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My first spud gun (and problems)

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 first spud gun (and problems)

Unread postAuthor: Wave Rider » Mon May 12, 2008 5:55 pm

I just discovered this place a couple of days ago, and I am amazed at the amount of new technology that has become available though this forum, especially the wealth of information about piston valves.

Naturally I set out right away to construct a small prototype coaxial launcher. Failing to find any pressure rated PVC in the shops around here I decided to go with what I had in my junk box, so I made this fairly run of the mill coax using a cast epoxy piston with a piece of bike tire for the seal. The chamber is from a desoldering sucker, the copper pipe is 1cm (3/8") inner diameter and the "end caps / bushings" are made from 5mm acrylic I cut to shape and glued in place with a generous amount of epoxy. I don't expect this to hold a lot of pressure, but it's just a prototype after all.

Trouble is: Even though nothing is leaking, and the piston definitely gives a good seal, nothing happens when I pump up the chamber and then trigger the valve. The pilot volume empties with a small whoosh of air, but nothing comes out the front. I am wondering if either the main chamber is not filled up at all (piston forming too tight a seal), or if all the air from the chamber exits through the pilot valve (piston too loose).
The fit seems about right, the piston moves easily and if I blow hard with my mouth I can get a little air past it. I guess I could just mold a few more pistons, since it's easy enough, but maybe I am just overlooking something very simple since I am new to this?

Any help would be appreciated!
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spud1.jpg
Assembled and glued up
spud1.jpg (28.25 KiB) Viewed 392 times
spud2.jpg
Schrader valved end cap, coaxial assembly and greased up piston
spud2.jpg (27.3 KiB) Viewed 392 times

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Unread postAuthor: bigbob12345 » Mon May 12, 2008 6:27 pm

Your problem is that your pilot is not suffecient, i would reccomend a presta valve rater than a schrader.
Or you could deacrease your pilot volume.
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Unread postAuthor: starman » Mon May 12, 2008 6:32 pm

Your piston may not be fitting close enough either...needs to be just close enough without catching the cylinder walls. In looking at your piston cylinder, the hole does not look round, rather a little out-of-round. That could be enough to let the air just slip on by without engaging the piston much or at all.
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Unread postAuthor: Davidvaini » Mon May 12, 2008 6:45 pm

Edit: Suggestion already noted..
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Unread postAuthor: Pyro Ninja » Tue May 13, 2008 2:30 am

First of all nice build. I'm glad you did the research. :)

1-As you noted, a likely problem could be that there is not a sufficient seal around the piston.

2- (Already said) may need a more efficient pilot valve

3- Id say opt for a smaller barrel, this would give you more chamber volume, which usually equals more power
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This pretty much had no power, little puff of air out the barrel.
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Unread postAuthor: psycix » Tue May 13, 2008 4:59 am

Exept for the piston and the pilot valve, you could also try higher pressures.
What pressure are you on now?
More pressure -> faster acting piston.
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Unread postAuthor: MaxuS the 2nd » Tue May 13, 2008 10:22 am

Copper is pressure rated...Which you seem to have.
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