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Influences of chip serration on micro-topography of machined surface in high-speed cutting
International Journal of Machine Tools & Manufacture 89 (2015) 202–207
Contents lists available at ScienceDirect
International Journal of Machine Tools & Manufacture journal homepage: www.elsevier.com/locate/ijmactool
Short Communication
Influences of chip serration on micro-topography of machined surface in high-speed cutting Guosheng Su a,b, Zhanqiang Liu c,d,n, Liang Li e, Bing Wang c,d a
School of Mechanical & Automotive Engineering, Qilu University of Technology, Jinan, Shandong 250353, China Key Laboratory of Advanced Manufacturing and Measurement and Control Technology for Light Industry, Qilu University of Technology, Jinan 250353, China c School of Mechanical Engineering, Shandong University, Jinan, Shandong 250061, China d Key Laboratory of High Efficiency and Clean Mechanical Manufacture (Shandong University), Ministry of Education, China e College of Mechanical and Electrical Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China b
art ic l e i nf o
a b s t r a c t
Article history: Received 28 August 2014 Received in revised form 22 October 2014 Accepted 28 October 2014 Available online 3 December 2014
Saw-tooth chip changes from macroscopically continuous ribbon to separated segments with the increase of cutting speed. The aim of this study is to find the correlations between chip morphology and machined surface micro-topography at different chip serration stages encountered in high speed cutting. High strength alloy steel AerMet100 was employed in orthogonal cutting experiments to obtain chips at different serration stages and corresponding machined surfaces. The chips and machined surfaces obtained were then examined with optical microscope (OM), scanning electron microscope (SEM), and white light interferometer (WLI). The result shows that chip serration causes micro-waves on machined surface, which increases machined surface roughness. However, wave amplitudes (surface roughness) at different serration stages are different. The principal factor influencing wave amplitude is the thickness of the sawed segment (tooth) of saw-tooth chip. With cutting parameters in this study, surface roughness contributed by chip serration ranges from 0.39 μm to 1.85 μm. This may bring on serious problems in the case of trying to replace grinding with high-speed cutting in rough machining. Some suggestions have been proposed to control the chip serration-caused surface roughness in high-speed cutting based on the results of the current study. & 2014 Elsevier Ltd. All rights reserved.
Keywords: Saw-tooth chip Chip morphology Surface integrity High-speed cutting
1. Introduction Today, high-speed cutting (HSC) is one of the most important rough machining techniques to form components for machinery [1]. Saw-tooth chip (also named as serrated chip) is the typical chip morphology produced in HSC when the critical cutting speed (CCS) for chip transition from continuous to serrated is reached [2]. In contrast to continuous chip in cutting of free-machine metals at low cutting speed, saw-tooth chip is of uneven strained chip consisting alternatively of localized shear band and nearly undeformed segment [2,3]. The chip morphology transforms from continuous to serrated at CCS is contributed by the catastrophic failure of primary shear zone (PSZ) caused by thermal softening [4,5] or cracking within PSZ [3,6]. When saw-tooth chip is forming the cutting force fluctuates intensely [7]. The chip morphology transition and the n Corresponding author at: School of Mechanical Engineering, Shandong University, Jinan, Shandong 250061, China. Fax: þ 86 531 88392045. E-mail address: [email protected] (Z. Liu).
http://dx.doi.org/10.1016/j.ijmachtools.2014.10.012 0890-6955/& 2014 Elsevier Ltd. All rights reserved.
cutting force fluctuation are generally believed harmful for cutting tool and surface integrity [1,4,7]. However, to the best of our knowledge, rare research has been carried out on this topic yet. In most of the previous researches, focus were put on influences of cutting process parameters on machined surface integrity without considering the effect of chip morphology even in case of sawtooth chip [8–10]. In 2008, Mabrouki et al. [11] proposed a numerical methodology concerning orthogonal cutting to study the dry cutting of an aeronautic aluminum alloy (A2024-T351). The simulation result shows that the chip serration may cause wavy stress distribution on the machined surface which is so high that rippled geometry of machined surface is yielded. Unfortunately, the machined surface ripple has never experimentally been observed by now. Moreover, for most of metals, saw-tooth chip develops gradually from macroscopically continuous into separated segments with the further increase of cutting speed higher than CCS [12,13], during which chip segmentation degree and chip size vary greatly. We wonder how these variations in chip morphology will affect the machined surface micro-topography and integrity. This paper focus on the effects of chip morphology variation on machined surface micro-topography and integrity in high-speed
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203
cutting. High strength alloy steel AerMet100 was employed in orthogonal cutting experiments with cutting speed ranging 100– 1200 m/min. Chips generated at different serration stages and corresponding machined surfaces were obtained, and then examined with optical microscope (OM), scanning electron microscope (SEM), and white light interferometer (WLI). The relationships between chip morphology and machined surface micro-topography (surface roughness) at different chip serration stages were setup and preliminarily discussed. The results of this study have potential applications for cutting parameter selection and machined surface integrity control in high speed cutting.
