Re: [CAD_CAM_EDM_DRO] Another EDM Approach

hi hugh
can u send me diagram of crunt sensing and all EDM working??
thanks
shabbir


At 01:59 PM 8/1/01 +0100, you wrote:

>----- 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
>
>
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