FAQ - Cleaning optical windows, insulators and chamber interiors

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Richard Hull
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FAQ - Cleaning optical windows, insulators and chamber interiors

Post by Richard Hull »

As the fusor operates with a bombarded cathode, the cathode material can end up depositied, over time, on optical windows, insulators, and chamber walls.

While deposition on chamber walls is not normally an issue, the windows slowly go opaque and insulator standoff voltages can degrade to arcing conditions.

An insulator can be successfully "occulted" or "shadowed" with a mask so long as it doesn't unduly upset the electrosatic field in the fusor.

A view port or optical window cannot be treated in the above manner and will always collect a deposit as the fusor is operated. At high input energies, this deposition can occur amazingly fast.

Cleaning a window or insulator depends on the cathode material's composition as well as the window and insulator's composition.

The temptation is to chemically remove the offending material. This can be done for many materials. A stainless steel grid construction will deposit a brownish material that is easily removed with concentrated hydrochloric acid. (muriatic acid). This can be obtained at most full service hardware stores or building supply outlets. Insulators, provided they are glazed, can also be treated and cleaned in this manner.

For my own and other fusors in our group that might utilize tungsten or tantalum grids. Only hydrofluoric acid or molten caustic will clean off these latter elemental depositions. With glass and glazed insulators, this is not tentable and glass is attacked by both of these very nasty agents.

For these grid material depositions, only mechanical abrasion works. A good plastic polish like Novus brand's coarsest polish will do the job well. (plastics supply house)

Any number of automotive paint polishing or rubbing compounds will also do well. The coarser compounds work very fast, but might leave a dull surface on glass, which must be gone over with a finer polishing agent to restore perfect trasparency. Regardless, abrasion works on everything, but is a bit more tedious and time consuming.

Needless to say, whether you chemically clean or mechanically abrade, you must be sure to leave all components exposed to the inside of the fusor scrupulously clean prior to re-assembly and operation or the working vacuum pressure might not be attained as left behind agents or chemicals outgass.

There are other methods that might ultimately be appended in the FAQ as time passes.

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Re: FAQ - Cleaning optical windows, insulators and chamber interiors

Post by Richard Hester »

One way to get around this nasty business is to use a piece of sacrificai glass in front of the view port, or a shutter. Insulators are a more complicated business. Some thin wall alumina (or fused quartz?) tubing slipped over the insulator stem may be the way to go. This can be cleaned or discarded when it gets gunked up. Alumina tubing is fairly expensive, but less so than your feedthrough. Fused quartz is actually fairly reasonable, if you find the right distributor.
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Re: FAQ - Cleaning optical windows, insulators and chamber interiors

Post by Carl Willis »

With my tungsten grids, I still find that the dip in HCl is sufficient to remove viewport deposits and deposits on glazed alumina and beryllia ceramic. I haven't had to crack open anything nastier than that.

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Re: FAQ - Cleaning optical windows, insulators and chamber interiors

Post by Steven Sesselmann »

If I can just add one more method to this..

Deposits on ceramic insulator stand offs can be easily removed by sandblasting with fine aluminum oxide, this method will leave the surface clean and dry.

Micro sandblasting pens are readily available and all you need is a bit of compressed air.

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Re: FAQ - Cleaning optical windows, insulators and chamber interiors

Post by Richard Hull »

Sandblasting is another form of abrasion. Few here have the materials or gear to sand blast, however, with microfine sand or jeweled beads they would be ideal for unglazed insulators. I have seen 1 ounce bags of man made diamond and sapphire dust on sale for special micro sandblasting jobs, much as Steven suggests.

Thanks Carl for noting that you get the acid to work off tungsten deposits from glass. Once I left SS grids, I have stuck with light polishing and rubbing compound abrasives.
Chemicals, if their action is rapid and positive, are always the best way to go.

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Re: FAQ - Cleaning optical windows, insulators and chamber interiors

Post by tligon »

The last time I had Dog and Pony II apart, I tried muriatic acid on the heavily tinted viewport. It had little effect. My grids have always been stainless and I can confirm the brown deposits. Mine must have ion-implanted to a depth the acid can't reach.

Thanks for posting this, Richard. I suspected abrasion/polishing was the next step but was hoping for some confirmation that the effort was worthwhile.

