Effect of Cutting Oil Viscosity on Tool Wear Reduction in Turning Using an MQL System

Transcription

1 Advances in Materials Science and Applications Sept. 214, Vol. 3 Iss. 3, PP Effect of Cutting Oil Viscosity on Tool Wear Reduction in Turning Using an MQL System Hironori Matsuoka *1, Hajime Ono 2, Takahiro Ryu 3, Takashi Nakae 4, Shuuichi Shutou 5, Tsukuru Kodera 6 1,3,4,5 Department of Mechanical and Energy Systems Engineering, Faculty of Engineering, Oita University, 7 Dannoharu, Oita, , Japan 2 Yushiro Chemical Industry Co., Ltd., 158 Tabata, Samukawa-cho, Kouza-gun, Kanagawa, Japan 6 Mechanical and Energy Systems Engineering, Graduate School of Engineering, Oita University, 7 Dannoharu, Oita, , Japan *1 hmatsuoka@oita-u.ac.jp; 2 h-ono@yushiro.co.jp; 3 ryu@oita-u.ac.jp; 4 tnakae@oita-u.ac.jp; 5 s-shuto@oita-u.ac.jp; 6 v12e113@oita-u.ac.jp Abstract-The present paper describes the influence of the viscosity of cutting oil on tool wear and the roughness of the finished surface when using a cemented carbide tool in dry and minimal quantity lubrication (MQL) systems for turning. The following results were obtained. (1) At a feed rate of.2 mm/rev, the side flank boundary wear reduction effect was obtained with cutting oil having a low viscosity of 4.9 mm 2 /s in the MQL system, irrespective of cutting speed. The front flank boundary wear increased in MQL compared to dry cutting, and double boundary wear was observed. (2) At a feed rate of.4 mm/rev, cutting oil having a moderate viscosity of 9.3 mm 2 /s provided suitable side flank boundary wear, irrespective of cutting speed. (3) The front flank boundary wear obtained using the MQL system was greater than that for dry cutting. The transcription of the groove of this front flank boundary wear formed the finished surface roughness. Keywords- Cutting; Turning; ; MQL; Viscosity Grade; Tool Wear; Finished Surface Roughness I. INTRODUCTION Recently, the technology of dry cutting without the need for cutting oil and semi-dry machining systems with an oil mist supply have been developed in an attempt to improve the working environment and prevent global environmental pollution, in general machining processes such as tuning, milling, and drilling [1, 2]. Minimal quantity lubrication (MQL) machining systems, as represented by semi-dry machining systems, show good cutting performance in drilling and reaming operations, in which a negligible amount of cutting oil penetrates the cutting zone. In addition, MQL machining offers advantages in terms of cost. Minimal quantity lubrication machining systems require a cutting oil with good lubricating properties, thus cooling ability is less important. In tribological studies of machine elements, the viscosity of the base oil used for lubrication has been reported to have a strong effect on lubricating performance [3, 4]. The contact conditions in turning between the cutting tool and cut away debris, and between the cutting tool and the finished surface are more significant than the contact conditions of general machine elements. However, since cutting phenomena are tribological, the viscosity of the cutting oil is expected to have a significant influence on tool wears and the finished surface roughness in turning. Although the viscosity of cutting oil in a tapping test has been investigated by Suda et al. [5] and in a turning test using a high-speed steel tool [6], few studies have investigated the viscosity of cutting oil in turning with a cemented carbide tool using an MQL system. Since turning is a continuous cutting process, sufficient penetration of cutting oil to the cutting edge, which contacts the shear plane and the contact surface between the rake face and the chips, is relatively obstructed. An infinitesimal quantity of cutting oil is anticipated to have a difficulty in penetrating to the contact surface. A previous paper [7] clarified that when changing the quantity of cutting oil in the MQL system, the MQL decreases the side flank boundary wear, and increases the quantity of oil supply extended the tool life. Moreover, in the MQL system, the front flank boundary wear increases, contrary to expectations and two-front flank boundary wear appears. These results support the idea that the cutting oil can penetrate to the contact region between the cutting tool and the work piece material in the MQL system. In the present study, the authors investigate the influence of the viscosity of the cutting oil on tool wear and the finished surface roughness using a cemented carbide tool in turning with an MQL system. II. EXPERIMENTAL METHOD AND CONDITIONS A tool holder and a cemented carbide tool (P2, Sumitomo Electric Hardmetal) were used as the cutting tool in turning tests, which were conducted under the conditions listed in Table 1. The geometrical shape of cutting tool is (-5, -6, 5, 6, 15, 15,.8 ). The work material used in the tests was S45C (HB19) carbon steel, and the test sample had a diameter of 8 mm and a length of 25 mm. The cutting speed was varied between 1 and 12 m/min. The feed was varied between.2 and

