Asian Journal of Research in Social Sciences and Humanities
Asian Research Consortium Asian Journal of Research in Social Sciences and Humanities Vol. 6, No. 3, March 2016, pp. 145-155 ISSN 2249-7315 A Journal Indexed in Indian Citation Index
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Development and Performance Study of Sparkgap Controller for Table Top Micro-EDM *
Jesudas T1, and Arunachalam RM2
1Mahendra Engineering College (Autonomous), Namakkkal, Tamil Nadu, India. 2Sultan Quobos University, Sultanete of Oman, Oman. *Corresponding author:
[email protected]
DOI Number: 10.5958/2249-7315.2016.00036.8
Abstract This research is mainly focused to develop a sparkgap controller for tabletop micro EDM setup and evaluating the performance in terms of micro hole drilling. To assess the performance of the controller, 27 experiments were conducted using full factorial design methodology. The input parameters of micro EDM operations such as voltage, capacitance and sparkgap were considered for experimentation to find out the Material Removal Rate (MRR). The experimental result shows that the voltage makes the major contribution and sparkgap provides minor contribution than capacitance and the performance of the developed sparkgap controller is satisfactory in micro EDM operations.
Keywords: Micro EDM, Sparkgap Controller, Micro Hole, MRR. ________________________________________________________________________________
1. Introduction Micro EDM is one of the non conventional techniques which is used to machine materials of recent development. In this process the metal is removed from the work due to erosion by spark discharge that takes place between the tool electrode and the work material. A thin sparkgap is maintained between the tool electrode and the work piece. In this process both tool electrode and work piece are submerged in a dielectric fluid like kerosene, EDM oil or deionized water. Bhattacharyya et. al. [1] developed an Electrochemical Micromachining (EMM) setup for performing basic and fundamental research in the area of EMM to satisfy the micromachining objectives. The developed setup consists of various sub-components and systems. The EMM setup thus developed provides various advantages, which mainly fulfills the requirements, and the needs of the micromachining operations. Yong et. al. [2] proposed a prototype micro EDM with an inchworm type of micro feed mechanism has been developed. It features high feed accuracy and 145
Jesudas & Arunachalam (2016). Asian Journal of Research in Social Sciences and Humanities, Vol. 6, No.3, pp. 145-155
quick response to maintain micro gap between electrodes and work piece during machining. In this developed setup, a micro hole of diameter less than 50 µm has been obtained. Said [3] developed and demonstrated an adaptive tip withdrawal technique. The technique was operated by monitoring the deposition current gradient rather than the current value, and triggered tip positioning at a speed proportional to the detected current gradient. Takezawa et. al. [4] developed micro EDM machining center that utilized rapidly sharpened electrodes to implement precession machining. They concluded that, they formed less than 20 µm diameter electrodes through ECM by single discharge. Hwa et. al. [5] studied the pulse on time, cutting speed, the width of slit or sparkgap on composite material. The surface roughness and sparkgap of machining significantly depend on the volume fraction. From the experimental investigation of sparkgap against pulse on-time it was found that the increasing pulse on time contributes to higher gap. Gwo et. al. [6] developed a micro EDM punching machine to produce micro holes and to fabricate micro-punch and die. From the results they concluded that the performances of the developed machine and geometry of punched micro holes were in satisfactory level. Hung et. al. [7] has proposed micro EDM combined with electropolishing for machining micro hole. Electro-polishing is used for finishing the hole wall with a high surface quality. Nakaoku et. al. [8] investigated the characteristic of µ-EDM by machining micro holes in sintered diamond. They reported that micro-holes with a diameter of 50 µm can be machined in sintered diamond. Murali et. al. [9] introduced an ultrasonic vibration to the workpiece to maximize the MRR and minimize the tool wear. From the investigation it was found that ultrasonic vibration and peak power with capacitance are significant for improving MRR. An investigation was made by Kun et. al. [10] was studied the performance on Si 3N4–TiN conductive ceramics to develop an ultra-miniature gas-turbine impeller about 20 mm diameter. They concluded that, in semi finishing with increase in voltage and discharge current, the MRR gets increased and RTW gets decreased. In finishing, surface quality is affected by increase in the open circuit voltage and discharge current. Takashi et. al. [11] introduced vibration assisted machining to micro EDM using PZT to flush out the debris between the electrodes. Using vibration assisted machining a small square shaft was fabricated. They concluded that the vibration assisted machining improves the machining stability and reduces machining time. A performance study was made by Jahan et. al. [12] on tungsten carbide to achieve good quality micro hole using transistor and RC type pulse generators. It was concluded that the RC pulse generator produced better quality micro holes in tungsten carbide, with rim free of burr-like recast layer, good dimensional accuracy and fine circularity. Kung et. al. [13] introduced powder mixed EDM when machining of cobalt-bonded tungsten carbide. The MRR and EWR were chosen as outputs. The response surface methodology (RSM) was used to plan and analyze the experiments. They concluded that the aluminium powder mixed with dielectric fluid increases the MRR and reduces the EWR. Adrian et. al. [14] studied the effect of process parameters on MRR when machining a hybrid metal matrix composite material (Al/SiC). The study revealed that the MRR gets increased with increase in current intensity and it was observed that low current intensity and pulse on time result in lesser electrode wear. Natarajan and Arunachalam [15] optimized the machining parameter using Taguchi method and Gray relational analysis. They concluded that the pulse on-time is the most significant parameter during machining. Himani et. al. [16] investigated to achieve highest MRR, good surface quality and low tool wear rate while machining Ni-Ti based Shape Memory Alloy. It was observed that the MRR was highly influenced by capacitance and discharge voltage and it depended on electrode material. TWR was found to be better at low energy levels. From the literatures it is learnt that not much
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work has been carried out to develop the sparkgap controller and study the effect of micro EDM process parameters. The present work focuses on the requirements of micro EDM developments.
