This is partly to show what can be done, and also a reminder to be cautious when playing around with pressurising things!

I decided to make one out of an out-of-date 4 kg gas bottle. These are tested to 480 psi, so are made from reasonable steel. First job was to remove the valve - I've removed three now, and it doesn't get any easier. Once that was out I filled it with water and let it sit for a couple of days (just in case). I then cut it in half, about an inch above the weld (suggestion: empty the water first!!). I forged two circular rings out of 1/2" steel bar to fit around each half at the rim and welded then in place, this was followed by a piece of thinner 1" stuff welded to the lower ring to make a rim into which the top half fits. Three clamps were made from 1/2" stock, using 3/8" threaded bolts as the screws. The clamps were pivoted on brackets welded below the lower rim. I made a rubber gasket to seal between the two halves, fitted a valve and pressure gauge and that was it.

First problem was my welding sucked! It leaked like a sieve, so I had to reweld most of the rim (still leaked a little, so I made a wider gasket, which seemed to work). Regarding the welding, the problem seems to be the steel that the tank is made of, it caused a lot of sputtering on the first pass (gasless MIG). I tried increasing the voltage, for better weld penetration, but promptly burned a hole in it.

Anyhow, there were only a couple of small pinhole leaks when I fitted the wider gasket, so I let it build up pressure. Got to around 55 psi and with an almighty CRACK the top half flew off and through the polycarbonate roof overhead! Very spectacular! (Luckily I was outside, under an awning, and not in the shed). Inspecting the damage showed that the welds on two of the clamps had given way. My fault, as I after welding them I ground them down to make them neater - DUMB!! The parts need to have a deeper V at the joins, so there is better weld penetration.

At the moment I've put it on hold while I consider my next step. I may forge the clamps out of one piece of thicker metal and then just weld the screw assembly on to each one, that way there are a minimum number of welds involved. A bit more work, but I have some 1" round stock that could be curved into the right shape. On the other hand, I may just reweld the existing clamps (properly), I only need to get to around 60 psi, so I'm almost there.

In short, a lot of work (but quite enjoyable) but also potentially very dangerous. I consider myself lucky, the results could have been much worse. So BE CAREFUL if playing with pressure equipment.

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

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G'day all !Enjoy your stay !!!

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.

Except where you have modified it

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

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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

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G'day all !Enjoy your stay !!!

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

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G'day all !Enjoy your stay !!!

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

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G'day all !Enjoy your stay !!!

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!!!