Citation: Boswell, Brian and Chandratilleke, Tilak. 2009. Sustainable Metal Cutting, in TIC-STH ommittee (ed), 2009 IEEE Toronto International Conferene Siene and Tehnology for Humanity, Sep 26 2009. Ryerson University - Toronto, Canada: IEEE Toronto Setion. Additional Information: If you wish to ontat a Curtin researher assoiated with this doument, you may obtain an email address from http://find.urtin.edu.au/staff Copyright 2009 IEEE This material is presented to ensure timely dissemination of sholarly and tehnial work. Copyright and all rights therein are retained by authors or by other opyright holders. All persons opying this information are expeted to adhere to the terms and onstraints invoked by eah author's opyright. In most ases, these works may not be reposted without the expliit permission of the opyright holder. Permanent Link: http://espae.library.urtin.edu.au/r?fun=dbin-jump-full&loal_base=gen01-era02&objet_id=135090 The attahed doument may provide the author's aepted version of a published work. See Citation for details of the published work.
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TIC-STH 2009 Sustainable Metal Cutting Dr Brian Boswell Department of Mehanial Engineering Curtin University of Tehnology Perth, Western Australia B.Boswell@urtin.edu.au A/Prof Tilak T Chandratilleke Department of Mehanial Engineering Curtin University of Tehnology Perth, Western Australia T.Chandratilleke@urtin.edu.au Abstrat Metal utting ompanies are being ompelled to redue their impat on the environment as more international and government environmental protetion legislation is introdued. Ensuring appropriate waste disposal measures are in plae will be essential to allow ompanies to operate. Partiular attention must be given to liquid oolant used in metal utting as this is a signifiant soure of environmental pollution. Dry mahining is obviously more eologially desirable for metal utting as there are no environmental issues or disposal osts for the oolant. Unfortunately though, there are issues that need to be resolved before manufaturing ompanies will adopt dry mahining, mainly assoiated with the redution of tool life. However, from previous researh, it has been shown that the introdution of old air direted at the utting zone signifiantly inreased tool life removing oolant waste disposal osts. To further improve the up take of air ooling the most sustainable and most effiient method of generating old air suitable for mahining must be sought. Comparing three methods of providing old air has shown that the vortex tube is the most suitable method for providing old air to the tool interfae. CO 2 /kwh [3], and it should be noted that this value is dependent on loal and ountry eletrial generating failities). Fig. 1 shows the shemati diagram of the old air utting tests whih onsists of a omputer, data logger to reord the tool tip temperatures, air ooling nozzle, and air ompressor to supply air to ooling nozzles. The temperature of the tool tip was measured by three imbedded thermoouples positioned 1mm from the tool interfae whih were able to determine the effetiveness of the ooling proess during mahining. Also, the volume of air direted at the tool tip was measured to determine the environmental ost of produing the old air. These measured flow rates allowed the power ontribution for eah old air system used to be alulated, enabling the most sustainable method of supplying old air to the tool tip to be identified. Keywords-tool life; vortex tube; environmental; mahining I. INTRODUCTION The need for optimum mahining parameters has always been of great onern to the manufaturing industry, where the eonomy of the mahining proess plays a key role in the ompetitiveness of the produt. Previous researhers [1] have shown that air-ooling an be effetive at prolonging the life of tools during mahining, so eliminating the ost of disposal of traditional liquid oolant. This researh will ompare three methods of providing old air to the tool tip during mahining, to establish the most effetive method. In addition to operational effetiveness during mahining the most sustainable method of produing the old air must be found. Lian-yi Chen et al. [2] developed a predition system for the environmental burden of mahine tool operation based on life yle analysis (LCA). This model enables the evaluation of the equivalent CO 2 emission during mahining. The CO 2 emission is alulated from the eletrial onsumption of the mahining proess: