Vaccum plate for bell jar
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charlie_mccartney
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- Real name: Charlie McCartney
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Vaccum plate for bell jar
I have decided to go with a bell jar for my demo fusor chamber. I have decided to choose this because not only is it a good visual choice due to the fact that everything is visible, but it is also very cheap compared to the SS chambers. For this I am going to be looking for some tips on the vacuum plate such as where to get a good on at a reasonable price or even make one (I love working with my hands as I am a engineer at heart). Also I am using most KF-25 pieces for my vacuum system (will start another conversation about an issue later) therefore would I just cut a circle and JB weld a KF-25 nipple into the hole or what. If anyone has any information I would love it, thank you.
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Rich Feldman
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- Location: Santa Clara County, CA, USA
Re: Vaccum plate for bell jar
Hi Charlie.
You can mate NW (aka KF) fittings directly to a flat metal plate that has a hole for the centering ring.
I've done that using "bulkhead clamps" as seen here at LDS store: http://vacuumshopper.stores.yahoo.net/qfbulclam1.html
and as posted to fusor.net: viewtopic.php?f=10&t=3977&p=25528#p25528
This forum has discussed one source of relatively inexpensive aluminum plates:
the remnants pile at S&S Machine, which also makes bicycle frame couplings.
http://www.sandsmachine.com/alumweb.htm
The material there is cast plate. Better than wrought (rolled) plate, because not full of internal stress that makes it warp when machined. Someone suggested that holes for screws can be tapped using forming taps instead of cutting taps. The result would be stronger threads, not so easily stripped.
You can mate NW (aka KF) fittings directly to a flat metal plate that has a hole for the centering ring.
I've done that using "bulkhead clamps" as seen here at LDS store: http://vacuumshopper.stores.yahoo.net/qfbulclam1.html
and as posted to fusor.net: viewtopic.php?f=10&t=3977&p=25528#p25528
This forum has discussed one source of relatively inexpensive aluminum plates:
the remnants pile at S&S Machine, which also makes bicycle frame couplings.
http://www.sandsmachine.com/alumweb.htm
The material there is cast plate. Better than wrought (rolled) plate, because not full of internal stress that makes it warp when machined. Someone suggested that holes for screws can be tapped using forming taps instead of cutting taps. The result would be stronger threads, not so easily stripped.
All models are wrong; some models are useful. -- George Box
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charlie_mccartney
- Posts: 139
- Joined: Sat Jun 29, 2013 3:45 pm
- Real name: Charlie McCartney
- Location: Dallas, Texas, U.S.A.
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Re: Vaccum plate for bell jar
I have already looked at all of this, as I do not have access to any kind of shop, welder, or any specialty tools, however I do have a barn full of generic tools that has proven useful. While I would love to make my own, I do not have the tools for working with Al. If anyone has any kind of premade vacuum plates with a KF-25 connection or would be willing to help me I would love it.
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Rich Feldman
- Posts: 1471
- Joined: Mon Dec 21, 2009 6:59 pm
- Real name: Rich Feldman
- Location: Santa Clara County, CA, USA
Re: Vaccum plate for bell jar
Charlie,
I suggested KF bulkhead clamps because the connection would be much stronger & more heat resistant than any glued connection between a KF half-nipple and metal baseplate.
What really matters is the kind of fitting on the big end of your diffusion pump, or vacuum throttling valve.
But you'll learn plenty by starting with a quick & dirty demo fusor & using mechanical pump only.
You can work aluminum with ordinary hardware store tools:
- drill (drill press would be nice but isn't strictly required)
- hole saw
- threading tap, probably 10-24 or 1/4-20.
Same goes for stainless steel, but you need to be much more selective about the hole saw, and should use cutting fluid, and a very slow-turning drill.
1/4 inch thick aluminum is strong & stiff enough for a 6" diameter bell jar, and probably for 8".
I got the job done with 3/8 inch thick HDPE from Tap Plastics, with barely tolerable deflection, but for a fusor you'll want metal.
Instead of drilling and tapping blind holes, you can drill and tap all the way through.
Then a) seal the holes on vacuum side with glued metal plates or very short pieces of threaded stock.
or b) glue threaded studs into the baseplate & use hex nuts on the other side of bulkhead clamps.
Perhaps c) use unthreaded clearance holes, and machine screws with heads on the vacuum side, sealed with epoxy.
The bulkhead clamps themselves, with a regular KF centering ring and nipple, can be used as a template for laying out the bolt pattern.
I suggested KF bulkhead clamps because the connection would be much stronger & more heat resistant than any glued connection between a KF half-nipple and metal baseplate.
What really matters is the kind of fitting on the big end of your diffusion pump, or vacuum throttling valve.
But you'll learn plenty by starting with a quick & dirty demo fusor & using mechanical pump only.
You can work aluminum with ordinary hardware store tools:
- drill (drill press would be nice but isn't strictly required)
- hole saw
- threading tap, probably 10-24 or 1/4-20.
Same goes for stainless steel, but you need to be much more selective about the hole saw, and should use cutting fluid, and a very slow-turning drill.
