Baseplate layers

Every fusor and fusion system seems to need a vacuum. This area is for detailed discussion of vacuum systems, materials, gauging, etc. related to fusor or fusion research.
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fpg
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Baseplate layers

Post by fpg » Tue Jan 05, 2010 9:47 pm

About 3 weeks ago I tried to build a simple demo fusor with the parts available at my school. But as it turns out their vacuumpump was way to weak for any fusor conditions.

So, I have to buy a vacuumpump. I have searched for a while and found this company that sells vacuumpumps and also makes high vacuum steel chambers (http://www.advanced-vacuum.se/index.php?p=1&id=1). But their cheapest vacuumpump costs about 1 700 $ that was well over my budget but they also informed me about one other company that sells a 2-stage vacuum pump with an ultimat vacuum of approximately 10 microns for a more reasonable price of 400$. Hopefully I will buy that pump next week if I don´t find any better offer until then.

As for my vacuum chamber I will start using this glass bowl(and yes I know it isn´t a good choice to use regular glass but I will also have another bigger safety bowl around it). I will only use it until I afford a real steel chamber.

I have not found any 1/4" steel plate to use as a base plate for the vacuum chamber. But I found an old stainless steel plate (see picture) which is 1.5mm thick so I thought that I could use quick epoxy to glue 4 pieces of my plate together so it would become about 1/4" thick. Will my baseplate be strong enought to withstand the pressure?

( Excuse my English. I have searched the forums with no results but I could be wrong)
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Doug Coulter
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Re: Baseplate layers

Post by Doug Coulter » Wed Jan 06, 2010 2:05 am

How thick it needs to be is a big function of how large it is (where it has a pressure differential from side to side) and how much flex you can tolerate. I'd avoid the glue thing, myself. No way are the tempcos going to match, and it will delaminate after a few thermal cycles, which it will see in use for sure and certain. Which may cause an accident at a bad time (is there any good time? But it's worse with high voltage on and so forth).

I can't tell how much flex you can tolerate because you've not mentioned how you intend to seal it to the glass piece. A properly designed O ring setup can handle a little of that. Not much else will, which includes a poorly designed O ring system. Don't even consider glue for that -- you'll need to get in there easily and it won't work anyway. You might do the old beeswax/rosin thing, but again, if it gets warm, you've got problems.

I'd say (and where are you?) find a metal supply house and ask for scraps, if you can't find decently thick plate in a hardware store -- here in the US you can always find things like that at Lowes and so on, horribly overpriced of course, but still not that much actual money.

A $400 vacuum pump that works right isn't a bad deal at all.
You will not get within about 2-3x of the ratings, those are very optimistic and measured with zero volume outside the pump to leak and outgas.
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fpg
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Re: Baseplate layers

Post by fpg » Wed Jan 06, 2010 7:19 pm

I will use a rubber O ring that came with the glass bowl and some silicon for some extra seal. I hope it will be enought to seal the bowl to the baseplate.

I´ve calculated that the pressure on the baseplate will be about 785.4N assuming the pressure difference is 100k Pa and the diameter of the hole is 10 cm(3.94"). But I don´t know how thick the baseplate will have to be. I know by reading in the forum that 0.66 cm (1/4") steel plate will probably be enought.

I live in sweden and somehow it feels like it´s much easier finding parts for a fusor in US than here . But I will do my best to find a plate that is thick enought.

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Re: Baseplate layers

Post by Doug Coulter » Wed Jan 06, 2010 7:42 pm

Silicone (note spelling) "goop" isn't very good for vacuum as it gives off acetic acid during curing (vinegar). This spoils vacuum and corrodes things. For 4" (sorry, I'm only metric when doing chemistry) you could doubtless get away with less than 1/4", again, if a little flex is OK in your setup. The O ring will need some kind of way to keep it from sucking in, of course. I'd bet if it's thick and hard to bend in the hands, it would do. You might be able to use something like 1/8" thick cover plate from some old electronic gear or the like for this.

A lot of people mention USA is a better place to get stuff -- this is likely true mostly because there are so many of us doing things like this and swapping parts with each other. I would guess the hardware stores are about the same everywhere? For new stuff, of course, most any company will ship overseas, it's just that the ship charges get bad. So a possible plan would be to get together with whoever else in your zone wants stuff and make some kind of group order to amortize the shipping across all. We do that here when we have to get things like BTI's from Canada or vacuum pump parts from Europe.

It may also be a little cultural. We tend to be a fairly brash people -- we will go to a junkyard and just assume we can go fishing there, and offer stupid low money for junk -- some of which we know is pearls, but the junkyard guy does not. I would guess a more polite culture would have more barriers to just going fishing around in junk, trash, etc than we do..But I would also guess some things are the same everywhere -- all universities sell their old gear somehow, and so on. But we don't find this by being timid and waiting for full page advertisements either -- we go and hassle people. This horse-trader mentality may indeed be a mainly American thing, I wouldn't know.

I'd love to hear from those overseas about how they approach things, how much the stuffy bureaucracy or just cultural rules affect how they can/will scrounge, it'd be an interesting topic.

I suspect that some things "simply aren't done" in "polite society" on the other side of the pond that fewer people here would have qualms about (we have our sheep too, of course, but we foxes do better than they at this and many other games). Here people will go right up to (and into) a big dumpster that has big signs saying "no taking things from this trash repository" and kind of dare the world to object -- and make some nice scores. Back when I was learning electronics, I got tons of stuff out of dumpsters behind repair shops for example. "Against the rules" but who really cares if you lighten their load of what they think is trash? Not many if you don't make a bigger mess doing it.

