- Email: [email protected]
Evaluation of mist flow characteristic and performance in Minimum Quantity Lubrication (MQL) machining
Accepted Manuscript Evaluation of mist flow characteristic and performance in minimum quantity lubrication (mql) machining Erween Abd Rahim, Hemarani Dorairaju PII: DOI: Reference:
S0263-2241(18)30191-X https://doi.org/10.1016/j.measurement.2018.03.015 MEASUR 5334
To appear in:
Measurement
Received Date: Revised Date: Accepted Date:
9 June 2016 5 December 2017 5 March 2018
Please cite this article as: E. Abd Rahim, H. Dorairaju, Evaluation of mist flow characteristic and performance in minimum quantity lubrication (mql) machining, Measurement (2018), doi: https://doi.org/10.1016/j.measurement. 2018.03.015
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
EVALUATION OF MIST FLOW CHARACTERISTIC AND PERFORMANCE IN MINIMUM QUANTITY LUBRICATION (MQL) MACHINING Erween Abd Rahim1, Hemarani Dorairaju Precision Machining Research Center (PREMACH), Faculty of Mechanical and Manufacturing Engineering, Universiti Tun Hussein Onn Malaysia, 86400 Parit Raja, Johor 1
Corresponding author: [email protected]
Abstract Minimum Quantity Lubrication (MQL) is an alternative method to supply the cutting fluid in the formation of mist. MQL has proven to reduce machining cost and increase machining performance. The effectiveness and the working principle of MQL are still questionable with very few explanations provided. The present study is conducted to investigate the performance of MQL technique with different combination of spray and machining parameters. The Phase Doppler Anemometry (PDA) was used to characterize the lubricant spray under different input pressure for various nozzle outlet diameter of 2.5mm (OD25) and 3.0mm (OD30). This device can measure the amount of droplet and size. From these results, the distance of the nozzle to the cutting tip can be estimated. The turning performance in terms of cutting force and cutting temperature was evaluated under three levels of cutting speed and two levels of feed rate and at a constant depth of cut. The result shows that the most suitable mist flow pattern during machining was the largest spray cone angle supplied under 0.4 MPa input air pressure. In addition, significant reduction of cutting force and cutting temperature were obtained when using OD30 nozzle at the nozzle distances of 6 to 9 mm and the input air pressure of 0.4 MPa.
Keywords: Droplet size, Minimum Quantity Lubrication (MQL), Phase Doppler Anemometry (PDA), Turning
1. Introduction
Machining process is greatly influenced by cutting fluid which is used to reduce the heat generated during machining process. Water was the first cutting fluid used in machining process and was replaced with other fluids as more studies done on the ability of water to perform as cutting fluid. Cutting fluid is widely used in machining process due to its ability to cool, lubricate and to convey the chips away from the cutting zone. As a result, it improves the tool life, protects the workpiece and tool from corrosion, improve surface finish and dimension accuracy of a product. Cutting fluid had acquired sufficient and proved to be necessary adjunct in high speed machining of difficult material such as the exotic steels and specially alloyed nonferrous metals. As the new methods of machining are constantly being developed, the cutting fluid industry provided machinist with a choice of several cutting fluid. Straight mineral oils, combinations of mineral oils and vegetable oils, animal fats, marine oils, mixes of free sulphur and mineral oil are commonly used as cutting fluids and suds [1]. Several cooling techniques have been introduced for to reduce the cutting temperature and improve the machinability during the machining process. Flood coolant method has been used extensively since cutting fluid was introduced in the machining industry. Cutting fluid is delivered excessively to cool and lubricate the cutting tool and workpiece subsequently reduces the heat generated at the tool-chip interface. Near dry cutting also known as minimum quantity lubricant (MQL) is another method of cooling. In MQL method a small amount of cutting fluid is carried by air jet directly to the cutting zone. The mists are produced under the atomization process and are supplied to the cutting zone [2, 3-7. The lubricant is directly sprayed into the tool-chip interface thus guarantees a good level of lubrication [8]. The chip produced is flushed away from the cutting zone due to the action of compressed air. The machined surface is almost completely dry after machining process. The compress air not only contributes in transporting the oil particles, but it also helps in the cooling and chip removal process. The high pressure jet cutting fluid with tiny droplets of oils decreases the friction between chip and the rake face during machining process. The tiny oil droplets penetrate into chip-tool interface thus reduces the cutting temperature and improves tool life to some extent [9]. From the previous research it was found that machining performance by using MQL towards cutting temperature and cutting force are affected by the lubricant type, flow rate, the distance between nozzle and tool tip, and the workpiece material [10,11]. All these parameters are found to be dependent on the work material and process conditions. Only the appropriate oil
quantity and appropriate distance between the nozzle to the tool tip provides the optimum process condition. Therefore, the correct amount of lubricant to be applied, type of lubricant to be used, and the position of the nozzle should be considered. For example, it was assumed that the oil can penetrate the tool-workpiece interface due to high pressure of the compressed air that serves as a vehicle for the oil droplets. No comparison was made between the contact pressure at this interface and that of compressed air. These important parameters are not reported in many research documents and papers on the MQL. Even though MQL is known for its many benefits but no one was able to prove that the statement is true or no one have ever suggested a systematic procedure to prove MQL’s efficiency. Insufficient recommendations can cause difficulties to make proper choices of the equipment and parameter in machining. The effectiveness and the working principle of MQL are still questionable with very few explanations provided. Meanwhile, the nozzle design and most preferable flow pattern have to be selected in order to achieve the best particle size for excellent penetration at chip-tool interface. The number of previous study on specific nozzle design and the best mist flow pattern are very limited thus it is difficult to explain on how MQL have greater cooling ability than conventionally supplied metal working fluid. Thus selecting the best design for nozzle head, nozzle distance from the tool tip and most desirable flow pattern for generating the best particle size would be a difficult task. The purpose of this study is to characterize the mist flow pattern and behaviour in order to improve the turning performance.
2. Experimental details 2.1 Measurement of Mist Spray Phase Doppler Anemometry (PDA) was used to characterize the lubricant spray under different input pressure for various nozzle outlet diameter of 2.5mm (OD25) and 3.0mm (OD30). This equipment is capable to measure the droplet size from 0.5 µm to 2500 µm with the estimated uncertainty to be ±2%. The system is equipped with transmitter and receiving optics and a traverse system as shown in Fig. 1. In order to characterize the lubricant spray at different distance, traverse system was used to move the transmitter probe and receiver probe. To ensure the measurement locations are around the spray cone, the spray cone photo of the lubricant (synthetic ester) was captured at the highest input air pressure of 0.4 MPa. The size of the cone
was measured under the highest input air pressure and the mesh was generated using the BSA Flow Software. Fig. 2 shows the measurement location of the spray. The measurement was only taken along the half of cone because it is assumed that the spray pattern is symmetrical. The experiment must be repeated in a short period of time to avoid any errors during measurement process. The standard deviation was calculated using Equation 1. The measurement was conducted at the input air pressure of 0.2, 0.3 and 0.4 MPa supplied by the compressor. The measurement was taken after the lubricant spray becomes stable with the air pressure valve regulated at desired input air pressure.
(1)
Where
σ = is the standard deviation N= the number of sample xi = mean value from all three measurement µ = value from measurement
Fig. 1. Schematic diagram of the measurement procedure setup
Fig. 2. Measurement location of the spray
Fig. 3. Lubricant spray image
The lubricant spray characteristics under various input air pressure for both nozzles (OD25 and OD30) was captured using DSLR camera in order to estimate the wide spread and the maximum length of the spray. It was done inside the acrylic chamber, fully covered with dark backgroud and combine lighting system to capture the spray image as shown in Figure 3. From
this image, the cone angle under different input pressure was measured more effectively by using a software. In addition, from the spray image, the measurement of distance using PDA can be roughly estimated.
2.2 Machining Setup The turning process was performed using a NC Harrison A400 Alpha lathe machine. Fig. 4 and Table 1 showed the experimental setup to measure cutting force and temperature. Cemented carbide TNGG220408R insert with 60 degree triangle shape was used as the cutting tool in this experiment. AISI 1045 medium carbon steel was used as the workpiece. The diameter and length of the workpiece were 100 mm and 200 mm respectively. The Kistler 9257BA dynamometer was mounted on the bed of the lathe machine. Meanwhile, the maximum cutting temperature was captured via thermal imager camera (FLIR T640) in a temperature range of 250 to 600 °C during the turning operation. It was fixed by placing the camera facing in the axial direction, near to the cutting zone and focused at the tool-workpiece interface. The measurement was taken for the highest, lowest and mean temperature. The cutting tool emissivity, Ɛ was set at 0.32 and calibrated within the measured temperature range. Mist flow during machining process was supplied using Kuroda KEP-R MQL atomizer unit. This unit has a capability to mix the lubricant with the high pressure compressed air to form mist under atomization process.
Table 1 Cutting parameter Cutting Parameter
Values
Cutting speed, Vc (m/min)
100, 160, 220
Feed rate, fr (mm/rev)
0.15, 0.30
Depth of cut, d (mm)
0.2
Coolant method
MQL
MQL Input Air Pressure (MPa)
0.2, 0.3, 0.4
Nozzle outlet diameter (mm)
2.5, 3.0
Nozzle distance (mm)
3, 6, 7, 9
Fig. 4. Schematic diagram of machining setup
The same nozzles as mentioned earlier were used in this experiment. The distance of nozzle was measured from the cutting tool to the outlet of the nozzle as shown in Fig. 5. The distances were determined from the results obtained from the PDA. Nozzle distances of 3, 6, 7 and 9 mm was used for both OD25 and OD30 nozzles. In addition, the nozzle was setup with an inclination of 450 from the cutting insert to ensure the lubricant spray can be more focused to the cutting edge. A synthetic ester was chosen as a lubricant because it provides an excellent cutting performance even in small quantities. It is also have outstanding eco-friendly compatibility. Synthetic esters have similar biodegradability and safety as the main components of vegetable oils, thus exhibits less negative impact to the people or environment.
Fig. 5. Nozzle setup for machining process at 45° inclination angle
3. Results and discussion 3.1 Formation of Cone Angle from the Mist Spray Spray cone angle was defined as an angle formed between the nozzle tip and two lines that delineate the maximum outer region of the injected spray. The spray cone angle was created owing to the nozzle design and input air pressure. The mist spray pattern was captured for OD 25 and OD30 nozzles under various input air pressure of 0.2, 0.3 and 0.4 MPa and the results are presented in Figs. 6 and 7.
