ABRASIVE JET MICRO BORE POLISHING - … · Abrasive Flow Polishing of Micro Bores 3 push the...

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STR/03/026/MT 1 Abrasive Flow Polishing of Micro Bores L. Yin, K. Ramesh, Y. M. S. Wan, X. D. Liu, H. Huang and Y. C. Liu Abstract - This report shares the feasibility study results of abrasive flow polishing of micro- bores of size φ500 µm or smaller and 2550 in aspect ratio for both metal and ceramic materi- als. A polishing device was designed and built up so as to create enough turbulence for facili- tating the polishing at the inner walls of the mi- cro-bore. Polishing of micro-bores on 3 materi- als: S45C, SS304 and Zr 2 O 3 of length 13.6 mm, were conducted. Surface roughness and topog- raphy of the polished inner walls were studied using profilometry and optical interferometry form the three dimensional point of view. Signifi- cant progress in surface roughness in all inner walls has been made in the polishing process. The results indicate that it is feasible to apply abrasive flow polishing for metal and ceramic micro-bores of diameters smaller than 500 µm. It is also found that the surface roughness of the inner wall decreases with the increasing number of slurry flow pass at a relatively constant ap- plied pressure. However there exists a critical number of flow passes beyond which the im- provement in surface quality is marginal. Keywords: Micro bores, Abrasive flow polishing, Three-dimensional surface assessment, Surface topography, Surface roughness 1 BACKGROUND Micromachining is a key technology enabling the manufacture of miniaturized products that are rapidly expanding [1-2]. One group of the miniaturised parts possesses micro bores of diameters of smaller than 500 µm. They are commonly found in various products such as fluidic filters, grids, bio-medical filters, ink-jet printer nozzles, fuel injection nozzles, optical ferrules, high-pressure orifices, standard defects for testing materials, micropipettes, pneumatic sensors and manipulators, guides for wire- bonders and spinning nozzles, and fuel injection nozzles [1]. The need of the microtreatment of materials for increased miniaturisation and complexity of me- chanical, optical and electronic components has led to the development of the new processes of micromanufacturing. Micro EDM (Electrical Dis- charge Machining) [3-4], micro-cutting [5-6], mi- cro USM (Ultrasonic Machining) [7], microform- ing and micromolding [8], micro ECM (Electro- Chemical Machining) [1], micropunching [9], and laser micro machining [10-11] have been devel- oped and applied for machining of micro bores. However, there are the limitations in proceeding these technologies in terms of workpiece mate- rials and dimensions. For example, micro EDM is capable only for machining conductive mate- rials, mainly metals. Furthermore, the final sur- face finishes obtained using these technologies are hardly satisfactory in many cases, especially when the quality requirements are stringent for optical and medical applications, such as optical ferrules and medical capillaries. To achieve high quality for micro bore inner walls, polishing processes are necessary. Recently, a meaning- ful technology for high precision polishing of mi- cro bores using a high speed slurry flowing has been developed, however it is still under devel- opment for holes of less than 1 mm inner diame- ters with the aspect ratios of smaller than 6.25 [12-16]. A gyration flow finishing method was used for polishing of 500 µm holes with only a 2.88 aspect ratio [17]. In fact, in the development of micro bore finish- ing, there are two challenges to conquer. One is the machining itself; the other is the measure- ment of the inner wall of a micro bore. It is diffi- cult to finish the inner walls of the bores by ordi- nary polishing methods, as micro bores are in- accessible by means of normal polishing tools. It is also difficult to assess a micro bore inner wall using profilometry and optical interferometry. 2 OBJECTIVE In this study, the abrasive flow technology was developed and applied to the polishing of micro bores of 260 ~ 500 µm inner diameters and 25 ~ 50 length/diameter ratios for both metal and ce- ramic materials. Three-dimensional assess- ments of the micro bore inner walls were con- ducted by means of a combination of stylus pro- filometry and optical interferometry to investi- gate the influence of polishing behaviours on micro bore inner wall surface roughness and topography.

