Have you ever dreamed of building a robot that could destroy walls and besiege castles? Well, it is not exactly that powerful or large. In fact the whole thing fits on top of a Roomba. The machines complement each other very well as the Roomba provides mobility and the ability to swivel for the catapult and the catapult provides defense for the Roomba.

Materials:

Two 12" 1 1/8 x 1 1/2 wood beams
Four 6" 1 1/8 x 1 1/2 wood beams
One 8" 1/2 x 1 1/2 wood
One 1/2" PVC pipe, length: 12"
One Roomba
One cup to hold the ammunition, 3d printed
16 3D printed blocks
2 large 3D printed balls
A box of 3" screws
Rope, bungee cord, etc.
Paint (optional)
Two Plywood Triangles 5" x 6" 83/500

Tools:

Cordless drill
Table Saw
Miter Saw
Clamps
Paintbrush

Step 1: Step 1: 3-D Printing

Design a sphere anywhere from 1" to 1.5" in diameter to be used as ammunition for the catapult. Print multiple for best efficiency at around 20% density and with support structure. If you do not have access to a 3D printer, ping pong balls or lightweight golf balls will suffice.
Design a cup or basket to hold the ammunition. This should be able to fit nicely over the PVC pipe, take the outer diameter of the pipe and add a little bit of distance to ensure that it will fit nicely even with glue. We used a 0.875" diameter circle as our model. The basket should be shallow enough to launch the ammunition properly but deep enough to hold it. If you do not have access to a printer, the bottom part of a plastic cup should suffice.
Design a simple block with dimensions similar to a domino, 1 x 2 x 0.5 inches approximately. Print around sixteen of these blocks at 10% density to make them slightly lighter and easier to knock over. Light wood blocks such as balsa wood blocks will work if a 3D printer is not available.

Step 2: Step 2: Building the Base

Lay one of the 12" pieces flat and set one of the 6" pieces perpendicular to the 12" five inches away from an end.Then screw the 5" in place.
Align the triangle to where the bases are lined up with the two boards. Then screw the triangle in all three corners.
Repeat for the other side of the base.
Now take two of the 5" pieces and position one at the end closer to the triangles and the other at the opposite end. Then take two screws to drill through one side of the base to the 5" and repeat for the other side and then the other board.
Attach the 8" board to the back of the two 5" to form the backboard.

Step 3: Step 3: Building the Arm

Take the PVC pipe and drill a 9/32" hole through it near one end
Take the cup we printed earlier and attach it to the other end of the pipe with Gorilla Glue and let it set
Drill a 13/32" hole in the left and right sides of the base. Make sure that they match up relatively
Add padding to the cross brace so that the throwing arm and the catapult itself lasts longer. We used sponge duct tapped to the catapult.

Step 4: Step 4:Applying Torsion

You will need about 7-8 feet of rope but you can use more.
Tie a clove hitch around one of the dowels and put the string through the left base hole, through the hole drilled in the pipe, out the right base hole and make a loop around the other dowel.
Then take the rope back through all the holes mentioned before, and repeat until there are four strings going through.
Now take the rope back through the left hole and go over the pipe and through the right hole and around the dowel. After making a loop on the dowel, go back through the hole and go under the pipe. Do this as many times as possible.
When you get towards the end of the rope, tie it to the closest dowel.
To make the catapult work, twist the dowel rods as tight as possible. Make sure that they are turned the same amount of times.

Step 5: Step 5:Mounting It to the Roomba

Try to center the catapult on the roomba.
Attach zipties to the handle of the roomba and the front of the catapult. Then attach one or two in the back of the roomba and catapult.