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Investigation in orthogonal turn-milling towards better surface finish
Journal of Materials Processing Technology 170 (2005) 487–493
Investigation in orthogonal turn-milling towards better surface finish S.K. Choudhury ∗ , J.B. Bajpai Department of Mechanical Engineering, Indian Institute of Technology, Kanpur 208016, India Received 22 May 2001; received in revised form 6 March 2002; accepted 12 December 2004
Abstract Turn-milling is a newly emerging machining process, which tends to make use of the advantages of both turning and milling, wherein both the work piece and the cutting tool are given rotary motion simultaneously. The objective of the present experimental work is to understand the phenomenon of orthogonal turn-milling especially in relation to the effects of work piece revolution, cutter diameter and depth of cut. Surface finish of the machined surface and the optimum work speed at which the surface roughness was minimum has been studied. It has been shown that surface quality obtained by turn-milling process is better than that of conventional milling process. The experiments have been conducted for orthogonal turn-milling of mild-steel work piece with high speed steel milling cutters using planning of experiments technique to study the surface finish achieved. © 2005 Elsevier B.V. All rights reserved. Keywords: Turn-milling; Surface finish; Planning of experiments; Optimum work speed
1. Introduction The turn-milling is a relatively new concept in manufacturing technology wherein both the work piece and the tool are given rotary motion simultaneously. According to the arrangement of the two rotating axes relative to each other there are two different ways by which the process can be classified: 1. orthogonal turn-milling in which the axis of rotation of the cutter is perpendicular to the work piece; 2. co-axial turn-milling in which the axis of rotation of the cutter is parallel to the work piece. Whereas, orthogonal turn-milling is suitable only for external machining of the work pieces, co-axial turn-milling is suitable for internal as well as external machining of rotationally symmetrical work pieces. This new innovative technology enables high productivity and high-machined surface quality at the same time. Especially at an increased number of tool revolutions (high speed machining) the quality of the process is reported to be rising very steeply [5]. The productivity could be much greater ∗
Corresponding author. E-mail address: [email protected] (S.K. Choudhury).
0924-0136/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2004.12.010
in comparison to the conventional processes like turning or milling. Recently it has been shown that when rotationally symmetrical work pieces are to be produced, turn-milling can be real alternative to grinding. In turn-milling high cutting speeds can be achieved since the two cutting speeds of tool and work piece are added up, resulting in high surface finish, low thermal stress of cutting edge and good chip removal due to short chip length. A very important advantage is also that it enables complete machining of work pieces with different shape elements without changing the set-up. Turn-milling offers the possibility of producing, besides flat and cylindrical shapes, more complicated ones, as for example, ellipses, screw-threads, etc. Shaw et al. [1] have described a disk-type turning tool rotating about its central axis. The authors have mentioned about the longer tool life, lower coefficient of friction and a greater effective rake angle as advantages of such rotary tools. The authors have also shown that under optimum conditions the total power required for a given rate of cutting is about 30% less with a rotary tool. The experimental studies conducted by Venuvinod et al. [2] on rotary tools in the form of a frustum of a cone show that with proper selection of rational speeds, the following can be achieved:
488
S.K. Choudhury, J.B. Bajpai / Journal of Materials Processing Technology 170 (2005) 487–493
(a) reduction in cutting forces up to 10%; (b) chip thickness ratio is much lesser than unity; (c) chip length ratios as high as 2:3. Armarego et al. [3] have explained the fundamentals of driven and self-propelled rotary tool cutting operations as wedge tool cutting process. They have observed that the driven oblique rotary tool process is the most efficient one. Yamaguchi et al. [4] conducted experiments on boring, in which a drilled hole is finished with a plug-type rotary cutting tool. On the basis of experimental studies following conclusions were made: (1) the rotation of plug-type tool has favorable effects on cutting mechanisms that improve the chip formation; (2) the mean diameter of the hole machined by plug-type tool is constant at every depth of finished hole. The creditable work in the field of turn-milling has been done in Germany by Schulz and Spur [5]. The experiments were conducted for machining of roller bearing races using high speed turn-milling with the objective to study the effect on surface finish, geometric accuracy, chip geometry, tool wear, etc. On the basis of experimental results the advantages of high-speed machining have been reported by the authors as high surface quality, low thermal stress of the cutting edge and low cutting forces. In addition, good chip removal has also been achieved due to short chip length. Schulz and Kneisel [6] have worked on turn-milling of hard materials as an alternative to turning. The authors have shown that surface quality of the work pieces obtained in turnmilling can be compared with those achieved by grinding. Daniel and Kneisel [7] conducted the experiments on coaxial turn-milling and shown that accessible surface quality mainly depends on the ratio of the rotational frequencies of tool and work piece. Investigations concerning the influence of cutting speed showed that tool wear and surface quality closely depend on this parameter. Kopac and Pogacnik [8] have described the variation of uncut chip thickness in orthogonal turn-milling. Experimental work has shown a strong influence of vibrations on turnmilling process. Authors suggested high dynamic stiffness, especially in the process frequency area as one of the solutions. As per the investigations conducted by Choudhury and Mangrulkar [9] surface roughness value; Ra achieved by orthogonal turn-milling is about 10 times lower than achieved in case of turning. The experimental results showed that the surface finish improves with increase in rotational speed of the cutter and deteriorates with increase in axial-feed rate. The aim of the present experimental work was to understand the phenomenon of turn-milling as well as to study the effects of various machining parameters on the surface texture generated by turn-milling. The emphasis has been laid mainly on the surface finish of the machined surface. The experiments were confined to constant cutter speed (1000 rpm) and axial-feed (8 mm/min) to study the effect of work speed,
diameter of cutter and depth of cut on the surface quality produced. Experiments were also conducted by reducing the work speed to zero rpm where the process was equivalent to the milling operation. The optimum work speed, at which the surface roughness was minimum, has been determined.