Transcript of ABRASIVE JET MICRO BORE POLISHING - … · Abrasive Flow Polishing of Micro Bores 3 push the...

Page 1: ABRASIVE JET MICRO BORE POLISHING - … · Abrasive Flow Polishing of Micro Bores 3 push the abrasive slurry for polishing of a micro bore. In this design, hydraulic oil 32 was pres-surized

STR/03/026/MT

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Abrasive Flow Polishing of Micro Bores

L. Yin, K. Ramesh, Y. M. S. Wan, X. D. Liu, H. Huang and Y. C. Liu

Abstract - This report shares the feasibility study results of abrasive flow polishing of micro-bores of size φ500 µm or smaller and 25∼50 in aspect ratio for both metal and ceramic materi-als. A polishing device was designed and built up so as to create enough turbulence for facili-tating the polishing at the inner walls of the mi-cro-bore. Polishing of micro-bores on 3 materi-als: S45C, SS304 and Zr2O3 of length 13.6 mm, were conducted. Surface roughness and topog-raphy of the polished inner walls were studied using profilometry and optical interferometry form the three dimensional point of view. Signifi-cant progress in surface roughness in all inner walls has been made in the polishing process. The results indicate that it is feasible to apply abrasive flow polishing for metal and ceramic micro-bores of diameters smaller than 500 µm. It is also found that the surface roughness of the inner wall decreases with the increasing number of slurry flow pass at a relatively constant ap-plied pressure. However there exists a critical number of flow passes beyond which the im-provement in surface quality is marginal. Keywords: Micro bores, Abrasive flow polishing, Three-dimensional surface assessment, Surface topography, Surface roughness 1 BACKGROUND Micromachining is a key technology enabling the manufacture of miniaturized products that are rapidly expanding [1-2]. One group of the miniaturised parts possesses micro bores of diameters of smaller than 500 µm. They are commonly found in various products such as fluidic filters, grids, bio-medical filters, ink-jet printer nozzles, fuel injection nozzles, optical ferrules, high-pressure orifices, standard defects for testing materials, micropipettes, pneumatic sensors and manipulators, guides for wire-bonders and spinning nozzles, and fuel injection nozzles [1]. The need of the microtreatment of materials for increased miniaturisation and complexity of me-chanical, optical and electronic components has led to the development of the new processes of micromanufacturing. Micro EDM (Electrical Dis-

charge Machining) [3-4], micro-cutting [5-6], mi-cro USM (Ultrasonic Machining) [7], microform-ing and micromolding [8], micro ECM (Electro-Chemical Machining) [1], micropunching [9], and laser micro machining [10-11] have been devel-oped and applied for machining of micro bores. However, there are the limitations in proceeding these technologies in terms of workpiece mate-rials and dimensions. For example, micro EDM is capable only for machining conductive mate-rials, mainly metals. Furthermore, the final sur-face finishes obtained using these technologies are hardly satisfactory in many cases, especially when the quality requirements are stringent for optical and medical applications, such as optical ferrules and medical capillaries. To achieve high quality for micro bore inner walls, polishing processes are necessary. Recently, a meaning-ful technology for high precision polishing of mi-cro bores using a high speed slurry flowing has been developed, however it is still under devel-opment for holes of less than 1 mm inner diame-ters with the aspect ratios of smaller than 6.25 [12-16]. A gyration flow finishing method was used for polishing of 500 µm holes with only a 2.88 aspect ratio [17]. In fact, in the development of micro bore finish-ing, there are two challenges to conquer. One is the machining itself; the other is the measure-ment of the inner wall of a micro bore. It is diffi-cult to finish the inner walls of the bores by ordi-nary polishing methods, as micro bores are in-accessible by means of normal polishing tools. It is also difficult to assess a micro bore inner wall using profilometry and optical interferometry. 2 OBJECTIVE In this study, the abrasive flow technology was developed and applied to the polishing of micro bores of 260 ~ 500 µm inner diameters and 25 ~ 50 length/diameter ratios for both metal and ce-ramic materials. Three-dimensional assess-ments of the micro bore inner walls were con-ducted by means of a combination of stylus pro-filometry and optical interferometry to investi-gate the influence of polishing behaviours on micro bore inner wall surface roughness and topography.

