Steel or aluminum baseplates and other questions

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Arun Luthra
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Steel or aluminum baseplates and other questions

Post by Arun Luthra » Mon Dec 02, 2019 9:53 pm

According to google image search, a popular fusor chamber design has a cylindrical glass wall (like a bell jar with no lid) with a metal base plate and metal upper plate.

Often there are four threaded rods around the chamber... what is the purpose of these? The vacuum itself should seal it tight against the gaskets.

I'm not making a fusor, but will be generating plasmas from tens of volts to a few thousand volts.

Can I use silicone rubber as the gasket material? I understand that I want to minimize stray plasma hitting this material.

Is aluminum sufficient for the base and top plates? I prefer this because it's easier to machine. Of course all the feedthroughs and piping into the chamber are steel. Is there a preferred method for "welding" the steel and aluminum together?


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Mark Rowley
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Re: Steel or aluminum baseplates and other questions

Post by Mark Rowley » Tue Dec 03, 2019 1:12 am

Aluminum is a decent baseplate material.

Those chambers you describe are atrocious at holding a vacuum. I would abandon the idea unless you want to go through a bunch of headache and ultimately come to the same conclusion. The glass cylinders do not have a wide base like a standard bell jar. This inadequacy makes them very leaky. Folks resort to tightly pressing all the parts together with those long bolts you describe (including the use of rubber) as a futile attempt at getting a better vacuum. At best, those cobblings attain 500mTorr of vacuum....and thats being generous.

Best course of action...
Use a nice 0.50" thick base plate with a Pyrex type heavy walled bell jar suited for vacuum work. Thick glass with a wide mating surface. See eBay item # 323068755231 for reference.

As a important side note....
All of this stuff has been discussed ad nauseam in FAQ's and recent posts. Baseplate material has been discussed a lot in the time span of this site. The chamber you describe has been discussed several times in the past month or two! Please spend a few hours or days reading past posts and FAQ's. Trust me, everything is here, many times over for that matter.

Mark Rowley

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Rich Feldman
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Re: Steel or aluminum baseplates and other questions

Post by Rich Feldman » Tue Dec 03, 2019 4:03 am

Here's a less pessimistic report, from the first project I ever reported on this forum.
We got down to about 20 microns at the science workshop where I was helping.
Using a polycarbonate bell jar with pretty narrow edge, a plastic baseplate only 3/8 inches thick, and a gasket of black rubber from the hardware store.
Bell jar held down only by suction.

We had the benefit of a good direct-drive rotary pump connected by a 1 inch hose (using a KF bulkhead clamp).
It took a couple hours of pumping before we saw the 20 microns.
First thing to do differently, if there's a next time, is to have a baseplate of thicker and/or stiffer stuff.

viewtopic.php?f=10&t=3977&p=25528&hilit ... ell#p25528

Looks like the pictures from 2010 survived!

p.s. Arun, I agree with Mark that seeing a design widely copied on the Internet doesn't mean it's any good.
Look at the silly Bourdon tube vacuum gauge, not useful even if "good enough vacuum" is electron mean free path of at least an inch.

[edit] Another glass option to consider (for your work, not for fusors) is Pyrex desiccator jars, like ebay 174081211784 today.
Perhaps more common than bell jars designed to safely withstand full vacuum.
For fat vacuum pipes and electrical feedthroughs you could replace the desiccator lid with a baseplate & gasket.
This one, ebay 184008862078, is lidless and seller is asking $19.
desicc.JPG (27.91 KiB) Viewed 2397 times
All models are wrong; some models are useful. -- George Box

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Mark Rowley
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Re: Steel or aluminum baseplates and other questions

Post by Mark Rowley » Tue Dec 03, 2019 5:20 am

Outside of this page I've seen quite a few folks (sadly absent the necessary in-person guidance of an expert) fail at obtaining a good vacuum with the "maker" fusor arrangement. FB, Youtube, Twitter, etc are packed folks who tried. The Pyrex bell jar is almost foolproof. A Disneyland E-Ticket to a good vacuum.

Nothing beats getting a good deep vacuum without all the gremlins we're so familiar with. Successes like that keep the momentum going and may enable the hobbyist to make it to the next level. Main reason I'm pretty firm with the classic bell jar or the dessicator you suggested. I had my fair share of mistakes, attempts, and failed vacuum chamber schemes in the early 2000's. I learned a lot but wasted a bunch of money too. It wasnt until I bought a good bell jar did I get below 30mTorr.

The new folks will have plenty to worry about keeping the feedthrough, vacuum gauge, and vacuum port airtight without the mess of the "maker" chamber.

Another distinct plus for the bell jar arrangement is it's ease of disassembly and simplistic vacuum tight reassembly.

Mark Rowley

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Richard Hull
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Re: Steel or aluminum baseplates and other questions

Post by Richard Hull » Tue Dec 03, 2019 5:31 am

The $19 desiccator lid or base is a great item for a small demo device. The beauty is the thick pyrex, and with a nice ground glass, wide lip...A suitable flat gasket with a hint of vacuum grease on both sides slapped on a 3/4-inch thick Aluminum plate and you can go very low if your pump is up to it. Nice find.

Richard Hull
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Dennis P Brown
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Re: Steel or aluminum baseplates and other questions

Post by Dennis P Brown » Tue Dec 03, 2019 7:51 pm

Aluminum is the best choice for 99.9% of people since hand drills and off-the-self hardware store drill bits (cheap chinese) can be used to drill holes. Steel isn't a metal most people can handle since high end drill bits and oil as well as a press are ussally required and even then, it is difficult to drill.

One cannot weld steel and alumium; rather, one uses a high grade epoxy for an easy fix (the hole should have a tight fit to the pipe that has as little epoxy as required); for higher end results, one threads the aluminum base plate and use proper fittings (also threaded) to seal the piping to the plate.

I have built very good vacuum grade sputtering systems using glass cylinders with an aluminum base plate and a glass disk top - vacuum grade flat o-ring shaped gaskets to seal. However, this also requires that the glass edges must be finely ground flat to be servicable for high vacuum work (under a few microns.)

Feed-thrus for glass pieces: Chinese glass drill bits (cyclinder type) have performed very well for me - just know how to use them: use a puddle of water (plumber's putty works great as a dam) with a drop of liquid soap/detergent, and run the drill press at low speed (never high!) I've made even 1/2 inch holes thru one inch plate glass with zero issues. Small glass dish shaped plates were trivial and even a hand drill can (with care) be used.

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Re: Steel or aluminum baseplates and other questions

Post by ian_krase » Thu Dec 05, 2019 5:52 am

I have a Pyrex cylinder chamber that I use for my sputter coater.

I achieve 1 micron pretty easily, and I think if I could just find my leak I would get to one millitorr. Grease and grinding is necessary to achieve this.

Jerry Biehler
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Re: Steel or aluminum baseplates and other questions

Post by Jerry Biehler » Thu Dec 05, 2019 7:24 am

Dont use steel, it is a mess. Stainless and aluminum is best. You can get nice rounds of aluminum tooling plate from S&S machine, I have used them a couple times:

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