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A study of materials swelling and recovery in single-point diamond turning of ductile materials
Journal of Materials Processing Technology 180 (2006) 210–215
A study of materials swelling and recovery in single-point diamond turning of ductile materials M.C. Kong ∗ , W.B. Lee, C.F. Cheung, S. To Advanced Optics Manufacturing Centre, Department of Industrial and Systems Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong Received 27 February 2006; received in revised form 27 February 2006; accepted 6 June 2006
Abstract This paper presents an investigation of the effect of materials swelling in ultra-precision machining of ductile materials. The combined influence of materials swelling and recovery was found to affect the surface roughness in single-point diamond turning. It is interesting to note that the effect of materials swelling for ductile materials would be overwhelmed by the impact of recovery when the depth of cut is extremely small and the front clearance is small. In addition, radically different surface roughness profiles were found for different materials even though they are machined under the same cutting conditions. The difference in the machining behaviour could not be accounted by the elastic recovery alone but by the plastic deformation induced in the machined layer. The findings in the present study provide an important means for improving the surface roughness in ultra-precision machining. © 2006 Elsevier B.V. All rights reserved. Keywords: Materials swelling; Recovery; Ductile materials; Single-point diamond turning; Surface roughness
1. Introduction The issue of surface generation especially the process factors affecting surface generation in machining operations has attracted a great deal of researchers to explore [1–4]. However, most of the previous work merely focused on the geometrical and process aspects. Relatively little attention has been paid to the effect of material properties in single-point diamond turning (SPDT), particularly on the analysis of materials swelling and the recovery in SPDT and its effect on surface roughness. Surface generation in SPDT is a complicated process which involves burnishing, elastic recovery, plastic deformation, and materials swelling. Unlike conventional machining, material factors have a larger influence on the cutting process as the depth of cut is often less than the grain size of materials. When machining is conducted at this small depth of cut and fine feed rate, cutting naturally becomes of a single crystal nature [5]. It is well known that single crystal materials are highly anisotropic in their physical and mechanical properties. In the same manner, the polycrystalline workpiece material is treated as a series of
single crystals in SPDT notwithstanding the consideration as an isotropic and homogeneous material in a conventional analysis [6]. Therefore, the generation of surface roughness is considered to be dependent on the crystallographic factors being cut. The phenomenon of burnishing and recovery basically appears when generating the tool-edge cut surface, while materials swelling mainly occurs when generating the tool-nose cut surface. Fig. 1 illustrates the surface roughness profiles of ideal tool-edge cut surface and ideal tool-nose cut surface, respectively. Both burnishing effect and swelling effect have an impact on the machined surface. However, the domination of influence depends on the main cutting force, thrust cutting force as well as Young’s modulus of the substrate materials. In short, their combined impact distorts the fidelity of transferring tool profiles on to a machined surface. In this paper, the effect of materials swelling on the surface quality of several types of aluminum and electroless nickel phosphorus is investigated. In addition, the formation of the tool-edge cut surface and the tool-nose cut surface of the materials are also discussed. 1.1. Tool-edge cut surface in SPDT
∗
Corresponding author. Tel.: +852 2766 6584. E-mail address: [email protected] (M.C. Kong).
0924-0136/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2006.06.006
Since cutting is done at a very high speed in SPDT, there is no build-up edge and hence the ideal tool-edge cut surface would
M.C. Kong et al. / Journal of Materials Processing Technology 180 (2006) 210–215
211
Nomenclature d DRA f Fz m R Ra Rt s Si V
depth of cut (m) degree of roughness anisotropy feed rate (mm min−1 ) thrust cutting force (N) number of angular spaced radial sections tool-nose radius (mm) arithmetic surface roughness (nm) maximum peak-to-valley height (mm) tool feed rate (mm rev−1 ) swelling significant index rotational spindle speed (rpm)
Greek letters ϕ angle of each radial section (◦ ) standard deviation of the arithmetic roughness σRa
be obtained theoretically for a perfectly sharp cutting tool as illustrated in Fig. 2(a). On the other hand, the cutting tool can have two facets. The cutting edge is for cutting the materials while the burnishing edge can be used to burnish the freshly machined surface. The burnishing motion following the cutting motion can yield a very fine surface finish [7]. Nevertheless, the case is not perfect in reality, as the single-point diamond tools always process a radius at the cutting edge and the depth of cut is extremely small. As a result, the burnishing edge burnishes
Fig. 2. (a and b) Generation of a tool-edge cut surface in an ideal and a real case.