It is clearly shown in Fig. 6 that the spray cone angle becomes wider as the input air pressure increases from 0.2 MPa to 0.4 MPa for both OD25 and OD30 nozzles. OD25 nozzle recorded the spray cone angle of 10.4°, 13.7°, and 15.7° at the input pressure of 0.2 MPa, 0.3 MPa and 0.4 MPa respectively. Meanwhile, OD30 nozzle recorded 12.8°, 15°, and 17° at the input pressure of 0.2 MPa, 0.3 MPa and 0.4 MPa respectively. In the case of OD25 nozzle, it can be seen that the cone angle increases 51% from 0.2 MPa to 0.4 MPa. The OD30 nozzle shows an increase of 33% from input air pressure of 0.2 MPa to 0.4 MPa. When comparing with both
nozzles, OD30 nozzle showed an increment of 23%, 9.5% and 8.3% at the input air pressure of 0.2, 0.3 and 0.4MPa respectively. The increase of spray cone angle for both OD25 and OD30 nozzles was due to the increase of the input air pressure. Ortman and Lefebvre [24] reported that the increase in pressure starting from atmospheric pressure resulted in the spray cone angle at first to widen but later contracted. However, this phenomenon does not occurred due to the low input air pressure supplied during the experiment. The same effect of pressure on the cone angle was discussed by Rashid et al. [12] in his research using water as the cutting fluid. The result shows that the hollow cone spray angle increases when the input air pressure increased. They also observed that the length of water sheet ligament decreases as the injection pressure increased and that increasing injection pressures leads to wider spray cone angle. Basically, the spray cone angle is mainly influenced by the properties of liquid, size of atomizer and input air pressure. Input air pressure during atomization process is a major parameter that plays significant role in the formation of spray cone angle. It was suggested that the compressed air generates high potential energy to the liquid during atomization process. The kinetic energy of the liquid and geometry of the nozzle causes the liquid to emerge in small ligaments, subsequently breakup further into very small droplets. On the other hand, Sovani et al. [13] stated that the injection pressure or input air pressure gives a major effects in spray cone angle. They found that the spray cone angle under higher injection pressure of shows an increase in the cone angle. When the injection pressure is increased again, the range spanned by cone half-angles is shifted further upward and the spray cone angle range increased even more than before. At the highest injection pressure studied, the cone half-angle continues to increase with an increase in injection pressure. It can be assumed that the angle of the spray cone widened when injection pressure was raised. The increase in the cone angle will increase the extent of the surrounding air which leads to improved the atomization. Galván et al. [14] also stated that when the input air pressure increases the spray cone increases in their study on effect of the spray cone angle in the spray cooling with R134a. Meanwhile, Rashid et al. [12] stated the most numbers of inlet slots produces the widest spray cone angle while the atomizer with the least numbers inlet slots produces the narrowest spray. The more inlet slot tends to increase the azimuthal velocity inside the swirl chamber. However, the increment of spray cone angle as the inlet slot number increased becomes less significant at lower injection pressure.
The larger the nozzle outlet diameter the wider spray cone can be obtained under high input air pressure. The increase in the cone angle will increase the extent of the surrounding air which leads to improved the atomization.The area of the nozzle outlet diameter was calculated using the equation of circle area. The area of OD25 and OD30 nozzle are 19.6 mm2 and 28.3 mm2 respectively. With a larger area, more cutting fluid can be supplied under high input air pressure, hence improve the lubrication at the tool-chip interfaces during machining process. The velocity of liquid that emerges from the injector orifice increases as the input air pressure increases thus creates a finer spray. This in turn produces smaller droplets which are relatively lighter allowing the droplets to go farther hence creating wider spray cone angle. These can be observed from the spray cone angle of OD30 nozzle as shown in Fig. 7. It is noticeable that the spray cone angle of OD30 nozzle was wider than OD25 nozzle. Galván et al. [14] have revealed that the increase in the nozzzle diameter orifice can effect the spray cone angle. In their study, nozzle with larger diameter orifice shows a larger spray cone angle compare with a nozzle of smaller diameter orifice.
Fig. 6. Spray cone angle
Fig. 7. Spray pattern under various nozzles and input air pressure
3.2 Effects of Nozzle Diameter and Pressure on Droplet Distribution Figs. 8 and 9 showed the droplet distribution under the input air pressure of 0.2, 0.3 and 0.4 MPa for OD25 and OD30 nozzle at various axial distances. The total number of droplet count at different axial distance was plotted against axial distance of the nozzle. For OD 25 nozzle under the input air pressure of 0.2 MPa, 237 droplets were captured at axial distance of 2 mm. This is followed by 289 and 412 droplets at axial distance of 4 and 6 mm respectively. Meanwhile the droplet count for axial distance of 8, 10 and 12 mm were 518, 453 and 357 droplets respectively. Furthermore, the droplets count under input air pressure of 0.2 MPa for OD 30 nozzle as shown in Figure 9. The number of droplet at axial distance of 2 mm was 269 followed by 456 for axial distance of 4 mm. The number of droplet increases as much as 780 droplets at axial distance of 6
mm. But the number of droplets decreases as much as 425 droplets at axial distance of 8 mm. At axial distance of 10 mm, the number of droplet counts was 398 droplets followed by 249 droplets at axial distance of 12 mm. Generally, the bar graph shows an increasing trend in the overall number of droplet count as the input air pressure increases. On the other hand, the number of droplet count at each axial distance varies for all input air pressure. By measuring the number of droplet count at each axial distance, the nozzle distance during machining process can be estimated. The result shows that more droplets are concentrated at the range of 4 to 10 mm of the axial distance from the nozzle outlet for both OD25 and OD30 nozzle under different input air pressure. By comparing with typical drop size distribution graph, the results obtained for OD 25 and OD30 nozzle are similar. By knowing the number of the droplets count, the nozzle distance was predicted at four different axial distances of 3, 6, 7 and 9 mm to understand the effect of nozzle distance during the turning process. These values can be considered as the appropriate distances, whereby the concentration of droplets help to improve lubrication thus reduces the cutting force and temperature. Shorter distance of nozzle also enables the tiny particle of droplets penetrating into the tool chip interfaces, thus reducing the friction at the interface. It can be suggested that the nearest distance of 3 mm can be placed to the workpiece. However, the distance less than 3 mm causes a collision between the nozzle and the workpiece or chips during turning process. Therefore, the closest nozzle distance of 3 mm was selected to evaluate the effect of supplying cutting fluid. Meanwhile, nozzle distance of 6 mm and 7 mm was selected based on the number of droplets count for OD25 and OD30 nozzle at different input air pressure. From the results, most droplets were concentrated at the axial nozzle distance of 6 and 8 mm. Meanwhile, nozzle distance of 7 mm was selected to understand more on the effect of droplets if it was placed in the range of between 6 and 8 mm. The same reason was used to select the nozzle distance of 9 mm. Since the next highest concentration of droplets count was focused at axial distance of 10 mm, however, nozzle distance of 9 mm was selected in order to study the effect if the nozzle was placed at a distance less than 10 mm. All of four selected nozzle distances was evaluated in turning process to observe the effectiveness of the nozzle location in terms of cutting force and cutting temperature. The results obtained for the number of droplet distribution shows a large difference for nozzle OD25 and OD30 under different input air pressure. The number of droplet increases as the
input air pressure increases. The same phenomena were observed when the nozzle diameter increases. The total number of droplets for input air pressure of 0.2 MPa for OD 25 and OD30 nozzles were 2266 and 2315 droplets respectively. The total number of droplets for OD 30 nozzle is higher than OD25 nozzle. Under input air pressure of 0.4 MPa, the total number of droplets was 5045 and 6281 for OD25 and OD30 nozzle respectively. As the cross section area of the nozzle outlet diameter increases the number of droplet produced increases. Therefore, the fluid flow rate passes through the OD30 nozzle was higher than OD25 nozzle. The process of generating droplets is called atomization, begins by forcing the liquid flow through the nozzle. The potential energy of the liquid along with the geometry of the nozzle causes the liquid to emerge as sheet and then forms small ligaments. These ligaments then break up further into very small pieces of droplets. More fluids sheets were broke up and more droplets were formed for OD30 nozzle under various input air pressure. Furthermore, more droplets were formed under higher input air pressure and these phenomena can be observed under input air pressure of 0.4 MPa for both nozzles. The number of droplet increases as much as 123% and 171% for both OD25 and OD30 nozzles respectively as the input air pressure was increased from 0.2 to 0.4 MPa. The higher the pressure, the more energy is available for spray breakup and droplet formation, which is beneficial for producing more number and fine droplets. In general, greater pressure gives smaller or tiny droplets [15].
Fig. 8. Droplet distribution under input air pressure of 0.2 MPa, 0.3 MPa, and 0.4 MPa for OD25 nozzle at various axial distances
Fig. 9. Droplet distribution under input air pressure of 0.2 MPa, 0.3 MPa, and 0.4 MPa for OD30 nozzle at various axial distances
3.3 Particle Size of Mist The Sauter Mean Diameter (SMD) of the spray was affected by two main parameters mainly the blending percentage and the input air pressure. Fig. 10 shows the effect of input air pressure on
SMD for OD25 and OD30. The trend shows an increase in SMD as the radial distance increases for both nozzles under various input air pressure. The smallest SMD value was obtained under the high input air pressure of 0.4 MPa. On the other hand, the highest SMD value of mist droplet was captured under the input pressure of 0.2 MPa. Generally, it can be observed that the size of droplets were not uniform in the spray formation. The SMD of the droplet varied from one point to the other. The bigger droplets were located at the outer boundaries of the spray. This phenomenon is attributed to the centrifugal force gained from the tangential motion of the droplets discharged from the atomizer in circulating motion. The largest drops posed the high mass and high centrifugal force, placing itself at the spray periphery while smaller droplets with lower mass are concentrated near the spray centre line. SMD decreases remarkably with increasing input air pressure due to promoted atomization. With the increase of input air pressure, the difference in SMD for the lubricant decreases. OD25 nozzle shows the results of producing much finer particles compare to OD30 nozzle. This can be observed by referring to Fig. 10 under input pressure of 0.2 MPa, where the SMD value for OD25 nozzle was 24.5 µm and the value decreases to 16.3 µm for input pressure 0.4 MPa. Meanwhile, the SMD value for OD30 nozzle for input air pressure 0.2 and 0.4 MPa was 33 µm and 23.6 µm respectively. The SMD value of particles produced for nozzle OD 30 is bigger than OD25 nozzle under the evaluated input air pressure. With the reduction of the diameters of nozzle, a more efficient formulation of atomization with finer droplets was observed in comparison to larger diameter. A smaller nozzle diameter means the smaller droplet diameters produced. The cutting fluid forced through the smaller diameter of nozzle with a small cross section area increases the energy level of the cutting fluid. This enables the spray to breakup into smaller droplets while forcing through a small cross section area [16]. The result of the SMD shows that smaller SMD droplet particles were captured near the spray centre line. As the radial distance increases, the SMD value of the droplet particles increases. Take an example of OD30 nozzle, the smallest SMD value ranging from 20 to 30 µm was obtained under the input pressure of 0.4 MPa. As the radial distance increase from the centre line to the periphery, the SMD value increases ranging from 60 to 70 µm under various radial distances. In general, the highest SMD value was obtained under the input air pressure of 0.2 MPa.
