How to Build a Catapult

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.