An Effective Method to Determine the Optimum Parameters for Minimum Quantity Lubrication(MQL) Grinding

Transcription

1 5 th International & 26 th All India Manufacturing Technology, Design and Research Conference (AIMTDR 2014) December 12 th 14 th, 2014, IIT Guwahati, Assam, India An Effective Method to Determine the Optimum Parameters for Minimum Quantity Lubrication(MQL) Grinding Dinesh Setti 1*, Manoj Kumar Sinha 1, Sudarsan Ghosh 2, P Venkateswara Rao 3 1* Department of Mechanical Engineering, IIT Delhi, , dineshsetti@gmail.com 1 Department of Mechanical Engineering, IIT Delhi, , manoj.coet@gmail.com 2 Department of Mechanical Engineering, IIT Delhi, , sudarsan.ghosh@gmail.com 3 Department of Mechanical Engineering, IIT Delhi, , pvrao@mech.iitd.ernet.in Abstract The present trend in the manufacturing industry is to make machining processes more environment friendly by adopting topractices such as dry machining, minimum usage of cutting fluid, and usage of non-reactivegas based coolants etc., The concept of Minimum Quantity Lubrication (MQL)has been suggested by several researchers long time back as a cause for addressing the environmental, healthand economic issues related with conventional bulk cooling processes. The MQL technique consists of atomizing a very small quantity of cutting fluid into fine droplets. In other words, the effectiveness of MQL system depends on the quality of these droplets. This paper presents the combination of microscopy and image processing techniques to ascertain the quality of droplets,by spraying the MQL fluidonto an acrylic sheet. The raw droplet features arecaptured with stereo zoom microscopeandanalyzed by measuring the average droplets size and number of droplets per unit area. Reliable MQL application is achieved when the MQL parameters like the distribution of the droplets,the applied carrier gas air pressure, fluid flow rate, nozzle tip to machining zone distance, and the angle at which the nozzle keptis optimal.the optimal MQL conditions as obtained from theseresults have been usedto improve the grindability of Ti-6Al-4V. Keywords: MQL, image processing, grinding, droplets 1 Introduction It is well knownthat the heat produced during the grinding process is very significant in terms of component quality. Relatively high friction in grinding causes heat generation, which leads to the thermal damage; cooling and lubrication consequently play a decisive role during grinding. Despite many advantages of the cutting fluids in the machining processes, there is serious concern about ecological and economic problems. Hence, several researches demandedin the last few years to minimize or even to eliminate the use of cutting fluids. This objectiveleads to the development/adaptation of sustainable alternativesfor reducing the consumption of cutting fluids in manufacturing processes. A large number of research works on this field can be found in the literature (Weinert et al.,2004). Techniques such as MQL, dry or near-dry machining, cryogenic machining is being used, especially in processes such as turning, drilling or even milling.however, now a days, notable number of researcherson track to experiment these things in grinding process also. One such alternative technique is being given more attention in grinding process is MQL. Typically, an MQL system supplies about few milliliters of cutting fluid per hour with pressurized air or other supplemental gases, whereas a conventional system supplies about several thousand ml/min cutting fluid. The conventional flood supply system demands more resources for operation, maintenance, and disposal, and results in higher environmental and health problems. The main benefits of MQL are the following ones (Barczak et al.,2011): 1. Environmentally friendly coolant delivery system: less waste disposal, biodegradable fluids, no pollutants, reduced power consumption; 2. Reduced hazard to operator health and working environment; 3. No need for component cleaning before further processing; 4. Cleaner and safer work place; 5. No unwanted thermal shock for workpiece and tool; 6. Reduced storage space requirements; 8. No need for an expensive coolant. From both ecological and economical point of view MQL is highly desirable. However, MQL relatively new concept in grinding processes and there is a lack of information regarding the effectiveness of MQL parameters in grinding. It is not yet possible to answer questions such as: Which kind of cutting fluid performs best with MQL? What should be the optimum standoff distance between nozzle and grinding zone? What should be the optimum flow rate? Therefore, it is hard to say the success of MQL without counting the above questions. Several number of publications explained the realization of MQL application in grinding operation and the summary is given in Table

