Re: [TCML] Tungsten electrode thickness

[Scott Bogard <sdbogard@xxxxxxxxx>]

Hi John,
     I'm coming in in the middle of this thread so take what I say
with some salt.  Firstly aluminum is the absolute worst metal you can
use in a spark cap, it has the lowest melting point of all commonly
available metals aside from lead or tin alloys.  Steel actually has a
substantially higher melting point than pure copper, hence why the
bronze age happened first; with that said it conducts poorly at RF
frequencies and will get really hot, for a propeller rotary gap not a
huge deal as a lot of cooling surface area, for a standard rotary, I
wouldn't...  As for the press fit vs no issue, again for a propeller
gap I would use a press fit, if anything the expanding rods during
operation will make things tighter, but on a standard one, too much
heat will make it to your disk, possibly warping it, (so you will have
one run in safety, then the melted disk will let electrodes fly after
the cool and shrink.)  Tungsten is the best, and yes it is expensive
but you only need to buy it once...  Bronze might be OK, it has a
lower melting point than pure copper but as was said it conducts
fairly well, and will move heat away from the contacts as it spins.  I
would not sue brass (actually I do, but I shouldn't) it "spits" and
leaves little bits of brass on the electrodes, not to mention zinc
fumes can kill, probably bronze is the best choice for an economy
spark gap, and again for a propeller gap I would not worry about a set
screw, for a disk type I would, regardless of material.  You could get
an awesome torch and melt some tungsten into copper and make your own
alloys, that would be pretty awesome.  Thanks for listening, have a
nice day.

Scott Bogard.

On 6/19/11, jhowson4@xxxxxxxxxxx <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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