Electric heating – Metal heating – Cutting or disintegrating
Reexamination Certificate
1999-08-30
2001-09-18
Evans, Geoffrey S. (Department: 1725)
Electric heating
Metal heating
Cutting or disintegrating
C200S08200C
Reexamination Certificate
active
06291791
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a switch that can be used within electric discharge machining fluid for changing the connection of a power supply circuit for supplying electric discharge machining power between a tool electrode and a workpiece.
BACKGROUND OF THE INVENTION
An electric discharge machine generally comprises a mechanical section, carrying a member for supporting a workpiece and mechanical components, and a power supply unit that by necessity must be positioned a physical distance away from the mechanical section. A train of controlled power pulses are produced in this power supply unit and applied to a gap between the tool electrode and the workpiece. As the inductance of a cable electrically connecting the power supply unit and the gap is large, the waveform of electrical current pulse generated in the gap suffers from increased distortion, thereby causing energy loss.
U.S. Pat. No. 5,750,951 discloses a transformer, for converting high frequency pulses transmitted from a power supply unit into high frequency alternating current, being provided as close as possible to the gap. The transformer disclosed in this patent comprises a ferrite ring core, a primary winding connected to a d.c. power source, and a secondary winding connected to the primary winding connecting a tool electrode to a workpiece. A case housing the transformer also houses a plurality of switches for connecting and disconnecting the transformer to and from a power supply circuit. When high frequency d.c. pulses are supplied to the gap, the transformer is disconnected from the power supply circuit, and when high frequency a.c. power is supplied to the gap, the transformer is connected to the power supply circuit. The case can be attached, for example, to a machining tank filled with dielectric fluid, or a work stand to which a workpiece is fixed.
FIG. 8
illustrates a conventional mechanical switch for use with a power supply circuit. The switch
100
comprises a case
101
formed of an insulating material, and a movable contact
102
and a fixed contact
102
A for respectively electrically connecting to a first terminal C and a second terminal T. The movable contact
102
is comprised of a piston
102
A and a piston rod
102
B extending from a bottom surface of the piston
102
A. A cylindrical hole
101
A is formed in the case
101
, and the piston
102
A is provided so as to be capable of reciprocating movement within the case
101
by being guided by walls of the hole
101
A. Openings
101
B and
101
C leading to the hole
101
A are respectively formed in opposite side walls of the case
101
. The piston rod
102
B extends to the outside of the case
101
through an opening
101
B in an axial direction of the piston
102
A, and the first terminal C is attached to a tip of the piston rod
102
B. An O ring type sealing member
105
is provided so that dielectric fluid does not encroach into the inside of the case from a gap between the piston rod
102
B and the case
101
. The fixed contact
104
is fitted into the case
101
and has a planar surface extending parallel to and opposite to the top planar surface of the piston
102
A. Part of the fixed contact
104
extends to the outside of the case
101
water-tightly blocking the opening
101
C, and the second terminal T is attached to a tip of the fixed contact
104
. Holes
101
F and
101
G for supplying air for driving the movable contact
102
to the hole
101
A are formed in the case
101
. If the top surface of the piston
102
A is brought into contact with the planar surface of the fixed contact
104
by supplying air from the hole
101
G to the hole
101
A, the first terminal C and the second terminal T are electrically connected. If the top surface of the piston
102
A is taken away from the fixed contact
104
by supplying air from the hole
101
F to the hole
101
A, the first terminal C and the second terminal T are electrically disconnected.
During machining the switch
100
is immersed in dielectric fluid or dielectric fluid that has splashed out from the machining section is attached to the switch
100
. Because of this, electrical current flows between the first and second terminals C and T, and the piston rod
102
B is gradually corroded as a result of this electrolytic action. The sealing member
105
will also be damaged as a result of corrosion on the piston rod
102
B. Additionally, fine metallic particles generated as a result of electric discharge machining become attached to the piston rod
102
B, and these metallic particles may damage the sealing member
105
. If the sealing member
105
is damaged in this way, dielectric fluid will encroach into the hole
101
A, and the contacts
102
and
104
will be corroded. Also, encroachment of the dielectric fluid increases energy loss due to the occurrence of insulation deficiency of the switch.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a compact switch for a power supply circuit of an electric discharge machine that makes it more difficult for insulation failure to occur.
A further object of the present invention is to provide a power supply circuit of an electric discharge machine that can supply specified high frequency a.c. power to a gap in a stable manner.
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 attain the above objects, a switch of the present invention for changing connection of a power supply circuit for supplying electric discharge power between a tool electrode and a workpiece comprises:
a movable contact (
12
) having a piston (
12
A) and a piston rod (
12
B) attached to a bottom surface of the piston;
a case (
11
) having a cylindrical hole (
11
A) enabling reciprocal movement of the piston;
a first fixed contact (
19
) water-tightly fitted into the case and having a hole (
19
A) enabling sliding of the piston rod;
a second fixed contact (
14
) water-tightly fitted into the case and having a planar surface extending parallel to and opposite to the top planar surface of the piston;
a first terminal (C) attached to the first fixed contact and at least partially exposed to the outside of the case;
a second terminal (T) attached to the second fixed contact and at least partially exposed to the outside of the case; and,
a driver (
15
) for moving the movable contact to change connection between the movable contact and the second fixed contact.
REFERENCES:
patent: 3941957 (1976-03-01), Tilman
patent: 5315088 (1994-05-01), Yamada et al.
patent: 5750951 (1998-05-01), Kaneko et al.
patent: 6-5171 (1994-01-01), None
Fujita Takeo
Hori Hidehito
Devinsky Paul
Evans Geoffrey S.
McDermott & Will & Emery
Sodick Co. Ltd.
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