Electrochemical Micromachining Setup

CHAPTER

ELECTROCHEMICAL MICROMACHINING SETUP

5

Electrochemical micromachining (EMM) has a great potential to machine microfeatures like surface structuring, patterning, microholes, microgrooves, microchannels, microcavities, and threedimensional (3D) microstructures of complex shapes with a high aspect ratio for various industrial and household microproducts such as micronozzles, microfluidic devices, microthermal devices, and micromolds. In recent years, applications of microproducts have been considerably increased in electronics, optical, medical, automotive, aerospace, and telecommunication fields, and the demand for microproducts will increase exponentially in the coming years due to the vast applications of microproducts and global competition among industries. To fulfill the tremendous market demand of microproducts in future, an industrial EMM machine needs to be developed, which will be robust, will have a low price, will be maintenance free, and will have a higher production rate. Keeping in mind the influence of predominant process characteristics of an electrochemical machining system for fulfilling the requirements of micromachining as discussed earlier, well-planned designs have been considered by researchers for the development of an EMM system setup, which can serve the various needs of micromachining considering different types of EMMs in a single unit. Attempts have been made by researchers, research laboratories, and microproduct industries to design and develop a multipurpose EMM setup. The developed system may consist of various subcomponents, for example, a mechanical machining unit, microtooling system, electrical power and controlling system, and controlled electrolyte flow system. The various system components of the developed EMM system setup must have more precise control on electrochemical dissolution [1]. The mechanical machining unit for the developed EMM system comprises the following elements: machine body, microtool feeding device, machining chamber, work mounting device, etc.

5.1 DETAILS OF EMM SETUP Figure 5.1 shows a schematic view of a basic unit of an EMM system setup, which consists of various subsystems, namely, a mechanical machining unit, controller unit, direct current (DC) pulsed power supply, electrolyte flow system, and machining chamber with a work holding arrangement. The basic functions of the different subsystems are as follows:

5.1.1 MECHANICAL MACHINING UNIT The basic function of a mechanical machining unit is to provide a rigid mechanical structure to support the various subunits. It also provides different axis movements to the microtool and workpiece, that is, Z-axis for microdrilling, and X, Y, and Z axes for machining of the 3D micropart or microfeature. It may consist of two or three long travel linear stages assembled in different arrangements for providing Electrochemical Micromachining for Nanofabrication, MEMS and Nanotechnology. http://dx.doi.org/10.1016/B978-0-323-32737-4.00005-0 Copyright © 2015 Elsevier Inc. All rights reserved.

83

84

CHAPTER 5 ELECTROCHEMICAL MICROMACHINING SETUP

Controller

Tool driving unit

Z

Electrolyte Tool Work piece Settling tank

Pulsed D.C power supply + Y X

Pump

Electrolytechamber

Sludge

FIGURE 5.1 Schematic view of an EMM system [2].

a suitable working space. It has a work table with the machining chamber, which is generally mounted on the X/Y or XY axis. Microtools along with the tool holder are mounted on the Z-axis. Long travel linear stages may be driven by a controlled stepper motor or servomotor. A higher resolution of the driving motors is preferred for better and precise control of the dissolution process. Based on the arrangement of the axes, the mechanical machining unit can be arranged into a column and knee-type and gantry-type structure as shown in Figs 5.2 and 5.3. In the Gantry-type

FIGURE 5.2 Column and knee-type structure.

5.1 DETAILS OF EMM SETUP

85

FIGURE 5.3 Gantry-type structure.

structure, more working space is available. The work table and machining chamber along with the workpiece holding arrangement are mounted on only one axis, that is, X-axis, which needs that stage to be of a higher load-carrying capacity. The remaining two axes Y and Z are comparatively free, and hence can be selected to have a less load-carrying capacity, which reduces the initial cost of the machine. Column and knee-type structures are more suitable for machining of microfeatures over the larger size components, since they provide free space to both sides of the column. It has limitations of movement of the Y-axis, and permits less machining area. Work table, Y-axis linear stage, and machining chamber along with the workpiece holding arrangement are mounted on the X-axis, where these X- and Y stages are to be selected so that they have higher load carrying capacities. In EMM, the material is removed from the workpiece surface by anodic dissolution with a very small inter-electrode gap (IEG). For continuation of the EMM process efficiently, there has to be enough supply of fresh electrolyte so that the by-products of the process such as sludge and heat generated can be carried out to provide fresh ions for electrochemical reactions. Electrolyte flow rate is very low or almost stagnant in EMM. Hence, rotatory movement or scanning type movement of the tool, which improves the supply of fresh electrolyte at a narrow IEG, improves the machining accuracy [3]. Rotational movement can be provided to the tool through a Z-axis rotary attachment as shown in Fig. 5.4. Although the development of spindles is a challenging task, it can give precise rotation, and is competent to transmit high-frequency current pulses with minimum radio frequency (RF) emission. Accuracy of the machine is directly related to the precision required for the final product. In EMM, an IEG <20 mm is usually required to be maintained during machining, which influences the resolution of the axis movements of the machine. Therefore, resolution of the axis movements would be expected to be <0.1 mm. The dynamics of the machine axis must be very high, in order to prevent the

