the cooling effect.
Fig. 5. (a) Measured temperatures variations in the working space; (b) Measured temperatures variations of the machine bed.
As expected, cooling with cooling lubricant in the form of full jet cooling has a considerable cooling effect. New findings are that this effect can be significantly improved by a suction. In addition, targeted air cooling can also show a significant cooling effect. 3. CFD-Simulations Using CFD-simulations, the experimental investigations can better be interpreted [12]. For example, the influence of different boundary conditions and cooling parameters on the target values (tool or workpiece temperatures) can be investigated. For this purpose, the same testing stand was modeled and prepared for the fluidic simulation (Figure 6).
Fig. 6. CFD-model of the testing stand
Based on this geometric model a finite element (FE) mesh was created for the solid and fluidic components. Subsequently, all boundary conditions, e.g. heat source of the tool, streaming velocity of the coolant nozzle and suction, and the outlet, are defined in the preprocessing. As cases of application were considered the main variants listed in Section 2 multi component fluids were regarded in the simulations. All calculations were done with ANSYS
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CFX. An example for such a multi component flow is the modeling of water as cooling lubricant and air in the working space. Here, free surfaces in a homogenous model were calculated. In a first step, water can be used as cooling lubricant because in our experimental tests the cooling material consisted of 94% water. In Figure 7, the flow velocities in the main yz-plane in the middle of the cooling nozzle and the tool were depicted. On the outlet of the nozzle, a volume flow of 1.25 L/min (or an outlet velocity of 0.74 m/s) and a water outlet temperature of the nozzle of 22°C was used. At the end of the tool, a heat flow was induced to fix the maximum temperature at 100°C in the case of no cooling and suction (Figure 8). With nozzle cooling (the distance of the nozzle outlet to the tool was 60 mm and the inclination to the tool axis was approximately 30 degrees), a significant decrease of the maximum temperature in the tool of 62°C was achieved in the steady-state case (Figure 9).
Fig. 7. Velocity of the water fraction in the steady-state case
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Fig. 8. Temperature distribution without cooling
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Fig. 9. Temperature distribution with cooling
For these calculations, a laminar flow regime without spindle rotating was used. Further investigations for turbulence modeling with the k- -, k- - and the SST-model verified the positive effects of turbulence for heat exchange. By using water (or parts of them) as cooling liquid buoyancy shall be regarded. All calculations showed that it is very time consuming to find suitable time incrementation parameters to ensure stable results and low calculation times. In some steady-state calculations, stable results could only be achieved by reducing the number of processors. Steady-state calculations are only possible in our case of non-rotating spindles. If the spindle rotates transient solutions shall be conducted because of different axes of rotation and gravity. Time steps of 0.01 s delivered good results for rotation velocities of 3000 revolutions per minute. Rotation of the tool leads to worse heat exchange for the drill. 4. Summary For a holistic consideration of the thermal behavior in the working space for different cooling scenarios in machine tools, both experimental and numerical investigations are necessary. There are three main results for the investigations. Due to the different heat inputs in a complex structure of moving and non-moving components, the previously known mathematical functions (Pt1, Pt2, tanh) are not applicable. There is a need for research for a correct description. The suction has a significant influence on the temperature field, and this is particularly evident in the workspace. The constant exchange of air results in a significant amount of heat being removed from the working space. This leads directly to lower air temperatures regardless of which cooling method is used. The CFX-Simulation is a simulation tool to describe the influence of the coolant lubricant in the working space of a machine tool. Therefore, a simplified CAD-Model was designed only with tool, workpiece and coolant nozzle. Especially, the calculated temperature field shows the big influence of the coolant lubricant on the tool. The future work is to extend the simulation model for the transient case of rotating spindles and to examine the influence of cooling lubricant to the heat transfer coefficient along the wall of the working space. For the resulting deformation field of the tool centre point, the calculation of correction values by using characteristic diagrams is intended for future work [13], [14], [15]. Acknowledgement The authors would like to thank the German Research Foundation (DFG) for the financial support within the Collaborative Research Centre Transregio 96 (SFB TR96).