Kyung et al. [17] reported that the droplet size at higher pressure was smaller than that of lower pressure. From the observation (Figs. 8, 9 and 10), it can be inferred that the higher input pressure provides more droplets with smaller SMD value. Increasing the input pressure led to a reduction in droplet size and the range of droplet distribution. It can be concluded that, although the input pressure has a strong effect on the atomization process. Elevated input pressure subsequently decreases the time to break-up of the jet but most droplets can be broken down completely at a high input pressure under ambient conditions [18]. Higher input pressure results in increasing of relative velocities between the input air pressure and the cutting fluid which increase the shear forces, thus producing smaller droplets. The maximum velocity of the droplet, close to the wall increases with increasing pressure owing to the higher air velocities. The velocity of the droplet decreases with increasing flow rates because more number of particles share the same amount of kinetic energy imparted by the compressed air [19]. As the input air pressure increases for smaller nozzle diameter, the diameters of droplets decreases to a certain limit of input air pressure [16]. General hypothesis on the performance of MQL technique stated that the high velocity of mist spray can easily penetrates into the machining zone, thus reducing the friction between the tool-workpiece interfaces [7, 21, 25]. Besides, this stream at high pressure produces tiny lubricant particles, subsequently provide good lubrication. Therefore, the result of spray cone angle, number of droplet count and SMD value have confirmed the aforementioned hypothesis. An analysis on the machining performance (cutting force and cutting temperature) was conducted to further understand the hypothesis. The input air pressure increase which will enhance the lubricant break up at nozzle tip and produce smaller droplet size. It facilitates an evaporation and result in larger spray cone angle and spray coverage.
Fig. 10. Effect of input pressure on SMD for OD25 and OD30 nozzle from axial distances of 2, 6 and 10 mm from the nozzle outlet
3.4 Analysis of Cutting Force
Figs. 11 and 12 showed the results of cutting force for OD25 and OD30 nozzles respectively. As expected, the cutting speed and feed rate significantly affected the result of cutting force. The cutting force tends to decrease as the cutting speed increases. Meanwhile, higher cutting force was obtained at the higher feed rate of 0.3 mm/rev. Furthermore, it is noticeable that the nozzle distance of 3 mm recorded the highest cutting force for both OD25 and OD30 nozzles. Meanwhile, the nozzle distances in the range of 6 to 9 mm provide lower cutting force value. For instance, by comparing the cutting force for both OD25 and OD30 nozzles at the feed rate of 0.15 mm/rev and input air pressure 0.2 MPa, it can be clearly seen that the results of cutting force are highest at the nozzle distance of 3 mm, which is the nearest distance in the experiment. In another example, at the nozzle distance of 7 mm for the OD25 nozzle, input air pressure of 0.2 MPa, feed rate of 0.15 mm/rev and cutting speed of 100 m/min, the cutting force value was much more lower than the distance of 3 mm. It shows a reduction of approximately 28%. In some cases, it was observed that the nozzle distance of 9 mm recorded slightly higher cutting force than the distance of 3 mm. This is because the mist supplied was unable to reach the cutting zone due to the surrounding temperature. Mist production is effected by the surrounding temperature which causes the mist to evaporate faster even before it reaches the cutting zone. Only small amount of mist spray supplied was able to lubricate and reduce the cutting force. For both OD25 and OD30 nozzle, it can be suggested that the nozzle distances of 6 and 7 mm provides the lowest cutting force at both feed rate of 0.15 and 0.30 mm/rev. At this nozzle distances, the cutting fluid in the form of mist can easily penetrates into the cutting zone. As mentioned earlier, the density of droplet count is higher in this range of distance. Therefore, with the combination of high input pressure more lubricants can be supplied and remained at the cutting zone. In addition, the recorded value of SMD at these distances is ranging from 30 to 40 µm thus enable the tiny particles or droplets to penetrate into the cutting zone, subsequently improved the lubricating effect. Hence, the cutting force was dramatically decreased. The droplet with small SMD value starts to move to the surrounding atmosphere before it reaches into the cutting zone. This might be the reason why far nozzle distance is ineffective and unable to reduce the cutting force. Most of tiny lubricant is wasted to the surrounding atmosphere and only a small amount reaches the cutting zone. This result also correlated with the generation of high cutting temperature.
Figs. 13 and 14 were drawn in order to obtain better understanding on the relationship of nozzle diameter and input air pressure toward the cutting force. In general, it can be observed that the OD30 nozzle recorded the lowest cutting force than OD25 nozzle. Finer spray was produced by the OD25 nozzle, however higher flow rate is obtained by the OD 30 nozzle. It is suggested that where more droplets are produced during atomization process. Size of nozzle diameter has a significant effect on the average size of lubricant’s droplet. Larger diameter of OD30 causes an increase in the droplet size due to increasing the energy in the breakup process. The cutting region in macro-scale machining is much larger, a comparatively higher flow rate, would likely to be needed. When the flow rate increases, the density of the droplets inside the nozzle becomes higher. It leads to high interaction of the droplets between themselves resulting in more fluid with high velocity are flushed out of the nozzle, thus improved the lubricating effect and reduced the friction during machining process. Larger diameter nozzle enables to reduce the cutting force more effectively by supplying more cutting fluid at the cutting zone [20, 21]. As mentioned earlier, higher pressure generates more energy to break up the lubricant and produces huge number of droplet count and finer particles spray. The huge amount of tiny droplets contributed to the effective lubricating at the tool-chip interfaces, thus reduces the value of cutting force. Even though the recorded data depicted in Figs. 13 and 14 were slightly scattered, however the general trend can be concluded. It shows that the cutting force decreases as the input air pressure increases. Sai et al. [19] reported that any increase in atomizing air pressure leads to notable increase in wetting area, offers better lubricate and reduces the cutting force which also reduces tensile residual stress.
Fig. 11. Cutting force, Fc against cutting speed, Vc for OD25 nozzle Fig. 12. Cutting force, Fc against cutting speed, Vc for OD30 nozzle
Fig. 13. Cutting force, Fc against various input air pressure for OD25 nozzle
Fig. 14. Cutting force, Fc against various input air pressure for OD30 nozzle
3.5 Analysis of Cutting Temperature Figs. 15 and 16 depicted the result of cutting temperature for OD25 and OD30 nozzles under various conditions. In this experiment, the value of cutting temperatures were taken and averaged once the turning process was at the steady state condition. The cutting temperature is evidently affected by the cutting speed. Moreover, it was noted that the value of cutting temperature decreases with an increase in the cutting speed from 100 to 220 m/min. For instance, for the OD25 nozzle (input air pressure of 0.2 MPa, feed rate 0.15 mm/rev), the cutting temperature increased at 48%, 50%, 28% and 24% for the nozzle distances of 3, 6, 7 and 9 mm. This result reveals that the adjustment of cutting speed embrace a significant effect on the cutting temperature. In the machining process, the cutting speed is always correlated with the chip thickness. As the cutting speed increases, the chip thickness was decreased. According to Rahim and Sasahara [10], this situation is corresponds with the high cutting energy and deformation strain rate. Moreover, the heat flux in the shearing zones is increased subsequently increased the cutting temperature during the machining process.
Further investigation was conducted and found that the cutting temperature increases with the feed rate. As the feed rate increases, the cutting temperature also increases and this occurrences is caused by the amount or material removed at the selected cutting speed increases thus more work are converted to heat generation. It was induced by higher mechanical load and higher temperatures generated with higher feed rates. At the increasing of the feed rate, an increment occurs on both the axial and radial residual stress. Meanwhile, the cutting speed had an inverse effect reducing the stress levels for both directions, this may be attributed to the thermal softening that increases at increasing the temperatures with the cutting speed. Generally, it was observed that the increase of feed rate for both OD 25 and OD30 nozzles increases the thickness of chips formed during machining. The feed rate influences the chip morphology obtained during the start of turning process. The chips formed at feed rate 0.15 mm/rev short helical chips formed, while for a feed rate of 0.30 mm/rev long helical chip resulted, provoking
chip entanglements around the workpiece and thus causing random scratches on the machined surface. At the highest level of feed rate, a sensible increment of burrs and adhered chip fragments were noted on the machined surfaces [22].
Fig. 15. Cutting temperature, Tc against cutting speed, Vc for OD25 nozzle
Fig. 16. Cutting temperature, Tc against cutting speed, Vc for OD30 nozzle
Generally, it can be observed that the cutting temperature is slightly higher at the distance of 3 mm under various conditions. Nozzle distances of 6 and 7 mm recorded a lower cutting temperature value for both OD25 and OD30 nozzle at feed rate 0.15 and 0.30 mm/rev. For input air pressure of 0.4 MPa and at the nozzle distances of 6 and 7 mm, the mist particles with smaller SMD value in the range of 20 to 30 µm can easily penetrate into the cutting zone thus reduced the cutting temperature. It is suggested that the spray pattern fully covers the cutting insert and the machined area at this selected nozzle distance thus leads to more successfully cooling effect. However, at the nozzle distance of 9 mm the cutting temperature was slightly higher but lower than 3 mm. Looking specifically at the input air pressure of 0.2 MPa, the droplet SMD value of 37.04 µm and 50 µm were recorded for OD25 and OD30 respectively at axial distance of 10 mm. It can be assumed that the particle size is bigger and causes less cutting fluid could be penetrated into the cutting zone thus poor lubricating and cooling effect were occurred. The cutting fluids has high tendency for condensation before it reaches to the cutting zone at the selected nozzle distance. The particles moves with low velocity and fluid flow rate leading to less cooling effects. At high input air pressure of 0.4 MPa, the lubricant accelerates at high velocity and produces smaller SMD value. Simultaneously, higher flow rate of lubricant is recorded at high input air pressure, causing more fluid can be supplied to the tool-chip interface. The particles can travel faster to the cutting zone before it goes through condensation at the surrounding atmosphere [20]. Therefore, it helps to improve cooling efficiency thus reducing the cutting temperature.
For instance, at the input air pressure of 0.2 MPa (Figs. 17 and 18), the recorded cutting temperatures are the highest for both OD25 and OD30 nozzles. Meaning that, the velocity of droplets are lower, consists of larger SMD value and unable to move faster to reach the cutting zone. The droplets with larger SMD value are unable to penetrate into the cutting zone. Moreover, the fluid flow rate is 0.04 and 0.06 l/hr for OD25 and OD30 nozzles for the input air pressure of 0.2 MPa is lower compare to higher input air pressure. For OD25 nozzle, at the high input air pressure the SMD value was smaller and the moving velocity of the cutting fluid is 27.5 m/s, which can be considered as high. With the increase in the velocity the cutting fluids are able to move faster and hit the cutting zone more effectively. The small SMD value of 16.3 µm at axial distance of 10 mm, the droplets can easily penetrated into the cutting zone and successfully reducing the cutting temperature. The input air pressure not only cools the cutting zone but also flushes away the chips formed and creates more wettable area during machining process. It indicates that an increment of the atomizing pressure substantially helps in dissipation of heat from the cutting zone. On the other hand, significantly increase of fluid flow rate predominantly improves the wettable of surface area. This is necessary for enhancement of lubrication and in turn for reduction of cutting force and temperature. Moreover, the high velocity of droplets at high input air pressure that impacted to the target surface absorbed the heat generated and evaporates quickly. This allows the continuously supplied fresh cutting fluid to access the cutting zone and reduce the heat [23]. Both pressure and flow rate independently have significant impact on reduction of cutting force and cutting temperature.
