AU452425B2 - Method and apparatus for electrical discharge machining - Google Patents

Description

7 09

This invention relates to an electrical machining method and apparatus for removing material from a conductive workpiece by spark'discharges across a machining gap between the workpiece and a tool electrode. In EDM (electrical discharge machining) apparatuses, It is desired to apply a high peak current for a relatively short period of time through the machining gap to produce the spark discharge. This arrangement produces the most effective metal removal of th workpiece. When utilizing a power source having a sinusoidal wave form, either clipped or not clipped, it is desirable to control when the current starts to flow through the gap and when the current ceases to flow through the gap. Thus, if the wave form is clipped, for example, it would be preferred to prevent current flow through the machining or spark dis- charge gap until the plateau of the voltage is reached. Likewise, it would be preferred to stop current flow through the gap before the voltage starts to decrease. The present invention satisfactorily solves the foregoing problem-by providing control means for determining when current flow from a power source through the machining gap starts and stops. Thus, the present invention permits the utilization of a sinusoidal wave form for an EDM appa- ratus in which a high peak energy is applied for a short period of time. An object of this invention is to provide a method

452.42 and apparatus for controllhg A spark discharge exists across a machining or spark discharge gap of an EDM apparatus so that an applied varying voltage is at its minimum rate of change. Another object of this invention is to provide a method and apparatus for controlling the supply of energy to a machin- ing gap of an EDM apparatus so that a selected level of energy is supplied to the machining gap during the machining. In one general form the invention provides an electrical machining apparatus for removing material from a conductive vorkpiece by spark discharges across a machining gap between the workpiece and a tool electrode, comprising a power source to repetatively produce a varying voltage, each repetition being for a predetermined period of time with the voltage during the predetermined period of time having an initial and ft nal period of relatively high rate of change of voltage and having an intermediate period of relatively low rate of change of voltage, means to connect said power source across the gap to &lAlfikY 44a supply current: theretokand means to siispend dupply of current from said power source to the gap for at !east ona of the initial and final periods of relatively high rate of change of voltage, said suspending means having means to render said suspending means ineffective during said intermediate pezbd of relatively low rate of change of voltage. In another general form the invention provides an electrical machining method for removing material from a conductive workpiece by spark discharges across a machining gap between the workpiece and a tool electrode, comprising repetatively producing a varying voltage, each repetition being for a predetermined period of time with the voltage durins

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10,709no the predetermined period of.time having an initial and fin- al period of relatively high rate of change of voltage and having an intermediate period of relatively low rate of change of voltage, using said varying voltage to produce current across the gap during at least part of said intermediate period of relatively low rate of change and suspending supply of said current to the gap for either or both the initial and final periods of relatively high rate of change of voltage.

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10.70 9 no The attached drawing illustrates a preferred

embodiment of the invention, in which: of the control of Figure 1 is a circuit diagram

the present invention; and showing the rela- Figures 2A-2C are timing charts with the control tion of current and voltage in accordance

of the present invention. Figure Referring to the drawing and particularly workpiece 10 and a tool elec- 1, there is shown a conductive supported on a suitable trode 11. The workpiece 10 may be manner. The tool elec- base (not shown) in the well-known workpiece 10 and is trode 11 is disposed adjacent the away from the workpiece adapted to be moved toward and there is a short cir- in the well-known manner such as when

cuit therebetween. a line 12 to one The workpiece 10 is connected by 15, which side of a winding or coil 14 of a transformer electrode 11 is con- functions as a power source. The tool the winding 14. nected by a line 16 to the other side of of the The winding 14 is the secondary winding winding 17 connected transformer 15, which has its primary with which the to an oscillator 18. Thus, the frequency gap between the work- current is supplied to the machining

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10,7 0 9 piece 10 and the tool electrode 11 is determined through

selecting the frequency of the oscillator 18.

During one half of each AC cycle, a capacitor 19,

which is in the line 16, is charged from the winding 14.

The capacitor 19 is variable to produce currents of different

magnitudes through the machining gap during the other half

of the AC cycle when a spark discharge is created across the

machining gap. The current flow through the machining gap

is directly proportional to the size of the capacitor 19.