I have new grids made and hope to install them momentarily, but now is a good opportunity to scrub the window.
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Re: FAQ - Cleaning optical windows, insulators and chamber interiors

Post by tligon »

Jeweler's rouge and elbow grease did the trick. It even removed the stubborn shadows of my front screen.
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Re: FAQ - Cleaning optical windows, insulators and chamber interiors

Post by lutzhoffman »

For very inert and difficult to remove metals, as a last resort, you may try molten sodium bisulfate. We used this to remove rhodium from some small military Quartz, and Pyrex substrate mirrors, because of tungsten contamination of the coating. Rhodium is pretty inert, it is what's left in the bottom of the flask when they dissolve platinum in aqua regia to refine it. It may work for other metals also since rhodium is quite the challenge. You can also make a poor mans aqua regia by mixing muriatic acid and potassium nitrate stump remover from the hardware store, and warming the mix. This even works for small scale gold refining, while avoiding the hazmat shipping charges.

Rhodium makes a really cool front surface mirror by the way, which will hold up under unbelievable conditions. Zeiss did a Rhodium mirror which was mounted outside in the industrial polluted Rhurr Valley in Germany, after 30 years the reflectivity was unchanged! The only way to remove it chemically, is via this molten salt method.
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Re: FAQ - Cleaning optical windows, insulators and chamber interiors

Post by John Futter »

Richard
I use KOH to remove tungsten and organic deposits off my SEM column apertures, anode and weinhelt apertures.

it has to be hot 80 degrees C plus and concentrate

Edit
As for abrasion we use Green Scotchbrite brand pot scouring pads ie the green plastic ones they work great on all surfaces except we keep them away from CF knife edges.
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Re: FAQ - Cleaning optical windows, insulators and chamber interiors

Post by Chris Bradley »

Sputtering is, surely, one process by which material can become ejected and find itself elsewhere. But is it known, or is it a presumption, that fusors suffer with this?

I had presumed that my 'browned' windows were also being similarly deposited-on by sputtering processes.

So I changed the electrode materials, changed them again, and again (Cu, SS, Al, CrMo, NiCr, W). Each time, no difference. So I put in test windows and after some browning then put those in an SEM for X-ray backscatter spectrum analysis. The browning was not metallic at any detectable level. It was all indicated as carbon.

Carbon gets pulled out of the background by electrons. The same effect can be seen in the real time xray machine at work (tungsten filament, SS accelerator tube, beryllium target, but carbon deposits) and also it is well known that items that spend a bit too long under the SEM electron beam end up with a noticeable carbon layer on them.

Therefore, by known analysis of deposits in my device, in an x-ray tube and in the other electron accelerating device I have access to [an SEM] carbon is the commonly deposited material that causes brown deposits. Carbon is ubiquitous and pumps much less readily than heavier metallic atoms in molecular flow so you can never really get rid of it.

Is there any objective evidence to show that fusors buck this trend and that their window deposits are dominantly metallic?
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Re: FAQ - Cleaning optical windows, insulators and chamber interiors

Post by tligon »

Carbon deposits would certainly explain the lack of effectiveness of HCl on my window.

I have a stainless steel "ratwire" screen between my fusor grids and the viewport, just the thing to keep stainless steel rats at bay, and with the hope of intercepting electron beams before they damage the glass. The screen is about 1 cm off the glass. I notice it leaves a sharp shadow pattern consistent with something coming from the central focus point.

Ions should not do this. We know electrons will. Possibly the glass "charges up" in this pattern and attracts various ions, be they metals, pump oil molecular fragments, etc. Carbon or hydrocarbon fragments would probably attract to the charged-up portions as in Xerography. Fast neutrals kicked by the main ion stream could also contribute.

Since I do not presently have a trap between my pump and vacuum chamber, backstreaming oil vapor could very likely be the culprit. While at EMC2 I operated the unit with a MicroMaze trap, and thinking back on it the deposits seem worse now than when I was there.
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Re: FAQ - Cleaning optical windows, insulators and chamber interiors

Post by Doug Coulter »

I have used, and I believe posted prior to this, all 3 main methods. I think anyone who actually runs a fusor for many hours runs into these things.

