Electric heating – Metal heating – Cutting or disintegrating
Reexamination Certificate
1999-06-04
2001-04-24
Evans, Geoffrey S. (Department: 1725)
Electric heating
Metal heating
Cutting or disintegrating
C219S069180
Reexamination Certificate
active
06222149
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an electric discharge machining apparatus for machining a workpiece by applying electrical pulses to a machining gap formed between the workpiece and a tool electrode, and particularly to an electric discharge machining apparatus provided with a switching circuit for generating high frequency a.c. voltage pulses from a d.c. power supply.
BACKGROUND OF THE INVENTION
When power is applied from a d.c. or a.c. power supply to a small gap formed between a tool electrode of an electric discharge machining apparatus and a conductive workpiece, simply known as a “gap”, the resistance of dielectric fluid across the gap is reduced. Then, when the insulation properties of the dielectric fluid are broken down, electric discharge is generated and “on-time” begins. During a controlled on-time, discharge current flows through the gap resulting in vaporization or melting of the workpiece material. When the on-time is completed, application of power is suspended during a controlled “off-time” in order to restore the insulation properties of the dielectric fluid. Decrease in on-time, in other words reduction in energy for one electric discharge, is known to contribute to improvements in surface roughness.
U.S. Pat. No. 5,149,931 discloses a power supply device for electric discharge machining, for applying a high frequency a.c. voltage from an a.c. power source to a gap, under the condition that series resonance, known as “gap resonance” is caused to occur between an capacitance component across the gap and a distributed inductance of a power supply circuit. If this gap resonance is maintained, electric discharge can occur in the gap even with high frequency a.c. voltage of 7 MHz or more. A low energy electric discharge generated in this way enables a shiny finished surface of 0.2 (m&mgr;Rmax or less. High frequency a.c. voltage of 7 MHz or more can cause electric discharge to be generated only when gap resonance is maintained. In order to maintain this gap resonance, the power supply device of U.S. Pat. No. 5,149,931 varies the frequency of the a.c. power source in response to variation in impedance of the gap. The resonant frequency F
0
is expressed by the equation F
0
=½·&pgr;·(LmCg)
½
. Lm represents the distributed inductance of the wiring, and Cg represents capacitance between the workpiece and the tool electrode. However, if the resonant frequency F
0
changes significantly due to changes in machining area, the surface roughness will vary. For example, in an extreme case, when a workpiece changing in thickness from 1 mm to 50 mm is machined using a wire electrode, the capacitance Cg is increased by 50 times and the resonant frequency F
0
varies by about {fraction (1/7)}. In this case, the surface roughness is increased in proportion to increase in thickness of the workpiece. The length of a wire for transmitting power into the gap also differs depending on the type of electric discharge machine, which means that the distributed inductance LM, and hence the surface roughness of the workpiece, differ depending on the type of the machine.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a power supply device for an electric discharge machining apparatus for generating high frequency a.c. voltage pulses, that can further refine the surface roughness of a workpiece.
Another object of the present invention is to provide a power supply device for an electric discharge machining apparatus for generating high frequency a.c. voltage pulses without utilizing gap resonance, and that can reliably realize favorable surface roughness.
Additional objects, advantages and novel features of the invention will be set forth in the description that follows and will become apparent to those skilled in the art upon reading this description or practicing the invention. The objects and advantages of the invention may be realized and attained by practicing the invention as recited in the appended claims.
In order to achieve the above described objects, a power supply device for electric discharge machining apparatus of the present invention, for supplying power to a gap formed between a workpiece and a tool electrode, comprises a d.c. power source; a bridge circuit supplied with d.c. voltage from the d.c. power source and having switching transistors provided on each side; a control circuit for controlling the on/off switching operation of switching transistors of the bridge circuit so that a.c. voltage pulses are applied from the bridge circuit to the gap; a first resistive element for limiting current flowing when the workpiece is negatively poled and the tool electrode is positively poled; and a second resistive element, having a larger value than the first resistive element, for limiting current flowing when the workpiece is positively poled and the tool electrode is negatively poled.
The power supply device preferably also comprises an inductance element connected in parallel across the gap cancel the influence of stray capacitance of the circuit for supplying power from the d.c. power source to the gap.
Another power supply device for electric discharge machining of the present invention, for supplying power to a gap formed between a workpiece and a tool electrode comprises an a.c. power source; and a d.c. power source which is connected in series with the a.c. power source and connected to the workpiece at its negative terminal and connected to the tool electrode at its negative terminal.
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Kaneko Yuji
Sadamitsu Daisuke
Toyonaga Tatsuo
Devinsky Paul
Evans Geoffrey S.
McDermott & Will & Emery
Sodick Co. Ltd.
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