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Fig. 1(a). The hydraulic principle design for abrasive flow.

Control Box Hydraulic Pump Pressure Gauge Direction Control Valve

Workpiece Unit Slurry Exit Hybrid Cylinder Slurry Tray

Fig. 1(b). The picture of the abrasive flow polishing machine.

Table 1. Polishing experimental conditions.

Micro Bore Diameter

500 µm, Stainless steel 304 500 µm, Steel S45C 400 µm, Steel S45C 260 µm, Zirconia

Bore Length 13.6 mm Abrasives Alumina of 17.5 µm grits Concentration 3.44 vol. % Number of polishing pass

5, 10, 15, 20 passes

Pressure 10 MPa Fluid Water

3 METHODOLOGY 3.1 Apparatus for abrasive flow polishing The abrasive flow polishing process developed in this investigation applied a pressurised slurry flow into a micro hole in the turbulent flow re-gime. To ensure that the abrasives remained turbulently suspended, the mean velocity U (m/s) of the abrasive slurry, and the kinematics vis-cosity v (m2/s) flowing through a hole of diame-ter D (m) must be qualified such that the Rey-nold’s number Re exceeds the critical value of 2300 [18]. The Reynold’s number (Re) can be computed using the equation below [18]:

Re = UD / v

where, U = Mean velocity (m/s), ν = Kinematics viscosity (m2/s), and D = Hydraulic diameter (m). In the apparatus designed, U of 12 to 29 m/s, ν of 1.1×10–6 m2/s, and D of 260 µm to 500 µm were selected. Hence, the Re values were cal-culated to be 8390 ~ 9090, much higher than the critical number of 2300 for turbulent flow, confirming the abrasive slurry moved turbulently in a micro hole.

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Fig. 3. Assessment of a micro bore using stylus pro-filometry. (a) Two-dimensional view, (b) Three-dimensional view, and (c) A view of a trace along a micro bore. The schematic design for the abrasive flow pol-ishing machine is shown in Fig. 1(a). It has a motor-driven hydraulic pump capable of gener-ating pressure up to 40 MPa, a direction control valve, a hybrid cylinder, a workpiece unit, two pressure gauges and a slurry tray. The hybrid cylinder used the pressurized hydraulic oil to

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push the abrasive slurry for polishing of a micro bore. In this design, hydraulic oil 32 was pres-surized in the hydraulic pump and supplied to the hybrid cylinder through a manually operated directional control valve to push the slurry to go through a micro bore. The photo of the polishing machine is shown in Fig. 1(b). In polishing, the pressurized hydraulic oil com-presses the pre-filled abrasive slurry, which was forced to flow through a micro bore inner wall. The micro bore workpiece was precisely aligned in the workpiece unit. O-rings were inserted at both ends of the workpiece in order to avoid the leakage of the slurry under a high flow pressure. When one pass polishing was finished, the spent slurry flowed to a tray and fresh slurry was added to the slurry portion of the hybrid cylinder for the next pass polishing. 3.2 Experimental conditions The workpiece materials in this investigation included stainless steel 304, steel S45C and powder-injection moulded zirconia. The sam-ples were 13.6 mm long, cylindrical in shape, 6

mm outer diameter and with 500 µm inner bore diameter for stainless steel 304, 400 µm and 500 µm for S45C, and 260 µm for zirconia. The bores in stainless steel and steel were drilled. The zirconia inner holes were injection-moulded. Polishing experimental conditions are listed in Table 1. The applied constant pressure in abra-sive flow polishing was 10 MPa; the abrasive slurry for polishing was alumina of 17.5 µm grit size, with a concentration of 3.44 vol%. The pol-ishing passes were 5, 10, 15 and 20. The corre-sponding speeds of slurry were 19.3 m/s for 500 µm holes, 25 m/s for 400 µm holes and 35.5 m/s for 260 µm holes. 3.3 Surface Characterization of Micro

Bores After polishing, the bores were ground off using stroke grinding with a silicon carbide grinding-wheel of a 500 grit size to exposure the inner wall surfaces of the micro bores. The inner walls of the polishing micro bores were characterized using profilometry and optical interferometry to study the surface roughness and topography.

