What kind of PVC should I use?

I must stress this again that no PVC is rated for air.  It was only designed to carry liquids even if it has a PSI rating.  

The only kind of PVC that should be used is SCH40 or heavier, and clearly has a PSI rating on the side.  Some PVC have the words Cellular Core written on the side, and should never be used.  Cellular Core is the name given for the manufacturing of this style pipe.  It is basically plastic whipped up that is filled with tiny air bubbles that effectively makes the PVC lighter, but weaker.  Do not use it if it is labeled Cellular Core.

Do not use ABS plastic.  This type of pipe is only rated for 10 PSI., and was designed for drain water where it would never be pressurized.  It should not be exposed to sunlight, and when heated will become flexible and not return to the original shape.

When you first apply air to the device, the diaphragm shifts forward towards the barrel and effectively seals the air from going out the barrel. (See the picture below.)  The air gets pushed out around the non-perfect edges of the seal and fills the outer chamber to the preset air pressure. As long as you have air pressure applied to the back of the diaphragm, the diaphragm will remain pressed against the barrel. Once the air is removed from the back of the diaphragm, the air pressure in the outer chamber pushes the diaphragm back against the threaded plug where the back gasket makes an airtight seal. This seal does not allow air to go back out through the threaded plug, and at the same time allows all of the air stored in the outer chamber to go out the barrel. This device is used in industry and is called a quick exhaust. It is used to allow air cylinders to vent quickly, which in turn allows a faster movement. The way we are using it, it allows the massive air flow that is required as our propellant.

Internal diagram of the SpudZooka.

It doesn't matter what air fittings you use as long as they attach your air hoses to the SpudZooka.  More than likely, you'll just be using regular air hose that you can purchase at your favorite hardware store.  You will also need some sort of reducer or couplings to attach the air line to the SpudZooka.  You will also need some sort of quick disconnect fitting.  This is the fitting that you will use to fire the device. 

Quick Disconnect Fittings, Also used as the Firing Trigger.

This is directly proportional to your skill level and available tools. Generally it takes 2 days to build a SpudZooka. You have to wait at least 24hrs. for the glue to dry. The hardest part to manufacture is the diaphragm assembly.  The rest of it is like building a conventional fuel style cannon.

What will happen if I shorten the barrel, or make the outer chamber smaller?

The velocity of the projectile will be greatly effected if you change either the barrel length, or the outer chamber diameter.  The potato is accelerating the entire time it is being pushed out of the barrel, therefore if you reduce the acceleration period, you reduce projectile speed.

What other materials can I use for the diaphragm?

The materials I suggest for the diaphragm are just that, a suggestion.  They are guaranteed to work if they constructed as outlined in the instruction booklet.  You can try using other materials, but they may or may not work for you.   The first couple of attempts I made failed.  I tried using thinner gasket materials, but the gasket would not slide properly, and would get cocked in the barrel.  The thickness of the materials must be taken into consideration for this style cannon.  If the overall thickness of the diaphragm assembly is to thick, this will reduce the volume of air flow through the cannon upon firing, hence reducing overall performance.  I designed the diaphragm to be easy to duplicate with as few tools as possible.  Not everybody has the same tools available, or the same skill level.  Best I can say is experiment with the materials that you have available, or purchase the diaphragm assembly if you are having problems building your own.

All the air comes back out the cleanout plug. Why?

I have heard of a couple of cannons doing this, and after speaking in depth with the persons constructing there own cannons, we still have not come up with the answer.  Somewhere in the construction, something is overlooked.  If you experience this problem, and then solve it (even if if it was stupid or obvious.) please send me an e-mail, and I'll include it here for the rest of the world.  Though this appears to be rare, I'd hate to see somebody give up building there cannon for this reason.

My diaphragm lets air by and it blows out the barrel. Why?

Obviously the diaphragm is not seated properly. With the SpudZooka 234, this may be because your diaphragm wasn’t tight enough inside the cleanout plug, and got cocked while sliding out. (The diaphragm should be snug.) I have not experienced this with the diaphragm assemblies I sell. I have seen this problem with other assemblies I was experimenting with and I believed this to be the reason. I have also seen this scenario when I tried to use thinner gasket material. To try reseating it again, remove the air. Then with the ram rod, push it all the way down the barrel and push the diaphragm back into the cleanout plug. Remove the ram rod, and try charging the cannon again.

