Re: Photoflash capacitors for EDM use

--- In CAD_CAM_EDM_DRO@yahoogroups.com, Dave Dillabough <dgd@b...>
wrote:

> If you are going to use these caps make sure that you monitor the
> temperature. Photo flash caps are probably not designed to be

repeatedly

> charged and discharged at the frequency that an EDM would use. If

an

> electrolytic cap gets hot enough it will vent or blow the end out.

Makes

> quite a bang and a mess.
>
> For use in an EDM you want a cap with a low ESR (Equivalent Series
> Resistance) so that it will not heat up when you charge and

discharge it at

> a high frequency. If you can find them in the right value oil

filled caps

> like the ones uses on motors work well.

I agree with the above and have read somewhere that motor starting
and/or running capacitors are not recommended for oscillator type
applications (we do have a sawtooth oscillator at work in the RC EDM
machines). I guess that allows only non electrolytic, non polarized
paper types with really secure foil to wire lead connections.
Actually I have been running my EDM ( a Langlois unit almost) for
quite a few years now and have been using motor start/run capacitors
for the driver. They don't get warm and seem ok for my use. I do a
lot more than tap busting, with electrodes as large as 1/4 inch brass
tubing, all shapes, where required. Rather than a large power
resistor in the charging path, I use whatever wattage light bulb I
happen to find. The stepper drive system as per Langlois and a 200
step motor is just perfect for the job.

With careful machine adjustment, the spark gap can be held fairly
constant allowing 'sparking', not arcing and it appears the electrode
never touches the work. It just purrs along. I believe I have done
enough playing to find that the the size of the lightbulb and/or the
capacitance value doesn't cause a very noticeable change in the time
required to spark a hole of a given size. I assume the frequency of
the sawtooth controls the cutting rate, and the frequency is
determined by the R and C components as well as the gap, dielectric,
water in my case and probably many other things not easily
controllable.