Fig. 17. Cutting temperature, Tc against various input air pressure for OD25 nozzle
Fig. 18. Cutting temperature, Tc against various input air pressure for OD30 nozzle
From the recorded cutting temperature value (Figs. 17 and 18), generally it shows that OD30 nozzle depicts lower cutting temperature compare to OD25 nozzle. As the nozzle diameter increases from 2.5 to 3.0 mm, the recorded cutting temperature was reduced. Larger nozzle
diameter enables more cutting fluid flow rate [19]. The fluid flow rates for OD25 nozzle were 0.04, 0.08 and 0.16 l/hr for the input air pressure of 0.2, 0.3 and 0.4 MPa respectively. On the other hand, the fluid flow rate increases for OD30 nozzle. The recorded fluid flow rates were 0.06, 0.12 and 0.22 l/hr for the input air pressure of 0.2, 0.3 and 0.4 MPa respectively and showed an increment of 50%, 50% and 38%. From this observation, it can be concluded that larger nozzle outlet diameter (OD30 nozzle) enable more fluid to flow through the nozzle compare to smaller nozzle outlet diameter (OD25 nozzle). Higher lubricant flow rate and input air pressure have improved heat dissipation efficiency. Furthermore, high lubricant flow rate subsequently improves the lubricating effect and reducing the friction. It was due to the smaller particle size and smaller SMD value, depicted from the analysis of spray characterization. As the flow rate increases, the density of the droplets inside the nozzle becomes higher and leads to high interaction of the droplets between themselves resulting in more fluid with high velocity are flushed out from the nozzle. For larger nozzle diameter the spray cone angle increases as the input air pressure increases. For OD 25 and OD30 nozzles the spray cone angle was 15.7° and 17° respectively under input air pressure of 0.4 MPa as mentioned in the previous section. OD30 nozzle shows a larger cone angle where this increases the wettable area at the cutting zone. Furthermore, the droplet distribution increases as the fluid flow rate increases. The total number of droplets captured for OD 30 nozzle is higher than OD25 nozzle under various input air pressure. Moreover, as the input air pressure increases the SMD value of the droplets decreases. More droplets of smaller SMD value and high moving velocity were captured under high input air pressure. The high velocity of moving particles can easily penetrate into cutting zone. Thus larger diameter nozzle enables to reduce cutting temperature more effectively.
4. Conclusion Based from the analysis, the performances of MQL spray with respect to the spray characteristic, cutting force and cutting temperature were determined. It shows that the performance OD30 nozzle is much more superior to OD25 nozzle. The OD30 nozzle exhibits wider cone angle, smaller SMD and faster velocity, hence provides better cooling effect during the turning process. Larger diameter nozzle enables to reduce the cutting force and cutting temperature more
effectively by supplying more cutting fluid at the cutting zone. In addition, the most preferable nozzle distance is in the range of 6 to 9 mm.
Acknowledgement The authors would like to acknowledge financial support from the Ministry of Higher Education of Malaysia under the FRGS (Vot 1058) financial scheme.
References [1]
J. P. Byers, (2006), Metal working fluids, Second edition. Philadelphia: Woodhead.
[2]
T. Obikawa, Y. Kamata, J. Shinozuka, High-speed grooving with applying MQL.
International Journal of Machine Tools & Manufacture 46(14) (2006) 1854-1861. [3]
E. A. Rahim, H. Sasahara, Effect of Machining Parameters and MQL Liquids on Surface
Integrity of High Speed Drilling Ti-6Al-4V, Key Engineering Materials 447-448 (2010) 816820. [4]
E. A. Rahim, H. Sasahara, Surface Integrity in MQL Drilling Nickel-Based Superalloy,
Key Engineering Materials 447-448 (2010) 811-815. [5]
E. A. Rahim, H. Sasahara, Surface Integrity when Drilling Nickel-Based Superalloy
under MQL Supply, Key Engineering Materials 443 (2010) 365-370. [6]
E. A. Rahim, H. Sasahara, Effect of MQL Liquids on Surface Integrity when High Speed
Drilling Titanium Alloy, Key Engineering Materials 443 (2010) 359-364. [7]
E. A. Rahim, Z. H. Samsudin, M. A. A. Rahim, Z. Mohid, Performance Investigation of
Modified Turning Tool Holder for MQL Application, Applied Mechanics and Materials 465-466 (2014) 1114-1118. [8] A. Attanasio, M. Gelfi, C. Giardini, C. Remino, Minimal quantity lubrication in turning: Effect on tool wear, Wear 260(3) (2005) 333 – 338.
[9] N. R. Dhar, M. Kamruzzaman, M. Ahmed, Effect of minimum quantity lubrication (MQL) on tool wear and surface roughness in turning AISI-4340 steel, Journal of Material Processing Technology 171(2) (2006) 299 – 304. [10] E. A. Rahim, H. Sasahara, A study of the effect of palm oil as MQL lubricant on high speed drilling of titanium alloy, Tribology International 44(3) (2011a) 309 – 317. [11]
E. A. Rahim, H. Sasahara, An Analysis of Surface Integrity when Drilling Inconel 718
using Palm Oil and Synthetic Ester under MQL Condition, Machining Science and Technology, 15(1) (2011) 76-90. [12]
M. S. Rashid, A. H. Hamid, O. C. Sheng, Z. Ghaffar, Effect of Inlet Slot Number on the
Spray Cone Angle and Discharge Coefficeient of Swirl Atomizer, Procedia Engineering 41 (2012) 1781 – 1786. [13]
S. D. Sovani, E. Chou, P. E. Sojka, J. P. Gore, W. A. Eckerle, J. D. Crofts, High pressure
effervescent atomization: Effect of ambient pressure on spray cone angle, Fuel 80(3) (2001), 427 – 435. [14]
E. M. Galván, R. Anton, J. C. Ramos, R. Khodabandeh, Effect of the spray cone angle in
the spray cooling with R134a, Experimental Thermal and Fluid Science 50 (2013) 127 – 138. [15]
B. P. Husted, P. Petersson, I. Lund, G. Holmstedt, Comparison of PIV and PDA droplet
velocity measurement techniques on two high-pressure water mist nozzles, Fire Safety Journal 44(8) (2009) 1030 – 1045. [16]
Z. Zhang, C. Zhao, Z. Xie, F. Zhang, Z. Zhao, Study on the Effect of the Nozzle
Diameter and Swirl Ratio on the Combustion Process for an Opposed-piston Two-stroke Diesel Engine, Energy Procedia 61 (2014) 542 – 546. [17]
H. P. Kyung, Y. J. Olortegui, C. Y. Moon, A study on droplets and their distribution for
minimum quantity lubricant (MQL), International Journal of Machine Tools & Manufacture 50(9) (2010) 824 – 833. [18]
S. Lee, S. Park, Experimental study on spray break-up and atomization processes from
GDI injector using high injection pressure up to 30 MPa, International Journal of Heat and Fluid Flow 45 (2014) 14 – 22.
[19]
S. S. Sai, K. Manojkumar, A. Ghosh, Assessment of spray quality from an external mix
nozzle and its impact on SQL grinding performance, International Journal of Machine Tools & Manufacture 89 (2015). 132 – 141. [20]
C. Nath, S. G. Kapoor, A.K. Srivastava, J. Iverson, Effect of fluid concentration in
titanium machining with an atomization-based cutting fluid (ACF) spray system, Journal of Manufacturing Processes 15(4) (2013) 419 – 425. [21]
E. A. Rahim, H. Sasahara, Application of Minimum Quantity Lubrication when Drilling
Nickel-Based Superalloy at High Cutting Speed, Key Engineering Materials 407-408 (2009) 612-615. [22]
A. Bordin, S. Bruschi, A. Ghiotti, The effect of cutting speed and feed rate on the surface
integrity in dry turning of CoCrMo alloy, Procedia CIRP 13 (2014) pp. 219 – 224. [23]
A.D. Jayal, A. Balaji, Effects of cutting fluid application on tool wear in machining:
Interactions with tool-coatings and tool surface features, Wear 267 (2009) 1723 – 1730. [24]
J. Ortman, A.H. Lefebvre, Fuel Distributions from Pressure-Swirl Atomizers, Journal of
Propulsion and Power 1(1) (1985) 11 – 15. [25] E.A. Rahim, H. Dorairaju, N. Asmuin, M.H.A.R. Mantari, Determination of mist flow characteristic for MQL technique using Particle Image Velocimetry (PIV) and Computer Fluid Dynamics (CFD), Applied Mechanics and Materials 773-774 (2015) 403-407.
Fig. 1. Schematic diagram of the measurement procedure setup
Fig. 2. Measurement location of the spray
Fig. 3. Lubricant spray image
Fig. 4. Schematic diagram of machining setup
Fig. 5. Nozzle setup for machining process at 45° inclination angle
Fig. 6. Spray cone angle
Fig. 7. Spray pattern under various nozzles and input air pressure
Fig. 8. Droplet distribution under input air pressure of 0.2 MPa, 0.3 MPa, and 0.4 MPa for OD25 nozzle at various axial distances
Fig. 9. Droplet distribution under input air pressure of 0.2 MPa, 0.3 MPa, and 0.4 MPa for OD30 nozzle at various axial distances
Fig. 10. Effect of input pressure on SMD for OD25 and OD30 nozzle from axial distances of 2, 6 and 10 mm from the nozzle outlet
Fig. 11. Cutting force, Fc against cutting speed, Vc for OD25 nozzle
Fig. 12. Cutting force, Fc against cutting speed, Vc for OD30 nozzle
Fig. 13. Cutting force, Fc against various input air pressure for OD25 nozzle
Fig. 14. Cutting force, Fc against various input air pressure for OD30 nozzle
Fig. 15. Cutting temperature, Tc against cutting speed, Vc for OD25 nozzle
Fig. 16. Cutting temperature, Tc against cutting speed, Vc for OD30 nozzle
Fig. 17. Cutting temperature, Tc against various input air pressure for OD25 nozzle
Fig. 18. Cutting temperature, Tc against various input air pressure for OD30 nozzle
HIGHLIGHT
Measurement of droplet was conducted to answer the hypothesis of MQL technique. Tiny droplet from MQL improves the lubricating effect . OD30 nozzle produces smallest SMD value under high flow rate. The most suitable nozzle distance is in the range of 6 to 9 mm. OD30 nozzle gives effective cooling during machining.
