Electric heating – Metal heating – Cutting or disintegrating
Patent
1979-08-13
1982-09-07
Shaw, C. C.
Electric heating
Metal heating
Cutting or disintegrating
219 69C, B23P 108
Patent
active
043485731
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to apparatus and methods for monitoring electrical machines and controlling such machines in accordance with the monitoring carried out. It is particularly, but not exclusively, related to monitoring electrical discharge machining (E.D.M.) otherwise known as spark erosion machining, and controlling E.D.M. machines.
Background Art
Some of the problems which arise in E.D.M. include establishing optimum pulse length and mark space ratio of pulses applied between a workpiece and an electrode machining the workpiece. Such parameters depend on for example the material of the workpiece and the shape to be machined. Other parameters which require optimisation are the size of the gap between the electrode and the workpiece, the speed of feed of the electrode towards the workpiece during machining and pulse voltages and currents. All these parameters are inter-related so that the adjustment of one affects another and consequently the effectiveness and efficiency of machining. Some of these parameters are discussed in more detail below.
Between the condition when the electrode touches the workpiece and a short circuit is present, and the condition in which the gap is too wide for an electrical discharge to take place, there are many ways in which an E.D.M. machine can be set up and an E.D.M. process carried out. Many known E.D.M. machines monitor the voltage across the gap and/or the current flowing through the electrode and the workpiece in order to give an operator a visual indication of how machining is progressing so that he can make adjustments. Usually in addition the monitored voltage and/or current is used to carry out some automatic adjustment of some of the above mentioned parameters.
Objects of the present invention include more effective monitoring of the discharge between the workpiece and the electrode, the provision of means for measuring the efficiency of E.D.M., and the provision of improved E.D.M. methods and apparatus.
Disclosure of Invention
According to a first aspect of the invention there is provided apparatus for monitoring electrical discharge machining comprising a receiver for sound of the type hereinafter specified emitted from the vicinity of an electrode and/or a workpiece when the electrode is being used to machine the workpiece, the sound having a characteristic which differs in some way or ways between first and second electrical-discharge conditions, and indicator means for providing a monitor signal representative of the said characteristic from output signals from the receiver.
In E.D.M. machining the type of discharge required is sparking rather than arcing. An arc is to be avoided because although metal removal rate is high an arc tends to remain stationary so that instead of obtaining even machining, metal is removed from one portion of the workpiece only. A spark on the other hand occurs at rapidly changing random positions across the gap so that even (that is generally homogeneous) machining is achieved. A spark can be regarded as the electrical discharge which precedes an arc. When an arc is set up a discharge first occurs across a gap and this discharge causes the dielectric, whether gas or fluid, to be ionised so that in a very short period an ionisation channel is set up. When such a channel is set up the electrical resistance along the channel is low and a discharge continues to follow the channel. In this condition an arc has been established. In other words a spark is present when a light emitting discharge occurs between the electrode and the workpiece but relatively little ionisation has taken place, certainly not enough to establish a "permanent" ionisation channel, while an arc is present when an electrical discharge along a low resistance ionisation channel occurs.
Voltage pulses applied between the workpiece and an electrode during E.D.M. machining have been mentioned, and the inventors have discovered that the discharge between the electrode and the workpiece during a voltage pulse is made up of a plurality of
REFERENCES:
patent: Re27707 (1973-07-01), Kauffman
patent: Re29398 (1977-09-01), Inoue
patent: 3673377 (1972-06-01), Jesnitzer et al.
patent: 3679865 (1972-07-01), Jesnitzer et al.
patent: 3816692 (1974-06-01), Ratmansky
"Controlling Machines by Ear" by J. D. Krouse, Machine Design, _Oct. 20, 1977, pp. 146-151.
Bhattacharyya Sushantha K.
El-Menshawy Mohamed F.
Woodrow Peter A.
National Research Development Corporation
Shaw C. C.
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