Step 2: build the catapult axle
next, build up the cross member to provide support for the axle. The length isn't a big deal, just use some scrap left over from cutting the cross members. I simply stapled these on. it takes a little finesse to get the staple gun in the right position. careful! watch your fingers!
take a dowel that is at least as long as the long members of the base - this will be the catapult arm. drill a hole in its end - through which you'll put the pipe that will become the axle. i zip tied the dowel above and below the drilled spot to keep it from splintering. (looking back on this step, i suspect you could just as easily zip tie the dowel to the axle without any need to drill a hole.)
center the dowel, then zip tie both sides to keep it from moving cross-wise on the axle.
hold the pipe in place while you screw the screws into it. the screws are snug to the wood, but not the pipe.
The screws i used were well over 1 inch long - providing a pivot point for the wind chime pipe to rotate on. That is to say that the pipe (the axle) is not secured to the catapult. it spins freely on the screws that protrude inside the pipe.