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Effect of various parameters on the surface roughness of an aluminium alloy burnished with a spherical surfaced polycrystalline diamond tool
International Journal of Machine Tools & Manufacture 39 (1999) 459–469
Effect of various parameters on the surface roughness of an aluminium alloy burnished with a spherical surfaced polycrystalline diamond tool Xingbo Yua,*, Lijiang Wangb a
Department of Mechanical Engineering, Jilin Institute of Technology, Changchun 130012, People’s Republic of China b Department of Mechanical Engineering, Jilin University of Technology, Changchun 130021, People’s Republic of China Received 28 February 1997; in final form 19 February 1998
Abstract The effect of various parameters on the surface roughness of an aluminium alloy burnished with a spherical surfaced polycrystalline diamond tool are studied experimentally with a theoretical analysis. Problems in selecting the optimum burnishing parameters and some burnishing mechanisms are discussed. With suitable parameters employed, the new no-chip finishing process developed can eliminate or reduce the cutting marks left on the workpiece surface by diamond cutting tools, with its surface roughness reduced to Ra ⫽ 0.026 m from the original 0.5 m. 1998 Elsevier Science Ltd. All rights reserved. Keywords: Burnishing machining; Precision machining; Polycrystalline diamond; Diamond tool
1. Introduction Application of diamond tools for the precision cutting of non-ferrous metals such as aluminium alloys are getting increasingly widespread in recent years. Nevertheless, cutting marks are still produced on the machined surfaces. To further improve the finish of the surface, cutting with very thin chips, grinding or polishing had been employed, although these processes are difficult and inefficient.
* Corresponding author. Tel: ⫹ 86-0431-592-0010; Fax: ⫹ 86-0431-595-2413; E-mail: [email protected] 0890-6955/98/$—see front matter 1998 Elsevier Science Ltd. All rights reserved. PII: S 0 8 9 0 - 6 9 5 5 ( 9 8 ) 0 0 0 3 3 - 9
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Having worked on aluminium and copper alloy cutting respectively, Yuan et al. [1] and Wang et al. [2] showed that it is possible to obtain a largely improved surface finish by cutting with diamond tools. Loh et al. [3] reported that the no-chip burnishing of steel workpieces by bearing steel or tungsten carbide balls is fruitful in smoothing down the original cutting marks. Also, works on surface rolling by cylindrical rollers to reduce the surface roughness of steel workpieces have been reviewed by Zhang [4]. Previous work [5] has indicated that the technique of burnishing with a spherical surfaced polyrystalline diamond (PCD) tool is effective in levelling off the cutting marks on the machined surface of Al-alloy workpieces formed by pre-machining diamond tool cutting, and it has been found that that the burnished surface roughness is not only related to the depth of tool penetration, but also to the initial surface roughness. However, further study on this topic is still demanded. It is the aim of this work to conduct a more extensive investigation on the burnishing of Alalloy with PCD tools and to examine and discuss the influences of various parameters on the surface roughness. 2. Experimental work 2.1. Experimental method Burnishing was conducted on a Chinese made ordinary lathe of the model CM0420M/2 with power feed, and dry burnishing was accomplished with a self prepared spherical surfaced PCD tool. As shown in Fig. 1, the penetration depth of the PCD tool into the surface of the workpiece, defined as burnishing depth app, was adjusted and controlled by two dialgauges (3) and (7), with (7) mounted on the tool head (5), app was fixed to 2 m in this investigation. Burnishing of metals is a process that leads to an accurate change in the surface profile of the workpiece by a minor amount of plastic deformation. In burnishing, the metal on the surface of the workpiece is redistributed without material loss. Microscopically, the profile of the machined
Fig. 1. Schematic illustration of the experimental set-up. 1—sample (spindle), 2—dog, 3 and 7—dial gauges, 4—tool rest, 5—tool head, 6—spherical surfaced PCD.
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Fig. 2. Deformation of workpiece surface during burnishing. Rsph—radius of the spherical surface of PCD, dw—diameter of sample, n—rpm of sample (spindle).
surfaces is composed of hills and valleys, and during burnishing, the hills are pressed down and the valleys distended up, resulting in a smooth surface of excellent finish. The process is schematically shown in Fig. 2. The dependence of the surface roughness after burnishing, denoted by Ra, on the burnishing speed V and feed f was examined, and the influence of other parameters like the diameter of the spherical surface of PCD tool Dsph, the initial surface roughness of the workpiece Ras and the rigidity coefficient of the spring in the tool head Ki were also investigated. Surface roughness was measured with Rank Taylor Hobson, Talysurf, with the arithmetic mean of the absolute distances from every point of the profile to the datum line taken as the roughness. 2.2. The burnishing tool head In fabricating the burnishing tool, natural diamond is too expensive. In this work, the much cheaper and more practical artificial polycrystalline diamond is employed. The structure of the tool head is shown in Fig. 3. Two spherical surfaced PCD tools (9) were
Fig. 3. Structure of the tool head. 1—tool shaft, 2—adjusting nut, 3—dial gauge, 4—compression spring, 5—body of tool head, 6—gauge mount, 7—dog, 8—steel ball, 9—PCD tool, 10—spherical surface of PCD, 11—keyway screw, 12—supporting frame.
