Design of arrow-head electrode in electropolishing of cylindrical part

Transcription

1 312 Int. J. Materials and Product Technology, Vol. 20, No. 4, 2004 Design of arrow-head electrode in electropolishing of cylindrical part H. Hocheng* Department of Power Mechanical Engineering, National Tsing Hua University, Hsinchu, Taiwan, R.O.C. *Corresponding author P.S. Pa Development of Medical Product, Globe Enterprises Inc., Taipei, Taiwan, R.O.C. Abstract: The current work investigates the improvement of surface finish of external cylindrical surface finish on several die materials beyond traditional rough turning or drawing by electropolishing using arrowhead electrodes. The electrode is designed effective and low-cost. The controlled factors in electropolishing test include the chemical composition and concentration of the electrolyte, the electrolyte temperature, the flow rate of electrolyte, and the initial gap width. The experimental parameters are die material, rotational speed of workpiece and tool electrode, current rating, the electrode design parameters, and electrode feed rate. The electrode travels across the surface with continuous direct current. Both cone and flat arrow-head features are studied. Small end radius and high rotational speed of electrode produce good polishing. Small end radius also leads to high current density and feed rate. Small taper angle and large flute rake angle provide sufficient discharge space, which is advantageous for electropolishing. The electrode with double discharge flutes is superior to the electrode with single flute. Thin electrode with large flute rake angle and deep flute performs well. Keywords: electropolishing; electrode design; surface roughness; arrow-head; cylindrical surface. Reference to this paper should be made as follows: Hocheng, H. and Pa, P.S. (2004) Design of arrow-head electrode in electropolishing of cylindrical part, Int. J. Materials and Product Technology, Vol. 20, No. 4, pp Biographical notes: Dr. H. Hocheng obtained his BSc from National Taiwan University, Taiwan, and later his Diplom-Ingenieur from Technische Hochschule Aachen, Germany. He received PhD from University of California, Berkeley. Presently he is a Professor at National Tsing Hua University. He was awarded outstanding research from national science council. His fields of interest are innovative manufacturing processes. Dr. P.S. Pa obtained his doctorate at National Tsing Hua University. His current research interests are ECM process for mould and die materials. Copyright 2004 Inderscience Enterprises Ltd.

2 Design of arrow-head electrode in electropolishing of cylindrical part Introduction Gusseff first filed a patent on electrochemical machining (ECM) in 1929 and found that ECM is suitable for high-strength and high-melting point alloys [1]. Electrochemical machining won more attention in industrial application, and the process became an important method of metal removal [2]. Electrochemical machining has been known as a highly effective metal removal method for machining the hardened metals without producing the process-induced stress and wear of the machining tool [3]. More industrial applications were realized throughout the decades, such as electrochemical drilling, electrochemical grinding, electrochemical deburring, and electropolishing [4]. The principle of ECM was presented by Faraday in the eighteenth century. The workpiece is made the anode of an electrolytic cell, the tool is made the cathode and is maintained at a gap of tenths of millimetre from the workpiece. An electrolyte is pumped between the gap while a large direct current is passed across, and the material of workpiece is removed by electrolytic dissolution of the anode. The metal removal in ECM depends on the current density between the electrodes, and the tolerances of workpiece depend on knowledge and control of this current distribution [5]. Hence the production of a desired work anode configuration by the electrochemical machining method requires correct design of the tool cathode [6]. The major factors in the removal rate of electrochemical grinding were found the conductivity of the workpiece, the rate of decomposition, the current capacity of power supply, and the ingredients, concentration, and temperature of electrolyte. The material removal rate per unit area (feed rate) is proportional to the current density multiplied by the current efficiency, and the feed rate of electrode [7]. Bannard correlates the current efficiency with current density and flow rate of the electrolyte. The maximum efficiency varies with the type of electrolyte [8]. Noto et al. put forward the study of the electrode gap using sodium nitrate solution [9]. Datta showed that the gap width between electrode and workpiece directly influences the current condition and the dreg discharge of the electrolyte [10]. Bejar et al. changed the machining gap width as well as the concentration of electrolyte to investigate the influence upon current efficiency. They found that current efficiency is raised with the increase of current density and electrolytic concentration [11]. Rajurkar et al. obtained the minimum gap width based on Ohm Law, Faraday Law, and the equation of conservation of energy, beyond which the electrolyte will be boiled in electrochemical machining. An on-line monitoring system was proposed [12]. When voltage is increased, the overcut also increases, the current passed between anode and cathode becomes higher [13]. The electropolishing can efficiently produce workpieces of good surface finish [14]. It is well suitable for difficult-to-machine materials. Plastic or press dies, wire-drawing dies, optical and electric parts can apply this technique as well [15]. The electrochemical honing of cylindrical holes improves the dimensional accuracy and relieves the surface layer stress [16]. While the NaNO 3 electrolyte reduces the current efficiency with current density, good dimensional control can be achieved [17]. Shen used NaNO 3 as the electrolyte to proceed the electropolishing on die surface. The result showed that the surface roughness of workpieces decreases with the increase of current density, flow rate and concentration of electrolyte. He also found polishing with pulse direct current is better than continuous direct current [18]. High-density pulse direct current improves the precision of workpiece, and significantly extends the range of electrochemical surface

