Thread: Home built pressure pot

Unless you have a pressure welding ticket and the facilities to test your welds, I would strongly advise against continuing what you are doing. You are lucky you didnt die when it let go.

The price of a properly made pressure pot is cheap compared to your life or the life of an innocent bystander.

Thanks Ironwood, that's why, when it blew, rather than re-weld it straight away I packed it in and watched a movie instead. I wanted to think about it a bit more first. I'm confidant that if I did the clamps properly it will be fine up to the 60 psi I want, but I may decide to keep it at a lower pressure, now that I have some idea of it's limitations. Any further testing will be out in the middle of my block, well away from anything (including me!).

If I decide not to proceed, it will still make a nice vacuum chamber.

This post may also serve as a warning to others, should they contemplate doing anything similar - they may not be as lucky.

My paint pressure pot from Supercheap autos cost 99 dollars and is set at 45 lbs on the safety ,sure its tested higher,but for casting pen blanks and such is more than enough I am a fitter and turner ,but never really got into the welding side of things ,I have discovered however that its best I don't chip off the slag ,as in most cases that is the strongest part of the weld lol. Cheers ~ John

Nice thing about the MIG is that there's very little slag. One of the reasons for using a (small) gas bottle is that they're pressure tested to far higher than I could ever get, so there's no worries about the container's integrity at the sort of pressures I'm interested in.

I always knew that the clamps were the potential weak link. Thinking about what happened, it appears that my design may have been an inadvertent safety factor. If one clamp failed, the other two would have held it enough to simply let the excess pressure bleed off from the unclamped part. If any two failed (as happened) the top would simply slip out of the third clamp and, because of the domed shape travel straight up. If it had been a flat top it would have gone sideways and that would have been far more dangerous.

Whenever I make/build anything, I normally tend to over-engineer things (a result of my training many years ago, I guess) and I thought I had done the same here, but in this particular case I've realised that any over-engineering needs to be several magnitudes higher than what I think I'll need.

All in all, a good learning experience.

Originally Posted by dikman

...so there's no worries about the container's integrity at the sort of pressures I'm interested in....

Except where you have modified it

Why would you be interested in such high pressures anyway? Not needed for casting.

I'm interested in stabilising wood for knife handles. Commercial stabilisers apparently use up to 130 psi, which obviously I wouldn't even consider, but reading here seems to indicate that 50 or so psi should be sufficient for my needs (possibly alternating vacuum and pressure to give the best results).

And yes, you're quite right, my modifications would certainly reduce the original structural integrity significantly, something I was aware of. The welding of the outer rings shouldn't be a problem, as they have considerably more welding material than the relatively minor amount on the clamps (in comparison).

I originally decided to use the smaller bottle because I didn't need a large tank, but it's now occurred to me that if I had used a 9 kg bottle it would probably be safer at the pressure I want. Although it would be a lot more work making the larger rings, the walls of the bottle are much thicker than the smaller one, so the welding could have deeper penetration. Plus I would use more clamps around it, being larger.

Still thinking......

Have you considered just vacuum and the right compounds to be used .one of the guys on the IAP site markets a vacuum only stabilising setup ,seems to work ok,probably depends on your materials of course cheers ~ John

IAP site? What's that?
My original intention was to use just vacuum, but after lots of reading I got the impression that vacuum by itself wasn't really adequate (although much safer to use, of course).

Sorry IAP International Association Penturners another forum for penturners lots of useful posts and tutorials Look for posts by MesquiteMan here,s a sample TEST
Cheers ~ John

Thanks John, most informative site . I've only started reading a little, but already I've realised that for what I want to do my original idea of using vacuum is the way to go.

You may have just saved me a lot of work. I'll just re-build the clamps and use the tank as a vacuum unit, and not bother with pressure.

Yep a ton of knowledge around and best of all most share freely hehe. Neil (Dai sensei) has played a bit with both ways of stabilising ,But I think he has settled down on vaccum
he got one of those vacuum outfits from mesquiteman .not cheap ,but does a good job.his is made from heavy duty acrylic or similar about 10mm walls ,works a treat for the small items we do for pens rotten spalted timbers hehe . The beauty of this outfit you can see the process and know when the bubbles are finished. If you make yourself one look into a solid window you can incorporate so you can view the action hehe. Cheers ~ John

When I became interested in knifemaking (and then blacksmithing) some time ago, I found that people there were also only too happy to share their knowledge and experience. I try to do the same thing, whichever forum I join - if I can help someone, I will.

I have no interest whatsoever in making pens, but mesquite man's information is of great value for other uses too. His very thorough testing of the effects of vacuum and pressure on penetration of the wood was excellent, and it's obvious that vacuum is the way to go for me.

My home-made vac pump can pull around 27", which is about as good as I'm going to get at my altitude (1800 ft). My other tank, made from a 9 kg LPG bottle, actually has a window in it, but I probably won't put one in the small tank because I can still put around 40-50 psi in it if I need to. (I wouldn't try that if I had a window in it!).

I've re-welded a fair bit of it, but I still seem to have one or two tiny pin-hole leaks, even through the welding!!!