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PACa Mk.II, French airgun

Built a pneumatic cannon? Then post it here! This section is for completed, finished cannons that you have built. Please include pictures and information.
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Unread postAuthor: jackssmirkingrevenge » Fri Sep 24, 2010 10:28 am

killerbanjo wrote:Its my eee pc webcam, i do this crap in the comfert of my own bed :P


Actually,this one makes you look like you've built your own epoxy batcave, especially with the green tint :D
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Unread postAuthor: Ragnarok » Fri Sep 24, 2010 11:36 am

killerbanjo wrote:i do this crap in the comfort of my own bed

Err... personally, I'd use the toilet.
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Does that thing kinda look like a big cat to you?
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Unread postAuthor: Technician1002 » Fri Sep 24, 2010 12:07 pm

Ragnarok wrote:
maverik94 wrote:How would a QDV have a faster opening time?


Not to say that people shouldn't use QDVs as appropriate, but they shouldn't be touted as inherently superior to a piston valve. I'd choose between the two on their genuine, not imagined, merits.


I have two items to cover in this post. In regards to the QDV opening time, If it was simply pulled open the above is completely true. With a sliding rod free from the piston, the rod gains speed, then strikes the piston. The time for the valve to crack open and then get air pressure to take over is greatly reduced. The rod is pushed in, then yanked out. This gives the piston a "running start" on the portion of the curve where the piston is mostly closed and the opening forces are still low. The second part is in regards to the mass the air is acting on. With the piston decoupled from the rod, the moving mass is lower. Force=Mass X Acceleration. Mass and Acceleration are inversely proportional for a given Force. 1/2 the mass, double the acceleration. Due to the acceleration early in the curve, the performance is closer to a burst disk in the beginning of the curve as this portion takes less time.

Next, on to valve cores. Any good auto parts store caries replacement valve cores and the tool to replace them. The core simply unscrews from the stem.
Image

I really like the cannon. It is a very nice build. Piston isolation from the trigger rod is the only improvement I can recommend. Nice build.
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Last edited by Technician1002 on Fri Sep 24, 2010 12:15 pm, edited 2 times in total.
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Unread postAuthor: jackssmirkingrevenge » Fri Sep 24, 2010 12:11 pm

Technician1002 wrote:Next, on to valve cores. Any good auto parts store caries replacement valve cores and the tool to replace them. The core simply unscrews from the stem.


Can be bought on eBay for very little too :)

I personally use a pair of tweezers, works just as well.
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Unread postAuthor: killerbanjo » Fri Sep 24, 2010 2:15 pm

jackssmirkingrevenge wrote:
killerbanjo wrote:Its my eee pc webcam, i do this crap in the comfert of my own bed :P


Actually,this one makes you look like you've built your own epoxy batcave, especially with the green tint :D


Thats from my PCB desk lamp, ill post a pic when im upstairs

Ragnarok wrote:
killerbanjo wrote:i do this crap in the comfort of my own bed

Err... personally, I'd use the toilet.


I lold pretty hard :P
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Unread postAuthor: Ragnarok » Fri Sep 24, 2010 2:23 pm

Technician1002 wrote:This gives the piston a "running start" on the portion of the curve where the piston is mostly closed and the opening forces are still low.

Sure. But even if I slowed things down a thousand fold, that part of the curve would STILL be a blink and you'll miss it event!
And at the same time, low forces on the piston means low forces on the projectile - it moves very little during this "slow part of the curve", and the distance it moves under full acceleration is scarcely changed.

With the piston decoupled from the rod, the moving mass is lower. Force=Mass X Acceleration.

Yes, but assuming a negligible initial velocity* doubling of acceleration only reduces the opening time by 30%. After all, s = ut + at<sup>2</sup>/2.
*In this case, a QDV piston's initial velocity is pretty small and won't much change that relationship

And anyway, the rod is irrelevant - a regular piston doesn't have that either, so saying "it doesn't count towards the accelerated mass" is moot - and as a consequence, you can't say that a QDV piston is lighter. It might be, it might not be.

Due to the acceleration early in the curve, the performance is closer to a burst disk in the beginning of the curve as this portion takes less time.

Yes, the QDV is usually faster to open, but not by a huge amount, and you've missed the more important point. The faster opening adds very little to performance.

Phrasing it as if it's some scientific law: "As a valve's opening time approaches zero, the effect on the muzzle velocity also approaches zero."
In other words, the faster a valve already is, the less improving it will help.

QDVs have other advantages (and disadvantages), and there are places I may well use the design. But the marginally better opening time does not make enough difference to performance for it to be a sensible reason to choose between the two.
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Does that thing kinda look like a big cat to you?
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Unread postAuthor: killerbanjo » Fri Sep 24, 2010 3:00 pm

Do you know what volume your air chamber is (what size is it?)

Also inspired by your gun im now designing my own here (Not to scale)


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Unread postAuthor: Gun Freak » Fri Sep 24, 2010 4:50 pm

At walmart they have the tool and 4 valve cores for like 2 bucks.
I have a couple of those long schraders with the spring on the outside that I could use for a pump. Also I have a core with white instead of black around it and the spring inside is very weak. I got it from a 1/8" threaded schrader from the hardware store.
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Unread postAuthor: jackssmirkingrevenge » Fri Sep 24, 2010 9:51 pm

killerbanjo wrote:(Not to scale)


With so much dead volume in the diagram I hope not!
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Unread postAuthor: G-man » Sat Sep 25, 2010 8:26 am

No, I don't know exactly the volume of my air chamber.

But I can show you:
Image

What is the green thing in your scheme ?
An industrial QEV ?
Don't you want to try a QDV ? :P
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Unread postAuthor: Lockednloaded » Sun Sep 26, 2010 1:38 pm

I have a schrader that I just pulled the core out of, the spring is shielded, so is there anyway to remove the spring from a shielded one?
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I love lamp
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Unread postAuthor: jerome » Fri Oct 01, 2010 3:49 am

the pump system is brilliant, the gun is well made and functional, you should be proud, well done!
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Unread postAuthor: Technician1002 » Mon Dec 20, 2010 12:31 pm

I really like the build quality on this launcher.

If you want to modify the piston and trigger rod so the piston can move without slamming the trigger back, take a look at my video that shows how it is done in my Marshmallow Cannon. The link to the video is in my sig below under the Competition QDV Cannons.

I did this out of necessity on the larger cannons as the piston force exceeds 100 lbs when triggered. I didn't want broken hands.
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Unread postAuthor: Selador » Mon Dec 20, 2010 4:06 pm

In pretty much it's simplest form, what you need is a way to 'trigger the trigger'.

Attach something to the valve rod.

Now, use the trigger you have, to pull back on that something.

But have no connection between the two. So that, once your trigger pulls back on the valve far enough to start it opening, your trigger stays in place, and the valve continues on it's merry way without affecting your trigger.

Here's a crude example, using your own valve pic.
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Attachments
trigger.jpg
Your trigger (blue) pulls back on attachment to valve (red) until valve starts to open.

Then valve opens, and valve and red part go on their merry way, without affecting your trigger (blue).
trigger.jpg (22.48 KiB) Viewed 1931 times
If you can't fix it, you don't own it.

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Unread postAuthor: GUNSANDBURGERS » Wed Mar 16, 2011 11:19 am

How does the pump work? and the gun is great. Im sure it would make a fine plinking rifle. Great work!
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