High Voltage Forum

Tesla coils => Spark Gap Tesla Coils (SGTC) => Topic started by: thedoc298 on July 03, 2019, 04:05:46 AM

Title: rotory spark gap
Post by: thedoc298 on July 03, 2019, 04:05:46 AM
What is the material that the actual rotor is made from, can't find cause don't know what its called. The brown stuff is what im looking for but any will do. Thanks
Title: Re: rotory spark gap
Post by: MRMILSTAR on July 03, 2019, 05:24:43 AM
I use FR4 (also called GP03 or Garolite) with a thickness of 1/2 inch. It is an excellent insulator and is incredibly strong. The brown stuff is most commonly called Bakelite or phenolic. The FR4 is much stronger.
Title: Re: rotory spark gap
Post by: profdc9 on July 03, 2019, 06:03:20 AM
FR4 is the circuit board material with copper on it.  G10 is the material without the copper (and usually the fire retardant).


By the way balancing spark gap wheels is very tough.  I used an angle grinder for mine, and I destroyed the bearings on one trying to balance the wheel.   I made a paper template and stuck it on the wheel to get the spark points about in the right place, and then I placed on the arbor of the angle grinder and spun it slowly and used a file to grind the end off into a circle.  Make sure you are wearing protective gloves and face mask for sure.


Title: Re: rotory spark gap
Post by: johnf on July 03, 2019, 08:40:34 AM
The brown stuff is either paper reinforced phenolic or cloth reinforced phenolic typically used for electrical switch boards
it is self extinguishing and much cheaper than G10 hence the use in switchboards
ps the brown stuff is much easier on tooling unlike G10

edit goes under the trade name "Tufnol" and can be bought in tubes, rod, sheet
https://nz.rs-online.com/web/p/solid-plastic-sheets/1941148/ etc etc
Title: Re: rotory spark gap
Post by: thedoc298 on July 03, 2019, 12:05:47 PM
Thanks for the info, im down to the rotor and the balanceing and have been dreading getting it right.
Title: Re: rotory spark gap
Post by: Mads Barnkob on July 03, 2019, 03:03:44 PM
Whatever material you end up using, spend a few minutes to calculate the speed at which the circumference of your RTG has, this should give you a good idea about how important sturdy materials and mountings are important for your own safety, as well as adding a blast shield around it to catch flying objects in the case of a failure.
Title: Re: rotory spark gap
Post by: MRMILSTAR on July 03, 2019, 05:26:05 PM
I don't know what I was thinking when I wrote GP03. G10 was what I meant to say. GP03 is a red plastic material with fiber reinforcement which is also very good. I use that material for other things. It could also be used to make a rotor. Its a lot easier to machine than G10. If you use G10, be sure and use a breathing mask. Inhaling the glass fibers when cutting or grinding is hazardous, much like asbestos fibers.

The precision drilling of the holes for the motor shaft and electrodes is extremely important. If you do this step correctly, the vast majority of your balancing problem will be solved. I highly recommend a drill press to get accurate hole placement. Before you attempt to drill a hole, first use a starter bit. These are short, stiff bits used by machinists that will not flex and are designed for starting holes. After the hole is started, then switch to a regular twist drill bit. If you try to drill through G10 with only a regular twist drill, the hole location will likely wonder away from exactly where you want it when you try to start the hole. This will then greatly complicate your balancing problem.

Concerning balancing, here is the way that I did it. First, the rotor must be perfectly circular. A lathe could be used. At the time that I made my G10 rotor I did not have a lathe. I came up with an alternative that worked well.  First, I rough-cut the rotor on a band saw. I then made it as round as possible using the band saw but I didn't worry about perfection. I then mounted the RSG motor to the rotor. Next, I secured the RSG motor to a stable platform. I then used a die grinder attached to a stable moveable tool holder to grind the rotor perfectly round as I slowly turned the rotor by hand. You can essentially do the same process with a table router. Insure that your tungsten electrodes are as close to identical length as possible. I then used brass bolts with holes drilled down the middle to hold the tungsten electrodes. I drilled and tapped two screw holes in each bolt head for 2 allen head grub screws to secure the electrodes.

If you have done the previous work with precision, balancing should be fairly simple. You may need no additional balancing. I have 4 flying electrodes, so I just used trial-and-error. I started the RSG and noted the vibration. If there was excessive vibration I added a small washer to one electrode bolt and tried again. If the vibration got better I then moved the washer to another electrode location until I found the least vibration. After finding the location with the least vibration, I tried several different washer sizes on that location until I found the one with the least vibration. It sounds tedious but it really didn't take that long. If you can make a good knife-edge balance, that would be a great way to balance it also but I didn't have one. I figured I could get it balanced by trial-and-error faster than it would take me to make a good knife-edge balance.
Title: Re: rotory spark gap
Post by: thedoc298 on July 06, 2019, 02:21:20 PM
Thanks for all the tips, I have a syncronous 3600 motor and going with 120 bpm. Being able to get great tips like these, I belive I will succeed. So far everything has been fun.
Title: Re: rotory spark gap
Post by: thedoc298 on July 06, 2019, 04:00:03 PM
Meant to say 120 bps :)
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