I can't really speak to the practical effect of non-sphericity (off hand, I wouldn't think it makes a heck of a lot of difference... People have used electrodes that are pretty non-spherical (polyhedrons, for instance) and which certainly don't have a truly uniform field (the joins between the wires and the electrode support(s).
But, if you want sphericity, why is it such a big deal to cut the excess off the hemispheres. I assume the flange has a big hole in the middle. Stick the hemisphere up through it, scribe a line around, take the hemisphere off, and have at it with a bandsaw or grinder. Since you are welding it together, the weld bead will take up any small variations in the cut, and you were going to grind it smooth after welding anyway, right?
For that matter, I assume you have a hole in the top of the hemisphere (for running the electrode support through?). You could use that to clamp it in a lathe and use a cutoff tool to cut it down just right. (or clamp it in a drill press to spin it slowly, and use a dremel tool on a rest to make a nice clean cut.
BTW, watch out for inadvertently making virtual leaks in the grinding process, when you mash little bits of metal over and create blind gas filled pockets in the surface. Sharp tools!
F
Created on Tuesday, January 09, 2001 12:41 PM EDT by James Lux