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Characteristics of high speed micro-cutting of tungsten carbide
Journal of Materials Processing Technology 140 (2003) 352–357
Characteristics of high speed micro-cutting of tungsten carbide K. Liu, X.P. Li∗ , M. Rahman Department of Mechanical Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260, Singapore
Abstract In this study, experiments are carried out to evaluate the characteristics of high speed cutting of tungsten carbide material using a Makino V55 high speed machine tool with cubic boron nitride (CBN) tool inserts. The cutting forces were measured using a three-component dynamometer, the surface roughness of the machined workpiece was measured using a Mitutoyo SURFTEST 301, and the machined workpiece surfaces and the chip formation were examined using a scanning electron microscope (SEM). Experimental results indicate that the radial force Fx is much larger than the tangential force Fz and the axial force Fy . Two types of surfaces of the machined workpiece are achieved: ductile cutting surface and fracture surface. Continuous chips and discontinuous chips are formed under different cutting conditions. Depth of cut and feed rate almost have no significant effect on the surface roughness of the machined workpiece. The SEM observations on the machined workpiece surfaces and chip formation indicate that the ductile mode cutting is mainly determined by the undeformed chip thickness when the tool cutting edge radius is fixed. Ductile cutting can be achieved when the undeformed chip thickness is less than a critical value. © 2003 Elsevier B.V. All rights reserved. Keywords: High speed; Micro-cutting; Tungsten carbide; CBN
1. Introduction Tungsten carbide as a brittle material is becoming an important engineering material due to its excellent properties. Ductile cutting of brittle materials has been widely recognized as an increasingly important technology for the industry. The grooving test of tungsten carbide has been conducted recently [1,2], which shows that in cutting of tungsten carbide when the undeformed chip thickness is small enough and the ratio of tool cutting edge radius to undeformed chip thickness is larger than 1, ductile cutting of tungsten carbide can be achieved. During the last few decades, much work has been done on ductile machining of brittle materials so as to make brittle materials more applicable. The possibility of grinding brittle materials in a ductile manner was proposed as early as 1954 [3]. By 1975, improvement in precision diamond grinding mechanism allowed the first reproducible evidence of grinding ductility in brittle glass workpiece [4]. Toh and McPherson [5] found that plastically deformed chips are formed in the machining of ceramic materials if the scale of the ma-
∗ Corresponding author. Tel.: +65-6874-3429; fax: +65-6779-1459. E-mail address: [email protected] (X.P. Li).
0924-0136/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0924-0136(03)00758-1
chining operation is small (less than 1 m depth of cut), that is, ductile mode cutting of brittle materials could be achieved if the depth of cut is extremely small. Similar ductile chip formation has been observed in fine scale machining debris from a wide range of ceramics, glasses, semiconductors and crystals [4–11]. Ductile-regime response during the diamond turning of brittle germanium crystals was evident from the damage-free surfaces obtained by Blackley and Scattergood [6]. The chip topography provided insight into the ductile-regime machining of germanium that occurred along the tool nose. The development of a research apparatus capable of ductile-regime was described and an analytical and experimental investigation of the infeed rates necessary for ductile-regime grinding of brittle materials was presented [7]. Using different diamond tools with rake angles of 0◦ and negative 25◦ at different cutting speeds, taper cutting experiments were carried out with increasing depth of cut on silicon [8]. The cutting groove formation changed from ductile mode to brittle mode as the depth of cut exceeded a critical value. Ultra-precision ductile cutting of soda-lime glass was conducted by applying ultrasonic vibration on a single-crystal diamond tool along the cutting velocity direction [9]. It was found that ultrasonic vibration would improve the ductile cutting performance
K. Liu et al. / Journal of Materials Processing Technology 140 (2003) 352–357
of glass. A germanium surface and the chips produced from a single-point diamond turning process operated in the ductile-regime had been analyzed by transmission electron microscopy and parallel electron-energy-loss spectroscopy [10]. Lack of fracture damage on the finished surface and continue chip formation were indicative of a ductile removal process. These results suggest that any material, in spite of its ductility, could be machined in ductile mode under the sufficiently small scale of machining. Plastic flow and fracture in WC single crystals and WC–Co materials showed that deformation resulted in the development of high compressive stresses that encouraged slip in WC [11]. Optical and transmission electron microscopy studies demonstrated that plastic flow in the carbide phase always preceded fracture. The grooving wear of single-crystal tungsten carbide against diamond was evaluated in single-tip scratch testing [12]. The single-tip grooves were made with a Vickers diamond indenter and the abrasion tests were performed with diamond and silicon grits. The experimental results indicated that there was difference in both the amount of wear and wear mechanisms between different crystallographic directions of WC. Depending on the direction of the slip planes in relation to the groove direction, the wear mechanisms changed from ductile (grooves parallel to the slip planes) to brittle (grooves perpendicular to the slip planes). Although tremendous work has been done on study of the ductile-regime machining of brittle materials and the grooving of tungsten carbide against diamond and cubic boron nitride (CBN), so far study of ductile cutting of tungsten carbide work material has not been reported. In this study, ductile cutting experiments of tungsten carbide material will be conducted to evaluate the cutting performances of tungsten carbide using high cutting speed with CBN cutting tools. The cutting forces will be measured using a three-component force dynamometer. The surface roughness of machined workpiece will be measured using a Mitutoyo SURFTEST 301. The machined workpiece topography and chip formation will be examined using a scanning electron microscope (SEM).