I do use a sacrificial layer in front of my viewports. Sometimes it's just a sheet of mica stuck on with a tiny bit of apeizon wax, sometimes another full-thickness piece of pyrex. I'd rather have to toss a ten buck piece of that than replace the window in my viewport/door at vacuum supplier prices -- they use odd sizes that are hard to find elsewhere.

More often than not, plain old HCL will do the trick on many things. For ceramic, I follow with a rinse in something basic (ammonia or NaOH solution) then a final set of rinses with Di water/alcohol in an ultrasonic cleaner, but for the ceramics, the acid doesn't always do. I use both tungsten and titanium grids here with sometimes some exposed Cu or Al for the HV stalk. The shadow method is good -- prevention is worth more than cure to the extent it works.

Particularly if the stuff doing the shadowing is disposable or arranged in such a way that it doesn't hurt things if it becomes conductive. I have had mica fail horribly in this case because once it got somewhat conductive, it attracted enough ions to heat up and thermally decompose. Gives a pretty light show while it dies and takes the fusor down -- you'll be getting inside again to fix that before you can run again. Mica just doesn't work well in that place, but seems fine in front of a viewport.

Sandblasting works well on ceramics, especially with the little gun like Steven pictured. It's really cheap, the whole kit is on the order of 10 bucks at Harbor freight and some very cheap compressors meant for airbrush painting will do in this case -- you don't need 120 psi shop air or even a portable tank you fill at the gas station, and the media that comes with the kit works, as does other sandblasting media. You may want to avoid silicon carbide though -- conductive. Since I do have a shop compressor, I never considered that as a problem though, and they are getting cheap for the crummy and noisy ones anyway. If you do sandblasting -- do it outdoors, as it can really make a mess in the shop...you will have sand down your back, and regular safety glasses may not keep it out of your eyes, either, as the stuff bounces around everywhere. I use a cheap hood when doing that.

I too have had interesting problems with browned glass (but interestingly not with sapphire, which is getting hit enough to glow a pretty blue during runs). Luckily it was the 3/8" thick sacrificial piece of pyrex from McMaster -- cheap, and they sell it in round pieces the right sizes. When nothing seemed to work to remove it, I broke it to see how deep the brown went and it was virtually all the way through the glass (which had no doubt been placed in there either way facing in at one time or another), so I am not sure which theory is correct there -- X rays could be part of it too, as well as things driven deeply into the glass, but I've not run energies here that wold drive things that deep. I bet someone here who does ion implant could chime in with a number for expected penetration depth vs the few tens of Kv we tend to run -- making the assumption that any ions actually reach that going *away* from the grid (reaction products might, and they can be plenty hot). To me it would seem more reasonable that it is electrons because of other things we see -- as in the glass lighting up white under their bombardment and a faraday probe saying that it's electrons coming that way (or negative ions?).

Warm ammonium bifluoride solution works pretty well on things other than glass....and if you are desperate, even glass, which you follow with cerium oxide and elbow grease to un-frost it again. This is pretty tame stuff compared to real HF. I happened to have a quantity of it provided by my electroplating supplier (Caswell Plating) as a pickle/cleaning solution mix.

One thing that may lend some credence to the carbon theory is that I had a lot more trouble when the fusor was pretty new, and presumably had more contamination in it. However, for the last fairly long time, I've been using high grade graphite (McMaster) for endcaps on my cylindrical grids to hold the tungsten rods and not noticed any big increase in deposits, even though I can see some degradation on the graphite. Basically it started off with a mirror polish, and got more like flat black over time, a somewhat roughened surface. Nothing else seemed specially affected at least so far. The McMaster graphite rod stock is really good stuff -- not carbon, but really graphite, no binders to outgas (or mess up lathe tools), and of course it easily takes incandescent heat in fusor conditions when you put in too much power.

My policy, which seems to be working out, is to prevent deposition on sensitive stuff as far as possible,
following the old saw, an ounce of prevention....is worth a lot of effort in a cure. I don't worry about the tank walls at all. If I sputter some Ti or W onto them, it only seems to help, though I'd avoid getting too much W on there as it will then make X rays more efficiently under electron bombardment than Ti or stainless steel elements. Not an issue here, we bit the bullet and totally shielded our fusor with thick lead sheet. A lot of work, but no worries now.

