Building a new cannon

[maggotman]

im not sure if you could see in the picture the blowgun is a pilot for a 1/2 inch
sprinkler type valve with then pilots the main piston

[ALIHISGREAT]

the brass should be good for 500psi.

[maggotman]

fired it today at 100psi it put a 400g cold roled 25mm steel bar thro 18mm plywood and threw some 45mm worktop but got stuck in the work top
do you think i should be using lighter projectiles or heavier i need to make a new piston ass my arodie one squashed and jammed up after the first fire anyone got any idears as what to use as a piston it has to be cheap and about 51mm

[ALIHISGREAT]

i would fire lighter stuff to not put strain on the cannon... and for a piston, solid plastic rod can be obtained... at 50mm and 55mm from directplastics.co.uk if you have access to a lathe, it would be a good option... or the 50mm might do.

[maggotman]

i colud probaly make somthing in colage on the lathe thay have white plastic and it is low friction would that hold up i think it might be Teflon

[D_Hall]

i have now got a fridge compressor so can it up to 600psi

And yet you've only hydro tested to 350.... Stuuuupid. Sorry, but that really is the only word to describe what you've implying here.

providing the welds are stronger than the material

That's one hell of an assumption. It MAY be true. It may not be. Further, it may be that welding damaged the parent material which means that the tank that claims to be good to pressure XXX may only be good for half that since you welded on it (in which case your assumption about weld strength is true but your assumption about the tank is not).

Do NOT assume.

Hydrostatically test it again before you try going for higher pressures.

AND.... Hydrostatically test it annually.




Aside: No offense, but from the looks of it, your welding is sub-standard for use on pressure vessels. I'm not saying that to insult you; I'm sure it's better than I could do (I'm a crappy welder by any standard!). I'm saying that to drive home the point that you really do need to hydrostat your gun again before you even think of bumping the pressures up.

[maggotman]

ime onleay using it about 100psi for now if i want to use more pressure than that i will test it for more pressure i also have left in the safty relice valve in the tank top of the tank

[POLAND_SPUD]

I completely agree with D_hall... you are taking to much risk... to use the gun at 600 psi you should have it hydro tested up to twice this pressure - so up to 1200 psi

first of all, since you don't have anything that can pump up to 1200 psi (I suppose that some fridge compressors might be able to reach that - normally no one would try it as it can be considered as pushing the limits a little bit to far) I suggest you to test it up to 500-600 psi and then normally use only 250-300 psi...
ohhh I wouldn't trust that fittings at 600 psi too (eventhough they look great - where did you get them?

secondly, there is a 90 degree angle between the barrel and the tank... it's not always a bad thing but since:
+you have already fired something rather heavy with it
+your cannon looks like it must have a lot of power
+you plan to use high pressures and it has been already pointed out that some of the parts migh not be strong enough
+you welded the tank yourself...
...I suggest you to follow ali's advice and stick to firing lighter projectiles... there are just too many things that make this cannon prone to failing due to recoil, stress and pressure...

you know what?? I have had stuff that failed at high pressures and I'll tell you I didn't like it.... but it thought me not to f*** with high pressures...

I've been taking some basic safety precautions - yet things have failed. Mostly becasue I missed something, I turned off the compressor a little too late, I didn't realize that the pressure hose got hot or something like that

However, what you do seems like ignoring the danger



LOL sorry for being harsh I think you should be made aware of that (even if you are already aware of that think about it one more time)


overall, I like you gun it must be very powerful it seems you have done your research well as you knew how to build a piston valve and you even included an additional valve to help with piloting the main one...
sure it isn't the pretties gun but I am not good at it neither

[maggotman]

fair comment i doubt i will ever fire it at 600 if i do i will not be standing next to it i can tell you that

eventhough they look great - where did you get them?

my friend borrowed them from were he works for me if you no what i mean

im thinking of putting the barrel parallel to the tank but thro by cuting a pice of 35mm pipe then a 90 bend and firmly connecting it to the tank it it produces altot of kickback

[maggotman]

some damage pics
at 80 to 100psi
i wish i could use it to its full potential but its just to loud

firing a 7/8 socket its about 31mm wide no wadding and about 125 grams









when i fired it it wenth thro both itemd and punched a hole thro the backstop ans was stopped buy a car battery in one of the boxes behind the wood

vid of the shot no sound sorry

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