2. Experimental setup The high speed cutting experiments were carried out on a Makino V55 high speed machine tool using CBN cutting tools. Commercial tungsten carbide inserts (SNMM433, Sumitomo) were used as the workpiece material. Fig. 2 shows the schematic illustration of the high speed cutting experimental setup, where v is the cutting speed and ao the depth of cut in the axial direction. The diameter of cutting tool was 23.6 mm, axial rake angle γ p was 13◦ , radial rake angle γ f was −4◦ , peripheral cutting edge angle ψ was 48◦ , clearance angle α was 20◦ , cutting edge inclination angle λs was 0◦ , minor cutting edge angle κr was
353
Table 1 Cutting conditions of the high speed cutting experiments No.
Cutting conditions Cutting speed
1 2 3 4 5 6 7 8 9 a
v (m/min)
na (rpm)
741.4 741.4 741.4 741.4 741.4 741.4 741.4 741.4 741.4
10000 10000 10000 10000 10000 10000 10000 10000 10000
f (mm/rev)
ao (m)
dmax (nm)
0.01 0.01 0.01 0.01 0.01 0.005 0.015 0.02 0.025
1 2 3 4 5 2 2 2 2
437 644 803 937 1054 338 920 1164 1377
Spindle rotation speed.
3◦ and the tool included angle or point angle εr was 120◦ . The radius of CBN tool cutting edge, r, was 6.0 m, the tool corner radius R was 0.8 mm and the rake angle of its chamfer, γ, was −41.4◦ . Only one fresh CBN insert actually contributed on the work material removal and another balance CBN insert did not contribute on the work material removal. Before the experiments, every tungsten carbide workpiece together with the fixture used was ground using a grinder (Okamoto, PSG-63AN), so as to ensure that the workpiece top surface is parallel to the bottom surface of the fixture. The grinding parameters were 38 mm wide and 355 mm diameter SiC wheel, 30 m/s grinding speed and 10 m depth of cut. The cutting conditions of high speed cutting experiments are shown in Table 1. All experiments were conducted under dry cutting. The cutting forces were measured using a three-component KISTLER force dynamometer. The machined workpiece surface topography and the chip formation were examined using an SEM. Each cutting experiment was repeated three times and the average cutting forces of three tests were taken. In this study, the depth of cut was in a micron scale. Nevertheless, the positioning resolution of the machine tool was ±1 m and its repeatability was ±1 m. Comparing the precision of the depth of cut used with the machine tool’s resolution, it is necessary to study the effect of the machine tool’s resolution on the cutting accuracy. Pre-experiments were carried out to verify that the cutting accuracy can be guaranteed when fresh tungsten carbide specimen was used as the workpiece and fresh CBN insert was used as the tool for each experiment. The nanometer scale values for undeformed chip thickness were achieved by arranging combinations of the radius of tool corner R, depth of cut ao and feed rate f, as shown in Fig. 1. The maximum undeformed chip thickness dmax can be determined using the equation: dmax = R −
R2 + f 2 − 2f 2Rao − ao2
(1)
354
K. Liu et al. / Journal of Materials Processing Technology 140 (2003) 352–357 100
MAKINO V55
Fx Fy Fz
Cutting Force (N)
80
Tool holder CBN insert
rt v
Workpiece
Fixture Bolt
ao
60 40 20 0 -20 0
0.002
0.004
0.006
0.008
0.01
0.012
-40
KISTLER
Time (second)
Dynamometer
(a) f = 0.01 mm/rev & ao = 2 µ m 120
O2 f
R
Fx Fy Fz
90 Cutting Force (N)
O1
dmax ao
60 30 0 -30
Fig. 1. Schematic illustration of micro-cutting experimental apparatus.
