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Rotary gaps

I am planning to build a rotary gap and would appreciate any suggestions or
comments from those who have already done this.

I am planning to use a 12.0" diameter nylon disc probably 3/8" or 1/2" thick.
 If I remember what I've read, my first choice for electrodes should be
tungsten, then brass then copper.  Is this correct?  I can only find tungsten
in 5/16" dia.  I think it is available in up to 3/8" but I haven't found any
yet.  I was thinking that I wanted about 1/2" dia. electrodes.  Any comment?
 I will use set screws in from the outside to hold the electrodes in place.
 I know I want to make the two stationary electrodes adjustable so I can vary
the gap.  If using this in series with a quenched gap what do you set the
gaps for?  Something like .050"?

This gap will be asynchronous.  I want to be able to control the speed so it
will fire 4 or 5 times during each half cycle (I have a transformer that will
supply plenty of current so I might as well take advantage of it) - does this
make sense?  If I use 8 electrodes, I need to run at up to 3600 to 4000 RPM.
 I would like to use a universal DC motor so I can control the speed with a
variac and a full wave bridge.  Can a small sewing machine motor handle this?
 The bearings in the motor may not support it.  How about mounting the disc
on a separate shaft with bearings on each side and driving it with a belt?

I would think it needs to be a sturdy construction (so the gaps aren't moving
in relation to the disc) so I probably want to build it on a steel frame.

Any comments would be appreciated.

Thanks,  Ed Sonderman