1- 4'x8'x3/4" plywood
1- double acting cylinder
1- 4/3 control valve
1- pressure regulator
8'- 1/2" air hose
Step 1: Mini Model
I began with building a model of the catapult using small syringes containing water. I created the larger version from this design and changed the hydraulic component to pneumatics.
Step 2: Tracing the Pieces
After testing the original model, I took it apart and simply traced the pieces on a piece of paper. Then, I scanned that drawing onto the computer and created an Autocad file which I then converted into a v-carve file to run on the CNC router.
Step 3: Assembly
After the pieces were cut out of 3/4" plywood on the CNC router, I assembled the pieces as I did with the model. I used 3/4" dowel as the pegs and screws to add strength and stability.
Step 4: Pneumatics
Next, I added a base and the cylinder. The base needs to be screwed (or attached in some manner) to the ground due to the power of the pneumatics. I choose a double acting cylinder with a 3/4" bore. I also added a manual stop (in the form of a steel pipe) to the rod of the cylinder to keep it from retracting all the way.
Step 5: Plumbing
I used 1/2" hose to plumb the cylinder. In order to control the speed of the extend and retract of the cylinder, I also added a regulator. Using a 4/3, lever operated valve, I can control the extend and retract of the cylinder and also have a neutral state.
Step 6: R&D
The last step is simply testing the mechanics and some slight tweaking to the design in order to get the throw distance desired. I was able to get the catapult to launch the tennis ball about 5'.