0
0.002
0.004
0.006
0.008
0.01
0.012
-60 Time (second)
3. Experimental results and discussions (b) f = 0.02 mm/rev & ao = 2 µ m
3.1. Cutting forces
Table 2 Cutting forces obtained under different cutting conditions No.
a b c
v (m/min)
741.4 741.4 741.4
f (mm/rev)
0.01 0.02 0.01
ao (m)
2 2 4
Cutting forces Fx (N)
Fy (N)
Fz (N)
6.0 8.6 15.9
43.1 60.1 117.8
74.8 111.4 229.0
250
Fx Fy Fz
200 Cutting Force (N)
Typical experimental cutting forces under different cutting conditions are shown in Fig. 2 and the corresponding experimental values of cutting forces are shown in Table 2. The displayed in Fig. 2 is cutting force variations in the duration of two cutting rotations. It was found that in the all high speed cutting tests, the cutting force Fz was the largest one among those cutting forces; meanwhile, the cutting force Fx was extremely smaller than others. Normally, the cutting force Fx was only about one-seventh of the cutting force Fy , and one-twelfth to one-fourteenth of the cutting force Fz . Forced vibrations were also found to coexist with the work material removal process, having the nature frequency of dynamometer 55 kHz within the measured cutting force signal. But in the high speed cutting process, forced vibrations were much more serious than that in the face milling process, as shown in Fig. 2. Particularly, comparing the force signals shown in Fig. 2(a) with Fig. 2(b) and (c), it seems that the larger feed rate and depth of cut produced larger forced vibration amplitude.
150 100 50 0 -50 0
0.003
0.006
0.009
0.012
-100 Time (second)
(c) f = 0.01 mm/rev & ao = 4 µm Fig. 2. Cutting forces of typical experiments in the high speed cutting.
The influence of depth of cut on cutting forces in high speed cutting of tungsten carbide is shown in Fig. 3 and the influence of feed rate on cutting forces is shown in Fig. 4, where the cutting speed was 741.4 m/min (10 000 rpm), and feed rate was 0.01 mm/rev for Fig. 3 while depth of cut was 2 m for Fig. 4. It can be apparently seen that all cutting forces Fx , Fy and Fz were increased monotonously when depth of cut and feed rate were increased in the high speed cutting, as shown in Figs. 3 and 4. 3.2. Machined workpiece surface texture SEM photographs of the machined tungsten carbide surfaces achieved in the high speed cutting are shown in Fig. 5,
K. Liu et al. / Journal of Materials Processing Technology 140 (2003) 352–357
355
Cutting Forces (N)
300 250
Fx Fy
200
Fz
150 100 50 0 0
1
2
3
4
5
6
Depth of Cut (µm)
Fig. 3. Cutting forces obtained under different depths of cut.
where the cutting speed was 741.4 m/min (10 000 rpm). SEM observations on the machined workpiece surface indicated that a good surface integrity was achieved in the high speed cutting as shown in Fig. 5(a) (depth of cut: 2 m; feed rate: 0.01 mm/rev), which was much better than that of the original ground workpiece surface as shown in Fig. 5(d). Feed marks were left on the machined surface as it moved across the workpiece. When feed rate was 0.02 mm/rev (undeformed chip thickness of 1164 nm) or depth of cut exceeded 5 m (undeformed chip thickness of 1054 nm) in the high speed cutting, some fracture features was achieved on its surface and the produced surface was not satisfied as shown in Fig. 5(b) and (c). 3.3. Surface roughness The surface roughness of machined tungsten carbide workpiece influenced by depth of cut and feed rate in the high speed cutting are shown in Figs. 6 and 7, respectively. Here, the cutting conditions were cutting speed of 741.4 m/min (10 000 rpm) and feed rate of 0.01 mm/rev for Fig. 6, cutting speed of 741.4 m/min and depth of cut of 2 m for Fig. 7. The machined tungsten carbide surface roughness was increased monotonously first, and then reached a maximum value when the depth of cut was 4 m, and decreased, as shown in Fig. 6. However, the surface roughness of machined tungsten carbide workpiece was only increased monotonously when cutting speed was increased.