2. Planning of experiments Planning of experiments technique helps in extracting useful information by performing minimum number of experiments. The central composite rotatable design [10] for three input variables, i.e. work speed, cutter diameter and depth of cut was selected in this work. The general form of a quadratic polynomial for three variables is illustrated as: yu = b0u + b1u x1 + b2u x2 + b3u x3 + b11u x12 + b22u x22 + b33u x32 + b12u x1 x2 + b13u x1 x3 + b23u x2 x3
(1)
Here, yu represents the response, b0u , b1u , . . ., b23u are regression coefficients, x1 , x2 and x3 are input variables. The regression coefficients were calculated by using the standard procedure [10]. In present analysis effect of three input variables viz., work speed, cutter diameter and depth of cut have been studied on the surface finish (Ra value) of the work piece. Relations between the coded x-scales and original scales, in which the levels were recorded, were established initially. In design scale, the lowest and the highest values of the input variables, ‘x’ were −1.68 and 1.68 [10]. For present experimentation, the ranges of work speed, diameter of cutter and depth of cut were selected depending on the available gear train, cutters and the overall technological capabilities of the machine and the fixture. They are, work speed, Nw from 12 to 40 rpm; diameter of cutter, Dc from φ6 to φ25 mm; depth of cut, t from 0.1 to 0.5 mm. In general x = a + b log (variable) [10] In which, a and b were chosen to satisfy the desired conditions at the ends of the scale as explained below. In case of work speed, Nw taken as variable, x = a + b log Nw , when x = −1.68; Nw = 12 and −1.68 = a + b log 12, when x = 1.68; Nw = 40 and 1.68 = a + b log 40.On solving these two equations simultaneously, we get, a = −8.60 and b = 6.42, hence, for work speed, Nw XN = −8.60 + 6.42 log Nw
(2)
Similarly for diameter of cutter, Dc XD = −5.89 + 5.42 log Dc
(3)
and for depth of cut, t Xt = 3.12 + 4.80 log t
(4)
From Eqs. (2)–(4), work speed, diameter of cutter and depth of cut corresponding to five levels of x as −1.68, −1, 0, 1, 1.68
S.K. Choudhury, J.B. Bajpai / Journal of Materials Processing Technology 170 (2005) 487–493
489
Table 1 Specifications of milling cutters Serial number
Type
Diameter (mm)
Number of teeth
Primary clearance angle
1 2 3 4 5
Shank type end-mill Shank type end-mill Shank type end-mill Shank type end-mill Shank type end-mill
φ6 φ8 φ12 φ18 φ25
4 4 4 6 6
5◦ 5◦ 5◦ 8◦ 8◦
were determined. Since the exact work speed, cutter diameter and depth of cut as obtained above were not available, values were slightly rounded off to their nearest ones.
