Wednesday, March 3, 2010

more mad scientists light switch project

In an effort to get even more parts off of my workbench, I pushed a bit further on the MSLS today last week month. (I, um, sat on this post a while, through sheer absent-mindedness)

If I could have kept up that pace - just a few hours a day, maybe only two, I might have had the thing finished in time for the monthly Mad Scientists party, but my energy levels and mood have been unpredictable at best, so I couldn't make it happen. At least it was fairly photogenic by the time the Feb party came around.

One part which has been needed for some time, and which I've been thinking hard about, was a purty way to attach the pipe elbow which supports the miniature Jacob's Ladder to the side of The Device. For some time I've been assuming this would turn out to be some sort of custom-turned brass flange, and today I brought it closer to fruition:


Now what prompted me to get to where this photo is is a bit of a story:

I had in my Brass Parts drawer, a blivit that was perfect (especially after some quick mods on the lathe) for soldering into a copper pipe, but which didn't have sufficient diameter to provide good solid rigid support for the thin sheet metal I need to mount it on.

But I had in that same drawer a bit more of the .125" alpha (I think - must remember to test a scrap cutting) brass sheet sheet from which I cut the trigger plane electrode for the triggered spark gap switch (more about that in an upcoming post).

What you're looking at above is two bits of metal, joined together by a bolt, several springy washers, and a nut.

Prior to assembling that mess, I first faced the two surfaces on the lathe to ensure they are very flat, and then tinned the faced surfaces with as thin a layer of silver solder as I could manage. Once I tightened the bolt and nut, the compression of the spring washers would force the two together as soon as the solder melted, and force any excess out of the joint, leaving a very thin layer of solder indeed. I did this as an experiment to learn about making very low resistance connections between brass (later, will also try copper). I did a four-wire ohms measurement on this joint with my six-decimal-place lab grade VOM, and measured the joint at around .00015 ohms or thereabouts. That's really not all that impressive. If I were to pass 100 kA through that joint -- and I assure you I could do that fairly easily, for a very, very short period of time -- the solder joint would dissipate 1.5 million watts peak. Erp. I might need better solder.

So that bit being done now, there's another bit that will be either soldered or heat-shrunk onto the part you see here which has threads on it. The threads will be turned off first. I am sorta half-heartedly working on that now. My lathe is a piece of shit made in China in the 1970s, worse news it was abused by its previous owners, so it can be challenging to hold any sort of tolerances on it. But we'll see. If I blow past the dimension I need, I can always fall back to soldering it on.


Jon Singer said...

Steve Waltman has noted to me that even silver solder tends to be dicey for microwave work, because the skin depth is so tiny. From your results, it begins to look like the stuff ain't quite good enough for high-current work either. [Sigh.] OTOH, if you are dissipating 500 MWE in your device when you push 100 kA through it, a mere 1.5 MWE is peanuts. Gotta look at the big picture. ;o)

Bill Lemieux said...

Yah, I begin to see why bare bolted connections are so popular with the hard-core pulsed power jocks.