2. Experiments AerMet100 steel was employed for the cutting experiment. The chemical compositions and major physical and mechanical properties of AerMet100 can be referenced in [13]. The specimen is disk in shape with 130 mm in diameter and 2 mm in thickness. The disk was screwed on a shaft which was fixed on the spindle with three screws. The arrangement of the workpiece and cutting tool are shown schematically as Fig. 1. The orthogonal cutting condition is ensured by making the cutting edge parallel to the axis of the spindle. The type of insert applied is KENNAMETA NG3142R with no chip breaker. The cutting conditions are listed in Table 1. To minimize the influence of cutting tool wear on chip formation, the cutting length is limited in three circle of the workpiece, and the insert was replaced by a new one after each cutting. Chips and correlated machined surfaces generated were collected. Some of the chips were intercepted, mounted, polished, and etched with 25% nitric acid for one minute. The machined surface and mounted and unmounted chips were photographed by OM, SEM, and machined surface roughness was tested with WLI.
3. Machined surface micro-topography at different chip serration stages The chip transformation from continuous to serrated occurs at CCS. The segmentation degree of saw-tooth chip increases with the further increase of cutting speed till the saw-tooth chip turns from macroscopically continuous ribbon into separated segments [12,13]. The OM, SEM, and WLI photos of the chips and correlated machined surfaces of workpiece at different serration stages are listed in Table 2. It shows in Table 2 (Row 1 and Row 2) that, with the cutting parameters in current study, the chip begins to be serrated at cutting speed Vc ¼120 m/min and ends serration (segmentation) at Vc¼ 1200 m/min at which the macroscopically continuous chip develops into separated segments. It can be seen from OM and WFI photos of machined surface (Table 2, Row 3 and Row 4), especially at Vc ¼200 m/min, 400 m/ min, and 600 m/min, micro-waves on machined surface have been caused by chip serration, as is numerically predicted by Mabrouki et al. [11]. However, the amplitudes of the micro-waves at different chip serration stages seem different; the micro-waves at midway of chip serration stage (Vc¼ 200 m/min, 400 m/min, and 600 m/ min) are apparent, while the micro-waves on the machined surface at beginning of serration at Vc ¼120 m/min and ending of serration at Vc¼ 1200 m/min are hardly visible. Fig. 2 shows the evolutions of the micro-wave amplitude A and related machined surface roughness Ra in cutting direction (as shown in the picture in Table 2 Row 4, Column 2) with the increase of cutting speed. The maximum of the amplitude of the micro-wave appears at Vc¼ 200 m/min, and the minimum of the
Fig. 1. Experimental setup.
Table 1 Cutting condition. Cutting speed, V (m/min) Uncut chip thickness, ac (mm/rev) Width of cut, w (mm) Rake angle, α (deg) Cutting fluid
100–1200 0.1 2 20 Dry cutting
amplitude occurs at Vc ¼1200 m/min. The chip serration makes the machined surface roughness Ra in cutting direction increase. It can be seen that the machined surface roughness Ra develops coincidentally with the micro-wave amplitude with the increase of cutting speed. The surface roughness Ra increases rapidly from 0.66 μm at Vc ¼120 m/min to 1.85 μm at Vc ¼200 m/min after which it decreases quickly to 0.82 μm at Vc¼600 m/min and then becomes flat. The minimum of the surface roughness Ra¼ 0.39 μm occurs at Vc¼1200 m/min where the fully separated segment is formed. Fig. 3 is the SEM micrograph of the peak (a) and valley (b) of the machined surface micro-wave at Vc¼ 200 m/min. It shows that there are apparently transition zones between the peak and valley about the dashed lines. In the transition zone from valley to peak, step faults are obvious, while in the zone from peak to valley dimples are apparent. On the surfaces of the peak and the valley scratches caused by cutting tool are apparent.
4. Relationships between chip morphology and machined surface roughness 4.1. Relationships between chip segmentation degree and machined surface roughness Segmentation degree Gs defined as Gs ¼(h1 h2)/h1 (h1, h2 are as shown in Fig. 4) can be used to depict the serration level of sawtooth chip at different serration stage. Based on the chip crosssectional micrographs (Table 2, Row 2) obtained in the cutting experiment, segmentation degree at different cutting speeds were calculated as shown in Fig. 4. For comparison, machined surface roughness values measured by WFI are also visualized in Fig. 4. From Fig. 4, the segmentation degree Gs and the surface roughness Ra increase rapidly to 0.84 and 1.85 μm before the
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Table 2 Saw-tooth chips and machined surfaces at different serration stages.
