Solderless Grid Construction
Posted: Sun Apr 22, 2007 9:03 pm
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.
>viewtopic.php?f=6&t=2484#p11994
>viewtopic.php?f=6&t=2484#p14888
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.
>viewtopic.php?f=6&t=2484#p11994
>viewtopic.php?f=6&t=2484#p14888