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Design on a Coaxial Piston Cannon

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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Unread postAuthor: Gun Freak » Thu Aug 25, 2011 9:36 pm

What are you gonna do about the seam in the barrel? Why not just use copper?
If you use copper, you can put a nipple in the middle of the barrel to attach it to the reducer and you won't need to modify the reducer at all. You could reinforce the seat with a thicker piece of pipe around the copper to make it easier on the sealing face.
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Unread postAuthor: Atattack » Thu Aug 25, 2011 10:02 pm

Reason for using galvanized steel is I was able to get a good deal on it compared to copper. What do you mean by seam in barrel gun freak?
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Unread postAuthor: Mr.Tallahassee » Thu Aug 25, 2011 10:06 pm

Welded pipe has a small ridge on the inside of the pipe that creates an issue with sealing. You may have to have machine work done to the galvanized pipe as well to remove that ridge.
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Unread postAuthor: Atattack » Thu Aug 25, 2011 10:14 pm

Mr. Tallahassee, I am new to this so sorry for not knowing what you mean. The only two seals on the chamber will be an end cap that is threaded on with teflon tape, and a bell reducer that is threaded on same way and has a pipe that will be welded into 2" hole of the reducer. Where is the ridge you are talking about.
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Unread postAuthor: Lockednloaded » Thu Aug 25, 2011 10:16 pm

The seam is a ridge that goes down the length of the pipe where it was welded. If your pipe has this seam it will make for a bad barrel, and a bad piston housing
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Unread postAuthor: Atattack » Thu Aug 25, 2011 10:18 pm

Oh, this is a brand new piece of galvanized pipe so I'm pretty sure it is not welded.
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Unread postAuthor: Mr.Tallahassee » Thu Aug 25, 2011 10:19 pm

It's on the inside surface of the pipe. The piston can't seal if a ridge is in the pipe. It may seal the barrel but the pilot and the chamber will be able to flow too easily if the piston even fits in the pipe. It looks kinda like this inside the pipe. Image
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Unread postAuthor: Atattack » Thu Aug 25, 2011 10:21 pm

I gotcha, I don't have my pipe with me since it is at machine shop so they can make piston to fit in it tightly but I'm pretty sure it is not in pipe. If it is I will have them sand it but I'm almost certain it is not welded.

Thanks for the heads up though, that would have put a damper on the project.
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Unread postAuthor: Mr.Tallahassee » Thu Aug 25, 2011 10:24 pm

It is pretty uncommon to find seamless pipe at local stores. Most pipe is rolled and electrically welded unless it was specifically designed for a purpose that requires no seam. Definitely have them sand out the seam.
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Unread postAuthor: Atattack » Thu Aug 25, 2011 10:43 pm

I went to my car and checked out the 2" that I have in my car and you are right. After closer inspection I can see the electrical weld on the outside. The ridge on this pipe on the inside is very small though which is good. I guess I just need to sand down on the 6" or so the piston will sit and travel. Any other precautions to take? I'm still assuming I am still ok to take this gun up to 450 psi correct?
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Unread postAuthor: Mr.Tallahassee » Thu Aug 25, 2011 11:00 pm

Atattack wrote:Any other precautions to take?
Just use a good bumper for a UHMW piston. You may want to upgrade to better materials though.
Atattack wrote: I'm still assuming I am still ok to take this gun up to 450 psi correct?
Easily. The bursting pressure is ridiculously high. It's upwards of 2000PSI or higher for larger diameter pipe. You can go to 600PSI with a HUGE safety margin even with 4'' steel pipe.
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Unread postAuthor: Atattack » Thu Aug 25, 2011 11:03 pm

What materials would you upgrade?
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Unread postAuthor: Mr.Tallahassee » Thu Aug 25, 2011 11:15 pm

jhalek90 wrote:Just a warning...200 psi on a 3.9" piston and it might shatter.
Consider a aluminum piston, and a damn good bumper.
I'd say jhalek was right. Aluminum is a great idea. As for a bumper, there really isn't much that will hold up to that much force while slowing and stopping the piston. Air hose sections are common bumpers. Maybe that will work if you put in a couple segments instead of just one.
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Unread postAuthor: jackssmirkingrevenge » Fri Aug 26, 2011 12:05 am

Atattack wrote:Thanks for the rule on d/4.


Some thoughts on the matter.

I would consider Delrin as a piston material, it's lighter than aluminium, self lubricating (like a woman :D) and less likely to deform permanently. Incorporating an air bumper as POLAND_SPUD suggested in a previous thread would reduce the impact on the piston and prolong its life.
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Unread postAuthor: Gippeto » Fri Aug 26, 2011 1:03 am

As a suggestion, I would make the piston hollow (soup can), and put the seal on the inside...think Jack may have suggested it recently.

Reduces the weight of the piston and makes life easier on the bumper. If you can maximize the contact area of the piston/bumper 1/4" 90 duro neoprene sheet was working well for me...even in the co-ax... 3oz piston 1.5"dia with 1500psi.

Reducing "unnecessary" piston travel will also reduce it's impact velocity...making things more durable.

Air bumper is an interesting idea...piston bounce might be an issue with that strategy...don't know for sure.
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