in this ase only the CO 2 emission produed by generating old air will be onsidered. The CO 2 emission intensities fator for eah ooling system is based on how muh eletrial energy is onsumed in produing old air (CO 2 emission intensity for eletriity used was 0.304 kg- 1. Supply of ompressed air, 2. Control valve, 3. Air-ooling nozzle, 4. Flow meter, 5. Pressure gauge, 6. Thermoouples, 7. Computer, 8. Data logger, 9. Cutting tool. Figure 1. Air-ooling test blok diagram. II. PERFORMANCE COMPARISON OF AIR COOLING METHODS This air ooling omparison onsiders three air ooling methods. The first is impingement ooling, the seond method onsiders the vortex tube, and the third method uses the semiondutor refrigeration effet to ool the air. Eah ooling method will be examined for ease of use, and environmental burden ontribution made during mahining. 978-1-4244-3878-5/09/$25.00 2009 IEEE 831
A. Impingement Cooling Air is direted onto the tool fae as lose to the tool interfae as possible as shown by the arrow in Fig. 2. It is known that the effetiveness of the air-ooling is improved by using jetimpinging jet Mao-Yu Wen et al. How effetive this will be in metal utting needs to be asertained. A series of metal utting tests were performed to determine how impingement airooling performed on ooling the tool tip. The most effetive impingement ooling arrangement was found by hanging jet parameters systematially, while keeping the utting onditions onstant throughout all the tests. Previous researhers Jung-Yang San et al. [4] have examined the aspet of determining the most suitable jet diameter with respet to the air gap between the jet nozzle and a flat objet. However, in this appliation there are other fators that have to be onsidered suh as the position of the jet. Seletion of the jet diameter is important in order to find the orret gap between the jet and the tool, but it is now diffiult to position the jet lose to the tool tip. Basially there is a ompromise between the theoretial ratio of the jet height and diameter to suit the mahining appliation. Tool tip temperature ( o C) 215 195 175 155 135 115 95 75 5 7 9 11 13 15 Nozzle gap (mm) Figure 3. Jet optimum height from tool tip [7]. 0.8 Mpa 1.0 Mpa 0.6 Mpa The temperatures of the three thermoouples in the tool tip showed the lowest temperature being reoded by hannel 13 (Ch13), whih is the position nearest to the tool tip, whih indiates heat from the tool is being removed. Tool tip temperature ( o C) 180 170 160 150 140 130 120 110 100 90 80 0 5 10 15 Nozzle gap (mm) Ch13 Ch14 Ch15 Figure 2. Adjustable Impingement nozzle holder. The effet of varying the jet gap, with the different supply pressures of 1 MPa, 0.8 MPa and 0.6 MPa, is shown in Fig. 3 and 4, where it learly shows that a jet height of 10 mm from the top fae of the tool is the most effetive ooling height. As the jet diameter is 2 mm the resulting optimum (diameter to jet gap) G D optimum ratio is therefore 5 whih is higher than the G D ratio based on previous researh of an impinging jet on a flat plate Lee et al. [5], Tong [6], proving that this previous theory may not be suitable for this mahining appliation. A number of fators suh as the shape of the surfae of the tool or the effet of the rotating workpiee disturbing the air steam from the jet may have affeted the performane of the impingement proess. Figure 4. Reorded temperatures at the tool tip for a number of air gaps at 0.8 MPa [7]. The 2 mm diameter air-ooling jet at 0.8 MPa was able to ool the tool tip lose to the onventional wet ooling utting temperature (100 o C) as shown in Fig. 5. However, although this method is apable of ooling the tool tip, it was found that after a number of utting tests the temperature in the workpiee beame extremely hot. This therefore indiates that additional air-ooling needs to be applied to the workpiee to prevent dimensional inauraies ourring. Fig. 3 showed that there was no real benefit in inreasing the pressure to 1 MPa from 0.8 MPa. Tool tip temperature ( o C) 120 100 80 60 40 20 Ch13 Ch14 Ch15 0 0 5 10 15 20 25 30 Time (s) Figure. 5. Tool tip temperatures at 0.8 MPa with an air gap of 10 mm [7]. 832
