Polishing Chamber

[Jerry Biehler]

Most people heat the outside of the chamber; using radiant energy would be very inefficient. One can get heating tape at some hardware stores (for outside pipes.) A variac could control the temperature.

For a fusor, as I understand from posts here, 10^-6 torr is fine relative to removing unwanted trace gasses. If the chamber can bottom that out range, then it is doing very well.

I get down to the -8 range on my chamber without much issue, it does take a couple days to do it, which for a chamber the size of mine is not too surprising. And the whole thing is nitrile or viton seals. Of course I do have a 2000l/s turbo on it...

You can heat the outside. What I do is use a 1kw linear quartz halogen lamp to heat things up from the inside. This way you get heating on any surface within line of sight of the lamp. You can also use shortwave mercury UV lamps instead. The UV knocks the water molecules off the wall.

IMG_8204 by macona, on Flickr

IMG_8209 by macona, on Flickr

[Dennis P Brown]

That is impressive - to reach 10^-8 torr routinely! You have a very good system.

As for using a UV source to breakup water on the chamber walls, an excellent trick used by a number of professional systems, too. Very good method if one needs to reach very high purity levels in their chamber but not for most fusors much less beginners. Still, external chamber wall heating is the easiest and most efficient method for clearing the walls of trap gasses for someone concerned with that issue (all types of gasses; but you are correct that water is the most difficult and common and that a hard UV lamp could be used. But it is important to add information on the danger of using a UV lamp - then NO quartz windows can be used on the chamber!)

Dry nitrogen would be useful as would (cheaper) dry air.

I notice you have a quartz thickness monitor in one picture - what use is that in a fusor or is that a coating system you are showing pictures for? If so, what do you coat?

[David Kunkle]

Most people heat the outside of the chamber; using radiant energy would be very inefficient. One can get heating tape at some hardware stores (for outside pipes.) A variac could control the temperature.

Never would have thought of that. So, basically wrap the chamber with as much heating tape as you dare (or can afford) while the pumps are running?

You mention controlling the temp. Wouldn't it be the hotter the better to vaporize everything off the walls?

Thanks.

[David Kunkle]

I get down to the -8 range on my chamber without much issue, it does take a couple days to do it, which for a chamber the size of mine is not too surprising. And the whole thing is nitrile or viton seals. Of course I do have a 2000l/s turbo on it...

You can heat the outside. What I do is use a 1kw linear quartz halogen lamp to heat things up from the inside. This way you get heating on any surface within line of sight of the lamp. You can also use shortwave mercury UV lamps instead. The UV knocks the water molecules off the wall.

I must have run across your old posts using non-metal seals and reaching -8. What is the size of your chamber?

[prestonbarrows]

... 2000l/s turbo on it...

Yup, that is how you get to 1E-8 using elastomer seals; throw around $50,000 worth of pumping on a smallish chamber.

[Jerry Biehler]

I must have run across your old posts using non-metal seals and reaching -8. What is the size of your chamber?

26" in diameter and about 30" tall. Just a wee thing! viewtopic.php?f=10&t=9501

Dry nitrogen would be useful as would (cheaper) dry air.

I notice you have a quartz thickness monitor in one picture - what use is that in a fusor or is that a coating system you are showing pictures for? If so, what do you coat?

Or use argon, argon is much easier to pump out.

I am building a system for doing coating. E-beam, sputtering, and thermal dep all in one. Hoping to use it to make dielectric mirror and filters. I finished up the passbank for the filament controller on the thermal size of things, a professor-friend at Portland State has some sputter gun and small e-beam gun designs I am going to try.

[Jerry Biehler]

Most people heat the outside of the chamber; using radiant energy would be very inefficient. One can get heating tape at some hardware stores (for outside pipes.) A variac could control the temperature.

Never would have thought of that. So, basically wrap the chamber with as much heating tape as you dare (or can afford) while the pumps are running?

You mention controlling the temp. Wouldn't it be the hotter the better to vaporize everything off the walls?

Thanks.

I set up a PID temp controller with my tape and a thermocouple. I dont run much past 250F. No normal heat tape will not go this high, this is kind of special silicone heat tape. You can find them on ebay sometimes, be prepared for sticker shock. Be sure to watch your temps if you do have rubber seals. They dont like much past 200F.

