Introduction: How to Build a Catapult

About: I'm a just a not-so-average human with a passion for making. I see a world of hackers, makers, and producers as the sun rises in the near future. I'm starting olopede, an educational electronics kits compa…

There are already a few large-scale Trebuchet Instructables out there, but for the low-tech medieval enthusiasts, here is the first large-scale catapult Instructable!

This was constructed during my internship @ Instructables this past summer, with
DerangedMoose7, Tetranitrate, and noahw

Oh, and if you were wondering... Yes, "Project X" is finally here.

Camera work done by [ noahw]
Video editing done by [ bofthem]

Step 1: Acquire Materials

You're going to need:
  • 9 - 8' two-by-four's
  • 1 - 12' four-by-four
  • 3 - 8' four-by-four
  • 50' of surgical tubing
  • Fiberglass windsurfing pole or a large section of steel pipe (Fiberglass pole preferred)
  • 20' thick nylon rope
  • 1- 4 ft section of 4" diameter ABS pipe
  • 1 -4 ft section of 2.5" diameter ABS pipe
  • 2 - 4"-2.5" step down ABS adapter
  • T-adapter for ABS pipe
  • ABS cement
  • 2" X 4' aluminum pipe
  • "S" hook
  • "U" bolts and nuts
  • Large Eye bolts
  • 12 - 6" lag bolts
  • 3 - 5" carriage bolts, nuts, and washers
  • Assorted deck screws (at least 3" long)

  • Chop Saw
  • Reciprocating saw
  • Drill Press
  • Assorted Drill Bits (including large hole-saw or adjustable spade bit)
  • Drill
  • Wrenches
  • Welder (if using steel pole)

Step 2: Plan Out the Frame

As Jason and I walked the shop floor, trying to picture how to layout the frame, we came up with this:

  • The base of the frame would be 3' x 8' and made out of 2X4's
  • The frame would be shaped like an off-balance "A"
  • The frame will be 6' tall

Lay out two of the 8' 2X4's on the floor, and space them 4' apart.

Cut two 3' sections of 2x4's on the miter saw and lay one of them on one end of the frame.
Lay the other 3' section about 2' in from the end of the 8' sections, this is so that when the arm comes forward, it doesn't throw the whole 'pult forward as well.

Cut the 4X4's to 6' in length, and stand them up so that they are on the inside of the frame and flush up the indented 2x4.

Cut one length of 4x4 to 3' and save it for later - this will be used as the cross-brace at the top.

Step 3: Start Screwing Stuff Together

Assemble the base of the frame first -
Use 3" screws and don't forget to pre-drill.
You should use between 2 and 3 screws at each joint, we don't want this falling apart.

Next, use your lag bolts to connect the 4x4's to the base of the frame.
You're going to need to pre-drill these, otherwise you'll never get them in the wood.
You' should use two bolts per post, one through the side of the base frame, and the other through the "indented" 2x4.

Cut four 88" pieces of 2x4 to use as the back braces. You'll need to miter them by 60 degrees on one end.
Once they're cut, you can make two pairs of them by screwing two of them together - then screw them into the frame. Remember: Pre-drill!

Cut four 1' pieces of 2x4 as corner braces, and miter them by 45 degrees on both ends. Screw them into the corners of the frame. (See pictures)

Step 4: The Cross-Brace

So we originally planned to have a cross-brace, which would stop the catapult arm from aiming down and force the release of the projectile at a 90 degree angle to the ground.
Later on, we found that this really didn't work, and ended breaking a really nice fiberglass pole because of it (and the insane momentum it had).

If you want to go true catapult, the cross-brace is essential, but I highly advise against it.

If you don't want a cross-brace, you'll have to get two more u-bolts and bolt them to the top of the posts so that they face eachother. Otherwise, follow the instructions below.

To install a cross-brace, first pre-drill four holes in that leftover 3' 4x4 that you cut earlier (two on each side), after clamping it to the top of the two 4x4 posts we put up in the last step. The cross-brace's top should be flush with the top of the two posts, and should be closer to the shorter side of the base of the frame.

Now, start screwing your lag bolts into the cross-brace piece. You can take it down from the posts now if it makes it any easier. Keep turning until the little pointy part sticks out of the other end of the wood, then re-clamp it and finish up. Turning lag bolts in 4x4's is pretty annoying, isn't it?

Drill two huge (but not too big... say 1.5") holes into the cross-brace on both sides. This is there the surgical tubing will go, so try and make it nice and smooth. Our tubing ripped a few times because the holes had sharp wood daggers eating the tubing. It's pretty scary when one of those snaps at full tension.

Also, on one side, install the other U-bolt, facing inwards, through one of the posts - this will be a tie-off point.