2 An Effective Method to Determine the Optimum Parameters for Minimum Quantity Lubrication(MQL) Grinding Author Table 1 Summary of available literature on MQL application in grinding MQL Fluid flow rate (ml/h) MQL parameters Air pressure (bar) Nozzle distance (mm) Spray angle Objective Nguyen et al.,2003 Soluble Coolant F, SR, H, RS da Silva et al.,2007 MQL oil SR, H, RS, M Shen et al.,2008 Synthetic oil,nanofluids,pure water F, SR,G-Ratio,T Tawakoli et al.,2009 MQL oil F, SR, SM, CM Sadeghi et al.,2009 Veg. oil &synthetic oil , 4, 5, F, SR, H, SM, M Barczak et al.,2010 Synthetic oil F, T, SR Sadeghi et al.,2010 Veg. &synthetic oil , 3, 4, 5, F, SR, H, SM, M Tawakoli et al.,2010 MQL oil 20, Influence of nozzle position, air 2, 3,4,7 50,100 pressure on MQL performance Tawakoli et al.,2011 Various oils F, SR, SM, CM Barczak et al.,2011 Pure synthetic oil F, SR, T Morgan et al.,2012 Pure synthetic oil T Mao et al.,2012 Pure oil,oil + water F, SR, T, SM,M Lee et al.,2012 Nanofluid F, SR, SM Mao et al.,2012 Al 2 O 3 nanofluid F,SR, T, SM Kalita et al.,2012 Oils with MoS 2 micro particles Paraffin oil with MoS 2 micro & nano particles SE, µ SE, µ, M,Gratio,T Kalita et al., Influence of MQL spraying direction, 2, 4, 6, Mao et al.,2013 nanofluid 60 spraying distance, air pressure on 8 performance 60, 80, Balan et al.,2013 MQL oil 2,4,6 F, SR, T, Droplet size & Velocity 100 F Forces, SR Surface roughness, SM Surface Morphology, CM Chip Morphology,T Temperature, H Hardness, RS Residual Stresses, M Microstructure, SE Specific energy,µ - Coefficient of Friction From the literature it has been observed that many researchers proved the applicability of MQL technique in grinding. But, very few researchers considered the account of optimal MQL parameters such as fluid flow rate, carrying gas/air pressure, spraying angle, standoff distances between the nozzle and grinding zone etc. The competence of any aerosol spray system depends upon the quality of the mist produced from the system, which will significantly affected by the above mentioned parameters. Number of droplets, droplets size and distribution are the main parameters which governs the mist quality.tawakoli et al.(2010) studied the influence of MQL parameters on grinding performance and their results showed that the setting location of the nozzle is an important factor and the efficient transportation of oil droplets to the contact zone requires higher mass flowrate of the oil mist towards the grains flat area and longer deposition distance of an oil droplet.mao et al.(2013)also investigated the grinding performance of nanofluid under different spraying parameters and they concluded that: the air pressure is critical in order to enhance the nanofluid mist to penetrate into the grinding zone. The grinding forces, surface roughness, and grinding temperature are decreased with the increase of the air pressure. They also found that mist size is increased with the increase of spraying distance and mist velocity is decreased along the spraying direction. Therefore, the grinding performance in the shorter spraying distance is better than that in the longer spraying distance. For an effective MQL system, Tawakoli et al.(2010) and Mao et al.(2013)outcomes can be summarize as : Nozzle location is an important factor 67-2

3 5 th International & 26 th All India Manufacturing Guwahati, Assam, India Technology, Design and Research Conference (AIMTDR 2014) December 12 th 14 th, 2014, IIT The spraying distance should be optimum Higher air pressure is required Higher mass flow rate of mist Based on thesenear optimum conditions, droplets quality may vary from fluid to fluid depends upon the viscosity. In order to determine these e optimum values for new cutting fluid or concentration one has to do extensive experimental work. Hence, in this paper we presented the combination of microscopy and image processing techniques to ascertain the quality of droplets by measuring the average droplets size and number of droplets per unit area in an easyapproach. Moreover, force measurement method also used to determine the optimum spraying distance. In this work, we adopted the conceptdeveloped by Park et al.(2010) to collect and study the droplets distribution. 2 Experimental Method The experiments have been carried out in three stages to determine the optimum spraying distance, droplets quality and nozzle position successively. with the increase of spraying distance but after this optimum distance mist velocity is decreased and the complete mist is unable to reach the targeted surface. Figure 2 Variation in force along with standoff distance 2.2. Droplets Quality In order to collect the droplets on targeted surface, very slow table movement has been given (1m/min). This movement remains same for all other experiments and the standoff distance maintained as 72mm for all experiments. The fluid flow rates varied from 50, 100, 150, 200, 250ml/h. At each flow rate the droplets have been collected on an acrylic surface and captured with stereo zoom microscope. The captured images converted into bitmap images. For identifying the edges of the droplets, threshold operation has been performed. For droplets analysis purpose, in ImageJ software analyze particles module has been used.the general procedure to calculate the droplet size and distribution is presented in Figure 3. Figure 1 Setup for optimumm distance experiments 2.1. Optimum spray distance Indigenously developed MQL system wasused to spray the soluble oil with 1:20 ratio. The MQL droplets were sprayed onto anacrylic sheet. The set-up for MQL spray experiment is shown in Figure 1. Kistler 9257B dynamometer has been used to measure the force exerted by the spray. The standoff distance varied from 10mm to 150mm with 10mm interval, at each distance, the flow rates varied from 50, 80, 100, 150, 200, 250 ml/h. All the experiments were conducted at an air pressure of 8bar. Figure2 shows the variation in force along with standoff distance. It can be observed that at a distance of 72mm the MQL system is able to deliver maximum force output. Because mist size is gradually increased Microscopic Image Converted to bitmap image using ImageJ software 67-3