2 Advances in Materials Science and Applications Sept. 214, Vol. 3 Iss. 3, PP mm/rev, and a constant depth of cut of 1. mm was maintained. Fatty acid esters with three different viscosities were used as the cutting oils (Oils A, B, and C) in the MQL tests, the kinematic viscosities of which are shown in Table 2. In addition, mineral oil having the same viscosity as Oil B was used in the tests for comparison. The oil supply rate was 15 ml/h, and the compressed air pressure was.5 MPa. cutting was also performed for comparison. The lathe used in the tests was an LS high-speed lathe (45 55, with an infinite variable speed attachment) manufactured by Okuma Tekkosyo Co. The MQL supply system was a FK-type external applicator produced by Fuji BC Engineering Co., Ltd. The oil was supplied from the rake face and the flank face sides. TABLE 1 CUTTING TOOL AND CUTTING CONDITIONS Tool holder Throw away chip Depth of cut Feed rate Cutting speed Oil supply rate Pressure of compressed air PSBNR22 SNGG1248R-UM, P2 1. mm.2,.4 mm/rev 1, 12 m/min 15 ml/h.5 MPa TABLE 2 CUTTING OIL USED IN THE TEST Mark Oil A Oil B Oil C Mineral Oil Higher fatty acid ester Mixture of Oil A and Oil C Medium fatty acid ester Mineral oil Kinematic viscosity 4 C, mm 2 /s The length of the side flank boundary wear VK1, the width of the side flank wear VB, and the length of the front flank boundary wear VK2, as illustrated in Fig. 1, were estimated. In the present experiments, the cutting distance was 6, m for a feed rate of.2 mm/rev and 3,5 m for a feed rate of.4 mm/rev. Crater wear Face VB VK1 : Length of side flank boundary wear VB : Width of side flank wear land VK2 : Length of front flank boundary wear Fig. 1 Wear of cutting tool III. EXPERIMENTAL RESULTS AND DISCUSSIONS A. Tool Wear In order to confirm the reproducibility of tool wear in the present study, tests were conducted twice under several conditions. The reproducibility of these tests was rather good. Fig. 2 shows the reproducibility of the progress curves of tool wear obtained using Oil B at a cutting speed of 1 m/min and a feed rate of.2 mm/rev. Here, two regions of front flank boundary wear, with lengths and VK2(2), were found to occur in the test with MQL. In these tests, the increase in each tool wear exhibited approximately the same tendency, and the amount of each tool wear after cutting for 6, m is considered to be approximately the same. Fig. 3 shows the tool wear for cutting speeds of 1 and 12 m/min and a feed rate of f =.2 mm/rev. At a cutting speed of 1 m/min, Oil A (low viscosity) shows the smallest side flank boundary wear VK1, the magnitude of which increases in the order of Oil C (high viscosity) and Oil B (moderate viscosity). The values of VK1 obtained with Oils A and C are smaller than that

3 Tool wear mm Advances in Materials Science and Applications Sept. 214, Vol. 3 Iss. 3, PP obtained by dry cutting. The values of VK1 are approximately the same for Oil B and mineral oil, which have the same viscosity. At 12 m/min, the values of VK1 obtained for Oils A and C are small and approximately the same. On the other hand, the value of VK1 obtained for Oil B is large. The value of VK1 obtained for Oil B is smaller than that for mineral oil. The values of VK1 obtained for Oils A and C decrease by approximately 5% compared to that by dry cutting. The wear reduction effect with the MQL system at a cutting speed of 12 m/min is greater than that at 1 m/min. At a cutting speed of 1 m/min, front flank boundary wear VK2 occurs characteristically at two locations in the case of MQL, compared to one location for dry cutting. At the first location, for MQL was larger than that for dry cutting. Both and VK2(2) tend to increase with increasing viscosity. Both and VK2(2) are smaller for Oil B than for mineral oil of the same viscosity. At 12 m/min, two front flank boundary wear regions also occur for MQL, compared to just one for dry cutting. The value of obtained for MQL is larger than that obtained for dry cutting. Both and VK2(2) are small in the case of using Oil B (moderate viscosity). On the other hand, a failure occurs at the position of VK2(2) when using Oil A. Both VK2s obtained while using Oil B are smaller than those obtained while using mineral oil..5.4 VK1.3.2 VK2(2).1 VB Cutting distance m Fig. 2 Reproducibility of tool wear (Oil B, cutting speed: 1 m/min, f =.2 mm/rev) VK1 mm D : A : Oil A (4.9 mm 2 /s) B : Oil B (9.3 mm 2 /s) C : Oil C (14.7 mm 2 /s) M : Mineral (9.3 mm 2 /s), VK2(2) mm m/min 12 m/min (a) Side flank boundary wear Failure 1 m/min 12 m/min (b) Front flank boundary wear VK2 (2)

