CIRCUIT DESIGN FOR ELECTRICAL DISCHARGE MACHINING
Janardhanan Niranjan Srinivasan
Department of Electrical and Computer Engineering
Mentor: Prof. Mustafizur Rahman
Department of Mechanical Engineering
National University of Singapore
Singapore – 119576
INTRODUCTION
Background – Electrical Discharge Machining
Electrical Discharge Machining is an effective non-contact machining process. It is a precision metal
removing process using an accurately controlled electrical discharge (spark) in a dielectric liquid.1
Process: Electro-Discharge Machining (EDM) involves two electrodes - a microelectrode of a
predetermined shape that needs to be fabricated and the work piece on which the machining is to be carried
out. A series of voltage pulses are applied between the electrodes, which are immersed in a dielectric
liquid. The arcs erode the work piece forming a cavity in the exact shape of the electrode.
Circuit: The electrical parts of the machine include a power supply, an EDM circuit and a motion control
circuit. The EDM circuit is a simple RC circuit. There are generally a number of resistors and capacitors to
choose from. A transistor toggles the EDM circuit at high frequencies. When the circuit is open, the
capacitor gets charged from the RC circuit. When the circuit is closed, the capacitor discharges forming an
electric arc across the electrodes.
The controller circuits that are currently in use commercially are normally complex and inconvenient to the
end user, thus the need for a simple and user-friendly circuit design for the EDM, which is accessible to the
small-scale buyer.
Objective – EDM Circuit
•
•
To come up with the complete circuit design for the EDM.
The EDM circuit should comprise the following parts:
1. Power Supply circuit or Transformer circuit.
2. RC circuit.
3. Relay board to facilitate choosing the resistors and capacitors in the RC circuit.
4. CPU circuit to control the relays and the transistor ON time.
5. Feedback Circuit to send the signal back to the computer for analog feedback.
6. Power Indicator Circuit
SOFTWARE USED
The circuits were designed using OrCAD Capture for Windows version v7.00.
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RESULTS AND DISCUSSION
The complete circuit diagram was developed based on the simple RC circuit. It was repeatedly analyzed
and modified to suit requirements. After many iterations and a lot of optimization, the circuit diagram was
finalized as shown in figures 1 through 5. The various circuit blocks are explained below.
1. EDM Transformer – Resistor Circuit
This part of the circuit consists of the transformers, a bridge rectifier and the resistors of the RC Circuit.
From the normal 230 V ac input voltage, we can choose the voltage from the transformer using the two
relays R1 and R2. Using a simple logic as shown in table 1.
There is also a provision to use an external power supply. This voltage is then converted to dc value across
capacitor C1 by passing it through a rectifier. C1 now becomes our voltage source for the RC circuit. This
voltage passes through the resistor before charging the capacitor. The resistors can be chosen using relays
Re5 – Re7. The logic involved is as shown in table 2.
R1
R2
L
H
L
H
L
L
H
H
Relay 5
L
L
L
L
H
H
H
H
AC
Voltage
55 V
110 V
165 V
220 V
Table 1: Voltage Selection Logic.
(above)
Table 2: Resistor Selection Logic
(right)
Relay 6
L
L
H
H
L
L
H
H
Relay 7
L
H
L
H
L
H
L
H
Resistance
R7
R6
R5||R7
R5||R6
R2||R4
R2||R3
R1||R4
R1||R4
R1
INPUT
VOLTAGE
AC 230 V
Transformers
100 VA
Choosing
the
Voltage
Choosing
Resistors
Ext/Int Source
S3
S1
R3
GND
Relay 4
Ext V
Relay 1
+24v
Relay 6
S4
LN1
LN2
S6
R2
EDM ON/OFF
100 VA
S5
Transistor
S7
J1
1
2
3
4
C-Box
R4
Relay 7
-
+
Relay 5
R5
Feedback
R6
C1
S2
Bridge Rectifier
R7
Relay 2
1
Figure 1: EDM Transformer – Resistor circuit
2. EDM Power Indicator Circuit
This circuit indicates the amount of voltage across the capacitor at any point of time. It uses a 20-element
bar display to indicate the voltage level and alert the user of high voltages.