Positive radial rake angle 6◦ 8◦ 10◦ 10◦ 10◦
Helix angle
Material
20◦ 20◦ 20◦ 35◦ 35◦
HSS HSS HSS HSS HSS
least count, which provided a numerical assessment of the surface roughness in terms of Ra values. 3.2. Experimental procedure
3. Experimental set-up and procedure 3.1. Experimental set-up Present work was conducted on a vertical milling machine since the axes of the work piece rotation had to be perpendicular to the axes of milling cutter rotation. Cylindrical work piece of mild-steel of φ65 mm diameter and 170 mm length was fixed on the machine using special attachment, consisting of an arrangement of gear train to achieve various rotational speeds of the work piece. A three-phase ac electrical motor of 0.3 kW power with rotation of 25 rpm was used for rotating the work piece. The details of five different cutters used in experimentation are shown in Table 1. Schematic diagram of the experimental set-up is shown in Fig. 1. Surface roughness was measured using SURTONIC 10 instrument of 0.1 m
Experiments were conducted in two stages to understand the effect of various machining parameters on the surface finish, to determine the optimum rotational work speed, Nw and to achieve the best possible surface finish. In stage-I the experiments were conducted to establish the effect of following parameters on the surface finish: • rotational speed of the work piece, Nw (rpm): 12, 16, 20, 31, 40; • diameter of the cutter, Dc (mm): φ6, φ8, φ12, φ18, φ25; • depth of cut, t (mm): 0.1, 0.12, 0.22, 0.36, 0.5. In the second stage the experiments were conducted to determine the optimum rotational work speed, Nw at which the surface roughness was minimum. This was accomplished by further reduction in rotational speed of the work piece up
Table 2 Experimental results of orthogonal turn-milling Experiment number
Work speed ‘Nw ’ (rpm)
Diameter of cutter ‘Dc ’ (mm)
Depth of cut ‘t’ (mm)
Surface roughness ‘Ra ’ value (m) Measured
Calculated
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
16 31 16 31 16 31 16 31 12 40 20 20 20 20 16 31 20 20 20 20 20 20 20 20
8 8 18 18 8 8 18 18 12 12 6 25 12 12 12 12 8 18 12 12 12 12 12 12
0.12 0.12 0.12 0.12 0.36 0.36 0.36 0.36 0.22 0.22 0.22 0.22 0.1 0.5 0.22 0.22 0.22 0.22 0.12 0.36 0.22 0.22 0.22 0.22
2.4 2.8 2.2 2.4 2.9 3.1 2.5 2.8 2.2 2.9 2.8 2.2 2.3 3.2 2.4 2.7 2.6 2.3 2.4 2.9 2.7 2.3 2.5 2.6
2.396 2.784 2.107 2.446 2.871 3.170 2.527 2.777 2.290 2.857 2.746 2.209 2.355 3.216 2.406 2.738 2.661 2.336 2.375 2.804 2.511 2.511 2.511 2.511
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S.K. Choudhury, J.B. Bajpai / Journal of Materials Processing Technology 170 (2005) 487–493
Fig. 1. Schematic diagram of the experimental set-up.
S.K. Choudhury, J.B. Bajpai / Journal of Materials Processing Technology 170 (2005) 487–493
Fig. 3. Effect of work speed, Nw on the surface roughness, Ra .
Fig. 2. Experimental vs. predicted values of surface roughness, Ra .
to zero rpm where the process approached to conventional milling process. The experiments were conducted by utilizing various work speeds of 0, 6, 9, 12, 20, 31 and 40 rpm. Similarly, effect of cutter diameter, Dc on the surface finish at optimum work speed has been studied.
4. Results and discussion 4.1. Stage-I Table 2 depicts the values of input variables and the measured and calculated (predicted) surface roughness for each experiment. From the experimental results given in Table 2, the constants in the regression Eq. (1) were evaluated. The following response surface equation was obtained for evaluating the surface roughness, ysf (Ra value) in terms of the variables, x1 (work speed), x2 (diameter of cutter) and x3 (depth of cut) ysf = 1.931 + 0.049x1 − 0.043x2 + 1.106x3 − 0.0004x12 + 0.0008x22 + 3.036x32 − 0.003x1 x2 − 0.025x1 x3 − 0.023x2 x3
491
(5)
ysf = 2.291 + 0.332x3 + 3.036x32
Using Eq. (6), surface roughness was calculated for a range of work speed from 12 to 40 rpm. Fig. 3 shows the variation of surface roughness, Ra with work speed, Nw . It is observed that within this range of speed the surface roughness, Ra increases with the increase in work speed, Nw . Vibration was considered to be one of the reasons for deterioration of the surface finish. While conducting these experiments range of rotational work speed selected was from 12 to 40 rpm as per the planning of experiments. However, the influence of rotational work speed in the range from 0 to 12 rpm has been investigated in the later stage. Using Eq. (7), surface roughness, ysf was calculated for the whole range of cutter diameter from φ6 to φ25 mm. Fig. 4 shows the variation of surface roughness, Ra with diameter of cutter, Dc . It was observed that the surface roughness decreased with increase in cutter diameter. Increase in the cutting speed due to increase in cutter diameter has been considered to be the reason for improvement in the surface finish. Using Eq. (8), surface roughness, ysf was calculated for the whole ranges of depth of cut from 0.1 to 0.5 mm. Fig. 5 shows the variation of surface roughness, Ra with depth of cut, t. It is
Using Eq. (5) surface roughness was predicted for each set of twenty-four experiments. A plot was drawn between measured surface roughness value (x-axis) and calculated surface roughness value (y-axis) as shown in Fig. 2. A good correlation between these two sets of values has been observed. In the scatter diagram all the points are on or very near to the 45◦ line which indicates that the predicted and the measured values are very close for all the twenty-four set of experiments. Moreover, using Eq. (5), the response surface equations in terms of work speed, diameter of cutter and depth of cut were obtained as follows ysf = 1.869 + 0.0394x1 − 0.00037x12
(6)
ysf = 3.042 − 0.054x2 + 0.00084x22
(7)
(8)
Fig. 4. Effect of cutter diameter, Dc on surface roughness, Ra .
