Re: [CAD_CAM_EDM_DRO] Re: EDM spark control system

Larry Edington wrote:

> 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?

A sensor (either resistor or current transformer) detects the start of the
discharge. A delay allows the discharge to continue for some adjustable
time. then, the electronic switch is turned off, and kept off for an
adjustable
time. then, the voltage is turned on again, and it waits for the next spark
to start.

> An additional 'pause time' setting could then be added which would ensure X
> amount of time passed between sparks.

Yes, this is needed for the ions in the earlier discharge to recombine,
otherwise
the arc will just stay in one spot.

>
> 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.

The drives will probably work, but I suspect there would be a need for much
software change, or addition of sensors, etc. Anyway, I think these discharges

are in the range of hundreds of amps. The MOSFET's series resistance will
lower the peak current, but one 47 A MOSFET may not handle it. I was thinking
of using about 4 similar sized units in parallel.

>

>
> 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?

Close. What you want is to increase feed when there is no discharge, and
stop feed when the electrode shorts to the work. Within that range, you want
to modulate feed based on how many sparks / second there are.

>
> 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?

I have used 30 - 60 V with an average current of 1/2 A. But, the oil-paper
caps of several uF are dumping this energy directly across the electrode-
workpiece gap. The only series resistance is the electrode and workpiece
wires and switches to select the number of caps. I have no idea what the
peak current is. 100 A would be expected - and the sparks produced sure
look like VERY short duration pulses in that range. 1000 Amps would not
surprise me a lot! I do have the gear to measure it, but it will take a bit of

gear to set it up. I will need my digital storage scope to capture the
discharge.

> 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?

This may be similar to my favorite - alum-tap. A thin, oil-based clear
liquid that evaporates very slowly. It is flammable, however. most people
suggest water! you have to keep the voltage down around 30 or the
water starts to electrolyze.

Jon