Cutting Forces (N)
150 Fx Fy Fz
120 90 60
Fig. 5. SEM photographs of the machined workpiece surfaces.
30 0 0
0.005
0.01
0.015
0.02 0.025
0.03
Feed Rate (mm/rev)
Fig. 4. Cutting forces obtained under different feed rates.
3.4. Chip formation SEM micrographs of chips formed under different cutting conditions in high speed cutting of tungsten carbide
K. Liu et al. / Journal of Materials Processing Technology 140 (2003) 352–357
Surface Roughness Ra (mm)
356
1 0.8 0.6 0.4 0.2 0 0
1
2
3
4
5
6
Depth of Cut (mm) Fig. 6. Surface roughness of the machined workpiece under different depths of cut.
Surface Roughness Ra (µm)
are shown in Fig. 8. The cutting conditions were (a) cutting speed of 741.4 m/min, depth of cut of 2 m and feed rate of 0.01 mm/rev; (b) cutting speed of 741.4 m/min, depth of cut of 2 m and feed rate of 0.015 mm/rev; (c) cutting speed of 741.4 m/min, depth of cut of 2 m and feed rate of 0.02 mm/rev; (d) cutting speed of 741.4 m/min, depth of cut of 5 m and feed rate of 0.01 mm/rev. Observations on the chip formation using SEM indicated that in high speed cutting of tungsten carbide, continuous slice chips were formed with ductile cutting mode when feed rate was less than 0.02 mm/rev under cutting speed of 741.4 m/min and depth of cut of 2 m as shown in Fig. 8(a) and (b). Continuous chips were also formed in a ductile cutting mode when depth of cut was 5 m (undeformed chip thickness of 1054 nm) as shown in Fig. 8(d). Discontinuous chip was formed in a brittle cutting mode when feed rate was equal to or more than 0.02 mm/rev (undeformed chip thickness of 1164 nm) as shown in Fig. 8(c). That is, in high speed cutting of tungsten carbide, chips are formed in a ductile cutting mode when undeformed chip thickness is not larger than 1054 nm. As a result, undeformed chip thickness is the key factor influenced the chip formation mode, which is determined by the depth of cut, feed rate and cutting tool geometry.
1 0.8 0.6 0.4 0.2 0 0
0.005
0.01
0.015
0.02
0.025
0.03
Feed Rate (mm/rev) Fig. 7. Surface roughness of the machined workpiece under different feed rates.
Fig. 8. SEM photographs of chips formed under different cutting conditions.
K. Liu et al. / Journal of Materials Processing Technology 140 (2003) 352–357
4. Conclusions • High speed cutting experiments have been carried out using a high speed machine tool with CBN tools. The influence of feed rate and depth of cut on cutting performances, such as cutting force, chip formation and surface integrity, in high speed cutting of tungsten carbide work material were investigated. • The results indicated that cutting force Fz was much larger than cutting force Fx and Fy , and all cutting forces were increased with the increasing of feed rate and depth of cut. Two types of surfaces, such as ductile cutting surface and fracture surface, were achieved. • SEM observations on the machined workpiece surfaces and chip formation indicated that ductile mode cutting was mainly determined by undeformed chip thickness, which was mostly influenced by feed rate and depth of cut used when the cutting tool nose radius remained unchanged. Ductile cutting of tungsten carbide was achieved when undeformed chip thickness was less than a critical value.
References [1] K. Liu, X.P. Li, Ductile cutting of tungsten carbide, J. Mater. Process. Technol. 113 (2001) 348.
