Well, I'd recently tried using Ti for the grid ends - one carved from 1" dia rod, one made of end-welded 1/8" wire.
It worked, but Ti is a lousy conductor of heat, and the ends got hotter than I'd like. Also it was evident I'd made that one too long.
So, despite some concerns about hydrocarbon contamination (which I think I have a fix for later), I decided to do it all once again. Since how I did this isn't totally obvious to anyone but a pro machinist, here's what I did, in what order.
Starting with 1" graphite rod stock chucked into the lathe, I made the HV stalk-end first. This involved turning down the last .4" to .375 - I'm going to put a Ti washer to cover most of the back (feedthrough facing) end to reduce carbon sputtering onto the boron nitride I'm now using as an HV insulator right up to the grid (which, BTW, solved all my FT problems - it's great, if expensive). I then cut it off with a cutoff tool, so it was 140 mils thick where the rods are going to go in.
After re-facing the graphite stock end, I ran a 3/4" Forstner bit into it so as to create a 1/8" cross section ring - in fact, I went a little deep, and I can get another ring out of this piece later on.
People who've been paying attention to my grid designs will notice that this time I made the HV end a female 10-32 thread instead of trying to make a threaded 10-32 graphite end. I might try that later, but this stuff is on the brittle side, and I can simply use a good heat conductor metal threaded rod to mount to the female 10-32 on the end of my HV stalk.
This just looked like a better way - This kind of thing is a one mistake and all your work is lost thing, so you get conservative in design.
Then I applied a degree wheel to the lathe chuck, using magnets (like auto guys use for setting cam timing) and have a pointer mounted on the lathe (just copper wire). I then mounted up my homebrew toolpost grinder, which accepts 1/8th inch stuff - diamond wheels for cutting glass and so on, or solid carbide tiny drill bits - they have a 1/8th shank, and in this case are then necked down to .020 inch. Using the degree wheel to get exact (well, a tiny fraction of a degree) 45 degree
increments, and the tool slide to control drill depth, I drilled 8 holes in the ring, 100 mils deep each, and then cut it off the main chunk of stock.
I then repeated the process, going a bit deeper - 125 mils - for the HV end piece. Since I had turned down part of it to 3/8", I had a nice way to chuck it up, and got the exact same drill circle, since no other part of the setup had to change at all.
I did a bit of bevelling and sanding along the way, some while in the lathe, and some by hand - still needs a little more, as I don't want sharp edges - but I also don't want any of the holes to be "all the way through" - so I'm cutting a fine line here.
Here's what it looks like as a test-fit.
Then I had to cut (at least) 8 rods for this design. I wanted them 1.75" long, which will give me about 1.525" of active length for the rods. You really want this to have lens-like accuracy, though I'm not even to good optical, much less ion-wavelenth precision here - but you never know - it might be perfect for some part of its length. Since I've not yet adapted my dremel drill press for the new tool (the old one literally let out the smoke), I made this jig for a newer dremel (thanks BillF) that's a lot nicer motor and in better shape anyway.
These rods come very straight - the far more expensive wire from Alpha-Aesar, say, don't - and these are pretty cheap at the welding supply store. I'd used the "E3" type in the other grid, but that stuff (doped with various rare earth oxides) was just too good at emitting electrons, especially after being hit by a bunch of hot D (reduces them). So this time we're going with tried and true pure tungsten, 20 mils diameter. It tends to get pretty hot in the middle - even with a cylinder shape, and so far, almost no matter the length as long as it's a bit longer than it is in diameter, the poisser concentrates in the center of the length anyway - and that's where the wires get hot. The ends get hot too - because they are hanging out there, big, and at full volts, so they attract ion hits. At least with carbon, it's not a great secondary electron emitter, but it seems I do get some hydrocarbons from the sputtered off carbon atoms interacting with the hot D ions. I may vacuum coat this with titanium or some other high work function metal - I now don't care if it gets orange or yellow hot, since the graphite is what's holding the forces anyway, and it's quite happy well past white hot (at which point I'm more worried about my copper feedthrough stalk...). With all titanium, it got orange hot - at which point Ti has the strength of butter...this may get as hot (I'd bet it doesn't, since if nothing else, carbon radiates heat better than any metal as well as being thermally very conductive), but it won't be at risk of slumping, which I've had happen to other grids.