86

CHAPTER 5 ELECTROCHEMICAL MICROMACHINING SETUP

FIGURE 5.4 Structure with a rotatory arrangement.

microtool from damage, when a short circuit occurs. For realization of the end gap, response time of the mechanical structure for the corresponding axis should be very low. In order to have high dynamics and precision motion, the machine must use frictionless slides and other supporting devices [4]. All the elements of the machine structure, jigs, and fixtures must be made of noncorrosive materials, as well as some parts of the machine setup have to be electrically insulated to satisfy electrical and chemical constraints. The electrical contacts to the workpiece should not be exposed to the electrolyte to avoid corrosion of the contact point as well as to minimize electrical affinity of the metals to influence the flow of current.

5.1.2 OTHER SUPPORTING SUBUNITS 1. Machine Controller Unit: The stepper motors or servomotors of each linear stage representing different axes are controlled by a controller unit. The controller unit is interfaced with the desktop computer. Various feeds can be given to all or any of the three motors at a time through a motor controller unit using position controller software or labview programs installed on a desktop computer. For machining of complex microprofiles and 3D microstructures, Computer Numerical Controlled (CNC) controllers are preferred. 2. Pulsed DC Power supply unit: This is one of the important subsystems of the EMM setup. It provides a pulsed power supply to the workpiece (anode) and microtool (cathode) during the dissolution process. The pulse supplied consists of various parameters such as voltage amplitude, current, pulse frequency, and duty ratio. Accuracy of the machined microfeature is directly affected by the parameters of the pulse; hence, pulse power specifications plays a major role in EMM.

5.2 CURRENT STATUS OF DEVELOPMENTS IN EMM SETUP

87

3. Machining Chamber: Machining chambers are generally fabricated from transparent and lightweight materials such as Perspex material so that the microtool and workpiece positions can be observed during machining. It has a work holding arrangement and is mounted over the work table. In EMM, a very low or almost stagnant flow rate of electrolyte is required. However, to exploit the full potential of EMM, research is still needed to improve its accuracy and compactness so that a developed EMM machine can be accepted by the industries for production purpose. Higher degrees of freedom of the machine setup increase the flexibility of the machine, which is helpful to manufacture free-form surfaces [5]. However, addition of more degrees of freedom makes the machine tedious and complex to operate. For some specific applications, for example, throughmask EMM and surface structuring and patterning, it may require special purpose arrangements, which need design and development of the EMM cell to accomplish the potential flexibility of controlled anodic dissolution, that can be utilized successfully for different purposes [6].

5.2 CURRENT STATUS OF DEVELOPMENTS IN EMM SETUP To bring EMM into the industry, for mass production of microfeatures, robust, maintenance -free, low cost, easy to operate machines need to be developed. Considering the complexity of the process, mechatronics involved in controlling the machine, etc., EMM machines are still in the developmental phase. Different EMM experimental setups have been developed by researchers for carrying out experiments in various laboratories around the world. Some of these laboratory setups with different features are reported hereunder with the intention of providing more information about the current trends in the development process of the EMM setup. Mithu et al. developed an EMM work cell, at the University of Pisa, Italy, a closed feedback computer-controlled system for feeding the tool and the workpiece for in-process monitoring of electrochemical microdrilling on a nickel plate using a tungsten microtool. The work cell consisted of a function generator, oscilloscope, linear travel guide, and microcontroller as shown in Fig. 5.5. To regulate the fixed IEG and to avoid the physical contact

FIGURE 5.5 EMM cell [7].