A lot can be solved by getting the geometry right so the things that get coated don't matter, but that takes significant skull sweat and a few tries, usually. My new HV feedthrough design accomplishes this pretty well. This: http://www.coultersmithing.com/AuxCP/FT.html describes that, and a couple samples of this have run hundreds of hours without any problems or noticeable degradation. The very tip of the quartz tube gets a little deposition on it, that's all, and it doesn't affect how well it works at all, it seems. Key are the dimensions near the end that sticks into the tank -- and the aluminum design pictured there is the best so far -- note the stalk is turned down thinner near the end where the action is -- that turns out to solve a lot of things. Small changes there make large differences.

If that fails, harbor freight sells some nifty small diamond hole saws that easily make clean holes in glass or quartz (not so good on alumina sheet) using the normal procedures -- slow drill press, water lube in a dam made of modeling clay, and patience - slow rotation and light pressure gets it done. You can put them over the HV stalk and they catch the stuff before it gets on the harder to clean ceramic, and no compunction about either putting them into nasty chemicals to clean or simply replace them.

www.quartz.com is where I get my quartz, good people to deal with, but quartz isn't cheap no matter where you get it -- I generally buy in bulk to get the discount. I sometimes resort to McMaster for small round quartz windows even though they tend to be a little pricey there, still not too bad when you only need one or two small ones.

I find it interesting that you can pretty much tell who really runs their gear by the problems they run into, and if they don't see this (or some other issues) -- they must not be running very much!

I am wondering where the carbon might be coming from, assuming Chris is correct, as I only generally see the usual small amounts of CO and CO2 on the mass spec. Could it be impurities in the normal CP grades of D that people are using? There is always a tiny bit in the glass already, but I'd think that it may not be the first thing to be reduced by electron bombardment. I do know that I am often making enough X rays to do the "color center" thing in glass and brown it that way. I am finding that really pure quartz (as obtained at quartz.com, not vycor but the good stuff) gets reduced *or* browned a lot less easily than most ceramics or glass, for whatever reason - I am finding a lot of good uses for that material around here. It is one heck of a lot trickier to do glass work on than pyrex, though. The score and snap technique does NOT work with quartz, and the melting point and boiling points are fairly close together - it takes good tools and some practice to work with even once you get good at doing the same things with pyrex, but drilled holes are easy in either case. To cut tubing I pretty much have to use a thin diamond wheel in a toolpost grinder on the lathe to get good results every time. As usual, use water lube and go slow and it works fine.
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Re: FAQ - Cleaning optical windows, insulators and chamber interiors

Post by Richard Hester »

Huh - the "brown all the way through" may be some sort of solarization, I've seen it happen in CaF2 exposed to EUV (193 nM) as well - in that case, the solarization was in the form of deep blue inclusions in the window. If it's solarization, annealing the glass may fix it. It'd be interesting to try it on a lark if you were inclined to otherwise throw the glass away.
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Re: FAQ - Cleaning optical windows, insulators and chamber interiors

Post by tligon »

More than once I have been tempted to use a sacrificial glass round to protect the viewport (my 8" CF viewport, intended for audiences, costs nearly $700). However, I never got around tuit.

You spurred me to go on-line at MMC and order one. A borosilicate round tuit of 6" diameter is about $29.
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Re: FAQ - Cleaning optical windows, insulators and chamber interiors

Post by Doug Coulter »

Thanks for the input, Tom and Richard, and yes I will try annealing the stuff and see if that helps, just for grins -- I have a metallurgical heat-treat oven that ought to do fine for that. I paid a bit more than that for my window/door from Lesker, but find that it was a great investment and way-way worth it. Especially for diagnosing little arc problems, too much heat and suchlike. And I'm real glad the cheap sacrificial piece guarding it seems to be working fine after hundreds of hours of operation -- I'd hate to have to buy that glass piece and new gaskets retail from Lesker. Sure is nice to have a big door to get into the tank through and not have to buy copper gaskets each time. Makes it far easier to try new things and make progress quicker.