Thus, when the size of the capacitor 19 is increased, the

magnitude of the current flow through the gap is increased. During the next half of the cycle after the

capacitor 19 has been charged, current flows from the winding

14 and the capacitor 19 through the line 12 to the workpiece When the dielectric in the gap ionizes, the current

flows through the gap to the tool electrode 11 and the line 16. The voltage wave form, P hich Q41 odudcs,between

the workpiece 10 and the tool electrode 11, is shown in

Figure 2A. The wave form is a clipped sinusoidal wave form

although it could be a sinusoidal wave form if desired. Without the present invention, the wave form of

the current would be similar to that shown for the voltageA

However, by using the present invention, the current through

the machining gap may be controlled to produce a wave form

such as that shown in Figure 2B. The line 16 has a silicon controlled rectifier 21

therein. When the silicon controlled rectifier 21 is not

energized and a manually operable switch 22, which is in

parallel with the silicon controlled rectifier 21, is open, 6 45 2,4 2

10,7 09 nr Insteaa, the current cannot flow through the machining gap. current flows through a line 23, which connects the lines 12 and 16 to each other and is in parallel with the machining gap to provide a shunt path for the current. The line 23 has the a resistor 23' and a variable resistor 24 therein. When current flows through the line 23, a capacitor 25 is charged through the resistor 24 and a potentiometer 26. With the the present invention, the voltage across the line 23, between line 12 and the cathode of the silicon controlled rectifier 21, is the same as that shown in Fig. 2A. When the

capacitor 25 charges to a predetermined potential a Zener

diode 27, which connects the capacitor 25 with the gate of the silicon controlled rectifier 21, breaks down whereby of the potential on the capacitor 25 is supplied to the gate the silicon controlled rectifier 21. When this occurs, the

silicon controlled rectifier 21 is turned on so that it ceases to block the line 16 and allows current to flow through the gap to produce a spark discharge between the workpiece in and the tool electrode 11. This is shown occurring at 28 Figures 2A-2C. It will be observed that this is along the plateau of the clipped voltage wave form. By varying the resistance of the resistor 24 and/or the potentiometer 26, the time at which the silicon controlled rectifier 21 is turned on may be varied as desired. Likewise, the capacitor 25 could be variable, if desired, to permit further selection of the time when the silicon controlled rectifier 21 is turned on. If it is desired to turn off the current flow through the machining gap before the flow of current from the winding 14 changes directn, a manually operable switch 452.425

'10.709 no 29 must be open and a manually operable switch 30 must be closed before machining starts. The switches 29 and 30 are connected to each other so that one is open when the other is closed. When the manually operable switch 29 is closed as

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10,709 /o connected diodes shown in Figure i, a plurality of series open and 31-35 is bypassed. However, when the switch 29 is

must flow through the the switch 30 is closed, current rectifier 21 is diodes 31-35 when the silicon controlled drop across the diodes turned on. .This produces a voltage

31-35. of fixed This voltage drop across the diodes 31-35 charged through a magnitude results in a capacitor 36 being 38 by resistor 37 of fixed magnitude and a potentiometer gap. When the the current flowing through the machining potential, a capacitor 36 is charged to a predetermined 36 with the Zener diode 39, which connects the capacitor breaks down gate of a silicon controlled rectifier 40, supplied to the whereby the charge on the capacitor 36 is 40. When this gate of the silicon controlled rectifier turns on and occurs, the silicon controlled rectifier 40 the line 16. provides a shunt path from the line 12 to gap to be This causes the spark discharge across the the gap since extinguished and no current to flow through

it flows through the silicon controlled rectifier Thus, the silicon controlled rectifier 40 may (see Figures 2A be employed to turn off the current at 41 may be and 2B). The time, which is indicated by 41, resistance of the changed as desired through varying the

potentiometer 38. Likewise, the capacitor 36 could be

when the silicon variable, if desired, to permit varying

controlled rectifier 40 is turned on.

