CA2109569A1

CA2109569A1 - Electric spark discharge machine

Info

Publication number: CA2109569A1
Application number: CA 2109569
Authority: CA
Inventors: Martin Randall; Taibali Dossaji; Gordon J. Pocock
Original assignee: Individual
Current assignee: HURCO HOLDINGS Ltd
Priority date: 1991-05-31
Filing date: 1992-05-29
Publication date: 1992-12-10
Also published as: JPH06507843A; GB9111719D0; WO1992021475A1; SK134493A3; EP0587691A1; AU1905892A; CZ251593A3; HU9303391D0

Abstract

2109569 9221475 PCTABScor01 An electric discharge machine is provided with first and second power supply sources which are connected in parallel during a duty cycle and diodes are arranged in series for the output of the first supply to prevent reverse current in the second supply when the loaded output voltage of the first supply is higher than that of the second supply.

Description

W092/21475 ~ t O ~ ~ b 9 PCT/GBg2/OOg77 ELECTRIC SPARK DISCHARGE MACHINE

i . This invention relates to electric spark discharge machines.

I Electric spark discharge machines exist in various forms all having the common characteristic that a spark is struc:k acros5 a ¦ spark gap between a workpiece and an electrode to erode, and thus shape, the workpiece. The conditions in wh~ ch a spark is struck vary dependant on the intended work rate, ~inish, materials and so on, but to ensure the spark strikes reliably it is often desired to strike at a much higher voltage ~cross the) gap than ~: that needed to maintain the spark. Conventionally series resistance ~including lnternal resistance~, is used to regulate ~the output o~ t~e ~generator providing the dlscharge current, so . ~ :tha~ be~ore the spark ~strikes the (open circuit) ou~put voltage ~across:the spaxk~ gap i5 high, and so that as~the spark discharge ~draws a rapidly lncreasing current,~the voltage across the spar~
gap falls ~to that requlred to maintain the discharge current at a desired le~el. It is observed that this arrangement wastes large :amounts o power in th~e~:,regulating resistance. For example, if the striking voltage is iOO volts, the:'discharge curren~:is 100 ~ :
amps and the gap voltags required to ~aintain the discharge is 27 i~.
volts (ali typical examples3, then once the spark has been struck, 7~00 watts îs: dis:sipated ~as wasted: heat during the remainder of the duty cycle. In addition to the waste of energy, ~' hls leads to the need for high voltage generators capable of :
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WO92/21475 ~ 9 ~j 6 `~ PCT/GB97/00977 delivering a high output current, which generators are Pxpensive~

GB-A-ll96644 discloses an electric ~ischarge machine having means to produce a high voltage high impedance low current followed by low voltage high current~ P~wer switchlng means ~r~ provided to connect the power suppliës to ~he workgap. The switching means is complex, and there is a transition period between switching out of the high voltage supply and the switching in of the low voltage supply during which the spark will extinguish.
This results in poor machining.

, GB~A-947363 discloses a machine in which the strike current and the low voltage supply are drawn from the main output transformer or the high ~oltage~supply is drawn from the transfoFm2r and a translstorised switched low voltage hLgh current supply is used to~generate the low current. It is:difficult to ensure the timing of the strlke current~with:respec:t to the~ low voltage hlgh~
current supply in such a`system and as a result the correct spark .
:~ g~p will not always~be present.
,~
It is one ob~ect~of this invention to pro~ide a:simple and relative inexpensi~e system for provlding the power for an :.
~- electric discharge machine in such~a way that there is little or no waste o~ power.

~ , ~¦ In a~cordance with one aspect o~ the invention th~re is provided :
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2 I V ~ 9 PCT/GB92/U0977 an electric discharge machine for generating a spark across a spar~ gap between an electrode and a workpiece to erode the workpiece, the machlne including: a first power supply having a relatively low short circuit output current and a relatively high open circui~ output voltage for striking a spar~ discharg~
between the workpiece and the el~ctrode; and a second power supply having a relatively low open circuit output voltage and a relatively hi~h short clrcuit output current fo~ maintaining ~park discharge ~etwee~ the electrode and the workpiece, characterised in that the first and second supplie are connected in parallel during a duty ~ycle and in that means are present to prevent reverse current in the second supply when the loaded higher: output voltage of the~first supply ls higher than that of the second~supply~

