For my final project, I built a spring powered dart blaster (aka nerf gun). It is built primarily out of pvc plastic piping. A wooden rod with a spring and rubber washer forms a plunger that creates air pressure to launch the dart out of the barrel.
Motivation for Project
The main components are as follows:
This is made from a piece of 1.25" diameter pvc 10" long, an endcap, and a coupler. The coupler has reducing bushing glued inside it, and the barrel is attached there. The reducing bushing fits snugly in the 1.25" coupler while allowing a .5" piece of pvc to be attached.
The barrel, pictured above, is made of a piece of .5" pvc and another piece of waxy material that Smitty made with an inner diameter the same width as the width of a nerf dart.
The plunger is made from a .5" diameter wooden rod 14" long, 9.5" spring, two .75" pvc endcaps, and a 2" length of .75" pvc. The seal is created with a 1.5" diameter, 3/16" width rubber fender washer, which is screwed in with two metal washers, a screw, and a nut.
The handle is made from two 4" pieces of 1" pvc. The second piece is cut in half lengthwise, and one of the halves is hot glued to the intact piece, as shown below. Electrical tape is wrapped around the result.
The trigger, shown above, is made from one and a half clothespins, a brass ring, a screw with stripped threads, hot glue, and electrical tape. When the ring is pulled back, the clothespin opens and the screw retracts. Normally the screw rests inside a hole drilled in the pressure chamber, but when the trigger is pulled the screw comes out.
Smitty already had most of the materials, and he cut lengths of pvc of each type that I used. He also had the screws, the wooden rod, the spring, the brass ring, and all the pvc endcaps. The rest of the materials I bought at a hardware store.
I made the plunger first; this entailed drilling holes in the endcaps, putting them on the .75" pvc, screwing on the rubber washer along with two metal washers, and attaching this to the wooden rod. I initially only taped the pvc-washer piece to the wooden rod, but the rod later came out when I was testing the final blaster so I hot glued the rod to the pvc as well as taping it.
The pressure chamber only needed holes drilled in the endcaps; then the plunger went in and the endcaps went on. The barrel fit right into the reducing bushing.
Smitty cut the 1" pvc lengthwise for the handle; I glued it together and glued it to the pressure chamber. I wrapped electrical tape around the handle.
The trigger required some experimentation to find a good position in which opening the clothespin would smoothly pull the screw out; I finally found that using half of a clothespin to add some distance between the first clothespin and the pressure chamber worked well. To determine where to drill the hole in the pressure chamber for the screw to fit in, I pulled back the plunger a comfortable distance and put a rod in the front end of the pressure chamber. I marked how far in the rod went, and used that as a guideline on how far to drill the hole. I drilled the hole in the pressure chamber and in the clothespins that the screw would go through, glued and taped the brass ring to the clothespin, and glued the clothespin to the pressure chamber and the end of the screw to the clothespin. Initially the trigger always got stuck when I tried to fire the blaster, so I filed off the threads of the screw and greased it. The friction was significantly reduced, so the trigger then worked smoothly.
How it works
Pulling back the plunger compresses the spring, and when the plunger is released it shoots forward. The tight seal against the pressure chamber (given by a rubber washer; friction is reduced with grease) causes air pressure to build and launch the dart forward. The trigger must be pulled back while drawing the plunger, and then when the trigger is released again the screw enters the pressure chamber and holds the plunger steady while the spring is compressed. Pulling the trigger removes the screw from the pressure chamber, freeing the plunger to shoot forward.
The dart blaster worked very smoothly in the end. I tested the range and found that it shoots 46-54 ft. The standard factory-built nerf pistol shoots only about 40 ft.
I also tried pulling back the plunger farther than the point where the trigger catches it and releasing the plunger by hand,
but it turned out that it went about 5 ft less distance despite a longer draw. Probable reasons are that hand-releasing the plunger is slower than trigger-releasing it, and also that the hole for the trigger screw would leak a lot of air pressure when the plunger is pulled back farther than the hole.
The final, working blaster is pictured above.