What is a Quick Exhaust and how does it work?

A quick exhaust is an industrial component that is mainly used with air cylinders. The quick exhaust allows the air cylinder to vent as quickly as possible, therefore move as quickly as possible. A quick exhaust has three ports, an IN, OUT and EXHAUST. Air is always applied to the IN port, your load is always attached to the OUT port, and the EXHAUST is vented to the atmosphere. When you apply air to the IN port, it passes directly through to the OUT port filling the air cylinder.  Once air is removed from the IN port, the load on the air cylinder causes the air to push out of the cylinder back into OUT port, but now an internal diaphragm shift and allows the air to vent quickly through the EXHAUST port.  Generally this is used if the source of air is located away from the air cylinder, and the air cylinder needs to move faster (vent faster) than when it was originally charged.  In our case, the air behind the diaphragm acts like the air cylinder.  Once the SpudZooka is full, we remove the airline from the IN port. An internal diaphragm in the quick exhaust switches, and the air behind the diaphragm gets dumped from the OUT port to the EXHAUST port. This way, we can remotely fire the device because the return path is not going out through our airline to the SpudZooka, but through the quick exhaust located directly at the SpudZooka.  The diaphragm assembly itself acts like a 2nd quick exhaust.  When it moves, it allows all the air stored in the outer chamber to instantly rush up the barrel.

Typical Quick-Exhaust

Do I need to use a quick exhaust?

No, you do not. The SpudZooka may still fire if you are not using a quick exhaust. If you are out for maximum potato speed and distance, then a quick exhaust must be used. If you do not use a quick exhaust and the air line to the SpudZooka is long, the diaphragm may shift to slowly, and instead of a sudden "thump" sound, you will get a "tttttthhhhhump" sound, and your distance and speed will greatly decrease.

Where can I find a quick Exhaust?

You will only be able to purchase them through suppliers that sell pneumatic components. The quick exhaust can be purchased through different industrial suppliers, or from me. I bulk purchase them to keep the price low. There are different types out there, but since I did not try them all, I do not endorse any other types than the one’s I sell, since I have no experience with them.  Try these suppliers . . .

McMaster-Carr  (Nice site, this is one I like to use for ordering industrial stuff.)

Grainger   (Requires you to Log In to there site)

I remove the air, and the SpudZooka doesn’t fire, or fires poorly. How come?

I get this one a lot. My 1^st question I ask is , "Did you use the spacer when you glued the barrel into place?" The spacer sets up the distance that the diagram can move. With out the spacer, the diaphragm has no where to move. 2^nd, If for some reason the diaphragm is to tight, the air may not be able to seep around the edges and charge the outer chamber. This could also happen if the metal plate in the diaphragm is to large. It may not allow the gaskets enough room to flex to let the air seep by while charging. If this happens, there will be little or no air to push the spud.

Here is an answer form a fellow Spudder . . .

11-2010

“All the air comes back out the cleanout plug. Why?

 Hi,

I had this problem too.  When I read your Q&A and learned no solution had yet been found, this struck me as something I just had to solve.  And I did.

What puzzled me was that I thought I had built an well-crafted diaphragm (essentially as per your plans).  When I opened the cleanout plug of the cannon for inspection, it was clear the diaphragm moved smoothly in its ½” of travel, and was built to fit well within the plug.

Nonetheless, it seemed obvious that the only reason the air would all blow out the back was because the diaphragm was not being released from the barrel.  Further reflection lead me to believe there would only be two possible reasons for this.

Possibility #1 - The diaphragm was physically incapable of moving, as it was held tight against the barrel by the plug.  I quickly eliminated this possibility by re-inserting the positioning plug and confirming that there was indeed a good ½” of travel between the barrel and the back of the cleanout plug (even allowing for the thickness of the diaphragm itself).

Possibility #2 – The diaphragm, though capable of moving, was not moving when the air rushed out the back.  At first, I thought this possibility most unlikely because I assumed the rush of air out of the plug ought to have caused any reasonably constructed diaphragm to slam back.  However, upon reflection, I realized there was another possibility – the air pressure was causing the barrel sealing face of the diaphragm to “stick” ever so slightly to the barrel, and the air was simply escaping around the diaphragm and exiting from the rear without exerting another force on the front sealing face to push it back.  I double checked the tolerance between the diaphragm and the inner diameter of the plug and noticed that although the diaphragm was a nearly perfect circle, and although it was built to fit well within the plug, there was an approximately 1mm gap between the diaphragm and the plug.  So I rebuilt the diaphragm and eliminated that tiny gap.  Presto!  The thrill of a perfect operation!!!