Tool tip temperature ( o C) 145 140 135 130 125 120 115 110 105 100 0 20 40 Time (s) one 2 mm dia hole three 2 mm dia holes three 1.1 mm dia holes Figure. 6. Tool temperatures for thermoouple loses to the tool tip [7]. Tool temperature ( o C) 4 3 2 1 0-1 0 5 10 15-2 -3-4 -5 Time (s) Ch13 Figure. 7. Tool temperature before utting using three 2 mm diameter jet [7]. Fig. 6 learly showed that the three 2 mm diameter holes of the jet produed the lowest tool tip temperature losest to the tool interfae during mahining. The volume of air used during this test was 849 SLPM at the maximum test pressure (1MPa) if it was neessary. This is not an extremely large supply of ompressed air as a typial two-stage, two-ylinder air ompressor an provide 1050 SLPM at a maximum working pressure of 1MPa with a Motor load of 7.5 kw. This would be relatively heap to supply as an be alulated from (1). The ost of ompressed air for one hour was alulated to be (AU) $1.5 using this ompressor. Cost (AU) $ = ( MLxOHxCxTxFL ) ME Where: ML = Motor full load in kw OH = Number of operating hours C = Cost per kwh (0.16kWh) T = % of time running at this operating level FL = % of full-load at this operating level ME = Motor effiieny at this level For air-ooling the main onsideration for environmental onsideration is the volume of air being passed over the tool tip, therefore the CO 2 produed ooling an be alulated from (2). e CP e (1) C = CUT CS (2) Where: C e = environmental burden of oolant (air) ($/kg) CUT = oolant usage time (s) CS = utting fluid disharge (l/s) CP e = environmental burden of oolant prodution (kg-co 2 /l) Figure. 8. Frost on impingement jet nozzle before mahining. The tool temperature in Fig. 7 was obtained before any mahining had taken plae and learly indiates the effetiveness of the impingement ooling of the tool tip. Fig. 8 shows a build up of frost on the jet nozzle. In pratie, however, the effiieny of the ooling of the tool tip is redued due to the utting ation during mahining. B. Vortex Cooling The Unlike the impingement ooling proess there is no differene to the ooling effet by adjusting the distane that the old tube nozzle is from the tool tip. The nozzle does not need to be perpendiular to the rake fae - unlike the impingement jet nozzle - to be effetive. Although, the loser the old tube nozzle jet is to the tool fae, the less time there is for the surrounding ambient temperature to begin to affet the old stream of air. Fig. 13 shows the vortex tube direting the old airflow onto the tool fae as lose as the mahining restraints will allow. The vortex tube shown in Fig. 9 was produed for ooling the tool tip during mahining, and was designed from the original onepts of the Ranque-Hilsh Tube [8]. A vortex tube onsists of three important parts: the old tube, hot tube and the vortex generator. When ompressed air enters the inlet it is direted tangentially to the vortex generator, ausing it to spin around produing a spiral airflow. Between the Vortex Generator and the old tube there is a diaphragm fitted - with a entral hole whih an be easily altered - allowing diaphragms with large or small hole to inrease or derease the temperature obtained at the old exit. 833
where Δ H is obtained from: ΔH COP = (4) W Figure. 9. Setional view of the vortex tube showing the diaphragm. The volume and temperature of these two air streams an be hanged by adjusting the onial valve built into the hot air exhaust exit. It was possible to obtain temperatures as low as - 46 C and as high as +127 C when using the vortex tube under test onditions. The ompressed air pressure was set at 0.206 MPa, 0.275 MPa, and 0.344 MPa respetively to investigate how air pressures affet the vortex tube performane during mahining [9]. Obviously, the higher the air pressure the higher the ost of produing the ompressed air. Therefore the parameters that use the lowest pressure while produing the oldest temperature should be used to aid redutions in the operating ost of air-ooling. The oeffiient of performane estimation for any ooling devie an be used to give a good measure of the pratial refrigeration performane of the ooling system. This an therefore be used in determining the performane of the vortex tube, whih an in turn be ompared with a onventional refrigeration system. The vortex tube s oldest airflow needs to be used in determining the oeffiient of performane for this vortex tube. In this ase, the lowest old temperature is obtained from the vortex tube with a 3 mm diameter jet, and inlet air pressure 0.275 MPa. The lowest temperature measured was T = -16.8 o C. The related values for this temperature are given in Table 1: ( T T ) ΔH = m (5) i The oeffiient of performane for the vortex tube an now be alulated by using (4), whih gave a value of 1.38. The temperatures shown in Fig. 