Introduction: Build a Crossbow for an Awesome Lesson on Newton's Laws!
In this instructable, we are going to build a crossbow that can launch 3-foot sections of 1/2" PVC pipe up to 30 feet! The crossbow will have about 40 lbs. of force (tested using a luggage scale), and can be adjusted down to 20 lbs. so that students can see the effect of different amounts of force. If you are concerned that you cannot pull the bow back, don't worry! There is an easy loading and easy firing feature built right in!
This crossbow was built for an 8th grade science class in order to teach Newton's laws, and not only did the activity show how different masses or forces changed the acceleration of the PVC bolts, but more importantly, the kids loved it!
Step 1: Safety First!
We are going to be working with wood, PVC pipe, rope, and a variety of hand tools. Make sure you know how to use each tool, and know the proper safety equipment and procedures. Safety glasses and a dust mask are the absolute minimum!
Step 2: Acquire Materials
For needed parts, you will need to get the following from your local hardware store:
1: 2" by 3" board, 8' long
1: 2" by 2" board, 4' long
1: 1" by 2" board, 4' long
2: 1/2" PVC pipe, 10' long (firing bows + 3 bolts)
3: 1/2" PVC slip caps
1: 1/4" braided nylon rope, 50 feet (minimum 60 lb. workload, more is better)
*optional: you can buy paracord for extra strength, but it will add significant cost!
3: caribiners (the 150lb. "not for climbing" ones are fine, once again 60 lb.+ is the minimum working load!)
1: 90 degree angle bracket, ~5-6 inches on each side
2: 1/4" by 6" carriage bolts (threaded all the way)
3: 1/4" by 2" bolts (threaded all the way)
5: 1/4" washers
5: 1/4" locking washers
5: 1/4" nuts
10: 2" wood screws
8: 2 1/2" or 3" wood screws
Miter box and saw
Pencil (mark wood)
Sharpie marker (mark PVC)
1/4" and 1/2" drill bits
1" hole saw
Pipe wrench pliers or a second adjustable wrench
Tool for prying wood out of hole saw
A pair of clamps to hold wood down
Step 3: Cut Materials Into Crossbow Parts!
Let's get cutting!
For most of the work you will need the pictured tools. Cut the 2" x 3" board into two 4' pieces, and cut two 4' pieces of PVC pipe from the first 10' piece. Mark the middle of both pipes all the way around. Next, cut the 1" x 2" board into a 12" piece, a 6" piece, two 3" pieces, and two 2" pieces.
Cut two ~2 1/2" pieces from the 2" x 2" board so that the pieces are flush with your 2" x 3" boards.
Sand it all down!
Any time you make a cut, sand the edges smooth with the file! This will not only make the crossbow look much nicer, but it will also dramatically reduce wear on the ropes, and the chance of splinters!
Step 4: Build the Trigger Mechanism
Cut the trigger mechanism
Cut one more piece from the 2" x 2" board, about 1 1/8 inch long. When we build the crossbow frame, this piece will need to easily slide between our 2" x 3" boards when they are put together as shown in Picture 2. This final piece is the action piece of the "trigger mechanism."
Drill a 1/2" hole into the block about halfway through, as shown, and then drill a 1/4" hole all the way through (Picture 1). Use the remaining parts of the 1" x 2" and 2" x 2" boards and a clamp to hold the trigger mechanism in place (Picture 3). Cut a V so it looks like the 4th picture. This is by far the trickiest cut of the entire project!
Assemble the trigger mechanism
Put a 1/4" bolt through the angle bracket as shown in picture 5. Use the adjustable wrench to tighten. Note that you can put enough force on when tightening to make the head of the bolt dig into the baord! You want this to happen; but don't go too far or the action piece will break!
Now, mark where you will drill 1/4" holes in the 6" piece of 1" x 2" board (Picture 6). Drill the holes, and put the bolts through them so that the bolt heads are on the wood side (Picture 7). Use a twist tie or piece of wire to complete your awesome trigger! It should look something like Picture 8.
Step 5: Assemble the Crossbow Frame and Bows
Gather the tools shown in Picture 1. Arrange the 4' board and the 2 1/2" 2"x2" blocks as shown in Picture 2, and use the longer (2 1/2" or 3") screws to hold the boards and blocks together. You should have a nice rectangle shape with a long channel in between. Use the electric drill and a 1 inch hole saw to drill holes through both boards at 4 inches and 7 inches away from the front edge of your crossbow (Picture 3). The resulting hole pieces can be sanded down and used in other projects (Picture 4).