Furthermore, because of the Zener diode 39 the silicon con- breaking down at a selected potential, tool electrode 11 trolled rectifier 40 also protects the t 452425

10.709/no from being damaged due to high energy short conditions. Thus, if there were a short, the capacitor 36 would charge much more rapidly whereby the silicon controlled rectifier would turn on much more rapidly to shunt the. current from the discharge gap. Therefore, the control of the present invention not only regulates when the current flow through the gap is stopped so as to obtain a maximum amount of removal of metal from the workpiece but it also protects the tool electrode 11 from damage by high energy shorts. Considering the operation of the present invention, the capacitor 19 is charged during one half of the AC cycle by current flow through the capacitor 19 and a diode 42. A diode 43 blocks the current from flowing through the line 16 to the tool electrode 11 during this half of the cycle. During the next half cycle, the current flows from the winding 14 of the transformer 15 to the line 16. The capacitor 19 also discharges in this direction at this time. With the manually operable switch 22 open, the current flows through the line 23 and the resistors 231 and 24 during the first portion of the period of the half cycle in which current is flowing to the line 16 from the trans- former winding 14. When the capacitor 25 charges to the selected potential, the Zener diode 27 breaks down and the silicon controlled rectifier 21 turns on. When this occurs, current flows through the gap because of its much smaller resistance in comparison with the resistors 231 and 24. If the switch 30 is closed and the switch 29 is opened, the capacitor 36 is charged when current flows through the machining gap. When the capacitor 36 reaches a selected potential as determined by the values of h 2 4 54 2

10.7 09rM 38, 37, and the potentiometer capacitor 36, the resistor controlled down, and the silicon the Zener diode 39 breaks path for the This produces a shunt rectifier 40 turns on. 19 whereby 14 and the capacitor current from the transformer gap and the through the machining current ceases to flow

spark discharge is extinguished. 21 controlled rectifiers While both the silicon should be as being employed, it and 40 have been described rectifiers one of the silicon controlled understood that only example, the if desired. Thus, for 21 and 40 may be used in Figure be in the position shown switches 29 and 30 could 21 would be controlled rectifier 1 whereby only the silicon the current starts determine only when employed; this would current would machining gap, and the to flow through the wind- the gap until the transformer continue to flow through This would to change direction. ing 14 causes the current form as shown in Figure produce a current wave 22 is closed, the switch Likewise, if the switch silicon 30 is closed, only the 29 is open, and the switch this would result 40 would be employed; controlled rectifier gap as soon as to flow through the in the current starting to the line 12. 14 supplies current the transformer winding current flow only the time when the In this arrangement, causes the before the winding 14 through the gap is stopped would be determined flow to the line 12 current to cease to

or controlled. of the circuit are as An example of parameters

follows:

11 452.425

10,709 /7 Resistors in Ohms

23' 24 40 (Variable from 0 to 20 in circuit to gate of silicon

controlled rectifier 21)

26 Variable from 0 to 125

37 1 38 Variable from 0 to

Capacitors

1 microfarad

36 5 microfarads Zener Diodes

Both of the Zener diodes break down at 3.3 volts.

With the foregoing circuit, the frequency at which

the spark discharges occur may be changed by changing the

frequency of the oscillator 18. The time at which the cur-

rent starts to flow through the spark discharge gap is deter-

mined by when the silicon controlled rectifier 21 turns on.

The time when the current stops flowing through the gap is

determined by when the silicon controlled rectifier 40 turns

on. This assumes that both the silicon controlled rectifiers

21 and 40 are employed.

While the present invention has been described

with the silicon controlled rectifiers 21 and 40 used as

the switch control means, it should be understood that any

other type of control means, which is responsive to a

signal, may be employed. For example, transistors could

be utilized instead of the silicon controlled rectifiers

21 and An advantage of this invention is that it insures 12 452,425

10.7 09 no that energy is dissipated across the machining gap of an

EDM apparatus in a short period of time. Another advantage

of this invention is that it allows selection as to the

time when current flow starts and/or stops through a machin-

ing gap of an EDM apparatus.