or indents the freshly machined surface but does not remove the material. It brings detrimental physical properties such as residual stress onto the surface. After burnishing, the material left behind the front clearance recovers (i.e., springs back). This amount of recovery is determined by the forces on the clearance face and by the Young’s modulus of the substrate material. Once again, this amount of recovery relies on the grain orientation. Different grain orientations have different amounts of recovery, and consequently give a wavy surface with elastic and plastic deformation as shown in Fig. 2(b). 1.2. Tool-nose cut surface in SPDT The ideal case of maximum peak-to-valley height of surface roughness Rt in the absence of the recovery of the plastic deformation is derived by tool geometry and tool feed only as described in Eq. (1). Sata reported the existence of materials swelling. He pointed out that the workpiece swelled, causing greater tool marks due to tool-nose geometry, thus resulting in higher surface roughness than in the theoretical case [8]. Rt =
Fig. 1. Graphical illustration of ideal tool-edge cut profile and tool-nose cut profile.
f2 8RV 2
as s R
(1)
Fundamentally, there are two types of materials swelling: side swelling and deep swelling. Side swelling is due to the plastic side flow, in which the metal left behind the cutting edge has undergone high pressure and causes the metal to flow to the side of the active cutting edge [9]. The side flow of the workpiece material is facilitated by high temperature and normal stress in the cutting zone, where the workpiece material at the chip/tool
212
M.C. Kong et al. / Journal of Materials Processing Technology 180 (2006) 210–215 Table 1 Materials and cutting conditions
Fig. 3. Illustration of the effect of materials swelling and recovery cross the tool-nose surface.
and workpiece/tool interface behaves as a viscous fluid. Thus, the metal at the trailing edge of the tool flows to the side to relieve the stress. This implies that the plastic deformation is not only along the feed marks but also in between the feed marks [10]. Although it has been reported that side swelling would lead to slightly wider tool marks in some cases, the effect is relatively insignificant when compared with deep swelling [11]. Deep swelling is due to the thrust cutting force along the main cutting edge of the tool as shown in Fig. 3. It causes the work material near the nose of the tool to flow to the free surface. The effect of swelling increases the surface roughness of the diamond turned surface. Fig. 3 shows the tool profile in the absence and in the presence of deep swelling as well as the influence of recovery. The depth of the tool marks after materials swelling Hs is greater than that of the ideal case Hi when deep swelling is present. However, the depth of tool marks after the combined effect of swelling and recovery Hr may be less than that of the ideal case Hi , which depends on the amount of recovery of the machined surface. As discussed previously, the font clearance of the diamond tool burnishes the machined surface, and the material recovers after burnishing during diamond turning. The amount of recovery is determined by the forces on the clearance face and by the crystallographic orientation of the substrate material being cut. The combined effect of burnishing, materials swelling, and recovery on the workpiece material affects the fidelity of the tool profile transfer to the diamond turned surface.
Materials
Specimen A: electroless nickel phosphorus Specimen B: rod aluminum alloy (6061) Specimen C: plate aluminum alloy (6061) Specimen D: straight-rolled aluminum alloy (6061) Specimen E: aluminum single crystal (1 1 0)
Spindle rotational speed (rpm) Feed rate (mm min−1 ) Depth of cut (m) Tool rake angle (◦ ) Front clearance angle (◦ ) Tool-nose radius (mm)
2000 40 2 0.0 7.0 0.5
avoiding the phenomenon of tool interference and generating a clear tool mark for the ease of analysis. The cutting tests were performed on a two-axis CNC ultra-precision machine (Nanoform® 200 from Precitech, Inc., USA). The surface roughness of the machined surfaces was examined by a non-contact type surface measurement system, Wyko NT 8000 (Veeco Metrology Group) and then by a contact type measurement system, Form Talysurf PGI 1240 (Taylor Hobson Ltd.), respectively. The magnification objectives used in the Wyko NT 8000 was 20× and the field of view was 309 m × 231 m for each machined sample. The phaseshifting interferometry (PSI) mode, which is typically used to test smooth surfaces (roughness less than 30 nm), was chosen in the experiment as the theoretical surface roughness is approximately 25 nm. The surface roughness profiles were then plotted by Vision software from Veeco Instruments, Inc. Regarding the Form Talysurf PGI 1240, the standard stylus with a tool tip radius 2 m was used. The surface roughness profiles at 12 radial sections (i.e., ϕ = 30◦ ) on a diamond turned surface were measured with the Form Talysurf system equipped with a rotary table. The measured length and the cut-off length were fixed at 1.25 and 0.25 mm, respectively.
