Science/Engineering/Technical forums

I have a design issue in my fusor that seems to require some machining of high alumina ceramics (no other material seems to work). Now, I have no troubles cutting tubing and rods with a diamond wheel, the ceramic PCB substrate stuff is a lot harder to do on the mill, but possible if you can hack rough edges (this app can't), and I've never managed to get a hole in that either, even with diamonds.

So, does anyone know how to machine this stuff to tiny sizes and get any yield at all? I've broken plenty in the attempts -- that last bit shatters when cutting with diamonds at even very low feed pressures and makes a mess, usually destroying the piece. And rats, no holes is a pretty large obstacle.

This is for making fusor grid parts, and nothing else insulating will live in or near a grid. Every other material is reduced by the hot H ions to metal and fails. Alumina makes a hydride that evaporates....so even this is sacrificial, but it's good while it lasts.

There are several companies that specialize in just this stuff. All specialized grinding equipment. Full water flood.

This probably isn't going to work because of the environment at the grid (high Si content), but I thought I'd throw it out there for those that are unfamiliar with the stuff since it didn't show up in a search. Macor is a machinable ceramic that is pretty easy to work with - machines very easily, even with HSS tooling. The downside is that it's pretty pricey stuff.

Macor would work for something I don't need to be a good insulator (like a grid) but not for the feedthrough stalk, which needs to stay insulating. The prices of this stuff in perspective isn't that big a deal unless it fails a lot. I've been using for example quartz for feedthroughs, and that isn't cheap either. But since ions reduce it and it fails, it's expensive.

OOh, lookie what I found --
http://accuratus.com/alumoxszs.html

No info on prices yet, but this is tubing big enough for a feedthrough outer layer (1") and rod small enough to insulate grid vanes from direct hits from outside (1/16 and down).

Even alumina isn't that great an insulator, easy breakdown, high D and high loss so the idea for a feed through is to use it as outer layer to catch ion impacts, and quartz on the inside to be the insulator with good DC and RF properties.

Doug
You have to be careful with macor
I found out it melts @1100degrees celcius. I did have an insulating (heat) stage made out of it in our annealler I had a accident while adjusting the controller on the electron beam and the beam went to full power 30kV @ 40mA the macor melted in the 2mm beam spot area and shattered through where I hit it with the beam. I'm now using a Tantalum holder and the x-rays are fierce.

Excuse me for extending beyond my knowledge-zone, but the way I see this is to contemplate what materials are the products of extreme heat, if you want something to survive extreme heat. So how about working granite? I see plenty of fine and highly polished pieces in gem shops...... just a thought....

I have no plans to use macor for this (Kohl explains all).
If heat were the problem, the quartz I use would live forever.

But heat isn't the problem. It's chemical reduction by fast hydrogen ions (isotope of no significance in this), which makes most things back into non-insulating metals, which now attract the very ions you made them insulating to prevent. This is a very weird environment, no commercial products I've seen actually handle it, which is why I did the DIY thing in the first place.

Things that work awesomely well in high vacuum just don't fly in a fusor.

It depends on whether we are talking grids, or feed through "outer shells" here, but the effect is the same. Quartz quickly becomes silicon if it's in the path of the ions, and now that it's conductive, attracts more, and intense spot heating is the result -- not the cause. If I'm careful, I can get some 10's of hours out of the quartz working fine, then bam. Ceramics that aren't pure Al2O3 -- worse. H hitting that makes aluminum hydroxide which isn't a great insulator, but gets sputtered off slowly and all that happens is the ceramic gets thinner very slowly. All other minerals (macor included) have other metal atoms, Mg, Mn, Fe and stuff that become conductive in hot H+ as it eats off the O and makes (heavy) water.

Anything else I've tried (most of the pantheon) reduces to conductive metal and fails in a spectacular way after that, without ever having reached even red heat.
Until it becomes conductive, when it attracts more ions, then whammo.

Yeah, high Z metals are great for making X rays and tantalum is right on up there. How about trying something lighter? Ti, Be, B come to mind. Lower energy K lines and all that.

Doug
Okay you don't like Partheon parts

but a few years back I was invited to see the restoration of one of NZ's first power plants.
A pelton wheel system 300ft head and the turbine alternator outputted at 11kV. The control panel with the mechanical AVR was built on a large marble slab to get the 11kV insulation. The original paper insulated stator had been rewound and fibreglass tape insulated (Their only concession to modern times).
Marble is cheap!!
sort of along the Chrismb idea

Ah, John, don't take me wrong -- I love the old stuff, actually. It's why I have some legacy tech things up here for one thing. The old guys were clever indeed, just didn't have the cooler materials and infrastructure we have -- in fact, they might have been cleverer than we -- here I'm thinking of the many auto-rod-feed designs on old arc lamps, and the variable valve timing in steam engines and similar things. I have an old (1800's) machinist book where they show how to get sub-mil accuracy out of machines of that era -- no small thing. Easy now, with high class bearings and computer controls, not so much so then.

But in this case, it's not what I like -- it's what hot D+ ions do to the materials. If granite, or leather piston rings would get me there, I'd be all over it, frankly.
I guess if I was in the same class of clever as those older generation guys, I'd find a way to keep the hot D+ off the stuff it was wrecking, and indeed that's been something on my mind, and to the extent it's doable, it works out well. So far, I'm just not clever enough.