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Fig. 4. Inner wall surface roughness for 500 µm stainless steel 304 bores obtained in polishing and assessed with profilometry.

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Fig. 5. Inner wall surface roughness for 500 µm steel S45C bores obtained in polishing andassessed with profilometry.

A stylus profilometer was used to assess the surface roughness of the inner walls. The three-dimensional profile traces were taken around the area covering the whole exposed inner wall of the polished micro bore. The two- and three-dimensional profile views for a micro bore are shown in Figs. 3(a) and 3(b). For each inner wall, the analyses for surface roughness were con-ducted along the length in the micro bore Fig. 3(c). Each measurement was repeated three times. The mean values and the standard devia-tions of the arithmetic average values of Ra, the root- mean-square values of Rq, the maximum values of Rt, and the ten points of Rz were cal-culated. In the measurements, the Gaussian filter of 0.8 mm cut-off was applied. The exposed inner walls of the polished micro bores were further viewed using an optical inter-ference profiler (Wyko NT 3300) to assess the surface texture and topography. A measured area of 46 × 60 µm and the measuring mode of the vertical scanning interferometer (VSI) were

used. The optical interactive image analyses are also used to identify the material removal, material defects and damage. 4 RESULTS 4.1 Surface roughness The surface roughness values obtained on 500 µm diameter stainless steel 304 bores with pol-ishing passes are plotted in Fig. 4. The trends are very clear that the inner walls of the polished bores were improved when increasing the pol-ishing passes. The Ra values dropped from 1.58 µm to 0.48 µm, or, 70%, and Rq values from about 2.05 µm to 0.57 µm, or 72%, after polishing with 20 passes. The plot for Rt vs. pass shows that the peak points did not change in the first 5 pass polishing but quickly dimin-ished from about average 15.90 µm to 3.96 µm, or 75% when the passes increased to 20. The

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ten-point average values of Rz reveal that in-creasing polishing passes to 20, the Rz values could drop down 6.58 µm from 9.22 µm, or 71%, at 2.64 µm. The summary of the surface roughness values for 500 µm steel 45C bores is given in Fig. 5. It also indicates that the abrasive flow polishing processes smooth the inner walls of the micro bores for steel. The Ra values dropped 3.22 µm, about 80%, to 0.75 µm, after 20 pass polishing. The Rq values reduced from 4.98 µm to 0.91 µm, or 81%. The peak point Rt values dimin-ished from average 41.2 µm to 7.30 µm, or 82%, and the ten-point Rz from 23.43 µm to 3.79 µm, or 84%. The plots for the surface roughness vs. polish-ing passes for 400 µm steel S45C bores are summarized in Fig. 6. The mean values of Ra and Rq exhibit that the surfaces are improved when increasing the polishing passes. The Ra values dropped from 2.31 µm to 0.80 µm, or 65%, and Rq values from about 2.91 µm to 1.02 µm, or 65%, after polishing with 20 passes. The plot for Rt vs. pass shows that the peak points may not have been lowered when the pass number was small, however, the peak points did get lowered from about average 22 µm to 6.39 µm, or 71%, when there were enough abrasive polishing passes through the inner wall. The ten-point average values of Rz also reveal that increasing polishing passes to 20, the Rz values could drop down 8.84 µm, or 63%, at 5.16 µm. Meanwhile, it is observed that the uncertainty for the surface roughness of the S45C inner walls decreased when the polishing passes increased. The summary for the measurement of the zirco-nia inner walls shown in Fig. 7. The mean and average representatives of Ra and Rq dropped down 3 µm to 0.44 µm, or 87%, and 4.64 µm to 0.84 µm, or 82%, respectively, after 20 pass polishing. The peaks of Rt were clearly dimin-ished from about 36 µm to 16 µm, or 56%. The ten point averages of Rz dropped from 27.67 µm to 6.73 µm, or 76%. 4.2 Surface topography The two- and three-dimensional optical interac tive images in Fig. 8 show the inner wall sur-faces for 500 µm stainless steel bores obtained in drilling and 20-pass polishing. Fig. 8(a) repre-sents a typical surface produced by drilling. The drilling marks are easily to observe and the sur-faces consist of long grooves parallel to the drill-