4 An Effective Method to Determine the Optimum Parame eters for Minimum Quantity Lubrication(MQL) Grinding Image after threshold operation iii. An angular position 15 degrees to the grinding wheel centerline. Number of droplets in measured area Total area covered by all 49 droplets in sq. microns Average area of droplet in sq. microns Percentage area covered by droplets A = π (r ) 2 avg avg r = 13.57µm avg Figure 4Several positions of nozzle to overcome the air boundary effect (Ebbrell et al.,2000) Analyze Particle module output Figure 3 Procedure to determine the droplets quality Table 2 gives the summary of experimental results conducted for the droplets quality. It has been observed that, size of droplets increases with increase in flow rate. For an efficient MQL system, there should always be more number of droplets, maximum surface area should be covered by fluid and size of droplets should be minimum. Hence, form the observed results it can be found that, for selected soluble oil with 150ml/h flow rate is able to generate mist with good quality. Table 2 Results of droplets quality experiments MQL Flowrate (ml/h) Number of droplets Average radius of droplets (µm) %Area covered by droplets Nozzle Location To overcome the influence of surrounding air boundary by the cutting fluid, in literature researchers suggested three possible nozzle locations. They are: i. Tangential nozzle position ii. An intermediate position where the nozzle was raised above the area of reversed flow Figure 5 Delivery of MQL mist with Tangential, Intermediate angular and intermediate positions 67-4

5 5 th International & 26 th All India Manufacturing Guwahati, Assam, India Technology, Design and Research Conference (AIMTDR 2014) December 12 th 14 th, 2014, IIT In order to observe the mist reachability to the grinding zone, several images have been captured by keeping the nozzle at above mentioned locations when the wheel is rotating. From the captured images (Figure 5), it has been observed that, MQL is able to overcome the surrounding air boundary at tangential and intermediate angular positions. The reason for this can be explained based on the Figure 6. Very near to grinding zone, the effect of surrounding air boundary can be overcome by high pressure jet, because the distance to be travelled is minimumaway from grinding zone, in order to take air boundary assistance, the nozzle should be inclined in air flow direction. But when the workpiece length is more than the optimum nozzle standoff distance, the tangentially positioned nozzle may get disturbed by the work material. Figure 6 Simulation studies of surrounding air boundary velocity direction (Ebbrell et al.,2000) Based on the above series of experiments, the optimum parameters for the present MQL system can be summarized as: Nozzle standoff distance: 72mm Air pressure: Higher is desirable, 8bar MQL flow rate: 150ml/h Nozzle position: Intermediate angular position (15 ) 3 Validation of Optimum conditions For validating the above mentioned optimum conditions, actual grinding experiments have been performed on a Chevalier Smart H1224 CNC Surface grinder with conventional SiC abrasive wheel on Ti- effect of 6Al-4V material to observe the anti-frictional MQL system. The wheel was Silicon carbide grinding wheel (Carborundum Universal Ltd., India, CG60K5V8). The size of the workpiece is 70mmX70mmX15mm. The size of the wheel is 340mmX50mmX127mm. To maintain the wheel topography uniform, dressing operation was performed before every experiment with single point diamond dresser with the following parameters: dressing depth - 10µm, dressing lead - 100mm/min, and number of passes 4. During the grinding operation, the wheel cutting speed maintained as15m/s, work table speedas 9m/min and given depth of cut is 5µm. The experiments were conducted under three different environments: dry, wet (soluble oil, 1:20 ratio), MQL with soluble oil 1:20 ratio. Grinding forces were measured online using a piezoelectric dynamometer (Kistler, Switzerland, 9257B), coupled to charge amplifier (Kistler, Switzerland, 5070 multichannel) and computer data acquisition dynoware software. For MQL system, the optimum conditions mentioned in section 2 has been maintained. Moreover, the MQL flow rate has been varied from ml/h. The ratio between tangential to normal forces taken as the coefficient of friction. The readings has been taken after 60 passes. 0.7 Coefficient of Friction Dry Wet Number of Passes Figure 7 Variation in coefficient of friction with number of passes From the Figure 7, it can be observed that, MQL with derived optimum flow rate condition has been able to give better friction reduction nature than the other conditions. Conclusions The following conclusions can be drawn from the present work: The significant MQL parameters like, nozzle stand of distance, MQL flow rate in terms of droplets quality, nozzle location has been studied. The experimental technique to measure the droplet sizes and distribution for MQL combining microscopy and image processing analysis to be successful. It has been observed that, size of droplets increases with increase in flow rate. Hence, form the observed results it can be found that, for selected soluble oil with 150ml/h flow rate is able to generate mist with good quality. Nozzle intermediate angular position (15 ) able to overcome the influence of surrounding air boundary by the cutting fluid. 67-5

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