Solderless Grid Construction

For posts specifically relating to fusor design, construction, and operation.
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Wilfried Heil
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Solderless Grid Construction

Post by Wilfried Heil » Mon Apr 23, 2007 1:03 am

The design of the grid is one of the more creative aspects in building a fusor and can be a technical challenge.

This one was inspired by Jon Rosenstiel's technique for the grid assembly.

A small metal barrel was machined from SS-304, which holds four loops of wire. The position of this barrel on the stalk of the HV feedthrough can be adjusted, so that grid diameters from 24-38 mm would be possible. The exact center position of the grid can't be seen in the fusor, it is taken from a drawing. The barrel is then fixed to its position with a small set screw.

The grid has an O.D. of 32 mm and is made of 0.7 mm thick NiCr heating wire.

One image shows the grid during assembly, along with its special alignment tool. By slightly nicking the wire near the lower end of the rings, on the part that slides into the holes, enough spring tension is produced to hold the loops in place after they are inserted into the barrel. The nicking was done by crossing two wires at right angles and pressing them together with pliers. The assembly method makes it possible to easily exchange wire loops, if that should become necessary in the future.

The wires are stiff enough to hold their form when heated to yellow-red glow. As we found out later, the thickness of the wires enables them to radiate away enough heat to allow operation at 500 W without overheating or thermal runaway from thermionic emissions. They should also be thick enough to withstand sputtering damage and occasional arcing for a while.

The last image gives an idea on how a number of perfectly round rings of the same size can be made. The procedure was described by Richard Hull elsewhere in this forum.

The NiCr wire is wound tightly around a mandrel (a round piece of tubing). When it springs open, it forms a spool of a diameter slightly larger than the tubing. From this spool the loops are cut with enough overlap for mounting. They can then be opened or closed a bit to adjust the exact diameter.

When they are mounted, the wire loops have to be without tension, as otherwise they will readjust and bend when heated.

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Jon Rosenstiel
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Re: Solderless Grid Construction

Post by Jon Rosenstiel » Mon Apr 23, 2007 4:49 pm

Excellent craftsmanship, Wilfried. Could you post a close-up of the barrel, and its dimensions? (Mainly diameter and length).

Jon Rosenstiel

Justin Nichols
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Re: Solderless Grid Construction

Post by Justin Nichols » Wed May 21, 2014 5:01 pm

What material did you use as your inside-the-chamber insulator?

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Richard Hull
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Re: Solderless Grid Construction

Post by Richard Hull » Thu May 22, 2014 2:22 pm

Nice piece of work, indeed! Best of luck in doing fusion.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

Tom Dressel
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Re: Solderless Grid Construction

Post by Tom Dressel » Sun Mar 15, 2015 8:00 pm

I donated my fusor to the University of Minnesota in November of 2012.
Since then a group of Physics students, the Tesla group, has been working on the fusor and getting to know how to run the device. The most important development is that we managed to get our hands on a 2KW / 30 KV power supply. We have the deuterium delivery system, the home made BC-720 scintillator and PMT neutron detector etc. and are getting ready to make neutrons, but we had to upgrade the fusor grid from a stainless steel silver soldered grid to a Nichrome solder less grid.

As you can see, old grid was trash. The new construction was made easier by using an inexpensive rotary table that I bought on Ebay last week, to make the eight hole grid hub with a slight variation of Wilfried Heil's technique. I added radial slots that facilitate the positioning of the grid rings. The grid rings are 20 gauge Nichrome Wire. The grid is hung from the HV feed through with a single 4-40 screw. A short length of 3/8/' ID alumina tube was added which will slide down to cover the grid hub.
090.JPG
Old grid after 12 years in demo fursor
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Rotary table
006.JPG
Grid hub with radial slots
001.JPG
completed grid of 20 gauge Nichrome wire
103.JPG
Final grid assembly with added alumina tube to cover the hub
Last edited by Tom Dressel on Sun Mar 15, 2015 9:09 pm, edited 4 times in total.

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Adam Szendrey
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Re: Solderless Grid Construction

Post by Adam Szendrey » Sun Mar 15, 2015 8:11 pm

Hmm, somehow I went totally by this post back in the day. I used the same grid structure back in 2006, with the difference that I added a single "blob" of solder at the "north pole" to make it more robust. I didn't make a custom barrel, I simply used the hole in the connector of the spark plug that I used as the feedthrough. Nice design though, well done!

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Richard Hull
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Re: Solderless Grid Construction

Post by Richard Hull » Mon Mar 16, 2015 9:25 pm

I am glad to see that folks are going back....Way back! They find out how many of us made up our grids in the distant past. Nice to bring some of this stuff forward for the Newbs who are often too lazy or busy to use the "way-back machine" (read and research).

Speaking of way-back machines, It has been a long time since we have heard from the old timer Tom Dressel. He was heavily involved in the early fusor effort.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

Sasha Cohen
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Re: Solderless Grid Construction

Post by Sasha Cohen » Wed Apr 01, 2015 8:55 pm

After some trouble figuring out how to silver solder, I found this thread. Thanks so much, I'll definitely be using this design!

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