4 Advances in Materials Science and Applications Sept. 214, Vol. 3 Iss. 3, PP VB mm m/min 12 m/min (c) Side flank wear Fig. 3 Comparison of tool wear (cutting speeds: 1 and 12 m/min, f =.2 mm/rev, cutting distance: 6 m) The width of the side flank wear VB tends to increase with oil viscosity and has values similar to those for dry cutting at cutting speeds of 1 and 12 m/min. The VB values obtained at 12 m/min are larger than those obtained at 1 m/min. As mentioned above, for a feed rate of.2 mm/rev, the low-viscosity Oil A (4.9 mm 2 /s) is effective at reducing VK1. The wear reduction effect of MQL is large at a cutting speed of 12 m/min, but MQL has no effect on VK2 or VB. Fig. 4 shows the conditions of the rake and the side flank faces after cutting for 6, m at a cutting speed of 12 m/min. Although the deposited metal adheres to the rake face both in MQL and dry cutting, the amount of deposited metal in the case of MQL cutting appeared to be smaller than that in the case of dry cutting. No difference in the amount of deposited metal was observed among the different viscosities. A deep groove, which was caused by rubbing of the chips in the outflow direction of the chips, appeared at the position of occurrence of the side flank boundary wear. In particular, a long, clear groove was observed in dry cutting. When using Oils A, B, and C, the groove length became shorter for higher viscosity oils. 1 mm Deposited metal Deposited metal Deposited metal Deposited metal Deposited metal Groove W Rake face Side flank Oil A Oil B Oil C Mineral (4.9 mm 2 /s) (9.3 mm 2 /s) (14.7 mm 2 /s) (9.3 mm 2 /s) Fig. 4 Conditions of the rake face and the side flank boundary wear (cutting speed: 12 m/min, f =.2 mm/rev, cutting distance: 6 m) On the side flank face, VB, which is thought to have formed as a result of the mechanical abrasion caused by the friction between the tool flank face and the work material, was approximately the same for MQL and dry cutting. Although the VK1 values obtained for MQL and dry cutting are different, they are similar and are considered to be caused by mechanical abrasion at the position corresponding of depth of cut. The width of VK1 (W in the figure) is approximately.3 mm for both the MQL and dry cutting. Fig. 5 shows conditions of front flank boundary wear for dry cutting and using Oils A and B at a cutting speed of 1 m/min. In dry cutting, one wear region is observed, whereas using oil, two regions are present with a spacing almost equal to the amount of feed during a single revolution (.2 mm). The reason for this is suggested in a previous paper [7]. As shown in Fig. 6, at the initial stage, wear first occurs at a position equivalent to half the feed rate from the contact point (point A) between the cutting edge and the work material. When the groove reaches a certain depth, the maximum height of the finished surface roughness, which occurred at almost the same interval as the feed rate because of its transcription, is increased. Therefore, the second wear region appears with a separation corresponding to the amount of feed in a single rotation. 1 mm (1) (2) (1) (2) (1) Chipping Oil A Oil B Fig. 5 Conditions of the front flank boundary wear (cutting speed: 1 m/min, f =.2 mm/rev, cutting distance: 6 m) - 1 -

5 1. mm Advances in Materials Science and Applications Sept. 214, Vol. 3 Iss. 3, PP Fig. 7 shows the results for a feed rate of.4 mm/rev. The cutting distance is 3,5 m. Cutting speeds of 1 and 12 m/min were used. The VK1 obtained with Oils A and B shows the same values, which are smaller than that obtained with Oil C at a cutting speed of 1 m/min. The VK1 values are smaller for MQL cutting than for dry cutting, even though the viscosity of the oil was changed. At 12 m/min, an optimum viscosity exists among Oils A, B, and C, where Oil B is the most suitable. The value of VK1 for all of the tested oils decreases approximately 3% compared with that obtained by dry cutting. At cutting speeds of 1 and 12 m/min, the VK1 obtained with Oil B (a fatty acid ester) is smaller than that obtained with mineral oil. The VK2 value is different from that for a feed rate of.2 mm/rev, and wear occurred at only one location, even for MQL. At a cutting speed of 1 m/min, tends to increase with increasing viscosity, but at 12 m/min, obtained using Oil B is the smallest among the conditions considered herein. When using Oil A at 1 m/min, the failure occurred at the portion of. Minimal quantity lubrication cutting also increases as compared with dry cutting. Compared to mineral oil, Oil B decreases. At a cutting speed of 1 m/min, the VB obtained using Oil B is somewhat small. The VB obtained by MQL cutting is approximately the same as that obtained by dry cutting. At 12 m/min, the VB tends to decease with increasing viscosity. The VB obtained for MQL cutting is slightly smaller than that obtained by dry cutting. On the whole, Oil B (moderate viscosity) is effective. A.2 mm Work VK2(2) Throw away chip, Face VK1 Fig. 6 Geometric factors affecting the occurrence of front flank boundary wear (f =.2 mm/rev) VK1 mm D : A : Oil A (4.9 mm 2 /s) B : Oil B (9.3 mm 2 /s) C : Oil C (14.7 mm 2 /s) M : Mineral (9.3 mm 2 /s), VK2(2) mm m/min 12 m/min (a) Side flank boundary wear Failure 1 m/min 12 m/min (b) Front flank boundary wear

