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Rotary spark gap electrodes

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Roger

Sat Apr 08 2006, 08:52PM

Registered Member #221 Joined: Mon Feb 20 2006, 05:36PM
Location: Chillicothe Ohio
Posts: 12

I'm building a rotary spark gap that uses a phenolic disk with electrodes around the circumference that make close contact with stationary electrodes. This is a very common design and I've seen pictures of them before. I was just wondering where people get the tungsten tip electrodes from. The electrodes look like something that can be bought off the self from somewhere. Dose anybody know?

Steve Conner

Sun Apr 09 2006, 11:13PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Hi roger

Most people buy tungsten rod from welding supply stores and cut it to length. It's used as electrodes for TIG welders. Tungsten is incredibly hard, so try and get the store to cut it for you. If you have to do it yourself, I believe the best way is to grind it.

I was given a kit of parts for a rotary gap by a coiler who never got round to finishing it. There was a plastic packet in there with some lengths of 1/4" tungsten rod and a warning sticker about the dangers of arc welding, so I guess those came from a welding supplier.

I'm pretty hopeless when it comes to precision metalwork, and if I tried to build it myself I would probably have ruined the good parts. So I sent the whole lot to a friend who is a professional machinist and it came back as a lovely rotary gap that runs at 5000rpm with practically no vibration.

Hazmatt_(The Underdog)

Sun Apr 09 2006, 11:25PM

Registered Member #135 Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735

There are several.

One source which is convient for me is broken circuit board drills. I break them sometimes, then keep them lying around for electrodes. They are larger then TIG rod so they give you just a little bit more dwell time. Their dia. is 1/8" and many are solid carbide. Once in a while you run across a carbide core drill. If you use one of these you will find it errodes horrendously.

Another source is Richard Hull of formerly TCBOR. For some time they were offering electrode kits. He mentions this in his construciton video.

The source where R.H. gets his electrodes would then be Miller. Their older welding machines use high frequency start electrodes in 2 paris closely gapped. Tungsten faced and I think 3/8" diameter.
I just recently ordered these through Cameron Welding from Miller. If you write to their technical staff they can give you the part number, which I am sorry, I have forgotten.
I will be at college tomorrow and I will see if I can get that part number off the PO and send it to you via PM. The faced electrodes cost ~$7 each.

Desmogod

Mon Apr 10 2006, 01:51AM

Registered Member #139 Joined: Sat Feb 11 2006, 11:01AM
Location: Perth, Western Australia
Posts: 358

Try that link, there's a fair few companies listed.
You might even be able to get the stock cut to the length you need.

Roger

Mon Apr 10 2006, 08:48PM

Registered Member #221 Joined: Mon Feb 20 2006, 05:36PM
Location: Chillicothe Ohio
Posts: 12

Thanks for all the good info. I never considered using carbide before. We use a lot of carbide drill bits where I work. Many of them are around 3/8 of an inch in diameter.

HV Enthusiast

Tue Apr 11 2006, 12:50AM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068

Several of the coilers on the TCML list (

) use pure tungsten electrodes, and they discuss them frequently there. You might want to post your question there to get a source. Of course, you still have to cut them yourself, but there is 3/8" and larger diameters available.

Tue Apr 11 2006, 02:21AM

Registered Member #256 Joined:
Location:
Posts: 124

cutting wouldnt present too big of a problem, i have cut tungsten with an angle grinder and cut off wheel

Cesiumsponge

Thu Apr 20 2006, 01:37AM

Registered Member #397 Joined: Wed Apr 19 2006, 12:56AM
Location: Western Washington
Posts: 125

Machine shops will commonly have larger tungsten carbide end mills which are dulled and no longer suitable for resharpening as they've already been resharpened too much. They also occasionally break so they're completely worthless at that point.

The nice thing about considering those as electrodes is that there is a LOT of mass. Also, the shank-end already has a chamfer on the edge of the piece so there is no real further machining needed to get a good, flat, parallel electrode surface. If you have little access to carbide shaping/grinding tooling, there is only so much you can do with abrasive wheels.

They also have a nice recessed flat on the shank body for attaching to taper spindles. This benefits you because physically, it is simple as drilling a hole wherever you want the electrode and keeping it in place with an allen/hex set screw or similar, and it'll stay in place.

We horde off a bunch of these worn mills and drills in a corner and throw it away as scrap once the pile gets big. I would assume they're free for the asking although I haven't bothered looking up electrical properties of tungsten carbide versus traditional tungsten and tungsten with trace dopants.

Chris

Fri Apr 21 2006, 02:22AM

Registered Member #8 Joined: Thu Feb 02 2006, 04:34AM
Location: Harlowton, MT, United States
Posts: 214

Tungsten carbide has an electrical conductivity similar to high carbon steels, similar to pure tungsten metal. It would make fine and efficient electrodes for a high voltage system like a tesla coil.

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