S.K. Choudhury, J.B. Bajpai / Journal of Materials Processing Technology 170 (2005) 487–493
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Table 4 Experimental results at optimum work speed, for Nw = 9 rpm and t = 0.22 mm
Fig. 5. Effect of depth of cut, t on surface roughness, Ra . Table 3 Experimental results of orthogonal turn-milling for Dc = 12 mm and t = 0.22 mm (Stage-II) Experiment number
Work speed ‘Nw ’ (rpm)
Measured ‘Ra ’ value (m)
1 2 3 4 5 6 7
0 6 9 12 20 31 40
3.8 2.1 1.9 1.9 2.2 2.7 3.1
observed that, the surface roughness increased with increase in depth of cut. The higher cutting forces due to larger chip cross-section area must be the reason for deterioration of the surface finish. In Figs. 3–5 experimental points taken from Table 2 have been superimposed on the curves to show close relationship between the results obtained analytically and those obtained experimentally. 4.2. Stage-II
Experiment number
Diameter of cutter ‘Dc ’ (mm)
Measured ‘Ra ’ value (m)
1 2 3 4 5
φ6 φ8 φ12 φ18 φ25
2.9 2.7 2.4 2.2 2.2
shows the variation of surface roughness, Ra with work speed, Nw for the entire range from 0 to 40 rpm. It is observed from the figure that the surface roughness reduces to minimum at the rotational work speed of around 10 rpm. When the rotational work speed approaches to zero rpm, the turnmilling process becomes similar to conventional milling process, where the milling cutter is only rotated together with the axial-feed given to the work piece. At this stage surface roughness value, Ra was found to be maximum. Moreover, it is clear from the Fig. 6 that, when work piece starts revolving, the conventional milling process converts into turn-milling and the maximum surface roughness value, Ra , obtained with the orthogonal turn-milling was still lower than that obtained with the conventional milling. Hence it is concluded that within the specified condition, a work piece rotational speed of about 10 rpm would result in good surface quality. Table 4 depicts the input variables, i.e. cutter diameter and measured surface roughness for each experiment. Fig. 7 shows the variation of surface roughness, Ra with the diameter of cutter, Dc at optimum work speed, Nw = 9 rpm. It is observed from the figure that, better surface finish can be realized at high cutting speeds, i.e., for larger cutter diameters where continuous chips without built-up-edge are formed. Moreover, it can be observed that, increase in the cutter diameter beyond certain value does not improve the surface quality.
Table 3 depicts the input variables, i.e. work speed and measured surface roughness for each experiment. Fig. 6
Fig. 6. Effect of rotational work speed, Nw on the surface roughness.
Fig. 7. Effect of diameter of cutter, Dc on surface roughness at optimum work speed, Nw .
S.K. Choudhury, J.B. Bajpai / Journal of Materials Processing Technology 170 (2005) 487–493
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5. Conclusions
References
From the experimental results following conclusions are drawn:
[1] M.C. Shaw, P.A. Smith, N.H. Cook, The rotary cutting tool, Trans. ASME 74 (1952) 1065–1076. [2] P.K. Venuvinod, W.S. Lau, P. Narasimha Reddy, Some investigation into machining with driven rotary tools, J. Eng. Ind., Trans. ASME 103 (1981) 469–477. [3] E.J.A. Armarego, V. Karri, A.J.R. Smith, Fundamental studies of driven and self-propelled rotary tool cutting processes I, theoretical investigation, Int. J. Mach. Tools Manuf. 34 (6) (1994) 785–801. [4] K. Yamaguchi, T. Nakamura, S. Kato, Boring by plug-type rotary cutting tool, J. Eng. Ind., Trans. ASME 112 (1990) 136–141. [5] H. Schulz, G. Spur, High speed turn-milling—a new precision manufacturing technology for machining of rotationally symmetrical work pieces, Ann. CIRP 39 (1) (1990) 107–109. [6] H. Schulz, T. Kneisel, Turn-milling of hardened steel—an alternative to turning, Ann. CIRP 43 (1) (1994) 93–96. [7] A. Daniel, T. Kneisel, High Precision Cutting by Turn-Milling, Institute of Production Engineering and Machine Tools, Technical University of Darmstadt, Germany, 1993, pp. 133–140. [8] J. Kopac, M. Pogacnik, Theory and practice of achieving quality surface in turn-milling, Int. J. Mach. Tools Manuf. 37 (5) (1997) 709–715. [9] S.K. Choudhury, K.S. Mangrulkar, Investigation of orthogonal turnmilling for the machining of rotationally symmetrical work pieces, J. Mat. Proc. Tech. 99 (2000) 120–128. [10] W.G. Cochran, G.M. Cox, Experimental Designs, Asia Publishing House, 1962.
1. In case of orthogonal turn-milling, surface finish of the machined surface varies with the rotational speed of the work piece. When the work piece is stationary, the surface roughness was maximum. The surface finish of the machined surface improves with the rotational speed of the work piece up to a certain limit, i.e. 10 rpm. However, with further increase in the rotational speed of the work piece the surface finish deteriorates. 2. The surface finish of the machined surface improves with increase in the cutter diameter up to a certain limit. Similarly, the surface finish of the machined surface deteriorates with increase in depth of cut. 3. The surface finish of the machined surface produced by orthogonal turn-milling is better than that of conventional milling. 4. Experimental work has shown strong influence of vibrations on the turn-milling process. Hence, the process of turn-milling should be carried out utilizing high dynamic stiffness, especially in the process frequency area.
