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m72 LAW Design questions

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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m72 LAW Design questions

Unread postAuthor: Zorlord » Sun Jul 22, 2012 1:17 pm

Hello All,

Fairly new here, although i've been looking through countless threads and pictures looking for help in designing a launcher. I've decided on using the "Toolies Valve" design, and i have a couple questions.
First, will a Toolies vale work in this kind of set up? Im looking to create a lightweight, relatively short distance (100 or 200 ft) launcher for scenario airsoft, designed to take out small buildings and bunkers at my local field. I need something to operate at low- medium pressures (50-100 psi), that can be filled either by a normal air tank, or a regulated HPA tank. Here is a diagram that i drew up:

Image
(Not to scale, just a rough estimate on how i'd like to have it laid out)

It's going to be filled via 1/4 male quick release with a ball valve for on/off, and piloted by a a blowgun attached to tubing on the back that's going to be routed up to the top of the launcher so it can be triggered much like the real one, i didn't draw it because my questions are in regards to the valve/ body of the launcher.

I guess we'll start with the piston itself. Would something like this (see below) be adequate? it seems that all the threads i can find are dedicated to coaxial guns, so i wasnt sure if the piston design would be able to transfer between the two.

Gun Freak wrote:Image
Image
Image
Image



Finally, i'd like to keep the overall length of the launcher at or a little over 2 feet. Any recommendations on chamber vs barrel length? From what i've read, it seems that the valve is more important than the cb ratio, but i'd like to try to get the max amount of efficiency out of this thing as possible, which is why i bring it up here.
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Unread postAuthor: mattyzip77 » Sun Jul 22, 2012 4:25 pm

If all you are going for is the short distance you noted a sprinkler valve would be plenty. This piston is overkill in my opinion and is designed to be used in a tee housing.
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Unread postAuthor: Zorlord » Sun Jul 22, 2012 9:25 pm

mattyzip77 wrote:If all you are going for is the short distance you noted a sprinkler valve would be plenty. This piston is overkill in my opinion and is designed to be used in a tee housing.


Unfortunately a sprinkler valve wont fit within 2 inch housing, otherwise I would've done that. Im trying to keep it as true to the shape/ structure of the real thing if possible.
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Unread postAuthor: iknowmy3tables » Sun Jul 22, 2012 9:29 pm

I think 2feet is very ambitious, but with 3 feet I think you could do it nerf footballs are cooperative projectiles
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Unread postAuthor: Zorlord » Sun Jul 22, 2012 9:33 pm

iknowmy3tables wrote:I think 2feet is very ambitious, but with 3 feet I think you could do it nerf footballs are cooperative projectiles


Could you elaborate on what you mean? Also, if a longer length would aid in accuracy/ efficiency, then maybe ill just make it into a Bazooka.
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Unread postAuthor: jackssmirkingrevenge » Sun Jul 22, 2012 10:43 pm

iknowmy3tables wrote:I think 2feet is very ambitious, but with 3 feet I think you could do it nerf footballs are cooperative projectiles


In my experience you can get decent performance from nerf footballs even from a 1 foot barrel with 150 psi or so, with a 1" porting piston valve you should be OK.

A 1 foot barrel and 1 foot chamber is probably a good idea. As to the piston, the one you posted would work though of course the front part which seals against the seat would need to be smaller in order to allow the air to flow past it.
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Unread postAuthor: Zorlord » Sun Jul 22, 2012 11:18 pm

jackssmirkingrevenge wrote:
iknowmy3tables wrote:I think 2feet is very ambitious, but with 3 feet I think you could do it nerf footballs are cooperative projectiles


In my experience you can get decent performance from nerf footballs even from a 1 foot barrel with 150 psi or so, with a 1" porting piston valve you should be OK.

A 1 foot barrel and 1 foot chamber is probably a good idea. As to the piston, the one you posted would work though of course the front part which seals against the seat would need to be smaller in order to allow the air to flow past it.


It is. With the way i'm hoping to have this work is the sealing face is created by a 1 1/2 inch neoprene washer supported by a metal washer, sealing onto a 1 inch pvc pipe that i've pushed through the reducing bushing by removing the lip inside the bushing. The one (or two, im not sure if it makes a difference) sealing "pistons" will be constructed with 2 inch orings suspended between two 2 inch washers. That should allow the valve to function properly correct?

