Re: [TCML] Tungsten electrode thickness

[jhowson4@xxxxxxxxxxx]

Quick Question for everyone based on the deans response below, 

Is thermal expansion going to be a problem if I try to utilize this press fit sleeve approach. I would absolutely hate for the press fit to be perfect and room temp and then have the little tungsten pellets flying out of the sleeves at the high operating temps a spark gap would have the endure. 
Without getting into a discussion of thermodynamics is this an issue I have to worry about, and If so how can I avoid it. 

Thanks, 
John "Jay" Howson IV 

----- Original Message ----- 
From: "David Dean" <deano@xxxxxxxxxxxxxxxx> 
To: "Tesla Coil Mailing List" <tesla@xxxxxxxxxx> 
Sent: Saturday, June 11, 2011 11:18:27 AM 
Subject: Re: [TCML] Tungsten electrode thickness 

Steel melts. Hence the desire for tungsten which has the highest melting point of the readily 
available materials that are suitable for the job. 

If you use copper, which has a melting point a little above that of steel and has much better 
electrical conductivity (less heat generated from I^2*R losses) and better thermal conductivity 
(heat that is generated gets conducted away better, faster), you can get very good results without 
going to the expense of tungsten. 

Copper is not easy to find in rods or bars of a suitable size, so most folks use brass, which is 
easily machinable, readily available, relatively inexpensive, and mostly copper. Brass has its 
drawbacks though, there are many alloys of brass / bronze, some work better than others. 

I have found that there are "copper bolts" made for electrical switchgear, or for marine use, that 
are made of a modern silicon bronze alloy which is almost as good as pure copper. You can get them 
from McMaster Carr Supply ( http://mcmaster.com/ ) and they are not that expensive. I face them off 
in the lathe, and turn the hex heads round, then put a "rounded chamfer" on the end of the spark gap 
end of the head. 

If these are going on a rotary gap, the machining needs to be precise to keep it all in balance. I 
used one of those cheezy fits in your pocket electronic scales I picked up at a pawn shop a few 
years back to check the "static" balance, just removed a bit of material from each of the heavier 
ones until it weighs the same as the lightest. 

I made spacer sleeves from 3/4 inch diameter brass rod with a through center hole bored out with a 
"letter size U" drill bit (just a tad over 23/64", press fit for a 3/8" bolt) and smeared some of 
that silver bearing heat sink grease made for cpu's on the bolts before pressing them into the 
sleeves. Of course the sleeves were "static ballanced" too..., and I could have used aluminum 
because it is lighter, but I had the brass, and I could have machined "fins" on the sleeves to aid 
in heat dissipation, but I didn't. 

One advantage to this method is that in order to "upgrade" to tungsten all one would have to do is 
bore an undersized hole in the "bussiness" end of the electrode and press in a tungsten slug. I have 
a piece of 1/4" tungsten rod just for that purpose around here somewhere. Just he copper bolts work 
so well, and I have too many other things to do. 

later 
deano 
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