Here, viewports and bellows and feedthroughs are the main sources of X rays escaping, and to solve the viewport issues we found a very thick piece of PbO glass from a radiology dept that still allows good stereo (human) vision and also very nice high resolution photography without getting a chest X ray in the process. I just built a frame for the 18" by 24" piece so we can set it in front of the door when we are running, but remove it to get the door open other times. We tried the webcam thing and basically rejected it as those are too low quality (rez and color rendition both) to tell much about what is going on in there. So far we're not seeing very significant leakage through the thick CF flanges themselves -- that's a lot of SS to go through for all but the very high energy (and also fairly rare) gammas from the reaction itself. But they obviously get through bellows and ceramics like they weren't even there.

BTW, before we got a little more careful, and somewhat smarter, we were getting enough X rays through the window (with the thick sacrificial pyrex behind it) to actually mess up pixels in a high quality Kodak CCD camera. After looking at that picture with the random white dots, I got religion real quick-like.

To solve the other X ray escape problems we've been able to make articulated lead shields for the bellows (wobble sticks are way cool!) and low volt feedthroughs (probes and thermocouples), but for the high voltage one the only thing we've managed is to have that "leak" pointed away from the experimenter -- in my case out through the wall of the lab, which is also safer as it makes it harder to get too close to the HV as well.
As I live in the far boonies, there's no risk to anything but the deer and the lawn getting hit outside my walls. I'd worry if I still lived in an apartment, though.

Doing up all that lead sheet for a complex shape tank was truly a pain (several days full time), but worth it, as now all our counters stay quiet during a run, and I'm pretty sure I'm not frying my wife as she watches TV upstairs -- we actually did check that one with a very sensitive scintillator. But it was one heck of a lot of cutting complex shapes and soldering overlapping joints to get there on my tank, shown before that process on my homepage.
www.coultersmithing.com for those who haven't seen it. It's pretty ugly now to all but the eyes of those who understand the radiation issues. We run over 50kv sometimes so the SS tank is way not-enough.
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Re: FAQ - Cleaning optical windows, insulators and chamber interiors

Post by Starfire »

Doug - X-ray is easy to deal with - use a mirror at 45* and a lead shield alongside the camera which should be at right angle to the view port.


BTW Thanks to Richard for another useul FAQ's.
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Re: FAQ - Cleaning optical windows, insulators and chamber interiors

Post by Doug Coulter »

We did that, but we didn't like the results too well -- the mirror tended to jiggle, the field of view wasn't so good (just having to tilt it 45 deg makes it not so good if you need a view at an angle) with what we could lash up, and it was just a pain because of random other light scattering off the mirror and so forth. The lead glass is the greatest stuff since sliced bread, really. You can put it up against the viewport and look in from all angles and suchlike, which with a mirror usually means having to adjust that -- in other words, putting parts of you into the fierce field to do so. I am not a scaredy cat, and I believe in hormesis, but there's a limit....
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Re: FAQ - Cleaning optical windows, insulators and chamber interiors

Post by tligon »

I ordered the 6" round on a whim, without checking the diameter of my chamber. My actual working diameter is 5.9 inches.

Not a problem for a guy with a Moto-Tool and a diamond wheel, although dust control was a non-trivial problem. You really don't wanna be breathing finely powdered glass. I looked like a bandit wearing a damp flannel bandana and full face shield.

DP-II is now back together with new grids, and the extra piece of 1/4" thick borosilicate glass outside my front Faraday screen but inside the front viewport. As soon as I got to low pressure and high voltage, circa 8 kV, the glass started to flouresce a yellow-green, in patches. I'm in the habit of manipulating the e-beam that characteristically comes from the Poissor with a magnet. The magnet also shifted the flourescent glow pattern. The pattern shows sharp shadows of both the Faraday shield and the outer grid wires. I conclude it is probably electron-driven, a classic "cathode ray" such as Thomson demonstrated, a flood of electrons that can be masked to cast a shadow.

Without deflection, the old grids tended to shoot an e-beam at an edge of the viewport, and I worried I might shatter it one day with the public gawking at it. I feel better now that the new glass will protect the viewport.
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Re: FAQ - Cleaning optical windows, insulators and chamber interiors

Post by Jerry Biehler »

Dont go at it with a dremel. Too easy to create a hot spot and shatter. Use a slow running 6" Silicon Carbide wheel wet. There are special coolant mixes used for glass that will help. They sell the stuff at stained glass shops.
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Re: FAQ - Cleaning optical windows, insulators and chamber interiors

Post by tligon »

The glass chipped a little on the edges when I got carried away, but nothing worse. Borosilicate is low expansion and much more tolerant than the ordinary variety.

