Electric heating – Metal heating – Cutting or disintegrating
Patent
1993-04-15
1994-04-26
Evans, Geoffrey S.
Electric heating
Metal heating
Cutting or disintegrating
B23H 704
Patent
active
053068890
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a wire cut electrical-discharge machining apparatus having a spark-status detector for detecting any bias in spark discharge positions, thereby preventing the wire electrode from breaking during wire cut electric discharge machining. The present invention also relates to a wire cut electrical discharge machining apparatus having a device for detecting a condition, such as a concentration of spark discharges, indicative of an impending wire electrode breakage and, more specifically, to a device for generating signals indicative of a biased discharge state. The present invention also relates to a device for measuring the break limit of a wire electrode from the conditions which gave rise to the formation of the detection signals indicative of a biased spark discharge or spark concentration.
BACKGROUND
A wire cut electrical discharge machining apparatus is an apparatus designed to use a thin wire of about 0.05 to 0.35 mm.phi. in diameter as an electrode tool. The wire is held in tension between a pair of positioning guide members which permit renewal of the wire electrode in the work zone by allowing movement of the electrode wire in its axial direction while maintaining a predetermined tension on it. During normal machining, the wire electrode approaches the workpiece to be machined at approximately a right angle with respect to the axis of the wire electrode, while maintaining a very small machining gap between the workpiece and the wire electrode. Machining is done by generating spark discharges on an intermittent basis. The discharges are generated by applying periodic voltage pulses across the working gap between the wire electrode and the workpiece under conditions where a working fluid fills the working gap. In such an apparatus, the machining feed device causes relative movement between the wire electrode and the workpiece as machining progresses. Unless otherwise prevented by suitable machining condition settings and control of working conditions, machining may be interrupted as a result of wire electrode breakage. Some recent arrangements for dealing with wire electrode breakage include self-recovery devices, such as, for example, a function which automatically joins broken wire electrode ends or otherwise rethreads the electrode through the workpiece. Such joining or rethreading functions are not necessarily 100 percent successful and, in any event, even if successfully accomplished, are not only wasteful of time, but also generally impair machining accuracy and the like. It is, therefore, preferable to avoid wire breakage. However, to do so in the context of prior art wire cut electrical discharge machines creates other problems. In order to machine accurately and with high performance, i.e., efficiently and at higher machining speed, working conditions, and in particular machining voltage pulse width and off time duration, mean amplitude of the discharge current, and machining feed control, must be established and controlled so that the machining is done under heavier load conditions at which there is a higher risk of wire breakage. Moreover, it is impossible to carry out precision machining wherein the wire electrode is highly tensioned without risking wire breakage, unless the machining is done at relatively low efficiency working conditions.
There are various causes of wire electrode breakage. However, wire breakage during machining may, to some extent, be avoided by detecting spark conditions, and controlling the working conditions to light-load conditions if a concentration of spark positions is detected; in other words, a condition where the sparks are only partially distributed along a limited area of the machining gap. Wire electrode breakage due to causes such as spark concentration may be avoided through the use of gap condition detection and adaptive control based on that detection. Appropriate detection, for the purpose of preventing wire electrode breakage, requires detecting the position of the sparks being generated within the wo
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Japan Society of Electrical-Machining Engineers, vol. 23/No. 45, Dec. 1989, pp. 22-28--"Obara, et al.".
Japan Society of Electrical-Machining Engineers, vol. 24/No. 47, Nov. 1990, pp. 12-22--"Obara, et al.".
Kaneko Yuji
Nishimura Hideo
Evans Geoffrey S.
Sodick Co. Ltd.
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