Vc=120m/min
Vc=200 m/min
Vc=400 m/min
Vc=600 m/min
Vc=1200 m/min G. Su et al. / International Journal of Machine Tools & Manufacture 89 (2015) 202–207
Cutting speed
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Fig. 2. Evolution of machined surface micro-wave amplitude A and roughness Ra with increase of cutting speed.
cutting speed of 200 m/min, respectively. However, evolutions of segmentation degree and surface roughness are apparently different when the cutting speed is higher than Vc ¼200 m/min; the segmentation degree increases gradually to unit, while the surface roughness decreases quickly to 0.82 μm at Vc¼ 600 m/min and then becomes flat. On the whole, the surface roughness is nonmonotonic in the chip serration range from beginning to be serrated to turning into isolated segments, while the segmentation degree increases monotonically. This means that there is weak correlation between segmentation degree and the chip serrationcaused surface roughness.
and simultaneously, the surface roughness Ra increases from 0.66 μm to its maximum of 1.85 μm at Vc ¼200 m/min. After 200 m/min, Ts/ac decreases quickly to 0.35 at Vc¼600 m/min and then becomes flat, which has the same trend as the surface roughness Ra in the same cutting speed range. However, as for the chip segment width W/ac, it almost monotonically decreases with the increase of cutting speed. The evolution trend of segment width W/ac is apparently different from the surface roughness Ra. As a summary, the thickness of the sawed segment of saw-tooth chip has great influences on machined surface roughness, while segment width has weak influences on machined surface roughness.
4.2. Relationships between chip size and machined surface roughness
4.3. Discussion
In Fig. 5, the variables Ts and W are thickness and width of sawed segment of saw-tooth chip, which are defined in the appended figure in Fig. 5. Ts/ac and W/ac are normalizations of Ts and W in which ac is uncut chip thickness. The evolutions of Ts/ac, W/ac, and machined surface roughness Ra with the increase of cutting speed are compared in Fig. 5. It shows that the evolution trend of Ts/ac is very similar to the trend of surface roughness Ra with increase of cutting speed. Ts/ac increases rapidly from 0.34 to 0.88 when the cutting speed rises from 120 m/min to 200 m/min,
It has been proved that the cutting force fluctuation when saw tooth is forming is caused by the chip segmentation process [14]. The fluctuation of cutting force will cause wavy stress states on machined surface, which are high enough to cause waves on the machined surface [11]. According to Mabrouki et al. [11], the evolution of residual stresses s11 (in the cutting direction) and s22 versus workpiece depth in the valley and peak zones shows that the s11 stress (in the cutting direction) in the peak is positive, while s22 stress is negative in the valley and positive in the peak,
Fig. 3. Peak and valley of the machined surface micro-wave under SEM.
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G. Su et al. / International Journal of Machine Tools & Manufacture 89 (2015) 202–207
Fig. 4. Evolution of segmentation degree Gs and surface roughness Ra with increase of cutting speed.
which may explain the periodicity of the micro-wave on the machined surface. So, the amplitude of cutting force fluctuation determines the amplitude of micro-waves and related roughness on the machined surface. During the formation of saw-tooth chip, the bulk (size) of the accumulated segment on cutting tool rake face before the catastrophic failure of PSZ, which is controlled by the competition level between thermal softening and combined strengthening of strain hardening and strain rate hardening [4,5], determines the increment of the cutting force [14]. Cutting experiments show the segment bulk of saw-tooth chip always increases initially from CCS to a special cutting speed and then decreases with the further increase of cutting speed [12,13], resulting the same evolution of the amplitude variation of cutting force and resultant surface roughness with the increase of cutting speed [14]. In this paper, the chip segment size is decomposed into two normalized variables as Ts/ac and W/ac, and Ts/ac is proved that it has stronger relationship with the machined surface roughness than W/ac. This maybe explained in terms of the different evolutions of Ts and W at different cutting speeds. At CCS, the strain hardening retains relatively high though thermal softening overweighs combined strengthening of strain hardening and strain rate hardening. So the thickness of localized shear band then is relatively high and the segment Ts is low [12,13]. With the
increase of cutting speed the thermal softening rises, which make the localized shear band thinner and chip segment thicker. However, with the further increase of cutting speed shear angle increases, causing the average chip thickness lower. This makes the chip segment width W and thickness Ts decrease simultaneously. So, as a whole, the chip segment thickness has the similar evolution with the amplitude variation of cutting force and stronger relationships with the resultant surface roughness, but segment width W has no. As for segmentation degree Gs, it is controlled by thermal softening and increase monotonously with the increase of cutting speed [15]. The more thermal softening overweighs the combined strengthening of strain hardening and strain rate hardening, the higher the Gs is. As mentioned above the amplitude variation of cutting force is basically controlled by the chip segment bulk but not the segmentation degree. At higher cutting speed, though segmentation degree Gs is higher the chip segment bulk is smaller, resulting smaller amplitude variation of cutting force and resultant machined surface roughness Ra. So there is no obvious relationship between Gs and machined surface roughness Ra. It has to be noted that though the correlations between Ra and Ts/ac, W/ac, and Gs may be explained in terms of cutting force, the formation mechanism of the micro-waves caused by chip serration is not completely clear. With the increase of cutting speed,
Fig. 5. Evolution of normalized chip sizes and surface roughness Ra with increase of cutting speed.