10 were reorded at the tool tip during adjustment of the onial valve on the vortex tube at an inlet pressure of 0.8 MPa. Setting the valve normally takes about thirty seonds to ahieve the optimum old exit air. Having obtained the minimum air temperature, mahining an ommene. During this time the old air had already started to ool the tool tip down, with two of the thermoouples reording temperatures of approximately 5 o C. Reduing the inlet air pressure to 0.4 MPa the old exit air temperature was approximately -15 o C, whih orrespondingly redues the tool tip temperature down to approximately -3 o C in double the time ahieve by 0.8 MPa. The time fator is important in ooling the tool tip, as the rise in temperature during mahining is extremely fast. It is therefore imperative to be able to dissipate the generated heat in the tool tip as quikly as possible. TABLE 1 Reorded readings from test Parameter Value Cold Mass Fration 0.605 Inlet Temperature ( o C) 22.4 Cold Outlet Temperature ( o C) -16.8 Hot Outlet Temperature ( o C) 66.6 Inlet Volumetri Flow Rate (SLPM) 1095 Hot Outlet Volumetri Flow Rate (SLPM) 425 Cold Outlet Volumetri Flow Rate (SLPM) 651 To determine the oeffiient of performane of the vortex tube for these onditions, it is neessary to alulate the ompressor exit temperature T 2 (314 K) by using (3) when: P 1 = P Ambient = 101.325 kpa, T 1 =T Ambient = 296 K, P 2 = 376.35 kpa and n =1.4 and W in the present ase is the work done to ompress the air from atmospheri pressure, and temperature to the inlet onditions of the tube. Assuming reversible ompression (isentropi, minimum work), W is then obtained from: The vortex tube used a 3 mm diameter orifie plate at a pressure of 0.8 MPa Figure. 10. Tool tip temperatures before mahining [10]. When the vortex exit air has reahed approximately -30 o C, mahining is ommened. As indiated by the rise in the tool tip temperatures, the tip rises to a steady state temperature of 60 o C as shown in Fig. 11. The temperature drop at the end indiates the point when the feed is stopped with no more hips being generated. Allowing the ooling air to flow unimpeded aross the tool tip gives a better heat dissipation from the tool reduing the temperature in the tool rapidly. ( T ) mr T2 1 n W = (3) n 1 834
produe an effetive heat exhanger to ool the air. This ooling system an provide ooling air of different temperatures by supplying the thermopile with different urrents. Figure. 11. The graph shows the tool tip temperatures during mahining, and after the tool feed has stoped utting [10]. Fig. 11 illustrates that a steady state temperature is reahed in the tool tip in approximately 10 seonds during mahining when being ooled by the vortex tube. Fig. 12 showed that the vortex tube used in these tests was able to generate very old air at the exit of the old jet. A temperature of -55 o C was obtained by using an inlet pressure 1MPa to the vortex generator. Nozzle exit temperature (o C) 10 0-10 0 100 200-20 -30-40 -50-60 Figure. 14. The temperature of ooling air against time when supplying the thermopile with different urrent [1]. Fig. 14 shows the temperature of ooling air against time for different urrents being supplied to the thermopile. From the graph it is shown that the temperature of ooling the air reahes a steady value after 3 minutes, indiating that this system has a high refrigeration speed and stability. Time (s) Figure 12. Vortex tube old air outlet temperature [10]. Figure. 15. The temperature of ooling air against time for various air supply pressure [1]. From the graph shown in Fig. 15 it an be seen that an inrease in the air supply pressure leads to a redution in the temperature of the ooling air. It is also shown in Fig. 16 that it only takes three minutes to adjust the temperature of the ooling air. Figure. 13. Frost forming on vortex tube old outlet. C. Refrigeration System A ooling system ombining a vapour-ompression refrigeration system, and semiondutor refrigeration effet was adopted by Y. Su et al. [1] to produe old-air used for ooling tool tips when metal utting. A vapour-ompression refrigeration system was designed to redue the temperature of water in order to improve the effiieny of semiondutor ooling. By applying a diret urrent to a thermopile heat energy is transferred from one side to the other side of the semiondutor due to the Peltier effet. The ombination of the old water and semiondutor ooling is now used to Figure. 16. Response urve of the new ooling gas equipment when adjusting the temperature of ooling air ontinuously [1]. 835