S0263-2241(18)30191-X https://doi.org/10.1016/j.measurement.2018.03.015 MEASUR 5334
To appear in:
Measurement
Received Date: Revised Date: Accepted Date:
9 June 2016 5 December 2017 5 March 2018
Please cite this article as: E. Abd Rahim, H. Dorairaju, Evaluation of mist flow characteristic and performance in minimum quantity lubrication (mql) machining, Measurement (2018), doi: https://doi.org/10.1016/j.measurement. 2018.03.015
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
EVALUATION OF MIST FLOW CHARACTERISTIC AND PERFORMANCE IN MINIMUM QUANTITY LUBRICATION (MQL) MACHINING Erween Abd Rahim1, Hemarani Dorairaju Precision Machining Research Center (PREMACH), Faculty of Mechanical and Manufacturing Engineering, Universiti Tun Hussein Onn Malaysia, 86400 Parit Raja, Johor 1
Corresponding author: [email protected]
Abstract Minimum Quantity Lubrication (MQL) is an alternative method to supply the cutting fluid in the formation of mist. MQL has proven to reduce machining cost and increase machining performance. The effectiveness and the working principle of MQL are still questionable with very few explanations provided. The present study is conducted to investigate the performance of MQL technique with different combination of spray and machining parameters. The Phase Doppler Anemometry (PDA) was used to characterize the lubricant spray under different input pressure for various nozzle outlet diameter of 2.5mm (OD25) and 3.0mm (OD30). This device can measure the amount of droplet and size. From these results, the distance of the nozzle to the cutting tip can be estimated. The turning performance in terms of cutting force and cutting temperature was evaluated under three levels of cutting speed and two levels of feed rate and at a constant depth of cut. The result shows that the most suitable mist flow pattern during machining was the largest spray cone angle supplied under 0.4 MPa input air pressure. In addition, significant reduction of cutting force and cutting temperature were obtained when using OD30 nozzle at the nozzle distances of 6 to 9 mm and the input air pressure of 0.4 MPa.
Keywords: Droplet size, Minimum Quantity Lubrication (MQL), Phase Doppler Anemometry (PDA), Turning
1. Introduction
Machining process is greatly influenced by cutting fluid which is used to reduce the heat generated during machining process. Water was the first cutting fluid used in machining process and was replaced with other fluids as more studies done on the ability of water to perform as cutting fluid. Cutting fluid is widely used in machining process due to its ability to cool, lubricate and to convey the chips away from the cutting zone. As a result, it improves the tool life, protects the workpiece and tool from corrosion, improve surface finish and dimension accuracy of a product. Cutting fluid had acquired sufficient and proved to be necessary adjunct in high speed machining of difficult material such as the exotic steels and specially alloyed nonferrous metals. As the new methods of machining are constantly being developed, the cutting fluid industry provided machinist with a choice of several cutting fluid. Straight mineral oils, combinations of mineral oils and vegetable oils, animal fats, marine oils, mixes of free sulphur and mineral oil are commonly used as cutting fluids and suds [1]. Several cooling techniques have been introduced for to reduce the cutting temperature and improve the machinability during the machining process. Flood coolant method has been used extensively since cutting fluid was introduced in the machining industry. Cutting fluid is delivered excessively to cool and lubricate the cutting tool and workpiece subsequently reduces the heat generated at the tool-chip interface. Near dry cutting also known as minimum quantity lubricant (MQL) is another method of cooling. In MQL method a small amount of cutting fluid is carried by air jet directly to the cutting zone. The mists are produced under the atomization process and are supplied to the cutting zone [2, 3-7. The lubricant is directly sprayed into the tool-chip interface thus guarantees a good level of lubrication [8]. The chip produced is flushed away from the cutting zone due to the action of compressed air. The machined surface is almost completely dry after machining process. The compress air not only contributes in transporting the oil particles, but it also helps in the cooling and chip removal process. The high pressure jet cutting fluid with tiny droplets of oils decreases the friction between chip and the rake face during machining process. The tiny oil droplets penetrate into chip-tool interface thus reduces the cutting temperature and improves tool life to some extent [9]. From the previous research it was found that machining performance by using MQL towards cutting temperature and cutting force are affected by the lubricant type, flow rate, the distance between nozzle and tool tip, and the workpiece material [10,11]. All these parameters are found to be dependent on the work material and process conditions. Only the appropriate oil
quantity and appropriate distance between the nozzle to the tool tip provides the optimum process condition. Therefore, the correct amount of lubricant to be applied, type of lubricant to be used, and the position of the nozzle should be considered. For example, it was assumed that the oil can penetrate the tool-workpiece interface due to high pressure of the compressed air that serves as a vehicle for the oil droplets. No comparison was made between the contact pressure at this interface and that of compressed air. These important parameters are not reported in many research documents and papers on the MQL. Even though MQL is known for its many benefits but no one was able to prove that the statement is true or no one have ever suggested a systematic procedure to prove MQL’s efficiency. Insufficient recommendations can cause difficulties to make proper choices of the equipment and parameter in machining. The effectiveness and the working principle of MQL are still questionable with very few explanations provided. Meanwhile, the nozzle design and most preferable flow pattern have to be selected in order to achieve the best particle size for excellent penetration at chip-tool interface. The number of previous study on specific nozzle design and the best mist flow pattern are very limited thus it is difficult to explain on how MQL have greater cooling ability than conventionally supplied metal working fluid. Thus selecting the best design for nozzle head, nozzle distance from the tool tip and most desirable flow pattern for generating the best particle size would be a difficult task. The purpose of this study is to characterize the mist flow pattern and behaviour in order to improve the turning performance.
2. Experimental details 2.1 Measurement of Mist Spray Phase Doppler Anemometry (PDA) was used to characterize the lubricant spray under different input pressure for various nozzle outlet diameter of 2.5mm (OD25) and 3.0mm (OD30). This equipment is capable to measure the droplet size from 0.5 µm to 2500 µm with the estimated uncertainty to be ±2%. The system is equipped with transmitter and receiving optics and a traverse system as shown in Fig. 1. In order to characterize the lubricant spray at different distance, traverse system was used to move the transmitter probe and receiver probe. To ensure the measurement locations are around the spray cone, the spray cone photo of the lubricant (synthetic ester) was captured at the highest input air pressure of 0.4 MPa. The size of the cone
was measured under the highest input air pressure and the mesh was generated using the BSA Flow Software. Fig. 2 shows the measurement location of the spray. The measurement was only taken along the half of cone because it is assumed that the spray pattern is symmetrical. The experiment must be repeated in a short period of time to avoid any errors during measurement process. The standard deviation was calculated using Equation 1. The measurement was conducted at the input air pressure of 0.2, 0.3 and 0.4 MPa supplied by the compressor. The measurement was taken after the lubricant spray becomes stable with the air pressure valve regulated at desired input air pressure.
(1)
Where
σ = is the standard deviation N= the number of sample xi = mean value from all three measurement µ = value from measurement
Fig. 1. Schematic diagram of the measurement procedure setup
Fig. 2. Measurement location of the spray
Fig. 3. Lubricant spray image
The lubricant spray characteristics under various input air pressure for both nozzles (OD25 and OD30) was captured using DSLR camera in order to estimate the wide spread and the maximum length of the spray. It was done inside the acrylic chamber, fully covered with dark backgroud and combine lighting system to capture the spray image as shown in Figure 3. From
this image, the cone angle under different input pressure was measured more effectively by using a software. In addition, from the spray image, the measurement of distance using PDA can be roughly estimated.
2.2 Machining Setup The turning process was performed using a NC Harrison A400 Alpha lathe machine. Fig. 4 and Table 1 showed the experimental setup to measure cutting force and temperature. Cemented carbide TNGG220408R insert with 60 degree triangle shape was used as the cutting tool in this experiment. AISI 1045 medium carbon steel was used as the workpiece. The diameter and length of the workpiece were 100 mm and 200 mm respectively. The Kistler 9257BA dynamometer was mounted on the bed of the lathe machine. Meanwhile, the maximum cutting temperature was captured via thermal imager camera (FLIR T640) in a temperature range of 250 to 600 °C during the turning operation. It was fixed by placing the camera facing in the axial direction, near to the cutting zone and focused at the tool-workpiece interface. The measurement was taken for the highest, lowest and mean temperature. The cutting tool emissivity, Ɛ was set at 0.32 and calibrated within the measured temperature range. Mist flow during machining process was supplied using Kuroda KEP-R MQL atomizer unit. This unit has a capability to mix the lubricant with the high pressure compressed air to form mist under atomization process.
Table 1 Cutting parameter Cutting Parameter
Values
Cutting speed, Vc (m/min)
100, 160, 220
Feed rate, fr (mm/rev)
0.15, 0.30
Depth of cut, d (mm)
0.2
Coolant method
MQL
MQL Input Air Pressure (MPa)
0.2, 0.3, 0.4
Nozzle outlet diameter (mm)
2.5, 3.0
Nozzle distance (mm)
3, 6, 7, 9
Fig. 4. Schematic diagram of machining setup
The same nozzles as mentioned earlier were used in this experiment. The distance of nozzle was measured from the cutting tool to the outlet of the nozzle as shown in Fig. 5. The distances were determined from the results obtained from the PDA. Nozzle distances of 3, 6, 7 and 9 mm was used for both OD25 and OD30 nozzles. In addition, the nozzle was setup with an inclination of 450 from the cutting insert to ensure the lubricant spray can be more focused to the cutting edge. A synthetic ester was chosen as a lubricant because it provides an excellent cutting performance even in small quantities. It is also have outstanding eco-friendly compatibility. Synthetic esters have similar biodegradability and safety as the main components of vegetable oils, thus exhibits less negative impact to the people or environment.
Fig. 5. Nozzle setup for machining process at 45° inclination angle
3. Results and discussion 3.1 Formation of Cone Angle from the Mist Spray Spray cone angle was defined as an angle formed between the nozzle tip and two lines that delineate the maximum outer region of the injected spray. The spray cone angle was created owing to the nozzle design and input air pressure. The mist spray pattern was captured for OD 25 and OD30 nozzles under various input air pressure of 0.2, 0.3 and 0.4 MPa and the results are presented in Figs. 6 and 7.