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Table 1 Parameters of PCD sphere No. 1
Sphere diameter Dsph (mm)
Surface roughness Ra (m)
1# 2#
1.0 0.5
0.035 0.035
Table 2 Rigidity coefficients of springs in tool head Rigidity coefficient
Ki (N/mm)
Rigid burnishing (no spring)
⬁ (K0)
Elastic burnishing
spring No. 1
spring No. 2
spring No. 3
6.71 (K1)
2.70 (K2)
0.45 (K3)
prepared, Table 1 illustrates their parameters. The PCD material was the COMPAX thin piece from the E.G. Co. of the US. Average grain size of the PCD is 20 m. The material was first welded to the tool rod by high frequency induction welding with an Ag-Cu alloy as the filler metal, and then ground into a spherical surface on a Swiss made diamond grinding machine of model RS12 and a diamond grinder installed by the first author. The diameters of the PCD tool Dsph were 0.5 mm and 1 mm respectively. Three compression springs (4) with different rigidity coefficients were prepared for the tool head (see Table 2). By the use of the screw (11), the tool shaft (1) can be tightened, causing the spring to lose its action, which corresponds to a state of rigid burnishing. When the screw is loosened, burnishing becomes elastic. 2.3. Sample preparation A cast Al-alloy was used as the sample material. The nominal composition of which is as follows: 12% Si, 0.8% Cu, 0.1% Zn, 0.5% Mn, 0.1% Fe. Its hardness reads BHN50. Other details of sample preparation is shown in Table 3. Table 3 Conditions of sample preparation Sample Cutting tool
Cutting regimes Machine-tool Vc—cutting speed
Roughness Ras (m) Table 4 and 5 Edge-angle K␥, K’␥ 12° K␥ ⫽ 38° K’␥ ⫽ 20° Depth of cut, ap (mm) 0.02
Material Al-alloy Material
Diameter dw (mm) Length L (mm) ⌽36 180 Structure Rake ␥0 Clearance ␣0
PCD
Turning tool
n (r/min), Vc (m/min) 565 Vc ⫽ 64 CM0420/No. 2 Lathe
1° Cutting feed, fc (mm/r) 0.0425
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3. Results and discussion 3.1. Effect of burnishing speed on the surface roughness The relationship curves between the surface roughness Ra and the burnishing speed V are shown in Fig. 4. The surface roughness of all the curves decreases first with the increase of speed until a minimum is reached. Then with the increase of speed, roughness increases also. At every minimum, best surface finish is obtained, and corresponds to an optimum burnishing speed Vopt. It is evident that the smaller roughness Ramin and the larger absolute value 兩Ramin-Ras兩 obtained after burnishing, imply that a larger improvement of surface finish is achieved, from which a relative evaluation on the burnishing quality of every curve can be made. The parameters of the curves are listed in Table 4. To explain the trend of the curves, it is thought that the temperature at the burnishing area increases with the burnishing speed. This will lower the shear resistance of the movement and multiplication of the dislocations in the metal, or plastic deformation will proceed more easily and more sufficiently. It follows that the microscopic hills and valleys on the workpiece surface will be levelled off in a larger extent, and as a result, the surface roughness decrease with the increase of the speed [2,6]. When the speed increases beyond the minimum point (Vopt), chattering of the burnishing system plays the principal role, then surface roughness deteriorates.
Fig. 4.
Relationship between speed and surface roughness.
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Table 4 Parameters of curves for Ra vs. V Curves
Dsph (mm)
app (m)
Ras (m)
Vopt (m/min) app/Ras
Ramin (m)
兩Ramin-Ras兩 (m)
No. No. No. No. No. No. No. No.
1.0 1.0 1.0 1.0 0.5 0.5 0.5 0.5
2 2 2 2 2 2 2 2
0.50 1.20 0.39 0.15 0.90 0.66 0.78 0.53
128 102 80 102 102 102 102 102
0.026 0.11 0.10 0.06 0.18 0.18 0.27 0.28
0.474 1.09 0.29 0.09 0.72 0.48 0.51 0.25
1 5 9 13 2 6 10 14
4.0 1.67 5.13 13.33 2.22 3.04 2.56 3.77
3.2. Effect of feed on the surface roughness In Fig. 5, surface roughness is plotted as a function of feed f. It can be seen that every curve is roughly a semi-parabola opening upward and lies in the first quadrant. Ra increases monotonically with f. Obviously, the minimum roughness Ramin of every curve corresponds to the minimum feed fmin employed, which is the optimum value fopt. In this work, fopt is taken to be 10.1 m/r. The parameters of the curves are listed in Table 5. For burnishing with spherical surface, the following equation can be derived from Fig. 6;
Fig. 5. Relationship between feed and surface roughness.
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Table 5 Parameters of curves for Ra vs. f Curves
Dsph (mm)
app (m)
Ras (m)
fopt (m/r)
app/Ras
Ramin (m)
兩Ramin-Ras兩 (m)
No. No. No. No. No. No. No. No.