Also, does anyone have a source for 2 inch washers? I couldn't seem to locate any at my local home depot.. biggest they had was 1 1/2.
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Unread postAuthor: iknowmy3tables » Mon Jul 23, 2012 9:02 am

Also, does anyone have a source for 2 inch washers? I couldn't seem to locate any at my local home depot.. biggest they had was 1 1/2.

well since you only need the full 2" washers for spacing I recommend you make it out of plastic or wood, it would help keep the weight of the piston down
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Unread postAuthor: Zorlord » Mon Jul 23, 2012 10:44 am

iknowmy3tables wrote:
Also, does anyone have a source for 2 inch washers? I couldn't seem to locate any at my local home depot.. biggest they had was 1 1/2.

well since you only need the full 2" washers for spacing I recommend you make it out of plastic or wood, it would help keep the weight of the piston down


I tried wood, but they didnt come out too well.. Also if i were to do that, how would i get them to seal off the pilot side when it was triggered?
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Unread postAuthor: iknowmy3tables » Mon Jul 23, 2012 6:12 pm

you may need to try a harder wood, or make washerers out sheet PVC which can be made by heating and flattening pvc pipe (a heat gun works well for this), it isn't necessary to make a perfect seal on the pilot side especially when you have a port diameter half the size of your piston, however if you mange to make good washers you can sandwich some sheet rubber between them to make a very strong seal, I recall brianthebrain used this method
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Unread postAuthor: Zorlord » Mon Jul 23, 2012 8:45 pm

iknowmy3tables wrote:you may need to try a harder wood, or make washerers out sheet PVC which can be made by heating and flattening pvc pipe (a heat gun works well for this), it isn't necessary to make a perfect seal on the pilot side especially when you have a port diameter half the size of your piston, however if you mange to make good washers you can sandwich some sheet rubber between them to make a very strong seal, I recall brianthebrain used this method


Okay.. I do have a heat gun.. And LOTS of PVC haha.

How would you recommend i cut out washers that are square without a drill press? I have drill bits to cut out a 2inch hole, but being able to cut something that's square is next to impossible
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Unread postAuthor: iknowmy3tables » Tue Jul 24, 2012 8:22 am

the last time I made a pvc washer I drew the outline of the circle and cut it out with a band saw, then I clamped between two nuts on an all-thread rod put the rod in a drill bit chuck and used the drill as a makeshift lathe
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Unread postAuthor: Live1nce » Tue Jul 24, 2012 11:13 am

iknowmy3tables wrote:the last time I made a pvc washer I drew the outline of the circle and cut it out with a band saw, then I clamped between two nuts on an all-thread rod put the rod in a drill bit chuck and used the drill as a makeshift lathe


I think this would be the best method for PVC washers. But if you don't have a band saw, you can use a jig saw. If you don't have one of those, you could get one of those handheld detail wood working saws that have the very fine blades and cut it by hand. I think I picked mine up from harbor freight for $5.99 and it came with some different blades for fine/rough cuts. This would be a perfect cheap alternative for ya if you don't have higher end tools.
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Unread postAuthor: Zorlord » Tue Jul 24, 2012 11:53 am

Live1nce wrote:
iknowmy3tables wrote:the last time I made a pvc washer I drew the outline of the circle and cut it out with a band saw, then I clamped between two nuts on an all-thread rod put the rod in a drill bit chuck and used the drill as a makeshift lathe


I think this would be the best method for PVC washers. But if you don't have a band saw, you can use a jig saw. If you don't have one of those, you could get one of those handheld detail wood working saws that have the very fine blades and cut it by hand. I think I picked mine up from harbor freight for $5.99 and it came with some different blades for fine/rough cuts. This would be a perfect cheap alternative for ya if you don't have higher end tools.


Okay, maybe ill try that. I think i have a jig saw around here somewhere.. I don't have any bigger tool (lathe, drill press, band saw), so my ability to machine things accurately is a bit limited :(
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Unread postAuthor: Live1nce » Tue Jul 24, 2012 11:40 pm

You might be able to get away with using a power drill that you clamp into a vise. Just make sure you dont clamp it too tight, and dont clamp it around the main motor of the drill, clamp it around the hand grip or something and find whatever you can to wedge it into place so it doesn't move on you. If you don't have a vise, then I don't know what to tell ya. You can pick up a vise at harbor freight for about 30 bucks though. I love that place :)
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