Re: [CAD_CAM_EDM_DRO] Another EDM Approach

----- Original Message -----
From: "Hugh Prescott" <hugh@...>
>
> Grab any blown PC powersupply and you can salvage at least one toroid,
somes
> times three from it that will make a nice current transformer.
>

Hi,

I'm a little bit puzzled how this could work on an EDM for current sensing
as it seems normal to use a DC voltage. Granted, when the thing is actually
sparking, the voltage/current will be chopped but, at times other than this,
surely, the coil method won't sense anything. As someone else mentioned, the
circuit which was published in the Model Engineer magazine some time ago,
and which formed the basis of my first EDM experiments, used a resistive
divider across the main spark current lines to feed current to the base of a
transistor, the output of which was then used to flip the direction input of
an SAA1024 stepper controller chip feeding the electrode positioning
stepper. So, when the macine was first switched on, the stepper drove the
electrode into the work (a simple 555 oscillator circuit fed the 'step'
input of the SAA1024) until the electrode made contact. This then shorted
across the resistive divider and grounded the base of the transistor which
reversed the direction of the drive motor so raising the electrode again. As
the contact broke, sparks were produced which ate away at the workpiece
until the current level in the resistive divider fell enough to allow the
transistor base to be pulled high again. The circuit was a bit crude as it
didn't actually monitor the current in the electrode/work and so there was
no real control over the quality of the cut. It also suffered from the
problem of blowing the transistor regularly as the sparking produced a lot
of RF spikes which seemed to easily get past the zener diode voltage limiter
across the base/emitter junction. When the circuit was working, it did a
fairly good job of tap removal etc. but just wasn't refined enough to do any
really useful work.
This system of hitting the work and backing up the electrode would be no use
whatever on a wire EDM machine where the reversal of direction would have to
be on two axes simultaneously in the exact direcvtion they had just moved
and so it is imperative we find a reliable method of current sensing which
will work on both DC and chopped DC and will tolerate large, fast voltage
spikes and which we can then use to control the speed of feed of the CNC
part.

Ian
--
Ian W. Wright
Sheffield UK
www.iw63.freeserve.co.uk