1.0 1.0 1.0 1.0 0.5 0.5 0.5 0.5
2 2 2 2 2 2 2 2
0.50 1.20 0.39 0.15 0.90 0.66 0.78 0.53
10.1 10.1 10.1 10.1 10.1 10.1 10.1 10.1
4.0 1.67 5.13 13.33 2.22 3.04 2.56 3.77
0.19 0.24 0.10 0.075 0.32 0.16 0.33 0.35
0.31 0.96 0.29 0.075 0.58 0.50 0.45 0.18
3 7 11 15 4 8 12 16
Fig. 6. Comparison of theoretical roughness resulted from turning and burnishing. ①—profile of residual area (turning marks) formed by turning, ②—profile of residual area (burnishing marks) formed by burnishing with PCD tool, R⬘max— maximum height of the profile of residual area formed by turning., fc—turning (cutting) feed, f—burnishing feed, *— starting position of turning, o—starting position of burnishing, (a) homothetic burnishing (both the starting positions and feeds of turning and burnishing are the same), (b) heterosteric burnishing (starting positions of turning and burnishing are different; feeds of turning and burnishing are the same), (c) heterosteric burnishing (feeds of turning and burnishing are different; starting positions of turning and burnishing are the same).
Rmax ⬇
f2 8Rsph
(1)
where Rsph—radius of spherical surface on the PCD tool and Rmax—maximum height of the profile of residual area formed by burnishing. Because Ra ⬇ Rmax/4 [6], Eq. (1) thus expresses approximately the theoretical roughness value
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obtainable in burnishing. It is evident that the relationship between roughness Ra (or Rmax) and feed f is roughly parabolic, and is basically similar to the curves shown in Fig. 5, indicating that the above analysis is in accord with the experimental results. Fig. 6 explains schematically the influence of the starting position of burnishing on the burnished roughness in addition to the influence of feed. When Fig. 6(a) and Fig. 6(b) are compared, it can be found that if the feed used in pre-burnishing turning and that used in burnishing are all the same; burnishing at a heterosteric position as shown in Fig. 6(b) results in a smaller Ra than that obtained by a burnishing at homothetic position [Fig. 6(a)]. By homothetic position, it means the starting point of burnishing is right at the valley of the surface profile formed by pre-burnishing turning, provided the burnishing feed is the same as the turning feed; while by heterosteric position, that is in front of the valley. In Fig. 6(c), burnishing feed is smaller than turning feed, once burnishing has started, it is heterosteric although the starting point of burnishing is right at the valley. In this experiment, by adjusting the burnishing PCD tool under a microscope, better Ra obtained by heterosteric burnishing had been observed. However, more systematic studies on this point remain to be carried out. 3.3. Effects of parameters of the PCD spherical surface and the rigidity of the spring As shown in Figs. 4 and 5, if Fig. 4(a) with Fig. 4(b) and Fig. 5(a) with Fig. 5(b) are compared, it can be seen that the burnishing quality is better when a larger Rsph is used. This is because with a larger Rsph, the stability of the burnishing system is higher. Besides, it is obvious that the better the finish of the spherical surface, the smaller the resulted roughness of the workpiece. From Fig. 4, it is clear that under lower burnishing speeds (V ⬍ 100 m/min), elastic burnishing brings out better results than that with rigid burnishing (rigidity coefficient K0 ⫽ ⬁), see left parts of curve No. 1 in Fig. 4(a) and curve No. 2 in Fig. 4(b) compared with the other corresponding curves. While under higher burnishing speeds (V > 100 m/min), rigid burnishing results in a finer finish, see right parts of the above curves. This can be explained as follows: With lower speeds, in addition to that the burnishing system is more stable, the radial force produced in burnishing can be regulated by the spring in the tool head; while with higher speeds, the system is influenced by chattering, rigid burnishing is more stable. From Fig. 4(b) and Fig. 5(b), it is also found that when a Dsph ⫽ 0.5 mm PCD tool is used under elastic burnishing, better results are obtained with springs of larger rigidity coefficients, this is because the overall stability of the burnishing system is relatively lower when a spring of smaller rigidity coefficient is matched with a PCD tool of smaller Dsph. 3.4. Effect of depth ratio of burnishing app/Ras on the surface roughness As aforementioned through Tables 4 and 5, the selection and control of the burnishing depth app have a very crucial influence on the quality of surface finish; and also, the larger the initial surface roughness Ras, the larger the extent of roughness decrement 兩Ramin-Ras兩 after burnishing. To examine the effect of the depth ratio of burnishing app/Ras on the roughness, two sets of rigid burnishing curves are presented in Fig. 7, and from the data of which, the surface roughness as a function of depth ratio of burnishing can be plotted as shown in Fig. 8. It can be seen from the three curves in Fig. 7(a), when V ⬍ 100 m/min, the smallest roughness is obtained in the
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Fig. 7. Relationship between speed feed and surface roughness.
Fig. 8. Effect of depth ratio of burnishing on surface roughness for different feeds and at different speeds.
workpiece with the smallest initial roughness (0.39 m); when V > 160 m/min, the contrary occurs; and when 100 m/min ⬍ V ⬍ 160 m/min, the Ra value of curve No. 3 with the smallest Ras value became higher than that of curve No. 2 and lower than that of curve No. 1. The parameters of these three curves, including the app ( ⫽ 2 m), are all the same except V and Ras.
