Electric heating – Metal heating – Cutting or disintegrating
Reexamination Certificate
2001-10-11
2003-12-09
Evans, Geoffrey S. (Department: 1725)
Electric heating
Metal heating
Cutting or disintegrating
Reexamination Certificate
active
06660957
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a power supply apparatus for electric discharge machining. More particularly, this invention to a transistor type power supply apparatus for electric discharge machining, which can generate an intermittent pulse current using a semiconductor switching element.
BACKGROUND ART
Conventionally, there is known a power supply apparatus for an electric discharge machine which supplies an intermittent pulse current to a working distance formed between an electrode and a workpiece via a working fluid, and carries out electric discharge machining while controlling a relative position between the electrode and the workpiece by numerical control. The transistor type power supply apparatus for electric discharge machining is, for example, a representative of the above-mentioned power supply apparatus. This transistor type power supply apparatus for electric discharge machining generates an intermittent pulse current by a semiconductor switching element repeating an on-off operation.
This type of power supply apparatus for electric discharge machining will be described below with reference to FIG.
7
(
a
) and FIG.
7
(
b
). FIG.
7
(
a
) shows a circuit configuration of the conventional power supply apparatus for electric discharge machining, and FIG.
7
(
b
) shows a drive control system thereof.
The above power supply apparatus for electric discharge machining has a switching circuit for supplying a pulse current to a workpiece W and an electrode E. This switching circuit includes the first switching circuit
20
and the second switching circuit
30
connected parallel with each other.
The first switching circuit
20
is composed of the direct current voltage source V
21
, semiconductor switching elements S
21
, S
22
, S
23
and S
24
such as a FET or the like, and the current limiting resistor R
21
. On the other hand, the second switching circuit
30
is composed of the direct current voltage source V
31
, semiconductor switching elements S
31
and S
32
, and diodes D
31
and D
32
.
In FIG.
7
(
a
), L
21
, L
22
, L
31
and L
32
denote a stray inductance of circuit, and C
11
denotes a stray capacitance.
A drive control system of the power supply apparatus for electric discharge machining includes a discharge detecting circuit
31
, an oscillation control circuit
32
, a drive circuit
33
and a drive circuit
34
. In this case, the drive circuit
33
drives and controls the semiconductor switching elements S
21
, S
22
, S
23
and S
24
of the above first switching circuit
20
. On the other hand, the drive circuit
34
drives and controls the semiconductor switching elements S
31
and S
32
of the above second switching circuit
30
.
Subsequently, operation of the power supply apparatus for electric discharge machining will be explained below. Assuming that a gap between the electrode E and the workpiece W (“between the electrodes”) is such that discharge or short-circuit does not occur, and when the switching elements S
22
and S
23
are turned off while the switching elements S
21
and S
24
are turning on, a voltage of the direct current voltage source V
21
appears between the electrodes. Simultaneously, the stray capacitance C
11
of the circuit is charged by the voltage of the direct current voltage source V
21
. A distance between the electrode E and the workpiece W is controlled by a numerical control device (not shown) and a servo drive control device so that a discharge is generated between the electrodes. When a discharge is generated by an output voltage of the direct current voltage source V
21
, first, a charge charged in the stray capacitance C
11
of the circuit is discharged as capacitor to the inter-electrode, and thereby, a discharge start current Ic flows through there. By doing so, a conductive path is formed in the inter-electrode.
In order to maintain the conductive path thus formed, a current must be continuously supplied to the inter-electrode after the charge of the stray capacitance C
11
of the circuit has been fully discharged; therefore, the switching elements S
21
and S
24
are kept as they are turned on.
From the direct current voltage source V
21
, a discharge holding current I
R
flows to the resistor R
21
, switching element S
21
, circuit inductance L
21
, workpiece W, electrode E, circuit inductance L
22
, switching element S
24
and direct current voltage source V
21
in succession, and thereby, the conductive path formed between the electrodes is maintained. In this case, the discharge holding current I
R
flows through the resistor R
21
; therefore, the maximum value of the discharge holding current I
R
is limited to I
R
(max)=V
21
/R
21
by the resistor R
21
.
The discharge holding current I
R
is a relatively small current, and it is too weak for machining. Therefore, the discharge holding current I
R
has a function as pre-discharge current for supplying a large-current discharge machining current I
S
, which will be described latter.
Moreover, when turning off the switching elements S
21
and S
24
while turning on the switching elements S
22
and S
23
, the above operation is carried out in a pattern of reversing a polarity of output voltage and current with respect to the gap between the electrodes.
The discharge holding current I
R
is a current appearing in between the electrodes at the same time with the generation of discharge. On the other hand, the large-current discharge machining current I
S
is supplied between the electrodes after the generation of discharge is detected. In this case, the large-current discharge machining current I
S
is output between the electrodes in a state of being delayed for a certain time from the first generation of discharge, as described latter.
The discharge detecting circuit
31
detects a drop of voltage between the electrodes (“inter-electrode voltage”) by the generation of discharge between the electrodes, and gives an instruction of large-current output to the oscillation control circuit
32
. The oscillation control circuit
32
outputs a pulse signal having a time width set by a machining state between the electrodes to the drive circuit
34
. The drive circuit
34
simultaneously drives on (turns on) the switching elements S
31
and S
32
only for the time width set in the oscillation control circuit
32
.
When the switching elements S
21
, S
24
, S
31
and S
32
are all in an on state, a circuit is formed such that a plurality of direct current voltage sources having different voltage is connected. For this reason, there is a possibility of breaking down these elements of the circuit by a potential difference including a serge voltage. Thus, in the case of turning on the switching elements S
31
and S
32
, the switching elements S
21
and S
24
are turned off as safety measures.
The switching elements S
31
and S
32
are simultaneously turned on, and thereby, from the direct current voltage source V
31
, the large-current discharge machining current I
S
flows to the switching element S
31
, circuit inductance L
31
, workpiece W, electrode E, circuit inductance L
32
, switching element S
32
and direct current voltage source V
31
in succession.
When no pulse signal is output from the oscillation control circuit
32
, the drive circuit
34
drives off the switching elements S
31
and S
32
. The discharge machining current I
S
continuously flows through the circuit by the induction of the circuit inductances L
31
and L
32
; however, it is fed back and regenerated to the direct current voltage source V
31
via the diode D
32
, circuit inductance L
31
, workpiece W, electrode E, circuit inductance L
32
, diode
31
and direct current voltage source V
31
.
FIG. 8
shows a waveform of discharge machining current obtained by the above operation in the conventional power supply apparatus and an output timing of each control signal. In
FIG. 8
, V
WE
denotes the inter-electrode voltage, and I
C
denotes a discharge start current by capacitor discharge of the stray capacitance C
11
of circuit. Further, I
R
denotes a discharge holding cu
Iwata Akihiko
Ohguro Hiroyuki
Satou Seiji
Suzuki Akihiro
Evans Geoffrey S.
Mitsubishi Denki & Kabushiki Kaisha
Sughrue & Mion, PLLC
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