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New Coax Design and idea, HELP !

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: mark.f » Sat Apr 23, 2011 11:29 am

I see you just glued the barrel too far in front of the rear of the chamber.

Not only does this require your piston to be longer, it requires it to be heavier, which is NOT good for a PVC cannon. Your piston should be only long enough to remain stable in the chamber pipe.

One way to fix this is to cut some 4" disks from plywood, and build them up between the very rear of the chamber and the piston until you have a distance d between the rear of the barrel and the plywood disks, where d equals the length of your piston (as short as possible!), your bumper length, and your desired opening distance (limit this as well to D/2 at most to limit piston travel). Glue these disks together and enlarge the hole through the center to allow air to flow (you can leave it at the standard 1/4" formed by the hole saw or enlarge it slightly for better flow). Finally, I recommend sealing the wood with polyurethane or similar to prevent swelling with moisture from condensation.

Yet ANOTHER tip to reduce pilot volume is to redo your piston track. If your barrel is 2.5" SCH-40, a 4" diameter piston is really quite oversized if you are concerned with efficiency and performance. I would get rid of the rear cap you've fashioned, and construct a new one, except in this one, use a modified 4x3" bushing in the 4" male adapter, and pass a piece of 3" pipe through, so that when tightened onto the cannon, the rear of the pipe comes the same d distance from the rear of the barrel as I mentioned before. Then, use the same plywood disk method as before (with a 3" hole saw) to limit the piston travel as before.

Hopefully you've learned a valuable lesson about planning ahead. Good luck on getting it working!
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Unread postAuthor: Shrike88 » Sat Apr 23, 2011 1:26 pm

@ Warhead, Yea I can tip it but it is seating fine after I tried this trick, GREAT IDEA.

Because of installation imperfections on any Coax Cannon Place a circle of sandpaper and bolt it in front of a rubber neoprene cushon so it wont spin, bolt the whole assembly to the threaded rod and put the end in your Drill Chuck. Insert into your cannon as it would be installed, with the sandpaper touching the barrel instead of the rubber Neoprene and LET HER SPIN ! Sand the crap out of that barrel, then go back over with 220 or finer paper to smooth and that worked like a Charm.

@ Tech. I checked the Rod and it isnt bent. I am hoping that it is short enough to last a while without replacing.

As for the single solid piston, Yea that was the origional Idea, however as my previous post I said because of some poor planning on my part the 3" couple (piston) will fit in the 4" pipe however will not go past the Couplers at the end of the cannon when moving backwards upon firing. Therefore there is 4-5" of dead pilot volume still behind the piston. One top of this there is really no good way to buffer the recoil when that much dead space behind the coupler. Check out my Sweet ass drawing if you dont understand.

So the Drawing shows why I needed to make a smaller Diameter piston still attached to the larger one
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Piston Problem.jpg
The Piston Dilemma, So I made the modification extension
Piston Cap.jpg

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Unread postAuthor: Technician1002 » Sat Apr 23, 2011 8:15 pm

My solution to the problem was to use a female adaptor with a short length of pipe inside it so the piston is removable out the breech. This is used in 3 of my cannons. They are the Mouse Musket, The Dragon, and the incognito golf ball cannon. The end of the barrel is placed about 1/2 the barrel diameter distance from the end of the pipe in the coupler for the piston.
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Unread postAuthor: Shrike88 » Sun Apr 24, 2011 9:38 pm

OK another quick one for the cosmetic Department. Anyone know of an idea if I wanted the finished cannon to have an almost seamless Barrel design to hide all the guts and valves on the back end ? I was thinking of cutting another piece of 4" pipe about 12 inches long to cover all the stuff on the back end. I wanted it to look like a bazooka and for a covering on the back to

1. protect the end from bumping or damage or cracking
2. make it look pretty :)

I dont think gluing 4" pipe to the 4" bushing on the back is the way to go...

and ideas ?
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Rear Cover Idea.jpg
Any Ideas How to do this without making it out of wood ? Or attaching it to the end ?

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Unread postAuthor: Shrike88 » Thu Apr 28, 2011 3:16 pm

Ok guys all done with the piston. I sanded it all down, painted and sealed it after cutting the O-ring grooves. Put on the O-Rings. Greased it up REALLY good with Vasolene, put it all together and BOOM she fires. However after the first shot. I start pumping air back into it and it just goes i am thinking around the piston and out the barrel. The Piston Fits snug pushing and first inserting it into the rear of the chamber however once inside i would say there is 1-2mm of space around the piston. Its tight however not air tight so I dont need an equalization hole.

I am thinking either my compresser is not putting out enough air out through the schrader attachment or there is not good enough of a seal on that piston. i could move the piston forwards and backwards before i put it all together with a shop vac or by sucking and blowing on the end of the tube myself. So I am thinking its just not enough air pressure to reseat that piston. I took it apart. Checked it, put it back together again and it shot just fine. However again the second shot didnt work. I tried tilting it up to try and make that piston drop on the barrel, and also tried filling it inverted hoping gravity would push the piston onto the barrel, but nope didnt work. Any ideas ?