88

CHAPTER 5 ELECTROCHEMICAL MICROMACHINING SETUP

between the tool and the workpiece, a tailored electronic circuit was used that automatically stops and retracts the tool. After the set time, the circuit automatically restarts by feeding the microtool and maintaining the predefined gap between the tool and the workpiece. The guides were controlled by a three-axis microstep controller system interfaced with a desktop computer. The maximum travel range of the precision guides was 155 mm (X), 155 mm (Y), 40 mm (Z) with a resolution of 0.1 mm in the X, Y, and Z directions. The linear stage guides also provided the facility of unidirectional repeatability lower than 0.05 mm. During microtool fabrication, the straight tungsten specimen was fixed to the holder unit, positioned vertically downward, and immersed through a circular electrode dipped into the electrolyte basin. The depth of immersion was controlled by the servocontrolled feed mechanism of the Z stage. Again, during microdrilling, the fabricated microtool was clamped in the tool holder, positioned vertically downward, and fed along the Z-axis while the workpiece was positioned horizontally and moved by X–Y axis travel guides. The developed EMM setup was utilized for microhole machining experiments to study the influence of different process parameters. Malapati and Bhattacharyya, at the Jadavpur University, Kolkata, India, developed an EMM system for experimentation. The system consists of various subcomponents, for example, mechanical machining unit, electrical power and controlling system, and controlled electrolyte flow system, as shown in Fig. 5.6. The mechanical machining unit is the main part of the EMM setup, which helps in the precision movement of the microtool. The XYZ stage “1” has a gantry moving bridge-type configuration. Three stepper motors, the X-axis motor is inside the base, Y-axis motor “2” and Z-axis motor “3” are controlled by a controller unit “4,” which is interfaced with a personal computer “5.” The machining chamber “6” for EMM is made of Perspex material. The workpiece “7” can be clamped by four sets of stainless steel nut and bolts on the work table. Microtool “8” is held by an attachment on Z-axis slide “9.” The main components of the electrical power and controlling system are pulse generator “10,” digital storage oscilloscope “11,” and multimeter. The pulse generator used is Phillips PM 5711, capable of generating high-frequency pulses. For digital storage and monitoring of pulse power parameters, a Yokogawa DL 1520 oscilloscope is used. A Tektronix multimeter is used to

FIGURE 5.6 Schematic and photographic view of the EMM setup [8].

5.2 CURRENT STATUS OF DEVELOPMENTS IN EMM SETUP

89

measure the current in the machining zone, which is an indication of the gap between the microtool and workpiece. During an electrochemical reaction, metallic oxides and hydroxides are formed as precipitates. The electrolyte is flushed through nozzle “12” to remove the machining by-products, that is, precipitates from the micromachining zone. Finally, the electrolyte is drained out through the outlet and collected at electrolyte storage tank “13.” Here sludge settles down and fresh electrolyte is circulated to the machining zone by pump “14.” In the same organization in India, Ghoshal and Bhattacharyya further developed another EMM experimental setup for micromachining and microtool fabrication using a tool vibration system to improve the performance of EMM. The setup consisted of various subcomponents such as an electrical power and controlling system, stepper motor controlled long travel linear stages for motion control of the tool and job, a mechanical unit for holding the tool and job, and a piezoelectric transducer (PZT) for vibrating the tool longitudinally. PZT is also very helpful for easy supply of the electrolyte to the IEG during EMM. Thus, material removal becomes possible at a very small diameter microdrilling or microchannel formation. PZT was connected to a 230 V main power supply through an amplifier module ( 20–130 V) for feed control in the nanometer range (resolution 0.12 nm) having a maximum stroke of 66 mm and a modulation input from a function generator for controlling the amplitude of vibration, which changes with the variation of voltage. The frequency of vibration of the PZT was controlled by the frequency of the input voltage. Electrical power and control comprised a function generator for the supply of a pulsed DC voltage to the microtool and workpiece. A digital storage oscilloscope acted as a data collection system, and is inevitable for the control IEG during EMM. Stepper motors were used to move the tool and workpiece along the X, Y, and Z axes. However, for a very low feed rate and accurate control of IEG (<10 mm), feed by PZT is preferred. Figure 5.7 shows the various subunits of the developed EMM experimental setup, used for the microtool fabrication as well for machining of complex microfeatures, during EMM experimentations. Motion controller

Stepper motor

+

Micro processor

Piezo actuator

Function generator – Tool holder

AC power supply 220V/50 Hz

Vibrating tool

+

Z

Oscilloscope

Function generator

Stainless steel ring

– Machining chamber Y

Mounting base

FIGURE 5.7 Experimental EMM setup for micromachining and microtool fabrication [9].

