Introduction: Wooden Desktop Trebuchet

The trebuchet is a medieval invention originally designed to lay siege to fortresses and castles. The largest of them could hurl immense stones hundreds of yards. Now you can have one to play with in your own home. The whole device stands about foot tall when unloaded and flings various things ten feet or more. Moreover, this is a nice introductory woodworking project that you can do with mostly basic tools. Make sure to check all of the images. A lot of the details are in the notes there.

Step 1: Materials and Tools

3/8" Hardwood Square
1/8" Hardwood Dowel
3/16" Metal Rod
Jumbo Craft Sticks
Wood Glue
Heavy Stuff as a Counterweight

Miter Box
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

Time to start cutting up all of that wood. We're going to need pieces of the following 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

We're going to employ a lap joint in the long frame rails (Part A). That involves removing half the thickness of the wood at each of the ends so the short sides have something to notch into. Three-eighths of an inch from each end, saw half-way through the stick. Then, using your chisel or sharp knife, cut along the grain to remove the half of the material at the end. You may find it easier to take a little at a time off so you can get it just right. Do the same thing to the other end on the same side.

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

The frame crossmember (Part C) is going to seat across both long frame rails so it needs two sections removed. Rather than measure it all out, it's easier to just clamp the four sides of the bottom frame together and lay the crossmember in grooves in the long rails. Get it lined up by eye and mark out the sections where the rails intersect. That's where you're cross-lapping the crossmember just like the rails so mates up tightly and sits flat.

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

You might be able to get away with just gluing all of the parts together but I pinned it all together with glue and poplar dowel; it's not likely to ever come apart and it adds a nice visual element. Clamp together the bottom frame and drill through where ever they overlap. Get a little glue on the joint surfaces and then put a little on the end of your dowel. and work it into your hole and cut or break the dowel off. Once it firms up a little you can shave the leftover nub off with your knife or sand it down. I suppose if you're really bored, you could precut a bunch of little 3/8" and 3/4" lengths of dowel and tap them in. Then you wouldn't need to come back and cut them off.

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

Now here's the tricky part: drilling and mounting the diagonal braces. It helps here to employ the buddy system in order to get a couple of extra hands or to use a bunch of clamps. Either way, try to get your pieces sturdily affixed in a temporary fashion and just drill carefully and slowly. Don't use a lot of pressure or you're likely to force it out of whack. If you've got some patience, you could glue the parts in place and let it dry before pinning.

Step 7: Throwing Arm Assembly

The arm is easy. The lower arms overlap the upper arms by about an inch and a quarter. You just need to line it up and glue it together. Then you need to drill out the axel holes. The axels will be about an inch and a half apart. It's important that you try to get them straight up and down as much as possible. If you have access to a drill press, I recommend you take advantage of it. You should have a little more than six inches of arm above the main axel depending on how close to the end of the lower arm that you drilled the holes for the lower axel.

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

Cut two little strips of craft stick to glue to the sides of the uprights. This will block out the main axel and keep it from sliding from side to side. Make them a little wide so you can trim them down just right.

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!

To fire your trebuchet, drop your arm into the uprights and hook the loop at the end of your projectile over the pin at the end of the throwing arm. Pull the arm down and place the projectile in the trough. When you're ready to fire, release the arm.

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.


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