Chong Nam Chu

Micro electrochemical milling

In this paper, electrochemical machining (ECM) for fabricating micro structures is presented. By applying ultra short pulses, dissolution of a workpiece can be restricted to the region very close to an electrode. Using this method, 3D micro structures were machined on stainless steel. Good surface quality of the structures was obtained in the low concentration electrolyte, 0.1 M H2SO4. In ECM, when the machining depth increases, structures taper. To reduce the taper, a disc-type electrode is introduced. By electrochemical milling, various 3D micro structures including a hemisphere with 60 µm diameter were fabricated.

Cutting force prediction of sculptured surface ball-end milling using Z-map

The cutting force in ball-end milling of sculptured surfaces is calculated. In sculptured surface machining, a simple method to determine the cutter contact area is necessary since cutting geometry is complicated and cutter contact area changes continuously. In this study, the cutter contact area is determined from the Z-map of the surface geometry and current cutter location. To determine cutting edge element engagement, the cutting edge elements are projected onto the cutter plane normal to the Z-axis and compared with the cutter contact area obtained from the Z-map. Cutting forces acting on the engaged cutting edge elements are calculated using an empirical method. Empirical cutting mechanism parameters are set as functions of cutting edge element position angle in order to consider the cutting action variation along the cutting edge. The relationship between undeformed chip geometry and the cutter feed inclination angle is also analyzed. The resultant cutting force is calculated by numerical integration of cutting forces acting on the engaged cutting edge elements. A series of experiments were performed to verify the proposed cutting force estimation model. It is shown that the proposed method predicts cutting force effectively for any geometry including sculptured surfaces with cusp marks and a hole.

Estimation of cutter deflection and form error in ball-end milling processes

This paper presents a method to analyze the 3-dimensional form error of a ball-end milled surface due to the elastic compliance of the cutting tool. In order to estimate the form error in various cutting modes, the cutting force and the cutter deflection models including the effect of the surface inclination were established. The cutting forces were calculated by using the cutter contact area determined from the Z-map of the surface geometry and the current cutter location. The tool deflection responding to the cutting force was then calculated by considering the cutter and the holder stiffness. The cutter was modeled as a cantilever beam consisting of the shank and the flute. The stiffness of the holder was measured experimentally. Various experimental works have been performed to verify the validity of the proposed model. It is shown that the proposed method is capable of accurate prediction of cutting forces and the surface form error.

Analysis of the side gap resulting from micro electrochemical machining with a tungsten wire and ultrashort voltage pulses

Micro wire electrochemical machining (ECM) using a O 10 µm tungsten wire as the tool electrode is presented. Since electrochemical machining does not wear out the tool, it can be easily applied to the fabrication of micro grooves by moving the tool electrode along a programmed toolpath. To minimize the side gap, ultrashort voltage pulses were applied between the tool electrode and the workpiece. Changes in the side gap according to the applied pulse voltage, pulse on-time and pulse period were investigated, and the optimal pulse condition for stable machining was obtained. By using this method, micro features such as micro grooves and gears were fabricated into stainless steel plates.

Fabrication of multiple electrodes by reverse EDM and their application in micro ECM

Micro electrodes of various shapes were fabricated by reverse electrical discharge machining (REDM). The machining characteristics of REDM were investigated under varying capacitances and applied voltages, and optimal conditions for stable machining were suggested. Multiple-tipped micro electrodes fabricated by REDM were then introduced for micro electrochemical machining in order to increase productivity over single micro electrodes. By using various multiple electrodes, micro hole arrays, grooves and channels were machined on stainless steel.

Straight hole micro EDM with a cylindrical tool using a variable capacitance method accompanied by ultrasonic vibration

A micro hole machined by EDM with a cylindrical electrode has different hole entrance and exit diameters. This taper shape of the micro hole is caused both by tool wear and by eroded particles that induce secondary discharge during hole machining. In this paper, a novel method is proposed to reduce the difference between the entrance and exit diameters of the holes. The effects of feeding depth, applied capacitance and machining time on the formation of the tapered shape were observed experimentally. Based on these experiments, ultrasonic vibration was applied to reduce the secondary discharge and electrode wear, and capacitance was varied when the tool was approaching the exit of the hole. Using the proposed method, a straight micro hole was successfully fabricated.

