* Original msg to: Esondrmn-at-aol-dot-com

Quoting Ed Sonderman:

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

Remember, you asked for it :-)

 ES> I am planning to use a 12.0" diameter nylon disc probably    
 ES> 3/8" or 1/2" thick.

I would avoid plastic rotors. The advantages are obvious, but I
have had them disintegrate at speed. I know of others who have
had failures of plastic rotors with disastrous results. If you do
use a plastic rotor, you must build a shield box to absorb the
impact of flying electrodes in the event of a rotor failure. Ed
Wingate in New York swears by a fiberglas re-enforced resinous
plastic called "G-10 epoxy". He claims no rotor failures in the
gaps he has built with this material.

 ES> If I remember what I've read, my first choice for electrodes 
 ES> should be tungsten, then brass then copper. Is this correct? 

Not bad. I would include zinc on the list as well. Zinc has a
very low ionization constant in an arc.

 ES> I can only find tungsten in 5/16" dia.  I think it is        
 ES> available in up to 3/8" but I haven't found any yet.  I was  
 ES> thinking that I wanted about 1/2" dia. electrodes.  Any      
 ES> comment? 

1/2 inch will be too large in diameter. The larger the diameter
of the moving electrodes on a rotary, the longer the dwell time.
It has been pretty well established that the shorter the dwell,
the better the spark. 

 ES> I will use set screws in from the outside to hold the        
 ES> electrodes in place. 

A common and accepted practice.

 ES> I know I want to make the two stationary electrodes          
 ES> adjustable so I can vary the gap.  If using this in series   
 ES> with a quenched gap what do you set the gaps for?  Something
 ES> like .050"? 

If you are talking about the rotary gap electrodes... set the
stationaries as close as possible to the rotary electrodes.
Almost touching, or as close as the tolerances of the gap will
allow. The rotary should be responsible for commutating the arc
 ES> This gap will be asynchronous.  I want to be able to control 
 ES> the speed so it will fire 4 or 5 times during each half      
 ES> cycle (I have a transformer that will supply plenty of       
 ES> current so I might as well take advantage of it) - does this 
 ES> make sense?  

Perfect sense.

 ES> If I use 8 electrodes, I need to run at up to 3600 to 4000   
 ES> RPM. I would like to use a universal DC motor so I can       
 ES> control the speed with a variac and a full wave bridge. 
 ES> Can a small sewing machine motor handle this? The bearings   
 ES> in the motor may not support it. 

Not enough torque at speed. One of the problems I have seen with
small high speed motors is that voltage drops in the line when
the coil is fired, or firing, can cause a drop in the rotor
speed. I have also seen oscillations set up between the gap motor
and the coil voltage draw. Coils are rarely prefectly smooth,
especially pig coils running with little or no resistive ballast.
The rotary gap will actually end up being the current limiter for
the supply circuit when the coil is run flat out. When the coil
fires heavy, it draws a lot of current, and the gap motor lags
due to the arc and a slight voltage drop in the supply lines. 
The gap motor lagging decreases the break rate, which decreases
the current draw by the tank circuit, which increases the line
voltage, and increases rotor speed, which increases the current
draw, which causes a slight voltage drop... The oscillation has
about the same frequency as the lag time of the inductive delay
in the control circuits: figure about 1 second give or take. It
is not critical or anything, just unsettling. It does not make
for very smooth coil operation. 

Running a distinct power line for the gap motor is only a partial
solution, as the force of the arc connecting the electrodes also
accounts for some of the lag. The only good solution is to equip
the small motor with a large flywheel; use a heavier motor; or

 ES> How about mounting the disc on a separate shaft with         
 ES> bearings on each side and driving it with a belt?