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Thus it is considered that reason for the occurrence of the above three cases can only be that the parameters, Ras and app/Ras are in action simultaneously in addition to the speed V. This can be explained further by Fig. 8(a). In Fig. 8(a), the five Ra vs. app/Ras curves are in the order of V1, V2, V5, V3, and V4. It can be seen that the three curves with speeds V3, V4 and V5 are all concaved upward, the roughness at every minimum is Ramin, and the corresponding optimum depth ratio of burnishing (app/Ras)opt ⫽ 4. The curves with speed V1 and V2 all decrease monotonically, i.e., Ra decreases gradually with the increase of app/Ras. Now, the curves with speeds V3 and V4, in Fig. 8(a) are just in the range 100 m/min ⬍ V ⬍ 160 m/min. In Fig. 7(a), because the app/Ras of curve No. 3 is 5.13; that of curve No. 2 is 4 and of curve No. 1 is 1.67, the surface roughness after burnishing of curve No. 3 is smaller than that of curve No. 1 and larger than that of curve No. 2. When V ⬍ 100 m/min, it corresponds to the monotonically decreasing curves with speed V1 and V2 in Fig. 8(a), and in Fig. 7(a), curve No. 3 has the largest depth ratio of burnishing, so its roughness after burnishing is smaller than those of curves No. 1 and No. 2. When V > 160 m/min, it corresponds to the curve with V5 in Fig. 8(a). On this curve, Ra at app/Ras ⫽ 5.13 is larger than those at app/Ras ⫽ 4 and 1.67, consequently, among the three curves in Fig. 7(a), curve No. 3 inevitably has the worst burnishing effect. Considering the difference in Ras of every sample and the elastic recovery of the workpiece material following burnishing, for rigid burnishing, the optimum position of burnishing point on the PCD spherical surface should be adjacent to the bottom of the valley on the residual area of the pre-machined workpiece surface at the radial direction, i.e., app ⫽ Rmax ⫽ 4 Ras, or app/Ras ⫽ 4 is the optimum depth ratio of burnishing. For elastic burnishing, the optimum burnishing point should pass over the bottom of the valley; and the smaller the rigidity coefficient of the spring, the larger the distance surpassed the bottom of the valley should be. Thus it is reasonable to select a suitable depth ratio of burnishing in the range app/Ras > 4 according to the value of Ki (or the radial burnishing force). The three Ra versus f curves in Fig. 7(b) are all parabolas with increasing tendencies similar to those in Fig. 5, except that when feed f ⬍ 40 m/r, an abnormal phenomena occurs, i.e., for curve No. 6 with the smallest Ras, the Ra value after burnishing is not the smallest. This can be analyzed through Fig. 8(b), in which the five Ra versus app/Ras curves with different f values are in the order of the value of f, among these curves, curve f1 exhibits a distinct minimum with an optimum (app/Ras)opt ⫽ 4. While in Fig. 7(b), the depth ratio of burnishing of curve No. 6 is app/Ras ⫽ 5.13, which makes its post-burnish roughness still higher than that of curve No. 5 with app/Ras ⫽ 4, although the initial roughness of curve No. 6 is the smallest. This is probably because under the condition of small feed, when app/Ras increases beyond the minimal point, work hardening of the workpiece material becomes intense, which deteriorates the surface roughness.
4. Conclusions 1. The workpiece surface of the aluminium alloy can be burnished to a roughness of Ra 0.026 m from the initial 0.5 m with a spherical surfaced PCD tool on an ordinary lathe. 2. Every Ra versus V curve has a minimum, which corresponds to a best burnished roughness
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3. 4. 5.
6.
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Ramin produced by an optimum burnishing speed Vopt; and every Ra vs. f curve is a parabola with progressively increasing tendency, the minimum roughness occurs at the smallest feed employed—10.1 m/r. Surface finish obtained by heterosteric burnishing is superior to that obtained by homothetic burnishing. Taking surface finish as the criterion, when a PCD tool with its Dsph ⫽ 1 mm is employed, burnishing quality is better than that with Dsph ⫽ 0.5 mm. Under lower speeds (V ⬍ 100 m/ ⬍ min), roughness obtained by elastic burnishing is smaller than that obtained by rigid burnishing; while with higher speeds (100 m/min ⬍ V ⬍ 160 m/min), the opposite is true. A spring with larger rigidity coefficient (K1) produces better burnishing quality than those with smaller rigidity coefficients (K2 and K3). Introducing the concept of the depth ratio of burnishing app/Ras, not only can the effect of the two parameters app and Ras on the burnished roughness be considered comprehensively, but also helps to select the critical parameter—the optimum burnishing depth app. In this study, the optimum depth ratio of burnishing (app/Ras)opt is found to be 4 for rigid burnishing, and > 4 for elastic burnishing.
References [1] Z.J. Yuan, et al., Influences of the sharpness of diamond tool cutting edge on the cutting deformation and quality of cutting surface, Machine Tool 11 (1992) 21–24 (in Chinese). [2] L. Wang, et al., Investigations on mirror surface turning conducted with PCD tool, Progress of Natural Science (Chinese edition) 2(2) (1992) 180–184 (in Chinese). [3] N.H. Loh, Investigations on the surface roughness produced by ball burnishing, Int. J. Mach. Tools Manufact 31 (1) (1991) 75–81. [4] Z.M. Zhang, Rolling—a method of ultra-precision machining (a review), Modular Machine Tool and Automatic Manufacturing Technique 6 (1991) 16–20 (in Chinese). [5] X. Yu, L. Wang, An experimental investigation for pressing Al-alloy with PCD tool, Proc. 7th Int. Conf. of Manufacturing, Harbin, China, October 1995, pp. 235–240. [6] Z.H. Zhou, Principles of Metal Cutting, Shanghai Publishing House of Science and Technology 135, (1995) 121– 122, (in Chinese).