2. Development of Sparkgap Control In micro EDM process the gap between the tool (cathode) and the work-piece (anode) plays a major important role. Sparkgap control is an important process which would help to remove metal and to generate accurate shape and size. The possibility of applying micro EDM to micromachining is increased by keeping the sparkgap to a small value. Figure 1 shows the flow chart of sparkgap control strategy.
Fig. 1 Flowchart for Sparkgap Control 2.1 Various functions of the micro controller unit Fast forward In this operation the tool moves rapidly towards the workpiece in the „Z „axis. Fast backward The fast backward operation is used to withdraw the tool electrode rapidly in the „Z‟ axis. Pulse forward The pulse forward operation is very important to set the required machining gap between the electrodes. In this operation the tool electrode moves in the „Z‟ axis towards the workpiece pulse by pulse. As per the lead screw dimension, it moves four microns for each pulse movement in full step mode. Pulse backward In the pulse backward operation, the tool electrode is withdrawn from its previous position pulse by pulse. The backward movement of the tool electrode is also four microns per pulse in full step mode.
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System reset The system reset function is used to erase the data (previous reference current value) which has already been stored in the system memory. It is necessary to reset the system memory. Figure 2 shows the sparkgap control circuit.
Fig. 2 Sparkgap control circuit 2.2 Working of Sparkgap Controller The microcontroller unit consists of two major modes of operations, namely, Initialization mode and Operation mode. Initialization mode This mode is used to set the required machining gap between tool electrode and workpiece. Operation mode In micro EDM operations, sparkgap plays an important role for better machining accuracy. Therefore it is important to maintain the constant gap between the electrodes. The activity is achieved through the developed sparkgap controller. The positions of the micro tool electrode and the workpiece are determined through contact sensing function (Bhattacharyya et. al.) [2] and tool electrode is withdrawn by 28 µm to form the minimum machining gap. After setting up the required machining gap, the power supply is switched on. As soon as the power is switched on, the current value between the electrodes is measured continuously using current sensor. The output of the current sensor value is amplified and converted into digital signal. Figure 3 shows the instrumentation amplifier circuit.
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Fig. 3 Instrumentation amplifier circuit The microcontroller unit continuously monitors the current in between the tool electrode and the workpiece through current sensor. Having set up the required sparkgap, the actual machining is carried out in the operating mode. During machining, if the gap between the electrodes increases then the current value gets decreased. The micro controller unit checks the measured current value with reference to current value, and the tool moves down one pulse to maintain the reference sparkgap. Figure 4 shows the sparkgap controller unit.
Fig. 4 Sparkgap controller unit By validating the accuracy of the machined micro holes, the performance of the developed sparkgap controller unit is evaluated.
3.
Experimental Planning
To evaluate the performance of the developed sparkgap controller it is necessary to perform the micro hole drilling operation. The experiments were conducted to study the effect of process parameters such as Voltage, Capacitance and Sparkgap on MRR. Three levels in all the three parameters were chosen for conducting the experiments. A full factorial design (Montgomery) [2] was used to conduct the experiments. The plan of experiment considered of 27 tests and repeated once for consistency. Table 1 shows the machining parameters and their levels used for the experiments. Tungsten wire of diameter 380 µm was used as a tool electrode and made as cathode. A 316L stainless steel of thickness 200 µm was used as the workpiece material, since it has many applications such as in micro punches and dies and it was made as anode. The composition of the work material is shown in Table 2. De-ionized water was used as dielectric medium in the experiments. A RC type of power supply of 300V and 5A with the capability for varying voltage, 149
Jesudas & Arunachalam (2016). Asian Journal of Research in Social Sciences and Humanities, Vol. 6, No.3, pp. 145-155
capacitance and resistance to the required values within the specified range was used. The RC type power supply was used because of its advantages of better surface finish and circularity (Jahan et.al.) [7]. Figure 5 shows the developed tabletop micro EDM setup.
