In this space, visitors are invited to post any comments, questions, or skeptical observations about Philo T. Farnsworth's contributions to the field of Nuclear Fusion research.
Subject: Re: Spot Welding?
Date: Oct 06, 09:26 am
Poster: Richard Hull
On Oct 06, 09:26 am, Richard Hull wrote:
Yes, the grids are a bear.
If Stainless Steel, they may be silver soldered or spot welded.
Fusor I and II were all stainless grided systems which were silver soldered and gave no real problems once outgassed. I know for a fact that the grids reached temps far beyond the brazing solder's melting temp on numerous occassions. I was stunned that they didn't pop open. It seems that after many heating cycles, short of melting the solder, the grids temper and settle into their shape and hold against what was "spring tension" when first assembled. In addition hydrogen embrittlement removes any springiness. This makes all grids of most all metals very delicate after being used in a fusor for a while. They will shatter like glass when stressed mechanically during service intervals if hit accidentally.
Silver soldering will outgas in vacuo for a while and may even pickup moisture when exposed to air, depending on how well you removed the flux agents. Silver soldering is more of an art and requires a hot acetylene/air flame as in a prestolite torch or an oxy/acetylene torch. Use the smallest tip possible and a micro flame.
I made up a crude, but effective spot welder out of an old 2.5 volt 60 amp filament transformer and a large capacitor of about 2000ufd @ 500 volts. This, I could charge with any voltage up to its maximum. I dumped this into the 120 volt winding. The electrodes were honest to goodness spot welding electrode picked up at a welding supply store. A jig was made to hold the electrodes and allow me to clamp and apply variying pressure to the joints. It is crude, but works for the smallish wires I am welding.
Never practice on grid rings. practice on cut off pieces and determine the "good" weld energy long before attempting something real. A good weld should not blast too deep into the pieces (approx half way), they should fuse smoothly without a lot of left over flashing or attached spelter. Test them by trying to separate your finished trial joints. They should hold fast up to a point of rather extreme abuse or flexure.
With Tantalum and Tungsten only spot welds will work well over time. (severe embritttlement) Still, these are the very best of grid materials. I have tungsten wire too, but it welds a bit better and is very close to tungsten in melting point, so I chose it.
I have mused over many grid making schemes. just a simple loop and hook followed by a crimp should suffice in any demo fusor provided the wire is properly annealed to avoid breaking when crimped. This will not look great but should work fine. Likewise, attacthement and connecting could, in theory be handled well with a micro tungsten or stainless wire wrap technique where 30 or more turns of .001" wire are wound along the wire on both side of a flattened overlap joint. A reflow of silver solder in this type of reinforced joint would make a joint many times stronger than a simple silver soldered joint. The silver solder should not be needed, however for just holding the thing together.
I have also mused over spot wleded "butt joints" which would be the ultimate ring system, but it requires a special jig be made to secure the wires for spot welding.
I have not had to force myself unduly on this issue as I have the skills and materials on hand to accomplish the task rather easily
I am sure there are many ways of making, joining and mounting grids which elude me, necessity being the mother of invention. These new methods will be obvious to others. I hope they share those thoughts.
- Re: Spot Welding? - Jim Lux Oct 06, 7:48 pm