Electric heating – Metal heating – Cutting or disintegrating
Reexamination Certificate
2000-04-06
2001-08-28
Riley, Shawn (Department: 2838)
Electric heating
Metal heating
Cutting or disintegrating
C307S075000
Reexamination Certificate
active
06281463
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an electric discharge machine for machining a workpiece by causing an electric discharge in a work gap formed between the workpiece and a tool electrode; more and particularly, the present invention relates to a power supply device for supplying power to the work gap for machining the workpiece.
BACKGROUND OF THE INVENTION
FIG. 5
shows a conventional power supply for an electric discharge machine. A work gap
6
formed between a tool electrode
4
and a conductive workpiece
2
is filled with dielectric fluid, and the size of the work gap
6
is maintained at a fixed microscopic value. A D.C. power supply
8
, a current limiting resistance
12
and a switching element
10
are connected in series and to the work gap
6
. The switching element
10
is closed when a gate pulse signal G
10
is ON, and is open when the gate pulse signal G
10
is OFF. The gate pulse signal G
10
has a controlled ON time and a controlled OFF time. When voltage is supplied from the D.C. power supply
8
to the work gap
6
by the switching element
10
being closed, the resistance of the dielectric fluid in the work gap
6
is reduced and electric discharge starts. When discharge current flows through the work gap
6
, material of the workpiece
2
is removed. The peak value of the discharge current is determined by the current limiting resistance
12
. When a set time has elapsed from detection of start of electric discharge, the gate pulse signal G
10
becomes OFF and electric discharge is terminated.
U.S. Pat. No. 5,534,675 discloses the circuit of
FIG. 6
containing substantially no current limiting resistance or inductance in order to conserve power. The circuit of
FIG. 6
comprises a comparator
20
for comparing a voltage at the two ends of a current detecting resistor
16
with a reference voltage
18
, and an AND gate
21
for supplying an output that has an ON state when the output from the comparator
20
and the gate pulse signal G
10
are both ON to the switching element
10
. As shown in
FIG. 7A
, if the gate pulse signal G
10
becomes ON at time t
1
, a voltage of 50V is supplied from the D.C. power supply
8
to the work gap
6
.
FIGS. 7C and 7D
respectively show a voltage Vgap and a current Igap across the work gap in FIG.
6
. When electric discharge starts at time t
2
, the current Igap starts to flow through the work gap
6
, as shown in FIG.
7
D. When the current flowing in the current detecting resistor
16
exceeds the set value at time t
3
, the output of the comparator
20
becomes OFF and the output of the AND gate
21
also becomes OFF. As a result, the switching element
10
is opened, no current at all flows in the current detecting resistor
16
, and so the outputs of the comparator
20
and the AND gate
21
both become ON again. As shown in
FIG. 7B
, the AND gate
21
generates high frequency pulses from time t
3
until time t
4
when the gate pulse signal G
10
goes OFF. During the period from t
3
to t
4
, the pulsed current Igap is maintained at a peak value determined by the reference voltage
18
, while the voltage Vgap is physically maintained at 30V.
FIGS. 7E and 7F
show voltage Vgap and current Igap across the work gap
6
when a higher voltage D.C. power supply
8
of 90V is used. As shown in
FIG. 7F
, current pulses Igap reach a set peak value a very short time &Dgr;t after time t
2
when electric discharge starts. In this way, if the slope of the rising edge of the current pulse Igap is made steeper, the machining rate is increased. However, if the D.C. power supply
8
is made a higher voltage, current ripple become larger and higher in frequency. If ripple of the current pulse Igap is large, it is probable that the current pulse Igap will be terminated before the set time. High frequency ripple has the undesirable effects of increasing heat generated in the switching element
10
. Also, at the time of electric discharge a voltage difference between the voltage of the D.C. power supply
8
and the voltage across the work gap
6
is expended on the switching element
10
etc., which means that a higher voltage D.C. power supply
8
outweighs the energy conservation effects of the circuit of FIG.
6
.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a power supply for an electric discharge machine that conserves energy and supplies current pulses having a steep rising edge and small ripple to a work gap.
In order to achieve the above and other objects, a first aspect of a power supply device for an electric discharge machine according may comprise
a parallel circuit in which a first power supply (
8
), a second power supply (
36
) are connected in parallel, the second power supply switchably supplying a larger voltage and a smaller current than the first power supply to the work gap (
6
).
Another aspect of a power supply device for an electric discharge machine according to the present invention may comprise
first and second power supplies (
36
) and (
8
) supplies connected in parallel with the work gap (
6
);
the second power supply applying a larger voltage than the first power supply to the work gap;
switching elements (
10
,
10
A), (
40
) for controlling current flow from the first and second power supplies to the work gap;
a detector (
42
) for detecting the occurrence of electric discharge; and
a controller (
44
)connected to the detector for controlling the switching elements so that current flows from the first and second power supplies to the work gap for a time period communicating with the start of electric discharge but current flow from the second power supply to the work gap only for a relatively time period.
Additional objects, advantages and novel features of the invention will be set forth in the description that follows, and 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.
REFERENCES:
patent: 3532850 (1970-10-01), Schulz et al.
patent: 3604885 (1971-09-01), Inoue
patent: 3832510 (1974-08-01), Pfau et al.
patent: 3892936 (1975-07-01), Kobayashi et al.
patent: 5534675 (1996-07-01), Kaneko et al.
patent: 5539178 (1996-07-01), Taneda et al.
Hayashi Hidetaka
Kaneko Yuji
Yamada Kuniharu
Devinsky Paul
McDermott & Will & Emery
Riley Shawn
Sodick Co. Ltd.
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