It is clearly shown in Fig. 6 that the spray cone angle becomes wider as the input air pressure increases from 0.2 MPa to 0.4 MPa for both OD25 and OD30 nozzles. OD25 nozzle recorded the spray cone angle of 10.4°, 13.7°, and 15.7° at the input pressure of 0.2 MPa, 0.3 MPa and 0.4 MPa respectively. Meanwhile, OD30 nozzle recorded 12.8°, 15°, and 17° at the input pressure of 0.2 MPa, 0.3 MPa and 0.4 MPa respectively. In the case of OD25 nozzle, it can be seen that the cone angle increases 51% from 0.2 MPa to 0.4 MPa. The OD30 nozzle shows an increase of 33% from input air pressure of 0.2 MPa to 0.4 MPa. When comparing with both
nozzles, OD30 nozzle showed an increment of 23%, 9.5% and 8.3% at the input air pressure of 0.2, 0.3 and 0.4MPa respectively. The increase of spray cone angle for both OD25 and OD30 nozzles was due to the increase of the input air pressure. Ortman and Lefebvre [24] reported that the increase in pressure starting from atmospheric pressure resulted in the spray cone angle at first to widen but later contracted. However, this phenomenon does not occurred due to the low input air pressure supplied during the experiment. The same effect of pressure on the cone angle was discussed by Rashid et al. [12] in his research using water as the cutting fluid. The result shows that the hollow cone spray angle increases when the input air pressure increased. They also observed that the length of water sheet ligament decreases as the injection pressure increased and that increasing injection pressures leads to wider spray cone angle. Basically, the spray cone angle is mainly influenced by the properties of liquid, size of atomizer and input air pressure. Input air pressure during atomization process is a major parameter that plays significant role in the formation of spray cone angle. It was suggested that the compressed air generates high potential energy to the liquid during atomization process. The kinetic energy of the liquid and geometry of the nozzle causes the liquid to emerge in small ligaments, subsequently breakup further into very small droplets. On the other hand, Sovani et al. [13] stated that the injection pressure or input air pressure gives a major effects in spray cone angle. They found that the spray cone angle under higher injection pressure of shows an increase in the cone angle. When the injection pressure is increased again, the range spanned by cone half-angles is shifted further upward and the spray cone angle range increased even more than before. At the highest injection pressure studied, the cone half-angle continues to increase with an increase in injection pressure. It can be assumed that the angle of the spray cone widened when injection pressure was raised. The increase in the cone angle will increase the extent of the surrounding air which leads to improved the atomization. Galván et al. [14] also stated that when the input air pressure increases the spray cone increases in their study on effect of the spray cone angle in the spray cooling with R134a. Meanwhile, Rashid et al. [12] stated the most numbers of inlet slots produces the widest spray cone angle while the atomizer with the least numbers inlet slots produces the narrowest spray. The more inlet slot tends to increase the azimuthal velocity inside the swirl chamber. However, the increment of spray cone angle as the inlet slot number increased becomes less significant at lower injection pressure.
The larger the nozzle outlet diameter the wider spray cone can be obtained under high input air pressure. The increase in the cone angle will increase the extent of the surrounding air which leads to improved the atomization.The area of the nozzle outlet diameter was calculated using the equation of circle area. The area of OD25 and OD30 nozzle are 19.6 mm2 and 28.3 mm2 respectively. With a larger area, more cutting fluid can be supplied under high input air pressure, hence improve the lubrication at the tool-chip interfaces during machining process. The velocity of liquid that emerges from the injector orifice increases as the input air pressure increases thus creates a finer spray. This in turn produces smaller droplets which are relatively lighter allowing the droplets to go farther hence creating wider spray cone angle. These can be observed from the spray cone angle of OD30 nozzle as shown in Fig. 7. It is noticeable that the spray cone angle of OD30 nozzle was wider than OD25 nozzle. Galván et al. [14] have revealed that the increase in the nozzzle diameter orifice can effect the spray cone angle. In their study, nozzle with larger diameter orifice shows a larger spray cone angle compare with a nozzle of smaller diameter orifice.
Fig. 6. Spray cone angle
Fig. 7. Spray pattern under various nozzles and input air pressure
3.2 Effects of Nozzle Diameter and Pressure on Droplet Distribution Figs. 8 and 9 showed the droplet distribution under the input air pressure of 0.2, 0.3 and 0.4 MPa for OD25 and OD30 nozzle at various axial distances. The total number of droplet count at different axial distance was plotted against axial distance of the nozzle. For OD 25 nozzle under the input air pressure of 0.2 MPa, 237 droplets were captured at axial distance of 2 mm. This is followed by 289 and 412 droplets at axial distance of 4 and 6 mm respectively. Meanwhile the droplet count for axial distance of 8, 10 and 12 mm were 518, 453 and 357 droplets respectively. Furthermore, the droplets count under input air pressure of 0.2 MPa for OD 30 nozzle as shown in Figure 9. The number of droplet at axial distance of 2 mm was 269 followed by 456 for axial distance of 4 mm. The number of droplet increases as much as 780 droplets at axial distance of 6
mm. But the number of droplets decreases as much as 425 droplets at axial distance of 8 mm. At axial distance of 10 mm, the number of droplet counts was 398 droplets followed by 249 droplets at axial distance of 12 mm. Generally, the bar graph shows an increasing trend in the overall number of droplet count as the input air pressure increases. On the other hand, the number of droplet count at each axial distance varies for all input air pressure. By measuring the number of droplet count at each axial distance, the nozzle distance during machining process can be estimated. The result shows that more droplets are concentrated at the range of 4 to 10 mm of the axial distance from the nozzle outlet for both OD25 and OD30 nozzle under different input air pressure. By comparing with typical drop size distribution graph, the results obtained for OD 25 and OD30 nozzle are similar. By knowing the number of the droplets count, the nozzle distance was predicted at four different axial distances of 3, 6, 7 and 9 mm to understand the effect of nozzle distance during the turning process. These values can be considered as the appropriate distances, whereby the concentration of droplets help to improve lubrication thus reduces the cutting force and temperature. Shorter distance of nozzle also enables the tiny particle of droplets penetrating into the tool chip interfaces, thus reducing the friction at the interface. It can be suggested that the nearest distance of 3 mm can be placed to the workpiece. However, the distance less than 3 mm causes a collision between the nozzle and the workpiece or chips during turning process. Therefore, the closest nozzle distance of 3 mm was selected to evaluate the effect of supplying cutting fluid. Meanwhile, nozzle distance of 6 mm and 7 mm was selected based on the number of droplets count for OD25 and OD30 nozzle at different input air pressure. From the results, most droplets were concentrated at the axial nozzle distance of 6 and 8 mm. Meanwhile, nozzle distance of 7 mm was selected to understand more on the effect of droplets if it was placed in the range of between 6 and 8 mm. The same reason was used to select the nozzle distance of 9 mm. Since the next highest concentration of droplets count was focused at axial distance of 10 mm, however, nozzle distance of 9 mm was selected in order to study the effect if the nozzle was placed at a distance less than 10 mm. All of four selected nozzle distances was evaluated in turning process to observe the effectiveness of the nozzle location in terms of cutting force and cutting temperature. The results obtained for the number of droplet distribution shows a large difference for nozzle OD25 and OD30 under different input air pressure. The number of droplet increases as the
input air pressure increases. The same phenomena were observed when the nozzle diameter increases. The total number of droplets for input air pressure of 0.2 MPa for OD 25 and OD30 nozzles were 2266 and 2315 droplets respectively. The total number of droplets for OD 30 nozzle is higher than OD25 nozzle. Under input air pressure of 0.4 MPa, the total number of droplets was 5045 and 6281 for OD25 and OD30 nozzle respectively. As the cross section area of the nozzle outlet diameter increases the number of droplet produced increases. Therefore, the fluid flow rate passes through the OD30 nozzle was higher than OD25 nozzle. The process of generating droplets is called atomization, begins by forcing the liquid flow through the nozzle. The potential energy of the liquid along with the geometry of the nozzle causes the liquid to emerge as sheet and then forms small ligaments. These ligaments then break up further into very small pieces of droplets. More fluids sheets were broke up and more droplets were formed for OD30 nozzle under various input air pressure. Furthermore, more droplets were formed under higher input air pressure and these phenomena can be observed under input air pressure of 0.4 MPa for both nozzles. The number of droplet increases as much as 123% and 171% for both OD25 and OD30 nozzles respectively as the input air pressure was increased from 0.2 to 0.4 MPa. The higher the pressure, the more energy is available for spray breakup and droplet formation, which is beneficial for producing more number and fine droplets. In general, greater pressure gives smaller or tiny droplets [15].
Fig. 8. Droplet distribution under input air pressure of 0.2 MPa, 0.3 MPa, and 0.4 MPa for OD25 nozzle at various axial distances
Fig. 9. Droplet distribution under input air pressure of 0.2 MPa, 0.3 MPa, and 0.4 MPa for OD30 nozzle at various axial distances
3.3 Particle Size of Mist The Sauter Mean Diameter (SMD) of the spray was affected by two main parameters mainly the blending percentage and the input air pressure. Fig. 10 shows the effect of input air pressure on
SMD for OD25 and OD30. The trend shows an increase in SMD as the radial distance increases for both nozzles under various input air pressure. The smallest SMD value was obtained under the high input air pressure of 0.4 MPa. On the other hand, the highest SMD value of mist droplet was captured under the input pressure of 0.2 MPa. Generally, it can be observed that the size of droplets were not uniform in the spray formation. The SMD of the droplet varied from one point to the other. The bigger droplets were located at the outer boundaries of the spray. This phenomenon is attributed to the centrifugal force gained from the tangential motion of the droplets discharged from the atomizer in circulating motion. The largest drops posed the high mass and high centrifugal force, placing itself at the spray periphery while smaller droplets with lower mass are concentrated near the spray centre line. SMD decreases remarkably with increasing input air pressure due to promoted atomization. With the increase of input air pressure, the difference in SMD for the lubricant decreases. OD25 nozzle shows the results of producing much finer particles compare to OD30 nozzle. This can be observed by referring to Fig. 10 under input pressure of 0.2 MPa, where the SMD value for OD25 nozzle was 24.5 µm and the value decreases to 16.3 µm for input pressure 0.4 MPa. Meanwhile, the SMD value for OD30 nozzle for input air pressure 0.2 and 0.4 MPa was 33 µm and 23.6 µm respectively. The SMD value of particles produced for nozzle OD 30 is bigger than OD25 nozzle under the evaluated input air pressure. With the reduction of the diameters of nozzle, a more efficient formulation of atomization with finer droplets was observed in comparison to larger diameter. A smaller nozzle diameter means the smaller droplet diameters produced. The cutting fluid forced through the smaller diameter of nozzle with a small cross section area increases the energy level of the cutting fluid. This enables the spray to breakup into smaller droplets while forcing through a small cross section area [16]. The result of the SMD shows that smaller SMD droplet particles were captured near the spray centre line. As the radial distance increases, the SMD value of the droplet particles increases. Take an example of OD30 nozzle, the smallest SMD value ranging from 20 to 30 µm was obtained under the input pressure of 0.4 MPa. As the radial distance increase from the centre line to the periphery, the SMD value increases ranging from 60 to 70 µm under various radial distances. In general, the highest SMD value was obtained under the input air pressure of 0.2 MPa.
