I've made lots of advances toward reliable and safe homemade air blasters in the past. After seeing Ice Nine's work on an exhaust valve homemade, however, I noticed there was still a need for doing things cheaply. After all, the valves being used cost a pretty penny. So, here I am with new construction methods and observations.
*These instructions are used for a backpressure tank roughly 1.66" in diameter. With some thought and a few changes in the process, it can be used for other sizes, as well as other kinds of firing chambers.
Our first task is the most time consuming one, as well. We need to make washers for the ends of the tank. Due to bonding concerns, I only want to use PVC or another material that can be easily bonded together. So no metal washers, or nylon washers.
You'll need to buy a large diameter PVC pressure pipe (Sch.40 2" pipe at minimum). DO NOT USE FOAM CORE PVC. Some large hardware chains might have pressure pipe that large, but often it just depends on the store in question. Be sure to also check some smaller chains like Do-It-Best, which may allow free ship-to-store on things not normally in stock.
After cutting the pipe lengthwise into two or three equal arcs, you're going to flatten it into sheets. There are several methods, two of which I'll link HERE and HERE. Personally, I use the top surface of the woodburning stove in the basement during the winter, placing oven bricks on top of the PVC as it bends. Works like a charm.
Once you have your sheets of PVC, head over to a drill press and use a hole saw to make 1.5" diameter washers. Or, more correctly, to make them as close to 1.5" without going under. Once you get the washers out, use your drill bits to expand the center holes until you get to 5/8".
Using a large file, go evenly around your washers until you get to 1.5" OD. Use calipers to confirm and see what edges still need worn away. DO NOT BE SURPRISED IF YOUR FIRST ONE IS OFF. It's all part of the learning process. Just go around the edge and keep turning the washer in place.
Now we can proceed with assembly! Take a short length of 1/2" CPVC, and drive it into a half inch long piece of Sch.40 1/2" PVC. You may need to ream the inside edge of the PVC before hammering the CPVC inside. Slide one washer over the CPVC, then hammer on another piece of 1/2" PVC to make a washer sandwich. You can now attach your air tank material (in this case, thinwall 1 1/4" PVC).
If you have any available (check Home Depot), 1/2" PVC/CPVC adapter bushings will work wonders for assembly, instead of hammering pipe sections together. Once you've lined things up as straight as you can (I would stand the whole thing on end, and then place a level on top), you can use solvent weld to bond things together. I just want you to see how things assemble before you "glue" them.
In previous projects, I used a 3/4" CPVC cap as my piston, with foam padding glued in place to help seal things up. This time, I've switched over to 7/8" plastic leg tips.
Since I was making PVC washers with large enough holes for CPVC, I made things easy for the front end. I used a copper reducing coupling as the surface the piston will seal against, with a decently large difference in cross-section to ensure the piston fires properly.
Fully assembled, the basic blaster looks like this!
As you can see, I've used this ~$7 shower diverting valve as my three-way valve. According to the manufacturer (I emailed them), it handles water pressures of 50-60 psi just fine. Given that you really don't need pressures over 30 for Nerf applications, it should work well. Also, it would make it extremely easy for adding on an air reservoir for semiauto capabilities. You need to spring-load the valve, though, so that it will return to its starting position.
While I'm fiddling around with different setups (I've been chopping up and recombining parts to see what I could do), I thought I'd also give these observations.
It's actually not too hard to make something akin to a 1st-gen Panther or an Exteme Blastzooka tank. Danco sealing washers generally have a 3/16" inner diameter, so using one at either end of a 3/16" diameter rod or tube lets you reclaim volume taken up by the CPVC. It also means that, should you use that drill bit with your hole saw, you have an easy way to make firing-pin tanks.
That's all for now; I'm still just experimenting with tank construction. But it's not that hard to make a solid air tank. Just be sure to have some kind of OPV or something designed to fail (like using vinyl tubing to connect things) at a safe pressure.