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CF vs KF flanges

Posted: Tue Mar 20, 2012 7:06 pm
by Rob Pope
So I've been looking at actually constructing my fusor, and one thing getting me down is the $$$$ needed for ConFlat flanges

KF flanges look much cheaper, so, is there a reason to use CF over KF?

if KF works, it looks simpler and cheaper, and as a highschooler, $ doesn't enter the wallet very often.

thoughts on the ups and downs of either?


Re: CF vs KF flanges

Posted: Tue Mar 20, 2012 7:20 pm
by Tyler Christensen
My chamber was a combination of CF and KF. CF on the large hemisphere (pretty much the only option there, assuming you're going for the standard spherical chamber). I have one CF2.75 for the feedthrough since almost all surplus feedthroughs you find appropriate for fusors will be CF. I have KF on the rest, and I think that's a very good setup. It makes for easy access to ports that are frequently changed.

The only thing in favor of all CF I can see is it'd be a higher quality high vacuum research grade chamber, since higher vacuum can be achieved with CF. For a fusor though, you don't need UHV levels.

Re: CF vs KF flanges

Posted: Tue Mar 20, 2012 10:41 pm
by MarkS
Hi Rob, I was in the same position that you are when I first started my fusor (in 2008). I had two jobs at the time. One for each of my loves, the fusor, and a girl @___@. You're right that KF (aka NW aka QF) is much cheaper than ConFlat, and that CF takes more work to use than KF. What I did and what I would recommend, is to us KF for the vacuum manifold, and to use it for components that will need to be removed frequently. ConFlat is great for making long term and high quality seals (the metal gaskets actually fill in any machine defects in the knife edges thus making a perfect seal). So you'll want to use it for stuff like HV feed, gauge heads, etc etc. In your vacuum chamber, if you can afford it, its always good to have an extra flange or two that you can keep blanked off for future experiments. Good luck with the budget though! It's quite possible to make a fusor on a HS budget!
PS Ask around to all the vacuum companies if they have anything that theyre going to throw away that maybe you can use for your "High school science project". I got quite a bit of help from a few companies!

Re: CF vs KF flanges

Posted: Wed Mar 21, 2012 6:05 am
by Jerry Biehler
Another thing is CF takes heat where the polymer seals in KFs get rather unhappy.

They do make a soft aluminum seal for KFs though

Re: CF vs KF flanges

Posted: Wed Mar 21, 2012 6:50 am
by John Futter
I've seen enough banter

If I can get a Vacuum of high ten to the minus 9 Torr with around 6 yards total of viton o-ring in a system using Iso and KF flanges (these both use elastomeric seals) I feel that these are more than enough for an amateur fusor effort.
Yes CF will let you get almost two magnitudes better vacuum but a mean free path of ten metres or a thousand metres matters little for what you guys are about.
Baking out means that you are limited to 200 degrees celsius with Viton--around 120 degrees with butyl or other rubbers. (note both these temps are well above the boiling point of water which is the major enemy of of any vacuum system CF or KF.
I note that non of you wear lint free gloves when pics are posted of you assembling this already limits you to high ten to the minus 8's for days / weeks of continuous pumping till the finger prints evaporate.

KF /ISO is more than suitable for all amateur fusion efforts

this does not stop you getting a CF bargain you can get reusable viton gaskets to fit CF. Reusing CF copper gaskets is possible but usually ends in grief

Re: CF vs KF flanges

Posted: Wed Mar 21, 2012 2:20 pm
by Richard Hull
John's right in his approach. Use whatever comes to hand that will mate with the next piece inline on the system. Surplus purchases are probably 80-90% of the materials gathered to build amateur fusors. Highly useful and valuable stuff that comes cheap may dictate your choice of mating flange or connection.

This can make for an unsightly and confused looking mess, but a string of such items, if they serve to work a fusor, can be just fine.

Personally, I would rather have an all KF fitting system for ease of maintenance and modification, but some cool surplus stuff that suits a crying need is often found in ASA, ISO, CF and flangeless conditions and really cheap.

Here is where machining/welding skills can shine. I have sawed off a conflat fitting and welded on a KF in a couple of instances to make two valuable parts mate up.

Making a fusor can be about many things and take many paths, from not lifting a finger and contracting the assembly out for several thousands, to horrid and atrocious, but working kludges that were mostly made up from gifts and adaptations that cost a couple of hundred or more bucks and takes a year or two to put together due to scrounging necessities. Most folks are somewhere between these two extremes.

Richard Hull

Re: CF vs KF flanges

Posted: Wed Mar 21, 2012 6:52 pm
by Rob Pope
Thanks for the input all!
It really cheered me up, since KF is all so cheap!!
(well, not all, but your basic flanges are about a magnitude cheaper than CF)

I have a drill press/lathe/milling machine that is designed for metal, and a friend who is good at welding with tons of equipment, and willing to both help out, and teach me if need be. He can definitley lay down a vacuum quality bead.
All this adds up to me going the DIY approach, and using what I can get cheap.

On the same note, is a 4-6 way cross with KF fittings a suitable fusion chamber?
Or should I go with a CF chamber for the compatibility needed for some things like a HV feedthrough?

Re: CF vs KF flanges

Posted: Thu Mar 22, 2012 5:36 pm
by Richard Hull
You are going to have to get high voltage in your fusor somehow so the size of whatever chamber you pick will be important. Several folks have used larger, (4-inch or larger), 4-6 way crosses in the past and had good luck with them. Smaller crosses have been used, but it is a bit cramped and voltage limited.

Richard Hull

Re: CF vs KF flanges

Posted: Thu Mar 22, 2012 6:11 pm
by Rob Pope
I have some experience with high voltage (planning on re-using my Tesla Coil NST for the fusor)
I wouldn't feel comfortable with less than 3" for arcing to the case.

Re: CF vs KF flanges

Posted: Sun Mar 25, 2012 2:56 am
by Rob Pope
So this is somewhat related-

Is it at all possible to achieve fusion in a KF 50 sized cross?

how small can one go?
is 2" ID too small?
Because past that they shoot quite out of what I want to pay (I seriously only make $20/week, right now my budget is $1000 in a box)