6 Advances in Materials Science and Applications Sept. 214, Vol. 3 Iss. 3, PP VB mm m/min 12 m/min (c) Side flank wear Fig. 7 Comparison of tool wear (cutting speeds: 1 and 12 m/min, f =.4 mm/rev, cutting distance: 35 m) As mentioned above, in the case of a feed rate of.4 mm/rev, the moderate-viscosity (9.3 mm 2 /s) Oil B is effective irrespective of the change in cutting speed, and the wear reduction effect of Oil B occurs at a high cutting speed of 12 m/min. Comparison of Figs. 3 and 6 reveals that the appropriate viscosity of the oil differs depending on the feed rate. Fig. 8 shows the conditions of rake and flank faces after cutting for 3,5 m at a cutting speed of 12 m/min. In both dry cutting and MQL cutting, although a small amount of deposited metal on the rake is recognized, there is little difference in the amount of deposited metal. Moreover, differences in the amount of deposited metal according to viscosity were not observed. The amount of deposited metal obtained when using mineral oil is approximately the same as that obtained using fatty acid ester. Although not shown in the figure, the amount of deposited metal obtained at a cutting speed of 12 m/min is smaller than that obtained at 1 m/min. Furthermore, a deep groove, which is cut by metal chips in the flowing direction of the chips, appears, and the length of the groove becomes longer in the case of dry cutting. When using Oils A, B, and C, the length of the groove becomes shorter than in the dry cutting condition. 1 mm Deposited metal Deposited metal Deposited metal Deposited metal Deposited metal Groove W Rake face Side flank Oil A Oil B Oil C Mineral (4.9 mm 2 /s) (9.3 mm 2 /s) (14.7 mm 2 /s) (9.3 mm 2 /s) Fig. 8 Conditions of the rake face ant the side flank boundary wear (cutting speeds: 1 and 12 m/min, f =.4 mm/rev, cutting distance: 35 m) In both dry cutting and MQL cutting, the conditions of the flank wear (VB) are approximately the same, which is thought to be due to the mechanical abrasion caused by the friction phenomenon between the tool flank face and the machining surface. The side flank boundary wear (VK1) under both dry machining and MQL machining exhibited similar mechanical abrasion. However, the wear occurred at a location ( ) at which the cutting edge does not contact the work material far from the position of cutting depth. Therefore, the distance of VK1 (W) is expanded to approximately.5 mm. The reason for this is considered to be that the outside edge of VK1 reaches a high temperature, and the area contacts oxygen in the air. Consequently, the oxidative wear is promoted [8]. B. Discussions Generally, crater wear of the cemented carbide cutting tool is a diffusion wear caused by a chemical reaction between the tool material and the work material. The mechanism of diffusion wear has been reported to be as follows: In the heating test involving contact between the cemented carbide and the work material (steel), the tungsten carbide (WC) in the cemented carbide is resolved, and the carbon (C) diffuses into/onto the work materials. On the other hand, the steel (Fe) in the work material diffuses into/onto the cemented carbide, which generates sub-carbide, and cobalt (Co) flows out from the cemented carbide as a crystal phase, which accumulates at the interface between the cemented carbide and the work material [9]. Furthermore, the tool rake face reaches a high temperature due to rubbing by the chips during cutting. Therefore, Fe and other components in the work material and WC in the cemented carbide undergo a chemical reaction, resulting in the formation of a diffusion layer on the rake face. This diffusion layer is said to be removed by the friction generated by the backside of the chips, where the face wear gradually progresses [1]. However, the face wear generated under conditions in these experiments is assumed to be primarily welding wear and/or adhesive wear. The diffusion phenomenon between the cemented carbide and the steel occurs at a temperature of over 1,3 C

7 Advances in Materials Science and Applications Sept. 214, Vol. 3 Iss. 3, PP [9]. Under the cutting speed range in these experiments, the cutting temperature is considered to be less than 1,3 C. Therefore, the experiment was conducted under dry cutting conditions at a cutting speed of 14 m/min. However, a fracture occurred from the front flank boundary wear position to the rounded portion within a machining time of 5 minutes, which made cutting impossible. Therefore, the experiments were conducted up to a cutting speed of 12 m/min. Furthermore, when using Oils A, B, and C, a small amount of oil penetrates to the contact region between the chip and the rake face. For this reason, the adhesion of the deposited metal on the rake face is assumed to have decreased because of the anti-welding