Electric heating – Metal heating – Cutting or disintegrating
Utility Patent
1998-06-12
2001-01-02
Evans, Geoffrey S. (Department: 1725)
Electric heating
Metal heating
Cutting or disintegrating
C219S069180, C219S069190
Utility Patent
active
06169261
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to machine tools, and more particularly, to an apparatus and method for generating an electric discharge in an alternating direction on a workpiece for line cutting of the workpiece.
2. Description of Related Art
An electric-discharge cutting apparatus is a machine tool that can generate and apply an electric discharge onto a workpiece for line cutting of the workpiece. Conventionally, the electric-discharge cutting apparatus utilizes a DC (direct current) voltage source for ignition and arc generation. One example of such an electric-discharge cutting apparatus is shown in FIG.
1
. As shown, the conventional electric-discharge cutting apparatus is used to generate and apply an electric discharge for use in the line cutting of a workpiece
5
. The electric-discharge cutting apparatus includes a pulse generator
1
, a high-voltage source
2
, a low-voltage source
3
, and an electrode
4
. In more detail, the high-voltage source
2
, a low-voltage source
3
, and an electrode
4
. In more detail, the high-voltage source
2
is composed of a DC voltage source
11
for supplying a DC voltage V
2
and a switch S
2
10
connected to the negative end of the DC voltage source
11
. The low-voltage source
3
is composed of a DC voltage source
7
for supplying a DC voltage V
1
, a switch S
1
6
connected to the negative end of the DC voltage source
7
, a resistor
8
, and a diode
9
.
In operation, the pulse generator
1
generates a pulsed control signal that can cause the switch S
1
6
to be switched into the conducing state, thereby allowing the DC voltage V
1
from the negative end of the voltage source
7
to be conducted successively through the switch S
1
6
, the resistor
8
, and the diode
9
to the electrode
4
. If the potential difference between the electrode
4
and the workpiece
5
is lower than a reference voltage, it indicates that the ignition is successful. After this, the pulse generator
1
turns off the switch S
1
6
and then turns on the switch S
2
10
, allowing the voltage from the negative end of the voltage source
11
to be conducted through the switch S
2
10
to the electrode
4
, thereby causing the electrode
4
to generate and apply an arc onto the workpiece
5
for line cutting. After a preset period, the switch S
2
10
is turned off so as to terminate the arc for a preset duration. After this, the switch S
1
6
is turned on again for the next cycle of ignition.
In the low-voltage source
3
, the resistor
8
is used for current-limiting purpose to prevent an excessive discharging current at the electrode
4
. The diode
9
can prevent the negative voltage V
2
from the DC voltage source
11
from entering into the low-voltage source
3
. In this electric-discharge cutting apparatus, since both the voltage source
7
in the low-voltage source
3
and the voltage source
11
in the high-voltage source
2
are DC-voltage source, the voltage applied to the electrode
4
is always negative in polarity, causing the workpiece
5
also to be always negative in polarity when the electric discharge is being applied to is. Where water is used as the insulating liquid in the operation of the workpiece
5
, the workpiece
5
is subjected to electrolysis, which degrades the surface quality of the workpiece
5
and reduces the useful life of the molding.
SUMMARY OF THE INVENTION
It is therefore an objective of the present invention to provide an electric-arc cutting apparatus and method, which can generate an electric discharge based on a given ignition.
It is another objective of the present invention to provide an electric-discharge cutting apparatus and method, which can generate an electric discharge in an alternating direction so as to prevent the workpiece from being subjected to electrolysis that would otherwise occur if the electric arc is generated from DC voltage sources only.
In accordance with the foregoing and other objectives of the present invention, a new electric-discharge cutting apparatus and method is provided. The electric-discharge cutting apparatus and method of the invention, in particular, can generate and apply an electric discharge in an alternating direction onto the workpiece for line cutting of the workpiece.
The method of the invention includes the following steps of:
(1) performing a low-voltage ignition process at the discharge gap; if the total time of using the same polarity for ignition is still less than a preset time T
4
, then using the current polarity for the next cycle of ignition is used; otherwise, the polarity is inverted to the opposite;
(2) detecting the polarity of the current low-voltage ignition; if negative, applying a negative ignition voltage to the electrode; otherwise, if positive, applying a positive ignition voltage to the electrode;
(3) detecting the gap voltage at the discharges gap based on a first reference time T
1
and a second reference time T
2
where T
1
<T
2
, and comparing the detected gap voltage V
gap
with a preset reference voltage range between a first reference voltage V
ref1
and a second reference voltage V
ref2
;
if during T
1
the gap voltage V
gap
is within the range between
Vref1
and V
ref2
, it indicates that the discharge gap is currently operating under arc discharging conditions;
if during the period between T
1
and T
2
the gap voltage V
gap
is within the range between
Vref1
and V
ref2
it indicates that the discharge gap is currently operating under normal discharging conditions; and
if beyond T
2
the gap voltage V
gap
is still beyond the range between
Vref1
and V
ref2
, it indicates that the discharge gap is currently operating under open circuit conditions;
(4) in the event of normal or arc discharging conditions, the discharging voltage is applied to the discharge gap for a predetermined on-time, and the arc-on-time is set to be half of the normal-on-time; and after the arc discharge is completed, the discharging voltage is switched off for a predetermined off-time; and
(5) in the even of open-circuited conditions, the discharging voltage to be applied is withheld from the discharge gap for an off-time T
3
, and the polarity for the next cycle of ignition is inverted in order to apply the ignition voltage in an alternating direction.
The apparatus of the invention includes an electrode for applying an electric discharge onto the workpiece; a negative ignition voltage source for supplying a negative ignition voltage to the discharge gap between the electrode and the workpiece to test the condition thereof; a positive ignition voltage source for supplying a positive ignition voltage to the discharge gap between the electrode and the workpiece to test the condition thereof; a discharging voltage source for supply a discharging voltage to the discharge gap between the electrode and the workpiece to generate an electric discharge for line cutting of the workpiece; and a pulse generator for generating a plurality of control signals in response to the current discharging condition at the discharge gap for controlling the negative ignition voltage source; the positive ignition voltage source; and the discharging voltage source. Prior to the discharging voltage source applying a discharge voltage to the discharge gap, the negative and positive ignition voltage sources are connected to the electrode to detect whether the current discharging conditions at the discharge gap are normal, arc, or open-circuited. The power levels of the discharging voltage are then adjusted accordingly and after which the electric discharge is applied onto the workpiece in an alternating direction.
The foregoing apparatus and method of the invention can be used to generate an electric discharge in an alternating direction that allows the benefits of preventing the workpiece from being cut apart due to excessive power and also prevents the electrolysis that can degrade the surface quality of the workpiece.
REFERENCES:
patent: 3997753 (1976-12-01), Inoue
patent: 4504722 (1985-03-01), Kishi et al.
patent: 4806719 (1989-02-0
Day Yea-June
Lee Hsiang Kuo
Li Hsi-Pin
Evans Geoffrey S.
Industrial Technology Research Institute
Smith , Gambrell & Russell, LLP
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