Electrical circuit for electro-discharge machines

An electrical discharge machining circuit for a machine working a workpiece by electro-erosive discharges through a dielectric medium in a machining gap between an electrode-tool and said workpiece which constitutes itself the second electrode of said gap, comprising:

• erosive discharge means having energy delivering output terminals which repetitively deliver high current erosive pulses to said electrodes and which exhibit corresponding voltage pulses, and
• preignition means for providing, in prologue to said erosive pulses, preignition pulses of a higher voltage but of a lower energy than said erosive pulses, said preignition pulses ionizing said dielectric medium in said machining gap so as to create each time a channel which thereafter allows an erosive discharge to flow therethrough,
• said preignition means comprising a step-up transformer and a preignition control circuit connected to both the primary winding of said transformer and to said output terminals of said erosive discharge means, said preignition control circuit comprising a Zener diode and being arranged to respond to the arrival of a certain value of voltage across said terminals to cause a current to flow from said terminals through said primary winding,
• said preignition control circuit comprising a preignition control transistor the base of which is connected to said Zener diode and the emitter-collector track of which is connected in series with said primary winding of said transformer, said preignition control circuit further comprising a pulse feedback loop that includes an amplifying transistor and a capacitor through which a reactive potential can be applied to the base of said preignition control transistor so as to give an abrupt reinforcement of a current in said base once initialized through said Zener diode, and said preignition control circuit including an input for receiving a signal which, independently of voltage changes on said output terminals, causes a one-shot double triggering of said transistors for generating a complementary preignition pulse, said signal being applied in replacement of a normal preignition pulse.

An electrical discharge machining circuit for a machine working a workpiece by electro-erosive discharges through a dielectric medium in a machining gap between an electrode-tool and said workpiece which constitutes itself the second electrode of said gap, comprising:

• erosive discharge means comprising a capacitor and means for charging and discharging said capacitor, and means responsive to the discharge of said capacitor for supplying said electrodes with repetitive pulses of erosive electrical energy, and
• preignition means for providing, in prologue to said erosive pulses, preignition pulses of a higher voltage but of a lower energy than said erosive pulses, said preignition pulses ionizing said dielectric medium in said machining gap so as to create each time a channel which thereafter allows an erosive discharge to flow therethrough,
• said preignition means comprising a step-up transformer having a primary winding and a secondary winding, first clamping means including a first Zener diode connecting said capacitor to said primary winding of said transformer, said first clamping means being operative to control the initiation of current flow through said primary winding during the charging of said capacitor, the secondary winding of said transformer being connected across said machining gap, and second clamping means connected across said machining gap on the secondary side of said transformer, said second clamping means including a second Zener diode and having high voltage and low energy parameters which correspond to the voltage and energy parameters of said preignition pulses, said second clamping means being operative to limit the voltage of said preignition pulses across said gap if the voltage of said preignition pulses tends to exceed a reference value related to said high voltage parameter of said second clamping means.