Assembled with background removed
Posed on one of my two Bambu A1 printers
Valve Mechanism
Getting the valve to work took the better part of two months of troubleshooting. I originally used a much thinner o-ring (.103”) because I wanted reduced friction. Unfortunately this made it nearly impossible to seal with at the piston’s size and my printer’s tolerances. I then tried floating o-rings which did reduce sliding friction by quite a bit, but never sealed very reliably. Eventually I looked up the Parker O-ring manual and sized the piston, bore, and o-ring grooves accordingly. I also used Apple’s calculator to confirm the values from Parker. Using those dimensions, I was finally able to get a reliable air tight seal.
I designed the bore so it had an ideal seal where it would mate with the o-rings and then slightly widen when the valve opened to reduce dynamic friction.
Even with this reduced friction, I couldn’t get the spring behind the piston to apply enough force to re-seat the piston after each shot. Again, it turned out that I undersized the component. I was only using a spring with a 5 lb/in spring with a half inch of preload hoping that the initial acceleration from the bottom of the stroke would overcome the friction needed to seat the piston in the bore’s mating section. I put the cannon on a bathroom scale and pushed the piston into position and found out that I actually needed 20 lb of force. I moved up to a spring with an 11 lb/in spring rate and 2.5 in of preload and now have no issues re-seating the piston into the closed position after each shot.
Initial prototypes didn’t use a spring at all which caused the piston to accelerate way too fast. With a 12 mm 20A silicone bumper, the piston was cracking the back plate. Even after adding the final spring, the piston was breaking where the spring pressed on it. I solved this by adding a bit more material to the piston and adding a small cut out so when the piston opened, air pressure would slightly balance out the spring force instead of the large air gap I had between the piston face and piston spring surface.
Trigger Mechanism
I read that the first Quick Dump Cannon needed extra care to avoid accidentally triggering the valve when loading it. I actually used this to my advantage by using a small floating section of barrel to trigger it. There’s an interrupted breach that allows me to rotate it into a locked position for loading and seating the barrel extension. When it’s ready to fire, you rotate the barrel so the barrel can slide into the tank and manually push the piston back 10 mm until the barrel and tank face meet their stops. The piston spring also acts as a trigger spring. At this point the piston is pushed past the bore’s mating surfaces and snaps back.
Printing Settings
I use transparent Elegoo Rapid PETG. The air tank’s walls at 4.5 mm. All other small air tight components have a wall size of 2.5 mm.The stock butt pad was printed in a high flow 95A TPU.
I think I used under 2 kg of filament. Using a 0.8 mm nozzle, the print finished in about 12 hours using two printers in parallel. I use Bambi’s Support for PLA/PETG for overhang support interfaces. It uses a difference in polarity to make support removal incredibly easy with no surface defects. I modeled all of my supports in CAD to avoid unnecessary waste.
Room for Improvement[\b]
Recoil is nuts and the trigger is very heavy. I’ll likely design a custom sprinkler valve since the diaphragm will always be lighter than a piston. I’ll aim for an internal flow path diameter of 3” to ensure it’s never flow restricted.
Last thing- I’ll link the youtube video of it firing once it finishes uploading
