GB1594967A - Spark erosion machine - Google Patents

Description

(54) SPARK EROSION MACHINE (71) We, LINDERFORT LIMITED, a British Company, of Barton Road, Water Eaton Industrial Estate, Bletchley, Buckinghamshire, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to a spark erosion machine suitable for use in the machining of die steels and extremely hard materials such as tungsten carbide and hardened alloy steels.

In a spark erosion machine, sparks are caused to travel from an electrode to a workpiece and the latter is eroded. The potential difference to generate the sparks was originally supplied from an RC relaxation circuit, but such a circuit, which was very inefficient has now been superceded by a combination of a rectifier arrangement providing a low voltage, smoothed, direct current supply and an inverter which chops the direct supply into a series of pulses whose frequency, height and mark/space ration can be varied to suit different operating conditions and requirements.

The output stage of the inverter is connected to the electrode and workpiece, and because these are very close together the possibility of their touching during operating of the machine is quite high. When the electrode and workpiece touch they shortcircuit the output stage of the inverter and the full open-circuit voltage (which may be 85 volts) is applied across the output stage. A high current thus flows through the transistor(s) of the output stage and would cause damage if it was not limited. In order to limit this short-circuit current each transistor (there are normally several) must have a resistance between it and (say) the electrode sufficient to limit the short-circuit current to a safe (i.e. non-damaging) value. This leads to a severe disadvantage of the known machine in that these resistors are always in series with the spark gap.Because currents used in spark erosion are quite high, 100 amps is not uncommon, these protective resistors must be rated for high current and must be effectively air cooled. This usually leads to them forming a bulky unit almost additional to the essential parts of the machine. By far the most serious disadvantage of these resistors, however, is in power consumption and thus economy. A typical known machine having a 100 amp operating current requires 18 KVA input. Not only does this mean that the running costs are high, but the existence of a high voltage three-phase supply is necessary. In some countries of the world this is not the case and such machines cannot be used there.

An object of the invention is to provide a spark erosion machine wherein the above disadvantages are obviated or minimised.

Accordingly the invention provides a spark erosion machine wherein operating current is supplied to an electrode from a direct current source via circuitry including a waveform generator and a power transistor arrangement, there being provided means responsive to the condition at the electrode/workpiece gap effective to disable the waveform generator when the current flowing across the gap reaches a predetermined value, the maximum current rating of the power transistor arrangement being greater than said predetermined value.

The transistor arrangement can be a single transistor of high current rating, or can be a plurality of individual transistors arranged in parallel, or a plurality of high current rating transistors for higher outputs than 100 amp.

Preferably the transistor arrangement is so chosen than when disablement of the waveform generator occurs, the current flowing is considerably lower than the maximum permissible current, through the transistor arrangement. This has the advantage that, in use, the transistor arrangement operates only at a fraction of its normal operating current, which leads to great reliability and reduced cooling requirements.

The means reaponsive to the condition of the electrode/workpiece gap can be responsive to the actual current flowing between electrode and workpiece and may be an additional output winding on a transformer of the direct current supply, this winding being connected in series with a resistance and a current sensor which, when the current through the resistance reaches a desired value, cuts off the supply to the inverter. The resistance should have little or no change of value with temperature so that the cut off value remains constant.

Preferably, however, said means is responsive to the voltage between the electrode and the workpiece (it being appreciated that as a short-circuit approaches current rises and voltage falls). The means can be a simple voltage-sensitive circuit which, when the voltage drops below a predetermined value, for example 20 volts, acts to reduce the pulse rate of the inverter to zero. Thus, no pulses pass to the transistor amplifier output stage and supply to the electrode and workpiece is cut off.

In the machine of the invention the bulky, highly-rated, resistors are dispensed with and a machine having a 100 amp output needs an input of only 3 KVA. This naturally means that the machine of the invention is cheaper to run and can, if necessary, be operated from single-phase 230 volt supply. Further units can be paralleled to give outputs in excess of 100 amps, i.e. 200, 300, 400 etc, but these will require 3 phase inputs.