357
[2] K. Liu, X.P. Li, Modeling of ductile cutting of tungsten carbide, Trans. NAMRI/SME XXIX (2001) 251. [3] R.F. King, D. Tabor, The strength properties and frictional behavior of brittle solids, Proc. Roy. Soc. Lond. A 223 (1954) 225. [4] M. Huerta, S. Malkin, Grinding of glass: the mechanics of the process, Trans. ASME J. Eng. Ind. 98 (1976) 459. [5] S.B. Toh, R. McPherson, Fine scale abrasive wear of ceramics by a plastic cutting process, in: E.A. Almond, C.A. Brookes, R. Warren (Eds.), Science of Hard Materials, Adam Hilger, Bristol, 1986. [6] W.S. Blackley, R.O. Scattergood, Chip topography for ductile-regime machining of germanium, Trans. ASME J. Eng. Ind. 116 (1994) 263. [7] T.G. Bifano, T.A. Dow, R.O. Scattergood, Ductile-regime grinding: a new technology for machining brittle materials, Trans. ASME J. Eng. Ind. 113 (1991) 184. [8] F.Z. Fang, V.C. Venkatesh, Diamond cutting of silicon with nanometric finish, Ann. CIRP 47 (1998) 45. [9] T. Moriwaki, E. Shamoto, K. Inoue, Ultraprecision ductile cutting of glass by applying ultrasonic vibration, Ann. CIRP 41 (1992) 141. [10] J.C. Morris, D.L. Callahan, J. Kulik, J.A. Patten, R.O. Scattergood, Origins of the ductile regime in single-point diamond turning of semiconductors, J. Am. Ceram. Soc. 78 (1995) 2015. [11] D.J. Rowcliffe, V. Jayaram, M.K. Hibbs, R. Sinclair, Compressive deformation and fracture in WC materials, Mater. Sci. Eng. A 105/106 (1988) 299. [12] H. Engqvist, S. Ederyd, N. Axen, S. Hogmark, Grooving wear of single-crystal tungsten carbide, Wear 230 (1999) 165.
Characteristics of high speed micro-cutting of tungsten carbide K. Liu, X.P. Li∗ , M. Rahman Department of Mechanical Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260, Singapore
Abstract In this study, experiments are carried out to evaluate the characteristics of high speed cutting of tungsten carbide material using a Makino V55 high speed machine tool with cubic boron nitride (CBN) tool inserts. The cutting forces were measured using a three-component dynamometer, the surface roughness of the machined workpiece was measured using a Mitutoyo SURFTEST 301, and the machined workpiece surfaces and the chip formation were examined using a scanning electron microscope (SEM). Experimental results indicate that the radial force Fx is much larger than the tangential force Fz and the axial force Fy . Two types of surfaces of the machined workpiece are achieved: ductile cutting surface and fracture surface. Continuous chips and discontinuous chips are formed under different cutting conditions. Depth of cut and feed rate almost have no significant effect on the surface roughness of the machined workpiece. The SEM observations on the machined workpiece surfaces and chip formation indicate that the ductile mode cutting is mainly determined by the undeformed chip thickness when the tool cutting edge radius is fixed. Ductile cutting can be achieved when the undeformed chip thickness is less than a critical value. © 2003 Elsevier B.V. All rights reserved. Keywords: High speed; Micro-cutting; Tungsten carbide; CBN
1. Introduction Tungsten carbide as a brittle material is becoming an important engineering material due to its excellent properties. Ductile cutting of brittle materials has been widely recognized as an increasingly important technology for the industry. The grooving test of tungsten carbide has been conducted recently [1,2], which shows that in cutting of tungsten carbide when the undeformed chip thickness is small enough and the ratio of tool cutting edge radius to undeformed chip thickness is larger than 1, ductile cutting of tungsten carbide can be achieved. During the last few decades, much work has been done on ductile machining of brittle materials so as to make brittle materials more applicable. The possibility of grinding brittle materials in a ductile manner was proposed as early as 1954 [3]. By 1975, improvement in precision diamond grinding mechanism allowed the first reproducible evidence of grinding ductility in brittle glass workpiece [4]. Toh and McPherson [5] found that plastically deformed chips are formed in the machining of ceramic materials if the scale of the ma-
∗ Corresponding author. Tel.: +65-6874-3429; fax: +65-6779-1459. E-mail address: [email protected] (X.P. Li).