G. Su et al. / International Journal of Machine Tools & Manufacture 89 (2015) 202–207
mechanical properties of workpiece change. This may lead to shear stress state variation in the region before cutting edge when the accumulated chip segment upsets or slips along the localized shear band, and results in variations in cutting force and resultant micro-waves on machined surface. This is a complicate and important problem which we are to study.
5. Conclusions In this paper, we aim to find the correlations between chip morphologies and machined surface micro-topographies at different serration stages of saw-tooth chip encountered in high speed cutting. Orthogonal cutting experiments have been carried out, in which saw-tooth chips at different serration stages and correlated machined surfaces were obtained. The chips and machined surfaces obtained were examined by OM, SEM, and WLI. Machined surface micro-topographies were presented, and relationships between machined surface roughness and chip morphologies and chip sizes were investigated. The main points of the current study can be concluded as follows: 1. Chip serration causes the formation of micro-waves on the machined surface, and this increases surface roughness. At different serration stages the influences of chip serration on wave amplitude are different, resulting in different surface roughness at different cutting speeds. The maximum value of machined surface roughness appears at about the middle rather than beginning or ending of serration range. The minimum surface roughness is at the chip serration stage corresponding to separated segments. This hints the machining with higher cutting speed at separated segments chip is feasible under the permission of tool wear. 2. Chip segmentation degree has no effects on the chip serrationcaused surface roughness. The principal factor influencing the machined surface roughness is the thickness of sawed segment of saw-tooth chip. The width of the sawed segment has weak effect on the machined surface roughness. 3. With cutting parameters in the current study, the surface chip serration-caused surface roughness by chip serration ranges from 0.39 μm to 1.85 μm. This may cause serious problems in the case of trying to use high-speed cutting to replace grinding in roughness machining. Increasing cutting speed or decreasing uncut chip thickness to reduce the chip serration-caused surface roughness is the suggestion based on the result of the current study.
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Acknowledgments The authors would like to acknowledge the financial support by the Project of Shandong Province Higher Educational Science and Technology Program (J13LB02), the China Postdoctoral Science Foundation (2013M531593), the Shandong Province Natural Science Foundation (ZR2013EEM022), the National Natural Science Foundation of China (51425503, 51375272, U1201245), the Major Science and Technology Program of High-end CNC Machine Tools and Basic Manufacturing Equipment (2012ZX04003-041), the Outstanding Young Teacher Domestic Visiting Scholar Project for Colleges and Universities in Shandong Province, and the Tai Shan Scholar Foundation.