III. CONCLUSION Previous researh has shown that old air used in metal utting improves the tool life to be just as effetive as traditional ooling methods [11]. Current work investigated three separate methods of providing old air to the tool tip, to determine the most sustainable and effetive method of removing the heat generated during metal utting. Even though the impingement ooling method removed heat from the tool tip, it was found to be impratial to be used in a manufaturing environment due to the neessary orret positioning of the nozzle. The vortex ooling method has no restritions on the positioning of the ooling nozzle making this a more user friendly system for the mahine operator to use in prodution. However, the high air mass flow rate of the vortex tube may in first instane be onsidered a disadvantage to this ooling method. This has been found not to be the ase as the exess ooling air removes the heat from the mahined part ensuring dimensional auray of the utting proess. The third ooling method onsidered used a vapour-ompression refrigeration system and semiondutor thermopile to ool the ompressed air that is direted at the tool interfae. Y. Su et al. [1] have shown that heat is removed from the tool tip during metal utting, however having less old air presented to the utting zone leads to heat not being removed as effetively from the mahined part, whih inreases after eah ut. The impingement ooling method of providing ooling air is rejeted on two ounts. Firstly it is unwieldy to use and uses the most energy, making it the least sustainable method of ooling the tool tip. The final two methods are both userfriendly in positioning the air flow or to make slight adjustments to the old air temperature. Although, there is an additional ooling plant needed for the thermopile ooling system and the additional ost to the environment to manufature the semiondutor materials used in this system. The most eonomial use of energy when these two ooling methods are used to produe old air for utting the same parameters needs to be used. The parameters that were taken for omparison were: old air temperature of -16 o C at a flow rate of 650 SLPM, the resulting test and alulations revealed that the vortex tube used the least energy as it only needed a soure of ompressed air. While the thermopile ooling system needed additional energy for the auxiliary equipment used in ooling the ompressed air. Therefore, for ease of use, no additional equipment was required and used the least power. The vortex tube is learly an exellent method for ooling air for metal utting purposes. REFERENCES [1] Y. Su, N. He, L. Li, A. Iqbal, M. H. Xiao, S. Xu, and B. G. Qiu, "Refrigerated ooling air utting of diffiult-to-ut materials," International Journal of Mahine Tools and Manufature, vol. 47, pp. 927-933, 2007. [2] L. Chen, F. Hideo, N. Hirohisa, K. Hiroshi, H. Takao, and N. Takashi, "Development of Predition System for Environmental Burden for Mahine Tool Operation," JSME International Journal, vol. 49, p. 8, 2006. [3] I. T. Tokyo Eletri Power Company, "Sustainability Report 2007," Tokyo 2007. [4] J.-Y. San, C.-H. Huang, and M.-H. Shu, "Impingement ooling of a onfined irular air jet," International Journal of Heat and Mass Transfer, vol. 40, pp. 1355-1364, 1997. [5] J. Lee and S.-J. Lee, "The effet of nozzle onfiguration on stagnation region heat transfer enhanement of axisymmetri jet impingement," International Journal of Heat and Mass Transfer, vol. 43, pp. 3497-3509, 2000. [6] A. Y. Tong, "On the impingement heat transfer of an oblique free surfae plane jet," International Journal of Heat and Mass Transfer, vol. 46, pp. 2077-2085, 2003. [7] B. Boswell, "Use of air ooling and its effetiveness in dry mahining proesses," in Mehanial Engineering Perth: Curtin University of Tehnology, 2008, p. 192. [8] M. P. Silverman, "The Wirbelrohr's roar," Cambridge University Press, p. 20, 1993. [9] M. H. Saidi and M. R. Allaf Yazdi, "Exergy model of a vortex tube system with experimental results," Energy, vol. 24, pp. 625-632, 1999. [10] B. B. a. T. T. Chandratilleke, "Air-Cooling Used For Metal Cutting," Amerian Journal of Applied Sienes, vol. 6, p. 11, 2009. [11] J. Liu and Y. Kevin Chou, "On temperatures and tool wear in mahining hypereuteti Al-Si alloys with vortex-tube ooling," International Journal of Mahine Tools and Manufature, vol. 47, pp. 635-645, 2007. 836