Investigation in orthogonal turn-milling towards better surface finish S.K. Choudhury ∗ , J.B. Bajpai Department of Mechanical Engineering, Indian Institute of Technology, Kanpur 208016, India Received 22 May 2001; received in revised form 6 March 2002; accepted 12 December 2004
Abstract Turn-milling is a newly emerging machining process, which tends to make use of the advantages of both turning and milling, wherein both the work piece and the cutting tool are given rotary motion simultaneously. The objective of the present experimental work is to understand the phenomenon of orthogonal turn-milling especially in relation to the effects of work piece revolution, cutter diameter and depth of cut. Surface finish of the machined surface and the optimum work speed at which the surface roughness was minimum has been studied. It has been shown that surface quality obtained by turn-milling process is better than that of conventional milling process. The experiments have been conducted for orthogonal turn-milling of mild-steel work piece with high speed steel milling cutters using planning of experiments technique to study the surface finish achieved. © 2005 Elsevier B.V. All rights reserved. Keywords: Turn-milling; Surface finish; Planning of experiments; Optimum work speed
1. Introduction The turn-milling is a relatively new concept in manufacturing technology wherein both the work piece and the tool are given rotary motion simultaneously. According to the arrangement of the two rotating axes relative to each other there are two different ways by which the process can be classified: 1. orthogonal turn-milling in which the axis of rotation of the cutter is perpendicular to the work piece; 2. co-axial turn-milling in which the axis of rotation of the cutter is parallel to the work piece. Whereas, orthogonal turn-milling is suitable only for external machining of the work pieces, co-axial turn-milling is suitable for internal as well as external machining of rotationally symmetrical work pieces. This new innovative technology enables high productivity and high-machined surface quality at the same time. Especially at an increased number of tool revolutions (high speed machining) the quality of the process is reported to be rising very steeply [5]. The productivity could be much greater ∗
Corresponding author. E-mail address: [email protected] (S.K. Choudhury).
0924-0136/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2004.12.010
in comparison to the conventional processes like turning or milling. Recently it has been shown that when rotationally symmetrical work pieces are to be produced, turn-milling can be real alternative to grinding. In turn-milling high cutting speeds can be achieved since the two cutting speeds of tool and work piece are added up, resulting in high surface finish, low thermal stress of cutting edge and good chip removal due to short chip length. A very important advantage is also that it enables complete machining of work pieces with different shape elements without changing the set-up. Turn-milling offers the possibility of producing, besides flat and cylindrical shapes, more complicated ones, as for example, ellipses, screw-threads, etc. Shaw et al. [1] have described a disk-type turning tool rotating about its central axis. The authors have mentioned about the longer tool life, lower coefficient of friction and a greater effective rake angle as advantages of such rotary tools. The authors have also shown that under optimum conditions the total power required for a given rate of cutting is about 30% less with a rotary tool. The experimental studies conducted by Venuvinod et al. [2] on rotary tools in the form of a frustum of a cone show that with proper selection of rational speeds, the following can be achieved:
488
S.K. Choudhury, J.B. Bajpai / Journal of Materials Processing Technology 170 (2005) 487–493
(a) reduction in cutting forces up to 10%; (b) chip thickness ratio is much lesser than unity; (c) chip length ratios as high as 2:3. Armarego et al. [3] have explained the fundamentals of driven and self-propelled rotary tool cutting operations as wedge tool cutting process. They have observed that the driven oblique rotary tool process is the most efficient one. Yamaguchi et al. [4] conducted experiments on boring, in which a drilled hole is finished with a plug-type rotary cutting tool. On the basis of experimental studies following conclusions were made: (1) the rotation of plug-type tool has favorable effects on cutting mechanisms that improve the chip formation; (2) the mean diameter of the hole machined by plug-type tool is constant at every depth of finished hole. The creditable work in the field of turn-milling has been done in Germany by Schulz and Spur [5]. The experiments were conducted for machining of roller bearing races using high speed turn-milling with the objective to study the effect on surface finish, geometric accuracy, chip geometry, tool wear, etc. On the basis of experimental results the advantages of high-speed machining have been reported by the authors as high surface quality, low thermal stress of the cutting edge and low cutting forces. In addition, good chip removal has also been achieved due to short chip length. Schulz and Kneisel [6] have worked on turn-milling of hard materials as an alternative to turning. The authors have shown that surface quality of the work pieces obtained in turnmilling can be compared with those achieved by grinding. Daniel and Kneisel [7] conducted the experiments on coaxial turn-milling and shown that accessible surface quality mainly depends on the ratio of the rotational frequencies of tool and work piece. Investigations concerning the influence of cutting speed showed that tool wear and surface quality closely depend on this parameter. Kopac and Pogacnik [8] have described the variation of uncut chip thickness in orthogonal turn-milling. Experimental work has shown a strong influence of vibrations on turnmilling process. Authors suggested high dynamic stiffness, especially in the process frequency area as one of the solutions. As per the investigations conducted by Choudhury and Mangrulkar [9] surface roughness value; Ra achieved by orthogonal turn-milling is about 10 times lower than achieved in case of turning. The experimental results showed that the surface finish improves with increase in rotational speed of the cutter and deteriorates with increase in axial-feed rate. The aim of the present experimental work was to understand the phenomenon of turn-milling as well as to study the effects of various machining parameters on the surface texture generated by turn-milling. The emphasis has been laid mainly on the surface finish of the machined surface. The experiments were confined to constant cutter speed (1000 rpm) and axial-feed (8 mm/min) to study the effect of work speed,
diameter of cutter and depth of cut on the surface quality produced. Experiments were also conducted by reducing the work speed to zero rpm where the process was equivalent to the milling operation. The optimum work speed, at which the surface roughness was minimum, has been determined.
