Step 6: Attach the Actuated Gear
First, cut out a hole slightly larger than the aperture in the center of the brass gear whose diameter is larger than the iris aperture's OD. By slightly, I mean somewhere between 1/100" and 1/32". To do this, I drilled out the center of my gear and mounted it to a plastic disk with a screw. I then put this in a lathe chuck and turned out the center of the gear.
Next, cut a slit the width of the iris aperture's pin into the brass gear. The slit should be along a radius of the gear, so it may be helpful to cut the slit first, depending on your method of cutting the hole. I used a bandsaw to cut the slit with pretty poor accuracy, but I still had a couple of 64ths of an inch to play with. To correct for this, I sanded down the gear until the slit looked nearly radial.
Finally, place the ring-gear around the aperture so that the actuator pin fits within the slot in the ring-gear. Stand-off the gear so that the gear is in the center of the actuator pin when looking at it from the top of the pin. Turn up your soldering iron and start to heat the junction. Try and heat the gear first, tin it, then move on to the pin. Tin the pin, then bridge the gap. Use flux if needed.