Hey folks! I am looking for help troubleshooting a problem with the pistons in some of my cannons. The piston keeps getting stuck in the up position, preventing the next round from loading. A firm tap on top of the cannon un-sticks the piston, but it gets stuck again 1-2 rounds later. This has been a consistently inconsistent problem in my 3d-printed cannons. Some individual cannons work perfectly every time, while others jam every 10 shots, while still others jam after every single shot. I have tried a variety of pistons, from 3d-printed ABS to injection-molded ABS to stainless steel rivets. I had best success with the SS rivets, but all three options would sometimes yield a lemon cannon. I have also tried a variety of geometries and tolerances, both beveled and flat, loose-tolerance and tight. None of that appears to make any difference. I have also tried several combinations of springs from two different manufacturers, both single springs and doubled-up springs. There was no noticeable difference between the two different springs, but doubling up made the cannons spurt a lot and failed to alleviate the problem. At this point, I have been through twenty design variations and over a hundred individual prints, and have yet to get satisfactory results. I finally gave up on a 3d-printed T-section, and designed a new cannon with a printed magazine and traditional brass T-fitting for the rest of the cannon. The result? It STILL jams, every other shot, with the piston in the up position. I'm pulling my hair out here! What is going on here, and what can I do to prevent it? This does not happen to every cannon I print. About one in four cannons works great. But I don't want to throw out three of every four prints because of an unknown fault. This is the last significant bug with my cannon design and I really want it eliminated.
Would you want to send me some of your buggy ones to play with? Hard to diagnose without having the actual gun.
It's possible that your spring isn't strong enough, or that you have a bind. Sometimes a screw/rivet is not centered, and when it's in the up position, it will bind to one side or the other. Sometimes the T isn't centered either. Fun times. Some of us resorted to using a nylon 'top hat' type thing to prevent the spring from getting crushed. It can be quite tricky sometimes.
What springs are you using? A few ideas: Comparing this gun to a standard T, the portion that aligns the piston seems much shorter (guessing around 1/8" on yours vs 1/4" on standard), which could allow the piston to get crooked and the head to jam on the side of its chamber, so I'd consider increasing that distance (and using longer piston, I use .61" shaft). If you don't drill that portion of the gun out, I would do so. Likewise, maybe increase the diameter of the chamber where the piston head is, so it can't contact the sides. The other note would be to decrease the stroke length a hair, probably unrelated but might limit how much of piston gets pinned by the next BB in line - I typically have piston cover about half of opening when in the up position
I am willing to bet it is a tolerance issue. FDM 3d printers arent perfect with extreme accuracy and repeatability. I would print it with extra perimeter walls and ream them to size .
I don't understand the second step. My guess is the spring is being "caught" in this area. Everything needs to be as smooth as a baby's bottom to avoid any impingent anywhere.
I hadn't considered smoothness of the walls. I will try under-sizing the holes and reaming them out. That'll leave a much smoother finish. I may also play around with the height of the piston hole, but I would prefer not to as that raises overall height of the cannon.
Usually when I found Pistons jam it comes to either smoothness of the walls or the bore is a little on the big side and and one part of the spring can get stuck. I see you have the three different bore diameter setup which is good I was going to mention that for only two different bore diameters if you over compress the spring I have had that also resulted jammed Pistons
On the gun I sent to you in a PM based on your design, I needed to gently ream out this area with a file to get the cannon to cycle reliably. It would occasionally (once or twice out of fifty) jam up like you are describing. After some minor clearancing, life is good.
Problem solved! I think... Based on the comments here, I made a few changes to the piston areas. I shrank down the piston slider region and spring recess, and then carefully reamed them out with the correct size drill bit after printing. I also measured the maximum compression of my springs, and reduced the height of the spring recess as much as practicable. The extra height was added to the slider region, giving it more length for alignment. I then printed one and prepped it for testing. Then I found my problem. Since both the spring recess and piston base region have flat horizontal surfaces, they require support to print properly. I had been using a 1/4" end mill to clean out leftover bits of support material from the spring recess, and a 5/16" end mill to clean out the piston base region. This works fine for the spring recess, since the end mill is a perfect fit. The piston base region, otoh, is slightly larger than 5/16", so I was accidentally creating a small step if I pressed just a little too hard during the cleaning process. This small step was just a hair smaller than the base of my pistons, so it would grab the piston when it was forced up by 120PSI gas during firing. I printed another one and used a poker for the cleanup instead. Test was perfect, 50 trigger pulls and 50 rounds, for five full magazine-loads. My next step is to print a bunch more of these, and test them all to see if the problem is truly solved.
