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Clear Plexiglass Piston Problems. FIXED!!! WORKING GUN NOW

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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Clear Plexiglass Piston Problems. FIXED!!! WORKING GUN NOW

Unread postAuthor: demilus » Tue Apr 10, 2007 3:12 pm

Alright, so this is my first piston type gun. I'll admit it, I've been putting off building one (I've been content with my modded sprinkler valves okay 8) ).

The chamber and tee fitting are both 3/4" Type M copper piping. The chamber is 2.5 ft long. The barrel is 4 ft. of 1/2" Type M copper tubing.

The tee fitting is 3/4 brass and was ground down with a dremel tool because the inside was rough.

When I pressurize the gun, the piston hits forward and seals the gun. It is pressurized through the pressure washer handle. Then, when I exhaust the gun, it lets a short "whoosh" and then stops. Absolutely no air out the barrel and the piston doesn't move at all. The piston fits well and I can make it slide easily just by blowing into the barrel or pressure washer handle.

I think that some of the problem may lie in the fact that the threaded copper part on the exhaust end is slightly smaller in diameter than the brass fitting (not much, but the piston fits tighter when I put it in through there).

I have oiled the piston and that hasn't helped. The original piston was the shorter one and didn't have any equalization holes. Then, after it didn't work, I attached a piece of neoprene to the back end in an attempt to make it seal better. That failed.

After that, I made a longer piston and made equalization holes immediately. I made sure that they connected with the chamber.

The pressure washer handle goes to 1/4" close nipple that was threaded directly into the brass cap.

I'm kinda frustrated and am wondering if I should try a new piston.

ANY PISTON DESIGNS WOULD BE NICE FOR MY GUN :D

WTF IS WRONG WITH THIS GUN?? Any ideas would be greatly appreciated. Thanks in advance.
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Attachments
HPIM3077.jpg
The whole cannon.
HPIM3073.jpg
The pressure washer handle. Also known as my exhaust vent.
HPIM3081.jpg
Close up of the valve. This is where all the action happens :). The shorter of the two pistons used to have neoprene on the front, but my epoxy gave way and it popped of. I like super glue better, it holds the neoprene the best.
HPIM3086.jpg
This is a view of the inside of my valve. It is all 3/4" except for a 3/4" to 1/2" bushing in the barrel area. You also get to see my sexy knee 8)
HPIM3088.jpg
This picture shows how tightly my piston sits in the one end of the tee.
Last edited by demilus on Thu Apr 19, 2007 9:07 am, edited 1 time in total.
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Unread postAuthor: Matheusilla » Tue Apr 10, 2007 4:08 pm

Maybe your equalization hole is too big. Everything else seems pretty peachy to me.
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Unread postAuthor: demilus » Tue Apr 10, 2007 4:18 pm

Well when I first tried it, I had no equalization hole. Then it seemed like the chamber wasn't pressurizing, so I put one in.
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Unread postAuthor: rad14701 » Tue Apr 10, 2007 4:34 pm

It's just a thought but my guess would be that the upward pressure onto the piston from the chamber is too great and too far back from the barrel seal to cause an effective rearward movement... The result is that the pressure slowly releases around the piston without ever piloting... The problem most likely arises due to the relative lack of chamber space in front of the piston... I'm not sure if there is an easy fix to this theoretical design flaw...
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Unread postAuthor: mopherman » Tue Apr 10, 2007 4:34 pm

your barrel should be half way into the tee so the piston doesnt need to go so far. unfrortunatly, you soldered them on so you could attempt something like
this
http://www.spudfiles.com/forums/viewtopic.php?t=6214&
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Unread postAuthor: demilus » Tue Apr 10, 2007 4:44 pm

Awww crap.... I completely forgot about that. I'll try to dissassemble the barrel, grind out the bushing, push the barrel in farther, and reassemble it. Thanks guys, I think you hit the problem right on.
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Unread postAuthor: spudthug » Tue Apr 10, 2007 4:44 pm

i can almost guarentee that the pressure washer handle doesnt port it quick enough...put a ball valve on instead of the pressure washer and put the quick connect in the other side of the ball valve...try it then with alot of lube...should work extremely well...btw that plexi glass wont shatter hitting something with like 1000 punds of force will it???


sarcasm not 1000 but pretty dam hard

and ur piston probly needs to be greased and ur barrel has to be halfway into the tee
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Last edited by spudthug on Wed Apr 18, 2007 7:33 pm, edited 1 time in total.
4" piston valved cannon-half done..( i spilt my cement...)

Hybrid- 75% done. need to build propane holder and drill/tap sparkplug hole..
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Unread postAuthor: demilus » Tue Apr 10, 2007 4:46 pm

I checked the plexiglass with a hammer :D . I was worried about that too, but it's actually a lot more sturdy than you'd imagine.
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Unread postAuthor: Hawkeye » Tue Apr 10, 2007 6:34 pm

I can almost guarantee that if you can't exhaust a piston that size with a schrader there is something wrong with your piston.
Definitely move the barrel back in the T and shorten the piston.
You can very easily fit a spring in there as a bumper also.
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Unread postAuthor: mopherman » Tue Apr 10, 2007 6:41 pm

hes using metal fittings, i dont think its going balistic if it lacks a bumper :wink:
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Unread postAuthor: demilus » Tue Apr 10, 2007 7:51 pm

Alright, I finally grinded out the bushing for the barrel and moved it back. Holy sh!t, it's a lot harder tha it sounds. I just finished resoldering the barrel back in. I'm really excited/nervous for the test. I'll post my results back in like 10 minutes. Oh and I'm putting a bumper in. There's been one from the begining, I just forgot to take a picture of it :)
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Unread postAuthor: mopherman » Tue Apr 10, 2007 7:53 pm

good luck! i hope it works
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Unread postAuthor: demilus » Tue Apr 10, 2007 8:02 pm

Dammit! For my piston, I put neoprene on the front of the plexiglass. Super Glue is the best because it melts the neoprene straight onto my piston! I accidentally cut the rubber too small so now it doesn't seat on the barrel! I'm waiting for one right now as I'm typing!!

DRY SUPER GLUE!!! DRY DRY DRY!!!
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Unread postAuthor: mopherman » Tue Apr 10, 2007 8:04 pm

CRAP, when i saw this post i thought i was gonna find out if it worked! the suspence is killing me! good luck, i love the washer handle
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Unread postAuthor: chaos » Tue Apr 10, 2007 8:18 pm

i reckon you problem exists with the shape of the piston, i think you need more "gas" flowing around the piston itself

==_==
==--==
soz the drawing is the best i can do, but anyways i think you either need to change the shape of the piston or put and o-ring at the back of it.

peace
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Last edited by chaos on Tue Apr 10, 2007 8:23 pm, edited 2 times in total.
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