ing direction that are caused by the microrough-ness of the cutting edge. The different types of surface damage are: plastically deformed mate-rial that has not been removed from the surface but remained on the feed marks as material side flow and porous structure, which is discontinu-ous flow of the material in the cutting zone. When the abrasive slurry polished the inner walls of the stainless steel, the peaks of the plastic grooves gradually diminished and the surface became smoother, as shown in Fig. 8(b). The material removal in polishing is considered as plastic deformation impacted by the flowing abrasives. Fig. 9 shows the two- and three-inner wall sur-faces for 500 µm steel S45C bores generated in drilling and 20-pass polishing. Fig. 9(a) demon-strates the very rough and discontinuous drilled surface. After polishing, the optical interactive images in Fig. 9(b) indicate of a significant im-provement in the steel bore inner surfaces. Fig. 10 shows the two- and three-dimensional optical interactive images of the inner wall sur-faces for 400 µm steel S45C bores obtained in drilling and 20-pass polishing. The drilled sur-face images in Fig. 10(a) for the 400 µm steel bore inner wall show a discontinuity and coarseness. Fig. 10(b) revealed a much smoother surface after 20-pass polishing. Fig. 11 shows the two- and three-dimensional optical interactive images of the inner wall sur-faces for 260 µm zirconia bores made from in-jection-moulding and 20-pass polishing. In Fig. 11(a), the injection moulded surface reveals ma-terial defect such as porosity, which is randomly distributed in the zirconia material. This porosity contributed to the surface roughness, especially to the peak points of Rt and ten-point values of Rz values. Porosity was not removed after pol-ishing shown in Fig. 11(b). But polishing has significantly smoothed the zirconia bore inner walls as shown in Fig. 11(b). 5 CONCULSIONS • Abrasive flow polishing appears to be a fea-

sible technique for polishing stainless steel, steel and ceramic bores.

• Abrasive flow polishing can be used to pol-ish micro bores of diameters of 260 µm or larger, and high length/diameter ratios.

• Surface roughness of micro bore inner walls decreases with the increase of number of slurry flow passes at a relatively constant applied pressure.

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Fig. 6. Inner wall surface roughness for 400 µm steel S45C bores

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Fig. 7. Inner wall surface roughness for 260 µm zirconia bores

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Fig. 8(a). The two- and three-dimensional optical in-teractive images of 500 µm stainless steel inner wall obtained in drilling.

Fig. 8(b). The two- and three- dimensional optical interactive images of 500 µm stainless steel inner wall obtained in 20-pass polishing.

Fig. 9(a). The two- and three-dimensional optical in-teractive images of 500 µm steel inner wall obtained in drilling.

Fig. 9(b). The two- and three-dimensional optical in-teractive images of 500 µm steel inner wall obtained in 20-pass polishing.

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Fig. 10(a). The two- and three-dimensional optical interactive images of 400 µm steel inner wall obtained in drilling.

Fig. 10(b). The two- and three-dimensional optical interactive images of 400 µm steel inner wall obtained in 20-pass polishing.

Fig. 11(a). The two- and three-dimensional optical interactive images of 260 µm zirconia inner wall ob-tained in injection moulding.

Fig. 11(b). The two- and three-dimensional optical interactive images of 260 µm zirconia inner wall ob-tained in 20-pass polishing.

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6 INDUSTRIAL SIGNIFICANT This project provides the industry the feasible solutions for polishing of micro bores and meth-odologies for assessment of micro bore inner walls. REFERENCES [1] T. Masuzawa, “State of art of Micromachin-

ing”, Annals of the CIRP, Vol. 49(2), pp. 473-488, (2000).

