Halloween has been a pirate theme around here for a while. I had the sword, I had the treasure chest, I needed a cannon. More than just a prop, this cannon will fire a potato slug a couple of hundred yards. For Halloween we dialed back the psi and shot harmless "cannonballs" made from tissue paper filled with baby powder. The result was a VERY popular addition to the pirate party we had.
Step 1: Build the Jig
The jig is made from 2X4 and MDF. First I determined the diameter of the front and rear of the finished cannon and the overall length I wanted. I then drew the profile out on a sheet of MDF and cut the profile out with a jigsaw.
Then I split the profile in half to attatch to the sides of the jig. This profile would provide the template I would use to carve out the material.
A piece of 1.5 inch Schedule 40 PVC is used to lay up the layers of foam and will also act as the barrel.
Step 2: Cut the Material
I used two inch ridgid foam for the main material of the cannon. One 4 X 8 sheet of two inch runs about $20.00. I marked out 12 inch by 12 inch squares and cut down the foam.
I marked the center point and cut a hole with a two inch hole saw for the barrel.
Step 3: Load the Jig
I cut two circles of MDF the diameter of the two ends. I placed a piece of 1.5 inch schedule 40 pipe into the jig then slid on the large MDF circle. I held it in place with a screw.
I stacked the squares of foam onto the pipe until the length I requried was achieved. The smaller circle of MDF went on. Then I got some help to compress the material to mark the pipe and drill a small pilot hole. I'ts important to compress the material a bit to hold it in place. The foam will not be glued until after it is shaped.
Step 4: Carve the Material
A few weeks prior I ordered a hot knife (Sculpting Tool) and some foam coat from Hot Wire Foam Factory.
The "Sculpting Tool" worked well but next time I would simply build my own as in this instructable
To extend the small hot knife I mounted it to a dowel rod cut to the appropriate length. The ends of the dowel ride the template and carve out the shape.
To use the jig, run the knife down the length removing the excess material. Turn the pipe, and thus the foam a half inch or so and make another pass with the tool. Eventually you have you basic shape.
A hot knife makes it easy to cut the foam but any cutting tool could work. many people use an inexpensive "Electric Knife" right out of the kitchen isle.
Step 5: Glue It Up
Draw a reference line on the foam before you take it out of the jig. You could number the layers also, but I didn't have any problems. To glue the layers I used a Liquid Nails product rated for foam and I have used "Gorilla Glue" before with sucess.
Use a piece of pipe to help align the barrel slot and stack the layers on it. I added some wieght to the top and let the glue cure for a couple of days.
After I double checked that the layers were bonded, I cut the slot for the firing mechanism, which I should have done before the glue was applied. You won't make the same mistake.
To shape the cannon, I left the foam on the pipe and sanded with 60 then 140 grit sand paper.
The next layer was "Foam Coat" a commercial product that worked well, but in the future I would fiberglass the foam if the project would see a lot of use (like this cannon). For a prop that didn't need to be too durable I probably coat the exterior with plaster of paris or something similar.
Because the foam is coated, any paint will work. Use a latex based paint if it contacts the foam directly or it will melt your project.
Step 6: Fire in the Hole
The firing mechanism is in order from right to left:
1/4 inch quick release
1/4 inch to 3/4 inch fitting
Sprinkler Valve (3/4 inch inline modified) Tutorial Here
3/4 inch close nipple
3/4 inch to 1.5 inch flush bushing (schedule 40 PVC)
Schedule 40 1.5 inch coupler
Schedlue 40 pipe 1.5 inch cut to length (the barrel)
During the modification of the sprinkler valve, you will add a 1/4 inch threaded nipple, and to this I attatched a lenght of hose and then the "blow gun". The blow gun acts as the trigger for firing the cannon. The quick disconnect is connected to the hose that runs to the compressor.
Step 7: For Those About to Rock
Start things off on a low PSI, say 30 or so until you are comfortable with the operation and range. So far, I've only shot "stage" rounds out of it for the kids in the neighborhood. I wrap about a 1/4 cup of baby powder in a sheet of tissue paper and spray paint the ball black. To load I insert two sheets of wadding (tissue cut into 6 x 6 inch squares then the faux cannon ball on top. Slide this down the barrel with a dowel cut to the correct length so it is seated at about the bottom of the barrel. The result is a nice pop and a cloud of "smoke".
A real pyrotechnician might develop a load that would use the compressed air to dispurse a liquid also known as a flame projector, but only with the correct permits, safety equipment, and plenty of insurance.