Electric heating – Metal heating – Cutting or disintegrating
Reexamination Certificate
1999-06-04
2001-04-03
Evans, Geoffrey S. (Department: 1725)
Electric heating
Metal heating
Cutting or disintegrating
C219S069180
Reexamination Certificate
active
06211481
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an electric discharge machining apparatus for machining a workpiece by applying electrical pulses to a machining gap formed between the workpiece and a tool electrode, and particularly to an electric discharge machining apparatus provided with a switching circuit for generating high frequency a.c. voltage pulses from a d.c. power source.
BACKGROUND OF THE INVENTION
When power is applied from a d.c. or a.c. power supply to a small gap formed between a tool electrode of an electric discharge machining apparatus and a conductive workpiece, simply known as a “machining gap” or “gap”, the resistance of dielectric fluid across the gap is reduced. Then, when the insulation properties of the dielectric fluid are broken down, an electric discharge is generated and machining “on-time” begins. During a controlled on-time, a discharge current flows through the gap resulting in vaporization or melting of the workpiece material. When the on-time is completed, application of power is suspended resulting in controlled “off-time”, in order to restore the insulation properties of the dielectric fluid. A decrease in the on-time, in other words a reduction in energy for one electric discharge, is known to contribute to an improvement in surface roughness.
U.S. Pat. No. 5,149,931 discloses an electric discharge machining method for causing the surface roughness of a workpiece to be reduced to 1 mRmax or less by applying high frequency a.c. voltage from an a.c. power source to the gap.
This patent also describes that changing the polarity of the voltage applied to the machining gap for each electric discharge and distributing the position at which electric discharge occurs contributes to a good quality machined surface.
FIG. 3
illustrates a power supply device for use in an electric discharge machining apparatus that generates a.c. voltage pulses from a d.c. power source. The power supply includes a d.c. power source
2
for outputting a d.c. voltage E, an output capacitor C and a bridge circuit
3
. These electrical components are usually housed in a cabinet which also includes a controller which controls the on-time and off-time, and which is positioned at a distance away from the workpiece
61
and the wire electrode
62
. In the illustrated embodiment, the bridge circuit
3
is connected through a low capacitance cable
7
to the workpiece
61
and the wire electrode
62
. As shown in the drawing, the bridge circuit
3
comprises switching transistors
31
,
32
,
33
and
34
connected in series so as to form four nodes
3
A,
3
B,
3
C and
3
D. One pair of diagonally opposite nodes
3
A and
3
B are respectively connected to positive and negative terminals of the d.c. power source
2
. The other pair of nodes,
3
C and
3
D, are respectively connected to the workpiece
61
and the wire electrode
62
. A current limiting resistor
35
is connected between the nodes
3
A and
3
C, and a current limiting resistor
36
is connected between nodes
3
B and
3
D. A controller
4
generates a gate control pulse signal PA for controlling the on/off switching operation of one pair of switching transistors
31
and
34
, and a gate control pulse signal PB for controlling the on/off switching operation of the other pair of switching transistors
32
and
33
. The controller
4
generates the pulse signals PA and PB to alternately switch on and off the pair of switching transistors
31
and
34
and the pair of switching transistors
32
and
33
. As a result, an a.c. pulse voltage PV inverted in polarity at the same frequency as the pulse signal PA across the nodes
3
C and
3
D of the bridge circuit
3
is applied as a voltage V to a gap G, formed between the workpiece
61
and the wire electrode
62
, to machine the workpiece
61
.
Since the surface roughness of the workpiece
61
becomes smaller as the frequency of the a.c. pulse voltage VP increases, MOSFETs that have a high operating speed are often used as the switching transistors
31
,
32
,
33
and
34
. In order to operate these MOSFETs at a high frequency, i.e., on the order of a few MHz, a MOSFET of at least 50 W is necessary. Thus is true even in the lowest frequency case, taking into consideration the effects of stray capacitance and distributed inductance, etc. of the circuit from the nodes
3
C and
3
D to the gap G. Also, the rated voltage of a MOSFET for supplying a voltage of 50-100V necessary to generate electric discharge at the gap G is preferably at least 200V. Because of these requirements, a MOSFET for a bridge circuit for generating a high frequency a.c. voltage pulse from a d.c. power source has an input capacitance of 700-1600 pF. The relationship between the input capacitance C
in
and power loss P
d
occurring as a result of driving the MOSFET is given below where V
GS
is a voltage across the gate and source of the MOSFET, f is the frequency of the a.c. voltage pulses:
P
d
=C
in
·f·V
GS
2
(1)
Accordingly, if, for example, C
in
=700 pF, V
GS
=20 V and f=5 MHz, power loss will be 1.4 W. The rated value of a commercial DIP (Dual In-line Package) is less than 1 W. This means that if the drive loss is 1.4 W, the DIP will need to be fitted with a heat dissipation device, such as fins. However, the surface area occupied by such cooling fins on the printed substrate of the DIP is relatively large and as a result the physical distance between the drive circuit and the MOSFET is increased. There is also undesirable series resonance caused by the inductance between the drive circuit and the MOSFET and the input capacitance (C
in
) of the MOSFET.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a power supply device for generating high frequency a.c. voltage pulses without increasing the surface area of the switching transistor drive circuit on the printed substrate.
Additional objects, advantages and novel features of the invention will be set forth in the description that follows, and in part will become apparent to those skilled in the art upon reading this description or practicing the invention. The objects and advantages of the invention may be realized and attained by practicing the invention as recited in the appended claims.
In order to achieve the above described objects, and in according with the present invention, there is provided a power supply for an electric discharge machining apparatus; the power supply comprises a d.c. power source, a bridge circuit having a first pair of switches and a second pair of switches, and a controller for alternately turning on either the first or second pair of switches so that high frequency a.c. voltage pulses are applied from a d.c. power source to a gap between a workpiece and a tool electrode. The four switches include at least two switching transistors connected in parallel, and the controller alternately turns on at least two transistors for each of the four switches.
REFERENCES:
patent: 5149931 (1992-09-01), Magara
patent: 5317122 (1994-05-01), Ito et al.
patent: 5416290 (1995-05-01), Magara et al.
patent: 5488329 (1996-01-01), Ridgers
patent: 5874703 (1999-02-01), Derighetti et al.
patent: 5986232 (1999-11-01), Kaneko et al.
patent: 61-4620 (1996-04-01), None
Kaneko Yuji
Toyonaga Tatsuo
Devinsky Paul
Evans Geoffrey S.
McDermott & Will & Emery
Sodick Co. Ltd.
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