I also want to start by re-iterating his safety warnings as this device has risks related to fire, steam, pressure, metal-work, and projectiles. I recommend wearing eye protection during all steps and using common sense. I accept no liability for any risk in the manufacture or use of this device. Do not attempt to use power tools if you are not familiar with them and I don't recommend anyone under the age of 18 attempt to manufacture this device without competent adult supervision. Don't touch the barrel for a few minutes after it has been fired as it becomes extremely hot. Also, the soot will stain clothes/skin. Be careful with this thing guys.
I will leave the construction of the barrel/chamber assembly to robbtoberfest's design and will primarily focus on the design modifications which allow the fuel/heat source to be a "Zippo (tm)" brand lighter. The only modification I made to the barrel/chamber assembly is that I used "J.B. Weld (tm)" brand two part epoxy paste instead of solder. I find that it is less messy and I have limited experience with solder. The joints and air-tight seals of the "J.B. Weld" have held through all of my test firings. The farthest I have gotten it to shoot a plug his about 23 feet.
It works, but it has limitations. I feel that it would work well indoors with very minor drafts, but outside it is challenging to keep the flame on the chamber if there is any wind at all. Seems to take me at least a couple minutes of fairly steady flame on the chamber to get it to fire. I am not sure if this modification is faster/slower to fire. I think indoors it is probably about the same to a little faster to fire.
Step 1: Components/ Tools
***The only modification is I used J.B. Weld instead of solder.
Oak plank - In the "craft wood" section of Home Depot - about $2.50
- Pine would be cheaper and would probably work fine. I would expect pine to burn a little easier so I chose oak.
Anvil (or hard surface like a piece of metal or wood to hammer against) / hammer
Pliers. Two needle nose would work fine.
Zippo (tm) brand lighter - usually around 18 bucks. Of course you will also need lighter fluid.
Bolts and corresponding wing nuts/ short wood screws. (I used brass, but that was mostly for show. Frankly, I think it may have been better with steel as I broke the heads off of a couple of them trying to screw them into the oak even with pilot holes. I suggest using bar soap on the threads to lubricate them as they screw in.
A drill or dremel with a drill bit to make pilot holes and drill through a couple of copper pieces.
A good table mounted vice makes life a little easier to bend the copper fitting (to be explained).
A hack saw is one option for cutting the pipe. I used a pipe cutter.
A file for deburring and cleaning out the lip on the barrel and corer.
Leather scrap or denim or any thick material as a place to store your plug cutter. Attached with furniture brads.
- Apparently they come in terms of "numbers". I found that #5 size seems to fit the 1/2 inch copper pipe best.
Potato. Not too thick. I wouldn't go much thicker than an inch.
Eye dropper or other device that can dispense about 8-10 drops.
A long wood screw is useful when you accidentally load the potato before you put the water in. I know it sounds stupid, but I have done it twice now. Just screw it into the potato and pull. Also useful if it doesn't fire for any reason.
Step 2: Modifying Two 1/2" Copper Pipe Fittings for the Barrel.
I did this by modifying two copper fittings as seen in the photo.
Hammer the fitting flat on the anvil or hard surface. See the photos below to show the steps to turn the standard fitting into the fitting necessary to mount the barrel assembly to the stock. The main thing is to keep the piece symmetrical.
After the piece is flat go put it on the anvil with just a little past the hole.
Hold the long end down and hammer the part hanging off the anvil to about 90 degrees. Do this to both sides in the same direction.
Next grab the long pipe or the barrel assembly. You are going to bend the fitting around the pipe with your fingers. Bend it as far as you can then fine tune with a hammer or pliers.
Be careful as the edges can be a little sharp. Make sure the holes line up so you can get a bolt through.
Next you need to drill a hole in the center of the piece so you can screw it into the stock. This could be done with a hand drill, but I did it with a drill press with a drill press vice to hold it steady. Again, safety first with safety eyewear and safety equipment. Make two identical fittings.
Step 3: Prepare the Stock.
Also, attach the two fittings in such a way to hold the barrel where you want it. Keep them close enough together to give you some play when positioning the lighter.
Step 4: Modifying a 1" Copper Pipe Fitting for the Lighter.
Hammer it flat like the other fittings.
In order to figure out where to put it you have put the barrel assembly in and position the chamber and the open lighter so that they are only about 1/2 inch apart. Remember, you will be able to rotate the barrel assembly so that you can close the lighter later.
Once you determine the best place for the lighter hold it against the wood. You need to bend the ends down with your fingers/pliers to make them hold the lighter in place. You may have to fiddle with it a little in order to get it perfect. Try to get it as symmetrical as possible. I screwed mine in with the lighter in place, but later I could pull the lighter out fairly easily. Don't worry too much about it falling down through the bottom. It was tight enough for me that I didn't have any problems. Again, brass screws are tricky and need to be lubricated with soap and put in pretty big pilot holes. I turned the head off of two of them before I was done. (You can't see the long ends of the screws in the wood because I covered them with the fitting.)
Step 5: The Potato Corer/ Cork Holder/ Leather Strap
Grab a piece of leather scrap and wrap it around the corer pipe. Make sure you have enough for it to overlap about an inch on the other side.
Hammer some furniture brads through two layers of leather and into the wood. I used three, but two would probably work. This is an optional step and is mainly decorative.
Step 6: Firing
1. ) Put 10 drops of water (or 9 drops water and one drop 100% acetone to slightly reduce the boiling point) into the barrel and twist the barrel around until the water has been moved into the boiling chamber.
***remember after this you have to hold it upright until it's fired or the water may move out of the chamber.
2.) Core a potato and make a plug about 1/2 to 1 inch long, and put it into the barrel about one to two inches at most.
3.) Put in the #5 size cork. Make it snug, but not jammed tight. After this point the device should not be pointed at any living thing.
4.) Aim and strike the flint striker until it is lit.
5.) Here is the biggest difference from the original potato pistol. If you are in a zero wind atmosphere you will be fine, but if you are not you have to kind of manage the flame by moving the stock around to keep the flame on the chamber. This can be tricky and messes up your aim. Frankly, I feel it is the biggest issue with the design as it stands. I have considered putting on a large wind break made of metal fabric mesh, but that would be a different project.
The firing in this video used the 9-1 water to acetone ratio. The chamber was hot from a previous firing. This firing took almost exactly a minute with a few seconds not taped. I also noted I had a really good seal between the potato and the barrel.
Issues in firing:
If it does not fire there are several things which could have gone wrong. The chamber might not be sealed on one of the seams or through the cork. However, I have had successful firings where just a few moments before it fires it starts to bubble a little around the cork out of the barrel.
It may also not be boiling the water successfully and creating the pressure for firing. This is usually because the wind kept the flame from heating the chamber consistently.
Firing can take up to 5-6 minutes sometimes. Be patient with it.
As I said earlier, a long wood screw is handy getting the potato plug out of the barrel if it doesn't fire or you load it before you put the water in.
Step 7: Improvements That Could Be Made.
1.) Improved Wind Resistance. This would be a huge advantage outdoors and would probably improve aim and decrease firing time.
2.) Decreased Firing Time. Definitely would make it more impressive to show off. Standing there for 5 minutes in the wind is anti-climactic. Especially if they are expecting a showing similar to that of a traditional PVC potato cannon.
3.) Better cork system? Rubber cork? Pressure gauge? Some kind of quick release valve to release the steam? All possibilities.
I welcome your comments and thoughts. Good luck and be safe with this thing if you make it.