Add a ton of padding to the cross brace - you'll need this if you don't want the catapult/arm to break.

Step 5: Drilling the Posts

So far, we've created a pretty sturdy frame - time to get crazy!

We found this awesome 2" aluminum round "U" channel stock that was perfect for an axle, but there was no way of cutting a hole that size through the 4x4 post, so we went down to the local hardware store and picked up an adjustable spade bit! It was "hella" expensive, but we beat the system - if you use it once, and return it... you don't put a hole in your wallet, and you get a big gaping hole in that 4x4!

The whole drilling process was exciting, kind of like watching an auger, but slightly less insane.

Anyhow, this steps self-explanatory - just remember to wear goggles/glasses and go straight into the post! If you're off by even a little bit, you're going to need to open up the hole even more, and that will weaken the posts even more.

When you're done, slide the aluminum stock into place.

Step 6: Constructing the Rotating Arm Holder

Now that we have a nice, smooth axle, we need something that will rotate about the axle and hold the arm. Sounds like a job for the Rotating Arm Holder!

Get your ABS T-adapter, 2 sections of ABS pipe, and ABS cement.
You might want to wear a respiratory mask and GLOVES for this one, too - that cement stuff is narsty!

In this case, we're going to take the 4" pipe and cement it to the base of the "T", and on each side of the "t" we'll cement in a 4"-2.5" adapter and half of the 2.5" pipe (2' in each side).

To use ABS cement, first try and make sure that your surfaces are nice, clean, and smooth.
Unscrew the applicator from the can and wipe it on the edge of the can - the applicator tends to pick up way more cement than you actually need, and you don't want to waste any, do you?
Spread the cement on the inside of the T adapter, and on the outside of whatever you're putting inside the adapter. You want a nice, even coat on both members, and don't apply too much. The parts are going to be a tight fit, and the cement sets relatively quickly, so you have to apply a lot of pressure to make sure your members are in their correct places.

When you're done, it should look similar to the 2nd to last picture in this step.

Now, compare it to the width of your 'pult, center it, mark the edges, and cut off the excess 2.5" ABS.
I would take this opportunity to drill two holes, one 1/3 up the 4" pipe and the other 2/3 up the 4" pipe, one perpendicular to the other. The holes should be a bit bigger than your carriage bolts, and will be used to hold the arm in place.

Slide out the aluminum pipe, and slide it back on so that the Rotating Arm Holder is able to Rotate about it.

Step 7: Make the Winding Spool, Rope, and Firing Pin

A few feet in from the back of the catapult, on the upward sloping braces, drill a hole through each brace, so that a metal pipe can fit though the holes. Cut and place a metal pipe into the holes, and leave a bit sticking out on each end - this will be the spool. Weld another pipe onto the spool, as to create a means to wind the spool with some sort of crude metal handle.

Tie one end of the nylon rope around the spool and the other end around the S-hook. You'll use this later to pull down the arm, then unhook it to prepare to fire.

To make the firing pin, you're going to need a sturdy hinge, some more steel pipe, and any carriage bolt or smooth piece of metal. If you use the carriage bolt, you'll have to grind it down so that it's smooth. Weld the steel pipe to one side of the hinge, and then weld the carriage bolt perpendicular to the end of the pipe that's not attached to the hinge and that the hinge moves in a direction opposite the side that the bolt/smooth piece of metal is welded on. These welds should be X-TREMELY strong - this is critikal. The bolt or smooth piece of metal should be slightly slanted downwards (see picture), and the pipe should be no more than 2.5' long. Sorry about the run-ons

Screw the not-welded part of the hinge into the back of the catapult so that the pipe is in the center of the back of the catapult.

Step 8: Prepping the Arm

Before we attach the arm to the catapult, we need to drill quite a few holes in it.

Depending on whether you're using a non-metallic arm or a metallic arm, there are two different ways to go about this.

Metallic Arm:
Start by getting really long lengths of steel pipe. I think the total length of the steel arm that I made was 14', which had an 8' double-barrel base (2 pipes welded next to eachother) and a 6' top piece. Basically, just weld the pipes together, putting about 2 inch welds every 4 inches or so. When you're done, weld a U-bolt onto the arm about 68" from the bottom of the arm, and a large eye bolt right below it - the u-bolt will hold the surgical tubing in place and the eye bolt will be the winding catch. About a foot and a half up, weld another large eye bolt - this will be the firing pin catch.
At the very tippy top of the arm, welt a bolt bent at a 45 degree angle to the top. Make sure that the bolt is facing the opposite way that the u-bolt and eye bolts are facing (which should all be facing the same direction).