The invention will be described further by way of example, with reference to the accompanying drawings, which illustrate a preferred embodiment thereof, it being understood that the following description is illustrative and not limitative of the scope of the invention.

In the drawings: Fig. l is a circuit diagram of a power supply of a preferred machine of the invention; Fig. 2 is a circuit diagram of a waveform generator of the machine; and Fig. 3 is a circuit diagram of a final transistor amplifier stage connected to the waveform generator.

A preferred embodiment of spark erosion machine conforming to the invention comprises a quill mounted above a tank. In use the quill carries an electrode and a workpiece is secured to the base of the tank. Operating current is supplied to the electrode and tank (the latter being filled with a cooling and flushing dielectric fluid).

Movement of the quill is in response to a servo motor driven from an amplifier which is responsive to the voltage across the gap between the electrode and workpiece.

By keeping this voltage constant it is ensured that the gap remains constant, and this makes for efficient erosion. Naturally as the electrode approaches closer to the workpiece the gap voltage decreases, reaching zero when a short-circuit arises. When this happens the quill is automatically caused to rise, which opens the gap again. This has relevance to the supply of the operating current as will be evident later. Control circuitry is provided for the quill movement, but forms not part of the present invention and will not be described further. As the present invention relates to the supply of power to the electrode and workpiece the aforementioned parts, which are generally of conventional construction, have not been illustrated.

The power supply 10 for the operating current includes three transformers, T 1, T2, and T3 which, from a 415 volts three-phase supply, and in combination with full-wave rectifier and smoothing circuitry provide a main power supply at 70 volts and subsidiary power supplies of + 15V, - 15V and + 80V.

Fig. 2 shows a waveform generator circuit which includes an integrated circuit W.G., which produces a pulsed output at G.Y., the pulse rate of which, the pulse height of which, and the mark/space ration of which can be varied over wide limits to suit different conditions and requirements of operation.

The pulsed output of the waveform generator passes to a transistor amplifier output stage illustrated in Fig. 3. Here a series of power transistors Tp are controlled by the waveform generator to produce power pulses which are fed to the electrode via the quill.

Also illustrated in Fig. 2 is a cut out circuit B, which is sensitive to the voltage between the electrode and the workpiece. When a short circuit tends to occur between the electrode and workpiece, the current rises and the voltage falls. When the voltage at the quill, from terminal BL, falls to a predetermined value, corresponding to a current of 100 amps, circuit B influences the waveform generator via its terminal 4 to cause it to cease pulsing. This means that no pulses are supplied to the power transistor Tp which thus do not conduct and thus power to the electrode is cut off.

The maximum permissible current through the power transistors Tp is some 800 amps and this has two consequencs. Firstly, when a short-circuit tends to occur at the electrode/workpiece, the current rises rapidly but cannot rise above 100 amps, at which value the waveform generator ceases to operate. Thus, the current between electrode and workpiece can never rise high enough to cause a dangerously high current to flow through any one transistor Tp. Secondly, during normal use the transistors Tp are operating at only a fraction of their rated capacities and this makes for great reliability and less severe cooling requirements.

Because the transistors Tp do not have any significant resistances in series with them, there is little power loss in the resistors and this means that the machine of the invention, having a maximum operating current of 100 amps, needs an input of only 3 KVA. This not only reduces the operating costs, but enables the machine to be used (with a modified transformer) with single phase 230 volt mains supply. This is particularly important in some developing countries which do not have a three-phase mains supply.

Variations can be made to the above. For example, the large numer of transistors used (which each have a maximum permissible current of, say, 4 ampls) can be replaced by a smaller number of high power transistors each having a maximum permissible current of, say 100 amps.

As an alternative to the voltage-sensitive cut-out circuit there can be an auxiliary winding on the transformer, in which winding is induced a current proportional to the current being taken from the transformer and the thus proportional to the current passing from the electrode to the workpiece.