3. Results and discussion For the purpose of measuring quantitatively the local variation of surface roughness, a parameter named degree of roughness anisotropy (DRA) [12] is used based on the arithmetic roughness Ra . It is defined as the ratio of the standard deviation and the mean of the arithmetic roughness values at a finite number of equally angular spaced radial sections of the machined
2. Experimental A series of orthogonal face cutting tests was conducted among five types of materials including electroless nickel phosphorus (NiP), rod aluminum alloy (6061-T6), plate aluminum alloy (6061-T6), straight-rolled aluminum alloy (6061-T6) and aluminum single crystal (Al 1 1 0) under the same cutting conditions. All samples used in the experiments were available in the markets except the straight-rolled aluminum alloy. The specimens of straight-rolled aluminum alloy were prepared by rolling along the original hot band rolling direction in multi-passes to a thickness of reduction to 90%. Table 1 tabulates the materials and the cutting conditions used in the experiments. It should be highlighted that a small radius, a low spindle speed, a high feed rate were preferred for the sake of
Fig. 4. Plot of mean arithmetic roughness and degree of roughness anisotropy against the conducted materials.
M.C. Kong et al. / Journal of Materials Processing Technology 180 (2006) 210–215
surface, i.e.,
m m i−1 Ra,i − Ra σRa DRA = = √ Ra m−1 m i−1 Ra,i
(2)
where Ra,i , is the arithmetic roughness value at ith radial section of the workpiece and Ra and σRa are the mean and the standard deviation of the arithmetic roughness values for the m radial sections on the surface. Fig. 4 shows the mean value of arithmetic roughness and the degree of roughness anisotropy against different types of materials measured by Form Talysurf PGI 1240 as described in the above section, respectively. The ascending order of Ra among the workpiece materials is electroless nickel phospho-
213
rus, plate aluminum alloy, rod aluminum alloy, straight-rolled aluminum alloy and aluminum single crystal (Al 1 1 0). The ascending order of DRA among the workpiece materials is electroless nickel phosphorus, rod aluminum alloy, plate aluminum alloy, straight-rolled aluminum alloy and aluminum single crystal (Al 1 1 0). A large value of DRA implies a high anisotropy of surface roughness. It appears that both of the values of Ra and DRA of electroless nickel phosphorus are the smallest while that for aluminum single crystal are the largest among the materials being cut. It can be explained by the effect of strong crystallographic orientation in Al (1 1 0) as the same crystallographic plane of different types of single crystal resulted in the same periodicity of variation of surface roughness. In this case, (1 1 0) crystals would have two-fold symmetry [13]. In contrast, electro-
Table 2 Two-dimensional and three-dimensional pictures of surface roughness profiles measured by Wyko measurement system
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M.C. Kong et al. / Journal of Materials Processing Technology 180 (2006) 210–215
less nickel phosphorus possesses an amorphous microstructure in the absence of grain boundary [14]. The Ra and DRA of straight-rolled Al6061 is also relatively high when compared with other polycrystalline materials such as rod aluminum 6061 and plate aluminum 6061. It is thanks to its texture with preferred orientations. The straight-rolled Al6061 has preferred orientation but it is not as strong as the single crystal one. As a result, the measured values are above the same group of polycrystalline materials but below the single crystal material. In fact, Al6061 plate has a comparatively stronger preferred orientation than that of rod Al6061 and this can be revealed from their DRA values. Normally, the experimental surface roughness is expected to be higher than the theoretical one as some factors such as relative vibration between the tool and workpiece, spindle error motion, materials swelling, crystallographic orientation influencing the surface generation may add on the surface roughness. Nevertheless, the values of the machined surface were a little bit lower than the theoretical value. The unexpected better surface can be accounted for the effect of recovery at the set of cutting condition. In SPDT, as cutting tools always process a radius at the cutting edge and the depth of cut is extremely small (e.g., 2 m). As the depth of cut becomes of the same order as the tool-edge radius, the assumption regarding the perfectly sharp tool-edge is no longer valid and the machining process may involve significant sliding along the clearance face of the tool owing to the elastic recovery of the workpiece material [15]. It is interesting to note that the main cutting force is very small while the depth of cut is very small in the range of ultra-precision machining [16]. In this experiment, the depth of cut was 2 m which is relatively small in ultra-precision machining. Moreover, the small front clearance angle makes the friction due to the elastic recovery of the workpiece at the clearance face