References [1] G. Bartarya, S.K. Choudhury, State of the art in hard turning, Int. J. Mach. Tools Manuf. 53 (2012) 1–14. [2] R. Komanduri, Z.B. Hou, On thermoplastic shear instability in the machining of a titanium alloy (TI6Al4V), Metall. Mater. Trans. A 33 (9) (2002) 2889–3010. [3] M.A. Elbestawi, A.K. Srivastava, T.I. El-Wardany, A model for chip formation during machining of hardened steel, CIRP Ann.—Manuf. Technol. 45 (7) (1996) 71–76. [4] R. Recht, A dynamic analysis of high speed machining, J. Eng. Ind.—Trans. ASME 107 (1) (1985) 309–315. [5] J. Barry, G. Byrne, D. Lennon, Observations on chip formation and acoustic emission in machining Ti–6Al–4V alloy, Int. J. Mach. Tools Manuf. 41 (2001) 1055–1070. [6] M.C. Shaw, A. Vyas, Chip formation in the machining of hardened steel, CIRP Ann.—Manuf. Technol. 42 (1) (1993) 29–33. [7] M. Cotterell, G. Byrne, Dynamics of chip formation during orthogonal cutting of titanium alloy Ti–6Al–4V, CIRP Ann.—Manuf. Technol. 57 (2008) 93–96. [8] M.S.H. Bhuiyan, I.A. Choudhury, Y. Nukman, An innovative approach to monitor the chip formation effect on tool state using acoustic emission in turning, Int. J. Mach. Tools Manuf. 58 (2012) 19–28. [9] W. Li, Y.B. Guo, C.S. Guo, Superior surface integrity by sustainable dry hard milling and impact on fatigue, CIRP Ann.—Manuf. Technol. 62 (2013) 567–570. [10] K. Bouacha, M.A. Yallese, S. Khamel, S. Belhadi, Analysis and optimization of hard turning operation using cubic boron nitride tool, Int. J. Refract. Met. Hard Mater. 45 (2014) 160–178. [11] T. Mabrouki, F. Girardin, M. Asad, J.-F. Rigal, Numerical and experimental study of dry cutting for an aeronautic aluminum alloy (A2024-T351), Int. J. Mach. Tools Manuf. 48 (2008) 1187–1197. [12] R. Komanduri, T.A. Schroeder, On shear instability in machining a nickel-iron base superalloy, J. Eng. Ind. 108 (1986) 93–100. [13] Z.Q. Liu, G.S. Su, Characteristics of chip evolution with elevating cutting speed from low to very high, Int. J. Mach. Tools Manuf. 54–55 (2012) 82–85. [14] S. Sun, M. Brandt, M.S. Dargusch, Characteristics of cutting forces and chip formation in machining of titanium alloys, Int. J. Mach. Tools Manuf. 49 (2009) 561–568. [15] H. Schulz, E. Abele, A. Sahm, Material aspects of chip formation in HSC machining, CIRP Ann.—Manuf. Technol. 50 (1) (2001) 45–48.
Contents lists available at ScienceDirect
International Journal of Machine Tools & Manufacture journal homepage: www.elsevier.com/locate/ijmactool
Short Communication
Influences of chip serration on micro-topography of machined surface in high-speed cutting Guosheng Su a,b, Zhanqiang Liu c,d,n, Liang Li e, Bing Wang c,d a
School of Mechanical & Automotive Engineering, Qilu University of Technology, Jinan, Shandong 250353, China Key Laboratory of Advanced Manufacturing and Measurement and Control Technology for Light Industry, Qilu University of Technology, Jinan 250353, China c School of Mechanical Engineering, Shandong University, Jinan, Shandong 250061, China d Key Laboratory of High Efficiency and Clean Mechanical Manufacture (Shandong University), Ministry of Education, China e College of Mechanical and Electrical Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China b
art ic l e i nf o
a b s t r a c t
Article history: Received 28 August 2014 Received in revised form 22 October 2014 Accepted 28 October 2014 Available online 3 December 2014
Saw-tooth chip changes from macroscopically continuous ribbon to separated segments with the increase of cutting speed. The aim of this study is to find the correlations between chip morphology and machined surface micro-topography at different chip serration stages encountered in high speed cutting. High strength alloy steel AerMet100 was employed in orthogonal cutting experiments to obtain chips at different serration stages and corresponding machined surfaces. The chips and machined surfaces obtained were then examined with optical microscope (OM), scanning electron microscope (SEM), and white light interferometer (WLI). The result shows that chip serration causes micro-waves on machined surface, which increases machined surface roughness. However, wave amplitudes (surface roughness) at different serration stages are different. The principal factor influencing wave amplitude is the thickness of the sawed segment (tooth) of saw-tooth chip. With cutting parameters in this study, surface roughness contributed by chip serration ranges from 0.39 μm to 1.85 μm. This may bring on serious problems in the case of trying to replace grinding with high-speed cutting in rough machining. Some suggestions have been proposed to control the chip serration-caused surface roughness in high-speed cutting based on the results of the current study. & 2014 Elsevier Ltd. All rights reserved.
Keywords: Saw-tooth chip Chip morphology Surface integrity High-speed cutting
1. Introduction Today, high-speed cutting (HSC) is one of the most important rough machining techniques to form components for machinery [1]. Saw-tooth chip (also named as serrated chip) is the typical chip morphology produced in HSC when the critical cutting speed (CCS) for chip transition from continuous to serrated is reached [2]. In contrast to continuous chip in cutting of free-machine metals at low cutting speed, saw-tooth chip is of uneven strained chip consisting alternatively of localized shear band and nearly undeformed segment [2,3]. The chip morphology transforms from continuous to serrated at CCS is contributed by the catastrophic failure of primary shear zone (PSZ) caused by thermal softening [4,5] or cracking within PSZ [3,6]. When saw-tooth chip is forming the cutting force fluctuates intensely [7]. The chip morphology transition and the n Corresponding author at: School of Mechanical Engineering, Shandong University, Jinan, Shandong 250061, China. Fax: þ 86 531 88392045. E-mail address: [email protected] (Z. Liu).