Kyung et al. [17] reported that the droplet size at higher pressure was smaller than that of lower pressure. From the observation (Figs. 8, 9 and 10), it can be inferred that the higher input pressure provides more droplets with smaller SMD value. Increasing the input pressure led to a reduction in droplet size and the range of droplet distribution. It can be concluded that, although the input pressure has a strong effect on the atomization process. Elevated input pressure subsequently decreases the time to break-up of the jet but most droplets can be broken down completely at a high input pressure under ambient conditions [18]. Higher input pressure results in increasing of relative velocities between the input air pressure and the cutting fluid which increase the shear forces, thus producing smaller droplets. The maximum velocity of the droplet, close to the wall increases with increasing pressure owing to the higher air velocities. The velocity of the droplet decreases with increasing flow rates because more number of particles share the same amount of kinetic energy imparted by the compressed air [19]. As the input air pressure increases for smaller nozzle diameter, the diameters of droplets decreases to a certain limit of input air pressure [16]. General hypothesis on the performance of MQL technique stated that the high velocity of mist spray can easily penetrates into the machining zone, thus reducing the friction between the tool-workpiece interfaces [7, 21, 25]. Besides, this stream at high pressure produces tiny lubricant particles, subsequently provide good lubrication. Therefore, the result of spray cone angle, number of droplet count and SMD value have confirmed the aforementioned hypothesis. An analysis on the machining performance (cutting force and cutting temperature) was conducted to further understand the hypothesis. The input air pressure increase which will enhance the lubricant break up at nozzle tip and produce smaller droplet size. It facilitates an evaporation and result in larger spray cone angle and spray coverage.
Fig. 10. Effect of input pressure on SMD for OD25 and OD30 nozzle from axial distances of 2, 6 and 10 mm from the nozzle outlet
3.4 Analysis of Cutting Force
Figs. 11 and 12 showed the results of cutting force for OD25 and OD30 nozzles respectively. As expected, the cutting speed and feed rate significantly affected the result of cutting force. The cutting force tends to decrease as the cutting speed increases. Meanwhile, higher cutting force was obtained at the higher feed rate of 0.3 mm/rev. Furthermore, it is noticeable that the nozzle distance of 3 mm recorded the highest cutting force for both OD25 and OD30 nozzles. Meanwhile, the nozzle distances in the range of 6 to 9 mm provide lower cutting force value. For instance, by comparing the cutting force for both OD25 and OD30 nozzles at the feed rate of 0.15 mm/rev and input air pressure 0.2 MPa, it can be clearly seen that the results of cutting force are highest at the nozzle distance of 3 mm, which is the nearest distance in the experiment. In another example, at the nozzle distance of 7 mm for the OD25 nozzle, input air pressure of 0.2 MPa, feed rate of 0.15 mm/rev and cutting speed of 100 m/min, the cutting force value was much more lower than the distance of 3 mm. It shows a reduction of approximately 28%. In some cases, it was observed that the nozzle distance of 9 mm recorded slightly higher cutting force than the distance of 3 mm. This is because the mist supplied was unable to reach the cutting zone due to the surrounding temperature. Mist production is effected by the surrounding temperature which causes the mist to evaporate faster even before it reaches the cutting zone. Only small amount of mist spray supplied was able to lubricate and reduce the cutting force. For both OD25 and OD30 nozzle, it can be suggested that the nozzle distances of 6 and 7 mm provides the lowest cutting force at both feed rate of 0.15 and 0.30 mm/rev. At this nozzle distances, the cutting fluid in the form of mist can easily penetrates into the cutting zone. As mentioned earlier, the density of droplet count is higher in this range of distance. Therefore, with the combination of high input pressure more lubricants can be supplied and remained at the cutting zone. In addition, the recorded value of SMD at these distances is ranging from 30 to 40 µm thus enable the tiny particles or droplets to penetrate into the cutting zone, subsequently improved the lubricating effect. Hence, the cutting force was dramatically decreased. The droplet with small SMD value starts to move to the surrounding atmosphere before it reaches into the cutting zone. This might be the reason why far nozzle distance is ineffective and unable to reduce the cutting force. Most of tiny lubricant is wasted to the surrounding atmosphere and only a small amount reaches the cutting zone. This result also correlated with the generation of high cutting temperature.
Figs. 13 and 14 were drawn in order to obtain better understanding on the relationship of nozzle diameter and input air pressure toward the cutting force. In general, it can be observed that the OD30 nozzle recorded the lowest cutting force than OD25 nozzle. Finer spray was produced by the OD25 nozzle, however higher flow rate is obtained by the OD 30 nozzle. It is suggested that where more droplets are produced during atomization process. Size of nozzle diameter has a significant effect on the average size of lubricant’s droplet. Larger diameter of OD30 causes an increase in the droplet size due to increasing the energy in the breakup process. The cutting region in macro-scale machining is much larger, a comparatively higher flow rate, would likely to be needed. When the flow rate increases, the density of the droplets inside the nozzle becomes higher. It leads to high interaction of the droplets between themselves resulting in more fluid with high velocity are flushed out of the nozzle, thus improved the lubricating effect and reduced the friction during machining process. Larger diameter nozzle enables to reduce the cutting force more effectively by supplying more cutting fluid at the cutting zone [20, 21]. As mentioned earlier, higher pressure generates more energy to break up the lubricant and produces huge number of droplet count and finer particles spray. The huge amount of tiny droplets contributed to the effective lubricating at the tool-chip interfaces, thus reduces the value of cutting force. Even though the recorded data depicted in Figs. 13 and 14 were slightly scattered, however the general trend can be concluded. It shows that the cutting force decreases as the input air pressure increases. Sai et al. [19] reported that any increase in atomizing air pressure leads to notable increase in wetting area, offers better lubricate and reduces the cutting force which also reduces tensile residual stress.
Fig. 11. Cutting force, Fc against cutting speed, Vc for OD25 nozzle Fig. 12. Cutting force, Fc against cutting speed, Vc for OD30 nozzle
Fig. 13. Cutting force, Fc against various input air pressure for OD25 nozzle
Fig. 14. Cutting force, Fc against various input air pressure for OD30 nozzle
3.5 Analysis of Cutting Temperature Figs. 15 and 16 depicted the result of cutting temperature for OD25 and OD30 nozzles under various conditions. In this experiment, the value of cutting temperatures were taken and averaged once the turning process was at the steady state condition. The cutting temperature is evidently affected by the cutting speed. Moreover, it was noted that the value of cutting temperature decreases with an increase in the cutting speed from 100 to 220 m/min. For instance, for the OD25 nozzle (input air pressure of 0.2 MPa, feed rate 0.15 mm/rev), the cutting temperature increased at 48%, 50%, 28% and 24% for the nozzle distances of 3, 6, 7 and 9 mm. This result reveals that the adjustment of cutting speed embrace a significant effect on the cutting temperature. In the machining process, the cutting speed is always correlated with the chip thickness. As the cutting speed increases, the chip thickness was decreased. According to Rahim and Sasahara [10], this situation is corresponds with the high cutting energy and deformation strain rate. Moreover, the heat flux in the shearing zones is increased subsequently increased the cutting temperature during the machining process.
Further investigation was conducted and found that the cutting temperature increases with the feed rate. As the feed rate increases, the cutting temperature also increases and this occurrences is caused by the amount or material removed at the selected cutting speed increases thus more work are converted to heat generation. It was induced by higher mechanical load and higher temperatures generated with higher feed rates. At the increasing of the feed rate, an increment occurs on both the axial and radial residual stress. Meanwhile, the cutting speed had an inverse effect reducing the stress levels for both directions, this may be attributed to the thermal softening that increases at increasing the temperatures with the cutting speed. Generally, it was observed that the increase of feed rate for both OD 25 and OD30 nozzles increases the thickness of chips formed during machining. The feed rate influences the chip morphology obtained during the start of turning process. The chips formed at feed rate 0.15 mm/rev short helical chips formed, while for a feed rate of 0.30 mm/rev long helical chip resulted, provoking
chip entanglements around the workpiece and thus causing random scratches on the machined surface. At the highest level of feed rate, a sensible increment of burrs and adhered chip fragments were noted on the machined surfaces [22].
Fig. 15. Cutting temperature, Tc against cutting speed, Vc for OD25 nozzle
Fig. 16. Cutting temperature, Tc against cutting speed, Vc for OD30 nozzle
Generally, it can be observed that the cutting temperature is slightly higher at the distance of 3 mm under various conditions. Nozzle distances of 6 and 7 mm recorded a lower cutting temperature value for both OD25 and OD30 nozzle at feed rate 0.15 and 0.30 mm/rev. For input air pressure of 0.4 MPa and at the nozzle distances of 6 and 7 mm, the mist particles with smaller SMD value in the range of 20 to 30 µm can easily penetrate into the cutting zone thus reduced the cutting temperature. It is suggested that the spray pattern fully covers the cutting insert and the machined area at this selected nozzle distance thus leads to more successfully cooling effect. However, at the nozzle distance of 9 mm the cutting temperature was slightly higher but lower than 3 mm. Looking specifically at the input air pressure of 0.2 MPa, the droplet SMD value of 37.04 µm and 50 µm were recorded for OD25 and OD30 respectively at axial distance of 10 mm. It can be assumed that the particle size is bigger and causes less cutting fluid could be penetrated into the cutting zone thus poor lubricating and cooling effect were occurred. The cutting fluids has high tendency for condensation before it reaches to the cutting zone at the selected nozzle distance. The particles moves with low velocity and fluid flow rate leading to less cooling effects. At high input air pressure of 0.4 MPa, the lubricant accelerates at high velocity and produces smaller SMD value. Simultaneously, higher flow rate of lubricant is recorded at high input air pressure, causing more fluid can be supplied to the tool-chip interface. The particles can travel faster to the cutting zone before it goes through condensation at the surrounding atmosphere [20]. Therefore, it helps to improve cooling efficiency thus reducing the cutting temperature.
For instance, at the input air pressure of 0.2 MPa (Figs. 17 and 18), the recorded cutting temperatures are the highest for both OD25 and OD30 nozzles. Meaning that, the velocity of droplets are lower, consists of larger SMD value and unable to move faster to reach the cutting zone. The droplets with larger SMD value are unable to penetrate into the cutting zone. Moreover, the fluid flow rate is 0.04 and 0.06 l/hr for OD25 and OD30 nozzles for the input air pressure of 0.2 MPa is lower compare to higher input air pressure. For OD25 nozzle, at the high input air pressure the SMD value was smaller and the moving velocity of the cutting fluid is 27.5 m/s, which can be considered as high. With the increase in the velocity the cutting fluids are able to move faster and hit the cutting zone more effectively. The small SMD value of 16.3 µm at axial distance of 10 mm, the droplets can easily penetrated into the cutting zone and successfully reducing the cutting temperature. The input air pressure not only cools the cutting zone but also flushes away the chips formed and creates more wettable area during machining process. It indicates that an increment of the atomizing pressure substantially helps in dissipation of heat from the cutting zone. On the other hand, significantly increase of fluid flow rate predominantly improves the wettable of surface area. This is necessary for enhancement of lubrication and in turn for reduction of cutting force and temperature. Moreover, the high velocity of droplets at high input air pressure that impacted to the target surface absorbed the heat generated and evaporates quickly. This allows the continuously supplied fresh cutting fluid to access the cutting zone and reduce the heat [23]. Both pressure and flow rate independently have significant impact on reduction of cutting force and cutting temperature.