Effect of various parameters on the surface roughness of an aluminium alloy burnished with a spherical surfaced polycrystalline diamond tool Xingbo Yua,*, Lijiang Wangb a
Department of Mechanical Engineering, Jilin Institute of Technology, Changchun 130012, People’s Republic of China b Department of Mechanical Engineering, Jilin University of Technology, Changchun 130021, People’s Republic of China Received 28 February 1997; in final form 19 February 1998
Abstract The effect of various parameters on the surface roughness of an aluminium alloy burnished with a spherical surfaced polycrystalline diamond tool are studied experimentally with a theoretical analysis. Problems in selecting the optimum burnishing parameters and some burnishing mechanisms are discussed. With suitable parameters employed, the new no-chip finishing process developed can eliminate or reduce the cutting marks left on the workpiece surface by diamond cutting tools, with its surface roughness reduced to Ra ⫽ 0.026 m from the original 0.5 m. 1998 Elsevier Science Ltd. All rights reserved. Keywords: Burnishing machining; Precision machining; Polycrystalline diamond; Diamond tool
1. Introduction Application of diamond tools for the precision cutting of non-ferrous metals such as aluminium alloys are getting increasingly widespread in recent years. Nevertheless, cutting marks are still produced on the machined surfaces. To further improve the finish of the surface, cutting with very thin chips, grinding or polishing had been employed, although these processes are difficult and inefficient.
* Corresponding author. Tel: ⫹ 86-0431-592-0010; Fax: ⫹ 86-0431-595-2413; E-mail: [email protected] 0890-6955/98/$—see front matter 1998 Elsevier Science Ltd. All rights reserved. PII: S 0 8 9 0 - 6 9 5 5 ( 9 8 ) 0 0 0 3 3 - 9
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X. Yu, L. Wang / International Journal of Machine Tools & Manufacture 39 (1999) 459–469
Having worked on aluminium and copper alloy cutting respectively, Yuan et al. [1] and Wang et al. [2] showed that it is possible to obtain a largely improved surface finish by cutting with diamond tools. Loh et al. [3] reported that the no-chip burnishing of steel workpieces by bearing steel or tungsten carbide balls is fruitful in smoothing down the original cutting marks. Also, works on surface rolling by cylindrical rollers to reduce the surface roughness of steel workpieces have been reviewed by Zhang [4]. Previous work [5] has indicated that the technique of burnishing with a spherical surfaced polyrystalline diamond (PCD) tool is effective in levelling off the cutting marks on the machined surface of Al-alloy workpieces formed by pre-machining diamond tool cutting, and it has been found that that the burnished surface roughness is not only related to the depth of tool penetration, but also to the initial surface roughness. However, further study on this topic is still demanded. It is the aim of this work to conduct a more extensive investigation on the burnishing of Alalloy with PCD tools and to examine and discuss the influences of various parameters on the surface roughness. 2. Experimental work 2.1. Experimental method Burnishing was conducted on a Chinese made ordinary lathe of the model CM0420M/2 with power feed, and dry burnishing was accomplished with a self prepared spherical surfaced PCD tool. As shown in Fig. 1, the penetration depth of the PCD tool into the surface of the workpiece, defined as burnishing depth app, was adjusted and controlled by two dialgauges (3) and (7), with (7) mounted on the tool head (5), app was fixed to 2 m in this investigation. Burnishing of metals is a process that leads to an accurate change in the surface profile of the workpiece by a minor amount of plastic deformation. In burnishing, the metal on the surface of the workpiece is redistributed without material loss. Microscopically, the profile of the machined
Fig. 1. Schematic illustration of the experimental set-up. 1—sample (spindle), 2—dog, 3 and 7—dial gauges, 4—tool rest, 5—tool head, 6—spherical surfaced PCD.
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Fig. 2. Deformation of workpiece surface during burnishing. Rsph—radius of the spherical surface of PCD, dw—diameter of sample, n—rpm of sample (spindle).
surfaces is composed of hills and valleys, and during burnishing, the hills are pressed down and the valleys distended up, resulting in a smooth surface of excellent finish. The process is schematically shown in Fig. 2. The dependence of the surface roughness after burnishing, denoted by Ra, on the burnishing speed V and feed f was examined, and the influence of other parameters like the diameter of the spherical surface of PCD tool Dsph, the initial surface roughness of the workpiece Ras and the rigidity coefficient of the spring in the tool head Ki were also investigated. Surface roughness was measured with Rank Taylor Hobson, Talysurf, with the arithmetic mean of the absolute distances from every point of the profile to the datum line taken as the roughness. 2.2. The burnishing tool head In fabricating the burnishing tool, natural diamond is too expensive. In this work, the much cheaper and more practical artificial polycrystalline diamond is employed. The structure of the tool head is shown in Fig. 3. Two spherical surfaced PCD tools (9) were
Fig. 3. Structure of the tool head. 1—tool shaft, 2—adjusting nut, 3—dial gauge, 4—compression spring, 5—body of tool head, 6—gauge mount, 7—dog, 8—steel ball, 9—PCD tool, 10—spherical surface of PCD, 11—keyway screw, 12—supporting frame.
