Intro: The Fold Flat Trebuchet on the Cheep
I was looking over my Make mags and came across a catapult in Make 28. Now I didn't wright this to bash Mr. Gurstelle but, I didn't like his design and thought i could do better. The thing that caught my eye was how it was written. My background is in writing SOP and i couldn't stop thinking where was the editor? Isn't there a standard format for submissions. The format was different then any thing els there was a diagram that was labeled but the part list didn't follow this or any thing els except some random shots of a few parts. Sorry its a pet peeve of mine to much rewriting due to lack of direction. Please take this as constructive criticism.
There were also several things that bothered be. The first was that the moving parts were the pipe joints. My thinking were the pipes should be tight. Loosing them and tightening them with each shot might cause them to seise or come apart.
Second was I hate to buy parts especially hardware, It cost to much, there is over $100.00 in hardware if you were to buy all the parts at the hardware store. I can't afford to make anything where i can't find or repurpose at least 90% of the materials.
Third was using plywood as a support. Plywood is strong but there is a good bend in it from the tension on the wires.
So as I was cleaning out my basement and had most of the parts i had to build it, and write an ibble.
Step 1: Parts
- 4 by 4 two eight footers will work I got (4) them off several palets. (Got about 32 of them good hall)
- 2 - 20 inch squares of 3/4 plywood
- 2 - 12 by 27 pieces of 3/4 plywood
- 16 - 5 1/2 inch bolts with nuts and washers
- 4 eye bolts taken from a Christmas tree stand Course thread
- Several pieces of 3/4 black iron pipe 1- 6', 4-10" I save pipe too use for pipe clamps and had to piece some together to get what i needed.
- three end caps
- one floor flange
- one cross joint (This and the end caps is the only pieces i had to buy.)
- several tees or elbows or extra flanges
- 2 eye bolts and nuts taken from a Christmas tree stand
- 1-2" bolt and nut
- 1- 1/2 bolt, nut and two washers
- Pipe clamp
- chain with quick link
- eye bolt covered with copper pipe (or heavy spike)
- 1/8" nylon rope
About 100lb in weights i used three 25's and three 22
1/8" nylon cord
Projectiles i wanted to use pumpkins but tested with 4x4 blocks and taped on glow sticks for some night shots.
Step 2: The Frame
The Frame was simple post construction. See picks for construction tips.
I left two 4x4 posts uncut at 4' 4" for the base and the uprights i cut at 36" and made half lap joints. See picks. The heights of the uprights is determined by the length of the short arm and weight supports in this case is 27 inches plus clearance yours may vary depending on the pipes.
The upright posts are set one foot back from the front end.
The cross pieces were from another pallet, and the length was determined buy the size of the weights and the cross pieces of the pipe I had two 8" nipples and two 4" nipples which places the uprights about 27inches apart (measuring outside the posts) the weights are 12 inches wide and need about 3" on each side for a safe clearance.
I had to go back and forth between the frame and arm for measurements.
I then drilled1" holes for the pipe four inches from the top of the uprights.
Step 3: Supporting the Frame
While the frame holds together well all by itself I thought putting some plywood to stiffen it. This would be the best use of plywood to tie it together tension not compression.
First i put two pieces of foot wide plywood on the bottom of the base with four bolts one in each corner of the plywood with one in each half lap joint on the frame.
Second I made four triangles and bolted them one in each corner and one in middle of the long side through the half lap joint. To support the uprights. The picture is worth 1000 words I'm having trouble with those words ( tech writing is difficult). Look at the picture. I had to cut the corner off the inside triangle. To fold flat the uprights get unbolted and lay in the frame.
Step 4: The Weight and Link
The cross joint was the hardest to find you need to go to a plumbing supply house. The longest pipe goes on the front about a six to seven feet long will do.
Two eight inch nipples go on the sides(arms) of the cross a coupling or tee and a four inch nipple.
A ten inch nipple on the side opposite the long pipe.
Next drill hole in all three end caps start with a small drill bit and work your way up. I held the cap with a pipe wrench to drill the holes.
I put a eye bolt on two of the end caps secured with a nut. On the third I bolted a two inch bolt on
The end cap with the bolt on the end of the longest pipe. The end caps with the eye bolts go on ten inch nipples one the arm and the other will support the weight.
A large bolt goes through the eye bolts with washer on each side to hold it. Do this last with help after the trigger and weight is on the hanging ten inch nipple.
Step 5: The Firing Pin
I put two eye bolts close to the center of the back cross piece (follow the pink string). and one eye bolt at the corner the pin (an eye bolt inside a 1/4 section of copper tube and a long cord attached.) having the trigger on the frame reduces the chance that an full body yank from a small kid will not (in a non-release) pull the arm into a dangerous direction (that is also why i question the wheels on the make mag design.)
A ring is attache to the long pipe and the chain to the ring
To ready, place the copper covered bolt through one eye bolt the chain and just into the second eye bolt.the string goes through third eye bolt. This insures that a full body yank from a kid will pull the bolt straight out.
You will want to stand to one side and not behind the trebuchet.
Step 6: Ammo
For ammo I intended to use pumpkins but for testing and practice i saw several wood chucks with holes in them leftover from another project.
The key is the length of the string the shorter the quicker the release and higher the flight path. Go extra long and use a slipknot to shorten.
I added the glow sticks for night maneuvers.
Get ready to defend the boarder against a Canada invasion.
We got it shoot about 75 ft. Happy hunting and be safe
Step 7: Flat Pack
The triangles can be bolted to the bottom four corners and the uprights tucked inside for long term storage.