X

90

CHAPTER 5 ELECTROCHEMICAL MICROMACHINING SETUP

Li Yong et al. developed an EMM experimental setup at the Tsinghua University, China, for EMM with machining process detection and gap control functions. A microgap control strategy was proposed based on the fundamental experimental behavior of electrochemical machining current with the gap variance. The developed EMM setup is shown in Fig. 5.8. It comprises the following components: a microfeeding mechanism with a resolution of 0.1 mm, a precise XY stage with a resolution of 1 mm, a power supply for micro-ECM, a machining process detecting module, and a control computer. The machining process detecting module samples the signals that reflect the machining status and then inputs them into the computer, in which signals were processed and appropriate decisions were worked out to drive the microfeeding mechanism and the XY stage. Therefore, the electrode gap can be controlled automatically according to the situation of machining. At the same institute in China, Liu Yong et al. further developed an EMM system for machining of complex structures on nickel surfaces. The developed EMM setup consists of various subcomponents such as an electrode unit, a controlled electrolyte flow system, a servocontrol feed unit, and a data acquisition system. Figure 5.9 presents a schematic view of the various system components of the developed EMM setup with a rotary tool unit. The electrode unit consists of a tungsten cylinder cathode, workpiece, tool holder, and electricity-conductive device. The controlled electrolyte flow system consists of a machining chamber, micropump, filter, electrolyte tank, etc. It also has a current sensor and data acquisition system to regulate the predefined IEG by monitoring the current during machining. For example, when a short circuit is detected, the servomotor will stop and move backward along the preceding path immediately until the danger is cleared. All these elements have been designed so as to fulfill the design requirements of an EMM system. Zhaoyang Zhang et al. of the Jiangsu University, China, developed an EMM system based on electrochemical dissolution of the material as shown in Fig. 5.10. The system consists of various subcomponents, namely, main machining body, mechanical movement equipment, power supply, electrolyte circulation system, tool movement control component, and process monitoring component. Two analog output channels of the PCI_7344 control card were used to control the on-off state of the electrolyte pump and the machining power supply. The size and shape of microtool and microstructures fabricated with the electrochemical method can be monitored online through a charge coupled FIGURE 5.8 EMM setup [10].

5.2 CURRENT STATUS OF DEVELOPMENTS IN EMM SETUP

91

FIGURE 5.9 Motion control card

EMM setup with a rotatory tool arrangement [11].

Rotate axis Data acquisition card

Nano-second pulse power

Current sensor

–

Z

Electrode + Workpiece

Nozzle Filter

Electrode holder

Micro pump

Y X

Shake proof mounting

device and video collection card. Using the developed EMM system, in situ fabricated tungsten microtool was utilized for machining microgroove of a 30-mm width. Kurita et al. developed an ECmM system at AIST, Ibaraki, Japan, with X-, Y-, and Z-axis actuators. Figure 5.11 shows the system configuration. The system mainly consists of an electrode, electrolyte FIGURE 5.10 Schematic diagram of an EMM system [12].

92

CHAPTER 5 ELECTROCHEMICAL MICROMACHINING SETUP

PERSONAL COMPUTER

CONTROLLER

ACTUATOR DRIVER

Pulse motor (Z-axis)

Z LINER ACTUATOR D/A 3

A/D CONVERTER

XYZ piezo actuator

D/A CONVERTER

A/D 2 Z Work (+) Y

*X *Y

A/D 0

ECM IN

CURRENT MONITOR

Z-AXIS STAGE X Y Z

PZT ACTUATORS 100 µm MAX

TOOL

Electrode (–)

Pulse motor (X-axis)

–10~150V PZT DRIVER

POSITION SENSOR

A/D 1

Pulse motor (Y-axis)

D/A 0~2

ECM POWER AMP –5~+8V

CURRENT

ECM VOLTAGE

CURRENT SENSOR

ECM OUT

WORK XY *X X STAGE LINER ACTUATOR Y

*Y

FIGURE 5.11 System configuration of an EMM setup [13].

