Step 1: Build the Jig
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 marked the center point and cut a hole with a two inch hole saw for the barrel.
Step 3: Load the Jig
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
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
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
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
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.