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My Cannon so far...

Post questions and info about pneumatic (compressed gas) powered cannons here. This includes discussion about valves, pipe types, compressors, alternate gas setups, and anything else relevant.
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My Cannon so far...

Unread postAuthor: Floyd » Sat Jul 12, 2008 7:03 pm

Here is my cannon built from MrCrowely's howto with a different chamber shape. I have tried three different pistons, everyone of them holding the pressure, just not activating.

The piston fits snug, but not tight(its easy to move) and I followed everything I could find with the search button (which I have found isn't that great).

Could the pilot volume be too big? The store at the time didn't have any 2" endcaps so I had to use a coupling and a slip plug. The cement hardened too quickly (noob mistake) and I couldn't push it all the way down so i was left with a huge pilot volume....

Please tell if I did anything obviously wrong.

Please don't tell me to use the search button, I've been trying to find the answer to this with that the whole friggin day...
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Attachments
P7120003.JPG
The cannon, MrCrowely's design w/ a changed up chamber.
P7120002.JPG
The piston housing. The piston fits only in the middle of the tee.
P7120007.JPG
The piston w/ rubber inner tube sealing face and foam bumper.
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Unread postAuthor: SpudFarm » Sat Jul 12, 2008 7:12 pm

looks good!
you can make the piston longer to get less pilot volume but then it will be heavyer.

try a accurate ducktape layer around the piston
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Unread postAuthor: starman » Sat Jul 12, 2008 7:13 pm

This is a good effort here. Chances are the piston is letting too much air slip around it...I battled the same problem. You should be able to pull the piston in and out and feel a little air vacuum resisting it.

I ended up using multiple coats of white plastic paint to get the diameter of my piston up to near perfect fit. It will take some patience because you'll want each coat to dry fairly well. I think I remember 10-12 coats for mine.
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Unread postAuthor: jrrdw » Sat Jul 12, 2008 7:23 pm

Is the sealing face of the piston meeting the barrel in the middle of the tee?
Welcome tyo Spudfiles.
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Unread postAuthor: Floyd » Sat Jul 12, 2008 7:25 pm

Thanks for the replies, it is really snug, if I try and blow through the barrel I can push the piston out with my breath easily. I may try a few coats of paint to see if that helps...

fyi I used this 3" piston adapted to fit my tee and w/out o rings.

EDIT: yeah, the barrel is meeting the piston in the middle of the tee... I've tried a lot of different barrel placements...
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Unread postAuthor: MrCrowley » Sat Jul 12, 2008 8:11 pm

For the pilot volume get a knee board made from foam. They are usually sold in a garden centre, they are for kneeling on when gardening so you don't get sore knees. They're quite thick, about an inch.

Cut some circles out of it that will fit in the back of the piston housing, then cut about an inch wide hole in the middle of each disk. Not only will this act as a bumper but it will eat up most of your pilot volume.

Pilot volume would be a factor but with a pilot valve of that size you still should get some results. You can always put an end cap on later since it's bolted.

The only other thing I can think of is that your piston might need some lube, not to help with friction but to help a bit with sealing against the sides of the piston housing. It can create a vacuum, as Starman said, better.
Also that groove in your piston from where you joined the PVC to make the piston body could be letting air go through it.

I'd just get some bondo and put a small strip of it down the groove and then wait for it to dry then sand off the excess.
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Last edited by MrCrowley on Sat Jul 12, 2008 8:18 pm, edited 1 time in total.
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Unread postAuthor: pizlo » Sat Jul 12, 2008 8:18 pm

Damn, I would say it was a combination of what people have said, and your pilot valve. Looks like a .5 inch ball, I think a 1 inch sprinkler would be more adequate.
Yeah, thats it, big pilot volume small pilot valve.
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Unread postAuthor: MrCrowley » Sat Jul 12, 2008 8:22 pm

A 1/2" ball valve should still work fine, maybe not with the pilot volume but you should still get results beyond what you are getting know.
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Unread postAuthor: Floyd » Sat Jul 12, 2008 8:37 pm

Thanks for all your help, I'll do a combination of what you guys said and post results later. I try to be as un-nooby as possible.

Thanks again!
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Unread postAuthor: Floyd » Tue Jul 15, 2008 7:23 pm

Well, I couldn't get it working, it seals fine but it still won't actuate. I lubed it and when i push it down I can feel the air pressure holding it back. I also 1/2" thick 2" diameter plywood discs with holes drilled in the middle of them to reduce the pilot volume.

I think I'm going to give up on this cannon for now as no matter what I do I can't get it.

Would a coax be easier?
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Unread postAuthor: MrCrowley » Tue Jul 15, 2008 7:47 pm

I'm stumped.

Just for the hell of it try and make a piston from a deodorant can. There are some cans that fit perfectly, so if you make a piston from one it will narrow down the problem. I made a how-to on one in both my instructables.
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Unread postAuthor: Floyd » Tue Jul 15, 2008 7:57 pm

Okay, I'll try with that when , if it doesn't work I might try an O-ring design. Thanks for all your help with my cannon and tolerating my noobiness.

How long did you make your deodorant can? It isn't stated in any of your instructions.
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Unread postAuthor: MrCrowley » Tue Jul 15, 2008 8:04 pm

It depends on your setup. Make it as long as possible so it takes up as much pilot volume as possible but still has enough travel to allow a decent flow at the barrel when it opens.
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Unread postAuthor: Cosmic Muffin » Wed Jul 16, 2008 4:18 am

looks pretty kool, but yer, add a sprinkler valve, but before you do anything like that, you should try opening the ball valve really fast. it took me about two weeks to get my gun working, and all i did was just flick the valve open really quick. might work for you too. good luck in getting this beast working mate. :wink:
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