Step 1: Materials and Tools
3/8" Hardwood Square
1/8" Hardwood Dowel
3/16" Metal Rod
Jumbo Craft Sticks
Heavy Stuff as a Counterweight
Sharp Knife and/or Chisel(s)
Drill with 1/8" and 3/16" Drill bits
You can really make this project out of just about any type of wood. I chose poplar because it's cheap and sturdy. Most big hardware stores will also have oak and occasionally maple. The design is stout enough that you could make it out of something softer like basswood. In fact, it's probably not a bad idea if you don't have a sharp narrow chisel and you're going to be carving the joints with a knife. These measurements call for just a little over six feet of it. Most 3/8" square is sold in three foot pieces. You might get away with just two if you adjusted the size of a couple of parts, but you'll almost certainly have to recut a couple of pieces so you might as well just get three sticks.
Step 2: Dimensions
2 x 6" Long frame side rails - Part A
2 x 3" Short frame sides - Part B
1 x 7" Frame crossmember - Part C
2 x 4.5" Frame extension rails - Part D
2 x 4.75" Uprights - Part E
4 x ~3.5" Long diagonal supports - Part F
2 x 2.25" Short diagonal supports - Part G
4 x ~1" Launch ramp support blocks - Part H
1 x 7.5" Upper Arm
2 x 2.25" Lower Arms
Parts D, F, and G need 45 degree miters cut on each end as shown in the picture. The length measurements are, of course, along the long side.
As for the long diagonal supports (Part F), you'll need to finely adjust the length of these in order to get them to fit nicely. Check the fourth image in step six to see what I mean. The given length of 3 1/2" should get you pretty close depending on how well you cut the frame extension rails.
The launch ramp supports (Part H) are just little bits of scrap that are angled to make a trough with craft sticks. A 45 degree cut isn't quite long enough to attractively cradle a jumbo craft stick so the angle on those is shallower. I just eyeballed it and got something that looked nice. Putting a protractor on it shows it to be about 35 degrees or so. If you don't feel like going through the trouble, you could just glue a couple of craft sticks flat on the frame to make a plank bottom and it'll work fine but won't be quite as pretty and you'll have a little harder time lining up consistent shots.
Step 3: Cutting the Joinery - Part 1
Parts A and D need cross-lap joints (as does part C, the frame crossmember, but it's a little different so we'll get to that in a minute). To do these, we need to cut out the same amount of wood but in the middle of the stick. Mark it out so it's right in the center. Check the photos to get the orientation right.
Step 4: Cutting the Joinery - Part 2
Now we're going to carve the uprights (Part E). One end will have a half-lap and the other end will have a groove carved in the end for the axle to ride in. The half-lap is just like the ones on the side rails. For the axle groove drill a 3/16" hole about half an inch or so from the end. The next step is to clear out the wood between the hole and the end of the upright. Cut a slot from the end to the hole so you can have a little room to get your knife in there and carve it out. Alternately, if you don't mind it being hard to get the arm out, you can just drill a hole a little bigger and thread the axle straight through the hole.
Step 5: Frame Assembly - Part 1
Pin the uprights to the frame extension rails as shown in the picture. Now is also the time to attach the ramp supports if you've got them. My craft sticks weren't long enough to get all the way across bottom frame, so I supported it in the middle and at the rear.
Once that's all together, you can mount the uprights to the rest of the frame. The frame extension rails run right along on top of the main frame rails. Just glue and pin them in place. Things are starting to shape up.
Step 6: Frame Assembly - Part 2
Step 7: Throwing Arm Assembly
The main axels are made from 3/16" steel rod. You could use aluminum if you'd like. It'll be easier to cut but usually costs more. I suppose you could even use a wooden dowel but you'll get a bit more friction. Check the distance between your uprights and cut an appropriate length of rod to serve as your primary axel. Tap it through the hole in your throwing arm. Cut another short piece to go between the lower arms to support your counterweight.
Then it's time to add the release pin. This is the business end of the trebuchet and it's important to mount the pin securely. I used a little length of wire from a jumbo paper clip as my pin. The 1/16" hole I originally drilled to accomodate my pin proved too large. Forcing a bit from a smaller paper clip into the hole along with it made for a nice tight fit. You may even get away with chucking a bit of wire right into your drill and trying to drive it directly into the end of the arm.
Step 8: Finishing It Up
Then glue down two more sticks to the ramp blocks to make a channel for your payload. If you didn't do the blocks, at least glue a couple of sticks down flat so your projectile doesn't get caught in the frame during launch.
You'll need a weight. It can be just about anything, coins, batteries, stones, etc. What I've found works well, though, is lead fishing sinkers. Specifically, I melted down a bunch of them and put a loop of picture wire in it to make a tidy little weight. Then I sprayed it with a spray-on rubber coating to protect it and make it look nice.
The size of your weight will determine how far your treb will throw. With an eight ounce weight, it'll throw a small binder clip about ten or twelve feet at a height of about four and a half feet. A twelve ounce weight gets a height of about six feet or so with a comparable increase in distance. At a pound or more, it gets to be more than you can practically use indoors unless you're throwing things that are potentionally painfully heavy or you have very large rooms. Of course, with a heavier payload, you'll need a heavier counterweight to get the same distance.
Once you get something together, hang your weight from the the axel between the lower arms of your trebuchet.
Finally, find something to shoot. I haven't found a design for a sling that works well at this size so I just tie a cord to whatever it is I want to chuck. A good length to start with is about four or so inches. Experiment to find what works best with what you're trying to throw.
Step 9: Fire!
If everything is well, your whatzit will go launching off into the near distance. If your launch didn't go so smoothly, you may be able to do a few things to help it. If your launch is too high or too low, you can adjust your pin or the length of the cord on your projectile.
And there you have it. The whole thing is probably over engineered but it looks nice and works well. In fact, if you wanted to get crazy, the thing could probably hold pretty much as much weight as you could fit though the uprights, easily five pounds or more.
Thanks for looking. This is my first Instructable so I hope it's clear enough for you to make one for yourself if you like it. Please post your comments with any feedback.