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New wadding design and piston update - Also .wav file

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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New wadding design and piston update - Also .wav file

Unread postAuthor: boyntonstu » Mon Oct 19, 2009 5:21 pm

Foam wadding hot glued top marble.

Quite a tight fit.

Image

Rear of Oak piston showing counter bore

Image

Latest piston configuration using 8-32 screw to hold washer to Oak.

Image

Attached is the sound file of it being shot.

Almost through the plywood sheet, but not quite.

How much energy does it take to penetrate a sheet of 5/8" plywood with a glass marble?

Can you please help analyze it?

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foam wadding 10-19-09.rar
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Unread postAuthor: jor2daje » Mon Oct 19, 2009 5:43 pm

Could be a good idea but, why put wadding on the front, and why glue it on? With wadding on the front you have much greater air resistance, and if the wadding doesnt seperate it also adds lots of air resistance. Your piston looks pretty nice, but are those one way floating o-rings?
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Unread postAuthor: boyntonstu » Mon Oct 19, 2009 5:57 pm

jor2daje wrote:Could be a good idea but, why put wadding on the front, and why glue it on? With wadding on the front you have much greater air resistance, and if the wadding doesnt seperate it also adds lots of air resistance. Your piston looks pretty nice, but are those one way floating o-rings?


The wadding makes a good air seal.

The front wad prevents fall out.

However a cotton wadding may be better.

I used foam circles because I got tired of wrapping marbles with random pieces of cloth.

The "O" ring grooves have square edges.

I am not sure about floating.

I believe that they sit a little too low.

Thanks,

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Wave analysis

Unread postAuthor: Technician1002 » Tue Oct 20, 2009 12:38 pm

Sorry for the time to get back to you on the waveform. I listened to it and didn't hear separate events for the shot and an impact. In Audacity I couldn't find the two events either.

The hiss before the bang is overly long indicating a piston with too much blow by, friction, or small pilot valve. It hisses for almost 130ms.

The bang is unusual because the normal pressure wave associated with a shot is late in the waveform. See the photo. This may be due to a blast of air passing the compressible foam and beating the marble out the barrel with lots of hiss and then the main pressure wave exiting when the marble exits. This is only a guess. Anybody else have any takes on this unusual shot?

There is another interesting spot in this waveform that does tell of a piston problem. In the hiss portion, the hiss is fine up to a point and then the valve tries to open but is obviously stuck. The valve flutters for a while before it comes loose and fires. See the second photo to see this hum buzz or whatever before the bang.
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Attachments
Screenshot-foam wadding 10-19-09.jpg
Foam wadding analysis
Screenshot-foam wadding 10-19-09-1.jpg
Hiss, hum, bang.
Last edited by Technician1002 on Tue Oct 20, 2009 12:48 pm, edited 1 time in total.
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Unread postAuthor: Hubb » Tue Oct 20, 2009 12:44 pm

If the marble is glued to the foam, and the foam will make a good seal and not move in the barrel, then the front wadding is not needed. All it will do, as jor2daje said, is add air resistance.
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Unread postAuthor: Technician1002 » Tue Oct 20, 2009 12:54 pm

Hubb wrote:If the marble is glued to the foam, and the foam will make a good seal and not move in the barrel, then the front wadding is not needed. All it will do, as jor2daje said, is add air resistance.


The point I am making is closed cell foam is flexible. As the foam is placed in a location under pressure, it compresses the gas in the foam shrinking it. At 10 bar, air and possibly the foam will shrink to about 1/10 the original volume. A cotton wad does not do this. Closed cell foam does as it reacts to the pressure.
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Re: Wave analysis

Unread postAuthor: boyntonstu » Tue Oct 20, 2009 2:15 pm

Technician1002 wrote:Sorry for the time to get back to you on the waveform. I listened to it and didn't hear separate events for the shot and an impact. In Audacity I couldn't find the two events either.

The hiss before the bang is overly long indicating a piston with too much blow by, friction, or small pilot valve. It hisses for almost 130ms.

The bang is unusual because the normal pressure wave associated with a shot is late in the waveform. See the photo. This may be due to a blast of air passing the compressible foam and beating the marble out the barrel with lots of hiss and then the main pressure wave exiting when the marble exits. This is only a guess. Anybody else have any takes on this unusual shot?

There is another interesting spot in this waveform that does tell of a piston problem. In the hiss portion, the hiss is fine up to a point and then the valve tries to open but is obviously stuck. The valve flutters for a while before it comes loose and fires. See the second photo to see this hum buzz or whatever before the bang.


Thank you very much for your analysis.

I really appreciate your effort and what a .wav sound can tell someone as educated as you.

I couldn't see the second bang even though I saw and heard the marble hitting the plywood.

I am having a hard time sealing a 1/2" black flat faucet washer to the barrel.

Sometimes it seals 99% and other times about 80-90% to where I can see bubbles if I dip the muzzle into a bucket of water.