Now, draw a line 26 inches away from the front edge of the crossbow. Put the two 3 inch blocks of 1" x 2" wood so that they just cover the line (Picture 5), and secure them in place with three 2" screws each. The blocks will hold the rope when the crossbow is loaded, so make sure the edges and corners are well sanded! Take the two 2" blocks, and put them on the front of the crossbow as shown in Picture 6. These will hold the bolt in place when the bow is loaded, and should be positioned so that the bolt can easily slide all the way through. Secure these blocks with two 2" screws each.
Pipes and ropes
Finally, the pipes. Slide the pipes through the 1" holes we just cut with the hole saw. The middle marks should be visible between the two long boards of the frame (Picture 6). Cut two 7 foot pieces of rope, and thread a rope through each pipe (this might be easier to do before putting the pipes into the crossbow frame). Tie a bowline knot or similar non-slip knot at the ends of each rope (Picture 7). Connect the ends of the rope for both pipes with a carbiner. Finally, cut one more piece of rope, and tie bowlines in both ends of it, Adjust this middle rope so that the ends hook on to the caribiners from either side. When pulled tight, it should look like pictures 8 and 9. Note that you can also disconnect the ropes with the caribiners, so that you only fire with one rod instead of two.
Step 6: Add the Trigger and Loading Arm
If you pull the rope tight at this point, then you will only be able to release it by pulling it up by hand! To install the trigger, drill a 1/4" hole about 23 inches back from the front edge of the crossbow. Drill 1 inch down from the top of the boards. Use Pictures 1 and 2 as a guide of what it should look like; the action piece should be able to push the rope upward to release it. Push a 6" carriage bolt through this hole, and secure it with a washer, locking washer, and nut. Be careful not to over-tighten and squeeze the boards! Lay the angle bracket over the carriage bolt, and secure it with wire or a twist tie. You now have a trigger!
Since the bow requires 40 lbs. of force to load, this might prove hard for some teachers (BTW, I will mention it again later, but students should NOT load the crossbow! If doing this at home, adults should load the crossbow.) to load, so let's build a loading arm! Drill a 1/4" hole about 12 inches away from the BACK of the crossbow, in the middle of the boards. Drill another 1/4" hole about 1 inch from the bottom of the 12" piece of 1" x 2" wood. Drill a 1/2" hole in the top of that piece also. Push a 6" carriage bolt through the 1/4" holes so that the 12" piece is secured as shown in Picture 3, and secure the carriage bolt with a washer, locking washer, and nut. Just like the trigger, be careful not to over-tighten and squeeze the boards!
Now, push a piece of rope through the 1/2" hole, and tie on a caribiner (Picture 3, the second caribiner pictured ended up being overkill). The rope length should be just long enough that the firing rope can be pulled into place when the loading arm is pulled all the way back (Picture 4). Note that you will need to pull the firing rope a little bit to get it to attach to the loading arm's caribiner (Picture 5).
Finally, you can tie a loop of rope around the trigger, and have the loading arm connect to that loop so you can pull the trigger from the top instead of reaching under, which is not necessary but may help make firing easier!
Step 7: Time to Fire!
Make the bolts
Cut the remaining 10' PVC pipe into three 3' pipes. You can use the remaining foot of pipe in another project. Push the end caps onto the pipe; I didn't need to cement these. Use the file to "cut" notches into the other side of the pipe (Picture 2). This part will hold onto the rope. I later learned that it is useful to wrap a little duct tape on the notched end also, to minimize cracking after the bolts have been repeatedly fired.
Load and fire the crossbow
Picture 3 through 8 illustrate the entire process. Connect the loading arm's carbiner to the firing rope, and pull it back to position the rope onto the two 3" blocks where it will hold in place. Be very careful not to put your hands in front of the rope as much as possible! Disconnect the loading arm caribiner, and fit the notched end of the bolt onto the rope. Aim for a target board, and pull the trigger back. The bolt may hit with enough force to punch a hole through cardboard and particle board! Make sure that you have something that will stop the bolts if they miss the target.
Step 8: Cleanup and Storage
As my father always said, "Clean as you go!" Clean up your working area, and return the tools. To store your crossbow, disconnect the pipe ropes using the caribiners, and pull the pipes out. The crossbow takes up surprisingly little space!
Step 9: Acknowldgements
I would like to thank the Ames MakerSpace for the workshop and tools, Symbi GK12 for providing the opportunity to teach 8th grade science, and my own site, LaboratoryMike.com, since that project is what is inspiring me to publish directions to all of my science-related activities!