It seems like laying down there is not a good enough seal around the piston to push it onto the barrel. However wouldnt that be fixed by tilting the cannon bottom end up and trying to fill it ? I am just perplexed However when it does shoot it is awesome ! lol
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Attachments
Assembled Piston With Rin gs.jpg
Sanded, Grooves cut for rings, Painted then sealed. Installed O rings then greased up with Vasolene
Rear Assembly with Pilot Plug.jpg
This is the Rear Cap with Wood puck fillings to reduce the pilot area with a 1/2" - 3/4" hold drilled in them. Epoxied to the rear cap
Rear Cap With Bumper Installed.jpg
Rear Bumper Glued Onto Spacers on Endcap
Forward Grip Trigger Idea.jpg
Idea I had to Mount the Blowgun, Just need to make an assembly to mount onto cannon that is sturdy
Assembled Cannon 1.jpg
Ready for Test Fire
Assembled Cannon 2.jpg

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Unread postAuthor: Technician1002 » Thu Apr 28, 2011 6:02 pm

Add a small fill valve in the breech. Use air pressure to seal the sprinkler valve and the main valve. If you only fill in the chamber, the rising pressure can unseat the valve before there is enough pressure in the breech to hold the barrel seal closed.

I don't have a solution to protecting and beautifying the breech. On one of my golfball bazookas I placed a small ball valve inside the breech plug.

I do recommend a much thicker bumper. The little felt washer is a little thin.
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Unread postAuthor: Shrike88 » Thu Apr 28, 2011 9:33 pm

I am not using felt as a bumper, the white bumper is 3/4" rubber and silicone.

What do you mean put a full valve in the breech ? What kind of fill valve ? Can you draw an arrow on one of the pictures to show me where your talking about ? Also I dont really understand how you use air pressure to seal the sprinkler valve can you explain that and how rising air pressures can un seat the piston? I'm thought rising pressure pushes the piston tighter against the barrel
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Unread postAuthor: Technician1002 » Thu Apr 28, 2011 10:38 pm

The bumper is larger than it seemed in the photo.

Adding some way to put air between the piston and sprinkler valve is where you may want to put an initial blast of air to seat the piston.

A sprinkler valve with a rapid rise in pressure in the inlet may cause it to open until the eq port in the sprinkler valve allows it to close again. If you are filling your cannon in the chamber the initial flow past the piston may cause it to open or not seat until there is enough pressure behind the piston to hold it closed. Unfortunately the open piston may prevent the necessary pressure from holding it closed to build the pressure.

Rising pressure does hold the piston against the barrel. Before you have rising pressure, you have flow with low to no pressure. This can unseat the piston. With the piston unseated, it won't build pressure.

The initial pressure to seat the valves so it can be filled has always been an issue with piston valves. This is why many are filled from the pilot so the flow from filling pushes the piston closed and holds it until the chamber is filled.

Even my Mouse Musket suffered this problem. Before I could fill it directly into the chamber, I needed to blow into the pilot to close the piston and generate a small amount of chamber pressure to hold the valve closed. Then I could connect to the chamber fill valve and finish filling it.
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Unread postAuthor: Shrike88 » Fri Apr 29, 2011 8:23 am

Ok I understand, however if you look a couple of posts up of mine of my ready cover idea, I placed the schrader valve on a t just above the pressure gauge in between the sprinkler valve and the main piston, this is the pilot area that your talking about correct?

So maybe the problem is I am not getting thought of that initial "blast" of air to seat that piston wuth a schrader valve.?
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Unread postAuthor: Technician1002 » Fri Apr 29, 2011 9:14 am

That is the correct location. If air in that valve is not seating the piston, the piston has too much blow-by. The problem should not be too little flow from the compressor.

How well does the piston seal in the pilot?
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Unread postAuthor: mark.f » Fri Apr 29, 2011 9:43 am

How long does it take your chamber to fill to the required pressure on the attempts where the piston seals correctly? With the gauge on the pilot, you can judge this by listening, if it is slow, you can hear the hiss of air escaping past the piston and into the chamber.

If you hear no hiss, and your chamber is filling quickly, you have too much blowby.

Just a tip in the meantime while you sort this out, but try temporarily replacing that schrader valve with a male auto coupling and lubing your piston with something not quite as thick and with a tendency to stiffen up like vaseline. Silicone or white lithium grease is pretty cheap and will work much better, as well as helping to seal small leaks around the piston (but not relatively large gaps).

Good luck with getting it working.
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Unread postAuthor: Shrike88 » Fri Apr 29, 2011 10:42 am

I guess the piston does not deal as good as I thought,

Thanks also mark ill try the white lithium grease or something similar. I guess I am getting too much blow by like you say, cause the chamber does fill rather quickly. I am going to try some teflon tape or maybe a later of electrical tape over the o ring grooves to try and make the piston seal diameter a little larger. Hopefully this will work without me having to drill an equalization hole

Thanks again guys
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Unread postAuthor: dewey-1 » Fri Apr 29, 2011 12:07 pm

Shrike88 wrote:The Piston Fits snug pushing and first inserting it into the rear of the chamber however once inside i would say there is 1-2mm of space around the piston.onto the barrel, but nope didnt work. Any ideas ?


Here is a diagram of what a 1mm gap does for different sized pistons and the equivalet hole size for a 1mm gap.

Image

Here it is with a .25mm gap.

Image

This should give you an idea what is causing you problem.
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