increases. Therefore, the freshly machined surface along the cutting direction tends to recovery after burnishing as the main cutting force is extremely small. Table 2 gives two-dimensional as well as three-dimensional surface roughness profiles measured by Wyko measurement system. The X profile is one of the tool-nose cut surface profile extracted from the three-dimensional surface profile while the Y profile is tool-edge cut surface profile. The scales of X profile and Y profile are the same for all specimens. According to the results of the Y profile among different types of workpiece materials, the profile of electroless nickel phosphorus is very smooth and it is similar to a straight line while Al (1 1 0) is very varied and the waviness length is very small. This is because of the variation of crystallographic orientations. As a matter of fact, the results of Y profile can be co-related to the results of surface roughness and DRA as mentioned above. It is arousing that each type of workpiece has its distinctive pattern of tool marks as shown in Table 2. For instance, electroless nickel phosphorus (NiP) was generated into a repeatable pattern of tool mark and has a smooth surface. Aluminum single crystal (1 1 0) also has a clear shape of tool mark as NiP but there was surface crack on the valley of the machined surface thanks to the grains orientations. The straight-rolled aluminum alloy
(6061) and plate aluminum alloy (6061) has a similar pattern of tool marks as the nature of sample preparation is similar. But the straight-rolled aluminum alloy has a larger extent of variation. The rod aluminum alloy (6061) had a strong surface spikes on the smooth surface, which was caused by precipitation hardened aluminum. Notwithstanding the unexpected good surface, there is significant effect of anisotropy of surface roughness due to crystallographic orientations. This can be revealed from the parameter DRA which provides a measure of the normalized variation of the surface roughness over the diamond turned surface. 4. Conclusions The results from the degree of roughness anisotropy indicate that the surface roughness varies systematically at different radial sections of the workpiece. The variation depends on the crystallographic orientations of the workpiece material. The arithmetic roughness Ra is found to be strongly co-related to the degree of roughness anisotropy. The effect of materials swelling for ductile materials is overwhelmed by the effect of materials recovery when the depth of cut is extremely small and the degree of the front clearance angle is comparatively small. In addition, there are distinctive features of tool mark pattern for each kind of specimens. Although there is still a required further investigation on the critical depth of cut for the recovery-dominance, it can be found that there is a case of recovery overwhelming the influence of materials swelling. The findings from this research can yield a better understanding of the process of materials swelling and recovery as well as its relationships with various workpiece materials in SPDT. It is notably essential for the improvement of the surface finish of the diamond turned surfaces. Acknowledgment The authors would like to express their sincere thanks to the Research Grants Council of the Hong Kong Special Administrative Region of the People’s Republic of China for providing financial support for this research work under the project no. PolyU 5269/03E. References [1] K. Mitsui, H. Sato, Frequency characteristic of cutting process identified by an in-process measurement of surface roughness, Ann. CIRP 27 (1) (1978) 67–71. [2] T.P. Tai, Y.C. Yang, Y.C. Hwong, C.H. Ku, A new concept of cutting marks formation in metal cutting vibration, in: Proceedings of 20th MTDR, 1980, p. 449. [3] S. Takasu, M. Masuda, T. Nishiguchi, Influence of steady vibration with small amplitude upon surface roughness in diamond machining, Ann. CIRP 34 (1) (1985) 463–467. [4] T. Bispink, Performance analysis of feed-drive systems in diamond turning by machining specified test samples, Ann. CIRP 41 (1) (1992) 601–604. [5] W.B. Lee, M. Zhou, A theoretical analysis of the effect of crystallographic orientation on chip formation in micro-machining, Int. J. Mach. Tools Manuf. 33 (3) (1993) 439–447.
M.C. Kong et al. / Journal of Materials Processing Technology 180 (2006) 210–215 [6] Z.J. Yuan, W.B. Lee, Y.X. Yao, M. Zhou, Effect of crystallographic orientation on cutting forces and surface quality in diamond cutting of single crystal, Ann. CIRP 43 (1) (1994) 39–42. [7] D.J. Whitehouse, Handbook of Surface Metrology, first ed., Institute of Physics Publishing, Bristol, UK, 1994. [8] T. Sata, Surface finish in metal cutting, Ann. CIRP 12 (1964) 190. [9] M.C. Shaw, J.A. Crowell, Finish machining, Ann. CIRP 13 (1965) 5. [10] H.A. Kishawy, Chip formation and surface Integrity in high speed machining of hardened steel, Ph.D. Thesis, The McMaster University, Canada, 1998. [11] S. To, C.F. Cheung, W.B. Lee, Influence of Material swelling upon surface roughness in diamond turning of single crystals, Mater. Sci. Technol. 17 (1) (2001) 102–108.
