This is a catapult (ala torsion style) I built inspired from the video a contributor named schoondogs posted. His catapult looked simple enough so I thought I'd make a step-by-step with some improvements (that's the American way right? - and the Japanese way for that matter). I am not a master craftsman, just trying to be a Hero Dad* for my boys so look past the low grade finishing and more so on the end result. The design is not pretty but it is very functional and that's really all a 5 year old cares about anyway. It took about three hours to build (not counting the trips to Home Depot) and does not include any difficult cuts (all straight cuts) or elaborate tools.
2 - 8' 2x4s
25 or so 3" brass wood screws
24" 1/2 PVC pipe (SCH40)
6 feet of paracord cord
2 small wood dowels (I used a old drum stick)
some type of cup to hold ammo (with hardware for attaching to arm)
Approx 10 big nails (2 1/2 inch honkers)
9" Japanese pull saw (this is great for silent late night projects)
7/64 drill bit
1" drill bit
tape measure (or yard stick)
Range: after all modifications this baby can chuck a tennis ball easily 60 feet
*Hero Dad - any dad trying to impress his kids with some type of act or service - in this case (in the steps of my dad) it's building something really cool
Step 1: Building the Base
Cut the 2x4s into 14" and 30" lengths. Drill the 3" screws into each end to create a base for the catapult
Step 2: Add the Side Supports
Approx 17" from the back of the catapult nail in the upright supports. These supports are 13" tall
Step 3: Install the Block Supports
Most catapults use a triangular support for their uprights but since I just wanted to make a functional (not pretty) catapult I just opted for easy horizontal blocks.
I did not show this step but you will also need to add the horizontal (stopper) support - this is 14" wide.
Step 4: Drill Side Holes
I used a 1" drill bit to make these side holes - this is where the rope will be fed through. Placement is about 1" down and 1" over. Notice that this hold is not in the middle of the 2x4 - this will allow for the arm to swing and not scrap on the ground. I made sure that these allowed for the pvc pipe to stand vertical, flush with the cross supports
Step 5: Drill Catapult Arm Holes
I used the 7/64 drill bit to make these holes. The great thing about pvc is you can easily carve out the holes so they are a larger than your bit. This is nice if you don't have a larger bit. When drilling these holes make sure the end of the arm does not drag on the ground when the arm is in action.
Step 6: Threading the Para Cord Rope
This is a crucial step and not easy to explain so I took a lot of photos. I ended up using about 4 yards of rope to string up the arm. I found that if you tie down one side with the peg that helps in threading this baby up. I first threaded the rope through the two drilled holes and then alternated front and back until I ran out of rope.
Step 7: Crank It
Now it's time to crank. Alternate from each side and crank the pegs clockwise until the cord is really tight. The arm should be firmly standing at a 90 degree angle, tight against the stopper bar.
Step 8: Ammo Cup
This was my first ammo cup - I changed it later. My only recommendation is to not use a bowl or a curved cup, the ammo get's thrown to the ground. I used a old clay container - it has straight sides so that worked pretty good (until it broke due to the force of the arm hitting the stopper too many times). Depending on what ammo you use - we used mostly wiffle balls and tennis balls. Later on I made a more of squared platform with a cut out for the balls - see "Modifications for more detail"
Step 10: Modifications & Action Videos
After some field testing I realized the catapult needed some modifications. So I added a foot on the front and on the cross bar to be able to change the trajectory. I tried extending the arm to see if that had any baring on the power. I also added a stripe of surgical tubing that gave more tension to the arm. This was a big help and really gave the catapult more power.http://www.youtube.com/watch?v=US97RQglQzAhttp://www.youtube.com/watch?v=j-LiVs2BVOo
this can also be placed atop any skateboard so you can easily drag it to family picnics or the local park
here's an old video of one of my dad's catapultshttp://www.youtube.com/watch?v=bTaiYhzCaDg