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Table 1 Parameters of PCD sphere No. 1
Sphere diameter Dsph (mm)
Surface roughness Ra (m)
1# 2#
1.0 0.5
0.035 0.035
Table 2 Rigidity coefficients of springs in tool head Rigidity coefficient
Ki (N/mm)
Rigid burnishing (no spring)
⬁ (K0)
Elastic burnishing
spring No. 1
spring No. 2
spring No. 3
6.71 (K1)
2.70 (K2)
0.45 (K3)
prepared, Table 1 illustrates their parameters. The PCD material was the COMPAX thin piece from the E.G. Co. of the US. Average grain size of the PCD is 20 m. The material was first welded to the tool rod by high frequency induction welding with an Ag-Cu alloy as the filler metal, and then ground into a spherical surface on a Swiss made diamond grinding machine of model RS12 and a diamond grinder installed by the first author. The diameters of the PCD tool Dsph were 0.5 mm and 1 mm respectively. Three compression springs (4) with different rigidity coefficients were prepared for the tool head (see Table 2). By the use of the screw (11), the tool shaft (1) can be tightened, causing the spring to lose its action, which corresponds to a state of rigid burnishing. When the screw is loosened, burnishing becomes elastic. 2.3. Sample preparation A cast Al-alloy was used as the sample material. The nominal composition of which is as follows: 12% Si, 0.8% Cu, 0.1% Zn, 0.5% Mn, 0.1% Fe. Its hardness reads BHN50. Other details of sample preparation is shown in Table 3. Table 3 Conditions of sample preparation Sample Cutting tool
Cutting regimes Machine-tool Vc—cutting speed
Roughness Ras (m) Table 4 and 5 Edge-angle K␥, K’␥ 12° K␥ ⫽ 38° K’␥ ⫽ 20° Depth of cut, ap (mm) 0.02
Material Al-alloy Material
Diameter dw (mm) Length L (mm) ⌽36 180 Structure Rake ␥0 Clearance ␣0
PCD
Turning tool
n (r/min), Vc (m/min) 565 Vc ⫽ 64 CM0420/No. 2 Lathe
1° Cutting feed, fc (mm/r) 0.0425
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3. Results and discussion 3.1. Effect of burnishing speed on the surface roughness The relationship curves between the surface roughness Ra and the burnishing speed V are shown in Fig. 4. The surface roughness of all the curves decreases first with the increase of speed until a minimum is reached. Then with the increase of speed, roughness increases also. At every minimum, best surface finish is obtained, and corresponds to an optimum burnishing speed Vopt. It is evident that the smaller roughness Ramin and the larger absolute value 兩Ramin-Ras兩 obtained after burnishing, imply that a larger improvement of surface finish is achieved, from which a relative evaluation on the burnishing quality of every curve can be made. The parameters of the curves are listed in Table 4. To explain the trend of the curves, it is thought that the temperature at the burnishing area increases with the burnishing speed. This will lower the shear resistance of the movement and multiplication of the dislocations in the metal, or plastic deformation will proceed more easily and more sufficiently. It follows that the microscopic hills and valleys on the workpiece surface will be levelled off in a larger extent, and as a result, the surface roughness decrease with the increase of the speed [2,6]. When the speed increases beyond the minimum point (Vopt), chattering of the burnishing system plays the principal role, then surface roughness deteriorates.
Fig. 4.
Relationship between speed and surface roughness.
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Table 4 Parameters of curves for Ra vs. V Curves
Dsph (mm)
app (m)
Ras (m)
Vopt (m/min) app/Ras
Ramin (m)
兩Ramin-Ras兩 (m)
No. No. No. No. No. No. No. No.
1.0 1.0 1.0 1.0 0.5 0.5 0.5 0.5
2 2 2 2 2 2 2 2
0.50 1.20 0.39 0.15 0.90 0.66 0.78 0.53
128 102 80 102 102 102 102 102
0.026 0.11 0.10 0.06 0.18 0.18 0.27 0.28
0.474 1.09 0.29 0.09 0.72 0.48 0.51 0.25
1 5 9 13 2 6 10 14
4.0 1.67 5.13 13.33 2.22 3.04 2.56 3.77
3.2. Effect of feed on the surface roughness In Fig. 5, surface roughness is plotted as a function of feed f. It can be seen that every curve is roughly a semi-parabola opening upward and lies in the first quadrant. Ra increases monotonically with f. Obviously, the minimum roughness Ramin of every curve corresponds to the minimum feed fmin employed, which is the optimum value fopt. In this work, fopt is taken to be 10.1 m/r. The parameters of the curves are listed in Table 5. For burnishing with spherical surface, the following equation can be derived from Fig. 6;
Fig. 5. Relationship between feed and surface roughness.
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Table 5 Parameters of curves for Ra vs. f Curves
Dsph (mm)
app (m)
Ras (m)
fopt (m/r)
app/Ras
Ramin (m)
兩Ramin-Ras兩 (m)
No. No. No. No. No. No. No. No.
