Electric heating – Metal heating – Cutting or disintegrating
Reexamination Certificate
2001-05-04
2003-10-07
Evans, Geoffrey S. (Department: 1725)
Electric heating
Metal heating
Cutting or disintegrating
C219S069130
Reexamination Certificate
active
06630641
ABSTRACT:
FIELD OF THE INVENTION
The present invention in general relates to an electric discharge machining apparatus that generates an electric discharge between a workpiece and an electrode so as to carry out machining with respect to the workpiece. More particularly, this invention relates to an electric discharge machining apparatus which can perform machining at high speed.
BACKGROUND OF THE INVENTION
As a conventional electric discharge machining apparatus, there have been known a wire electric discharge machine carrying out wire electric discharge machining and a die sinking electric discharge machine carrying out die sinking electric discharge machining. In wire electric discharge machining, a conductive wire is used as an electrode so as to carry out machining; on the other hand, in die sinking electric discharge machining, an electrode having various shapes is used so as to carry out machining.
FIG. 23
is a view schematically showing a construction of a conventional wire electric discharge machine.
The conventional wire electric discharge machine includes: a conductive wire
51
used as an electrode; a voltage applying circuit
53
for applying a rectangular voltage pulse between the wire
51
and a workpiece
52
; a feeder cable
54
a
for connecting the workpiece
52
and the voltage applying circuit
53
; a feeder cable
54
b
for connecting the wire
51
and the voltage applying circuit
53
, and a feeder terminal
55
. Further, the conventional wire electric discharge machine includes: a feed reel
56
for feeding the wire
51
to the workpiece
52
side; a winding reel
57
for winding up the fed wire
51
; a brake
58
for stopping feed and winding of the wire
51
; a winding roller
59
for feeding the wire
51
to the winding reel
57
; a cross table
60
for fixing the workpiece
52
, and an X-axis motor
61
for moving the cross table
60
to a predetermined X-axis direction.
Moreover, the conventional wire electric discharge machine includes: a Y-axis motor
62
for moving the cross table
60
to a Y-axis direction perpendicular to the X-axis direction; a servo circuit
64
for driving the X-axis motor
61
and the Y-axis motor
62
via motor control cables
63
a
and
63
b
; a control circuit
65
, which outputs a control signal to the servo circuit
64
, and moves the cross table
60
and the workpiece
52
so as to control a machining position. Further, the conventional wire electric discharge machine includes: a working fluid tank
66
, which is filled with a working fluid; a pump
67
for pumping a working fluid out of the working fluid tank
66
; a working fluid supply pipe
68
a
for supplying a working fluid from the working fluid tank
66
to the pump
67
; a working fluid supply pipe
68
b
for supplying a working fluid from the pump
67
to the workpiece
52
side, and a guide
69
for feeding the wire
51
to the workpiece
52
side.
In the conventional wire electric discharge machine, the voltage applying circuit
53
applies a rectangular voltage pulse between the wire
51
and the workpiece
52
via the feeder cables
54
a
and
54
b
and the feeder terminal
55
. By doing so, an electric discharge occurs between the wire
51
and the workpiece
52
, and a part of the workpiece
52
is removed by this electric discharge. Subsequently, the workpiece
52
is moved so as to remove a desired portion of the workpiece
52
, and thereby, the workpiece is formed into a desired shape. In this case, by the electric discharge, a part of the workpiece
52
is removed while the surface of the wire
51
is being removed. For this reason, when the same portion of the wire
51
is continuously used, the wire
51
wears out. In order to prevent a breakdown of the wire, in the wire electric discharge machine, a portion of the wire
51
, where no electric discharge is applied, is fed to the workpiece
52
in succession, and then, machining is carried out while the portion, where electric discharge has been already applied, is wound up in succession.
The feeding of the wire
51
is carried out by the feed reel
56
via the brake
58
and the guide
69
; on the other hand, the winding of the wire
51
is carried out by the winding reel
57
via the winding roller
59
. The cross table
60
is used to fix the workpiece
52
. The X-axis motor
61
and the Y-axis motor
62
two-dimensionally move the cross table
60
. An NC device comprising the control circuit
65
and the servo circuit
64
drives the X-axis motor
61
and the Y-axis motor
62
so that the cross table
60
and the workpiece
52
are moved so that machining position is controlled. The working fluid tank
66
is filled with de-ionized water as a working fluid. The pump
67
pumps up a working fluid of the working fluid tank
66
via the working fluid supply pipe
68
a
, and then, supplies the working fluid to a discharge field via the working fluid supply pipe
68
b.
FIG. 24
is a view showing a configuration of the voltage applying circuit
53
shown in FIG.
23
. The voltage applying circuit
53
includes a resistor
74
, a switch SW
51
, a direct current constant voltage source
75
, and a switch SW
52
. More specifically, the resistor
74
has one end connected to the wire
51
via the voltage applying circuit and an inductance
73
included in a current path, and one end of the switch SW
51
is connected to the other end of the resistor
74
. The direct current constant voltage source
75
is constructed in a manner of connecting the other end of the switch SW
51
to a high voltage side, and connecting the workpiece
52
to a low voltage side. The switch SW
52
is interposed between the other end of the resistor
74
and the workpiece
52
. The direct current constant voltage source
75
generates a predetermined voltage. The resistor
74
is additionally provided for limiting a discharge current. The switch SW
51
is a switch for increasing a voltage between the wire
51
and the workpiece
52
(hereinafter, referred simply to as interelectrode); on the other hand, the switch SW
52
is a switch for setting a voltage of the inter-electrode to 0V. For example, a field effect transistor (FET) is used as each of these switches.
FIG. 25
is a view showing an operation of a conventional voltage applying circuit
53
. In the operation of the voltage applying circuit
53
, first, the circuit operation is changed from a state in which the switch SW
51
is turned off and the switch SW
52
is turned on to a state in which the switch SW
51
is turned on and the switch SW
52
is turned off. At this time, a voltage rises between the resistor
74
side P
51
of the switch SW
52
and the workpiece side P
52
of the switch SW
52
, and thus, an interelectrode voltage rises. An interelectrode static capacitance and a value of the inductance
73
are considerably small as compared with the resistor
74
; therefore, when the switch SW
51
is turned on, an interelectrode voltage rises at a extremely high speed.
After a discharge time lag td (described later) elapsed, an interelectrode discharge starts in the middle of voltage pulse application, and then, a discharge current flows through the interelectrode, and thereby, an interelectrode voltage decreases. Thereafter, when the switch SW
51
is turned off and the switch SW
52
is turned on, the voltage between P
51
and P
52
and the interelectrode voltage become 0V, and then, discharge is stopped; as a result, a discharge current becomes 0 ampere. The voltage applying circuit
53
repeats the above operation at a predetermined period, and thereby, intermittently generates a discharge in the interelectrode.
FIG. 26
is a graph showing a relation between a discharge time lag and a discharge probability in a conventional electric discharge machining apparatus. As shown in
FIG. 26
, a discharge time lag td until a discharge current starts to flow more than a predetermined value after the interelectrode voltage exceeds 10% of the maximum value, means the following time. More specifically, the discharge time lag td is a time adding the minimum formative time lag tf require
Hashimoto Takashi
Iwata Akihiko
Suzuki Akihiro
Tamida Taichiro
Taneda Atsushi
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