In use, the spark ~is :~s~truck by the fir~t power supply which, since~:it ~has a low short circuit current ls, firstly, not expensive~and, secondly, when the~spark strikes and the voltage acr:oss ~the gap falls~to,~say, 27 volts,:the~wasted power is low.
For examp~e,:i~ the:~:striking voltage is: th~ same:100 volts and :the sh~rt circuit current of the first supply:is: O.S amps, the waste power is only 36.:5~ watts.: Further,~since the second supply wh~ch, which supplie~s the buIk of the discharge current has a low open circult voltage,~`its cost is far less`and ~the power wastéd ., i , . ~
.j ; is again reduced. : For example, if~ the open circuit output :volta~e ~f the second supply is ~5 volts, at 100 amps discharge ~"

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wo 92/Z1475 ~ 6~ PCT/CB92/00977 current, the wasted power is only 1800 wattsO

The arrangement can be quite simple in principle. For example, a preferred embodiment i5 SO arranged that the first and second power supplies are connected in parallel during a duty cycle, one or more diodes being arranged in series with the output of the second supply to prevent reverse current in the second supply when the loaded output voltage of the first supply is higher than that of the second supply. There is thus no need for switch control gear to time the operation of the second supply relativ~
to the application of the strlking voltage to the spark gap. The term "prevent" in this context is intended to include: the : ~:
~ practical reality of there~ being s~all so-called "leakage"
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currents~

: The duty cycle of the machine~ prefera~ly i5 Gontrolled by first swltch means in parallel with the spark gap,~ second ~swltch means in series with the output of the second supply, and switch : ~
control means for opening:the; fir9t switch~means and closing the second switch means in a duty cycle and~for clo9ing the first switch means and opéning the second switch;~means in~s non-duty~
cycle. ~ ~

~:.. . . So ~hat different ~operatin~ conditions may be set, the first-~i~ supply may prefer~bly be set to provide one of a plurality of ~, ~ diffexent striking voltages. ~ :

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WO92~21475 ~ fj ~ PCT/GB92/00977 Because the second supply is not subject to high voltages, transistors may be used to control its output current. Thus the machine preferably includes transistor means ha~ing a controlled path in series with the output of the second supply, and means providing a current control signal to a control gate of the transistor means, to determine the current in the controlled pa~h. This has the advantage tha~ since the control signal can be varied automatically between one duty cycle and the next, the discharge current can be varied automatically. Experiments show that if suitable variations are chosen, a high work rate can be achieved at the same time as a fine finish. For example, the current control means may be arranged to control the transistor means~to~provide a ~periodic current profile, e.g. the ra~io of the currents~ in consecutive cycle~ is 1:2:4 or 1:4:16 in a repeating~ seguence. ~In an alternative; the current control means includes a pseudo-random`sequence generator arranged to determine the control signal so as to produce a pseudo-random sequence of output currents from the~second su~ply. ~ ;

In another aspect the invention provides.a method of treatlng a workpiece`by an electric spark discharge,~ the method comprising striking a spark disl~hal~rgè'between thë workpiece and an ~lectrode ,, ; fr~m a flrst power supply having a relatively~high open circui~t output voltage and providing a relatively low short circuit ~ outpu~ current, and maintaining the spark discharge between the :,, ~
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W092/2l47~ 3 PCT/GB92/00977 workpiece and the electrode by means of a second power supply having a relatively low open circuit currQnt until the treatme~t is finished, characterised_~ connecting the first and second :, .
supplies in parallel during a duty cycl~ and by preventing reverse current in the seco~sd supply when the loaded higher output voltage of the first supply exceeds that o th~3 second ' supE~ly.

One embodiment of the invention will now be described 0 by way of . , example, wlth reference to the accompanying drawings, ln which `
~: Figure 1 is a block diagram of the spark generator of an ; electric discharge~machine embodying~the invention, Figur~;e 2 ls a ~schematic detail of high voltage and~ high current~generators of Flgure l,~and~

Figure 3 shows ~he ~switching wave~forms.:

Referring to the a:rawings,- a spar3c: gap: 2~ is schema~ically~
: illustrate~d between~ a workpiece 4 ~and an electrode :6.~ In one :, ~
:- example, the electrode is~ an inver:se replica of the shape to : which it ~s: desir~d ItO ierode the~1workpiece ~and, in usP, is ; :
advanced towards the workpiece ~t a rate ~controlled:-by measured~
spark:gap p~rameters : ~ ~

