Understanding CF connections

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Pablo Llaguno
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Understanding CF connections

Post by Pablo Llaguno »

Hey,

I am doing a CAD design of what would be my vacuum chamber (6" spherical chamber), however, I was very unfamiliarized with ConFlat fittings, and though I understand them better at the moment I still have some doubts, and would like your points of view.

1) http://teslaboys.com/Fusor/Construction/index.html These guys made a vacuum chamber design that looks very good and professional, I would like to do something similar, how are the bolts attached to the 2.75" Conflat flange, if the flange is welded to the hemisphere?

2) Other designs have used tubes/pipes welded to a hole in the hemisphere and then this welded to a ConFlat, in which later you could add a feedthrough, viewport, etc. Does this give you some advantage over welding the ConFlat directly? If so, would I need any special adapter?

3) What type of connections should be used between the chamber - gate valve - diffusion pump?

4) In any case, when doing CF connections, you always have to bolt them right? So what teslaboys did in 1) is a good solution right? What are other options?

Thanks in advance. I have attached a preview of how my CAD is going on (AutoDesk Fusion 360).
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ian_krase
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Re: Understanding CF connections

Post by ian_krase »

While there's no reason not to do this kind of spherical chamber if you want to, people have had pretty good luck with fusors made out of large conflat or ISO flanged tees/crosses for the chamber. Of course, this mandates a somewhat tiny fusor.

Probably the bolts are put in from the outside, or the flanges on the chamber have tapped holes. Either way works. Also, don't forget the option of studs rather than bolts (i.e. small bits of threaded rod with nuts, rather than bolts with attached heads) as this can possibly make tight connections easier.

The two main advantages of having flanges be tubulated are that it gives you space to weld and it gives you space to put the bolts in. It also gives you space for... stuff, such as nude ion gauges or the inside extension of feedthroughs.

Conflat or ISO-LF are probably both options. It will heavily depend on what valve, etc you scrounge (if you are building your own chamber). The connection between the diffusion pump and whatever it attaches to will likely be an ASA flange or some manufacturer specific form of flange -- either way, this is a Plain Old Flat Pipe Flange meant to be bolted to an identical flange and sealed with a flat sheet gasket or an Econo-Ring (as sold by Duniway and others). Will depend on your diff pump. (Turbopumps on the other hand almost always use KF/LF/Conflat.) You might need to make your own.
Pablo Llaguno
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Re: Understanding CF connections

Post by Pablo Llaguno »

Thanks Ian, seems like a tubulated design might be the best way to go if I want to avoid future leaks.
Jerry Biehler
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Re: Understanding CF connections

Post by Jerry Biehler »

Conflats generally use through bolts. So this means the flange on the chamber must be mounted on a nipple so you have back access to the flanges for nuts. Generally you dont use blind holes with conflat since stainless has a tendency to gall and the last thing you want is a broken bolt in an expensive part.

That valve you have there has blind holes and the way you have it designed it is impossible to assemble.

Honestly conflat is overkill for a fusor. ISO flanges are more than good enough for the vacuum levels you need for a fusor.
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Richard Hull
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Re: Understanding CF connections

Post by Richard Hull »

I second Jerry's comments. ISO fittings for the chamber halves are fine.
My diff pump was ISO 63 and my chamber valve on hand was 2.75 conflat. A simple ISO to CF flush adapter solved the connectivity issue.

Richard Hull
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Pablo Llaguno
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Re: Understanding CF connections

Post by Pablo Llaguno »

Honestly I think I need a good reading on vacuum fittings, if anyone has a recommendation on any book, article, webpage, pdf, etc. it would really help. Also, Jerry, from what I have seen in Lesker I thought that Conflat was cheaper than ISO, at least in flanges, wouldn't it be better to use conflat? And also, thanks for the insight on the valve; Richard, do you have a post from your most recent fusor (fusor IV?), maybe I can learn something from yours.

Thanks!
Jerry Biehler
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Re: Understanding CF connections

Post by Jerry Biehler »

Look again, Clamp style ISO is about half the price of conflat, much more machining involved.

Only buy from lesker if you want to waste your money. Try LDS Vacuum. http://www.ldsvacuumshopper.com/shorwelflans1.html
ian_krase
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Re: Understanding CF connections

Post by ian_krase »

Yeah, LDS is much cheaper for a number of things. In particular, they are the only source of compression port fittings (useful for ion gauges and DIY feedthroughs) that are not super expensive.

Pchemlabs may also deserve attention.


Besides being cheaper and more re-usable, ISO-LF flanges are also much more compact.

However, Conflat seems to be kind of a "gold standard" so many surplus components will have CF flanges like it or not. (And VCR fittings for small tubes)
Jerry Biehler
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Re: Understanding CF connections

Post by Jerry Biehler »

It just depends, I have a lot of stuff with ISO flanges of one type or another (and the annoying JIS flange). Around here a lot of the surplus is coming from semiconductor equipment which a lot of it runs only at high vacuum levels so it is easy to get away with o-ring seals. On most of the semiconductor equipment I have taken apart it is all KF/ISO flanges.

When you get into stuff like FIBs and electron beam litho you start seeing a lot more conflat. And anything that needs to be baked out to any degree. These two old pfeiffer turbos are both conflat which is really annoying, it looks like the 1000l/s pump is going to work so I need to go from a 10" conflat to 6" ASA, fun...
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