http://dx.doi.org/10.1016/j.ijmachtools.2014.10.012 0890-6955/& 2014 Elsevier Ltd. All rights reserved.
cutting force fluctuation are generally believed harmful for cutting tool and surface integrity [1,4,7]. However, to the best of our knowledge, rare research has been carried out on this topic yet. In most of the previous researches, focus were put on influences of cutting process parameters on machined surface integrity without considering the effect of chip morphology even in case of sawtooth chip [8–10]. In 2008, Mabrouki et al. [11] proposed a numerical methodology concerning orthogonal cutting to study the dry cutting of an aeronautic aluminum alloy (A2024-T351). The simulation result shows that the chip serration may cause wavy stress distribution on the machined surface which is so high that rippled geometry of machined surface is yielded. Unfortunately, the machined surface ripple has never experimentally been observed by now. Moreover, for most of metals, saw-tooth chip develops gradually from macroscopically continuous into separated segments with the further increase of cutting speed higher than CCS [12,13], during which chip segmentation degree and chip size vary greatly. We wonder how these variations in chip morphology will affect the machined surface micro-topography and integrity. This paper focus on the effects of chip morphology variation on machined surface micro-topography and integrity in high-speed
G. Su et al. / International Journal of Machine Tools & Manufacture 89 (2015) 202–207
203
cutting. High strength alloy steel AerMet100 was employed in orthogonal cutting experiments with cutting speed ranging 100– 1200 m/min. Chips generated at different serration stages and corresponding machined surfaces were obtained, and then examined with optical microscope (OM), scanning electron microscope (SEM), and white light interferometer (WLI). The relationships between chip morphology and machined surface micro-topography (surface roughness) at different chip serration stages were setup and preliminarily discussed. The results of this study have potential applications for cutting parameter selection and machined surface integrity control in high speed cutting.
2. Experiments AerMet100 steel was employed for the cutting experiment. The chemical compositions and major physical and mechanical properties of AerMet100 can be referenced in [13]. The specimen is disk in shape with 130 mm in diameter and 2 mm in thickness. The disk was screwed on a shaft which was fixed on the spindle with three screws. The arrangement of the workpiece and cutting tool are shown schematically as Fig. 1. The orthogonal cutting condition is ensured by making the cutting edge parallel to the axis of the spindle. The type of insert applied is KENNAMETA NG3142R with no chip breaker. The cutting conditions are listed in Table 1. To minimize the influence of cutting tool wear on chip formation, the cutting length is limited in three circle of the workpiece, and the insert was replaced by a new one after each cutting. Chips and correlated machined surfaces generated were collected. Some of the chips were intercepted, mounted, polished, and etched with 25% nitric acid for one minute. The machined surface and mounted and unmounted chips were photographed by OM, SEM, and machined surface roughness was tested with WLI.
3. Machined surface micro-topography at different chip serration stages The chip transformation from continuous to serrated occurs at CCS. The segmentation degree of saw-tooth chip increases with the further increase of cutting speed till the saw-tooth chip turns from macroscopically continuous ribbon into separated segments [12,13]. The OM, SEM, and WLI photos of the chips and correlated machined surfaces of workpiece at different serration stages are listed in Table 2. It shows in Table 2 (Row 1 and Row 2) that, with the cutting parameters in current study, the chip begins to be serrated at cutting speed Vc ¼120 m/min and ends serration (segmentation) at Vc¼ 1200 m/min at which the macroscopically continuous chip develops into separated segments. It can be seen from OM and WFI photos of machined surface (Table 2, Row 3 and Row 4), especially at Vc ¼200 m/min, 400 m/ min, and 600 m/min, micro-waves on machined surface have been caused by chip serration, as is numerically predicted by Mabrouki et al. [11]. However, the amplitudes of the micro-waves at different chip serration stages seem different; the micro-waves at midway of chip serration stage (Vc¼ 200 m/min, 400 m/min, and 600 m/ min) are apparent, while the micro-waves on the machined surface at beginning of serration at Vc ¼120 m/min and ending of serration at Vc¼ 1200 m/min are hardly visible. Fig. 2 shows the evolutions of the micro-wave amplitude A and related machined surface roughness Ra in cutting direction (as shown in the picture in Table 2 Row 4, Column 2) with the increase of cutting speed. The maximum of the amplitude of the micro-wave appears at Vc¼ 200 m/min, and the minimum of the
Fig. 1. Experimental setup.