Fig. 5 Developed micro EDM setup
Parameters
Level 1 Level 2 Level 3
C 0.03
Mn 2.0
Si 0.75
Table 1 Process parameters and their levels Voltage Capacitance (V) (pF) 80 200 100 300 120 500
Sparkgap (µm) 28 32 36
Table 2 Composition of 316L stainless steels P S Cr Mo
Ni
0.045
0.03
18.0
3.00
14.0
N 0.10
In these experiments the MRR was calculated by weight loss method of the workpiece after machining by subtracting the machining time. The machining time is the total time required to complete the profile of the electrode on the workpiece. The initial and final weights of the workpiece were measured using SHIMADZU electronic balance having a resolution of 0.001g. The weight loss method of MRR calculation gives the actual material removed during machining. Table 3 shows the experimental values and their results.
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Table 3 Experimental values and results for MRR Voltage
Capacitance
Sparkgap
Average MRR
(V)
(pF)
(µm)
(*10-3 gms / min)
1
80
200
28
0.6365
2
100
200
28
0.7527
3
120
200
28
0.9022
4
80
300
28
0.7119
5
100
300
28
0.8527
6
120
300
28
0.9655
7
80
500
28
0.7998
8
100
500
28
0.9613
9
120
500
28
1.1080
10
80
200
32
0.5544
11
100
200
32
0.6751
12
120
200
32
0.8146
13
80
300
32
0.6391
14
100
300
32
0.7586
15
120
300
32
0.9898
16
80
500
32
0.6983
17
100
500
32
0.8699
18
120
500
32
1.0191
19
80
200
36
0.4671
20
100
200
36
0.5998
21
120
200
36
0.7054
22
80
300
36
0.5453
23
100
300
36
0.6977
24
120
300
36
0.8484
25
80
500
36
0.6308
26
100
500
36
0.8013
27
120
500
36
0.9884
Sl. No.
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4. Performance Studies on MRR 4.1 Effect of Voltage on MRR In micro EDM operations, the voltage is one of the important parameters. To study the effect of voltage, the sparkgap and capacitance were kept constant. From Figure 6 (a), (b) and (c), it is observed that, with increase in voltage, the MRR gets increased. This is because, the energy discharge from the electrode becomes increased with increase in voltage. Due to increase in discharge energy, higher temperature develops between the electrodes. This results in higher MRR.
Fig. 6 Effect of voltage at 200, 300 and 500 pF capacitance From Figure 6 (b) it is observed that at voltage 120, capacitance 300 pF and sparkgap 32 µm there is a sudden increase in MRR than voltage 120, capacitance 300 pF and sparkgap 28 µm. This may be due to the acceleration of machined particles, since they act as an additive during machining. 4.2
Effect of Capacitance on MRR The capacitance is a more influencing parameter in micro EDM especially when using RC circuit. To study the effect of capacitance, other two parameters namely voltage and sparkgap were kept constant. From Figure 7 (a), (b) and (c), it is observed that, with increase in capacitance the MRR also increased. This is because the energy discharged from the capacitor is high and this results in higher MRR.
Fig. 7 Effect of capacitance at sparkgap 28, 32 and 36 µm From Fig. 7 (b) it can be seen that at voltage 120, sparkgap 32 µm and 300pF capacitance there is an increase in MRR. This may be due to the acceleration of the machined particles, since they act as an additive during machining.
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4.3
Effect of Sparkgap on MRR In micro EDM operation, the workpiece and electrode represent positive and negative (anode & cathode) terminal and are separated by a well controlled gap which is constantly and continuously controlled by the machine. This gap is filled with dielectric fluid which acts as insulator, cooling as well as flushing mediator in order to flush away the eroded particles from the machining zone. The sparkgap is also known as discharge gap between the electrodes (tool and work) which must be maintained constant throughout the machining. To find the influence of the sparkgap on MRR, capacitance and voltage were kept constant. The effect of sparkgap on MRR is shown in Figure 8 (a), (b) and (c). With increase in sparkgap, the MRR gets decreased. Because, when the distance between electrodes increases, the discharge energy (thermal) concentration towards the work piece becomes less, resulting in lower MRR. Higher capacitance results in higher discharge energies and so the MRR gets increased. Also it is observed that, MRR is higher for higher capacitance and it becomes decreased with decrease in capacitance.
Fig. 8 Effect of sparkgap at 80, 100 and 120 Voltage From Figure 8 (c) it can be observed that at voltage 120, sparkgap 32 µm and 300pF capacitance there is an increase in MRR. This may be due to the acceleration of the machined particles, since they act as an additive during machining.
5. Conclusions 1.
2. 3. 4.
In the present work the sparkgap controller for a tabletop micro EDM setup was developed and the performance of the controller was studied in terms of micro hole drilling on 316L stainless steel. To study the effect of different process parameters such as voltage, capacitance and sparkgap was considered as input parameters to maximize the Material Removal Rate. From the experimental results it is found that the MRR gets increased with increase in voltage and capacitance. With simultaneous increase in capacitance and sparkgap, highest MRR could be achieved
Acknowledgement The 2nd author acknowledges the financial assistance provided by the All India Council for Technical Education (AICTE), New Delhi, void reference: CAYT File No1-51/FD/CA/18/2007-08 dated 31/03/2008.
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