supplier, triaxial pulse motor, PZT, power supply, and personal computer. The electrode can move in the Z direction at 0.0156 mm/pulse by using the pulse motor. Further, the PZT actuator enables the high-speed X, Y, and Z-direction movement of the electrode. The maximum amplitude of the actuator is 100 mm. Electrolyte was supplied with a dropper to the work material at a flow rate of 1–5 ml/min. The personal computer sets and stores the machining conditions, and controls the operation of the actuators and the machining power supply. The developed ECmM system consisted of an online machining current sensor and a tool positioning sensor for monitoring machining conditions. Movements of X, Y, and Z stages were controlled accordingly through a personal computer. IEG can be predefined and maintained constantly during the machining of microfeatures. Three-dimensional shape micromachining was successfully carried out by utilizing the scanning movement of a prismatic electrode under optimum parametric conditions. Wansheng Zhao et al. developed a new micro-ECM setup independently at the Harbin Institute of Technology, China, which was in fact a multifunctional machine tool, as shown in Fig. 5.12. There was a three-axis Numerical Controlled (NC) electrode feed mechanism. The motion parts of X, Y, and Z axes were driven by direct current servomotors through a precision ball-race feed screw with a resolution of 0.1 mm. The motion part of the Z-axis was equipped with the rotating precision spindle. The motion parts X-, Y-, or Z-axis can make the rotating precision spindle move in the direction of the X-, Y-, or Z-axis, respectively. The microtool electrode is clamped on the rotating precision spindle and rotates along with the spindle at a high speed. The rotating speed ranges from 1000 to 25,000 rpm, and the rotating speed can be regulated continuously. The experimental setup is equipped with two pulsed power supplies, respectively, for micro-EDM and micro-ECM. The online fabrication of a microelectrode with a high shaping accuracy and a high aspect ratio can also be realized by wire electrical discharge grinding through a Resistance Capacitance (RC) power supply on the same machine tool. The experimental setup was utilized for the successful fabrication of a high-quality 3D microscale and mesoscale structures by microelectrochemical milling.

5.2 CURRENT STATUS OF DEVELOPMENTS IN EMM SETUP

FIGURE 5.12

Control computer

Short circuit detection for EMM

X axis

Z axis

X axis Y axis

Y axis

CCDsystem

Micro feed system

Pulsed power Oscilloscope supply for EMM

93

Servo control card

Z axis Drivers

DC servo motor

Rotary spindle Electrode

Setup of an EMM experimental system [14]. Short circuit detection for EDM

Electrode fabrication

RC power supply for EDM

Granite base

Electrolytic cell Electrolyte

Work piece

Yuang-Cherng Chiou et al. at the National Sun Yat-Sen University, Taiwan, developed an EMM setup as shown in Fig. 5.13(a). This setup was located on a plate of a vertical work table so that it is easy to adjust its position of the vertical direction. In an experiment, a tungsten rod with a diameter of 200 mm was taken as the workpiece, and was attached to a jig that was precisely chucked to a rotating spindle mounted by a bearing support. The conductive wire was connected to the anode of the DC power supply and carbon brush, respectively, and then the carbon brush was in contact with the

FIGURE 5.13 Schematic diagram of (a) an electrochemical micromachining apparatus, (b) configuration of the tungsten workpiece being machined [15].

94

CHAPTER 5 ELECTROCHEMICAL MICROMACHINING SETUP

rotating spindle, so that the tungsten rod becomes the anode. Distilled water containing 2 wt% NaOH taken as the electrolyte, and an iron needle was used as the cathode. The rotating speed of the tungsten rod was controlled and adjusted by the first motor. In the experiment, the rotating speed was set to be 200 and 500 rpm. The bearing support and the first motor were attached to the linear slider, and this slider was driven by the screw of a nonrotating spindle-type micrometer head. A synchronous belt was used to connect the gear in the spindle of the second motor and the gear was mounted on the spindle of a micrometer head so that the slider could be driven by the second motor. The iron needle was surrounded by a layer of the epoxy resin of an insulated material so that its end face with a diameter of 50 mm was exposed to the electrolyte as the tool electrode, and another end was attached to an insulated holder, which concealed inside a conductive wire to connect the cathode of a DC power supply, as shown in Fig. 5.13(b). This holder was attached to the linear slider within the support, and this slider was connected to the screw of a nonrotating spindle-type micrometer head. Hence, the tool electrode could be horizontally moved by the rotation of the sleeve of the micrometer head through the linear slider. Therefore, the gap between the tool electrode and the tungsten rod could be adjusted. Further, during the machining, since the diameter of the tungsten rod was gradually reduced, the tool electrode was adjusted so that the gap between its end face and the tungsten rod remains 30 mm. The developed setup was successfully utilized to fabricate extremely thin and straight microtungsten rods of high aspect ratios.