Non-metallic Arm (fiberglass)

You're going to need a drill press and a whole bunch of drill bits. Since you're drilling into fiberglass, you have to slowly step through all of the sizes of the bits, until you reach the final bit.

You'll need to make out two holes for the u-bolt about 68" from the base of the pole, a hole for the eye bolt (winding catch) a few inches below that, and a hole for the other eye bolt (firing pin catch) about a foot or so above that.


Place the arm in its holder and mark where the holes are on the arm. Drill out the arm and test out how it fits. It should be pretty snug, and not able to move very much. This is GOOD!

You should check all of your measurements and hole alignments before drilling by putting the arm in its holder. The u-bolt should line up with the cross-brace, the winding catch should line up close to the winding "spool", and the firing pin catch should line up with the firing pin (this is a must).

Step 9: Fitting the Arm

This is easy - just slide the arm into the Rotating Arm Holder and put the carriage bolts through the holes you drilled already.

The best part about having a Rotating Arm Holder is that you can easily change arms within a few minutes.

Step 10: Make 'er Mobile!

If you want - and we really wanted to, you can add wheels to make the catapult easier to transport - you know, for all of those medieval battles you get into so often.

We found some wheels and axles on an old rusty trailer in the junk pile. We weren't sure if they were up for grabs or not, so we asked around and (if my memory serves me) Tim gave us the green light! We ripped the whole thing off of the trailer and eagerly screwed it into the bottom of the back of our near-finished catapult. We secured it with scrap wood galore, and added some 4x4 stilts to the front of the catapult to keep it level.

To make sure the wheels work, we tugged it around with a forklift for a bit! Hooray for fun!

Step 11: Tie 'im Up! (With Elastic?)

Get out your surgical tubing and tie one end off on the U-bolt that you bolted to one of the posts.
Weave the surgical tubing from the U-bolt through the U-bolt on the arm, through the hole on the other post, back through the hole on the post with the U-bolt, and continue the pattern (you don't need to tie it off any more). Make your windings tight, but don't stretch it, just try and minimize slack.
Generally, 3 winds works pretty well. With a heavier bar, use 4-6 winds.
Tie the other end off either on the U-bolt attached to the post or somewhere else - your choice.

Make sure you use good knots - you don't want this slipping under high tension.

Step 12: Operation: X

Here's how to play with this big toy:

Take the winding rope and hook it onto the winding catch. Turn the spool until the arm is far down enough to be locked into place by the firing pin.

Lock the arm in place with the firing pin, load your projectile, and take off the winding rope.

For dramatic purposes, yell "Fire in the hole!" twice, like how the bomb squad does it in Mythbusters.

Stand clear, and pull the firing pin out either with a rope or by large 2xz4 (might I suggest the rope).

Gaze as you lose sight of your projectile, then jump for joy and try to go find it!

Step 13: Test It Out!


You're done! (sort of)

You have to test out your catapult now, to make sure that it's not going to break, and that it works well.
We tested ours here too, and found that it really wasn't that great as is. In fact, it pretty much stunk - although we had results.

The problem? That goddarn cross-brace shattered our first arm (fiberglass), and kept on bending our second one (steel). Also, it caused the arm to slow too quickly, because it actually released the projectile (which was hung on a string) way too early.

Here's some photos of our first few tests, including its public debut at the Squid Labs Intern Barbecue! Featuring......: Eric!

Step 14: Fix It!

We decided to scrap the cross brace at this point - after deciding that it was our main problem, and ripped it open with a "sawsall".

After doing so, we tested it out again, and got AWESOME results! Then we switched to a fiberglass arm, and got even better results! (if you're using a fiberglass pole, a bunch of tape works really well as the ammo holder.)

Although, we had to make some adjustments.

We were kind of worried about drilling into the new fiberglass pole, so instead of putting all of those bolts through it, we just used some rope to make loops. The surgical tubing was content without having something to keep it where we wanted it, so we didn't even bother making a restraint for that.

We added a 12' 4x4 across the bottom back of the posts, and then a diagonal support from the end of the 12' "back brace" to the tops of the posts. Simple enough, and strong enough to take one hell of a beating.

Well - that's it!


Step 15: Throw Some Freaking Awesome Stuff!

You've gotta throw some cool stuff at this point, which led us to throw items such as:

  • Metal pipes
  • Tomatoes
  • Apples
  • Oranges
  • Frozen cantaloupe
  • Pseudo-flaming metal pipes
  • Champagne bottles
  • Scrap wood

Our personal record was somewhere around 100 yards, with a metal pipe, which isn't too shabby. In the video, you can see those 18-wheelers in the background... yeah, we hit those =).
Oh man - those were such good times!