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PVC Tee QDV build

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: aEx155 » Sat Mar 05, 2011 1:37 am

Okay everyone: the main valve body is finished. I glued it this afternoon and it is drying/curing. I went with two .5" b 1" slots separated by 1/4" of PVC. This is enough to keep the o-ring in but still have large airflow.

I am still having issues with the o-rings. I tried cutting up some large o-rings and gluing them together with super glue. One worked for a while until I tried to replace it: then the glue broke. I will try again with this strategy tomorrow.

Right now I have 1 3/8" OD x 1 3/16 ID x 3/16" wide o-rings. My o-ring slots are approximately 1/4" wide.

I am having problems with the seal: I am guessing the o-rings are too tight (even though they are "floating") and can't move to seal like they should against the outside of the piston. It barely works if I have a tight o-ring in the back and a floating o-ring in the front, then I move the piston backward a tiny bit while pressurizing to seal it . It pressurizes, but there is a slight leak in the front where the floating o-ring is.

Any ideas?
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Unread postAuthor: Crna Legija » Sat Mar 05, 2011 5:52 am

you can try sand the O-ring down its worked for me before, and if it don't work out your only down a couple of bucks
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Unread postAuthor: dewey-1 » Sat Mar 05, 2011 8:29 am

I think the simplest and best solution is to enlarge the ID of the pipe slightly. This should be done any way to "polish" or smooth the surface to reduce friction. Removing about .003" to .004" to pipe ID should be ample.

Wet sand with 400 grit using flap style rotary sander. Tech has mentioned this in one of his methods to smooth the pipe.

1.375"-1.360"= .015" is the o-ring OD to the pipe ID differential.

.015"/2 = .0075" ID enlargement is to much.
Hence the .003" to .004" initial amount. Then check for seal fit and function with lubricant applied.

Another method would be to use a brake cylinder hone with water as a cutting lubricant.

http://www.sears.com/shc/s/p_10153_1260 ... ockType=G1

http://www.sears.com/shc/s/p_10153_1260 ... ckType=G15
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Unread postAuthor: Technician1002 » Sat Mar 05, 2011 11:30 am

I don't recommend using a brake hone. The corners of the stones can get caught in a port and quickly ruin things.

My best results has been using stick on sandpaper on a smaller pipe. The Mouse Musket was initially sanded with 1 inch pipe in the 1.5 inch sleeve. The 2 inch was sanded with 1.5 inch ABS pipe, and the 2.5 inch has had much of the sanding done with 2 inch ABS pipe.

To apply the pressure to the sandpaper, I have used 2 methods. Both work.

A build up of paper towel under the edge of the sand paper to expand out a flap of paper and having the end of the pipe split and stuffing the end of the pipe with paper towels to expand out the 2 halves works well too.

Sandpaper with paper towel under the edge.
Image

I keep the paper towel away from the corners so they don't get caught in the ports.

Ace Hardware in the US carries the stuff.
Image

A 1 inch pipe cap and the 1.5 inch pipe for the valve cylinder were both carefully sanded until the pipe cap fit inside the pipe on the Mouse Musket. Once a tight fit was obtained, they were lapped together.
Image
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Unread postAuthor: aEx155 » Sat Mar 05, 2011 1:21 pm

Is that my only option? Are there any alternatives?
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Unread postAuthor: Gun Freak » Sat Mar 05, 2011 2:17 pm

aEx155 wrote:Is that my only option? Are there any alternatives?

Make a better piston, or get the correct rings, or use a different cylinder.
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Unread postAuthor: aEx155 » Sat Mar 05, 2011 10:05 pm

I finished the cannon today. It works with some minor leaking. I can't inflate it using a bike pump; I can only use my compressor (The o-rings seal at a higher pressure but my bike pump can't keep up with the initial leaking to get it that high)

I changed the way I cut and glued the o-rings I had: I used a box knife instead of scissors and that made a cleaner cut, and I switched to a better super glue. I managed to make two o-rings that just barely fit (something like 1.362" OD) and consequently, they barely work.

I also fashioned a barrel attachment system (to be detailed in the cannon writeup).

I will leave this thread here for anyone who wants to ask questions and link it to the new thread once I make it.

Thanks for the help everyone.
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Finding O rings

Unread postAuthor: Technician1002 » Sun Mar 06, 2011 12:15 am

Due to the difficulty I had finding rings for the ABS cannon to make it a floating ring, all my later builds had finding an o ring/pipe pair before beginning. I started with the 2 inch and followed with the 1 inch and then the 2.5 inch. I hope this helps any future builders of a QDV.
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Unread postAuthor: Gun Freak » Sun Mar 06, 2011 12:21 pm

aEx155 wrote:I will leave this thread here for anyone who wants to ask questions and link it to the new thread once I make it.

You cannot do either of those things :D
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