2. Planning of experiments Planning of experiments technique helps in extracting useful information by performing minimum number of experiments. The central composite rotatable design [10] for three input variables, i.e. work speed, cutter diameter and depth of cut was selected in this work. The general form of a quadratic polynomial for three variables is illustrated as: yu = b0u + b1u x1 + b2u x2 + b3u x3 + b11u x12 + b22u x22 + b33u x32 + b12u x1 x2 + b13u x1 x3 + b23u x2 x3
(1)
Here, yu represents the response, b0u , b1u , . . ., b23u are regression coefficients, x1 , x2 and x3 are input variables. The regression coefficients were calculated by using the standard procedure [10]. In present analysis effect of three input variables viz., work speed, cutter diameter and depth of cut have been studied on the surface finish (Ra value) of the work piece. Relations between the coded x-scales and original scales, in which the levels were recorded, were established initially. In design scale, the lowest and the highest values of the input variables, ‘x’ were −1.68 and 1.68 [10]. For present experimentation, the ranges of work speed, diameter of cutter and depth of cut were selected depending on the available gear train, cutters and the overall technological capabilities of the machine and the fixture. They are, work speed, Nw from 12 to 40 rpm; diameter of cutter, Dc from φ6 to φ25 mm; depth of cut, t from 0.1 to 0.5 mm. In general x = a + b log (variable) [10] In which, a and b were chosen to satisfy the desired conditions at the ends of the scale as explained below. In case of work speed, Nw taken as variable, x = a + b log Nw , when x = −1.68; Nw = 12 and −1.68 = a + b log 12, when x = 1.68; Nw = 40 and 1.68 = a + b log 40.On solving these two equations simultaneously, we get, a = −8.60 and b = 6.42, hence, for work speed, Nw XN = −8.60 + 6.42 log Nw
(2)
Similarly for diameter of cutter, Dc XD = −5.89 + 5.42 log Dc
(3)
and for depth of cut, t Xt = 3.12 + 4.80 log t
(4)
From Eqs. (2)–(4), work speed, diameter of cutter and depth of cut corresponding to five levels of x as −1.68, −1, 0, 1, 1.68
S.K. Choudhury, J.B. Bajpai / Journal of Materials Processing Technology 170 (2005) 487–493
489
Table 1 Specifications of milling cutters Serial number
Type
Diameter (mm)
Number of teeth
Primary clearance angle
1 2 3 4 5
Shank type end-mill Shank type end-mill Shank type end-mill Shank type end-mill Shank type end-mill
φ6 φ8 φ12 φ18 φ25
4 4 4 6 6
5◦ 5◦ 5◦ 8◦ 8◦
were determined. Since the exact work speed, cutter diameter and depth of cut as obtained above were not available, values were slightly rounded off to their nearest ones.
