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Old 03-31-2008, 01:45 AM   #1
6061
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Default My "not so" homemade watergun

For the last year or so I have been browsing this forum and isoaker to get ideas for my own homemade water gun.

I'm a second year manufacturing major at a large polytechnique institute (BCIT) and for my last term project I decided to build a water gun. I thought that you guys might like to check it out.

Now as a disclaimer I have to say that in no way is this gun suitable for a water war. It is more an excersise in enginnering and manufacturing.

The basic principal is a spring powered piston, but the spring I choose to use is from a high proformance off road motorcycle. The spring is rated at 750 lb/in for a total force of over 3000 lbs. This is spread over a fairly large 3.5" piston making the operating pressure somwhere in the 350-400 psi range.

The entire project is made of aircraft grade aluminum or high pressure brass and will be held together with high strenght steel machine screw.

As it stands right now I've made the main pressure chamber, the piston and a few other peripheral parts.



The image above is a solid rendering of a section view showing the spring (red) ball valve which is pnuematically actuated using the compressed air from behind the spring and the clear vacuformed holding tank at the rear.



This picture is the drilling of the main chamber with a 2.5 inch drill, which was latter bored out to 3.5 inches.

Anyway I'll keep you updated on the progress and hopefully there will be a youtube video eventually.
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Old 03-31-2008, 02:29 AM   #2
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Default Re: My "not so" homemade watergun

Me loves you long time. Very nice design work, can't wait to see it finished! Question though; how is the spring cocked? It seems you'd need an awful lot of force to do so.. Is there a lever to help?
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Old 03-31-2008, 07:25 AM   #3
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Default Re: My "not so" homemade watergun

Wow, I await a finished product as well.

Can you tell us more about the design? What kind of valve are you planning to use? It looks like a ball valve but I don't see any mechanism to open and close it. Also, the "barrel" seems far too long. Usually only an inch would be needed and after that, it's just not very useful at all unless you want to increase the length of the gun somehow or possible improve how laminar the flow is. Are you worried that the flow will be turbulent?

Also, how's the spring going to be cocked? I'm assuming that the handle on the front end of the gun is to a pump. Do you plan to just fill the chamber with water so that the piston just moves back while you pump the gun?

How long is gun supposed to be?

Other that, looks like a great design. I don't think I've seen anyone make one out of aluminum yet either. Again, I can't wait for the finished product.
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Old 03-31-2008, 02:10 PM   #4
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Default Re: My "not so" homemade watergun

Very impressive! As the others have noted, I, too, am looking forward to seeing pics and video of the final creation as well as it in action! Will the spring be somehow pulled back to pressurize the water or will water be pumped into the PC area, compressing the spring? Also, can't quite tell from the diagram, but how large is the ID of the tube from the PC-area to the nozzle? Basically, I'm curious how thick a stream you're planning on producing with this beast!

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Old 03-31-2008, 02:33 PM   #5
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Default Re: My "not so" homemade watergun

Welcome to SSC!

This is a really interesting project! It's about time someone did an aluminum water gun.

There's no obvious way to pull back the spring and even with mechanical advantage systems to pull the spring directly, at 750 lb/in. little can be done. I've toyed with that idea before, but it seems that to get decent pressure and water capacity, the force of the pull will either be too high for most people or much too long. The hydraulic advantage from smaller diameter pumps works best. The thing on the underside looks like a pump. 6061 will verify I'm sure.

Drenchenator, he mentioned an air actuated ball valve.

Post some more pictures and keep us informed on your development. If you want to share some helpful manufacturing methods, please do too. I'd love to make something from aluminum that's more practical, but I know nothing about building with aluminum. I might research it now...
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Old 03-31-2008, 05:20 PM   #6
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Default Re: My "not so" homemade watergun

Ben, the rod at the bottom is the steel machine screw that 6061 mentioned.

Looks like a neat water gun, 6061. My only suggestions are to connect the reservoir and the pressure chamber (to stabilize the reservoir - aluminum can break easily since it isn't very malleable) and to use larger tubing wherever possible - take advantage of the velocity gradient.

On a side note, how is Vancouver? I've heard that the city and the weather there are rather nice.
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Old 03-31-2008, 06:29 PM   #7
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Default Re: My "not so" homemade watergun

I thought he was holding things together with machine screws (i.e. putting the pieces together with them), but you can't tell if what he said was a typo. That part looks more like a pump to me. He'll explain I'm sure.
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Old 03-31-2008, 10:27 PM   #8
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Default Re: My "not so" homemade watergun

Just a random thought here.. but what if you'd use a small hand-cranked mechanical winch to pull back the spring and piston? Perhaps this action would suffice to simultaneously suck water into the cylinder if you'd use a top reservoir. I imagine the charging and trigger would function similarly to a medieval heavy crossbow then... Not sure about practicality, but it would be an interesting design exercise if northing else..
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Old 03-31-2008, 11:23 PM   #9
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Default Re: My "not so" homemade watergun

That would work too, but here it wouldn't be practical. The lever arm of the winch would have to be long at the very least. Unless you use some pulley/gear system on top of that, getting enough force to pull 750 lb/in., the lever arm will be extremely long (at least in this design). Pulleys and gears introduce additional complexity and cost. You'd also need to put a piece in that will allow the winch to rotate without moving the handle or else you'll have a rapidly moving handle that could be dangerous. I don't even know anything about that, but I bet it's on most winches that you can buy.