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~1 WO92t21475 2 1 ~ 9 ;~ ~ 9 PCT/G~g2/00977 A first, hiyh voltage, generator 8 is connected directly across the gap 2. The circuit of the high voltage generator 8 is illustrated schematically in Figure 2. 8ere a mains transformer T1 has a primary windin~ 10 connected to a mains electricity supply, not shown. A secondary winding 1 2 of the transformer T1, has taps giving ~45 V, -55 V, -155 V and -155 V relative to a 0 V
tap. As shown, the + 45 V ~ap is connected directly to on~ side of an input to a strlke supply rectificatlon and smoothing network 14. The other side of the input to the rectification and smoothing network 14 i~s connected to relay m~ans~ illustrated schematically at 16, which ena~les one of the 0 V, -55 V; -155 V
or~-255 V taps of the winding 12 to be seleGted so that the output~o the; rectify;i~ng ~and smoothing network 14,~connected across the~:~gap;2 is~sel~ectable between 45 V, 100 V, 200 V:or 300 V,~as;determined;by~a voltage select;lnput from a DIP swi~tch, not:
illustrated:.~The;~windi:ng 12 is not inteDd~d; to carry a heavy current ~a~nd the~:inte~rn;al resi~stance ~of :the windlng 12 and rectiyIng~and smooth~in~g~circult 1~4~combine~d is such that ~the short~circuit current~output of~the~generator:~8~ is;~ mi~ted to,~
:for~example~O S~amps;~at 300~v~and:~1 amp~at 100 V.:

Also conn~cted across the~gap 2 is~a series circuit containing a second, high~cu:rrënt ~en~eratori~16 and ~a current control circuit~
:8 illus~rated sch~ematlcally~in more~detai~l in~ Figure 2. The :transformer T~ has a~:~further secondary~ winding 20 which is capa~le of carr~ing heavy currents and, accordingly, has a low ,~.. , :, :
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WO92/2147~ ~ PCT/GB92/00977 int~rnal resistance. The winding has 45 V and 0 V outputs connected to the input of a high current recti~ying and smoothing circuit 22 to produce a rectified output having an open circuit voltage of 45 V and a short circuit current of 120 ~mps, for example. The current control circuit 18 comprises a bank of paralle} transistors, only one is illustrat2d in Figure 2. The ourrent control sys~em utilises the transistor characteristic that a consta,nt base-emitter voltage applied to a transistor .
~: prc,duces a con~,tan~ current in the base-collector path for a : :
: known temperature. The variables are ~ontrolled hy adding emitter resistors to each transistor, using a temperature , : reference diode and:thermally balancing the transistor heat sinks :
(not illustrated in the drawings).

In or~der to control the duty cycle of the machine a spark timebase generator:24 Ls provided by which the duty cycle and , epetition rate of a~ spark timebase on lead 26 can be varied.
The~ spark~ timebase is~ input to ~a para~1:lel F~,T drive 28 and a series FET drive 30j whlch:respectively control switch m,eans ~in the form of FETs 32`connected in paralle1~ across the gap 7l and FETs 24:~connected in~series with~the high current generator 16 and current controI circuit 18, so that the F~T3 32 go open c:ircuit andithe F~,T's;34 con~uct in a-~uty !cycle, wbereas th~;FETs 32 c~nduct and the FETs~ 34 go~open circuit in a non-duty cycle.;

In a~non-duty cycle, the gap is thus a) short cir~uited by the ,, :,, , ~:, ,~

wo 92/2lq7~ ~1 0 9; ~ PCT/GB92/00977 FETs 32, and b) isolatsd from the high voltage end of the supply 16 by the FETs 34.

.
In a duty cycle, initially the selected high voltage supply open circuit voltage appears across the gap 2, eg 300 V. In order to prevent reverse currents in the high current supply at the beginning of a duty cycle, schottky diodes 36 are connected in s~ries therewith. The high voltage rapidly establi~he~ spar~ng between the workpiece and the electrode, leading to a rapidly in~reasing current which, due to the regulation characteristics of the high voltage generator 8 rapidly pulls the gap ~oItage down ~owards 45 V. As soon as the schottky diodes 36 become forward biassed, discharge current is also supplied by the high current generator 16 which ~upplies the bulk o~ the gap current, .
~ às~ determin~d by the current:control means 18, up to 120 amps. ~ :
; The voltage across the gap 2 is sensed by a servo control signal ~:: circuit 38 and the res~lting control signal is used to control the position of the electrode relative t~the ~workpiece so that :~ th~ voltage~across the gap~ is regulated to 27 V. If ~he voltage - across ~the~gap i5 higher, the ~electrode lS advanced towards the workpiece. If the voltage across the gap is lower ~han 27 V
~: advances is stopped or the electrode lS backed o~f the workpiece.
The servo control signal i5 also supplied to ~a voltmeter via a : ~rive 4a and to a sp~aker via a drive 42 to pro~ide visual and ~ audio indications of conditions at the gap~ : :
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:: , W092/21475 cz~ ~93~ PCT~GBg~/00977 - The gap current is selected from a DIP switch (not shown) the setting of which is decoded by a digital to analog convertor 44 .` which, in ~urn, provi~es an input to a circuit 46 which drives .~ the bases of the transistor bank in the current control circuit .,~ 18~
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1 Since the gap current ls ~constant at a given setting, and since :'~ that determines a constant voltage drop in the generator 16, too ~ . I
low a gap voltage implles a higher ~oltage drop acros~, and thus heat dissipation in ~ the bank of transistors in the current control circuit 18~ ~lthough a low gap voltag~ will result in the servo control backlng the electrcde off the workpiec~ (or at least~stopping lts advance), the reaction is too slow to protect the current control~transistors.~ To:provlde protection the:gap voltage (essentiall:yj is~sensed a~: the node between the FET~ 34 and~the ourrent~:control~circuit by an arc suppression:clr~ult 48 and~`compared~ wlth a varlable reference s~lgnal on lead~S0. If the:
arc ~suppres~lon clrcui~: sets~the:output :of~the convertor 44 to zero, thus~ suppres;si~ng~ the arc~ until:the gap :~oltage has reco~ered.