Table 1 Cutting condition. Cutting speed, V (m/min) Uncut chip thickness, ac (mm/rev) Width of cut, w (mm) Rake angle, α (deg) Cutting fluid
100–1200 0.1 2 20 Dry cutting
amplitude occurs at Vc ¼1200 m/min. The chip serration makes the machined surface roughness Ra in cutting direction increase. It can be seen that the machined surface roughness Ra develops coincidentally with the micro-wave amplitude with the increase of cutting speed. The surface roughness Ra increases rapidly from 0.66 μm at Vc ¼120 m/min to 1.85 μm at Vc ¼200 m/min after which it decreases quickly to 0.82 μm at Vc¼600 m/min and then becomes flat. The minimum of the surface roughness Ra¼ 0.39 μm occurs at Vc¼1200 m/min where the fully separated segment is formed. Fig. 3 is the SEM micrograph of the peak (a) and valley (b) of the machined surface micro-wave at Vc¼ 200 m/min. It shows that there are apparently transition zones between the peak and valley about the dashed lines. In the transition zone from valley to peak, step faults are obvious, while in the zone from peak to valley dimples are apparent. On the surfaces of the peak and the valley scratches caused by cutting tool are apparent.
4. Relationships between chip morphology and machined surface roughness 4.1. Relationships between chip segmentation degree and machined surface roughness Segmentation degree Gs defined as Gs ¼(h1 h2)/h1 (h1, h2 are as shown in Fig. 4) can be used to depict the serration level of sawtooth chip at different serration stage. Based on the chip crosssectional micrographs (Table 2, Row 2) obtained in the cutting experiment, segmentation degree at different cutting speeds were calculated as shown in Fig. 4. For comparison, machined surface roughness values measured by WFI are also visualized in Fig. 4. From Fig. 4, the segmentation degree Gs and the surface roughness Ra increase rapidly to 0.84 and 1.85 μm before the
204
Table 2 Saw-tooth chips and machined surfaces at different serration stages.
Vc=120m/min
Vc=200 m/min
Vc=400 m/min
Vc=600 m/min
Vc=1200 m/min G. Su et al. / International Journal of Machine Tools & Manufacture 89 (2015) 202–207
Cutting speed
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Fig. 2. Evolution of machined surface micro-wave amplitude A and roughness Ra with increase of cutting speed.
cutting speed of 200 m/min, respectively. However, evolutions of segmentation degree and surface roughness are apparently different when the cutting speed is higher than Vc ¼200 m/min; the segmentation degree increases gradually to unit, while the surface roughness decreases quickly to 0.82 μm at Vc¼ 600 m/min and then becomes flat. On the whole, the surface roughness is nonmonotonic in the chip serration range from beginning to be serrated to turning into isolated segments, while the segmentation degree increases monotonically. This means that there is weak correlation between segmentation degree and the chip serrationcaused surface roughness.
and simultaneously, the surface roughness Ra increases from 0.66 μm to its maximum of 1.85 μm at Vc ¼200 m/min. After 200 m/min, Ts/ac decreases quickly to 0.35 at Vc¼600 m/min and then becomes flat, which has the same trend as the surface roughness Ra in the same cutting speed range. However, as for the chip segment width W/ac, it almost monotonically decreases with the increase of cutting speed. The evolution trend of segment width W/ac is apparently different from the surface roughness Ra. As a summary, the thickness of the sawed segment of saw-tooth chip has great influences on machined surface roughness, while segment width has weak influences on machined surface roughness.
4.2. Relationships between chip size and machined surface roughness
4.3. Discussion
In Fig. 5, the variables Ts and W are thickness and width of sawed segment of saw-tooth chip, which are defined in the appended figure in Fig. 5. Ts/ac and W/ac are normalizations of Ts and W in which ac is uncut chip thickness. The evolutions of Ts/ac, W/ac, and machined surface roughness Ra with the increase of cutting speed are compared in Fig. 5. It shows that the evolution trend of Ts/ac is very similar to the trend of surface roughness Ra with increase of cutting speed. Ts/ac increases rapidly from 0.34 to 0.88 when the cutting speed rises from 120 m/min to 200 m/min,
It has been proved that the cutting force fluctuation when saw tooth is forming is caused by the chip segmentation process [14]. The fluctuation of cutting force will cause wavy stress states on machined surface, which are high enough to cause waves on the machined surface [11]. According to Mabrouki et al. [11], the evolution of residual stresses s11 (in the cutting direction) and s22 versus workpiece depth in the valley and peak zones shows that the s11 stress (in the cutting direction) in the peak is positive, while s22 stress is negative in the valley and positive in the peak,
Fig. 3. Peak and valley of the machined surface micro-wave under SEM.
