Electric heating – Metal heating – Cutting or disintegrating
Reexamination Certificate
2002-04-26
2004-05-04
Evans, Geoffrey S. (Department: 1725)
Electric heating
Metal heating
Cutting or disintegrating
C219S069130, C219S069160
Reexamination Certificate
active
06730872
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method of and apparatus for wire electric-discharge machining.
BACKGROUND OF THE INVENTION
FIG. 14
shows the mechanism of general electric-discharge machining which is popularly used in the field of mold machining in the automotive industry, the home electric industry, the semiconductor industry, and the like. When a pulse-like voltage is applied to an inter-pole gap between an electrode and a workpiece which are dipped in a machining liquid, the processes are sequentially performed, such as (
1
) formation of an arc column by generation of discharge and local melting by discharge heat energy, (
2
) to (
3
) generation of evaporative explosive power of the machining liquid and scattering of the melted portion, and (
4
) to (
5
) cooling and solidification of the melted portion by the machining liquid and recovery of inter-pole insulation. These processes are repeated at a high frequency, and thereby the workpiece can be machined. In electric-discharge machining, an inter-pole gap between the electrode and the workpiece is kept to be very small, i.e., several &mgr;m to several ten &mgr;m. This is an important factor of precise machining.
Of such discharge machining techniques, a target of this invention is a wire discharging machining technique which machines a workpiece by using a wire electrode. The wire electric-discharge machining technique is used in piercing machining, cutting machining, and the like, and a demand for precise machining is especially enhanced. For example, a precise mold used in the semiconductor industry, a high precision of 1 to 2 &mgr;m has been demanded.
FIGS. 15A
,
15
B, and
15
C show machining processes performed in the wire electric-discharge machining. As shown in
FIG. 15A
, in the wire electric-discharge machining, rough machining called first cutting is performed first. This first cutting is machining in which a wire electrode is passed through an initial hole, and a workpiece is cut by the wire electrode. In general, in the first cutting, in order to perform finishing after the first cutting, severe surface roughness and severe precision are not demanded, but the most important factor is that a machining speed is increased. In order to increase the machining speed in the wire electric-discharge machining, a machining liquid is strongly sprayed so that machining waste is efficiently exhausted from the inter-pole gap between the wire electrode and the workpiece. In order to uniformly spray the machining liquid and to prevent disconnection of the wire electrode, a method of dipping the workpiece in the machining liquid collected in a machining tank is used.
Upon completion of the first cutting, a core (scrap) is removed, and, as shown in
FIG. 15B
, middle finishing called second cutting is performed. In addition, as shown in
FIG. 15C
, finishing called third cutting is performed. By the way, the second cutting and the third cutting are called for convenience in easy understanding, which means that all the machining procedures are not always completed in three processes. Some are completed by performing the second cutting, but some requires third or more processes when demands for surface roughness and dimensional precision are severe.
In finishing after the second cutting, surface roughness is made fine to adjust the shape. Therefore, when an amount of residue on a target shape is uneven, the shape is corrected, but when an amount of residue on a target shape becomes even, machining which uniformly removes the workpiece must be performed. When such finishing is performed, so-called “electrode position servo”, which controls drive speeds serving as a speed of relative movement of the wire electrode and the workpiece, is performed such that the drive speeds are equal to a predetermined value based on an inter-pole voltage between the wire electrode and the object to be machined.
On the other hand, in the wire electric-discharge machining, an adaptive control to prevent disconnection of the wire electrode which causes machining interruption is performed. As an adaptive control to prevent disconnection, the most ordinary method is one that changes a discharge stop time to decrease machining energy.
As explained above, in the wire electric-discharge machining, a plurality of control methods are often applied to a mechanical system, a power supply system, and the like. However, the application of the plurality of control methods may unexpectedly produce an inconvenience in a machining result.
An example of such an inconvenience will be explained below with reference to a wire electric-discharge machining apparatus shown in FIG.
16
. In
FIG. 16
, reference numeral
1001
denotes a wire electrode,
1002
a workpiece,
1003
a movable table,
1004
a machining power supply,
1005
a machining power supply control unit,
1006
a control unit,
1007
a servo mechanism, and
1008
denotes an inter-pole voltage detection unit.
The control unit
1006
sends a machining condition signal to the machining power supply control unit
1005
according to an input machining condition. The machining power supply control unit
1005
drives a switching element (not shown) of the machining power supply
1004
according to a signal from the control unit
1006
. The machining power supply
1004
applies a pulse-like voltage across the wire electrode
1001
and the workpiece
1002
to perform electric-discharge machining to the workpiece
1002
. An inter-pole voltage between the wire electrode
1001
and the workpiece
1002
in the electric-discharge machining is detected by the inter-pole voltage detection unit
1008
to be sent to the control unit
1006
. The control unit
1006
determines a drive speed of the movable table
1003
based on inter-pole voltage information sent from the inter-pole voltage detection unit
1008
and sends a command to the servo mechanism
1007
. As a result, the servo mechanism
1007
allows the movable table
1003
to move at the drive speed, and the wire electrode
1001
and the workpiece
1002
relatively move.
In general, a positional control of the wire electrode
1001
is performed based on a measured voltage in the inter-pole voltage detection unit
1008
. As a control method used in this case, the following method is used. That is, drive speeds serving as a speed of relative movement between the wire electrode
1001
and the workpiece
1002
are increased when the inter-pole voltage is high, and the drive speeds are decreased when the inter-pole voltage is low. However, it has been understood that the control method for the drive speeds cannot be used without various problems.
In the first cutting, it is mainly required that a machining speed is increased as explained above. Disconnection which is a factor of hindering the increase in machining speed must be avoided as much as possible. Research and development are earnestly performed to prevent disconnection. Although various methods are reported and practically used, a method which is the most effective method is that a discharge stop time is elongated.
However, since a power supply control which elongates the discharge stop time responses considerably quickly as compared to a drive control for the mechanical system, a vibration phenomenon may occur due to the difference between respective responses of both the controls.
The wire electric-discharge machining apparatus which performs a control to prevent the wire electrode
1001
from being disconnected recognizes a decrease in inter-pole voltage as a leading phenomenon in which the wire electrode
1001
is disconnected. Therefore, when the inter-pole voltage lowers, the operation of elongating a discharge stop time is performed. This operation is effective as means for preventing disconnection. When the discharge stop time becomes long, the inter-pole voltage more lowers and thereby the apparatus erroneously recognizes that the wire electrode
1001
is about to be disconnected, and performs the operation of further elongating the discharge stop time. Such a series of ope
Goto Akihiro
Nakashima Toshio
Sato Tatsushi
Shibata Jun'ichi
Evans Geoffrey S.
Mitsubishi Denki & Kabushiki Kaisha
Sughrue & Mion, PLLC
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