5.3 IEG CONTROL STRATEGY In EMM, the workpiece as the anode and the microtool as the cathode are immersed in an electrolyte, with a very small IEG. During machining, a micron-sized tool electrode moves with a constant feed rate toward the workpiece to maintain the IEG. Based on the machining strategy, the microtool tracks the scheduled path to fabricate the desired microfeatures such as microhole, microgroove, etc. IEG controls the dimensional accuracy of the machined feature in micromachining. In order to obtain an accurate shape, the IEG should be kept as small as possible. When the microtool feed rate is higher than the material removal rate, IEG decreases gradually and after some time it touches the workpiece causing a microspark. Sparking phenomena may damage both the microtool and the workpiece. When the tool feed rate is smaller than the material removal rate, the IEG increases gradually, and generates a higher machining gap that may affect the machining accuracy. Therefore, to maintain a constant IEG, the microtool feed rate should synchronize with the material removal rate, known as equilibrium speed. While machining microfeatures by EMM, the material removal rate varies according to various process parameters as well as the equilibrium speed of the microtool. Hence, to maintain a constant IEG during micromachining, different strategies have been developed and adopted by researchers. Some of the IEG control strategies for EMM adopted by researchers are reported as follows: Young Li et al. constructed an experimental setup for EMM, which has machining process detection and gap control functions. Figure 5.14 shows the flow chart of a gap control in the microECM process for microhole drilling. At first, the microtool electrode is positioned to form the gap between the tool and the workpiece. The machining current is sampled during feeding of the microtool toward the workpiece. The status of machining is judged by a computer via the current sample. It shows a short circuit when detecting the current jump-up. Then, a decision is made to withdraw the microtool electrode to maintain a tiny gap. When the microtool is moved backward, it stops at the

5.3 IEG CONTROL STRATEGY

95

FIGURE 5.14 Yes

Start

Back

Current Jump-up

Flow chart for IEG control [10].

Electrode Positioning No Gap forming Waiting and Feeding

Feeding forward And Waiting for Removal

Sampling

Yes No

Manual Stopped End

place several micrometers away from the jump-up place for a while to wait for removal of machined products, and is then moved forward. Then, the gap is maintained continuously within a tiny range. Kurita et al. developed an electrochemical micromachining (ECmM) for the 3D shape machining on an Ni plate. To maintain the machining gap shorter than 0.1 mm, the control sequence as shown in Fig. 5.15 is employed. First, a sensing voltage that is smaller than the machining voltage is applied between the microtool electrode and the workpiece. If no contact is detected, the electrode is fed farther downward. If the electrode and the workpiece come into contact with each other, the electrode

FIGURE 5.15 Control sequence of IEG [13].

96

CHAPTER 5 ELECTROCHEMICAL MICROMACHINING SETUP

is raised while the value of the current is monitored. After the separation between the electrode and the workpiece occurs, which is confirmed by the monitoring current, the electrode is raised further, and the distance is considered as the machining gap. Next, machining voltage pulses are supplied, and machining is started. In this study, the distance between the electrode and the workpiece (machining gap) was preset. As the machining shape becomes smaller, removal of the machining sludge becomes more difficult. This happens because the Reynolds number becomes smaller due to the stagnant nature of the electrolyte and the effect of the viscosity of the electrolyte becomes larger due to the addition of process by-products in the narrow machining zone. Therefore, in order to remove the machining sludge, the microtool moves up and down in the Z direction with the help of PZT. If contact is detected during the pulse-off period or a short when applying the machining pulses, the process restarts after the contact detection. Wansheng Zhao et al. machined microscale and mesoscale features using a cut-edge electrode on a developed experimental setup. Figure 5.16 shows the adopted flow chart of the gap control strategy. The positions of the microtool electrode and the clamped workpiece was determined through the contact sensing function of the multifunctional machine tool, and then the tool electrode was withdrawn about 10 mm to form a sound machining gap. The proper machining speed was set in advance through an NC system specially designed for micro-ECM according to the experience before machining. Then, the microtool electrode was fed to start machining and a uniform machining speed was adopted during micro-ECM. The short circuit detection system periodically detects whether or not a short circuit occurs through a Hall current sensor during micro-ECM. When the tool electrode touches the workpiece surface or the distance between the tool electrode and workpiece was only few microns, the machining current jumps up instantly, and the voltage of the sampling resistance soars. Thus, the short circuit could be detected by the short circuit detection module. When short circuit occurred, the pulsed power supply was switched off immediately, and the tool electrode was drawn back rapidly by several micrometers to avoid short circuit damage. The computer automatically regulates the feed rate and afterward switches on the pulse supply. Then, the microtool electrode was

