Electric heating – Metal heating – Cutting or disintegrating
Reexamination Certificate
2001-05-03
2003-06-17
Evans, Geoffrey S. (Department: 1725)
Electric heating
Metal heating
Cutting or disintegrating
C219S069180
Reexamination Certificate
active
06580048
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a discharge processing device which can improve the processing efficiency while reducing the damage to switching elements at the time of discharge-processing a target.
BACKGROUND OF THE INVENTION
The principle of target discharge processing and current waveforms required when the discharge processing is carried out will be explained first.
In a discharge processing, a discharge is started in the following manner: between a gap of an electrode and a target placed so as to face the electrode, after a minute conductive path not more than several tens gm has been found, a pulse current is allowed to flow through the minute conductive path so that the minute transport path or the electrode and a minute portion of the target contacting the path are forcefully evapotranspirated or fused and scattered by thermal energy generated at this point. In this case, the degree of evapotranspiration and fusing and scattering at the minute portion is determined by the following factors: the rate of change in time of the pulse current, that is, a current having an abrupt rising characteristic, the size of a current peak value, thermal characteristics of the electrode and target material and cooling characteristics of an insulating solution, etc.
When the target is made from a material having small electrical resistance, heat generation due to Joule heat becomes smaller. Furthermore, when the target is made from a material having superior thermal conductivity, the heat generation and temperature rise at the minute portion are lowered. Moreover, when the target is made from a material having high melting temperature, it is hardly melted even when heated. Furthermore, when the target is made from a material having high viscosity at the time of fusing, it is hardly scattered even when fused.
In an actual processing operation, these conditions are combined so that the resulting phenomena are a slow processing rate, a rough surface or a fine surface, susceptibility to short-circuiting, susceptibility to degradation in the processing efficiency and susceptibility to a concentrated discharge. Moreover, in the case of a wire discharge processing, the resulting phenomena are more short-circuiting and high frequency in wire disconnection.
Moreover, conventionally, in order to eliminate the short-circuiting phenomenon, alloys made of materials having a low fusing temperature and fusing latent heat or a low viscosity at the time of fusing, that is, materials having smoothness, have been used as the electrode material. Alloys such as brass are listed as these alloys; however, since these materials raise a problem of electrode consumption, etc., they are not used so much except for wire discharge processing machines and high-speed thin-pore processing machines. Here, a special wire which is coated with a material having a low fusing temperature and a small viscosity at the time of fusing has been developed as a wiring electrode for use in a wire discharge processing, and the application of such a special wire makes it possible to improve the processing efficiency while preventing the above-mentioned short-circuiting.
Moreover, after the start of the discharge, the insulating solution on the periphery of the discharge is evaporated so that a bubble that abruptly expands is formed. Thus, this internal pressure results in a reaction that scoops the fused portion out. As the discharging time elapses, the fused portion gradually expands, and as the bubble expands, the density of the generated inner pressure becomes smaller. Therefore, there is a greatest value in the amount of the scooped portion determined by the material and the discharging time, and both of a shortened discharging time (pulse width) and a lengthened discharging time cause degradation in the processing efficiency. In particular, even when the discharging time (pulse width) is lengthened exceeding the time required, the excessive discharging time is consumed as heat generation and fusing of the electrode and the target, resulting in a unwanted thick processed surface of the fused layer.
For this reason, respective conditions, which include the current peak value and the current rising rate that serve as discharge starting capabilities with the discharging time (pulse width) serving as a processing capability added thereto, are preferably set so as to be selected independently, depending on differences in thermal characteristics of the electrode, target and insulating solution. Moreover, when a pulse having a triangular waveform is used in such a very small area of a pulse width as described earlier, it is not possible to carry out an efficient processing operation.
When the same quantity of charge is applied in a rectangular waveform and a triangular waveform, the effective current value is smaller in the rectangular waveform. Therefore, when a shift is made from the triangular waveform to the rectangular waveform, heat input to the electrode can be reduced and the processing capability is improved. In particular, in the case of the wire discharge processing device, this arrangement is very effective since it prevents wire disconnection.
Japanese Laid-Open Patent Application No. 11-48039 discloses a discharge processing device in which the rising rate and pulse width of the above-mentioned electric current are independently controlled so as to provide an efficient processing operation.
FIG. 30
shows a circuit structure thereof. Reference number
101
denotes a main dc power supply and reference number
102
denotes a sub dc power supply which supplies a voltage lower than the output voltage of the main dc power supply
101
. Moreover, reference symbols T
101
, T
102
and T
103
denote first, second and third switching elements constituted by FETs.
The positive terminal of the main dc power supply
101
is connected to a target W through the first switching element T
101
, and the positive terminal of the sub dc power supply
102
is connected to the target W through the third switching element T
103
. Furthermore, the negative terminals of the main and sub dc power supplies
101
and
102
are connected to an electrode P through the second switching element T
102
. To the gates G
101
to G
103
of the FETs constituting the switching elements T
101
, T
102
and T
103
are connected switching element driving circuits (not shown), and the respective switching element driving circuits are allowed to on-off control the respective switching elements T
101
, T
102
and T
103
by using pulses output from a pulse distribution circuit (not shown).
FIG. 31
is a schematic drawing that exemplifies the relationship between the operational timing and the waveform of the discharging current (processing current) in the discharge processing device of FIG.
30
.
When the discharge processing operation is started by the discharge processing device, pulse width setting data t
1
and t
2
are set in accordance with a dischargeable state between the electrode P and the target W. Based upon these settings, a pulse signal having the pulse width t
2
is output from each of the switching element driving circuits, with the result that the second and third switching elements T
102
and T
103
are turned on as shown by (a) and (c) in FIG.
31
.
As a result, the voltage of the sub dc power supply
102
is applied between the target W and the electrode P through the third switching element T
103
and the second switching element T
102
so that a current I
1
(═I
0
) flows from the sub dc power supply
102
, thereby securing a current applying point (see(e) in FIG.
31
). This is also referred to as a preliminary discharge which aims to secure the current applying point, and a separate power supply system may be installed for use in the preliminary discharge. The rise of this current is gradual since the output voltage of the sub dc power supply
102
is low. However, after a delay time successively set, a pulse having the time width t
1
that has been set by the current peak value setting data is output from the rest
Hashimoto Takashi
Iwata Akihiko
Ooguro Hiroyuki
Suzuki Akihiro
Tamida Taichirou
Evans Geoffrey S.
Leydig , Voit & Mayer, Ltd.
Mitsubishi Denki & Kabushiki Kaisha
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