Introduction: Spider Hydrolics

Our catapult has a simple lever throwing arm. The stoping arm is adjustable: more blocks can be added to its length which in turn changes the angle at which the throwing arm releases the projectile. The arm's forward thrust comes from the torsion of the rope as well as from the stretch cord around the arm's neck pulling it forward. The arches will be marked then used as a guide to how far we pull back the arm. This will give us maximal accuracy.

Step 1: Materials and Methods Overview

What we used:

4' by 4' by .75" sheet of plywood



ShopBot CNC




Wood glue

Plastic cups


Elastic/Bungee cord

Step 2: Designing Our Catapult

We used Sketchup to design our catapult. We made fifteen pieces in total. Our first pieces were two identical sides to support the catapult.These sides were two feet high, two and a half feet wide, with a thickness of .75 inches. The front of these pieces curved downward from the highest point in the middle. An arc that reaches across the midpoint to the back served as a guide for our throwing arm. The next pieces were two identical 9" by 4" by .75" rectangles. These pieces separated the sides and add support in the front and back. Our throwing arm was a 3' by 2" by .75" rectangle that we planned to fasten directly between the midpoints of the side pieces. We decided to design the stopping arm (the bar that stops the throwing arm and determines the angle at which the projectile leaves the catapult) to be adjustable (see first image). The four horizontal bars can be slid in and out between the two bars on either side in order to change the angle at which the throwing arms stops. The more bars we remove, the larger the angle of the throwing arm will be. The second photo is a complete image of the SketchUp design of our catapult.

Step 3: Formatting Our Design for CNC

After we completed our 3D catapult design, we needed to format it in a way that CNC would be able to cut out our parts. To do this, we made a copy of our entire catapult on SketchUp and laid the copy's components flat within a 4' by 4' by .75" rectangle. That way, on our final SketchUp file, we had both the 3D model as well as the 2D version that CNC would cut.

Step 4: ​Exporting

We screenshot the 2D version of our catapult directly from above and exported it as a .dxf file . However, we later discovered that the .dxf file was less compatible with CNC's program than a .pdf would have been. With the .dxf, the CNC workers had to take a few additional steps to remove extra geometry that resulted from the .dxf.

Step 5: Run CNC-Shopbot

The file was finally converted to a VCarve file. The CNC ShopBot then read the VCarve file and cut out our flat pieces for us to assemble according to our original 3D plan. The ShopBot is able to cut two catapults from one 4' by 8' sheet of plywood: a 4' by 4' square per catapult.

Step 6: Assembly of the Catapult

The last step was to assemble the catapult. We began by sanding down all the sharp edges and corners of our parts. Then we wood glued all the pieces according to our 3D diagram EXCEPT for our adjustable pieces. We were extra careful around the area where the adjustable pieces were going to be held. Next we screwed all the pieces together as well for maximal stability.Then we took a string about two feet long and put it through the small 1/4 inch whole in one side, through one whole in the arm, then through the whole on the other side, around a small block, and back through all those wholes with a knot on the one end to secure it. Then we twisted this string blocks toward the front of the catapult for torsion (the first two images show this torsion string completed.) After making the torsion hinge, we added a 9 inch bar at the bottom of the catapult directly under the top bar for extra support of the sides. We also added an elastic/bungie cord at the top of the bar for more power. Last. we screwed a cup onto the arm to hold our projectile. Our catapult was now complete!