GB2099351A - Method of electrical discharge machining - Google Patents

Abstract

A method of spark erosion machining of metal by means of spark pulses of short duration generated by discharge of a capacitor and passing through the working gap (2), in which method the capacitor (5) lying in parallel with the working gap becomes charged up by means of a d.c. source and upon a breakdown voltage being reached at the working gap is discharged across it, characterised in that the d.c. source at the start of the spark discharge across the working gap is under the control of a discriminator (8) connected across the latter, whereby it is switched off from the working gap for a predetermined period of time, and is subsequently automatically switched on. Preferably the connection of the d.c. source to the working gap is effected after a predetermined voltage of charge across the capacitor has been reached, and is maintained only on the earth side. <IMAGE>

Description

SPECIFICATION Method of electrical discharge machining This invention relates to a method of spark-erosion machining of metal by means of spark pulses of short duration generated by discharge of a capacitor and passing across a working gap, in which method a capacitor lying in parallel with the working gap becomes charged by means of a d.c. source and upon a breakdown voltage being reached at the working gap, is discharged across it.

For fine spark-erosion machining (finishing) for the production of surfaces of low degree of roughness, the use of machining pulses of short duration is necessary. The machining pulses have a duration of the order of magnitude of one microsecond or less. Pulses of this short width (duration) can under practical conditions, i.e. in the case of a somewhat large distance between the pulse generator and the working gap in the machine, be generated only with difficulty because of inductance and capacitance effects always present in the supply leads.

For fine machining of metal surfaces by sparkerosion it is therefore usual to generate the machining pulses by discharge of a capacitor across the working gap. For that purpose a capacitor is connected in parallel with the working gap and charged up by a current source to the necessary breakdown voltage. Upon reaching the breakdown voltage, provided that the ionization conditions in the working gap are favourable, the capacitor is discharged across the working gap, resulting in the generation of machining pulses of short duration and adequate working frequency. For such purposes the capacitor is charged up by a timed d.c. source (i.e. a source of d.c. pulses) by the supply of a series of d.c. pulses, or from a plain d.c. source by the supply of an appropriate charging voltage.

However known spark-erosion method of fine machining suffers, in particulai- with the use of timed d.c. sources, from the disadvantage that under difficult operating conditions at the working gap in particular when using capacitors of low charge capacity, discharges occur which are attributable to direct access of the charging current source to the working gap. Such discharges disturb the fine machining at the metal surface since they often give rise to greater degrees of roughness.

The invention is concerned with the problem of evening-out the fine machining of the surfaces and freeing them from troublesome irregularities which are attributable to pulse discharges of greater energy.

To this end the invention provides a method of spark-erosion machining of metal by means of spark pulses of short duration generated by discharge of a capacitor and passing through the working gap, in which method the capacitor lying in parallel with the working gap gets charged up by means of a d.c.

source and upon a breakdown voltage being reached at the working gap is discharged across it, characterised in that the d.c. source at the start of the spark discharge across the working gap is under the control of a discriminator connected across the said working gap, whereby it is switched off from the working gap for a predetermined period of time, and is subsequently automatically switched on again.

The method in accordance with the invention is particularly useful when using a d.c. pulse generator.

In this instance the pulse interval is preselected in the usual way, whilst the pulse duration is not rigidiy predetermined but is determined directly by the discriminator at the working gap. Thus the inherent characteristic of capacitor discharge circuits, in that they adjust themselves to an optimum, come into full effect, whilst the disadvantages hereinbefore referred to are avoided. The charging up of the capacitor is effected after effective freeing of the working gap in the interval in the shortest possible time. In special instances it may be advantageous to interrupt the charging up also by shorter intervals.

The start of the spark discharge across the working gap may be determined by the discriminator in the usual way. For example as the criterion for the start of the discharge across the working gap after charging up of the capacitor has been effected, the dropping of the breakdown voltage below a predetermined voltage level may be employed. Also the rise of the discharge current above a predetermined level may be utilised as the criterion. The duration of the interval of shutdown of the charging current generator is in such circumstances chosen by experience in such a way that the length of interval is adequate for creating favourable conditions again for the next discharge in the working gap.

The method may also be applied in such a way that the length of the charging interval and/or the strength of the charging current of the capacitor are automatically brought to an optimum in dependence upon suitable criteria, e.g. upon the rate of rise of the charging voltage. Thus for example the interval may be extended if the capacitor voltage after a spark, rises too slowly. In this instance in accordance with experience, the gap after discharge is not being adequately cleaned and deionized so that a prolonged interval is advantageous. If on the contrary the gap is completely satisfactory, the rate of rise of the charging voltage reaches a maximum value which is dependent upon the setting of the charging current source. The duration of the interval may then be shortened and/or the strength of the charging current may be raised.