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Fig. 4. Evolution of segmentation degree Gs and surface roughness Ra with increase of cutting speed.
which may explain the periodicity of the micro-wave on the machined surface. So, the amplitude of cutting force fluctuation determines the amplitude of micro-waves and related roughness on the machined surface. During the formation of saw-tooth chip, the bulk (size) of the accumulated segment on cutting tool rake face before the catastrophic failure of PSZ, which is controlled by the competition level between thermal softening and combined strengthening of strain hardening and strain rate hardening [4,5], determines the increment of the cutting force [14]. Cutting experiments show the segment bulk of saw-tooth chip always increases initially from CCS to a special cutting speed and then decreases with the further increase of cutting speed [12,13], resulting the same evolution of the amplitude variation of cutting force and resultant surface roughness with the increase of cutting speed [14]. In this paper, the chip segment size is decomposed into two normalized variables as Ts/ac and W/ac, and Ts/ac is proved that it has stronger relationship with the machined surface roughness than W/ac. This maybe explained in terms of the different evolutions of Ts and W at different cutting speeds. At CCS, the strain hardening retains relatively high though thermal softening overweighs combined strengthening of strain hardening and strain rate hardening. So the thickness of localized shear band then is relatively high and the segment Ts is low [12,13]. With the
increase of cutting speed the thermal softening rises, which make the localized shear band thinner and chip segment thicker. However, with the further increase of cutting speed shear angle increases, causing the average chip thickness lower. This makes the chip segment width W and thickness Ts decrease simultaneously. So, as a whole, the chip segment thickness has the similar evolution with the amplitude variation of cutting force and stronger relationships with the resultant surface roughness, but segment width W has no. As for segmentation degree Gs, it is controlled by thermal softening and increase monotonously with the increase of cutting speed [15]. The more thermal softening overweighs the combined strengthening of strain hardening and strain rate hardening, the higher the Gs is. As mentioned above the amplitude variation of cutting force is basically controlled by the chip segment bulk but not the segmentation degree. At higher cutting speed, though segmentation degree Gs is higher the chip segment bulk is smaller, resulting smaller amplitude variation of cutting force and resultant machined surface roughness Ra. So there is no obvious relationship between Gs and machined surface roughness Ra. It has to be noted that though the correlations between Ra and Ts/ac, W/ac, and Gs may be explained in terms of cutting force, the formation mechanism of the micro-waves caused by chip serration is not completely clear. With the increase of cutting speed,
Fig. 5. Evolution of normalized chip sizes and surface roughness Ra with increase of cutting speed.
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mechanical properties of workpiece change. This may lead to shear stress state variation in the region before cutting edge when the accumulated chip segment upsets or slips along the localized shear band, and results in variations in cutting force and resultant micro-waves on machined surface. This is a complicate and important problem which we are to study.
5. Conclusions In this paper, we aim to find the correlations between chip morphologies and machined surface micro-topographies at different serration stages of saw-tooth chip encountered in high speed cutting. Orthogonal cutting experiments have been carried out, in which saw-tooth chips at different serration stages and correlated machined surfaces were obtained. The chips and machined surfaces obtained were examined by OM, SEM, and WLI. Machined surface micro-topographies were presented, and relationships between machined surface roughness and chip morphologies and chip sizes were investigated. The main points of the current study can be concluded as follows: 1. Chip serration causes the formation of micro-waves on the machined surface, and this increases surface roughness. At different serration stages the influences of chip serration on wave amplitude are different, resulting in different surface roughness at different cutting speeds. The maximum value of machined surface roughness appears at about the middle rather than beginning or ending of serration range. The minimum surface roughness is at the chip serration stage corresponding to separated segments. This hints the machining with higher cutting speed at separated segments chip is feasible under the permission of tool wear. 2. Chip segmentation degree has no effects on the chip serrationcaused surface roughness. The principal factor influencing the machined surface roughness is the thickness of sawed segment of saw-tooth chip. The width of the sawed segment has weak effect on the machined surface roughness. 3. With cutting parameters in the current study, the surface chip serration-caused surface roughness by chip serration ranges from 0.39 μm to 1.85 μm. This may cause serious problems in the case of trying to use high-speed cutting to replace grinding in roughness machining. Increasing cutting speed or decreasing uncut chip thickness to reduce the chip serration-caused surface roughness is the suggestion based on the result of the current study.
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Acknowledgments The authors would like to acknowledge the financial support by the Project of Shandong Province Higher Educational Science and Technology Program (J13LB02), the China Postdoctoral Science Foundation (2013M531593), the Shandong Province Natural Science Foundation (ZR2013EEM022), the National Natural Science Foundation of China (51425503, 51375272, U1201245), the Major Science and Technology Program of High-end CNC Machine Tools and Basic Manufacturing Equipment (2012ZX04003-041), the Outstanding Young Teacher Domestic Visiting Scholar Project for Colleges and Universities in Shandong Province, and the Tai Shan Scholar Foundation.
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