My current top load is two home made toroids - a small 4.5" x 15.5" toroid made from aluminum flex duct, stiffened and smoothed with 2-part wood filler and spakel, covered with aluminum tape, and stacked on top of that is a large 6" x 23" toroid made from aluminum flex duct, covered with aluminum tape. As I now get multiple streamers, even when I have a deliberate breakout point, I know that it's time for a larger toroid. The bulk-buy on the Pupman Tesla Coil Mailing List for the 6"x24" spun aluminum commercial toroid was tempting, but as it really wasn't larger than what I have now, wouldn't help me. And chances were that a commercial toroid as large as what I need would be very expensive. I could again go the route of using corrugated aluminum duct ,but this technique has real durability issues, as they're very easily dented. I did some experiments using aerosol foam to fill the duct to stiffen it. The foam hardened very well, but the problem is that it slowly continues to expand over 1-2 days, and ruptured the joint of the duct. Plus I'm still left with a less than smooth corrugated surface finish, even after covering with aluminum tape.

I came across a company in Ohio, Global Manufacturing Solutions, which makes custom plastic and foam parts, largely for theatrical props. I inquired and they were very willing to make me a toroid to my specifications. As they have a completely computer-controlled milling machine operation, it didn't matter how complex (my speculation here) the shape was. A pseudo-toroid (hole filled in) would be the same price as a normal toroid. And they can make it out of a wide variety of materials. As I was concerned with the final product being dent-resistant, I initially asked for the densest foam they could source. They normally work with 2 lb (per cu ft) styrofoam, but could also order 3 lb or 5 lb foam. It turned out that the material cost for the 3 and 5 lb foam added considerably to the total cost, so I settled for 2 lb styrofoam. For reference, the material used to make styrofoam coffee cups is 1 lb styrofoam.

With the corrugated duct toroids, it was always a bit of a trick to get the center disk to just the right diameter that I could glue it securely. To improve upon this I added a feature to my toroid, adding a route to create a ledge around the inner perimeter that the disk will rest against, so that the disk diameter isn't critical. Since the e-field inside the toroid is low, the edge of this route shouldn't cause any problems. Below is the drawing that I supplied to Global Mfg Solutions:
Dimensioned drawing of toroid
The finished product arrived well packed and in good shape. The foam density was better than what I had been expecting. It is much denser than a coffee cup, but not as dense as a Styrofoam bicycle helmet. If I press very hard with a fingertip it will leave a small dent. The surface finish was pretty good.

The cost was $115, plus $30 for shipping from Ohio to Massachusetts (!).My contact was:
William Bankes
Vice President
Global Manufacturing Solutions
9562 N. Dixie Hwy
Franklin, Ohio 45005-1106
Web Site:

Here is the delivered product. The yellowish coating is two coats of water based polyurethane (which really served no purpose):

Uncoated foam toroid

I added a thick coating of epoxy, the kind used for fiberglass repairs, to improve upon the foam's dent-resistance. I mounted the toroid on a plywood disk, and mounted this assembly like a tire on a threaded rod driven by an electric drill, powered by a variac to control the speed. To apply the epoxy I slowly spun the toroid and poured on the epoxy, and spread it with a foam rubber paint roller. MUCH better than a paint brush! The drill was not the best choice to spin the works. At low speeds, the RPM was not very stable, but it was all that I had. If the speed went too slow it would stall,and it the speed went too fast it would fling off the liquid epoxy. So I basically had to baby sit the drill RPM for an hour or so until the epoxy hardened enough not to run. I applied two very thick coats. I'm guessing that the total thickness is about 1/8". The results were beautiful- the finish was smooth like glass.

Applying epoxy with a paint roller

To make the surface conductive I used MANY strips of 2" wide aluminum tape. The process of applying this tape to such a large toroid took much longer than expected, perhaps 5 hours total. Each piece had to be burnished with a wooden stick to make it lie perfectly smooth.

For the center disk I used a piece of 1/4" plywood. I epoxied heavy-duty aluminum foil to both sides.

By themselves, each of the aluminum tape strips covering the toroid is not really connected to the others. An Ohm meter will in most cases show this. If I simply mounted the foil-covered plywood disk to the toroid, the contact to each aluminum tape segment would be variable and poor. To ensure a good connection to each tape segment, I mounted a length of EMI gasket around the perimeter of one side of the disk. This gasket is a continuous bead of low density open-cell foam, covered by a tube of thin, flexible, conductive metal mesh. Under compression, it provides good electrical contact between irregular surfaces - the disk and the tape-covered ledge on the toroid. Below is a photo showing the compliant nature of the gasket:
EMI gasket

Below is the competed toroid and disk ready to be fastened. The square in the center of the disk is just there to give the screw that holds the assembly to the top of the secondary something solid to bite against.
Disk and foil-covered toroid

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