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M. Bräunig, J. Regel, C. Richter, und M. Putz, „Industrial relevance and causes of thermal issues in machine tools“, in conference proceedings, 1st Conference on Thermal Issues in Machine Tools, Dresden, 2018. G. M. Perri, M. Bräunig, G. D. Gironimo, M. Putz, A. Tarallo, und V. Wittstock, „Numerical modelling and analysis of the influence of an air cooling system on a milling machine in virtual environment“, Int. J. Adv. Manuf. Technol., S. 1–12, Jan. 2016. M. Putz, V. Wittstock, U. Semmler, und M. Bräunig, „Simulation-based thermal investigation of the cutting tool in the environment of single-phase fluxes“, Int. J. Adv. Manuf. Technol., S. 117,121, Juli 2015. D. J. Michalek, W. W.-S. Hii, J. Sun, K. L. Gunter, und J. W. Sutherland, „Experimental and Analytical Efforts to Characterize Cutting Fluid Mist Formation and Behavior in Machining“, Appl. Occup. Environ. Hyg., Bd. 18, Nr. 11, S. 842–854, Nov. 2003. C. M. Daniel, K. V. C. Rao, W. W. Olson, und J. Sutherland, „Effect of cutting fluid properties and application variables on heat transfer in turning and boring operations“, Bd. 2, S. 1119–1126, Jan. 1996. L. N. López de Lacalle, C. Angulo, A. Lamikiz, und J. A. Sánchez, „Experimental and numerical investigation of the effect of spray cutting fluids in high speed milling“, J. Mater. Process. Technol., Bd. 172, Nr. 1, S. 11–15, Feb. 2006. M. Putz, J. Regel, M. Bräunig, S. Winkler, und A. Hüllmann, „Berührungslose Werkzeugerwärmung“, Zeitschr F Wirtsch Fabr., Nr. 07– 08, S. 1–5, Aug. 2016. W.-G. Drossel, V. Wittstock, M. Bräunig, und G. Schmidt, „Untersuchung der thermischen Werkzeugverformung“, Wt Werkstattstech. Online, Bd. 103, Nr. 11/12, S. 882–887, 2013. U. Semmler, M. Bräunig, W.-G. Drossel, G. Schmidt, und V. Wittstock, „Thermal deformations of cutting tools: measurement and numerical simulation“, Prod. Eng., Bd. 8, Nr. 4, S. 543–550, Aug. 2014. M. Putz, C. Oppermann, M. Bräunig, und U. Karagüzel, „Heat sources and fluxes in milling: Comparison of numerical, analytical and experimental results“, in 16th CIRP Conference on Modelling of Machining Operations, 2017. Gesellschaft Produktionstechnik, Hrsg., Praxis der Trockenbearbeitung: Tagung Karlsruhe, 8. und 9. März 1999, Bd. 1458. Düsseldorf: VDI-Verl, 1999. R. Neugebauer, S. Ihlenfeldt, C. Zwingenberger, J. Glänzel, C. Richter, „Modelling of Thermal Interaction between Environment and Machine Tool”, Thermo-energetic design of machine tools, Berlin, S. 111–24, 2014. J. Glänzel, S. Ihlenfeldt, C. Naumanna, M. Putz, “Decoupling of fluid and thermo-elastic simulations on machine tool using characteristic diagrams”, Procedia CIRP Volume 62, S. 340-345, 2016. J. Glänzel, R. Unger, S. Ihlenfeldt, „Clustering by optimal subsets to describe environment inter-dependencies. Proceedings in 1st CIRP Conference on Thermal Issues in Machine Tools, March, 21th-23th, Dresden (Germany), 2018. J. Glänzel, S. Ihlenfeldt, C. Naumann, M. Putz, „Efficient Quantification of Free and Forced Convection via the Decoupling of Thermomechanical and Thermo-fluidic Simulations of Machine Tools”. In: XXIX CIRP Sponsored Conference on Supervising and Diagnostics of Machining Systems, March, 11th -15th, Karpacz. 2018
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16th Global Conference on Sustainable Manufacturing - Sustainable Manufacturing for Global Circular Economy 16th Global Conference on Sustainable Manufacturing - Sustainable Manufacturing for Global Circular Economy
Effects of cooling lubricant on the thermal regime in the working Effects of cooling lubricant onmachine the thermal space of toolsregime in the working space of machine tools2017, Manufacturing Engineering Conference MESIC 2017,a,b28-30 June a, Society International a b Michael Bräunig *, Joachim Regel , Janine Glänzel 2017, Vigo (Pontevedra), Spain b, Matthias Putza,b a, a Michael Bräunig *, Joachim Regel , Janine Glänzel , Matthias Putz Institute for Machine Tools and Production Processes, Technische Universität Chemnitz, Reichenhainer Str. 70, 09126 Chemnitz, Germany
a
Fraunhofer forProduction Machine Tools and Forming Technology IWU,Chemnitz, Reichenhainer Str. 88, 09126 Chemnitz, GermanyGermany Institute for MachineInstitute Tools and Processes, Technische Universität Str. 70, 09126 Chemnitz, Costing models capacity optimization inReichenhainer Industry 4.0: Trade-off Fraunhofer Institute forfor Machine Tools and Forming Technology IWU, Reichenhainer Str. 88, 09126 Chemnitz, Germany between used capacity and operational efficiency Abstract a
b b