effect of the cutting oil. However, there is no difference in the influence of the viscosity of the oil. On the flank face in these experiments, the flank wear obtained for both dry cutting and MQL cutting are approximately the same conditions, which is considered to be the mechanical abrasion caused by the friction phenomenon between the tool flank face and the machining surface. The boundary wear is an abnormal wear that occurred at a point corresponding of the cutting depth. This wear is an oxidative wear caused in order to be easily in contact with the oxygen in the atmosphere. Therefore, the mechanism that generates the boundary wear is different from that which generates the face wear. When using Oils A, B, and C, the side flank boundary wear is reduced compared with dry cutting in order to prevent oxidative wear, because the oxygen supply in the atmosphere is cut off by the oil to some extent. Furthermore, the lubricating effect of the oil may be a factor in wear reduction. In order to reduce the cutting tool wear, it is necessary to choose a cutting oil with excellent lubricating properties, which would ensure reduced heat generation by reducing as much as possible the resistance to shearing in the shear plane and the resistance to friction between the tool and debris chips and between the tool and the finished surface. The oil should have a high ability to remove the generated heat, which would prevent the reduction of the cutting edge hardness and the deterioration of its wear resistance property. Therefore, when choosing a cutting oil, it is highly desirable to select an oil with a viscosity that would meet these two requirements. However, in practice, the lubrication property of mineral lubricating oils is always inversely related to their cooling ability, which means that oils with high viscosity have excellent lubrication properties but provide rather poor cooling [11, 12]. Furthermore, higher-viscosity oils have greater oil film formation ability. A thicker oil film can be easily formed on the contact surface between the tool and the worked surface, which provides significant protection against atmospheric oxygen. The wear reduction effect of the MQL system is considered to be dominated by the lubrication property rather than the cooling effect of the oil because of the small oil supply. Although high-viscosity Oil C with excellent lubricating properties was expected to be advantageous, low-viscosity Oil A or moderate-viscosity Oil B were more effective. The reason is that the relation between the tool wear and the viscosity of the oil is not simple, and high-viscosity oil has been proven to exhibit poor penetration to the contact surface between the tool and the worked surface, i.e., poor spreading ability. Therefore, the higher the viscosity of an oil, the poorer its penetration [13]. It is considered that with a high-viscosity oil, which generally has a high lubrication property, the penetration of the oil to the contact region between the cutting tool and the finished surface is extremely difficult. Furthermore, it is known that the viscosity of oil has a large effect on the producing mist rate [amount of mist (mg)/drop quantity of oil (mg) 1%], which is in inverse proportion to the viscosity of oil [14]. It is also considered that as the higher viscosity oil becomes the smaller producing mist rate, the amount of reached oil to the cutting point decreases. Although these experiments consider continuous cutting in turning, one of the authors has investigated the influence of the viscosity of oil on tool wear in intermittent cutting in hobbing [15]. This study clarified that low-viscosity oil (19.1 mm 2 /s) reduces tool wear in a viscosity range of from 19.1 to 47.6 mm 2 /s, which is different from the results of the present study. Under the cutting conditions in the viscosity range of oils in the turning test, low-viscosity oil was recognized to be effective for the wear reduction. In these experiments, in the case of MQL cutting, the side flank boundary wear decreases compared with dry cutting. However, the side flank wear is not reduced, in fact the front flank boundary wear increases. The boundary wear on the side flank and the front flank faces are said to be oxidative wear. Although cutting oils may act on both boundaries, the front flank boundary wears increases in the case of MQL machining. This is explained as follows. If the boundary wear is oxidative wear, the boundary wear may increase if a large amount of oxygen is supplied. Moreover, in an MQL system, compressed air is used to supply the oil, which increases the front flank boundary wear. Fig. 9 compares the tool wear when only compressed air is supplied at.5 MPa, identical to pressure used in an MQL system, and the tool wear in the case of dry cutting. The cutting conditions are a cutting speed of 1 m/min, a feed rate of.2 mm/rev, and a cutting distance of 6, m. When supplying compressed air, the side flank boundary wear VK1 increases slightly compared with dry cutting. Thus, the oil in the MQL system prevents the increase in VK1 caused by supplying compressed air, and Oils A and B, are considered to decrease tool wear (see Fig. 3). On the other hand, the front flank boundary wear VK2 may increase if compressed air is supplied, and VK2(2) and VK2() newly occurred at the rounded portion. Therefore, in the MQL system, there is assumed to be a high probability that oxidative wear is promoted by supplying compressed air, despite the wear reduction effect of the oil