FIGURE 5.16 Gap control strategy [14].

Start Electrode positioning Gap forming Set machining speed

Shortcircuit

Feeding

Yes

Electrode move backward

No Feeding forward and machining

Sampling No

Machining finish

Yes

End

5.3 IEG CONTROL STRATEGY

97

again fed to continue machining. By this machining gap control strategy, the machining gap can be controlled within about 10 mm. Mithu et al. developed a micro-ECM work cell for microhole generation experimentations. A smooth micromachining requires moving of the microtool electrode along the workpiece in order to maintain a constant predefined gap between them. The flow chart of the gap control system is illustrated in Fig. 5.17. The wave generator, the oscilloscope, and a tailored circuit were used as a signal source, signal analyzer, and tool feeding controller, respectively. The wave generator provided the signals during machining, and these signals were analyzed through the oscilloscope. The screen images of the oscilloscope produced during machining were recorded and transferred to the PC for storage and further processing. The control software performed two functions: (1) tracking control and (2) gap control. To avoid any physical contact between the workpiece and the tool, the tailored electronic circuit automatically stopped and retracted the tool. After the set time, the circuit automatically restarted feeding the tool for maintaining the predefined gap between the electrodes. Bhattacharyya et al. developed an EMM system for machining of microholes and microchannels on a copper foil. Figure 5.18 shows the flow chart for the EMM machining strategy, which also includes the IEG control strategy and sequence of operations for maintaining a narrow end gap. First, a sensing voltage, that is, 0.5–1 V, which is smaller than the machining voltage, is applied between the microtool and the workpiece in the absence of an electrolyte, and monitored through an ammeter. Initially, the microtool moves in the downward direction, if contact occurs, that is, in the no-gap condition the microtool is moved in the upward direction with the help of a stepper motor and positioned to the required IEG. Thereafter, the machining voltage is applied between the microtool and the workpiece for the machining operation in the presence of an electrolyte. Now the microtool starts moving in the downward direction to continue the desired machining with the help of a microprocessor-based controller. The microtool feed rate (downward motion) must be lower than material removal for

FIGURE 5.17 Flow chart of feedback control strategy [16].

98

CHAPTER 5 ELECTROCHEMICAL MICROMACHINING SETUP

START

1

2

Bring the tool close to the job Move the backward Send the single to stepper motor for one forward step through microprocessor Count number of backward step

Tool touch the job?

NO

YES YES

Send single to stepper motor through the microprocessor to move back to create the required end gap

Is the backward step count < required step?

NO Activate the electrolyte flow system and the electrode YES Start the program for fast forward and slow backward motion

Is the total net feed < depth of final shape? NO Set forward step and backward step count to zero

Move forward

Count number of forward step Deactivate the electrode, electrolyte flow system & stop the stepper motor Tool touches the job NO YES Is the forward step count < required ?

NO 1

2

FIGURE 5.18 Flow diagram of the EMM operation [17].

YES

Stop the stepper motor and deactivate the electrode

Send signal to retract the tool to initial position

STOP

REFERENCES

99

effective machining. If not, the microtool may touch the workpiece and cause a short circuit, which in turn could cut off the power supply. After the short circuit, the microtool moves in an upward direction and again continues in the same sequence of operation for the proper regulation of the IEG. Development of the EMM setup is still at the research level. All the developed setups used a specific type of microtool for one or two types of specific micromachining operations, which simplifies the construction of the machining setup. In a CNC-controlled machine setup, the determination of the work coordinate system according to the machine coordinate system and measuring the relative positions of the microtool according to the work coordinate system are important, which needs further simplifications in the machining setup procedure. The initial setting of IEG is based on the position of the workpiece surface, which may adversely affect the accuracy of the established IEG, and in turn accuracy of the machined features. A lot of work remains to be done on a reliable online measurement system in the setup during EMM operations, utilizing a very small microtool and reliable IEG controlling system.