[2] H. van Brussel, J. Peirs, D. Reynaerts, A. Delchambre, G. Reihart, N. Roth, M. Weck and E. Zussman, “Assembly of Microsys-tems”, Annals of the CIRP, Vol. 49(2), pp. 451-472, (2000).

[3] B. Odom, “Making Microholes”, Manufactur-ing Engineering, Vol. 126(2), pp. 1-8, (2001).

[4] K.P. Rajurkar and Z.Y. Yu, “Micro EDM Can Produce Micro Parts”, Manufacturing Engi-neering, Vol. 125(5), pp. 1-4, (2000).

[5] T. Masuzawa and H.K. Tonshoff, “Three-dimensional Micromachining by Machine Tools”, Annals of the CIRP, Vol. 46(2), pp. 621-628, (1997).

[6] H. Weule, V. Huntrup and H. Tritschler, “Mi-cro-cutting of Steel to Meet New Require-ments in Miniaturization”, Annals of the CIRP, Vol. 50(1), pp. 61-64, (2001).

[7] K. Egashira and T. Masuzawa, “Microultra-sonic Machining by the Application of Workpiece Vibration”, Annals of the CIRP, Vol. 48(10), pp. 131-134, (1999).

[8] M. Geiger, M. Kleiner, R. Ekstein, N. Tiesler and U. Engel, “Microforming”, Annals of the CIRP, Vol. 50(2), pp. 445-462, (2001).

[9] B.Y. Joo, S.I. Oh and B.H. Jeon, “Develop-ment of Micro Punching System”, Annals of the CIRP, Vol. 50(1), pp. 191-194, (2001).

[10] H.K. Tonshoff, L. Overmeyer, A. Ostendorf and H. Hesener, “New Machining Concept for Manufacturing Microstructures with Ex-

cimer Lasers”, Machining Science & Tech-nology, Vol. 1(1), pp. 153-162, (1997).

[11] M.C. Gower, “Industrial Application of Laser Micromachining”, Optics Express, Vol. 7(2), pp. 56-67, (2000).

[12] T. Kurobe, Y. Yamade and K. Yamamoto, “Development of High Speed Slurry Flow Finishing of the Inner Wall of Stainless Steel Capillary – Polishing and Gas Flow Characteristics of Various Size of Capillar-ies”, Precision Engineering, Vol. 25, pp. 100-106, (2001).

[13] T. Kurobe, Y. Yamade and K. Yamamoto, “Application of High Speed Slurry Flow Fin-ishing Method for Finishing of Inner Wall of Fine Hole Die – Effects of the Hardness of Die Materials on the Polishing Characteris-tics”, Precision Engineering, Vol. 26, pp. 155-161, (2002).

[14] T. Sakuyama, T. Kuriyagawa and K. Syoji, “Development of a New Abrasive Jet Ma-chining Device”, in World Scientific, J. Wang, W. Scott, L. Zhang (Ed.), Singapore, pp. 291-298, (1999).

[15] T. Kurobe, Y. Yamada and H. Sugimori, “High Speed Slurry Flow Finishing of the Inner Wall of a Stainless Steel Pipe”, Key Engineering Materials, Vol. 238-239, pp. 345-348, (2003).

[16] K. Yamamoto, T. Kurobe and K. Nakamori, “High Speed Flow Finishing of Inner Wall of Stainless Steel Orifice Plate”, Journal of Japan Society for Precision Engineering, Vol. 69(1), pp. 79-83 (in Japanese), (2003).

[17] H. Sugimori, T. Kurobe and S. Hirose, “Fin-ishing of the Inner Wall of Tiny Nozzle and Chamfering of Nozzle Edge by Gyration Flow Finishing Method”, Journal of Japan Society for Precision Engineering, Vol. 69(1), pp. 74-78 (in Japanese), (2003).

[18] R.V. Giles and D. Pollard, Schaum’s Out-line of Theory and Problems of Fluid Me-chanics and Hydraulics, McGraw-Hill, New York (1977).