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[12] C.F. Cheung, W.B. Lee, Study of factors affecting the surface quality in ultra-precision diamond turning, Mater. Manuf. Process. 15 (4) (2000) 481–502. [13] W.B. Lee, S. To, C.F. Cheung, Effect of crystallographic orientation in diamond turning of copper single crystals, Scr. Mater. 42 (2000) 937– 945. [14] B.D. Cullity, Elements of X-ray Diffraction, second print, Addison-Wesley Publishing Company Inc., October 1978. [15] J.D. Kim, D.S. Kim, Theoretical analysis of micro-machining characteristics in ultra-precision machining, J. Mater. Process. Technol. 49 (1995) 387–398. [16] W.B. Lee, C.F. Cheung, S. To, Characteristics of micro-cutting force variation in ultra-precision machining, Mater. Manuf. Process. 16 (2) (2001) 177–193.
A study of materials swelling and recovery in single-point diamond turning of ductile materials M.C. Kong ∗ , W.B. Lee, C.F. Cheung, S. To Advanced Optics Manufacturing Centre, Department of Industrial and Systems Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong Received 27 February 2006; received in revised form 27 February 2006; accepted 6 June 2006
Abstract This paper presents an investigation of the effect of materials swelling in ultra-precision machining of ductile materials. The combined influence of materials swelling and recovery was found to affect the surface roughness in single-point diamond turning. It is interesting to note that the effect of materials swelling for ductile materials would be overwhelmed by the impact of recovery when the depth of cut is extremely small and the front clearance is small. In addition, radically different surface roughness profiles were found for different materials even though they are machined under the same cutting conditions. The difference in the machining behaviour could not be accounted by the elastic recovery alone but by the plastic deformation induced in the machined layer. The findings in the present study provide an important means for improving the surface roughness in ultra-precision machining. © 2006 Elsevier B.V. All rights reserved. Keywords: Materials swelling; Recovery; Ductile materials; Single-point diamond turning; Surface roughness
1. Introduction The issue of surface generation especially the process factors affecting surface generation in machining operations has attracted a great deal of researchers to explore [1–4]. However, most of the previous work merely focused on the geometrical and process aspects. Relatively little attention has been paid to the effect of material properties in single-point diamond turning (SPDT), particularly on the analysis of materials swelling and the recovery in SPDT and its effect on surface roughness. Surface generation in SPDT is a complicated process which involves burnishing, elastic recovery, plastic deformation, and materials swelling. Unlike conventional machining, material factors have a larger influence on the cutting process as the depth of cut is often less than the grain size of materials. When machining is conducted at this small depth of cut and fine feed rate, cutting naturally becomes of a single crystal nature [5]. It is well known that single crystal materials are highly anisotropic in their physical and mechanical properties. In the same manner, the polycrystalline workpiece material is treated as a series of
single crystals in SPDT notwithstanding the consideration as an isotropic and homogeneous material in a conventional analysis [6]. Therefore, the generation of surface roughness is considered to be dependent on the crystallographic factors being cut. The phenomenon of burnishing and recovery basically appears when generating the tool-edge cut surface, while materials swelling mainly occurs when generating the tool-nose cut surface. Fig. 1 illustrates the surface roughness profiles of ideal tool-edge cut surface and ideal tool-nose cut surface, respectively. Both burnishing effect and swelling effect have an impact on the machined surface. However, the domination of influence depends on the main cutting force, thrust cutting force as well as Young’s modulus of the substrate materials. In short, their combined impact distorts the fidelity of transferring tool profiles on to a machined surface. In this paper, the effect of materials swelling on the surface quality of several types of aluminum and electroless nickel phosphorus is investigated. In addition, the formation of the tool-edge cut surface and the tool-nose cut surface of the materials are also discussed. 1.1. Tool-edge cut surface in SPDT
∗
Corresponding author. Tel.: +852 2766 6584. E-mail address: [email protected] (M.C. Kong).
