Machining steel balls for spark gaps: a better way

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Original poster: "Scott Hanson by way of Terry Fritz <teslalist-at-qwest-dot-net>" <huil888-at-surfside-dot-net>

Dan -

There's a much better way of mounting your steel balls for spark gaps. See 
photo at (http://surfboard.surfside-dot-net/huil888/ball-gaps.jpg) of a dual 
three-ball safety gap assembly I use in conjunction with Terry Fritz' R-C 
filter array described at hot-streamer-dot-com.

Regardless of whether they are bearing balls or unhardened balls, clamping, 
drilling and tapping will be an absolute nightmare. Even in a well equipped 
machine shop, this would be an unpleasant job. Additionally, if you use 
threaded rod to support the balls you will find that the sharp threads 
"leak" corona like crazy when HV is applied.

Much easier is to very slightly flare the end of a piece of copper tubing 
2-3 inches long and braze the ball into the seat formed by the flare. If 
you can get some, use thin layer of high-silver content brazing paste 
(powdered alloy & flux) smeared on the flared part of the tubing. 
Otherwise, use wire silver solder and paste flux. Hold the tubing upright 
in a vise, set the ball in the flared end of the tubing, and heat the ball 
with a propane torch. At red heat the brazing paste will flow into a 
perfect joint around the ball. Allow everything to cool, then solder a 
brass machine nut in the opposite end of the tubing as a means of 
connecting a solder lug.

Mount each ball assembly in a standoff insulator made from a piece of 1/2 
inch square Delrin, nylon, etc. Drill (or drill & ream) a through-hole in 
the standoff that will just accept the copper tubing in a sliding fit. 
Drill and tap another small hole in the top of the standoff for a lock 
screw to allow adjusting and locking the gap spacing.  Make three ball 
assemblies and three standoffs, and you have a very serviceable triple 
safety gap for NST protection, etc.  1/2" diameter copper tubing works 
great for 3/4" or 1" diameter balls, costs almost nothing, and a plumber's 
flaring tool costs less than $20 and will last a lifetime of tube flaring.

I have used this safety-gap/R-C filter array for several years on a 15 
kV/120 ma NST powered 6" X 32" coil. A set of triple ball gaps is used at 
both the "input" and "output" sides of the R-C filter. The filter is 
located right at the NST's.  An interesting observation:  under certain 
operating conditions, the "output" ball gaps will fire erratically, as 
would be expected. However, under other conditions the "input" gaps (right 
across the NST output) will fire, while there is no activity at the 
"output" gaps! There must be some strange resonances occurring to allow 
this. All four gaps are set with identical spacing.

The only downside of using carbon steel balls is that they will rust if 
exposed to moist air. McMaster-Carr offers stainless steel and bronze 
balls, but they get pricey in the larger sizes. I got a coffee can of 1" 
carbon steel balls for a few bucks and tried them first, but since I live 
on the coast rust has been a problem. Next time I'll use stainless steel 
balls if I can find them at the "right" price.

Regards,
Scott Hanson