Re: [CAD_CAM_EDM_DRO] Re: EDM spark control system

>Since I know nothing about EDM, but would like to build one as well, I shall
>ask some newbie questions.
>
>How would a chopper style drive that lets the current ramp up then shuts the
>driver off at a preset current level work?
>
>An additional 'pause time' setting could then be added which would ensure X
>amount of time passed between sparks.
>
>Second thought. How about a standard high frequency PWM drive? Varying PWM
>frequency and duty cycle would provide the spark on and off time variations.
>
>If either would work, I've already got both designs done that are
>controlling 120V DC motors with LCD displays for operator inputs. The LCD's
>connect to the control board via RS-485 links. These are optoisolated Mosfet
>drives.
>
>It would be trivial to change the boards to add more Mosfets to provide for
>higher current. I'm using a 200V 47A Mosfet for the 15A locked rotor current
>120 VDC motors now.
>
>I could also modify the code to provide for a simple variable on and off
>time.
>
>Am I correct in assuming that the EDM controller should assert a 'feed hold'
>signal to the CNC controller when firing pulses to the electrode then when
>current no longer flows after X number of pulses it then releases the feed
>hold signal while continuing to fire pulses to allow the CNC controller to
>move the electrode into the work piece?
>
>This I could also add to my existing code as it would be a trivial change.
>Right now, I look for a stalled motor over time to shut down the drive and
>turn on an alarm buzzer and blink an LED. The logic would just be reversed
>on the EDM.
>
>Since I missed the start of this discussion, could someone repeat the
>voltages and currents required for EDM?
>
>If my assumptions are correct, I've already got the basis for an EDM
>running.
>
>Is Kerosene still the most common dielectric fluid for EDM?
>
>later,
>Larry E.
>
>
>
>> > It looks like the "electronic switch" would be a power transistor
>> > (MOSFET or bipolar) that would have to tolerate fairly high current
>> > pulses. Perhaps much higher than the "1/4 to 1/2 amp" average
>> > current. Once that parameter is nailed down, it's easy to generate
>> > whatever pulse rate and duration you want.
>>
>> Since this is essentially a short circuit during the spark, a MOSFET would
>> be the best choice, as it resists secondary breakdown. i would guess
>> about 4 of a 25 A 100 V transistor in parallel would be a good start.
>> I already scribbled down a circuit with a current sense resistor (although
>> a transformer-type current pickup would also work) a 556 dual timer
>> and a MOSFET gate driver. It would have pots to adjust on and off
>> time, and could also output a signal to the CNC telling it when to proceed
>> and when to stop the feed.
>>
>> I could enter this as a schematic and post it to the drop box. I'm a bit
>> too busy with commercial work to do any construction and testing of it
>> right now, but would be glad to help anyone who wanted to build it.
>
>
>
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