0924-0136/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2006.06.006
Since cutting is done at a very high speed in SPDT, there is no build-up edge and hence the ideal tool-edge cut surface would
M.C. Kong et al. / Journal of Materials Processing Technology 180 (2006) 210–215
211
Nomenclature d DRA f Fz m R Ra Rt s Si V
depth of cut (m) degree of roughness anisotropy feed rate (mm min−1 ) thrust cutting force (N) number of angular spaced radial sections tool-nose radius (mm) arithmetic surface roughness (nm) maximum peak-to-valley height (mm) tool feed rate (mm rev−1 ) swelling significant index rotational spindle speed (rpm)
Greek letters ϕ angle of each radial section (◦ ) standard deviation of the arithmetic roughness σRa
be obtained theoretically for a perfectly sharp cutting tool as illustrated in Fig. 2(a). On the other hand, the cutting tool can have two facets. The cutting edge is for cutting the materials while the burnishing edge can be used to burnish the freshly machined surface. The burnishing motion following the cutting motion can yield a very fine surface finish [7]. Nevertheless, the case is not perfect in reality, as the single-point diamond tools always process a radius at the cutting edge and the depth of cut is extremely small. As a result, the burnishing edge burnishes
Fig. 2. (a and b) Generation of a tool-edge cut surface in an ideal and a real case.
or indents the freshly machined surface but does not remove the material. It brings detrimental physical properties such as residual stress onto the surface. After burnishing, the material left behind the front clearance recovers (i.e., springs back). This amount of recovery is determined by the forces on the clearance face and by the Young’s modulus of the substrate material. Once again, this amount of recovery relies on the grain orientation. Different grain orientations have different amounts of recovery, and consequently give a wavy surface with elastic and plastic deformation as shown in Fig. 2(b). 1.2. Tool-nose cut surface in SPDT The ideal case of maximum peak-to-valley height of surface roughness Rt in the absence of the recovery of the plastic deformation is derived by tool geometry and tool feed only as described in Eq. (1). Sata reported the existence of materials swelling. He pointed out that the workpiece swelled, causing greater tool marks due to tool-nose geometry, thus resulting in higher surface roughness than in the theoretical case [8]. Rt =
Fig. 1. Graphical illustration of ideal tool-edge cut profile and tool-nose cut profile.
f2 8RV 2
as s R
(1)
Fundamentally, there are two types of materials swelling: side swelling and deep swelling. Side swelling is due to the plastic side flow, in which the metal left behind the cutting edge has undergone high pressure and causes the metal to flow to the side of the active cutting edge [9]. The side flow of the workpiece material is facilitated by high temperature and normal stress in the cutting zone, where the workpiece material at the chip/tool
212
M.C. Kong et al. / Journal of Materials Processing Technology 180 (2006) 210–215 Table 1 Materials and cutting conditions
Fig. 3. Illustration of the effect of materials swelling and recovery cross the tool-nose surface.
and workpiece/tool interface behaves as a viscous fluid. Thus, the metal at the trailing edge of the tool flows to the side to relieve the stress. This implies that the plastic deformation is not only along the feed marks but also in between the feed marks [10]. Although it has been reported that side swelling would lead to slightly wider tool marks in some cases, the effect is relatively insignificant when compared with deep swelling [11]. Deep swelling is due to the thrust cutting force along the main cutting edge of the tool as shown in Fig. 3. It causes the work material near the nose of the tool to flow to the free surface. The effect of swelling increases the surface roughness of the diamond turned surface. Fig. 3 shows the tool profile in the absence and in the presence of deep swelling as well as the influence of recovery. The depth of the tool marks after materials swelling Hs is greater than that of the ideal case Hi when deep swelling is present. However, the depth of tool marks after the combined effect of swelling and recovery Hr may be less than that of the ideal case Hi , which depends on the amount of recovery of the machined surface. As discussed previously, the font clearance of the diamond tool burnishes the machined surface, and the material recovers after burnishing during diamond turning. The amount of recovery is determined by the forces on the clearance face and by the crystallographic orientation of the substrate material being cut. The combined effect of burnishing, materials swelling, and recovery on the workpiece material affects the fidelity of the tool profile transfer to the diamond turned surface.