For purposes of exemplification, a particular embodi.

ment of the invention has been shown and described according

to the best present understanding thereof. However, it will

be apparent that changes and modifications in the arrangement

and construction of the parts thereof may be resorted to with-

out departing from the spirit and scope of the invention.

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Claims (14)

1. e.0,7 0 9 no The claims definiln invention are as follows: 1. An electrical machining apparatus for removing material from a conductive workpiece by spark discharges across a machining gap between the workpiece and a tool electrode, comprising a power source to repetatively pro- duce a varying voltage, each repetition being for a pre- determined period of time with the voltage during the predetermined period of time having an initial and final period of relatively high rate of change of voltage and having an intermediate period of relatively low rate of change of voltage, means to connect said power source across the gap to supply current thereto during at least part of said intermediate period of relatively low rate of change, and means to suspend supply of current from said power source to the gap for either or both the initial and final periods of relatively high rate of change of voltage, said suspending means having means to render said suspending means in- effective during said intermediate period of relatively low rate of change of voltage.
2. 2. An electrical machining apparatus according to claim 1, wherein said suspending means is arranged to suspend supply of current from said power source to the gap for at least said initial period of relatively high rate of change of voltage.
3. 3. An electrical apparatus according to claim 2, in which said suspending means includes means to block current flow from one side of the gap to one side of said power source, means to connect the other side of said power source between said blocking means and said one side of said power source to provide a path for the current from the powe sorc when 4 1 2,425 s 10,709/70no said blocking means is effective and said rendering means renders said blocking means ineffective.
4. 4. An electrical machining apparatus according to claim i, 2 or 3, in which said. suspending means suspends supply of current from said power source to the gap at least for said final period of relatively high rate of change of voltage.
5. An electrical machining apparatus according to claim 2, 3 or 4, in which said suspending means is able to vary the time length of said period or periods of suspension.
6. 6. An electrical machining apparatus according to claim 4, in which said suspending means is effective to suspend supply of current for said final period when a selected energy level is produced due to current flow through the gap.
7. 7. An electrical machining apparatus according to claim 4, in which said suspending means includes means to shunt the power current from the gap by connecting the two sides of said source to each other; means to block said shunt means; and means to render said shunt blocking means ineffective for said final period.
8. 8. An electrical machining apparatus according to claim 7, in which said means to render said shunt blocking means ineffective includes means to store a charge created when current flows through the gap and means responsive to a total charge on said storage means reaching a selected total charge to render said shunt blocking means ineffective.
9. 9. An electrical machining apparatus substantially as hereinbefore described with reference to, and as illustrated

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10. 10.7 09 no in, the accompanying drawings. An electrical machining method for removing material from a conductive workplce by spark discharges across a machining gap between the workpiece and a tool electrode, comprising repetatively producing a varying voltage, each repetition being for a predetermined period of time with the voltage during the predetermined period of time having an initial and final period of relatively high rate of change of voltage and having an intermediate period of relatively low rate of change of voltage,using said varying voltage to produce current across the gap during at least part of said intermediate period of relatively low rate of change, and suspending supply of said current to the gap for either or both the initial and final periods of relatively high rate of change of voltage.
11. 11. An electrical machining method according to claim wherein said supply of current to the gap is suspended for at least said initial period of relatively high rate of change of voltage.
12. 12. An electrical machining method according to claim or 11, in which said supply of current to the gap is suspen ded for at least said final period of relatively high rate of change of voltage.
13. 13. An electrical machining method according to claim 11 or 12, including varying the time length of said period or periods of suspension.
14. 14. An electricE machining method according to claim 12, final in which sid suspension of supply of current for said period occurs when a selected level of energy is produced due to current flow through the gap. 16 452.42

    7 0 9 no An electrical machining method substantially as

    hereinbefore described with reference to the accompanying

    drawings.

    DATED THIS TH[IRTEENTH DAY OF AUGUST, 1974 THE CINCINNATI MILLING MACHINE CO.

    Patent Attorneys for the Applicant SPRUSON FERGUSON

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