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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 500 m 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 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 m. 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 m. 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 m 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 m 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.

Abrasive Flow Polishing of Micro Bores

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

Bore Length 13.6 mm Abrasives Alumina of 17.5 m 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-nolds number Re exceeds the critical value of 2300 [18]. The Reynolds number (Re) can be computed using the equation below [18]:

Re = UD / v

where, U = Mean velocity (m/s), = Kinematics viscosity (m2/s), and D = Hydraulic diameter (m). In the apparatus designed, U of 12 to 29 m/s, of 1.1106 m2/s, and D of 260 m to 500 m 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.

(a)

(b)

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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 m inner bore diameter for stainless steel 304, 400 m and 500 m for S45C, and 260 m 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 m 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 m holes, 25 m/s for 400 m holes and 35.5 m/s for 260 m 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 m stainless steel 304 bores obtained in polishing and assessed with profilometry.

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