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Thread: Need advice and guidance

  1. #1
    Associate Engineer
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    Jun 2013
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    Need advice and guidance

    Hi everyone, i'm new here and I am looking for some advice and guidance on a project. My grandfather and I built a potato cannon awhile back, and me being the builder and fabricator I want to improve it... And make it bigger. So, what I want to know is can I mount it to a cannon mount similar to revolutionary war cannons? what I would like to do is install gears and 2 cranks to adjust windage and elevation (left-right l up-down). back to my original point, how do I set up the gears to do so and where might I get gears for this task? im concerned I have to custom order them rather than cannibalize something. Secondly, how would I make an accurate aiming system for this? Third (and last, sorry this is so long), if I take and use 1/4in thick pipe 2in and 4in to make a new one out of steel, (the connectors will also be 1/4in hopefully I have to fins some first, if you know where please say so), will it withstand the pressures of approximately 10 seconds of flammable gas? (would propane be suitable? acetylene is spendy). im not sure on the PSI but I should put a gauage on it if I do so, who makes one and how should install it to prevent it from going boom itself? thanks in advance! hopefully I will one day be able to say I am an engineer as well, lifelong goal to be an explosives engineer!

  2. #2
    Technical Fellow
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    Feb 2011
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    Hmmm, welcome to the forum, but I suspect messing with this stuff with limited technical knowledge in Strength of Materials, you may well soon be an exploded Engineer. Making things that use explosive propellants "bigger" is working towards bigger material failures.

    Only an idiot would take advice from faceless but well meaning folk on the Internet to build such a device without Professional, well qualified and experienced Engineering help. You and your grandpa have been lucky so far. The cost of gears and propellant will be a minor issue compared to the Hospital costs.

    Try this on Google...
    potato cannon explosions

    If that serves you no good, then try this...
    homemade cannon explosions

  3. #3
    Associate Engineer
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    Jun 2013
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    Pinkerton, it's not that I don't know, but the expansion of gases that I'm unsure about. I come from a family of machinists welders gear heads etc and I know my choice of materials very well. Secondly, I'm not that stupid, I'm more concerned with the gears, aiming system and propellant. We use a remote detonation system for any of my experiments, usually down in a four foot hole about a hundred foot from us. Plus our bunker.
    The increase in energy should push the potato out but a crack or failure will destroy the system, I do know some chemistry and physics, only limited by what the high school offers for science courses :/

    this is a physics experiment revolving around 2 variables, one is propellant, the other is trajectory. All to find out how far the potato will go.

  4. #4
    Lead Engineer RWOLFEJR's Avatar
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    Ah yes... The good old spud guns. I still have mine in the garage...!! Need a new igniter for it though. It got lost at camp several years back. They are a lot of fun but you do know you're flirting with disaster right? Definitely need to check out the examples of failure Dave mentioned. If I remember right there was an example where a guy actually blasted himself in the face with a frog...

    A little story about our camp spud gun fun that came close to disaster... and probably how my igniter came to be missing.

    First you drink most of the day while pigging out and playing horseshoes etc.. Then one genius pulls out a bag-o-spuds and a can of hair spray and let the fun begin. Outhouse door at a long ago abandoned camp next door is the target. The thing is about fifty yards away so not a very easy shot. One of the least drunk (the sensible one) in the group isn't playing so another ribs him a bit. Next thing you know there's a small wager offered to the least drunk guy betting he can't hit the door with 2 to 1 odds on the bet. The least drunk accepts the offer and tells the other guy to load it up. The bet was to hit the outhouse door...

    So least drunk guy is loaded and ready to fire aiming at the outhouse when suddenly he twirls around about 45 degrees left and blows a hole through the crapper door of our own outhosue that's just about 15 feet away...!!! An outhouse door... The very same crapper door that another fellow and myself had just replaced about an hour or two before. Well we're all a little stunned by this sly move and then it happened. The crapper door opened slightly and Pete yells out... "Don't shoot...!!!" We all were pretty close to needing to use the seat on that note. Out comes Pete brushing splinters and potato bits off his head and shirt. The hole in the door was head level. Lucky for him he was bent down to pull up trow right when the door explosion took place. Had the hit happened a second or so sooner he'd of got blasted in the face.

    Could tell another one about the day the meat cannon exploded putting holes in cars and people narrowly escaping severe injury or death... but you get the gist... (What can I say... Grow up in the country and you gotta make your own fun?)

    I'll toss a weeee little bit out there that might help keep you from blowing yourself apart... or a "Pete" in your world. Don't use your spud gun when you have a rented double barrel slushie machine cranking out high octane frozen beverages all day. Never load your gun without making sure your charge chamber is open and COMPLETELY empty. Put an O.D. chamfer ONLY on your barrel end. A heavy I.D. chamfer will make your spud fit extremely tight and has been known to cause equipment failure. A heavy O.D. chamfer makes it easier to stuff the spud in. Always load before you charge your chamber.

    Far as your charge and the ability for the materials you select to hold up to the explosion goes... You're on your own there. I'll just suggest that when in doubt... make it stout... I mean you're probably gonna build this regardless of anyone's contrary advice so at least try to overkill it?

    Good luck and try not to kill anyone...
    Bob

  5. #5
    Associate Engineer
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    Ok, well.... Any idea where I can get the gears? An old transmission, custom make them? A start on setting them up? How about an aiming system too? Rise up iron sights used on the sharps carbine and spear guns? Or a side mounted glass optic?

    If nothing else I want it to sit there and look decent.

  6. #6
    Technical Fellow jboggs's Avatar
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    The gears on those old cannons were all cast iron. There are places where you can get large gears now but most of them are custom shops. I would recommend a completely different approach - roller chain. You can get it in any size you want from any industrial supply house, like Grainger or McMaster. Set up the small pinion sprocket to be turned by hand crank. And since the large "sprocket" won't be turning more than a few degrees, you don't need teeth on it. Just attach the chain to it with bolts, clamps, or some other method.

  7. #7
    Lead Engineer RWOLFEJR's Avatar
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    You could pick up a flexplate or flywheel from a junkyard cheap and a starter drive gear? Cheap and offer you some pretty fine tuning capability...
    Or maybe look on Craigslist or the like for an old portable concrete mixer and use the dump gear assembly off of one of those? Or if you're handy a tractor scrap yard the bull gears from the rearend of an old tractor are big impressive gears and one of the spur gears from the differential? Or the flywheel and starter gear from an old Deere if you want to bulk the thing up...

    The sites will depend on the look you're after more than anything else. Anything you put on it will be just to get you in the ballpark anyway. I mean you'll be launching spuds of varying weights and shapes. If it were me... I think I'd go with Sharp's style vernier sites. (Like Tom Selleck's character used in Quigley Down Under movie....) That'd look pretty cool. You can buy them from vintage firearm shops. Montana vintage arms was the first I found with a quick search...

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