Patent GB-1196644-A (GB)
Title: Improvements in Spark Erosion Process and Apparatus
IPC: B23H1/02
Filed: 1967-07-26
Abstract: 1,196,644. Pulse circuits. INGERSOLL MILLING MACHINE CO. 26 July, 1967 [27 July, 1966], No. 34262/67. Headings H3P and H3T. [Also in Division B3] In the spark erosion machining of a conductive workpiece 16, a relatively high voltage source 20 is first connected across the gap 17 to effect ionization followed by a relatively low voltage high power machining source 22. The source 20 is connected through a switch 28, consisting of a tube or transistor controlled by a pulse generator 24. Source 22 is connected through a similar switch 42 also controlled by the pulse generator through a delay timer 36. Instead of or in addition to the timer, a diode 50 may be used to prevent application of the power source 22 until the gap has been ionized. The voltage pulses produced across the gap have a loading peak of short duration merging into a low constant voltage portion of longer duration, Fig. 2 (not shown). In modified embodiments, Figs. 3, 3a, 4 (not shown), a pulse transformer (54) and/or an R-C network are connected between the switch 28 and the gap. In another embodiment, Fig. 7 (not shown), an "and" gate (140) is connected between the timer (154) and power switch (142). This gate is supplied with an reference voltage (160) and is arranged to open the power switch (142) in the event of a short circuit in the gap. In another embodiment, Fig. 5 (not shown), the high voltage switch (102) is controlled by a flip-flop circuit (98) and the power switch (116) by a monostable vibrator (112). A detection device (90) connected across the gap provides a signal to the flip-flop when the gap has been deionized and a further signal to the vibrator when the gap has been ionized by the high voltage source. The flip-flop circuit (98) ensures that the high voltage switch (102) is opened when the power switch (116) closes. In a further embodiment, Fig. 9, a pulse generator 230, whose frequency is variable between 100 c.p.s. and 500 k.c.p.s., and whose duty cycle is also variable between 5% and 95%, feeds a pulse through a differentiating circuit 234 to a firing pulse amplifier 238 which feeds it through conductor 240 to the gap 17. Simultaneously a high voltage switch circuit 250 is energized to provide the gap with sufficient power to erode away any burns in the gap causing a short circuit. A pulse is also fed through gate 258 and inverter circuits 270, 276-to a bank of transistors or tubes 286 acting as a power switch. The latter delivers a machining pulse from variable voltage source 282 to the gap 17 through rectifier 290 which blacks the pulse until the gap has been ionized. A voltage sensor 300 and a current sensor 332 detect conditions in the spark gap and act on the gate 25 through circuits 308, 312, 322, 326, 340 to black the triggering pulse producing the main machining pulse until the gap is ionized and to stop the pulse in the event of a short circuit. A detailed example of this embodiment is described, Fig. 11b, wherein the gap 17 between workpiece 16 and electrode 18 is fed with high voltage ionizing pulses from source 526 through firing pulse switch triode 472 in parallel with high voltage switch triode 470. High power machining pulses are supplied from variable voltage source 540 through diode 636 and a bank of parallel transistors 630, 632. The high voltage triodes 470, 472 are controlled through an amplifier-differentiating circuit 464 comprising a normally conducting transistor 477 and differentiating network 488, 490, 492. The output of this network is fed to transistor 468 through diode 498 which selects the negative spike from the network. The output of this transistor 468 is connected to the control grid 546 of pulse triode 472, which produces a firing pulse of 1 amp for 1 microsecond. Transistor 477 is also connected through resistor 560 and diode 564 to transistor 466 whose output controls the grid 580 of high voltage switch triode 470. Protection diodes 520, 622 are provided for the base-emitter circuits of transistors 468, 466. The differentiating circuit 464 receives control pulses along conductor 462 from a Schmitt Trigger circuit (400), Fig. 11a (not shown), supplied from the main pulse generator (350). The machining pulse transistors 630, 632 are controlled by driver transistor 640 supplied with pulses from the main pulse generator (350) through inverter amplifier (732), Fig. 11a (not shown), Schmitt Trigger circuit (670) and "and" gate (672). The voltage across the gap is detected by conductors 842, 844 and fed a voltage sensing circuit (840), ig. 11c (not shown) whose output is connected through a Schmitt Trigger circuit (900) to a flip-flop circuit (790), Fig. 11a (not shown). The current passing through the gap is detected by conductors 1002, 1004, Fig. 11c and passed to a currant sensing circuit (1000), Fig. 11c (not shown), whose output is also fed to the flip-flop circuit (790), Fig. 11a (not shown). The latter is connected to the "and" gate (672) controlling the supply of triggering pulses for the machining pulses. The spark gap is bathed in a dielectric coolant.