Re: micro wire-edm project (and software solution)

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "Graham Stabler" <eexgs@n...>
wrote:

>
>
> I have been interested in EDM for several years but recently after
> thinking about building a small injection moulding machine gained
> renewed enthusuasm for the project. I am aiming to build a small
> machine designed for submerged cutting of small parts with wires in
> the order of 0.05mm thick.
>
> I am doing a little wiki on the project which can be seen here:
>
> www.indoor.flyer.co.uk/edmwiki.htm
>
> it is not an online editable wiki but if you follow the instructions
> on the front page you can save it to disk and edit it (then email it
> to me if you want).
>
> The main part of the project that may interest people is the software:
>
> Control of a wire EDM machines is difficult due to the need for
> precise gap control based on the elecrical and physical conditions
> between electrode and workpiece. The control software must be able to
> increase/decrease the feed rate and on occasions reverse the machining
> path a little.
>
> My idea which I mentioned before was to record the step and direction
> pulses out of the parallel port generated by Mach3/turbocnc. Once
> recorded they can be "played" back and forth according to the detected
> gap conditions. Origionally I was going to drip feed the pulses to a
> PIC but after some thought decided that it would be better to store
> the whole program at once. To do this I will use a multimedia card as
> you find in your camera (up to 1Gb and cheap). It is quite easy to
> interface a pic with such a card and it will also allow stand alone
> operation of the EDM which will be handy because I doubt it will be
> fast. A second pic will control the playing of the card to the drives.
>
> The power supply is a work in progress, conventional u-EDM machines
> seem to use RC supplies for simple sub microsecond pulse generation
> with C's less than 1uF. However it may be nice to have faster cutting
> with thicker wires by eiher a current limited (resisters) pulsed
> supply or pulsed current-source switching inverter. I will start with
> RC and then look at the alternatives. It seems however that the
> inverter option is only suitable for longer pulse lengths as the
> switching frequencies required would otherwise be well into MHz. The
> second option is definately possible with the right mosfets but you
> are building a heater.
>
> I have looked into gap control rather a lot and it seems that although
> not optimal taking the average gap voltage or average voltage accross
> a shunt resister has been used quite sucessfully my several machines.
>
> If you are interested in the project please drop me a line.
>
> Cheers,
>
> Graham
>
> p.s. No lectures on the complexity of gap control and the use of fuzzy
> logic etc etc please.
>

Couple things,

a diode can be a great and cheap type of shunt resistor for gap current.

and, I have recentluy been thiking about using AC for faster spark times.

considder a diode on each leg of the AC, one can seperate the two paths.

then using a transitor (FET) on the spark, open the line between the
charging resistor and the power supply. that would deliver the charge
in the cap only and not fade as the power supply voltage leaks past
the resistor during the spark.

then, when that cap is discharged, a second transistor, this one
between the cap and the work electrode, opens and allows the cap to ge
charged without being connected to the work.

this would allow a spark to occur while the second cap is being charged.

I'm not totally convinced that it will add any great value to the
power supply, but it has been buzzing around in the back of my EDM
thoughts.

Dave