Positive radial rake angle 6◦ 8◦ 10◦ 10◦ 10◦
Helix angle
Material
20◦ 20◦ 20◦ 35◦ 35◦
HSS HSS HSS HSS HSS
least count, which provided a numerical assessment of the surface roughness in terms of Ra values. 3.2. Experimental procedure
3. Experimental set-up and procedure 3.1. Experimental set-up Present work was conducted on a vertical milling machine since the axes of the work piece rotation had to be perpendicular to the axes of milling cutter rotation. Cylindrical work piece of mild-steel of φ65 mm diameter and 170 mm length was fixed on the machine using special attachment, consisting of an arrangement of gear train to achieve various rotational speeds of the work piece. A three-phase ac electrical motor of 0.3 kW power with rotation of 25 rpm was used for rotating the work piece. The details of five different cutters used in experimentation are shown in Table 1. Schematic diagram of the experimental set-up is shown in Fig. 1. Surface roughness was measured using SURTONIC 10 instrument of 0.1 m
Experiments were conducted in two stages to understand the effect of various machining parameters on the surface finish, to determine the optimum rotational work speed, Nw and to achieve the best possible surface finish. In stage-I the experiments were conducted to establish the effect of following parameters on the surface finish: • rotational speed of the work piece, Nw (rpm): 12, 16, 20, 31, 40; • diameter of the cutter, Dc (mm): φ6, φ8, φ12, φ18, φ25; • depth of cut, t (mm): 0.1, 0.12, 0.22, 0.36, 0.5. In the second stage the experiments were conducted to determine the optimum rotational work speed, Nw at which the surface roughness was minimum. This was accomplished by further reduction in rotational speed of the work piece up
Table 2 Experimental results of orthogonal turn-milling Experiment number
Work speed ‘Nw ’ (rpm)
Diameter of cutter ‘Dc ’ (mm)
Depth of cut ‘t’ (mm)
Surface roughness ‘Ra ’ value (m) Measured
Calculated
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
16 31 16 31 16 31 16 31 12 40 20 20 20 20 16 31 20 20 20 20 20 20 20 20
8 8 18 18 8 8 18 18 12 12 6 25 12 12 12 12 8 18 12 12 12 12 12 12
0.12 0.12 0.12 0.12 0.36 0.36 0.36 0.36 0.22 0.22 0.22 0.22 0.1 0.5 0.22 0.22 0.22 0.22 0.12 0.36 0.22 0.22 0.22 0.22
2.4 2.8 2.2 2.4 2.9 3.1 2.5 2.8 2.2 2.9 2.8 2.2 2.3 3.2 2.4 2.7 2.6 2.3 2.4 2.9 2.7 2.3 2.5 2.6
2.396 2.784 2.107 2.446 2.871 3.170 2.527 2.777 2.290 2.857 2.746 2.209 2.355 3.216 2.406 2.738 2.661 2.336 2.375 2.804 2.511 2.511 2.511 2.511
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S.K. Choudhury, J.B. Bajpai / Journal of Materials Processing Technology 170 (2005) 487–493
Fig. 1. Schematic diagram of the experimental set-up.
S.K. Choudhury, J.B. Bajpai / Journal of Materials Processing Technology 170 (2005) 487–493
Fig. 3. Effect of work speed, Nw on the surface roughness, Ra .
Fig. 2. Experimental vs. predicted values of surface roughness, Ra .
to zero rpm where the process approached to conventional milling process. The experiments were conducted by utilizing various work speeds of 0, 6, 9, 12, 20, 31 and 40 rpm. Similarly, effect of cutter diameter, Dc on the surface finish at optimum work speed has been studied.
4. Results and discussion 4.1. Stage-I Table 2 depicts the values of input variables and the measured and calculated (predicted) surface roughness for each experiment. From the experimental results given in Table 2, the constants in the regression Eq. (1) were evaluated. The following response surface equation was obtained for evaluating the surface roughness, ysf (Ra value) in terms of the variables, x1 (work speed), x2 (diameter of cutter) and x3 (depth of cut) ysf = 1.931 + 0.049x1 − 0.043x2 + 1.106x3 − 0.0004x12 + 0.0008x22 + 3.036x32 − 0.003x1 x2 − 0.025x1 x3 − 0.023x2 x3
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(5)
ysf = 2.291 + 0.332x3 + 3.036x32
Using Eq. (6), surface roughness was calculated for a range of work speed from 12 to 40 rpm. Fig. 3 shows the variation of surface roughness, Ra with work speed, Nw . It is observed that within this range of speed the surface roughness, Ra increases with the increase in work speed, Nw . Vibration was considered to be one of the reasons for deterioration of the surface finish. While conducting these experiments range of rotational work speed selected was from 12 to 40 rpm as per the planning of experiments. However, the influence of rotational work speed in the range from 0 to 12 rpm has been investigated in the later stage. Using Eq. (7), surface roughness, ysf was calculated for the whole range of cutter diameter from φ6 to φ25 mm. Fig. 4 shows the variation of surface roughness, Ra with diameter of cutter, Dc . It was observed that the surface roughness decreased with increase in cutter diameter. Increase in the cutting speed due to increase in cutter diameter has been considered to be the reason for improvement in the surface finish. Using Eq. (8), surface roughness, ysf was calculated for the whole ranges of depth of cut from 0.1 to 0.5 mm. Fig. 5 shows the variation of surface roughness, Ra with depth of cut, t. It is
Using Eq. (5) surface roughness was predicted for each set of twenty-four experiments. A plot was drawn between measured surface roughness value (x-axis) and calculated surface roughness value (y-axis) as shown in Fig. 2. A good correlation between these two sets of values has been observed. In the scatter diagram all the points are on or very near to the 45◦ line which indicates that the predicted and the measured values are very close for all the twenty-four set of experiments. Moreover, using Eq. (5), the response surface equations in terms of work speed, diameter of cutter and depth of cut were obtained as follows ysf = 1.869 + 0.0394x1 − 0.00037x12
(6)
ysf = 3.042 − 0.054x2 + 0.00084x22
(7)
(8)
Fig. 4. Effect of cutter diameter, Dc on surface roughness, Ra .