Radiative heating works quite well. You dont need the heat so much as you need the energy. As long as you have energy hitting the inside of the chamber it will help, especially with anything else in the chamber as it will get heated or irradiated as well. The base plate for my chamber is 1" thick stainless, if I remember right, after about an hour it is actually quite hot and the lamp is a couple feet away.

[Jerry Biehler]

This is my old vacuum system and I installed a 250w Ushio Ultra High Pressure mercury lamp inside it and powered it off my tig welder. It worked very well! I had the RGA running when I fired it up monitoring mass 18 for water vapor, you can see the massive spike on the laptop screen from where the lamp turned on.

Ushio Mercury Lamp in vacuum chamber by macona, on Flickr

[Dennis P Brown]

Any varaic will control the heating tape temp and a good first point to try would be 50 C (digital thermometers with a remote sensor can be had rather cheap at most hardware stores.) See what that does to improve your base pressure. Walking up in temp can be done but I am no expert on what most seals on any given chamber can withstand so be careful (thermal expansion is an issue that cause or enhance leaks. I'd think standard KF o-rings could handle up to 100 C but might want to research that before going very high.) Internal heating requires feed through and filaments which aren't always easy/cheap for people but does work. You are correct that high temp heating tape is expensive ... .

I'd like to see a thread on your system, Mr. Biehler. Coatings do relate to fusors but this is a thread on chamber polishing ... .

[Bob Reite]

I wouldn't go above 50 C, unless your system is all metal seals.

[Jerry Biehler]

O-rings can take way more heat than 50C and even 100C:

http://www.duniway.com/images/_pg/viton ... orings.pdf

A thread on my vacuum system:

http://bbs.homeshopmachinist.net/thread ... uum-system

[David Kunkle]

O-rings can take way more heat than 50C and even 100C:

http://www.duniway.com/images/_pg/viton ... orings.pdf

Thanks for reminding me- I've actually had that link saved for some time since I will have one big Viton gasket. I probably would've forgotten about it by the time it comes to cooking it.

Started some grinding on my chamber pieces, and came to the realization that one guy on the internet was right about starting with 80 grit on a mill finish. Also talked to the welding shop, and he confirmed that I'll have to start with 80, then 200, 400, and then polishing compounds. Still be lots of work, but now that I finally know what equipment I need, at least I'll be able to really get started soon.

On the plus side, he thinks most all the welds can be done from the inside.

[Dennis P Brown]

To cut away weld joints, 80 grit would do it but to grind an already flat, and smooth surface with 80 grit? Most mill worked piece's I've cut are rather smooth. Maybe yours isn't smooth enough but 80 grit is very aggressive. Still, if you want to learn how to grind and polish steel, there is only one way to learn and that is to do it. As such, why not try it on a small test piece of steel to learn what you need to do before trying it on your chamber. That way, you can try anything and not worry about fixing a mistake on your expensive chamber ... .

[David Kunkle]

Hi all,

Working on polishing my chamber pieces. First I thought the SS was painted or had a coating. Tried removing it with acetone, then paint stripper. Nope. It's just the "mill finish". For the record in case anyone wants to accomplish the same thing down the road, here's what I found to work best so far to polish mill finish.

Turns out I did have to start with 80 grit. This still takes a fair amount of time to cut through the mill finish which I would describe as millions of tiny pits that, collectively, give the metal surface a dull, satin, greyish appearance. Flap disc on a grinder seems to work the best and the flap discs last seemingly forever for 10 bucks- unlike a sandpaper disc which is shot in a minute or two. Also, I have newfound respect (fear would probably be a better word for it) for angle grinders. It kicked back on me Sunday, and the 80 grit removed a significant strip/deep chunk of flesh from one finger in an instant. Never injured myself this badly with a power tool before. Nearly went to the ER, but decided to nurse it myself. So far so good- or at least it doesn't seem to be infected and the finger hasn't fallen off.

Then 120 grit flap disc (finest grit anyone makes them in) takes out the 80 grit scratches. Trying to go straight to 240 grit sandpaper discs takes too long and too much $$$. Next, 240 grit sanding discs go fairly quickly to remove the 120 grit scratches.

I'm now looking at 400 grit sanding discs which should go fast since all the deep scratches are now gone by the time the 240 grit is done. After that, coarse polishing compound, followed by 2 finer compounds to get to the finished mirror-like finish.

In the photo, the cylinder on the left is untouched mill finish- except for the bright strip where the weld was factory ground. The head on the right is done with the 240 grit.