You can read more about how I developed this design at ECN - DIY hot-air iron.
The comments to an article I posted to ECN Magazine a while back got me interested in making my own hot air soldering iron. A look around the web connected me with a do it yourself project to make just such an iron.
First off you will need a suitable iron. I checked the Variable Temperature Soldering Station I bought from Amazon for about $20 and found after checking it out that it was enough like the iron in the project to proceed so I did.
First you need to gather the project materials. For the iron conversion you will need about six inches of silicon rubber model airplane fuel tubing. It should be rated for 3/32" fittings. I liked this racing orange tubing. But blue/clear tubing would probably also work. This type of tubing is size rated according to the outside diameter of the metal tubing it will mate with. We will be needing such tubing in brass. One foot each of 3/32" (to mate with the fuel line) and 1/8" (to mate with the air line). You can pick it up at your nearest hubby shop. I got mine at Royal Hobby. I also bought a K&S Tubing Cutter in order to avoid ragged hacksaw edges on the cut tubing.
The first step is to open up the iron by removing the three screws that hold the heating element to the handle.
Find a suitable spot on the iron hand and drill a 3/32" hole pilot between the wires. I drilled my first hole straight into the iron (90 degs to the barrel) and then using that one as a starter hole drilled a 9/64" hole from the same point at 45 degrees to the barrel. It may be a good idea to reinstall the heater with at least one screw to keep the wires from absorbing excessive vibration during drilling. I hand drilled the final 15/64" fuel tubing hole using a pin vise/hand chuck to hold the drill. The point is to go slow so you are not drilling into the power wires at the end of the hole.
Cut off about 1" of a copper clad scouring pad (no soap please), twist it up and stuff as much as you can into the heater barrel being careful not to disturb the power wires.
You can now snake your fuel tubing through the handle and insert the (squared off) tip into the bottom of the heating element between the wires. Create an air seal where the tubing enters the heater with Permatex High Temp Red RTV. It is rated for one hour at 650°F. The area of the iron it is being used on probably gets no hotter than 300°F so the seal should last a while. The RTV will skin over in about an hour and be fully cured in 24. Let it sit.
Now put it all together. To do that you will need to cut one 1" (2.5cm) long piece of 3/32" brass tubing and another equal length of 1/8" brass tubing. Overlap them about 1/8" (3 mm) and solder, epoxy, or super glue them together. Be sure to ream the tubes out with a suitable drill if the tubing cutter necked them down. You may also have to sand down the end of the 3/32" tubing to make it fit. That little fitting will connect your iron tubing to the air line tubing.
The nozzle for the hot air iron is a Stimpson GS5-12 backed up by a 4-40 nut drilled out to 5/32".
Now we come to the question of pumps. I had an Whisper 10 (old square model) laying around and by itself it was barely adequate to give sufficient flow to melt solder paste. In tandem with an AQT3001 I also had around it was better. But the best was the Petco AC-9903 which has a flow/pressure adjustment on top. The AC-9903 is rated at 4.5 liters a minute and .014MPa. You can read more about my adventure with air pumps at DIY hot-air iron.
I used a ProsKit 9303C meter for the temperature measurements I did. I was looking for better/worse more than accuracy. And for that it was adequate.