8 Tool wear mm Tool wear mm Advances in Materials Science and Applications Sept. 214, Vol. 3 Iss. 3, PP VK mm.2.1 VB VK2() VK2 (2) () (2) (1) Compressed air, VK2 Compressed air Fig. 9 Comparison of tool wear (cutting speed: 1 m/min, f =.2 mm/rev, cutting distance: 6 m) Fig. 1 shows the experimental results obtained with the MQL and the flood using moderate-viscosity Oil B under the same condition as Fig. 9. Oil is supplied at 1 and 15 ml/h in the MQL and at 48 and 96 ml/h in the flood. The value of VB is the same for 1 and 15 ml/h. In the case of 1 ml/h, VK1 decreases, however VK2 increases. When a large amount of oil was used, as compared with MQL cutting, the observed wear was approximately the same as that in case of 1 ml/h. Furthermore, for the case in which oil was supplied at 96 ml/h, VB decreased slightly, however both VK1 and VK2 increased. Moreover, compared with dry cutting, decreased and VK2(2) occurs. These results indicate that VK2 did not increase due to the compressed air, but rather due to the oil..5.4 VK1.3.2 VK2 (2).1 VB 1 ml/h 15 ml/h 48 ml/h 96 ml/h MQL Flood Fig. 1 Influence of oil supply quantity (Oil B, cutting speed: 1 m/min, f =.2 mm/rev, cutting distance: 6 m) The oil film-forming capability and the reactivity between the frictional surface and the ester influence the wear reduction. The reaction of ester is believed to occur through hydrolysis, where fatty acids react with the metal to generate a metallic soap that functions as an excellent lubricant [16, 17], resulting in decreased the friction and tool wear. On the other hand, EP additives promote wear due to the chemical reaction of the additives and the metal [18-2]. By increasing the amount of oil supply from 48 ml/h to 96 ml/h, both VK1 and VK2 increased. This is considered to be a result of increased corrosion due to the increased oil supply. The active abrasion powder is believed to deposit on the surface of the tool and form a built-up edge, but the activation deteriorates when a fatty acid is adsorbed on the surface of the abrasion powder and the built-up edge is easily discharged outside the system without deposition. Moreover, VK2 is considered to increase because cutting the work-hardened layer of the work material caused by the cooling effect due to the large amount of cutting fluid and/or the work-hardened deposited metal adhered to the plastic flow of the work material [21]. Fig. 11 shows the colors of the chips at the beginning of the cut in the cases of dry cutting and cutting using Oil B. The color of the chip obtained at a cutting speed of 1 m/min and a feed rate of.2 mm/rev by dry cutting was dark blue/purple and that obtained by cutting using Oil B was brown. At a feed rate of.4 mm/rev, the color of the chip obtained by dry cutting was dark purple/red and that obtained by cutting using Oil B was brown. Even if the cutting speed was increased to 12 m/min, the color of the chip remained the same in both cases of dry cutting and cutting using Oil B, as compared with that obtained at a cutting speed of 1 m/min. Furthermore, the radius of curvature of the curled chip obtained at a feed rate of.4 mm/rev is smaller than that at a feed rate of.2 mm/rev

9 Surface roughness Rz mm Advances in Materials Science and Applications Sept. 214, Vol. 3 Iss. 3, PP mm Oil B Oil B f =.2 mm/rev f =.4 mm/rev 1 m/min 12 m/min Fig. 11 Condition of debris chips (at the beginning of cutting) Judging from the difference in chip color, as proposed by Fujimura [22], the cutting temperature is assumed to be lower when using the MQL system compared with dry cutting. The color of the chip was dark blue/purple or dark purple/red in the case of dry cutting and brown in the case of MQL cutting using Oil B. Fujimura has reported that the color of the chip is an interference color (temper color) of oxide film generated on the surface of the chip in an extremely short time when the chip is drastically heated by the cutting heat and is rapidly cooled in the atmosphere. Although this explains the color of the chip in dry cutting, the color of the chip obtained by MQL system can be compared with that in dry cutting, because the small amount of oil used has little influence on the color of the chip. C. Finished Surface Roughness Fig. 12 compares the finished surface roughness, Rz, at the beginning of cutting and the end of cutting at a feed