Re: [TCML] Tungsten electrode thickness

[Phil <phil@xxxxxxxxxxx>]

On Sun, 19 Jun 2011 04:13:58 +0000 (UTC)
jhowson4@xxxxxxxxxxx wrote:

> 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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Brass expands about 4 times more then Tungsten by volume (Wikipaedia),
so there may be some loss of grip. However the rotating electrodes do
stay fairly cool due to air flow.
Having said all that I would never just have a tolerance fit for
electrodes that are flying around at 3000 rpm (think bullets).

Phil
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