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.