If anyone is having problems with the air blowing back, and they’ve eliminated the possibility of not having left the ½” gap for travel, I highly recommend the reconstruct their diaphragm ever so slightly bigger.  Worked like a charm for me.

All the best, and thanks so much for the great design.

            D. Steinberg  

 

11-2010

By the way, I re-tested my theory by building another diaphragm that worked well.  I then shaved off about an 1/8” of an inch from the diaphragm.  Once more, the air blew out the back before the diaphragm sealed the rear.  Though it may seem hard to believe that such a small gap would cause the diaphragm to essentially fail, an 1/8” gap around a 3” diaphragm creates a full 2.3 square inch passageway of zero resistance from which the air can escape out the back.  The result is that virtually all the air is out the back before the diaphragm moves to cover the plug.

             D. Steinberg

 

Why do I have to use primer when gluing joints?

First let me state that with PVC glue, you are not actually gluing the pieces together.  You are actually more or less melting the pieces to form one solid piece.  More like welding. The primer first aids in cleaning of the PVC, then it actually preps and softens the PVC for up to .030" deep. Without primer, your joints will not be nearly as strong.  Purple primer is generally used so that you can see if primer was used or not. If you are going for looks, try locating some clear primer, but always use it!

Do not use ABS cement.  It is not compatible with PVC.

How far can a SpudZooka shoot?

I do not have this answer. I never tried to measure the distance, but if you were impressed with a combustion cannon, your jaw will drop after seeing a potato shot from a pneumatic.  I believe I have shot a potato half a mile already, but I couldn't see it very well. Just for a few minutes as the sun was setting, the light angle must have been just right and it gave me a good view of it sailing through the air, but I couldn't tell exactly where it was landing.

How fast can a SpudZooka shoot?

I have personally recorded speeds up to 400ft/sec. with the SpudZooka model 234, but I am not sure how accurate my home made chronograph is.  I believe the speed was actually higher.  As soon as I get a better answer to this one, I'll get it posted.  Paint ball guns are limited to 300ft/sec. For safety.

I have built a larger 4" proto-type that contained a larger volume of air, and allowed the diaphragm to shift much farther.  This was superior to the SpudZooka 234 but my diaphragm (rubber) swelled after one season, and never fired well after that.

Are they legal?

Click here to go to my Legal Page

Where can I find PVC? The hardware stores in my area only handles ABS plastic.

I have heard from a few people that live in the middle of no where that they have this problem. You’re shit out of luck if you want to use PVC. I believe shipping and handling costs would be ridiculous since you wouldn’t be ordering mass quantities.

Can I Use a Bike Pump?

Maybe if you are super man. 
1^st of all, It takes a lot of energy to make compressed air. You’ll be pumping a long time for every shot fired. You would have to use a small diameter pump. Those wide ones that fill up air mattresses won’t work for you, because of the size, you’ll never be able to compress the air to 100PSI.
2^nd, you may not be able to seat the diaphragm properly while pumping with a bike pump. You may need the sudden burst of air to make it shift and seal properly. I have never tried to use a bike pump, and I guess I never will. If anybody out there ever does, let me know. I’m curious to see if it is feasible.

I am Under Age 18, Now what?

Don't worry, you'll grow up someday.  Till then, your parents are responsible for you.  I do believe that if you are under 18, you should build this as a father/son project.  Even if your dad isn't cool, let him think he is, and let him in on what you are doing.  He will probably get that interested in the concept of shooting a potato that he will build you one, and then build one for him self.  Especially when you black mail him by saying, "yeah, but I'm the one with the plans."  Your mother on the other hand, won't understand and will ask, " . . . and why do you want to shoot a potato?"  (Women in general just don't understand.)

Stupid Question Hall of Shame . . .

I was once told that, "No question is stupid.  The only thing stupid would be you for not asking."  This statement is bull shit.  If your question is that bad, and I have gotten some from kids that shouldn't be building air cannons,  it will get posted here in this section.  You need to have some common sense before you even attempt at any project like this.