Heck, I still do things like that. I recently scored two large projection TV's, which were "bad" but which had some very nice fusor parts -- HV supplies, screens I could get phosphors off of, general electronics parts, etc. Would embarrass some people, but I just consider it fun. I will often see something, put some electric/recharable power tools in the car, and just go take the parts I want, and leave the rest there -- a little fun outing which you could consider "income producing" activity.
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Re: Baseplate layers

Post by DaveC » Wed Jan 06, 2010 8:55 pm

Fredrik-

For a small (10 cm dia. or less) pseudo bell jar type vacuum container, perhaps the simplest and least costly is a glass peanut butter or pickle jar... the type that came vacuum sealed.

Years ago I built a simple chamber using the screw top jar like that, with a soldered copper tube fitting in it, for low vacuum ( as in not very good) experiments. I think it was good for somewhere around 0.5 Torr or bit less.

I used the room temperature cure silicone rubber for the seal. And that worked, too.

Doug's point about the acetic acid being given off is very important to note. The vinegar-like emission will corrode electronics, particularly copper and nickel parts. It's not too agressive on cold rolled steel, and won't really bother stainless, unless you leave it for weeks.

I made the top gasket with a ribbon of the RTV, and then cured it very quickly... in the steam plume from a tea kettle of boiling water!! Took about 5 or 10 minutes instead of about 24 hrs. The steam seemed to also wash away the acetic acid residue. A good soak in hot water for an hr would also probably work. Just use your nose to tell... When you can't smell a trace of it... it's essentially gone. (Th' human mass spec to the amateur's rescue.)

For a real base plate... you should think of at least 1 cm thickness. Steel or aluminum plate is fine... Stainless steel is the best of course. Why worry about how muck flex with the thinner ones?

The O ring type seal will need to fit in a proper sized groove so it is contained laterally, and makes contact to the bottom edge of the jar on at least. Real bell jar seals have an L shaped cross-section, and are pretty expensive in the larger sizes. Don't know what the smallest size is.

Also, unless you have something already designed for a vacuum, application, the edge probably needs to be ground flat. I used glass polishing compound, but Silicon Carbide wet/dry sandpaper works well, or a diamond grit file. Use a glass plate so it's reasonably flat across the whole plane of the edge. I t should have a flat area about 3 mm wide or so.

Hope that helps a little. Be careful, with the glass, though. Implosions can be dangerous and messy. You need your eyes and fingers and hand.

Dave Cooper

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Re: Baseplate layers

Post by fpg » Sat Jan 09, 2010 12:52 am

Finally!
After a long search I found a steel workshop that made me a steelplate with 6 holes, four holes for the legs, one hole for the vacuum pump and one hole for the inner grid. The size was 250*250*10mm so it should meet the requirements for the vacuum. The cost of the plate was 28$

As you see in the picture below I have a steel bar through a rubber cork which I will use as the negative output to the inner grid. The cork and bar will go through the big hole in the center and right beside it will the vacuum pump hose be. the surface of the plate isn´t very smooth so I will use some sandpaper to make it fit perfectly with the bowl.

The bowl I have in the picture is just one of many. That is why I bought a quite large steelplate so I will be abled to upgrade to a bigger and better jar if the one I use fails. as I´ve said before I use a rubber seal O ring that was on the lid of the bowl when I bought it. It fits almost perfectly on the bowl and it looks almost like a suction cup. I will probably seal it with some kind of grease if silicon isn´t such a good choice.

If everything goes as planned I will order my vacuum pump tomorrow.
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Re: Baseplate layers

Post by Doug Coulter » Sat Jan 09, 2010 1:28 am

Looking good! You're going to have "interesting times" with that cork, but you'll learn something.
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Re: Baseplate layers

Post by lutzhoffman » Sat Jan 09, 2010 3:02 am

Hello:

Ref: Vacuum pumps, have you checked ebay? The 2 stage Robinair pumps are not bad, and I have seen new pumps go for as little as $250.00, I am talking about the larger 6CFM ones. At anything below 50-100 microns the cheap factory oil may give you trouble.

I would argue that a cheaper pump with good oil, will serve you much better than an expensive pump with cheap oil : ) Dunaway stockroom, and others have good reasonably priced oil, just search the forum.

Personally I like re-built belt drive pumps, its just personal, I like their simplicity and reliability. Try to buy direct with some direct drive pumps, sometimes third parties repackage the same pump, and pad the bill so to say. Good Luck

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Re: Baseplate layers

Post by Doug Coulter » Sat Jan 09, 2010 3:15 am

You might find this link interesting re prices of new vacuum pumps.

http://www.mcmaster.com/#vacuum-pumps/=5ak6ey
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Re: Baseplate layers

Post by fpg » Sat Jan 09, 2010 11:47 am

I´ve looked at ebay several times but there is very few pumps which actually ship outside the US. Those who ship worldwide also have a huge shipping cost. it somehow feels safer buying one here even if the price is a bit higher due to warranty and other factors.

The 2-stage vacuum pump I will buy have a warranty of 2 years, 1.8 cfm , ultimat vacuum of 10 microns and it includes an analog vacuum gauge. The cost is 400$ including shipping as I wrote before. The name is V-i220SV and here it is: http://www.worldvalue.cn/Products_item_big.asp?id=112

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