Materials
Specimen A: electroless nickel phosphorus Specimen B: rod aluminum alloy (6061) Specimen C: plate aluminum alloy (6061) Specimen D: straight-rolled aluminum alloy (6061) Specimen E: aluminum single crystal (1 1 0)
Spindle rotational speed (rpm) Feed rate (mm min−1 ) Depth of cut (m) Tool rake angle (◦ ) Front clearance angle (◦ ) Tool-nose radius (mm)
2000 40 2 0.0 7.0 0.5
avoiding the phenomenon of tool interference and generating a clear tool mark for the ease of analysis. The cutting tests were performed on a two-axis CNC ultra-precision machine (Nanoform® 200 from Precitech, Inc., USA). The surface roughness of the machined surfaces was examined by a non-contact type surface measurement system, Wyko NT 8000 (Veeco Metrology Group) and then by a contact type measurement system, Form Talysurf PGI 1240 (Taylor Hobson Ltd.), respectively. The magnification objectives used in the Wyko NT 8000 was 20× and the field of view was 309 m × 231 m for each machined sample. The phaseshifting interferometry (PSI) mode, which is typically used to test smooth surfaces (roughness less than 30 nm), was chosen in the experiment as the theoretical surface roughness is approximately 25 nm. The surface roughness profiles were then plotted by Vision software from Veeco Instruments, Inc. Regarding the Form Talysurf PGI 1240, the standard stylus with a tool tip radius 2 m was used. The surface roughness profiles at 12 radial sections (i.e., ϕ = 30◦ ) on a diamond turned surface were measured with the Form Talysurf system equipped with a rotary table. The measured length and the cut-off length were fixed at 1.25 and 0.25 mm, respectively.
3. Results and discussion For the purpose of measuring quantitatively the local variation of surface roughness, a parameter named degree of roughness anisotropy (DRA) [12] is used based on the arithmetic roughness Ra . It is defined as the ratio of the standard deviation and the mean of the arithmetic roughness values at a finite number of equally angular spaced radial sections of the machined
2. Experimental A series of orthogonal face cutting tests was conducted among five types of materials including electroless nickel phosphorus (NiP), rod aluminum alloy (6061-T6), plate aluminum alloy (6061-T6), straight-rolled aluminum alloy (6061-T6) and aluminum single crystal (Al 1 1 0) under the same cutting conditions. All samples used in the experiments were available in the markets except the straight-rolled aluminum alloy. The specimens of straight-rolled aluminum alloy were prepared by rolling along the original hot band rolling direction in multi-passes to a thickness of reduction to 90%. Table 1 tabulates the materials and the cutting conditions used in the experiments. It should be highlighted that a small radius, a low spindle speed, a high feed rate were preferred for the sake of
Fig. 4. Plot of mean arithmetic roughness and degree of roughness anisotropy against the conducted materials.
M.C. Kong et al. / Journal of Materials Processing Technology 180 (2006) 210–215
surface, i.e.,
m m i−1 Ra,i − Ra σRa DRA = = √ Ra m−1 m i−1 Ra,i
(2)
where Ra,i , is the arithmetic roughness value at ith radial section of the workpiece and Ra and σRa are the mean and the standard deviation of the arithmetic roughness values for the m radial sections on the surface. Fig. 4 shows the mean value of arithmetic roughness and the degree of roughness anisotropy against different types of materials measured by Form Talysurf PGI 1240 as described in the above section, respectively. The ascending order of Ra among the workpiece materials is electroless nickel phospho-
213
rus, plate aluminum alloy, rod aluminum alloy, straight-rolled aluminum alloy and aluminum single crystal (Al 1 1 0). The ascending order of DRA among the workpiece materials is electroless nickel phosphorus, rod aluminum alloy, plate aluminum alloy, straight-rolled aluminum alloy and aluminum single crystal (Al 1 1 0). A large value of DRA implies a high anisotropy of surface roughness. It appears that both of the values of Ra and DRA of electroless nickel phosphorus are the smallest while that for aluminum single crystal are the largest among the materials being cut. It can be explained by the effect of strong crystallographic orientation in Al (1 1 0) as the same crystallographic plane of different types of single crystal resulted in the same periodicity of variation of surface roughness. In this case, (1 1 0) crystals would have two-fold symmetry [13]. In contrast, electro-
Table 2 Two-dimensional and three-dimensional pictures of surface roughness profiles measured by Wyko measurement system
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less nickel phosphorus possesses an amorphous microstructure in the absence of grain boundary [14]. The Ra and DRA of straight-rolled Al6061 is also relatively high when compared with other polycrystalline materials such as rod aluminum 6061 and plate aluminum 6061. It is thanks to its texture with preferred orientations. The straight-rolled Al6061 has preferred orientation but it is not as strong as the single crystal one. As a result, the measured values are above the same group of polycrystalline materials but below the single crystal material. In fact, Al6061 plate has a comparatively stronger preferred orientation than that of rod Al6061 and this can be revealed from their DRA values. Normally, the experimental surface roughness is expected to be higher than the theoretical one as some factors such as relative vibration between