1/4 inch thick aluminum is strong & stiff enough for a 6" diameter bell jar, and probably for 8".
I got the job done with 3/8 inch thick HDPE from Tap Plastics, with barely tolerable deflection, but for a fusor you'll want metal.
Instead of drilling and tapping blind holes, you can drill and tap all the way through.
Then a) seal the holes on vacuum side with glued metal plates or very short pieces of threaded stock.
or b) glue threaded studs into the baseplate & use hex nuts on the other side of bulkhead clamps.
Perhaps c) use unthreaded clearance holes, and machine screws with heads on the vacuum side, sealed with epoxy.
The bulkhead clamps themselves, with a regular KF centering ring and nipple, can be used as a template for laying out the bolt pattern.
All models are wrong; some models are useful. -- George Box
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Chris Bradley
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Re: Vaccum plate for bell jar
I've posted quite a bit on my Al base plate arrangements, that you should be able to find.
The key things to bear in mind are to ensure flatness of the plate, and perpendicularity of any threads you might make through it.
You can get 1/2" NPT to KF25 adapters which work just fine, but the sealing doesn't happen 'in the threads', it happens with an o-ring between the flange and baseplate, so the axis of the thread, and thus flange, needs to be perpendicular.
'C-clamps' also work fine, where you thread a blind hole to receive the C-clamp bolts, then hold down any type of flange you want.
To ensure flatness, you may need to turn the whole plate in a large 4-jaw chuck. You can skim off just the area where the bell jar will sit. By skimming it down you should get microns accuracy and you'll then only need a really thin gasket, or no gasket at all and use vacuum grease (but it can be problematic to separate the ground glass from such a flat surface).
Because it is a good idea to skim the plate in a 4-jaw chuck, and tap perpendicular holes, it is a job best done by an engineering firm, if you can find a friendly one in the area. It should be a real quick job for them, and even if they want to charge you full price (if you can't persuade them with a few packets of biscuits or other gift treats!) then it is probably still a worthwhile expense to avoid the hassles you'll generate for yourself if you try to do the job with a hand-drill or other domestic limitations.
The key things to bear in mind are to ensure flatness of the plate, and perpendicularity of any threads you might make through it.
You can get 1/2" NPT to KF25 adapters which work just fine, but the sealing doesn't happen 'in the threads', it happens with an o-ring between the flange and baseplate, so the axis of the thread, and thus flange, needs to be perpendicular.
'C-clamps' also work fine, where you thread a blind hole to receive the C-clamp bolts, then hold down any type of flange you want.
To ensure flatness, you may need to turn the whole plate in a large 4-jaw chuck. You can skim off just the area where the bell jar will sit. By skimming it down you should get microns accuracy and you'll then only need a really thin gasket, or no gasket at all and use vacuum grease (but it can be problematic to separate the ground glass from such a flat surface).
Because it is a good idea to skim the plate in a 4-jaw chuck, and tap perpendicular holes, it is a job best done by an engineering firm, if you can find a friendly one in the area. It should be a real quick job for them, and even if they want to charge you full price (if you can't persuade them with a few packets of biscuits or other gift treats!) then it is probably still a worthwhile expense to avoid the hassles you'll generate for yourself if you try to do the job with a hand-drill or other domestic limitations.
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Jim Kovalchick
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Re: Vaccum plate for bell jar
My son's fusor used an aluminum base plate that was milled by his uncle to have a shoulder socket to partially set a KF-25 nipple for his feed through. The nipple was attached by JB Weld. Rough your plate and nipple so the epoxy sticks. Most of the epoxy was outside the vacuum side and did disturb the plasma. If you do this make sure your plate is stout enough that it doesn't flex under vacuum.
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charlie_mccartney
- Posts: 139
- Joined: Sat Jun 29, 2013 3:45 pm
- Real name: Charlie McCartney
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Re: Vaccum plate for bell jar
My two bellows valves are 2.75 CF and I have a 2.75 CF cross, also I already have a diffusion pump that has a input and output of kf-25, you can look at everything right here -
viewtopic.php?f=18&t=8820
viewtopic.php?f=20&t=8882&p=60965#p60965
I have purchased this - http://www.ebay.com/itm/370912253912?ss ... 1426.l2649
as my bell jar, I will soon be purchasing a Al disc for my plate, also if anyone has a few kf-25 to CF 2.75 adapters, I really need them, also a 2.75 CF vacuum hose.
I will probably end up having the disc professionally finished off once I start getting some funding, also would it be better to keep with CF for diffusion to chamber or should I switch to KF as it is not going to be any where close to high vacuum (single digits at most probably).
viewtopic.php?f=18&t=8820
viewtopic.php?f=20&t=8882&p=60965#p60965
I have purchased this - http://www.ebay.com/itm/370912253912?ss ... 1426.l2649
as my bell jar, I will soon be purchasing a Al disc for my plate, also if anyone has a few kf-25 to CF 2.75 adapters, I really need them, also a 2.75 CF vacuum hose.
I will probably end up having the disc professionally finished off once I start getting some funding, also would it be better to keep with CF for diffusion to chamber or should I switch to KF as it is not going to be any where close to high vacuum (single digits at most probably).