Abstract a many milling and b b to achieve the best possible The use of cooling lubricant isA. indispensable machining primarily Santanafor , P. Afonsoa,*, grinding A. Zanin , R. tasks, Wernke cooling lubricating effectisinindispensable the cutting zone to milling reduce wear. In addition, tempered cooling lubricant carriesthe outbest the function The use and of cooling lubricant for and many and grinding machining tasks, primarily to achieve possible a 4800-058 Portugal of heat removal from the working space.University A reduction due to resource-efficient or ecological efforts has carries a directout impact on the cooling and lubricating effect in the cutting zone andoftoMinho, reduce wear. InGuimarães, addition, tempered cooling lubricant the function b Unochapecó, 89809-000 Chapecó, SC, Brazil temperature regime in the the working working space. space, Awhich affectsdue thetothermal behavior oforthe frame structures anda can cause machining of heat removal from reduction resource-efficient ecological efforts has direct impact on the inaccuracies.regime In thisinarticle, these effects cooling lubricant on thebehavior thermal of behavior of exemplary assemblies aremachining modeled. temperature the working space, of which affects the thermal the frame structures and can cause Experimental In investigations, basedeffects on temperatures displacements, are compared simulation-based calculations. The inaccuracies. this article, these of cooling and lubricant on the thermal behaviorwith of exemplary assemblies are modeled. results includeinvestigations, the description based of the environmental conditions that must beare taken into account modeling frame structures The and Experimental on temperatures and displacements, compared with when simulation-based calculations. Abstract the influencing variables such of as the coolant temperature, suppliedthat volume and extracted air when volume. results include the description environmental conditions mustflow be taken into account modeling frame structures and the influencing variables such as coolant temperature, supplied volume flow and extracted air volume. Under the concept of "Industry 4.0", production processes will be pushed to be increasingly interconnected, © 2018 The Authors. Published by Elsevier Ltd. © 2019 The Authors. Published by Elsevier B.V. necessarily, information based on a realunder time basis and, more efficient. In this context, capacity optimization This is an open access article the CC BY-NC-ND licensemuch (https://creativecommons.org/licenses/by-nc-nd/4.0/) © 2018 The Authors. Published by Elsevier Ltd. This is an openthe access article under thecapacity CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/) goes beyond traditional aim of maximization, contributing for organization’s profitability and value. Peer-review under responsibility of the scientific committee of the 16th Globalalso Conference on Sustainable Manufacturing This is an open access articleunder under CC BY-NC-ND (https://creativecommons.org/licenses/by-nc-nd/4.0/) Selection and peer-review responsibility of thelicense scientific committee ofsuggest the 16thcapacity Global Conference on Sustainable Indeed, lean management and continuous improvement approaches optimization instead of (GCSM) Peer-review under responsibility of the scientific committee of the 16th Global Conference on Sustainable Manufacturing Manufacturing (GCSM). maximization. The study of capacity optimization and costing models is an important research topic that deserves (GCSM)
Keywords: machine tool;both thermal cooling CFD-simulation; thermal modeling contributions from theanalysis; practical and system; theoretical perspectives. This paper presents and discusses a mathematical Keywords: tool;management thermal analysis; cooling CFD-simulation; thermal (ABC modeling model formachine capacity based onsystem; different costing models and TDABC). A generic model has been developed and it was used to analyze idle capacity and to design strategies towards the maximization of organization’s 1. Introduction value. The trade-off capacity maximization vs operational efficiency is highlighted and it is shown that capacity 1. Introduction optimization might hide operational inefficiency. In machine tool Published operations, thermal © 2017 The Authors. by the Elsevier B.V. error in comparison with the geometric, static and dynamically induced In machine tool operations, the thermal error in comparison withscenarios the Engineering geometric, static and dynamically error can predominate [1]. Consequently, cooling are used in daily operations toinduced reduce Peer-review under responsibility of the scientificenergy-intensive committee of the Manufacturing Society International Conference error can predominate [1]. Consequently, energy-intensive cooling scenarios are of used in daily reduce 2017. thermally induced structural deformations in machine tools. The effects of the use coolants on operations the