0924-0136/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0924-0136(03)00758-1
chining operation is small (less than 1 m depth of cut), that is, ductile mode cutting of brittle materials could be achieved if the depth of cut is extremely small. Similar ductile chip formation has been observed in fine scale machining debris from a wide range of ceramics, glasses, semiconductors and crystals [4–11]. Ductile-regime response during the diamond turning of brittle germanium crystals was evident from the damage-free surfaces obtained by Blackley and Scattergood [6]. The chip topography provided insight into the ductile-regime machining of germanium that occurred along the tool nose. The development of a research apparatus capable of ductile-regime was described and an analytical and experimental investigation of the infeed rates necessary for ductile-regime grinding of brittle materials was presented [7]. Using different diamond tools with rake angles of 0◦ and negative 25◦ at different cutting speeds, taper cutting experiments were carried out with increasing depth of cut on silicon [8]. The cutting groove formation changed from ductile mode to brittle mode as the depth of cut exceeded a critical value. Ultra-precision ductile cutting of soda-lime glass was conducted by applying ultrasonic vibration on a single-crystal diamond tool along the cutting velocity direction [9]. It was found that ultrasonic vibration would improve the ductile cutting performance
K. Liu et al. / Journal of Materials Processing Technology 140 (2003) 352–357
of glass. A germanium surface and the chips produced from a single-point diamond turning process operated in the ductile-regime had been analyzed by transmission electron microscopy and parallel electron-energy-loss spectroscopy [10]. Lack of fracture damage on the finished surface and continue chip formation were indicative of a ductile removal process. These results suggest that any material, in spite of its ductility, could be machined in ductile mode under the sufficiently small scale of machining. Plastic flow and fracture in WC single crystals and WC–Co materials showed that deformation resulted in the development of high compressive stresses that encouraged slip in WC [11]. Optical and transmission electron microscopy studies demonstrated that plastic flow in the carbide phase always preceded fracture. The grooving wear of single-crystal tungsten carbide against diamond was evaluated in single-tip scratch testing [12]. The single-tip grooves were made with a Vickers diamond indenter and the abrasion tests were performed with diamond and silicon grits. The experimental results indicated that there was difference in both the amount of wear and wear mechanisms between different crystallographic directions of WC. Depending on the direction of the slip planes in relation to the groove direction, the wear mechanisms changed from ductile (grooves parallel to the slip planes) to brittle (grooves perpendicular to the slip planes). Although tremendous work has been done on study of the ductile-regime machining of brittle materials and the grooving of tungsten carbide against diamond and cubic boron nitride (CBN), so far study of ductile cutting of tungsten carbide work material has not been reported. In this study, ductile cutting experiments of tungsten carbide material will be conducted to evaluate the cutting performances of tungsten carbide using high cutting speed with CBN cutting tools. The cutting forces will be measured using a three-component force dynamometer. The surface roughness of machined workpiece will be measured using a Mitutoyo SURFTEST 301. The machined workpiece topography and chip formation will be examined using a scanning electron microscope (SEM).
2. Experimental setup The high speed cutting experiments were carried out on a Makino V55 high speed machine tool using CBN cutting tools. Commercial tungsten carbide inserts (SNMM433, Sumitomo) were used as the workpiece material. Fig. 2 shows the schematic illustration of the high speed cutting experimental setup, where v is the cutting speed and ao the depth of cut in the axial direction. The diameter of cutting tool was 23.6 mm, axial rake angle γ p was 13◦ , radial rake angle γ f was −4◦ , peripheral cutting edge angle ψ was 48◦ , clearance angle α was 20◦ , cutting edge inclination angle λs was 0◦ , minor cutting edge angle κr was
353
Table 1 Cutting conditions of the high speed cutting experiments No.
Cutting conditions Cutting speed
1 2 3 4 5 6 7 8 9 a
v (m/min)
na (rpm)
741.4 741.4 741.4 741.4 741.4 741.4 741.4 741.4 741.4
10000 10000 10000 10000 10000 10000 10000 10000 10000
f (mm/rev)
ao (m)
dmax (nm)
0.01 0.01 0.01 0.01 0.01 0.005 0.015 0.02 0.025
1 2 3 4 5 2 2 2 2
437 644 803 937 1054 338 920 1164 1377
Spindle rotation speed.