The~wave forms of the switching are shown in:Figure 3.
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Other digital inputs (not shown) to the digital to analog c~nvertor may be automatlcally cycled by an:lnput from the spark ~;;' time base. Thu~ currents in proportions 1::2:4 or 1:4:16, for ~, . :
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; W092/21475 2 1 ~ 9 ~ 5 ~ PCT/GBg2/00~77 example may be provided, or a pseudo-random digital sequence generator may be indexed by the time base 50 that the input to the digital to analog convertor is a pseudo-random sequence~
Experiments have shown that such arrangements enable a fine iniSll, 2g corresponding to a current of 2 amps in a conventional machine, may be attained at a work rate correisponding to a much higher current, eg 30 amps, .in a conventional ma~hine. The invention is not llmited to the embodiment shown. For example the secondary winding 12 of the taps of the trans~ormer may be 0.60 V (ac)~ 155 V (ac), 220 V ~ac); and that of the secondary winding 20 may be 18 V (ac) N; 18 V (ac) N; 18 V (ac~ N.
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Claims

1. An electric discharge machine for generating sparks across a spark gap between an electrode and a workpiece to erode the workpiece, the maching including: a first power supply having a relatively low short circuit output current and a relatively high open circuit output voltage for striking a spark discharge between the workpiece and the electrode; and a second power supply having a relatively low open circuit output voltage and a relatively high short circuit output current for maintaining spark discharge between the electrode and the workpiece, characterised in that the first and second supplies are connected in parallel during a duty cycle and in that means are present to prevent reverse current in the second supply when the loaded higher output voltage of the first supply is higher than that of the second supply.

2. A machine as claimed in Claim 1, characterised in that the means to prevent reverse current in the second supply when the loaded output voltage of the first supply is higher than that of the second supply comprises one or more diodes arranged in series with the output of the second supply.

3. A machine as claimed in Claim 1 or 2, characterised by first switch means in parallel with the spark gap, second switch means in series with the output of the second supply, and switch control means for opening the first switch means and closing the second switch means in a duty cycle, and for closing the first switch means and opening the second switch means in a non-duty cycle.

4. A machine as claimed in any preceding Claim, characterised by means whereby the first supply may be set to provide one of a plurality of different striking voltages.

5. A machine as claimed in any preceding Claim, characterised by transistor means having a controlled path in series with the output of the second supply, and means providing a current control signal to a control gate of the transistor means to determine the current in the controlled path.

6. A machine as claimed in Claim 5, characterised by the current control means for automatically changing the current control signal between one duty cycle and the next.

7. A machine as claimed in Claim 6, characterised in that the current control means is arranged to control the transistor means to provide a periodic current profile.

8. A machine as claimed in Claim 6, characterised in that the current control means includes a pseudo-random sequence generator arranged to determine the control signal so as to produce a pseudo-random sequence of an output currents from the second supply.

9. A method of treating a workpiece by an electric spark discharge, the method comprising striking a spark discharge between the workpiece and an electrode from a first power supply having a relatively high open circuit output current, and maintaining the spark discharge between the workpiece and the electrode, by means of a second power supply having a relatively low open circuit output voltage and providing a relatively high short circuit output current until the treatment is finished, characterised by connecting the first and second supplies in parallel during a duty cycle and by preventing reversed current in the second supply when the loaded higher output voltage of the first supply exceeds that of the second supply.

10. A method as claimed in Claim 9, characterised by arranging one or more diodes in series with the output of the second supply to prevent reverse current in the second supply when the loaded output voltage of the first supply is higher than that of the second supply.