thermaltobehavior thermally structural deformations in machine tools. The effects of the use of coolants on the thermal behavior are poorlyinduced understood. Keywords: Cost Models; ABC; TDABC; Capacity Management; Idle Capacity; Efficiency [2], [3]. The investigations have areIn poorly understood. other work, the effects of air cooling on the tool structureOperational were investigated In other the effectsfield of air theair tool structure investigated [3]. The investigations have shown that work, the temperature in cooling the tool on with cooling can were be reduced by up [2], to 50%, compared to machining shown the temperature field in the tool with air cooling can be reduced by up to 50%, compared to machining withoutthat coolant. 1.InIntroduction without coolant. addition to a better cooling effect, the use of cooling lubricant brings also a significant change in the air quality. In addition a better of cooling lubricant brings alsoparticles a significant in the air into quality. Depending ontothe main cooling spindleeffect, speed,thea use significant concentration of fog may change be evaporated the The cost of is a fundamental informationconcentration for companiesofandfog their management of extreme importance Depending onidle thecapacity main spindle speed, a significant particles may be evaporated into the in modern production systems. In general, it is defined as unused capacity or production potential and can be measured 2351-9789 © 2018 The Authors. Published by Elsevier Ltd. in several ways: tons of production, available hours of manufacturing, etc. The management of the idle capacity This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/) 2351-9789 © 2018 The Authors. Published by Elsevier Ltd. * Paulo Tel.: +351 253 510 +351 253license 604of741 Peer-review under responsibility of the761; scientific committee the 16th Global Conference on Sustainable Manufacturing (GCSM) This is anAfonso. open access article under CC fax: BY-NC-ND (https://creativecommons.org/licenses/by-nc-nd/4.0/) E-mail address: [email protected] Peer-review under responsibility of the scientific committee of the 16th Global Conference on Sustainable Manufacturing (GCSM)
2351-9789 Published by Elsevier B.V. B.V. 2351-9789©©2017 2019The TheAuthors. Authors. Published by Elsevier Peer-review underaccess responsibility the scientific committee oflicense the Manufacturing Engineering Society International Conference 2017. This is an open article of under the CC BY-NC-ND (https://creativecommons.org/licenses/by-nc-nd/4.0/) Selection and peer-review under responsibility of the scientific committee of the 16th Global Conference on Sustainable Manufacturing (GCSM). 10.1016/j.promfg.2019.04.040
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environment, which can be dangerous for health reasons [4]. The impact of fog particles to the machine will not be considered here. Further work is concerned with the calculation of heat transfer at the workpiece, which is essential, but which was determined for turning in one-dimensional view for different oils [5]. It is previously known by minimum quantity lubrication (MQL) that the emulsion coolant at high speed milling is inefficient because the inner zones of the tool teeth cannot be reached. The flow enters the cutting zone and acts in three different ways. The flow acts on the tool, act on the workpiece and removes chips. The nozzle position with respect to the feed direction is essential to achieve an optimal cooling effect. It is possible that the arrangement of several nozzles makes sense, which are activated and deactivated depending on the feed direction [6]. A holistic investigation has so far been carried out for different cooling scenarios. In particular, the impact on the machine structure is not known. The purpose of this article is to provide important insights in the identification of thermal behavior in machine tools through metrological and numerical investigation. 2. Experimental Investigations The metrological investigations were carried out on a testing stand, which is predestined for thermal analyses in particular. The structure comprises of a machine bed having mounted on a support motorspindle. An enclosure shields the working space from the environment. The tool and the chuck can be heated there in the same way as in the real machining process by a specially designed induction system [7]. A major advantage of this approach is the possibility of separate consideration of thermal and static deformations. The heating of the tool is carried out without contact and in a defined area. The induction system is shown and indicated in Figure 1. The workpiece heating and the heat input into the machine bed by chips are made possible by heating mats and heating rods.