3◦ and the tool included angle or point angle εr was 120◦ . The radius of CBN tool cutting edge, r, was 6.0 m, the tool corner radius R was 0.8 mm and the rake angle of its chamfer, γ, was −41.4◦ . Only one fresh CBN insert actually contributed on the work material removal and another balance CBN insert did not contribute on the work material removal. Before the experiments, every tungsten carbide workpiece together with the fixture used was ground using a grinder (Okamoto, PSG-63AN), so as to ensure that the workpiece top surface is parallel to the bottom surface of the fixture. The grinding parameters were 38 mm wide and 355 mm diameter SiC wheel, 30 m/s grinding speed and 10 m depth of cut. The cutting conditions of high speed cutting experiments are shown in Table 1. All experiments were conducted under dry cutting. The cutting forces were measured using a three-component KISTLER force dynamometer. The machined workpiece surface topography and the chip formation were examined using an SEM. Each cutting experiment was repeated three times and the average cutting forces of three tests were taken. In this study, the depth of cut was in a micron scale. Nevertheless, the positioning resolution of the machine tool was ±1 m and its repeatability was ±1 m. Comparing the precision of the depth of cut used with the machine tool’s resolution, it is necessary to study the effect of the machine tool’s resolution on the cutting accuracy. Pre-experiments were carried out to verify that the cutting accuracy can be guaranteed when fresh tungsten carbide specimen was used as the workpiece and fresh CBN insert was used as the tool for each experiment. The nanometer scale values for undeformed chip thickness were achieved by arranging combinations of the radius of tool corner R, depth of cut ao and feed rate f, as shown in Fig. 1. The maximum undeformed chip thickness dmax can be determined using the equation: dmax = R −
R2 + f 2 − 2f 2Rao − ao2
(1)
354
K. Liu et al. / Journal of Materials Processing Technology 140 (2003) 352–357 100
MAKINO V55
Fx Fy Fz
Cutting Force (N)
80
Tool holder CBN insert
rt v
Workpiece
Fixture Bolt
ao
60 40 20 0 -20 0
0.002
0.004
0.006
0.008
0.01
0.012
-40
KISTLER
Time (second)
Dynamometer
(a) f = 0.01 mm/rev & ao = 2 µ m 120
O2 f
R
Fx Fy Fz
90 Cutting Force (N)
O1
dmax ao
60 30 0 -30
Fig. 1. Schematic illustration of micro-cutting experimental apparatus.
0
0.002
0.004
0.006
0.008
0.01
0.012
-60 Time (second)
3. Experimental results and discussions (b) f = 0.02 mm/rev & ao = 2 µ m
3.1. Cutting forces
Table 2 Cutting forces obtained under different cutting conditions No.
a b c
v (m/min)
741.4 741.4 741.4
f (mm/rev)
0.01 0.02 0.01
ao (m)
2 2 4
Cutting forces Fx (N)
Fy (N)
Fz (N)
6.0 8.6 15.9
43.1 60.1 117.8
74.8 111.4 229.0
250
Fx Fy Fz
200 Cutting Force (N)
Typical experimental cutting forces under different cutting conditions are shown in Fig. 2 and the corresponding experimental values of cutting forces are shown in Table 2. The displayed in Fig. 2 is cutting force variations in the duration of two cutting rotations. It was found that in the all high speed cutting tests, the cutting force Fz was the largest one among those cutting forces; meanwhile, the cutting force Fx was extremely smaller than others. Normally, the cutting force Fx was only about one-seventh of the cutting force Fy , and one-twelfth to one-fourteenth of the cutting force Fz . Forced vibrations were also found to coexist with the work material removal process, having the nature frequency of dynamometer 55 kHz within the measured cutting force signal. But in the high speed cutting process, forced vibrations were much more serious than that in the face milling process, as shown in Fig. 2. Particularly, comparing the force signals shown in Fig. 2(a) with Fig. 2(b) and (c), it seems that the larger feed rate and depth of cut produced larger forced vibration amplitude.
150 100 50 0 -50 0
0.003
0.006
0.009
0.012
-100 Time (second)
(c) f = 0.01 mm/rev & ao = 4 µm Fig. 2. Cutting forces of typical experiments in the high speed cutting.
The influence of depth of cut on cutting forces in high speed cutting of tungsten carbide is shown in Fig. 3 and the influence of feed rate on cutting forces is shown in Fig. 4, where the cutting speed was 741.4 m/min (10 000 rpm), and feed rate was 0.01 mm/rev for Fig. 3 while depth of cut was 2 m for Fig. 4. It can be apparently seen that all cutting forces Fx , Fy and Fz were increased monotonously when depth of cut and feed rate were increased in the high speed cutting, as shown in Figs. 3 and 4. 3.2. Machined workpiece surface texture SEM photographs of the machined tungsten carbide surfaces achieved in the high speed cutting are shown in Fig. 5,
K. Liu et al. / Journal of Materials Processing Technology 140 (2003) 352–357
355
Cutting Forces (N)
300 250
Fx Fy
200
Fz
150 100 50 0 0
1
2
3
4
5
6
Depth of Cut (µm)
Fig. 3. Cutting forces obtained under different depths of cut.