REFERENCES [1] B. Bhattacharyya, S. Mitra, A.K. Boro, Electrochemical machining: new possibility for micromachining, Int. J. Rob. Comput. Integr. Manuf. 18 (2002) 283–289. [2] B. Bhattacharyya, J. Munda, M. Malapati, Advancement in electrochemical micro-machining, Int. J. Mach. Tools Manuf. 44 (2004) 1577–1589. [3] Z.-W. Fan, L.-W. Hourng, Electrochemical micro-drilling of deep holes by rotational cathode tools, Int. J. Adv. Manuf. Technol. 52 (2011) 555–563, http://dx.doi.org/10.1007/s00170-010-2744-x. [4] W. Gao, Y. Arai, A. Shibuya, et al., Measurement of multi-degree-of-freedom error motions of a precision linear air-bearing stage, Precis. Eng. 30 (2006) 96–103, http://dx.doi.org/10.1016/j.precisioneng. 2005.06.003. [5] A. Spieser, A. Ivanov, Recent developments and research challenges in electrochemical micromachining (mECM), Int. J. Adv. Manuf. Technol. 69 (2013) 563–581, http://dx.doi.org/10.1007/s00170-013-5024-8. [6] D. Landolt, P.F. Chauvy, O. Zinger, Electrochemical micro machining, polishing and surface structuring of metals: fundamental aspects and new developments, Electrochim. Acta 48 (2003) 3185–3201. [7] M.A.H. Mithu, G. Fantoni, J. Ciampi, A step towards the in process monitoring for electrochemical microdrilling, Int. J. Adv. Manuf. Technol. 57 (2011) 969–982, http://dx.doi.org/10.1007/s00170-0113355-x. [8] M. Malapati, B. Bhattacharyya, Investigation into electrochemical micromachining process during microchannel generation, Int. J. Mater. Manuf. Processes 26 (2011) 1019–1027. Taylor & Francis. [9] B. Ghoshal, B. Bhattacharyya, Influence of vibration on micro-tool fabrication by electrochemical machining, Int. J. Mach. Tools Manuf. 64 (2013) 49–59. [10] L. Yong, Z. Yunfei, Y. Guang, P. Liangqiang, Localized electrochemical micro machining with gap control, Sens. Actuators-A Phys. A108 (2003) 144–148. [11] Y. Liu, D. Zhu, L. Zhu, Micro electrochemical milling of complex structures by using in situ fabricated cylindrical electrode, Int. J. Adv. Manuf. Technol. 60 (2012) 977–984, http://dx.doi.org/10.1007/s00170011-3682-y. [12] Z. Zhang, Y. Wang, F. Chen, W. Mao, A micro-machining system based on electrochemical dissolution of material, Russ. J. Electrochem. 47 (7) (2011) 819–824. [13] T. Kuritaa, K. Chikamorib, S. Kubotac, M. Hattoria, A study of three-dimensional shape machining with an ECmM system, Int. J. Mach. Tools Manuf. 46 (2006) 1311–1318.

100

CHAPTER 5 ELECTROCHEMICAL MICROMACHINING SETUP

[14] W. Zhao, X. Li, Z. Wang, Study on micro electrochemical machining at micro to meso-scale, in: Proceedings of the 1st IEEE International Conference on Nano/Micro Engineered and Molecular Systems, January 18–21, 2006, pp. 325–329. Zhuhai, China. [15] Y.-C. Chiou, R.-T. Lee, T.-J. Chen, J.-M. Chiou, Fabrication of high aspect ratio micro-rod using a novel electrochemical micro-machining method, Precis. Eng. 36 (2012) 193–202. [16] M.A.H. Mithu, G. Fantoni, J. Ciampi, M. Santochi, On how tool geometry, applied frequency and machining parameters influence electrochemical microdrilling, CIRP J. Manuf. Sci. Technol. 5 (2012) 202–213. [17] B. Bhattacharyya, J. Munda, Experimental investigation on the influence of electrochemical machining parameters on machining rate and accuracy in micromachining domain, Int. J. Mach. Tools Manuf. 43 (2003) 1301–1310.