Fig. 1. Experimental setup and testing stand
The temperature measurement takes place with resistance thermometers in the form of thin-film sensors and larger platinum resistances. These are positioned on freely accessible areas and also glued on the tool. The signal transmission takes place there with slip rings. The arrangement of the sensors builds on previous investigations [8], [9]. In addition, free fluid flows are detected with an impeller anemometer and guided air flows with a compressed air meter. The setup is shown in Figure 1. The experiments were varied with the following conditions. The heating of the tool and workpiece structure as well as the mapping of chips by heating mats took place. The experiments were carried out: • without cooling and suction,
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• with suction, • with cooling lubricant, • with cooling lubricant and suction, • with MQL and suction, • with air cooling and suction conditions. Table 1 lists the main parameters for each cooling method. The cooling lubricant is water-soluble and intended for milling. The oil of the minimum quantity lubrication system is not soluble in water and is also used for milling, whereby it can only be used with a suction. Cutting experiments with MQL have shown that with metered oil supply, the main part of the oil quantity is a lubricating film on the tool and workpiece and this can be declared as dry machining [11]. Over a longer period, however, oil accumulates, especially in the air, which must be extracted. Table 1. Technical details of the cooling methods Coolant Details Cooling Lubricant
DAW AEROLAN 2200, 1.25 L/min coolant amount, 22°C coolant temperature
MQL
ITEC-M 4300, 1.2 bar over pressure, 50 ml/h oil amount, 23°C aerosol temperature
Air Cooling
10 Nm³/h supplied air volume, 21.5°C air temperature
Suction
9 m³/min exhausted air volume
So far, a regression analysis and a corresponding mathematical basic function can be used to evaluate the experimental results very well [10]. For these experiments, however, this is not possible with the proven models (Pt1, Pt2, tanh). This likely occurs because the inclusion of multiple heat sources results in an increased complexity. This is evident, for example, in the temperature behavior near the tool center point (Figure 2).
Fig. 2. Measured and calculated temperatures on the tool
For example, the Pearson correlation coefficient is 0.92 and could be significantly higher. Normally values of 0.99 are often possible. Above all, it is noticeable that the temperature increase of the experimental results deviates strongly from the calculations at the beginning of the measurement. This makes it clear that new mathematical models have to be found for such thermal differences, which can represent the experimental results better. In the following, this is limited to the evaluation of absolute temperatures. These results already show significant differences in the thermal microclimate of the working space for different cooling scenarios. The use of cooling lubricant has a direct effect on the tool and workpiece temperature (Figure 3a and 3b). The temperatures are much lower than with dry processing, and constancy is reached quickly. The heating time of the machine bed takes much longer. Cooling lubricants cause less severe effects on the tool and workpiece. Particularly extreme is the influence of the suction system (Figure 3d). The temperature in the working space can be significantly reduced by the suction. The influence of the cooling lubricant is less strong here.
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Fig. 3. Experimental results for different cooling scenarios (a)
Tool temperature; (b) Workpiece temperature; (c) Temperatures of the machine bed; (d) Temperatures in the working space.
This is also evident in Figure 4. Here, the resulting frame deformation was determined. Without cooling and suction, the thermal drift increases considerably. Only with the use of cooling lubricants, especially in the case of air extraction, are thermal displacements significantly lower than in dry machining.
Fig. 4. Spindle Support Deformation
In addition, individual experiments with MQL could be evaluated. This shows that minimally lubricated processes can be declared as dry processing. Air cooling and MQL show only the slightest differences. In the machine bed, the temperatures with air cooling are still significantly lower than with MQL. The addition of oil in the air stream does
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not cause a noticeable cooling effect. In the machine bed even significantly lower temperatures were measured by air cooling. While MQL can cause 5 K colder temperatures in the machine bed than air cooling, it is of particular importance that exactly the opposite effect can be observed on the workpiece. This likely occurs because the geometry of the proportion of turbulence flows on the workpieces is significantly higher than on the surface of the machine bed. This may encourage convective heat transfer and increase the cooling effect.