the tool and workpiece, spindle error motion, materials swelling, crystallographic orientation influencing the surface generation may add on the surface roughness. Nevertheless, the values of the machined surface were a little bit lower than the theoretical value. The unexpected better surface can be accounted for the effect of recovery at the set of cutting condition. In SPDT, as cutting tools always process a radius at the cutting edge and the depth of cut is extremely small (e.g., 2 m). As the depth of cut becomes of the same order as the tool-edge radius, the assumption regarding the perfectly sharp tool-edge is no longer valid and the machining process may involve significant sliding along the clearance face of the tool owing to the elastic recovery of the workpiece material [15]. It is interesting to note that the main cutting force is very small while the depth of cut is very small in the range of ultra-precision machining [16]. In this experiment, the depth of cut was 2 m which is relatively small in ultra-precision machining. Moreover, the small front clearance angle makes the friction due to the elastic recovery of the workpiece at the clearance face increases. Therefore, the freshly machined surface along the cutting direction tends to recovery after burnishing as the main cutting force is extremely small. Table 2 gives two-dimensional as well as three-dimensional surface roughness profiles measured by Wyko measurement system. The X profile is one of the tool-nose cut surface profile extracted from the three-dimensional surface profile while the Y profile is tool-edge cut surface profile. The scales of X profile and Y profile are the same for all specimens. According to the results of the Y profile among different types of workpiece materials, the profile of electroless nickel phosphorus is very smooth and it is similar to a straight line while Al (1 1 0) is very varied and the waviness length is very small. This is because of the variation of crystallographic orientations. As a matter of fact, the results of Y profile can be co-related to the results of surface roughness and DRA as mentioned above. It is arousing that each type of workpiece has its distinctive pattern of tool marks as shown in Table 2. For instance, electroless nickel phosphorus (NiP) was generated into a repeatable pattern of tool mark and has a smooth surface. Aluminum single crystal (1 1 0) also has a clear shape of tool mark as NiP but there was surface crack on the valley of the machined surface thanks to the grains orientations. The straight-rolled aluminum alloy
(6061) and plate aluminum alloy (6061) has a similar pattern of tool marks as the nature of sample preparation is similar. But the straight-rolled aluminum alloy has a larger extent of variation. The rod aluminum alloy (6061) had a strong surface spikes on the smooth surface, which was caused by precipitation hardened aluminum. Notwithstanding the unexpected good surface, there is significant effect of anisotropy of surface roughness due to crystallographic orientations. This can be revealed from the parameter DRA which provides a measure of the normalized variation of the surface roughness over the diamond turned surface. 4. Conclusions The results from the degree of roughness anisotropy indicate that the surface roughness varies systematically at different radial sections of the workpiece. The variation depends on the crystallographic orientations of the workpiece material. The arithmetic roughness Ra is found to be strongly co-related to the degree of roughness anisotropy. The effect of materials swelling for ductile materials is overwhelmed by the effect of materials recovery when the depth of cut is extremely small and the degree of the front clearance angle is comparatively small. In addition, there are distinctive features of tool mark pattern for each kind of specimens. Although there is still a required further investigation on the critical depth of cut for the recovery-dominance, it can be found that there is a case of recovery overwhelming the influence of materials swelling. The findings from this research can yield a better understanding of the process of materials swelling and recovery as well as its relationships with various workpiece materials in SPDT. It is notably essential for the improvement of the surface finish of the diamond turned surfaces. Acknowledgment The authors would like to express their sincere thanks to the Research Grants Council of the Hong Kong Special Administrative Region of the People’s Republic of China for providing financial support for this research work under the project no. PolyU 5269/03E. References [1] K. Mitsui, H. Sato, Frequency characteristic of cutting process identified by an in-process measurement of surface roughness, Ann. CIRP 27 (1) (1978) 67–71. [2] T.P. Tai, Y.C. Yang, Y.C. Hwong, C.H. Ku, A new concept of cutting marks formation in metal cutting vibration, in: Proceedings of 20th MTDR, 1980, p. 449. [3] S. Takasu, M. Masuda, T. Nishiguchi, Influence of steady vibration with small amplitude upon surface roughness in diamond machining, Ann. CIRP 34 (1) (1985) 463–467. [4] T. Bispink, Performance analysis of feed-drive systems in diamond turning by machining specified test samples, Ann. CIRP 41 (1) (1992) 601–604. [5] W.B. Lee, M. Zhou, A theoretical analysis of the effect of crystallographic orientation on chip formation in micro-machining, Int. J. Mach. Tools Manuf. 33 (3) (1993) 439–447.
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