1.0 1.0 1.0 1.0 0.5 0.5 0.5 0.5
2 2 2 2 2 2 2 2
0.50 1.20 0.39 0.15 0.90 0.66 0.78 0.53
10.1 10.1 10.1 10.1 10.1 10.1 10.1 10.1
4.0 1.67 5.13 13.33 2.22 3.04 2.56 3.77
0.19 0.24 0.10 0.075 0.32 0.16 0.33 0.35
0.31 0.96 0.29 0.075 0.58 0.50 0.45 0.18
3 7 11 15 4 8 12 16
Fig. 6. Comparison of theoretical roughness resulted from turning and burnishing. ①—profile of residual area (turning marks) formed by turning, ②—profile of residual area (burnishing marks) formed by burnishing with PCD tool, R⬘max— maximum height of the profile of residual area formed by turning., fc—turning (cutting) feed, f—burnishing feed, *— starting position of turning, o—starting position of burnishing, (a) homothetic burnishing (both the starting positions and feeds of turning and burnishing are the same), (b) heterosteric burnishing (starting positions of turning and burnishing are different; feeds of turning and burnishing are the same), (c) heterosteric burnishing (feeds of turning and burnishing are different; starting positions of turning and burnishing are the same).
Rmax ⬇
f2 8Rsph
(1)
where Rsph—radius of spherical surface on the PCD tool and Rmax—maximum height of the profile of residual area formed by burnishing. Because Ra ⬇ Rmax/4 [6], Eq. (1) thus expresses approximately the theoretical roughness value
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obtainable in burnishing. It is evident that the relationship between roughness Ra (or Rmax) and feed f is roughly parabolic, and is basically similar to the curves shown in Fig. 5, indicating that the above analysis is in accord with the experimental results. Fig. 6 explains schematically the influence of the starting position of burnishing on the burnished roughness in addition to the influence of feed. When Fig. 6(a) and Fig. 6(b) are compared, it can be found that if the feed used in pre-burnishing turning and that used in burnishing are all the same; burnishing at a heterosteric position as shown in Fig. 6(b) results in a smaller Ra than that obtained by a burnishing at homothetic position [Fig. 6(a)]. By homothetic position, it means the starting point of burnishing is right at the valley of the surface profile formed by pre-burnishing turning, provided the burnishing feed is the same as the turning feed; while by heterosteric position, that is in front of the valley. In Fig. 6(c), burnishing feed is smaller than turning feed, once burnishing has started, it is heterosteric although the starting point of burnishing is right at the valley. In this experiment, by adjusting the burnishing PCD tool under a microscope, better Ra obtained by heterosteric burnishing had been observed. However, more systematic studies on this point remain to be carried out. 3.3. Effects of parameters of the PCD spherical surface and the rigidity of the spring As shown in Figs. 4 and 5, if Fig. 4(a) with Fig. 4(b) and Fig. 5(a) with Fig. 5(b) are compared, it can be seen that the burnishing quality is better when a larger Rsph is used. This is because with a larger Rsph, the stability of the burnishing system is higher. Besides, it is obvious that the better the finish of the spherical surface, the smaller the resulted roughness of the workpiece. From Fig. 4, it is clear that under lower burnishing speeds (V ⬍ 100 m/min), elastic burnishing brings out better results than that with rigid burnishing (rigidity coefficient K0 ⫽ ⬁), see left parts of curve No. 1 in Fig. 4(a) and curve No. 2 in Fig. 4(b) compared with the other corresponding curves. While under higher burnishing speeds (V > 100 m/min), rigid burnishing results in a finer finish, see right parts of the above curves. This can be explained as follows: With lower speeds, in addition to that the burnishing system is more stable, the radial force produced in burnishing can be regulated by the spring in the tool head; while with higher speeds, the system is influenced by chattering, rigid burnishing is more stable. From Fig. 4(b) and Fig. 5(b), it is also found that when a Dsph ⫽ 0.5 mm PCD tool is used under elastic burnishing, better results are obtained with springs of larger rigidity coefficients, this is because the overall stability of the burnishing system is relatively lower when a spring of smaller rigidity coefficient is matched with a PCD tool of smaller Dsph. 3.4. Effect of depth ratio of burnishing app/Ras on the surface roughness As aforementioned through Tables 4 and 5, the selection and control of the burnishing depth app have a very crucial influence on the quality of surface finish; and also, the larger the initial surface roughness Ras, the larger the extent of roughness decrement 兩Ramin-Ras兩 after burnishing. To examine the effect of the depth ratio of burnishing app/Ras on the roughness, two sets of rigid burnishing curves are presented in Fig. 7, and from the data of which, the surface roughness as a function of depth ratio of burnishing can be plotted as shown in Fig. 8. It can be seen from the three curves in Fig. 7(a), when V ⬍ 100 m/min, the smallest roughness is obtained in the
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Fig. 7. Relationship between speed feed and surface roughness.
Fig. 8. Effect of depth ratio of burnishing on surface roughness for different feeds and at different speeds.
workpiece with the smallest initial roughness (0.39 m); when V > 160 m/min, the contrary occurs; and when 100 m/min ⬍ V ⬍ 160 m/min, the Ra value of curve No. 3 with the smallest Ras value became higher than that of curve No. 2 and lower than that of curve No. 1. The parameters of these three curves, including the app ( ⫽ 2 m), are all the same except V and Ras.