2
Display
EDM
Transistor
On/Off
1
2
3
4
S10
S8
C18
C17
C13
R29
C16
C-Box
R30
C15
R28
R27
1
2
+24v
J3
J1
C14
R26
Driver
Display
Driver
1
2
3
J4
Choosing the Capacitor
1
2
3
+24v
10-seg display
+24v
+24v
10-seg display
J3
Q4
R31
2
3
Figure 2: Power Indicator Circuit
Figure 3: Capacitor Box circuit
3. EDM Capacitor Box Circuit
The Capacitors of the RC circuit are shown in this circuit. Also shown is a buzzer circuit, which facilitates
easy positioning of the machine tool.
One of the most important parts of the EDM is the MOSFET (Metal Oxide Semiconductor Field Effect
Transistor) that features in this part of the circuit. It completes the circuit of the EDM when it is in the ON
state. This transistor is given a clock pulse input in the range of a few kHz so that it acts as a continuous
toggle switch to the EDM circuit. A MOSFET transistor is used since it turns ON very quickly.
4. Analog Feedback Circuit
+24v
Average Voltage Feedback
-
+
R12
C2
J2
C3
R14
1
2
3
D1
Computer
R13
R11
+
R10
R8
-
+24v
Minimum Voltage Feedback
+24v
-
Feedback
+
R9
4
Figure 4: Analog Feedback Circuit
The signal in the RC circuit is sampled and taken out using the usual amplifier circuitry and fed back into
the computer for signal analysis. Both the average voltage and the minimum voltage are fed back to the
computer. Provisions can be made to feed back the maximum voltage as well.
5. The CPU
3
5
Relay 1
Relay 2
Relay 4
Relay 5
Relay 6
Relay 7
5V
16
3
2
4
5
11
10
13
9
15
5V
C7 5V
6
C6
14
7
12
8
C8
40
39
38
37
36
35
34
33
32
31
30
29
28
27
26
25
24
23
22
21
Microprocessor
C4
1
Interface to the Computer
C5
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
1
2
3
4
5
6
7
8
22
23
19
20
21
24
5V
C12
9
11
10
Timer IC
C11
15
14
13
18
16
17
12
Output Pulse
Current
Booster
Circuit
Transistor
C9
Figure 5: The CPU Circuit
The processor used in the circuit is an 8-bit microcontroller. This chip controls the relays and the timer
control chip, which sends out a clock pulse. The frequency and the duty cycle of this pulse are controlled
by the user using the microcontroller. This clock pulse is passed through a current booster circuit and then
sent to the MOSFET in the capacitor Box.
CONCLUSION
The Electro – Discharge Machine circuit design was completed successfully. The various parts of the
circuit – the main RC circuit, the level indicator circuit and the CPU – were all tested separately and they
worked without hitches.
ACKNOWLEDGEMENTS
I would like to express my sincere feeling of gratitude to the following people:
• My main supervisor, Prof. Mustafizur Rahman for his continuous support and encouragement.
• My co-supervisors Dr. Lim Han Seok and A/P A. Senthil Kumar for their valuable guidance and
advice.
• My seniors Mr. Golam Kibria Chowdhury and Mr. Abu Bakar Md. Ali Asad for their technical
help and assistance.
• The staff of Advance Manufacturing Laboratory, NUS for their services throughout the course of
this project.
REFERENCES
1.
2.
3.
Development of a Hybrid System for Micro-Machining, Koh Ngiap Hong, Mark, Dept. of
Mechanical Engineering, National University of Singapore, 2002-03.
Modern Manufacturing Processes, James A. Brown, New York Industrial Press, c1991.
Manufacturing processes for Technology, William O. Fellers & William W. Hunt, Englewood
Cliffs, N.J., Prentice Hall, c1995.
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