S.K. Choudhury, J.B. Bajpai / Journal of Materials Processing Technology 170 (2005) 487–493
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Table 4 Experimental results at optimum work speed, for Nw = 9 rpm and t = 0.22 mm
Fig. 5. Effect of depth of cut, t on surface roughness, Ra . Table 3 Experimental results of orthogonal turn-milling for Dc = 12 mm and t = 0.22 mm (Stage-II) Experiment number
Work speed ‘Nw ’ (rpm)
Measured ‘Ra ’ value (m)
1 2 3 4 5 6 7
0 6 9 12 20 31 40
3.8 2.1 1.9 1.9 2.2 2.7 3.1
observed that, the surface roughness increased with increase in depth of cut. The higher cutting forces due to larger chip cross-section area must be the reason for deterioration of the surface finish. In Figs. 3–5 experimental points taken from Table 2 have been superimposed on the curves to show close relationship between the results obtained analytically and those obtained experimentally. 4.2. Stage-II
Experiment number
Diameter of cutter ‘Dc ’ (mm)
Measured ‘Ra ’ value (m)
1 2 3 4 5
φ6 φ8 φ12 φ18 φ25
2.9 2.7 2.4 2.2 2.2
shows the variation of surface roughness, Ra with work speed, Nw for the entire range from 0 to 40 rpm. It is observed from the figure that the surface roughness reduces to minimum at the rotational work speed of around 10 rpm. When the rotational work speed approaches to zero rpm, the turnmilling process becomes similar to conventional milling process, where the milling cutter is only rotated together with the axial-feed given to the work piece. At this stage surface roughness value, Ra was found to be maximum. Moreover, it is clear from the Fig. 6 that, when work piece starts revolving, the conventional milling process converts into turn-milling and the maximum surface roughness value, Ra , obtained with the orthogonal turn-milling was still lower than that obtained with the conventional milling. Hence it is concluded that within the specified condition, a work piece rotational speed of about 10 rpm would result in good surface quality. Table 4 depicts the input variables, i.e. cutter diameter and measured surface roughness for each experiment. Fig. 7 shows the variation of surface roughness, Ra with the diameter of cutter, Dc at optimum work speed, Nw = 9 rpm. It is observed from the figure that, better surface finish can be realized at high cutting speeds, i.e., for larger cutter diameters where continuous chips without built-up-edge are formed. Moreover, it can be observed that, increase in the cutter diameter beyond certain value does not improve the surface quality.
Table 3 depicts the input variables, i.e. work speed and measured surface roughness for each experiment. Fig. 6
Fig. 6. Effect of rotational work speed, Nw on the surface roughness.
Fig. 7. Effect of diameter of cutter, Dc on surface roughness at optimum work speed, Nw .
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5. Conclusions
References
From the experimental results following conclusions are drawn:
[1] M.C. Shaw, P.A. Smith, N.H. Cook, The rotary cutting tool, Trans. ASME 74 (1952) 1065–1076. [2] P.K. Venuvinod, W.S. Lau, P. Narasimha Reddy, Some investigation into machining with driven rotary tools, J. Eng. Ind., Trans. ASME 103 (1981) 469–477. [3] E.J.A. Armarego, V. Karri, A.J.R. Smith, Fundamental studies of driven and self-propelled rotary tool cutting processes I, theoretical investigation, Int. J. Mach. Tools Manuf. 34 (6) (1994) 785–801. [4] K. Yamaguchi, T. Nakamura, S. Kato, Boring by plug-type rotary cutting tool, J. Eng. Ind., Trans. ASME 112 (1990) 136–141. [5] H. Schulz, G. Spur, High speed turn-milling—a new precision manufacturing technology for machining of rotationally symmetrical work pieces, Ann. CIRP 39 (1) (1990) 107–109. [6] H. Schulz, T. Kneisel, Turn-milling of hardened steel—an alternative to turning, Ann. CIRP 43 (1) (1994) 93–96. [7] A. Daniel, T. Kneisel, High Precision Cutting by Turn-Milling, Institute of Production Engineering and Machine Tools, Technical University of Darmstadt, Germany, 1993, pp. 133–140. [8] J. Kopac, M. Pogacnik, Theory and practice of achieving quality surface in turn-milling, Int. J. Mach. Tools Manuf. 37 (5) (1997) 709–715. [9] S.K. Choudhury, K.S. Mangrulkar, Investigation of orthogonal turnmilling for the machining of rotationally symmetrical work pieces, J. Mat. Proc. Tech. 99 (2000) 120–128. [10] W.G. Cochran, G.M. Cox, Experimental Designs, Asia Publishing House, 1962.
1. In case of orthogonal turn-milling, surface finish of the machined surface varies with the rotational speed of the work piece. When the work piece is stationary, the surface roughness was maximum. The surface finish of the machined surface improves with the rotational speed of the work piece up to a certain limit, i.e. 10 rpm. However, with further increase in the rotational speed of the work piece the surface finish deteriorates. 2. The surface finish of the machined surface improves with increase in the cutter diameter up to a certain limit. Similarly, the surface finish of the machined surface deteriorates with increase in depth of cut. 3. The surface finish of the machined surface produced by orthogonal turn-milling is better than that of conventional milling. 4. Experimental work has shown strong influence of vibrations on the turn-milling process. Hence, the process of turn-milling should be carried out utilizing high dynamic stiffness, especially in the process frequency area.