Wednesday, May 6, 2009

further progress on the DTSGS

I made significant progress on the spark gap switch last night while Judy was finishing up with her last patient and before we headed down to the hospital. I drilled all the bolt holes in one end cap, and also counterbored them.

Why did I counterbore the holes? I have changed the design on the fly, as is my habit. My original design used button-head cap screws with the heads on top of the end cap, leaving a conductive projection, which is Undesirable for high voltage kit, because corona forms on projections, and if an undesired spark to ground (ie; flash-over or arc fault) occurs, it will start from a sharp projection. This is why you see strange rounded and smooth shapes (including donut-shaped "corona rings") applied to high voltage equipment. The smooth surfaces of large radius smooth out the electrical field, preventing the sharp voltage gradients which encourage corona or unwanted sparks. This is why the end caps have rounded edges in the drawing. So I decided to use socket-head cap screws countersunk into the caps so that nothing projects at all.

I also rounded off another corner of that end cap. The end caps were cut as squares from 1/2" plate, and will subsequently need to be turned on the lathe to their net diameter. Turning those big corners off on the lathe is stupid, excruciatingly slow, and the heavy interrupted cut makes damaging an insert more likely. If I had a bandsaw, that would be the best way to cut out circles like this, but I don't. So I decided to mill off the corners. It's slower than using a bandsaw, but at least I can leave a nearly circular edge for turning on the lathe. After that I had to quit because Judy finished with her last patient and it was time to grab some dinner on the way to the hospital to visit a friend.

I realized I don't have an easy way to hold these 1/2" thin plates in or on the lathe, since there can be only a tiny little 1/4" hole in the center for the main electrode bolts - not enough for a tapered arbor (which I don't have anyway, and would have to make even if it were thick enough to support the work).

So I've decided to make a custom fixture which will have the same bolt-circle pattern of threaded holes as the end caps, but a smaller diameter, leaving the edges free to be rounded off. Then I can simply bolt the end caps to the fixture and mount it in the 3-jaw chuck. This bugs me because it will take more time, but I can see no other solution. I dislike making single-purpose "white elephant" tools, but if I must, I must.

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