Micro electrical discharge milling using deionized water as a dielectric fluid

In electrical discharge machining, dielectric fluid is an important factor affecting machining characteristics. Generally, kerosene and deionized water have been used as dielectric fluids. In micro electrical discharge milling, which uses a micro electrode as a tool, the wear of the tool electrode decreases the machining accuracy. However, the use of deionized water instead of kerosene can reduce the tool wear and increase the machining speed. This paper investigates micro electrical discharge milling using deionized water. Deionized water with high resistivity was used to minimize the machining gap. Machining characteristics such as the tool wear, machining gap and machining rate were investigated according to resistivity of deionized water. As the resistivity of deionized water decreased, the tool wear was reduced, but the machining gap increased due to electrochemical dissolution. Micro hemispheres were machined for the purpose of investigating machining efficiency between dielectric fluids, kerosene and deionized water.

The form error prediction in side wall machining considering tool deflection

In this research, an effective method for the form error prediction in side wall machining with a flat end mill is suggested. The form error is predicted directly from the tool deflection without surface generation by cutting edge locus with time simulation. The developed model can predict the surface form error accurately about 300 times faster than the previous method. Cutting forces and tool deflection are calculated considering tool geometry, tool setting error and machine tool stiffness. The characteristics and the difference of generated surface shape in up milling and down milling are discussed. The usefulness of the presented method is verified from a set of experiments under various cutting conditions generally used in die and mold manufacturing. This study contributes to real time surface shape estimation and cutting process planning for the improvement of form accuracy.

One-step process for superhydrophobic metallic surfaces by wire electrical discharge machining.

We present a direct one-step method to fabricate dual-scale superhydrophobic metallic surfaces using wire electrical discharge machining (WEDM). A dual-scale structure was spontaneously formed by the nature of exfoliation characteristic of Al 7075 alloy surface during WEDM process. A primary microscale sinusoidal pattern was formed via a programmed WEDM process, with the wavelength in the range of 200 to 500 μm. Notably, a secondary roughness in the form of microcraters (average roughness, Ra: 4.16 to 0.41 μm) was generated during the exfoliation process without additional chemical treatment. The low surface energy of Al 7075 alloy (γ = 30.65 mJ/m(2)) together with the presence of dual-scale structures appears to contribute to the observed superhydrophobicity with a static contact angle of 156° and a hysteresis less than 3°. To explain the wetting characteristics on dual-scale structures, we used a simple theoretical model. It was found that Cassie state is likely to present on the secondary roughness in all fabricated surfaces. On the other hand, either Wenzel or Cassie state can present on the primary roughness depending on the characteristic length of sinusoidal pattern. In an optimal condition of the serial cutting steps with applied powers of ∼30 and ∼8 kW, respectively, a stable, superhydrophobic metallic surface was created with a sinusoidal pattern of 500 μm wavelength.

Micro electrical discharge drilling of tungsten carbide using deionized water

Micro electrical discharge machining (micro EDM) is an effective machining method for cobalt-bonded tungsten carbide (WC-Co); however, this material is susceptible to electrolytic corrosion when deionized water is used as the working fluid with a dc power source for the RC circuit. In this study, a bipolar pulse power source and a triangular electrode were used in order to reduce the electrolytic corrosion phenomenon during micro EDM using an RC discharge circuit. A bipolar pulse power source reduces the positive polarity period of the workpiece by periodically alternating the polarity of the workpiece and electrode and decreases the average gap voltage at the machining gap. Therefore, electrolytic corrosion, which is a type of electrochemical reaction on the positively charged workpiece, is reduced by these electrical conditions. The triangular electrode has a smaller side area as compared with the cylindrical electrode. Since the electrolytic corrosion is an electrochemical reaction between the side of the electrode and the surface of the workpiece, the small side area of the triangular electrode could reduce these reactions. With the aid of the bipolar pulse power source and the triangular electrode, an electrolytic-corrosion-free hole could be machined on the WC-Co workpiece using deionized water.