I am uncertain if it is a seal leak or a leak past the 8-32 machine threads.

I need some help here.

This is my slimmed down cannon after relocating the pressure gauge to the a/c pump.

The valve trigger is pulled towards the quick connect and it seems faster this way.

The marbles are slightly hot glued on one side to a closed cell foam wadd.

Image

Here is an attempt at a dart nail.

It does not fly true and it appears to tumble.

Image

Suggestions always welcome.


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Last edited by boyntonstu on Tue Oct 20, 2009 2:21 pm, edited 1 time in total.
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Re: Wave analysis

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Unread postAuthor: Hubb » Tue Oct 20, 2009 2:19 pm

Tie a piece of yarn to the head of the nail before inserting the nail into the foam.
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Unread postAuthor: boyntonstu » Tue Oct 20, 2009 2:24 pm

Hubb wrote:Tie a piece of yarn to the head of the nail before inserting the nail into the foam.


Thanks.

Will do.

Wool yarn or string?

About what length?
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Unread postAuthor: POLAND_SPUD » Tue Oct 20, 2009 2:32 pm

why do you use wadding?
if I were you I would find ammo that fits the barrel instead... that would give you more time for shooting stuff :D

also notice that marbles are really poor projectiles... 16mm (5 gram) marbles have only 50% of their muzzle energy at about 35th meter... you use smaller marbles so I suspect that they perform even worse... there is no point in saboting them really
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Unread postAuthor: boyntonstu » Tue Oct 20, 2009 2:38 pm

POLAND_SPUD wrote:why do you use wadding?
if I were you I would find ammo that fits the barrel instead... that would give you more time for shooting stuff :D

also notice that marbles are really poor projectiles... 16mm (5 gram) marbles have only 50% of their muzzle energy at about 35th meter... you use smaller marbles so I suspect that they perform even worse... there is no point in saboting them really



I would rather shoot common nails as my main projectile.

I bent a 6" long 5/16" carriage bolt when it tumbled.

For cannon testing at 20 feet, marbles are cheap and they almost penetrate 5/8" plywood.

I can reclaim both the foam and the marbles for another shot.

It takes only a second with hot glue to wad a marble with a foam disk.
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Unread postAuthor: Hubb » Tue Oct 20, 2009 2:50 pm

boyntonstu wrote:
Hubb wrote:Tie a piece of yarn to the head of the nail before inserting the nail into the foam.


Thanks.

Will do.

Wool yarn or string?

About what length?


I used to make them (with great success) out of yarn. I would make the length twice as long as the nail and tie it in the center, under the head. This would make two tails that were about the same length as the projectile. You could also just tie one length to the nail - which would create less drag, but not stabilize the dart as quickly.

Another option you could consider (if you want to take the time and make a purdy one) is to use fly fishing techniques. I've done this too and, although it seems to stabilize the dart much faster and have a slightly higher velocity, it takes a while to make a dart, and that's after you learn the technique of tying one.
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Harder wads

Unread postAuthor: nakarti » Tue Oct 20, 2009 3:01 pm

A foam wad is ok when it's thick(~2"), fails/leaks when it's too thin. That may be your extra pressure bang.
DO NOT do front wadding. That will really screw up your accuracy, nevermind the extra drag.
Hot glue doesn't hold well, but that's good if you use absolute minimum: the smushing of the wad plus the sudden drag will pull it off the marble, making the wad easier to recover(but heavier next time because the glue probably stays with it.)

Try a harder wad, find a cap to something(chapstick? Soda bottle?) that's a good fit, open side toward the barrel opening, and you have a fairly light, self-parachuting wad that isn't subject to the compression issues of foam.

I see you've got a rubber in valve, how well does that handle high (>200) pressure? I was worried about that on my design so grabbed a brass stem valve, but it was difficult to seal, not being designed for a thin wall, but it would be solderable to copper.
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Unread postAuthor: jor2daje » Tue Oct 20, 2009 4:11 pm

I think the best darts you can make are the ones from nails and milk jugs, I made an instructble on them here but I think I may have seen the original idea on this site.

As for your piston valve, how smooth and square is the surface your piston seals against?
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Unread postAuthor: boyntonstu » Tue Oct 20, 2009 5:42 pm

jor2daje wrote:I think the best darts you can make are the ones from nails and milk jugs, I made an instructble on them here but I think I may have seen the original idea on this site.

As for your piston valve, how smooth and square is the surface your piston seals against?


This is the sealing surface:
Image

It is a 3/4 > 1/2 Copper Fitting reducer with a 1/4" length of 1/2" tubing soldered to increase the surface area. It was smoothly sanded down to make a flat surface.

The black faucet washer and the face has held 500 psi for about 15 minutes.

No bubbles appeared when muzzle was placed into shallow water.

The problem is that I cannot depend on it.

I thought that it was O.K. and I removed the pressure gauge.

I won't give up.

Suggestions?

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