3 314 H. Hocheng and P.S. Pa finishing. Furthermore, the average current density is reduced [19]. The experimental results of Mileham et al. showed the quality of the machined surface is influenced by the current density, flow rate of electrolyte and the gap width [20]. The external cylindrical surface of commonly used die materials is electropolished in the current study. In rough turning or drawing, the average surface roughness lies between 3.0 and 6.3 µm, better surface ( µm) can be obtained either by fine turning or grinding [21]. When more severe requirement on surface finish (<0.8 µm) arises, subsequent conventional techniques, such as polishing by hand or machine, are often applied. These methods heavily depend on sophisticated experience, and either hand or machine polishing will result in nonuniform residual stress due to the contact between tool and workpiece. Surface crack and micro voids are often induced and deteriorate the service life of the parts. The electropolishing can efficiently produce workpieces free of the above-mentioned shortcomings [20]. However, the major difficulty of electropolishing is the cost of the mate tool electrode. An ECM using a universal tool and CNC path planning technique was presented [22]. It requires a sophisticated CNC controller and software. The electrochemical grinding (ECG) can be used for difficult-to-machine alloys to bring the surface roughness down to µm [20], whereas the equipment cost is a concern. An efficient polishing process for drilled holes using low-cost feeding electrode has been proposed [23]. For electropolishing of external cylindrical surface, various types of electrode were developed, including the turning tool and ring [24,25]. The current study is a serial work discussing the effects of the arrow-head tool design as another major effective alternative when turning and drawing can hardly achieve the desired fine surface finish for the die materials. 2 Electrode design The development of electrode is based on the following considerations: Effective discharge of electrolytic products The discharge of the electrolytic products out of the gap is crucial for the polishing. The advantages of small taper angle, workpiece and tool electrode rotation, large flute rake angles, thin electrode thickness, and deep discharge flute will be incorporated into the electrode design. Reduction of secondary machining To ensure the geometrical and dimensional accuracy of the polished surface, the secondary overcutting induced by the working gap should be avoided as possible. Fast feed rate A good electrode design should provide the electropolishing with sufficient electrical energy for fast feed rate in the operation. Cost of subsequent polishing After rough turning or drawing, the workpiece surface should be electropolished using the electrode of low cost. The manufacturability of the electrode should cause no concern.