This coil is connected in series with a fixed resistance having a zero or close to zero thermal coefficient, and a current sensor. The current sensor is operative to interrupt the power supply to the inverter when the operating current reaches a predetermined value, which can be 100 amps. When the power supply is cut off the voltage at the electrodes naturally falls to zero and via the amplifier the servo-motor acts to raise the quill, thus tending to break the short circuit which has occurred or was about to occur.

WHAT WE CLAIM IS: 1. A spark erosion machine wherein operating current is supplied to an electrode from a direct current source via circuitry including a waveform generator and a power transistor arrangement, there being providing means responsive to the condition at the electrode/workpiece gap effective to disable the waveform generator when the current flowing across the gap reaches a predetermined value, the maximum current rating of the power transistor arrangement being greater than said predetermined value.

2. A machine as claimed in Claim 1, wherein said means is responsive to the current in the gap and acts to disable the waveform generator when the current reaches a predetermined value.

3. A machine as claimed in Claim 2, wherein said means includes an extra winding on a transformer in a power supply of the machine and a current sensor arranged to monitor the current in a circuit including this winding and cut-off power to the waveform generator when such current reaches a value corresponding to said predetermined current in the gap.

4. A machine as claimed in Claim 1, wherein said means is responsive to the voltage across the gap and acts to disable the waveform generator when this voltage falls to a value corresponding to said predetermined current in the gap.

5. A machine as claimed in Claim 2, 3 or 4, wherein the power transistor arrangement consists of one transistor or a group of transistors in parallel, the maximum current rating of the transistor or the group being substantially greater than said predetermined value.

6. A spark erosion machine substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (6)

**WARNING** start of CLMS field may overlap end of DESC **. amps, needs an input of only 3 KVA. This not only reduces the operating costs, but enables the machine to be used (with a modified transformer) with single phase 230 volt mains supply. This is particularly important in some developing countries which do not have a three-phase mains supply. Variations can be made to the above. For example, the large numer of transistors used (which each have a maximum permissible current of, say, 4 ampls) can be replaced by a smaller number of high power transistors each having a maximum permissible current of, say 100 amps. As an alternative to the voltage-sensitive cut-out circuit there can be an auxiliary winding on the transformer, in which winding is induced a current proportional to the current being taken from the transformer and the thus proportional to the current passing from the electrode to the workpiece. This coil is connected in series with a fixed resistance having a zero or close to zero thermal coefficient, and a current sensor. The current sensor is operative to interrupt the power supply to the inverter when the operating current reaches a predetermined value, which can be 100 amps. When the power supply is cut off the voltage at the electrodes naturally falls to zero and via the amplifier the servo-motor acts to raise the quill, thus tending to break the short circuit which has occurred or was about to occur. WHAT WE CLAIM IS:

1. A spark erosion machine wherein operating current is supplied to an electrode from a direct current source via circuitry including a waveform generator and a power transistor arrangement, there being providing means responsive to the condition at the electrode/workpiece gap effective to disable the waveform generator when the current flowing across the gap reaches a predetermined value, the maximum current rating of the power transistor arrangement being greater than said predetermined value.

2. A machine as claimed in Claim 1, wherein said means is responsive to the current in the gap and acts to disable the waveform generator when the current reaches a predetermined value.

3. A machine as claimed in Claim 2, wherein said means includes an extra winding on a transformer in a power supply of the machine and a current sensor arranged to monitor the current in a circuit including this winding and cut-off power to the waveform generator when such current reaches a value corresponding to said predetermined current in the gap.

4. A machine as claimed in Claim 1, wherein said means is responsive to the voltage across the gap and acts to disable the waveform generator when this voltage falls to a value corresponding to said predetermined current in the gap.

5. A machine as claimed in Claim 2, 3 or 4, wherein the power transistor arrangement consists of one transistor or a group of transistors in parallel, the maximum current rating of the transistor or the group being substantially greater than said predetermined value.

6. A spark erosion machine substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.