Rabid's Coaxial Tennis Ball Cannon

[RabidDuck]

This is my 2nd Pneumatic cannon. It has a 3 foot long chamber, 4 inches in diamter and the barrel is 2.5 inches, 4.5 feet long. The piston is made out of fiber glass resign and i got the design idea from Jrrdw. He calls it a cup piston. The bumper is a hard cushy building block thing. The sealing face was hard to find since i couldn't find neoprene rubber, but i believe its a sink washer of some sort so i will see if it works or not. I am piloting the cannon with a sprinkler and blow gun valve. The sprinkler is 1 inch. The plug was the hardest part on this cannon and the worst looking but it should work. I don't have a lathe so i used a hack saw and then took a ice pick type of thing and smoothed the groove. I filled it with fiberglass so i wouldn't have to worry about leaks and i could screw the screws all the way in. I am excited to fire it tomorrow. I still need to put screws in the plug. I got a chrony so i will be able to see how fast it goes and yes i know it will need a paint job. Thanks to everyone who helped me with this and answered questions (especially Jrrdw.) Let me know what ya think.









http://img37.imageshack.us/gal.php?g=p1000057q.jpg

[spudtyrrant]

there ya go rabid, all those long hours in chat seemed to have payed off.

can't wait to fire her huh?

I'll let you know right now that back plug is going to leak, i would make a bigger groove and get a bigger o-ring, also its very important that you make the groove completely straight around the plug, other than that its a beautiful gun you got there.

[jrrdw]

Congrats on getting it together will be waiting to hear the rest.....

[jackssmirkingrevenge]

Looks good, some damage pics are in order

[RabidDuck]

would it work to double the grooves width and throw 2 rings in there? It sure seemed like it sealed well when i put it in there. Yeah i don't know what to fire at since I'm in the city and only have a backyard. And also speaking i may not fire many shots since it's so cold but as long as i fill inside and keep it warm and bring it out and shoot then bring it right back in. Also i screwed up somehow on my measurements when putting the barrel into the chamber so that's why i sanded out about half the plug so the piston had more travel room. I agree if i were to redo a part it would be the plug.

[jrrdw]

Just leave the 1 o-ring in and test it and see if it leaks and if so, at what pressure. I would not put 2 o-rings in 1 groove even if it leaks, make 2 grooves 1/4" apart.

[jackssmirkingrevenge]

Yeah I don't know what to fire at since I'm in the city and only have a backyard.

I feel your pain, this is why I don't really make big bore launchers.

[RabidDuck]

yeah i have a tennis ball and golf ball cannon now. No small bore but i'm planning on making one for my next project. So I'm about to put in the screws and then pump it up and shoot! I will try to get a video and some pics of something destroyed. I will see what i have around the house.

EDIT: when carrying everything upstairs i dropped the piston. I am going to make it more neat this time by molding the cup. Firing won't happen till tomorrow now.

[RabidDuck]

got it all together then to find that the piston isn't sealing. Have tried a few different things but it's still not sealing. I currently have my original sealing face, which is kinda squishy, then a hard plastic sheet, and then a flat piece of rubber. Tried the lip stick test and seals about 1/2 or 3/4 of the barrel. Not sure what else to try

[jrrdw]

Need to get the barrel end squared up and flat so the seal makes full contact around the entire sealing face.

Need to make a tool that will slide into the barrel that has a type of sanding disk then a handle to spin it with.

A stick with a washer in the middle for a quick description.

The part that slips into the barrel should fit with no side to side play and still spin freely inside the barrel. That will keep the cutter/sanding disk true/squared to the sealing face. Make for a nice sealing face. That's off the top of my head...

[RabidDuck]

the barrel was pre cut so i imagine its square and the problem is that the piston isn't flat. I may try putting some weatherstripping foam type of stuff under the sealing face to smooth it all out. If you think it really is the barrel i will go ahead and sand some of it down

[jrrdw]

If the end of the barrel is still the factory cut, yea it should be ok. I thought you cut the pipe yourself.

Try everything and anything at this point, maybe even place your piston on a level surface and pure just enough fiberglass in to cover the bottom of the cup, it should settel level, giving you a level flat bottom.

[RabidDuck]

Ok, and you don't think the problem is that there's not enough pressure on the piston so it's not sealing fully? Just thinking of what else the problem may be.

[jrrdw]

The piston should seal with low pressure. My piston seals before my sprinkler valve does most of the time. The lip stick test proved unevenness.

Re-pure the bottom of the piston cup, make very sure it's level so when it settles all you will have to do is put the seal in and retest. I think it's the most logical thing to try at this point.

[RabidDuck]

Centered barrel and tried filling but still no luck. May pour small layer in bottom of piston since i don't know what else it could be. Do you think i should try getting a new type of rubber like neoprene?