4 Design of arrow-head electrode in electropolishing of cylindrical part 315 The development sequence of the electrode design is illustrated in Figure 1, and the design of arrow-head electrode is shown in Figure 2. Figure 1 Development of electrode design 3 Experimental set-up The chemical compositions of workpiece materials are shown in Table 1, the experimental materials include SKD11, SKD61, NAK80, and SNCM8. The dimension of the workpiece is 10 mm in diameter and 50 mm in length, and the initial average surface roughness (R a ) of the workpiece from drawing or rough turning is 4 ~ 6 µm. The amount of the reduction of diameter after electropolishing lies between 0.02 mm and 0.2 mm, which is designed in the process for dimensional control of parts. The equipment of electropolishing includes DC power supply, pulse generators, electrolytic tank and pump, flow meter, and filter (see Figure 3). Table 1 Chemical composition of workpieces (wt%) Fe C Si Mn P S Cr Mo Al V Cu Ni SKD SKD NAK SNCM

5 316 H. Hocheng and P.S. Pa Figure 2 Design of arrow-head electrode

6 Design of arrow-head electrode in electropolishing of cylindrical part 317 Figure 3 Experimental set-up The electrolyte is NaNO 3 of 25 wt%, and the flow rate of electrolyte is 4l min The temperature of the electrolyte is maintained at 25 ± 5 C. The gap width between electrode and workpiece is set 0.3 mm. The main parameters include die material, the rotational speed of workpiece and tool electrode, the geometry of electrode, the electrical current, and the electrode feed rate. The ranges of these parameters are shown in Table 2. Table 2 Experimental parameters Rotation speed of workpiece (rpm) 200, 400, 600, 800, 1000, 1200 Rotation speed of electrode (rpm) 0, 200, 400, 600, 800, 1000, 1200 Current rating (A) 5, 10, 15, 20 Electrode feed rate (mm/min) 0.5, 1, 1.5, 2, 2.5, 3, 3.5, 4 End radius (mm) 0.5 to 10, at 0.5 increment Taper angle (degree) 30, 45, 60, 75, 90, 105 Back rake angle (degree) 0, 10, 20, 30, 40 Side rake angle (degree) 0, 10, 20, 30, 40 Tail angle (degree) 0, 5, 10, 15, 20, 25 Flat electrode thickness (mm) 5, 6, 7, 8 Flute depth (mm) 4, 6, 8, 10

7 318 H. Hocheng and P.S. Pa 4 Results and discussions Figure 4 shows that reduction of the taper angle is effective for the polishing, the surface roughness improves 7% when the taper angle is reduced by 15. The reduced taper angle is advantageous also for dreg flushing and the reduction of secondary overcutting. Figure 5 shows small taper angle and high rotational speed of electrode should be used. Figure 6 illustrates the optimum current rating varies with the feed rate for a material and an electrode. The adequate combinations of current rating and electrode feed rate in this case are 5 A and 1 mm/min, 10 A and 2 mm/min, 15 A and 3 mm/min, and 20 A and 4 mm/min. The design of flute opening can help remove the electropolishing products from the gap. Figure 7 shows electrode type C with double-discharge flute is better than type B (single-flute) and A, since the double-flute provides more discharge space and flushing mobility. The average improvement is about 9%. Large flute back rake angle and side rake angle will slightly improve the polishing, as illustrated in Figure 8. Large rake angle and high rotational speed of electrode produce better polishing thanks to the effective dreg discharge. Figure 4 Electropolishing at different taper angle (Type A 800 rpm, 4l min, continuous DC, 10 A, 2 mm/min, R = 2 mm, workpiece 600 rpm) Figure 5 Electropolishing at different taper angle and rotational speed of electrode. (Type A, 4l min, continuous DC, 10 A, 2 mm/min, R = 2 mm, workpiece 600 rpm)

8 Design of arrow-head electrode in electropolishing of cylindrical part 319 Figure 6 Electropolishing at different feed rate and current rating (SKD61, Type C 800 rpm, 4l min, continuous DC, R = 2 mm, θ = 45, α and β = 40, workpiece 600 rpm) Figure 7 Electropolishing with cone arrow-head electrode (800 rpm, 4l min, continuous DC, 10 A, 2 mm/min, R = 2 mm, θ = 45, α and β = 40, workpiece 600 rpm) Figure 8 Electropolishing at different electrode rotational speed and rake angles (SKD61, Type C, 4l min, continuous DC, R = 2 mm, θ = 45, workpiece 600 rpm)

