CIRP Annals - Manufacturing Technology 58 (2009) 181–184

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CIRP Annals - Manufacturing Technology

journal homepage: http://ees.elsevier.com/cirp/default.asp

Micro electrochemical machining for complex internal micro features

Chan Hee Jo a, Bo Hyun Kim b, Chong Nam Chu (2)a,*a School of Mechanical & Aerospace Engineering, Seoul National University, Seoul, Republic of Koreab School of Mechanical Engineering, Andong National University, Andong, Republic of Korea

A R T I C L E I N F O

Keywords:

Electrochemical machining (ECM)

Micromachining

Internal feature

A B S T R A C T

In this paper, the application of micro electrochemical machining (ECM) for the micromachining of

internal features is investigated. By controlling pulse conditions and machining time, micro features are

machined on the side wall of a micro hole. These methods can easily machine a micro hole with larger

internal diameters than the entrance diameter, which is very difficult to do by the conventional

processes. A micro disk-shaped electrode with an insulating layer on its surface is also introduced to

machine microgrooves inside the hole. This method is similar to the turning lathe process. The purpose of

this study was to confirm the various possibilities of making complex internal structures in a micro hole

by micro ECM.

� 2009 CIRP.

1. Introduction

Micro holes are the most basic products of micro machining,and are widely used in many applications, including micro nozzles,bio-medical filters, and micro dies. The fabrication of simple microholes is relatively easy, and utilizes many machining technologies,such as mechanical drilling, lasers, punching, and electricaldischarging machining (EDM) [1]. Recently, however, the demandfor micro holes with 3D shapes rather than simple cylindrical holeshas increased. If the hole contains some micro features, or thehole’s shape is a reverse taper in which the internal size of the holeis larger than its opening, fabrication becomes difficult because ofproblems not only from the tool fabrication, but from the processesthemselves [2]. Several studies have been conducted on internalstructures such as reverse-tapered holes and grooves in a microhole produced by using a customized tool electrode, in micro-EDM[2,3]. However, the tool electrode is not reusable because it is wornout during EDM.

Since electrochemical machining (ECM) is a conductivematerials removal process based on the electrochemical reactionof anode metals, tool wear is negligible [4–10]. Therefore, even amicro tool of complex shape can be reused. Since the working gapvaries with pulse voltage or pulse duration, various hole shapes canbe obtained with a single tool.

2. Tool electrode insulation

Usually, in micro ECM drilling using a cylindrical tool electrode,a micro hole with a taper shape is obtained. This is because thedissolution occurs not only on the bottom of the tool, but onthe side of the tool as well. During drilling, dissolution time at theentrance to the hole is much longer than that at the exit. Since the

* Corresponding author.

0007-8506/$ – see front matter � 2009 CIRP.

doi:10.1016/j.cirp.2009.03.072

machining gap increases with the machining time in ECM, atapering side wall will result. In order to control the hole shapeprecisely, or to make internal features within the micro hole, theside dissolution should be prevented. For this purpose, in thisstudy, insulation on the side of the electrode was used. When theside of the tool electrode is coated with an insulator, potential ischarged only in the double layer of the tool bottom, which is notcovered with the insulator. Therefore, dissolution does not occuralong the tool’s side, and tapering is thereby prevented. Moreover,since dissolution is restricted only to the bottom of the tool, thedissolution rate is not affected by the immersion depth of the tool,while the machining rate is kept constant [11]. In ECM drilling withun-insulated tools, as the tool electrode moves downward, the areaof the electrode–electrolyte interface increases. Consequently, asthe immersion area increases, the rising time of the double layerincreases, and the dissolution rate (that is, the machining rate)gradually decreases.

To utilize insulated electrodes for micro ECM, insulationmaterial satisfies some indispensable requisites. First, it shouldbe adhered well onto the surface of the tool electrode, which isusually made from metals. Since acid electrolytes are used in ECM,the insulation material must be resistant to the acid. To satisfy theabove requisites, polystyrene as an adhesive and a tetrahydrofuran(THF) as a solvent are utilized. In addition, a pigment is added to aidvisualization. Fig. 1 shows an SEM image of a section of theinsulated electrode. The insulation thickness is about 3 mm.

3. Reverse taper

3.1. Characteristics of pulse on-time and machining gap

The primary factors considered for taper machining are aminimum machining gap, a proper machining speed. In ECM, thepulse condition, tool size, concentration of electrolytes, and so on,determine the machining gap [8,11]. The pulse condition is one of

Fig. 1. Sectional images of insulated tool electrode (tool: Ø 34 mm, insulation layer:

3 mm).

Fig. 3. Diameter and machining gap according to pulse duration.

Fig. 4. Reverse-tapered hole: (a) cross-sectional view, (b) hole entrance, (c)

isometric view, (d) hole exit (tool Ø 35 mm, pulse amplitude: 6 V, pulse on-time

increased from 30 ns to 150 ns).

C.H. Jo et al. / CIRP Annals - Manufacturing Technology 58 (2009) 181–184182

the machining parameters that can be controlled during machin-ing. In this experiment, only the pulse on-time is changed toachieve various diameters of micro holes.

To estimate the machining gap, as shown in Fig. 2, microholes with 20 mm depth were machined by using a Ø 35 mminsulated electrode, and the hole sizes were measured. Thefeedrate was 0.1 mm/s. The period of pulse was 1 ms, and theapplied pulse duration was 30 ns to 160 ns. Fig. 3 shows therelationship between the pulse on-time and the machining gap.The figure shows that the machining gap increases as the pulseon-time increases. Consequently, an increase in the machininggap offers the possibility of making a reverse-tapered hole bycontrolling the pulse duration. However, when the pulse on-time was longer than specific duration (160 ns for thisexperiment), the machined surface was rough, as seen inFig. 2(d).