On short notice I had to go with what I had.

Another option, something I used at EMC2, is a diamond tile-cutting saw. These are amazingly cheap, a small table saw with a 4" diamond wheel dipping into a water bath. Ours cost about $95, and would easily cut alumina tubing.
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Re: FAQ - Cleaning optical windows, insulators and chamber interiors

Post by Richard Hull »

I have used one of these cheapo diamond table saws for tile to slab and shape U rock and other small mineral specimens as well as certain other ceramics.

It's wet and messy and you need to be in a swim suit to use it, but it works great outdoors.

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Re: FAQ - Cleaning optical windows, insulators and chamber interiors

Post by Doug Coulter »

I've had OK luck with borosilicate going after it pretty hard, even using a belt sander with 60 grit.

Of course it's nicer with more refined tools, and if you can, use water as a lube/coolant, it really helps amazingly.

Yes, breathing powdered glass is how you get silicosis -- don't. It seems inert stuff is worse for your lungs than even cigarette smoke -- at least your body knows how to "eat" that latter, but stuff that just stays in there and irritates, like this or like asbestos is to be really avoided. I always use water to keep that dust out of the air and out of my lungs.

But of course, next time just get the right size -- I ran into that one too -- the SS pipe with weld bead tends to be about 5.9" inside.
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Re: FAQ - Cleaning optical windows, insulators and chamber interiors

Post by tligon »

I tried the belt sander with a fresh belt. Half an hour of work reduced the diameter about 0.005".

The already used diamond wheel took off 0.050" in about 5 minutes. I was amazed at the difference.

I used the belt sander to put a smoother finish on the result.
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Re: FAQ - Cleaning optical windows, insulators and chamber interiors

Post by Doug Coulter »

Yeah, I use it mainly on thinner stuff -- a good belt will eat 1/4" tubing almost as fast as you can push it in there, or a thin (like 1/16") window. The thick stuff....probably mostly just strips the abrasive off the belt.

I use diamond wheels and hole saws extensively here in glass and quartz working -- no question that's the way to go. I built what amounts to a lathe mountable dremel tool to hold the wheels for cutting tubing, and it's the cat's meow for that, I just use an eyedropper to drip water on it and keep the dust down. It wouldn't occur to me to try the table saw or angle grinder versions -- too fast and too brutal -- and I don't like having to wear a suit.

I use the hole saws in the milling machine to hole even thin wall quartz tubing, it works fine, and saves work making Tees and so on. You use modeling clay to make a little dam to hold some water, which also cuts down chipping around the hole, and just be patient -- it doesn't actually take all that long to get through and does a nice job. The reason to use the mill is it will go nice and slow (80 rpm), and has less wobble in the quill than any drill press I've had.

I use cerium oxide for polishing when I bother to do that (Caswell plating sells it online), make a slurry on a cloth wheel for example, chucked into a hand drill.

I did that to make some old CRT lead glass usable for a viewport cover to keep X rays down, the inside side wasn't smooth enough without that step. That was a decent stopgap until we found some real good lead glass from a radiology dept that really stops them cold and is clear (but must be almost all PbO, that is one heavy chunk of glass).

With that, and the lead dressing on the fusor, I can now stand right there and observe and take high rez pictures with the detectors sitting at background levels (except for neutrons). Really makes it easier to learn and make progress on these -- worth doing. The webcam approach just wasn't getting it done for us here, and it's hard to move a wobble stick from across the room anyway.
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Re: FAQ - Cleaning optical windows, insulators and chamber interiors

Post by tligon »

Some people like wet and messy.

I recall a bit of a learning curve. Stand to the side, wear old clothes, and only do it in the late evening when there is nobody to give you a hard time about being all covered in speckles.

And do your own laundry.

I got diamond wheels for the Moto-Tool to cut alumina tubing. What resulted was worn out diamond wheels. Dry-cutting alumina peels the diamonds right off the wheel. But the wetted tile-cutting diamond wheels cut right thru alumina with no apparent wear to the diamond coating in the time I used it. The cuts were beautiful.

The neat thing about a tile-cutting saw is my wife will think I bought it for her projects! She is thinking about some tile in the kitchen and has a gemstone project in which it would be handy.
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