Fig. 5. (a) Measured temperatures variations in the working space; (b) Measured temperatures variations of the machine bed.
As expected, cooling with cooling lubricant in the form of full jet cooling has a considerable cooling effect. New findings are that this effect can be significantly improved by a suction. In addition, targeted air cooling can also show a significant cooling effect. 3. CFD-Simulations Using CFD-simulations, the experimental investigations can better be interpreted [12]. For example, the influence of different boundary conditions and cooling parameters on the target values (tool or workpiece temperatures) can be investigated. For this purpose, the same testing stand was modeled and prepared for the fluidic simulation (Figure 6).
Fig. 6. CFD-model of the testing stand
Based on this geometric model a finite element (FE) mesh was created for the solid and fluidic components. Subsequently, all boundary conditions, e.g. heat source of the tool, streaming velocity of the coolant nozzle and suction, and the outlet, are defined in the preprocessing. As cases of application were considered the main variants listed in Section 2 multi component fluids were regarded in the simulations. All calculations were done with ANSYS
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CFX. An example for such a multi component flow is the modeling of water as cooling lubricant and air in the working space. Here, free surfaces in a homogenous model were calculated. In a first step, water can be used as cooling lubricant because in our experimental tests the cooling material consisted of 94% water. In Figure 7, the flow velocities in the main yz-plane in the middle of the cooling nozzle and the tool were depicted. On the outlet of the nozzle, a volume flow of 1.25 L/min (or an outlet velocity of 0.74 m/s) and a water outlet temperature of the nozzle of 22°C was used. At the end of the tool, a heat flow was induced to fix the maximum temperature at 100°C in the case of no cooling and suction (Figure 8). With nozzle cooling (the distance of the nozzle outlet to the tool was 60 mm and the inclination to the tool axis was approximately 30 degrees), a significant decrease of the maximum temperature in the tool of 62°C was achieved in the steady-state case (Figure 9).
Fig. 7. Velocity of the water fraction in the steady-state case
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Fig. 8. Temperature distribution without cooling
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Fig. 9. Temperature distribution with cooling
For these calculations, a laminar flow regime without spindle rotating was used. Further investigations for turbulence modeling with the k- -, k- - and the SST-model verified the positive effects of turbulence for heat exchange. By using water (or parts of them) as cooling liquid buoyancy shall be regarded. All calculations showed that it is very time consuming to find suitable time incrementation parameters to ensure stable results and low calculation times. In some steady-state calculations, stable results could only be achieved by reducing the number of processors. Steady-state calculations are only possible in our case of non-rotating spindles. If the spindle rotates transient solutions shall be conducted because of different axes of rotation and gravity. Time steps of 0.01 s delivered good results for rotation velocities of 3000 revolutions per minute. Rotation of the tool leads to worse heat exchange for the drill. 4. Summary For a holistic consideration of the thermal behavior in the working space for different cooling scenarios in machine tools, both experimental and numerical investigations are necessary. There are three main results for the investigations. Due to the different heat inputs in a complex structure of moving and non-moving components, the previously known mathematical functions (Pt1, Pt2, tanh) are not applicable. There is a need for research for a correct description. The suction has a significant influence on the temperature field, and this is particularly evident in the workspace. The constant exchange of air results in a significant amount of heat being removed from the working space. This leads directly to lower air temperatures regardless of which cooling method is used. The CFX-Simulation is a simulation tool to describe the influence of the coolant lubricant in the working space of a machine tool. Therefore, a simplified CAD-Model was designed only with tool, workpiece and coolant nozzle. Especially, the calculated temperature field shows the big influence of the coolant lubricant on the tool. The future work is to extend the simulation model for the transient case of rotating spindles and to examine the influence of cooling lubricant to the heat transfer coefficient along the wall of the working space. For the resulting deformation field of the tool centre point, the calculation of correction values by using characteristic diagrams is intended for future work [13], [14], [15]. Acknowledgement The authors would like to thank the German Research Foundation (DFG) for the financial support within the Collaborative Research Centre Transregio 96 (SFB TR96).
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