3.2. Reverse-tapered hole

As the pulse on-time increases during machining, the machin-ing gap increases. The use of an insulated electrode restricts thedissolution that occurs on the side of the electrode. As a result, ifthe pulse on-time increases during drilling, the machining gapincreases at the bottom of the tool electrode, but the gap in the holeentrance does not increase. Therefore, a hole whose size becomeslarger as the depth increases can be fabricated by this method.

Fig. 2. SEM images of pit according to pulse duration: (a) 30 ns, (b) 90 ns, (c) 120 ns,

(d) 160 ns.

Fig. 4 shows a reverse-tapered hole, which was machined usingan insulated tool electrode. The applied amplitude of pulse was 6 V,and pulse on-time was increased from 30 ns for the hole entranceto 150 ns for the hole exit. The workpiece was a 50 mm thickstainless steel plate. When the 35 mm insulated cylindricalelectrode was used, the diameter of the entrance and exit were45 mm and 63 mm, respectively.

Similarly, a barrel-shaped hole can also be fabricated bycontrolling the pulse duration, as shown in Fig. 5. The pulseduration was increased, at a constant rate, from 30 ns to 150 ns forthe middle section of the hole, and decreased from 150 ns to 30 nsin the rest of the hole. The applied pulse’s amplitude was 6 V. Theseresults show that tool insulation and pulse control can make microholes of various shapes.

Fig. 5. Barrel-shaped hole (tool Ø 35 mm, pulse amplitude: 6 V, variable pulse on-

time).

Fig. 6. Hole diameter and machining gap according to stationary time. Fig. 8. Machining processes of disk electrode: (a) cylindrical electrode by WEDG, (b)

disk electrode by EDM with plate electrode.

Fig. 9. Schematic diagram of a groove made in the micro hole by using a disk

electrode: (a) drilling, (b) eccentric layer-by-layer rotation for grooving.

C.H. Jo et al. / CIRP Annals - Manufacturing Technology 58 (2009) 181–184 183

3.3. Spherical cavity

The machining gap can be controlled not only by changing thepulse duration, but also by changing the dissolution time. Fig. 6shows that the machining gap increases with increasing dissolutiontime. The control of dissolution time is used to make sphericalcavities, as shown in Fig. 7. To make these cavities, an insulated toolelectrode is moved downward to make a cylindrical hole of a fewtens of micrometers in depth (30 ns pulse on-time). Then, the toolelectrode was kept at this position for several minutes, with a longpulse on-time of 150 ns. In Fig. 7(a), the stationary time was 5 minand the diameter of the cavity was 58 mm. In Fig. 7(b), the stationarytime was 20 min and the diameter of the cavity was 75 mm.

4. The fabrication of microgrooves in the micro hole

4.1. Fabrication of disk electrode

To make micro features in a micro hole, a customized toolelectrode such as a disk shape tool is needed. Wire electricaldischarging grinding (WEDG) is commonly utilized to make amicroelectrode. However, since WEDG uses wire as a tool, it is noteasy to make a disk with a shaped edge. Thus, EDM with a plateelectrode is used here to achieve a disk shape [2,8]. The process ofdisk electrode fabrication is shown in Fig. 8. After making a microshaft using WEDG, the plate electrode was horizontally moved tothe rod, and the disk shape was machined. A stainless steel plate of300 mm thickness was used as the plate electrode.

4.2. Fabrication of micro grooves

ECM with a disk electrode can produce an internal groove in themicro-structure because the ECM transcribes the shape of theelectrode. The groove-machining process in the micro hole is shownin Fig. 9. The sequences are divided into drilling and grooving. Thefirst step of the process is to drill a hole in the workpiece. Then, thetool electrode is moved to the middle of the hole, and the electrode

Fig. 7. Micro cavity machined by controlling

eccentrically rotated. The distance between the axis of rotation andthe axis of the hole is increased step-by-step to dissolve the surfacelayer-by-layer. A relatively long pulse on-time is used for drilling,and a short pulse on-time is used for grooving, because the materialremoval rate for drilling is higher than it is for grooving. The internalgroove in the hole was fabricated as shown in Fig. 10. The dimensionsof the groove were 30 mm deep and 33 mm high, and the diameter ofthe hole was 130 mm.

A repeatable structure such as an array can be fabricated usingone electrode, since there is no tool wear during ECM. This is thebiggest advantage of ECM. Therefore, groove array can bemanufactured with a single disk electrode in one micro hole.Fig. 11 shows the sectional image of the groove array in the micro

dissolution time: (a) 5 min, (b) 20 min.

Fig. 10. Groove in the micro hole.

Fig. 11. Groove array in the micro hole.

C.H. Jo et al. / CIRP Annals - Manufacturing Technology 58 (2009) 181–184184

hole. The diameter of the hole is 130 mm. The depth and height ofthe groove are 30 mm and 33 mm, respectively.

5. Conclusion

In ECM, the machining gap can be controlled by increasing ordecreasing pulse on-time, pulse voltage, or machining time. By usingthis method, it is possible to control the diameter of the hole duringdrilling and to make the hole’s entrance size smaller than the inside.In this paper, reverse-tapered and barrel-shaped holes werefabricated. In order to prevent over-dissolution during the machin-ing, the use of insulation on the electrode was suggested. To make aninternal groove in the micro hole, a disk electrode was used. A groovearraycanalsobefabricated,becausethereisnotoolwear duringECM.

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