Since the voltage level across the capacitor at the instant of breakdown forms an important parameter of the erosion, it has proved to be advantageous with the method in accordance with the invention, if the generation of the d.c. source to the working gap is, after a predetermined voltage of charge across the capacitor has been reached, maintained only on the earth side, as is necessary in order to avoid redischarging of the capacitor through leakage currents. The breakdown voltage is thereby restricted to the value required at any time.

The method in accordance with the invention is hereinafter described in greater detail in the accompanying drawings, wherein Figure 1 is a diagrammatic representation of one circuit means for carrying out the invention, and Figure 2 is a graph of the variation with time, of the voltage and also the current at the working gap of a spark-erosion machine, operated by the method in accordance with the invention.

In Figure 1 a d.c. current/source 1 is connected across a working gap 2 of a spark-erosion machine between a machining electrode 3 and a workpiece 4 which is to be machined. The said working gap 2 is filled with a liquid dielectric, e.g. mineral oil. A storage capacitor 5 of suitable capacitance is connected in parallel with the working gap and is charged up by the charging current source 1 and discharged across the working gap 2 in a periodic cycle. With an adequately short duration of pulse there is effected at the workpiece 4 a removal of material leading to a relatively smooth surface of the workpiece.

The pulse generator 1 consists of a d.c. current source 6, a switch 11 controlled by switch means 10 and a current-limiting resistor 7 which is connected in conventional manner to the working gap. The said generator generates a d.c. pulse of adjustable pulse and interval duration. For fine finishing the capacitor 5 is in parallel with the working gap.

A discriminator 8 is connected to the working gap 2 (or to the capacitor 5) and detects the start of a spark discharge across the working gap 2. Forthat purpose the discriminator 8 supervises the voltage across the working gap 2 (or capacitor 5) and after a maximum voltage being applied across the working gap 2, indicates the drop in this voltage which starts at the start of a spark discharge.

The said discriminator influences the switch control in such a way that it terminates and restarts the current pulse at a preset interval. The next pulse, started after the said interval is however terminated, not after a preset time, but by the next discriminator signal.

Figure 2a shows the variation with time of the charging voltage across the capacitor 5. The voltage first rises during the charging up of the capacitor from the current source 1, to a maximum value which is the breakdown value of the voltage, and then drops at the start of the discharge. If required, by suitable control of the switch 11 this breakdown voltage can extend up to a predetermined value U2.

When the voltage drops below the set limit U1 the discriminator 8 switches the charging current source 1 off from the working gap 2 by actuation of the switch 11. After an interval tp the switch is closed again and the charging process across the capacitor 5 is repeated.

Figure 26 shows the variation with time of the charging voltage UL effective before the limiting resistor 7 and the succeeding interval tp.

Figure 2c shows diagrammatically the current flowing across the working gap 2.

In contrast to the embodiment described, the discriminator 8 may pick up the current flow across the working gap and make use of a corresponding current limit as the criterion for the start of the discharge across the working gap.

It is also possible to bring the duration tp of the interval to an optimum in dependence upon suitable parameters of the working gap 2. Usually it will be advantageous to employ as the charging current source the normal pulse generator for roughing first-finishing. In that instance it is a particular advantage of the method in accordance with the invention, that only slight control is necessary in the control of this generator. Short-circuit protection, feed adjustment and other devices associated with the generator may be employed for bringing the process to an optimum without or with only slight modification.

If it is desired to avoid even such slight modifications and/or if it is advantageous also to interrupt the charging pulse too, the method may likewise be applied in an arrangement similar to Figure 1. In this instance the d.c. current source 6 is a completely unaltered pulse generator which every time after the start of a discharge of the capacitor is separated for a predetermined time from the working gap by an additional switch 11 controiled by the discriminator 8 in the manner in accordance with the invention.

Claims (4)

1. A method of spark-erosion machining of metal by means of spark pulses of short duration generated by discharge of a capacitor and passing through the working gap, in which method the capacitor lying in parallel with the working gap gets charged up by means of a d.c. source and upon a breakdown voltage being reached at the working gap is discharged across it, characterised in that the d.c.

source at the start of the spark discharge across the working gap is underthe control of a discriminator connected across the said working gap, whereby it is switched off from the working gap for a predetermined period of time, and is subsequently automatically switched on again.

2. A method as in Claim 1, characterised in that for the charging up of the capacitor a d.c. pulse generator is employed.

3. A method as in Claim 1 or Claim 2, characterised in that the connection of the d.c. source to the working gap is effected after a predetermined voltage of charge across the capacitor has been reached, and is maintained only on the earth side.

4. A method of spark erosion machining of metal, substantially as hereinbefore described, with particular reference to the accompanying drawings.