rate of.2 mm/rev. At cutting speeds of 1 and 12 m/min, the finished surface roughnesses obtained by MQL cutting is greater than that obtained by dry cutting both at the beginning of cutting and the end of cutting. The finished surface roughness increases with increasing oil viscosity. The surface roughness obtained using Oil B is smaller than that obtained using mineral oil. At 12 m/min, the finished surface roughnesses obtained using Oils A and B are small compared with that obtained by dry cutting at the beginning of cutting but becomes large for MQL cutting at the end of cutting. The surface roughness obtained using Oil B is slightly smaller than that obtained using mineral oil. 4 3 D: A: Oil A B: Oil B C: Oil C M: Mineral Beginning of cutting End of cutting m/min 12 m/min Fig. 12 Comparison of surface roughness (cutting speeds: 1 and 12 m/min, f =.2 mm/rev, theoretical roughness: 6.3 μm) Fig. 13 shows the finished surface roughness at a feed rate of.4 mm/rev. Although the finished surface roughness obtained by MQL is approximately the same as that obtained by dry cutting at the beginning of cutting for each cutting speed, the finished surface roughness becomes larger at the end of cutting. At a cutting speed of 1 m/min, the finished surface roughness has a tendency to increase with increasing oil viscosity, but decreases at 12 m/min. Fig. 14 exemplifies the profiles of the finished surface roughness observed at a cutting speed of 12 m/min and a feed rate of.2 mm/rev. The black dashed line in the figure indicates the theoretical surface roughness. At the beginning of cutting, in the cases of dry cutting and cutting using Oils A, B and C, the finished surface roughness profiles are approximately the same, and although revolution marks appear at approximately the same interval at a feed rate of.2 mm/rev, the finished surface

10 25 mm Surface roughness Rz mm Advances in Materials Science and Applications Sept. 214, Vol. 3 Iss. 3, PP roughness indicates a rough profile caused by the built-up edge [23]. When using mineral oil, the disturbance of the profile is very large. At the end of cutting, for both dry cutting and MQL cutting, the profiles of the finished surface roughness are the same and revolution marks are observed. Here, the short dashed line shows the contour of the cutting tool (nose roundness), and the increase in the finished surface roughness is due to the increase in the amount of hatchings, which was transcribed by the profile of groove of the front flank boundary wear. Therefore, front flank boundary wear appears at two places on the cutting edge in the case of MQL because the peak of the finished surface roughness (shaded area in Fig. 14) becomes higher with increasing depth of groove of. Hence, the peak begins to contact at the position of approximately the same distance as the feed rate (.2 mm) from, consequently, the second wear position of VK2(2) may occur, as shown in Fig. 6. Furthermore, Fig. 13 indicates that the value for MQL cutting is larger than that for dry cutting. 6 5 D: A: Oil A B: Oil B C: Oil C M: Mineral Beginning of cutting End of cutting m/min 12 m/min Fig. 13 Comparison of surface roughness (cutting speeds: 1 and 12 m/min, f =.2mm/rev, theoretical roughness: 25 μm).1 mm Beginning of cutting End of cutting Work Oil A (4.9 mm 2 /s) Oil B (9.3 mm 2 /s) Oil C (14.7 mm 2 /s) Mineral (9.3 mm 2 /s) Fig. 14 Surface roughness profiles (cutting speed: 12 m/min, f =.2 mm/rev) Fig. 15 shows the profiles of the finished surface roughness observed at a cutting speed of 12 m/min and a feed rate of.4 mm/rev. At the beginning of cutting, the profiles of the finished surface roughness obtained by both MQL and dry cutting are approximately the same and are not affected by the built-up edge. Moreover, revolution marks can be seen at approximately the

11 25 mm Advances in Materials Science and Applications Sept. 214, Vol. 3 Iss. 3, PP same interval as the feed rate, i.e.,.4 mm/rev. At the end of cutting, in both dry cutting and MQL cutting, only the shaded areas in the figure have large surface roughness. If the increase in the finished surface roughness is assumed to be caused by the transcription caused by the profile of groove of the front flank boundary wear, the value for MQL is believed to become larger than that for dry cutting. However, as shown in Fig. 7, at a feed rate of.4 mm/rev, the second wear position does not appear because there is no contact with the first wear position, and the front cutting edge at a feed rate of.4 mm/rev is larger than that at.2 mm/rev..1 mm Beginning of cutting End of cutting Work Oil A (4.9 mm 2 /s) Oil B (9.3 mm 2 /s) Oil C (14.7 mm 2 /s) Mineral (9.3 mm 2 /s) Fig. 15 Surface roughness profiles (cutting speed: 12 m/min, f =.4 mm/rev) IV. CONCLUSIONS In this paper, the influence of cutting oil viscosity on cutting performance in turning using an MQL system was investigated. The cutting speed and feed rate was varied