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Thus it is considered that reason for the occurrence of the above three cases can only be that the parameters, Ras and app/Ras are in action simultaneously in addition to the speed V. This can be explained further by Fig. 8(a). In Fig. 8(a), the five Ra vs. app/Ras curves are in the order of V1, V2, V5, V3, and V4. It can be seen that the three curves with speeds V3, V4 and V5 are all concaved upward, the roughness at every minimum is Ramin, and the corresponding optimum depth ratio of burnishing (app/Ras)opt ⫽ 4. The curves with speed V1 and V2 all decrease monotonically, i.e., Ra decreases gradually with the increase of app/Ras. Now, the curves with speeds V3 and V4, in Fig. 8(a) are just in the range 100 m/min ⬍ V ⬍ 160 m/min. In Fig. 7(a), because the app/Ras of curve No. 3 is 5.13; that of curve No. 2 is 4 and of curve No. 1 is 1.67, the surface roughness after burnishing of curve No. 3 is smaller than that of curve No. 1 and larger than that of curve No. 2. When V ⬍ 100 m/min, it corresponds to the monotonically decreasing curves with speed V1 and V2 in Fig. 8(a), and in Fig. 7(a), curve No. 3 has the largest depth ratio of burnishing, so its roughness after burnishing is smaller than those of curves No. 1 and No. 2. When V > 160 m/min, it corresponds to the curve with V5 in Fig. 8(a). On this curve, Ra at app/Ras ⫽ 5.13 is larger than those at app/Ras ⫽ 4 and 1.67, consequently, among the three curves in Fig. 7(a), curve No. 3 inevitably has the worst burnishing effect. Considering the difference in Ras of every sample and the elastic recovery of the workpiece material following burnishing, for rigid burnishing, the optimum position of burnishing point on the PCD spherical surface should be adjacent to the bottom of the valley on the residual area of the pre-machined workpiece surface at the radial direction, i.e., app ⫽ Rmax ⫽ 4 Ras, or app/Ras ⫽ 4 is the optimum depth ratio of burnishing. For elastic burnishing, the optimum burnishing point should pass over the bottom of the valley; and the smaller the rigidity coefficient of the spring, the larger the distance surpassed the bottom of the valley should be. Thus it is reasonable to select a suitable depth ratio of burnishing in the range app/Ras > 4 according to the value of Ki (or the radial burnishing force). The three Ra versus f curves in Fig. 7(b) are all parabolas with increasing tendencies similar to those in Fig. 5, except that when feed f ⬍ 40 m/r, an abnormal phenomena occurs, i.e., for curve No. 6 with the smallest Ras, the Ra value after burnishing is not the smallest. This can be analyzed through Fig. 8(b), in which the five Ra versus app/Ras curves with different f values are in the order of the value of f, among these curves, curve f1 exhibits a distinct minimum with an optimum (app/Ras)opt ⫽ 4. While in Fig. 7(b), the depth ratio of burnishing of curve No. 6 is app/Ras ⫽ 5.13, which makes its post-burnish roughness still higher than that of curve No. 5 with app/Ras ⫽ 4, although the initial roughness of curve No. 6 is the smallest. This is probably because under the condition of small feed, when app/Ras increases beyond the minimal point, work hardening of the workpiece material becomes intense, which deteriorates the surface roughness.
4. Conclusions 1. The workpiece surface of the aluminium alloy can be burnished to a roughness of Ra 0.026 m from the initial 0.5 m with a spherical surfaced PCD tool on an ordinary lathe. 2. Every Ra versus V curve has a minimum, which corresponds to a best burnished roughness
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3. 4. 5.
6.
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Ramin produced by an optimum burnishing speed Vopt; and every Ra vs. f curve is a parabola with progressively increasing tendency, the minimum roughness occurs at the smallest feed employed—10.1 m/r. Surface finish obtained by heterosteric burnishing is superior to that obtained by homothetic burnishing. Taking surface finish as the criterion, when a PCD tool with its Dsph ⫽ 1 mm is employed, burnishing quality is better than that with Dsph ⫽ 0.5 mm. Under lower speeds (V ⬍ 100 m/ ⬍ min), roughness obtained by elastic burnishing is smaller than that obtained by rigid burnishing; while with higher speeds (100 m/min ⬍ V ⬍ 160 m/min), the opposite is true. A spring with larger rigidity coefficient (K1) produces better burnishing quality than those with smaller rigidity coefficients (K2 and K3). Introducing the concept of the depth ratio of burnishing app/Ras, not only can the effect of the two parameters app and Ras on the burnished roughness be considered comprehensively, but also helps to select the critical parameter—the optimum burnishing depth app. In this study, the optimum depth ratio of burnishing (app/Ras)opt is found to be 4 for rigid burnishing, and > 4 for elastic burnishing.
References [1] Z.J. Yuan, et al., Influences of the sharpness of diamond tool cutting edge on the cutting deformation and quality of cutting surface, Machine Tool 11 (1992) 21–24 (in Chinese). [2] L. Wang, et al., Investigations on mirror surface turning conducted with PCD tool, Progress of Natural Science (Chinese edition) 2(2) (1992) 180–184 (in Chinese). [3] N.H. Loh, Investigations on the surface roughness produced by ball burnishing, Int. J. Mach. Tools Manufact 31 (1) (1991) 75–81. [4] Z.M. Zhang, Rolling—a method of ultra-precision machining (a review), Modular Machine Tool and Automatic Manufacturing Technique 6 (1991) 16–20 (in Chinese). [5] X. Yu, L. Wang, An experimental investigation for pressing Al-alloy with PCD tool, Proc. 7th Int. Conf. of Manufacturing, Harbin, China, October 1995, pp. 235–240. [6] Z.H. Zhou, Principles of Metal Cutting, Shanghai Publishing House of Science and Technology 135, (1995) 121– 122, (in Chinese).