Electric heating – Metal heating – Cutting or disintegrating
Reexamination Certificate
2001-12-12
2004-04-13
Evans, Geoffrey S. (Department: 1725)
Electric heating
Metal heating
Cutting or disintegrating
C219S069190
Reexamination Certificate
active
06720516
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method and apparatus for electro discharge machining. More particularly, the present invention relates to a method and apparatus for electro discharge micromachining used to machine a micro-diameter hole of a nozzle for discharging ink used for an ink-jet printer, a hole of a nozzle for producing a chemical fiber, a fuel injection nozzle used for an automobile engine, and the like.
2. Description of the Related Art
Recently, a hole of a nozzle for discharging ink used for an ink-jet printer, a nozzle for discharging material used for a chemical fiber production machine, a fuel injection nozzle used for an automobile engine, or the like, are becoming smaller and more precise. As such, there is a demand for hole machining capable of producing a number of nozzle holes having such a small diameter and precise geometry. Such machining typically requires several hours and high-precision positioning.
To achieve such high-precision hole machining, an electro discharge micromachining apparatus capable of hole micromachining using electro discharge may be employed. Such an electro discharge micromachining apparatus is provided with an electro discharge machining apparatus capable of controlling an electro discharge operation of the electro discharge micromachining apparatus with high precision so as to machine a microhole with electro discharge.
FIG. 8
shows a conventional electro discharge machining apparatus
1300
. The electro discharge machining apparatus
1300
comprises: a holder
351
for holding a workpiece
350
to be subjected to electro discharge machining; an electrode
310
provided opposing the workpiece
350
held by the holder
351
; a head
320
for holding the electrode
310
; a capacitor
390
for supplying electric charge to the electrode
310
and the workpiece
350
; a power source
360
for applying a voltage between opposite ends of the capacitor
390
; electric resistances
370
and
380
provided between the capacitor
390
and the power source
360
; a current detecting element
301
for detecting a current flowing between the workpiece
350
and the electrode
310
; a control unit
300
for judging whether a short circuit occurs between the electrode
310
and the workpiece
350
; a table
321
for supporting the head
320
; a linear guide
330
capable of sliding in a Z axis direction for supporting the table
321
; a ball screw
340
for sliding the table
321
in the Z axis direction due to axial rotation; a motor
315
for rotating the ball screw
340
about an axis of the ball screw
340
; a motor driver
312
for driving the motor
315
; and a motor controller
310
for instructing the motor driver
312
to drive the motor
315
. The holder
351
may be a machining vessel supplied with a machining liquid.
The power source
360
which is a direct current power source applies a voltage between the opposite ends of the capacitor
390
, so that electric charge is accumulated in the capacitor
390
in accordance with a charge time constant. The charge time constant is determined based on values of the electric resistances
370
,
380
and the capacitor
390
. The electric charge accumulated in the capacitor
390
is supplied to the electrode
310
and the workpiece
350
, so that electro discharge occurs between the electrode
310
and the workpiece
350
. Repetition of accumulation of electric charge to the capacitor
390
and supply of the accumulated electric charge to the electrode
310
and the workpiece
350
in accordance with the charge time constant leads to generation of pulse electro discharge between the electrode
310
and the workpiece
350
. The workpiece
350
is machined by the pulse electro discharge.
Machining of the workpiece
350
is initiated by a machining initiation command by a user's key input, for example. The machining initiation command leads to accumulation of electric charge into the capacitor
390
, while the motor controller
310
outputs to the motor driver
312
a command to drive the motor
315
so as to move the electrode
310
downward. The motor drive causes the distance between the electrode
310
and the workpiece
350
to be small. When the distance becomes such that an electro discharge can be generated, pulse electro discharge is generated between the electrode
310
and the workpiece
350
to start machining of the workpiece
350
.
In the electro discharge machining apparatus
1300
, the current detecting element
301
detects a current flowing between the power source
360
and the capacitor
390
. The current detecting element
301
may also detect a voltage between the electrode
310
and the workpiece
350
. The current detecting element
301
outputs a result of detecting the current to the control unit
300
.
The control unit
300
judges, based on the current detecting result, whether a short circuit occurs between the electrode
310
and the workpiece
350
. If the control unit
300
judges that a short circuit occurs, the control unit
300
outputs a short circuit detection signal indicating the short circuit state to the motor controller
310
. In accordance with the short circuit detection signal, the motor controller
310
outputs to the motor driver
312
a command to drive the motor
315
so as to elevate the workpiece
390
as long as a short circuit continues between the electrode
310
and the workpiece
350
. The electrode
310
continues to be elevated in the Z axis direction until the short circuit halts between the workpiece
350
and the electrode
310
.
When by the elevation of the electrode
310
in the Z axis direction, a mechanical isolation distance between the workpiece
350
and the electrode
310
is secured, the short circuit halts. When the control unit
300
detects the halt of the short circuit, the motor driver
312
drives the motor
315
to move the electrode
310
downward to a position for initiation of electro discharge, so that electro discharge machining is resumed and continued.
By the above-described electro discharge operation, electro discharge machining is carried out for the workpiece
350
.
To achieve a high level of roundness in hole machining, the electrode
310
is rotated by a spindle motor during electro discharge machining. Typically, such machining is carried out for several hours.
However, in the above-described conventional electro discharge machining apparatus
1300
, every time a short circuit occurs, the electrode
310
moves upward and downward so as to halt the short circuit. Such upward and downward operations require much time, leading to an increase in the time required for overall machining processes.
Further, in the electro discharge machining apparatus
1300
, a high voltage remains applied between the workpiece
350
and the electrode
310
during a period of time from detection of the occurrence of a short circuit to restart of electro discharge machining. When a machining liquid (e.g., deionized water) supplied into the holder
351
(i.e., a machining vessel) is used to cool heat generated by electro discharge and remove swarfs during electro discharge machining, electrolysis occurs between the workpiece
350
and the electrode
310
due to a high voltage applied between the workpiece
350
and the electrode
310
. Abnormal machining occurs due to the generation of electrolysis in addition to intended machining, leading to a reduction in quality of the machined workpiece
350
.
Moreover, in the electro discharge machining apparatus
1300
, the continuous rotation of the motor
315
and the spindle motor generates heat which in turn propagates through the head
320
, the ball screw
340
, or the linear guide
330
made of stainless steel, casting iron (FC material), or the like which, as a result, thermally expands. Such thermal expansion causes displacement of the electrode
310
, leading to a reduction in machining precision. For example, it is assumed that the head
320
is made of stainless steel having the coefficient of thermal expansion of 17&
Masaki Takeshi
Nakagawa Toru
Nakata Hiroyuki
Wada Toshihiko
Yamada Yoshio
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