and the results compared to those for dry cutting. The tool wear and finished surface roughness were evaluated, and the following points were clarified: (1)At a feed rate of.2 mm/rev, MQL cutting provided reduced side flank boundary wear compared with dry cutting, irrespective of cutting speed. Low-viscosity cutting oil (4.9 mm 2 /s) is effective at reducing wear. The side flank wear was not reduced, and the front flank boundary wear became rather large. In the case of MQL cutting, front flank boundary wear occurs at two locations on the cutting edge. (2) At a feed rate of.4 mm/rev, the oils considered herein reduced the side flank boundary wear in the MQL system, irrespective of cutting speed. The cutting oil having a moderate viscosity of 9.3 mm 2 /s is suitable for reducing wear. A sufficient reduction in side flank wear cannot be obtained, and the front flank boundary wear becomes rather large. When using the MQL system, front flank boundary wear occurs at only one position. (3) The front flank boundary wear obtained by MQL cutting is larger than that obtained by dry cutting, irrespective of cutting speed or feed rate. The front flank boundary wear is closely related to the finished surface roughness. In the case of MQL, it is suggested that the depth of groove of the front flank boundary wear transcripts the finished surface roughness. (4) On the whole, low-viscosity oil is effective for high-cutting-speed conditions

12 Advances in Materials Science and Applications Sept. 214, Vol. 3 Iss. 3, PP In the future, it will be investigated the generation mechanism of the front flank boundary wear and the factors that may reduce it. REFERENCES [1] F. Klocke and G. Eisenblätter, Cutting, Annals of the CIRP, vol. 46, no. 2, pp , [2] K. Weinert, I. Inasaki, J. W. Sutherland, and T. Wakabayashi, Machining and Minimum Quantity Lubrication, Annals of the CIRP, vol. 53, no. 2, pp , 24. [3] Y. Yamamoto and S. Matsui, Effects of Mineral Oil Viscosity on Scuffing Resistance in Sliding Contacts (Part 1) -Relation between Scuffing Resistance and Operation Conditions-, Junkatsu (Lubrication), vol. 29, no. 4, pp , [4] T. Sakai, T. Murakami, Y. Yamamoto, and H. Sakamoto, Effect of Operation Conditions on Lubrication Performance of Mineral Oils in Four-Ball Testing (Part 1) -Variation of Lubrication Performance with Operation Conditions and Viscosity Grades-, Junkatsu (Lubrication), vol. 32, no. 1, pp , [5] S. Suda, H. Yokota, I. Inasaki, and T. Wakabayashi, A Synthetic Ester as an Optimal Cutting Fluid for Minimal Quantity Lubrication Machining, Annals of the CIRP, vol. 51, no. 1, pp , 22. [6] A. Katsuki, T. Sakai, and H. Matsuoka, Effect of the Viscosity Grade of Base Oil Used for Cutting Oil on Tool Wear, Japanese Journal of Tribology, vol. 37, no. 1, pp , [7] H. Matsuoka, H. Ono, T. Ryu, T. Nakae, and S. Shutou, Research on Minimal Quantity Lubrication of Cutting Oil in Turning (Effect of Quantity of Oil Supply), Transactions of the Japan Society of Mechanical Engineers, Series C, vol. 78, no. 794, pp , 212. [8] K. Nakayama, The Metal Cutting in its Principle, Corona Publishing, p. 163, [9] H. Takeyama, Cutting Process, Maruzen, pp , [1] K. Karino, Advanced Technology of Cutting Process, Kogyo Chosakai Publishing Co., Ltd., p. 513, [11] T. Sakai, F. Hirano, T. Imai, and Y. Sugimoto, Investigation on Boiling Heat Transfer from a Horizontal Wire to Mineral Oils (Part 1) -Experimental-, Junkatsu (Lubrication), vol. 29, no. 2, pp , [12] T. Sakai, F. Hirano, T. Imai, and Y. Sugimoto, Investigation on Boiling Heat Transfer from a Horizontal Wire to Mineral Oils (Part 2) -Discussions-, Junkatsu (Lubrication), vol. 29, no. 2, pp , [13] T. Sakai, F. Hirano, and N. Yamagata, Investigation on Evaporation of Films of Petroleum Oils (2nd Report) -Effects of Molecular Weight Distributions of Oils on Evaporation-, Junkatsu (Lubrication), vol. 22, no. 11, pp , [14] The Society of Cutting Fluids & Cutting Technology, Cutting and Grinding Fluids Handbook, Kogyo Chosakai Publishing Co., Ltd., pp , 24. [15] H. Matsuoka, Y. Tsuda, S. Suda, and H. Yokota, Fundamental Research on Hobbing with Minimal Quantity Lubrication of Cutting Oil (Influence of Viscosity Grade of Base Oil), Transactions of the Japan Society of Mechanical Engineers, Series C, vol. 72, no. 72, pp , 26. [16] S. Hiroi and Y. Nakayama, Cutting and Grinding Fluids, Saiwaishobo, pp , [17] F. P. Barden and D. Taber, Translated by N. Soda, The Friction and Lubrication of Solids, Maruzen, pp , [18] K. Kimura and H. Okabe, An Introduction of Tribology, Yokendo, pp , [19] H. Okabe, Wear in Corrosive Environments, Junkatsu (Lubrication), vol. 18, no. 4, pp , [2] A. Hida, On the Corrosive Wear, Junkatsu (Lubrication), vol. 3, no. 7, pp , [21] H. Takeyama, Cutting Process, Maruzen, p. 111, [22] Y. Fujimura, Machining Methods for Practical Use, Kyoritsu Shuppan, p. 172, [23] K. Nakayama, The Metal Cutting in its Principle, Corona Publishing, p. 144,