That idea should work fine on more normal powered water guns, but I didn't like the complexity and potential cost of the system. I don't remember what sort of figures I was calculating, but I think I designed one with about 50 PSI and 600 mL water capacity. I won't say any more about this because it's somewhat related to a recent secret project of mine. I will say that what I've said isn't important to the project at the moment though.

I've thought about these different pressurization methods in depth and it's probably not an improvement on regular pump systems due to the complexity. The way I think about it is that given that all possibilities are similarly efficient, you'd exert the same amount of energy to build up a certain amount of pressure (or volume in a CPS system). So it's really a matter of preference I suppose. Someone needs to try out some of these other possibilities I suppose. I hope someone else will jump in on this, but knowing the past it'll probably end up being me.
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Old 04-01-2008, 01:18 AM   #10
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Default Re: My "not so" homemade watergun

A ratcheting system to make sure the crank only turns in one direction could become complicated, too.

You're right that a crank won't use less energy than a pump. (For the most part.) Since a sliding pump lets you exert a force in a fairly comfortable direction all the time, it may be easier than a crank.
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Old 04-01-2008, 01:49 AM   #11
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Default Re: My "not so" homemade watergun

Your right, the underside of the front is a very small diameter piston pump: about a quarter inch (6mm) in diameter. This gives a huge mechanical advantage. When I designed the project one of the main objectives was to have it fully self contained. The maximum pumping force is about 15 lbs, but that is neglecting the friction in the system, so probably more like 20 lbs, which sounds like alot, but is pretty common in most pellet guns.

The small pumping chamber and the front "barrel" are airsoft bores because they use precision ground brass tubing with a fairly small diamter. I used a long barrel because I like the look, in all honesty this will probably be test fired a few times then hung on the wall. But the long barrel is also there because I wanted to try the Disney engineers laminar flow chamber that uses many parallel straws or tubes. I'm pretty sure that there is thread somewhere on SSC discussing this topic. But I have to have this done by mid may and that includes wrrting the 100 page paper that goes along with it, so I'm going to wait and see what the intial test firing results are like.

I choose aliminum because of the weight factor, but also because it is very strong. All of the pressurized components are built to a safety factor of thirty to ensure safety. Aluminum is actually very malleable, but it tends to work harden very quickly. Most of my parts are cut out of billet so I'm not overly conserned about hardening.

So here's a quick facts sheet that should answer a few more questions.
  • lenght: 56 inches
  • Weight: Solidworks calculated 40lbs
  • Max Pressure: 600 psi (ball valve limiting factor)
  • Trigger : Autococker paintball trigger linked to 3/2way pnuematic valve
  • Screws : 58 1/4" 20 UNC Shoulder head cap screws;
  • Piston Orings : Crown Seals (5000psi rated)

The water is delivered through 45 degree flare fittings and copper tubing and the pumping system uses two 2000psi rated ball flow control valves.

Here's a couple more pictures.


This a picture of the CNC mill cutting out the two struts that will hold the rear clear water tank.



These are the two clamps that will hold the main chamber to the aluminum backbone that runs the lenght of the watergun.
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Old 04-01-2008, 08:40 AM   #12
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Default Re: My "not so" homemade watergun

Yeah, testing should be necessary to determine if that long tube will help performance. At normal water gun pressures, I'd say a long tube like that wouldn't help performance and might lower it. Here the turbulence might be sufficiently high to make that a useful addition.

The straws are one pretty basic turbulence reducing method. There's a few others I discussed in these two pages:

http://www.sscentral.org/physics/nozzle-patents.html
http://www.sscentral.org/physics/nozzles.html

The first link I need to update after discussion with the inventor of the first thing mentioned in there, but it's a good listing nonetheless. The first thing in there uses the straws in addition to some other unseen technologies. The inventor himself told me that the most important new feature is the "strings" in the nozzle that help make the flow more laminar.

Edit: You wouldn't mind posting your 100 page paper here when you're done would you?
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Old 04-01-2008, 10:13 AM   #13
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Default Re: My "not so" homemade watergun

I can see the components of the pump - thanks for pointing that out. 15-20 pounds is the right range for pump force for most people, at least if you want to pump for a good period of time.

From what I've read, straws are used to simulate the lamination of a small bore in a large barrel by using many tubes of small diameter. Since your barrel is already rather narrow, how wide will the straws be?

When referring to aluminum's malleability, I was talking about at room temperature. Aluminum may break where steel would bend. I'm only bringing it up because there's a remote possibility that the reservoir could snap off should the gun be dropped (especially when filled and heavy).
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Old 04-01-2008, 10:34 AM   #14
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Default Re: My "not so" homemade watergun

40lbs all-up weight? Wow.. I guess that rules out running around with it all day, lol! If it's got a good range, it could be interesting to mount it on a tripod and use it as a sniper rifle, or a base defense weapon..

But I suppose you're not going to use it in a waterfight anyhow, so mehh, nevermind my ramblings.. :P

EDIT: Almost forgot.. the pump seems to be of fairly small volume. Any guesstimates on the number of pump strokes needed to fully charge it?
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Old 04-01-2008, 01:54 PM   #15
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Default Re: My "not so" homemade watergun

The math says that 65 pumps should get it to full power, that's with a 15 inch throw, so there will be a little sweating involved.

For the tubes I was thinking of screwing on a larger diameter tube, replacing the existing small bore barrel.

I will definatly read the two links... thanks for all the support, this forum is full of great people.
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