Fig. 17. Cutting temperature, Tc against various input air pressure for OD25 nozzle
Fig. 18. Cutting temperature, Tc against various input air pressure for OD30 nozzle
From the recorded cutting temperature value (Figs. 17 and 18), generally it shows that OD30 nozzle depicts lower cutting temperature compare to OD25 nozzle. As the nozzle diameter increases from 2.5 to 3.0 mm, the recorded cutting temperature was reduced. Larger nozzle
diameter enables more cutting fluid flow rate [19]. The fluid flow rates for OD25 nozzle were 0.04, 0.08 and 0.16 l/hr for the input air pressure of 0.2, 0.3 and 0.4 MPa respectively. On the other hand, the fluid flow rate increases for OD30 nozzle. The recorded fluid flow rates were 0.06, 0.12 and 0.22 l/hr for the input air pressure of 0.2, 0.3 and 0.4 MPa respectively and showed an increment of 50%, 50% and 38%. From this observation, it can be concluded that larger nozzle outlet diameter (OD30 nozzle) enable more fluid to flow through the nozzle compare to smaller nozzle outlet diameter (OD25 nozzle). Higher lubricant flow rate and input air pressure have improved heat dissipation efficiency. Furthermore, high lubricant flow rate subsequently improves the lubricating effect and reducing the friction. It was due to the smaller particle size and smaller SMD value, depicted from the analysis of spray characterization. As the flow rate increases, the density of the droplets inside the nozzle becomes higher and leads to high interaction of the droplets between themselves resulting in more fluid with high velocity are flushed out from the nozzle. For larger nozzle diameter the spray cone angle increases as the input air pressure increases. For OD 25 and OD30 nozzles the spray cone angle was 15.7° and 17° respectively under input air pressure of 0.4 MPa as mentioned in the previous section. OD30 nozzle shows a larger cone angle where this increases the wettable area at the cutting zone. Furthermore, the droplet distribution increases as the fluid flow rate increases. The total number of droplets captured for OD 30 nozzle is higher than OD25 nozzle under various input air pressure. Moreover, as the input air pressure increases the SMD value of the droplets decreases. More droplets of smaller SMD value and high moving velocity were captured under high input air pressure. The high velocity of moving particles can easily penetrate into cutting zone. Thus larger diameter nozzle enables to reduce cutting temperature more effectively.
4. Conclusion Based from the analysis, the performances of MQL spray with respect to the spray characteristic, cutting force and cutting temperature were determined. It shows that the performance OD30 nozzle is much more superior to OD25 nozzle. The OD30 nozzle exhibits wider cone angle, smaller SMD and faster velocity, hence provides better cooling effect during the turning process. Larger diameter nozzle enables to reduce the cutting force and cutting temperature more
effectively by supplying more cutting fluid at the cutting zone. In addition, the most preferable nozzle distance is in the range of 6 to 9 mm.
Acknowledgement The authors would like to acknowledge financial support from the Ministry of Higher Education of Malaysia under the FRGS (Vot 1058) financial scheme.
References [1]
J. P. Byers, (2006), Metal working fluids, Second edition. Philadelphia: Woodhead.
[2]
T. Obikawa, Y. Kamata, J. Shinozuka, High-speed grooving with applying MQL.
International Journal of Machine Tools & Manufacture 46(14) (2006) 1854-1861. [3]
E. A. Rahim, H. Sasahara, Effect of Machining Parameters and MQL Liquids on Surface
Integrity of High Speed Drilling Ti-6Al-4V, Key Engineering Materials 447-448 (2010) 816820. [4]
E. A. Rahim, H. Sasahara, Surface Integrity in MQL Drilling Nickel-Based Superalloy,
Key Engineering Materials 447-448 (2010) 811-815. [5]
E. A. Rahim, H. Sasahara, Surface Integrity when Drilling Nickel-Based Superalloy
under MQL Supply, Key Engineering Materials 443 (2010) 365-370. [6]
E. A. Rahim, H. Sasahara, Effect of MQL Liquids on Surface Integrity when High Speed
Drilling Titanium Alloy, Key Engineering Materials 443 (2010) 359-364. [7]
E. A. Rahim, Z. H. Samsudin, M. A. A. Rahim, Z. Mohid, Performance Investigation of
Modified Turning Tool Holder for MQL Application, Applied Mechanics and Materials 465-466 (2014) 1114-1118. [8] A. Attanasio, M. Gelfi, C. Giardini, C. Remino, Minimal quantity lubrication in turning: Effect on tool wear, Wear 260(3) (2005) 333 – 338.
[9] N. R. Dhar, M. Kamruzzaman, M. Ahmed, Effect of minimum quantity lubrication (MQL) on tool wear and surface roughness in turning AISI-4340 steel, Journal of Material Processing Technology 171(2) (2006) 299 – 304. [10] E. A. Rahim, H. Sasahara, A study of the effect of palm oil as MQL lubricant on high speed drilling of titanium alloy, Tribology International 44(3) (2011a) 309 – 317. [11]
E. A. Rahim, H. Sasahara, An Analysis of Surface Integrity when Drilling Inconel 718
using Palm Oil and Synthetic Ester under MQL Condition, Machining Science and Technology, 15(1) (2011) 76-90. [12]
M. S. Rashid, A. H. Hamid, O. C. Sheng, Z. Ghaffar, Effect of Inlet Slot Number on the
Spray Cone Angle and Discharge Coefficeient of Swirl Atomizer, Procedia Engineering 41 (2012) 1781 – 1786. [13]
S. D. Sovani, E. Chou, P. E. Sojka, J. P. Gore, W. A. Eckerle, J. D. Crofts, High pressure
effervescent atomization: Effect of ambient pressure on spray cone angle, Fuel 80(3) (2001), 427 – 435. [14]
E. M. Galván, R. Anton, J. C. Ramos, R. Khodabandeh, Effect of the spray cone angle in
the spray cooling with R134a, Experimental Thermal and Fluid Science 50 (2013) 127 – 138. [15]
B. P. Husted, P. Petersson, I. Lund, G. Holmstedt, Comparison of PIV and PDA droplet
velocity measurement techniques on two high-pressure water mist nozzles, Fire Safety Journal 44(8) (2009) 1030 – 1045. [16]
Z. Zhang, C. Zhao, Z. Xie, F. Zhang, Z. Zhao, Study on the Effect of the Nozzle
Diameter and Swirl Ratio on the Combustion Process for an Opposed-piston Two-stroke Diesel Engine, Energy Procedia 61 (2014) 542 – 546. [17]
H. P. Kyung, Y. J. Olortegui, C. Y. Moon, A study on droplets and their distribution for
minimum quantity lubricant (MQL), International Journal of Machine Tools & Manufacture 50(9) (2010) 824 – 833. [18]
S. Lee, S. Park, Experimental study on spray break-up and atomization processes from
GDI injector using high injection pressure up to 30 MPa, International Journal of Heat and Fluid Flow 45 (2014) 14 – 22.
[19]
S. S. Sai, K. Manojkumar, A. Ghosh, Assessment of spray quality from an external mix
nozzle and its impact on SQL grinding performance, International Journal of Machine Tools & Manufacture 89 (2015). 132 – 141. [20]
C. Nath, S. G. Kapoor, A.K. Srivastava, J. Iverson, Effect of fluid concentration in
titanium machining with an atomization-based cutting fluid (ACF) spray system, Journal of Manufacturing Processes 15(4) (2013) 419 – 425. [21]
E. A. Rahim, H. Sasahara, Application of Minimum Quantity Lubrication when Drilling
Nickel-Based Superalloy at High Cutting Speed, Key Engineering Materials 407-408 (2009) 612-615. [22]
A. Bordin, S. Bruschi, A. Ghiotti, The effect of cutting speed and feed rate on the surface
integrity in dry turning of CoCrMo alloy, Procedia CIRP 13 (2014) pp. 219 – 224. [23]
A.D. Jayal, A. Balaji, Effects of cutting fluid application on tool wear in machining:
Interactions with tool-coatings and tool surface features, Wear 267 (2009) 1723 – 1730. [24]
J. Ortman, A.H. Lefebvre, Fuel Distributions from Pressure-Swirl Atomizers, Journal of
Propulsion and Power 1(1) (1985) 11 – 15. [25] E.A. Rahim, H. Dorairaju, N. Asmuin, M.H.A.R. Mantari, Determination of mist flow characteristic for MQL technique using Particle Image Velocimetry (PIV) and Computer Fluid Dynamics (CFD), Applied Mechanics and Materials 773-774 (2015) 403-407.
Fig. 1. Schematic diagram of the measurement procedure setup
Fig. 2. Measurement location of the spray
Fig. 3. Lubricant spray image
Fig. 4. Schematic diagram of machining setup
Fig. 5. Nozzle setup for machining process at 45° inclination angle
Fig. 6. Spray cone angle
Fig. 7. Spray pattern under various nozzles and input air pressure
Fig. 8. Droplet distribution under input air pressure of 0.2 MPa, 0.3 MPa, and 0.4 MPa for OD25 nozzle at various axial distances
Fig. 9. Droplet distribution under input air pressure of 0.2 MPa, 0.3 MPa, and 0.4 MPa for OD30 nozzle at various axial distances
Fig. 10. Effect of input pressure on SMD for OD25 and OD30 nozzle from axial distances of 2, 6 and 10 mm from the nozzle outlet
Fig. 11. Cutting force, Fc against cutting speed, Vc for OD25 nozzle
Fig. 12. Cutting force, Fc against cutting speed, Vc for OD30 nozzle
Fig. 13. Cutting force, Fc against various input air pressure for OD25 nozzle
Fig. 14. Cutting force, Fc against various input air pressure for OD30 nozzle
Fig. 15. Cutting temperature, Tc against cutting speed, Vc for OD25 nozzle
Fig. 16. Cutting temperature, Tc against cutting speed, Vc for OD30 nozzle
Fig. 17. Cutting temperature, Tc against various input air pressure for OD25 nozzle
Fig. 18. Cutting temperature, Tc against various input air pressure for OD30 nozzle
HIGHLIGHT
Measurement of droplet was conducted to answer the hypothesis of MQL technique. Tiny droplet from MQL improves the lubricating effect . OD30 nozzle produces smallest SMD value under high flow rate. The most suitable nozzle distance is in the range of 6 to 9 mm. OD30 nozzle gives effective cooling during machining.