where the cutting speed was 741.4 m/min (10 000 rpm). SEM observations on the machined workpiece surface indicated that a good surface integrity was achieved in the high speed cutting as shown in Fig. 5(a) (depth of cut: 2 m; feed rate: 0.01 mm/rev), which was much better than that of the original ground workpiece surface as shown in Fig. 5(d). Feed marks were left on the machined surface as it moved across the workpiece. When feed rate was 0.02 mm/rev (undeformed chip thickness of 1164 nm) or depth of cut exceeded 5 m (undeformed chip thickness of 1054 nm) in the high speed cutting, some fracture features was achieved on its surface and the produced surface was not satisfied as shown in Fig. 5(b) and (c). 3.3. Surface roughness The surface roughness of machined tungsten carbide workpiece influenced by depth of cut and feed rate in the high speed cutting are shown in Figs. 6 and 7, respectively. Here, the cutting conditions were cutting speed of 741.4 m/min (10 000 rpm) and feed rate of 0.01 mm/rev for Fig. 6, cutting speed of 741.4 m/min and depth of cut of 2 m for Fig. 7. The machined tungsten carbide surface roughness was increased monotonously first, and then reached a maximum value when the depth of cut was 4 m, and decreased, as shown in Fig. 6. However, the surface roughness of machined tungsten carbide workpiece was only increased monotonously when cutting speed was increased.
Cutting Forces (N)
150 Fx Fy Fz
120 90 60
Fig. 5. SEM photographs of the machined workpiece surfaces.
30 0 0
0.005
0.01
0.015
0.02 0.025
0.03
Feed Rate (mm/rev)
Fig. 4. Cutting forces obtained under different feed rates.
3.4. Chip formation SEM micrographs of chips formed under different cutting conditions in high speed cutting of tungsten carbide
K. Liu et al. / Journal of Materials Processing Technology 140 (2003) 352–357
Surface Roughness Ra (mm)
356
1 0.8 0.6 0.4 0.2 0 0
1
2
3
4
5
6
Depth of Cut (mm) Fig. 6. Surface roughness of the machined workpiece under different depths of cut.
Surface Roughness Ra (µm)
are shown in Fig. 8. The cutting conditions were (a) cutting speed of 741.4 m/min, depth of cut of 2 m and feed rate of 0.01 mm/rev; (b) cutting speed of 741.4 m/min, depth of cut of 2 m and feed rate of 0.015 mm/rev; (c) cutting speed of 741.4 m/min, depth of cut of 2 m and feed rate of 0.02 mm/rev; (d) cutting speed of 741.4 m/min, depth of cut of 5 m and feed rate of 0.01 mm/rev. Observations on the chip formation using SEM indicated that in high speed cutting of tungsten carbide, continuous slice chips were formed with ductile cutting mode when feed rate was less than 0.02 mm/rev under cutting speed of 741.4 m/min and depth of cut of 2 m as shown in Fig. 8(a) and (b). Continuous chips were also formed in a ductile cutting mode when depth of cut was 5 m (undeformed chip thickness of 1054 nm) as shown in Fig. 8(d). Discontinuous chip was formed in a brittle cutting mode when feed rate was equal to or more than 0.02 mm/rev (undeformed chip thickness of 1164 nm) as shown in Fig. 8(c). That is, in high speed cutting of tungsten carbide, chips are formed in a ductile cutting mode when undeformed chip thickness is not larger than 1054 nm. As a result, undeformed chip thickness is the key factor influenced the chip formation mode, which is determined by the depth of cut, feed rate and cutting tool geometry.
1 0.8 0.6 0.4 0.2 0 0
0.005
0.01
0.015
0.02
0.025
0.03
Feed Rate (mm/rev) Fig. 7. Surface roughness of the machined workpiece under different feed rates.
Fig. 8. SEM photographs of chips formed under different cutting conditions.
K. Liu et al. / Journal of Materials Processing Technology 140 (2003) 352–357
4. Conclusions • High speed cutting experiments have been carried out using a high speed machine tool with CBN tools. The influence of feed rate and depth of cut on cutting performances, such as cutting force, chip formation and surface integrity, in high speed cutting of tungsten carbide work material were investigated. • The results indicated that cutting force Fz was much larger than cutting force Fx and Fy , and all cutting forces were increased with the increasing of feed rate and depth of cut. Two types of surfaces, such as ductile cutting surface and fracture surface, were achieved. • SEM observations on the machined workpiece surfaces and chip formation indicated that ductile mode cutting was mainly determined by undeformed chip thickness, which was mostly influenced by feed rate and depth of cut used when the cutting tool nose radius remained unchanged. Ductile cutting of tungsten carbide was achieved when undeformed chip thickness was less than a critical value.
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