9 320 H. Hocheng and P.S. Pa Figure 9 summarizes the contribution of surface finish improvement using cone arrow-head electrode type C. It is obtained from the electrode rotation (23%), the end radius (32%), the taper angle (22%), the flute back rake angle (7%), the flute side rake angle (4%), and the flute number (12%). One notices the end radius is the most effective factor in electrode design. Figure 9 The contribution pie of surface finish improvement of type C (SKD61, electrode 800 rpm, 4l min, continuous DC, 10 A, 2 mm/min, R = 2 mm, θ = 45, α and β = 40, workpiece 600 rpm) The arrow-head electrode can be also designed in flat (electrode type D V, D H, and E). The secondary machining can be reduced mildly with large tail angle, that more space for dreg discharge and thus the polishing is available, as shown in Figure 10. Thin electrode improves the polishing for the same reason, as seen in Figure 11. Larger the flute back rake angle or the side rake angle is, the more effective is the polishing. It provides the inclined plane allowing higher flow velocity of the electrolyte for effective flushing. In the meantime, the electrolytic products and heat can be brought away more rapidly. Deep flute depth is desired for the improvement of the polishing, since it enlarges the space for dreg discharge (see Figure 12). Figure 10 Electropolishing at different tail angle (Type D V 800 rpm, 4l min, continuous DC, 10 A, 2 mm/min, R = 2 mm, θ = 45, t = 5 mm, workpiece 600 rpm)

10 Design of arrow-head electrode in electropolishing of cylindrical part 321 Figure 11 Electropolishing at different electrode thickness (Type D V 800 rpm, 4l min, continuous DC, 10 A, 2 mm/min, R = 2 mm, θ = 45, γ = 25, workpiece 600 rpm) Figure 12 Electropolishing at different electrode flute depth (Type E 800 rpm, 4l min, continuous DC, 10 A, 2 mm/min, R = 2 mm, t = 5 mm, θ = 40, γ = 25, α = β = 20, workpiece 600 rpm) The contribution pie of surface finish improvement using the flat arrow-head electrode type E is shown in Figure 13. Among the electrode rotation (18%), the end radius (24%), the taper angle (17%), the tail angle (9%), the flute back rake angle (8%), the flute side rake angle (6%), the flute depth (11%), and the plate thickness (7%), the end radius remains the most effective factor in electrode design, as in the case of cone arrow-head electrode.

11 322 H. Hocheng and P.S. Pa Figure 13 The contribution pie of surface roughness improvement of type E (Type E 800 rpm, 4l min, continuous DC, 10 A, 2 mm/min, R = 2 mm, t = 5 mm, h = 10 mm, θ = 45, γ = 25, α = β = 20, workpiece 600 rpm) Figure 14 shows the polishing results of different type electrodes for various materials. SKD61 is the most suitable material, and the electrode type C and E are the best design. SKD61 also achieves the fastest feed in electropolishing, when compared with other materials at the same amount of material removal, as shown in Figure 15. Figure 16 shows the polishing results from different electrodes and current rating. The current rating of 10 A should be used and type C and E are recommended in the current study (using SKKD61, 4l min, continuous DC 10 A, 2 mm/min, R = 2 mm, θ = 45, electrode 800 rpm, workpiece 600 rpm, and the amount of the reduction of diameter after electropolishing is 0.2 mm). The comparison of improvement of surface roughness by electropolishing using different electrodes is shown in Figure 17. It indicates the best result is obtained by the electrode type C (cone arrow-head with double discharge flute, 43%) and type E (flat arrow-head with discharge flute opening and rake angle, 41%), followed by the electrode type B (35%), the electrode type D V and D H (29%), and the electrode type A (18%). In summary, the average contribution of surface finish improvement obtained from the factors of electrode, i.e., the electrode rotation, the electrode feed rate, and the tool geometry, is shown in Figure 18. The design of tool, including end radius, various angles, thickness, and flute depth, leads to 58% of the improvement, compared to 42% from the electrode rotation and feed rate.

12 Design of arrow-head electrode in electropolishing of cylindrical part 323 Figure 14 Electropolishing using various material and electrode (4l min, continuous DC, 2 mm/min, R = 2 mm, θ = 45, workpiece 600 rpm) Figure 15 Electropolishing using different material and electrode (4l min, continuous DC, R = 2 mm, θ = 45, workpiece 600 rpm)

13 324 H. Hocheng and P.S. Pa Figure 16 Electropolishing using different current rating and electrode (SKD61, 4l min, continuous DC, 2 mm/min, R = 2 mm, θ = 45, workpiece 600 rpm) Figure 17 The surface finish improvement obtained from different electrode (SKD61, electrode 800 rpm, 4l min, continuous DC, 2 mm/min, R = 2 mm, θ = 45, workpiece 600 rpm) Figure 18 The contribution pie of surface finish improvement (SKD61, 4l min, continuous DC, 2 mm/min, R = 2mm, θ = 45, workpiece 600 rpm)

14 Design of arrow-head electrode in electropolishing of cylindrical part Conclusions In order to achieve fast surface finishing and to reduce the residual stress on the external cylindrical surface of die materials beyond traditional rough turning or drawing, electropolishing using arrow-head electrode design is investigated. The optimal rotational speed of workpiece, current rating and electrode feed rate associated with fast electrode rotation produce good polishing. The electrode with small end radius, small taper angle, and large flute back rake angle and side rake angle provides large discharge space for dregs, thus performs well. Small end radius also leads to high current density and fast feed rate. The polishing effect of cone arrow-head electrode with double-flute surpasses the single-flute by increasing discharge space and flushing mobility during electropolishing. The same principle applies to the flute opening of the flat arrow-head. Large flute rake angle and deep flute of thin electrode also are advantageous for polishing. In summary, the design of electrode contributes the most to the surface finish improvement, while the operation parameters, such as rotational speed and feed, also play significant roles. The proposed electrode designs of type C and E are proved effective and low-cost as well. Acknowledgement The research is supported by National Science Council, Taiwan, R.O.C., under contract E References 1 McGeough, J.A. (1988) Advanced Methods of Machining, Chapman and Hall. 2 Gurklis, J.A. (1965) Metal Removal by Electrochemical Methods and its Effect on Mechanical Properties of Metals, Battelle Memorial Institute, Defence Metals Information Centre, DMIC Report Hoare, L.P. and LaBoda, M.A. (1969) An investigation of the difference between NaCl and NaClO 3 as electrolytes in electrochemical machining, Electrochemical Science, February, pp Wilson, J. (1971) Practice and Theory of Electrochemical Machining, pp Hopenfeld, J. and Cole, R.R. (1966) Electrochemical machining prediction and correlation of process variables, ASME Trans. J. of Engineering for Industry, November, Vol. 88, pp Hopenfeld, J. and Cole, R.R. (1969) Prediction of the one-dimensional equilibrium cutting gap in electrochemical machining, ASME Trans. J. of Engineering for Industry, August, Vol. 91, pp Louter, S.P. and Cook, N.H. (1973) High rate electrochemical machining, ASME Trans. J. of Engineering for Industry, Vol. 95, pp Bannard, J. (1977) Effect of flow on the dissolution efficiency of mild steel during ECM, J. of Applied Electrochemistry, Vol. 7, pp Noto, K., Okudairira, H. and Kawafune, K. (1973) The working gap in ECM using sodium nitrate solution, Annals of the C.I.R.P., Vol. 22, No.1, pp Datta, M. and Landolt, D. (1981) Electrochemical machining under pulsed current conditions, Electrochimica Acta, Vol. 26, No. 7, pp

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