For starters, this whole idea came about after I read Kotori's thread on using an online laser cutting service to build the magazine of a Big Gun style magazine. www.rcnavalcombat.com/Forum/tabid/5...fault.aspx So in my off time...aka the baby has gone to sleep and I can't work on other boat stuff, I've been working on designing a Big Gun style cannon. My goals for this project include: keeping current use features (rotation & elevation change), decreased space requirements, increased CO2 efficiency, and increase options for availability. - Rotation to be achieved through the current method of a servo and cable. - Elevation to use current method of servo beneath the barrels. - By increasing efficiency, the accumulation tank can be reduced in size. - Most of the improved efficiency is to come from using the O-ring style breach that fast gun cannons use. - Design the cannon in layers allowing someone to simply send off to an online company and have the parts laser cut. - Reduce the cost and/or increase ease of construction by using readily available parts less expensive methods of construction. okay so at this point I was going to upload a couple of screen shots from sketchup, however as it is past my bed time and my eyes are blurry, I think I will wait till tomorrow night.
Cool idea! I've started getting into sketchup to draw things for my 3D printer (nearly online). Look forward to seeing your work!
I am very interested in how you'll incorporate the o-ring breech into a Big Gun style cannon. I look forward to your pictures.
Okay so pictures! Note: I don't have the barrel assembly idea's in sketchup yet...come to think of it, I don't know why I haven't done that yet....oh well maybe tomorrow. These first three pics are of my 1st rendition. It is designed to use a current style accumulator and buna ball valve setup. This design is composed of laser cut plates either 1/8" or 1/16" thick. - The left most pic shows in good detail the bottom plate, the rotation bushing, the manifold plates, the piston shelf plate, the pistons, and the piston travel plates. Springs would be located around each piston. Springs take to long to model in sketchup, so I didn't. The pistons travel upwards to block additionally rounds from leaving the cannon. Each piston also allows CO2 to pass through it's center to propel the active round upwards toward the breach. Additionally, the pistons at max travel, should seat against the up tube and prevent large amounts of CO2 loss into the magazine. I hope anyway. - The center pic shows the piston shelf plate, you can see a bit of the pistons, the piston travel plates, and the magazine feed plates. - The right most pic show the magazine feed plates, the magazine containment plates, the up tubes, the magazine fill tube, and the cover plate. Another cool thing about this design is that if you want more ammo and have room height wise, add more magazine containment plates and longer bolts! I know the weak point is the pistons and as yet I don't know if they will work as hoped, but hey that is why I'm hoping people versed in Thermodynamics will call me an idiot and tell me how to fix it. It is also all drawn up in turbocad just waiting to be sent off, depending upon how many you idiots I recieve! These next pics are of the 2nd idea that I had just last week. This design is based around using pre-made solenoids such as kips or clippard, though I have clippard manifolds modeled at the moment. This design is less thought out as I just had it last week so forgive me if somethings screwball. - The left pic shows the clippard manifolds, the bottom plate, the piston shelf plate, the pistons (same design and potential issues as above), the internal accumulator, accumulator cap plate, and then the magazine feed plate etc. on up. - The right pic mostly just shows a better view of the internal accumulator. The internal accumulator has at least 1/4" ABS on the sides, above and below. Okay so the issue with this design is the accumulator is 3/8" thick and so the only way to get it laser cut effectively would be to cut it into slices. Problem with that is upon assembling it would need some sort of seal. Would RTV provide a good seal? I have some pics of the pistons in detail, but it is WAY past my bed time and my brain turned off like an hour ago...and I don't remember where I put them...anyway maybe tomorrow.
I've been looking at doing something similar. My biggest concern is that a hollow piston has very little surface area for the CO2 to act upon. After all, it not only must move the piston vertically, but must also overpower the tension of the spring. I think you'll have to attach the pistons to a plate of some kind to increase the surface area enough for the CO2 to close the breech. The plate will have holes that line up with holes in the pistons. I'm typing this from my phone, so I can't attach a drawing right now of what I'm trying to describe. I hope it makes sense...
did you peruse the old hybrid plans of mine? there should be some good ideas you can pillage from in there.
I have seen a set of cannons very similar to your first design, radollar. It was a traditional buna-ball valve design with sliding pistons to close each breech. It was more efficient than regular Indiana-style cannons without the closed breech, but not as efficient as canister-style cannons with their improved airflow path. The other problem was that its pistons required regular cleaning and lubrication to ensure they wouldn't stick. That added a lot of work to each sink recovery and after each battle day. If your design does not require said maintenance, then it would be very good. As to your 2nd design, I don't see how it is an improvement, since it is difficult to rotate. Each valve requires at least one, usually two air lines, each of which will do its level best to get tangled in whatever rotation mechanism you use. The only ships that try to rotate multi-barrel MJV valve cannons are the 1:72 scale heavy cruisers up in Washington. They have the space to make it work. All the other multi-barrel MJV cannons are fixed in place. Tugboat, do you know how 3D printed ABS compares, pressure-test wise, to extruded piping or machined blocks of ABS? The best way to find out may be to experiment. Print out several accumulators of various IDs and wall thicknesses, then pressure-test each of them inside a large bucket of water. Find out what pressure they fail at. Another thing to test is the regular store-bought ABS pipe, with your printed endcaps, to see if that is any improvement.
@Tugboat: Since PVC is used as an accumulator in many Big Gun cannons I figured ABS would work as well. @Greg: As far as the technical aspect on how much PSI it could take before rupture I have no Idea. I just know that from working with PVC piping even PVC 220 (the thinnest easily available) is rated at 220 PSI and it is only like 1/8" thick. I know there are different properties and all but I figured it would be over engineered enough to work. Concerning your hybrid cannon, I would really like to be able to incorporate a slightly modified version of your barrel assembly. @Dietzer: the pistons I purchased are (in the real world) the outer portion of a binding post set. I have pictures but I'm going to have to wait till this evening to post them...as I still haven't found them yet....They have a wide flange that I believe will suffice. @Kotori: Yeah so I won't know how maintenance goes until after I build it and try it a while. As far as rotation, I was going to have the rotation servo mounted higher up using the body of the magazine assembly, this way the CO2 lines are all at the bottom while the rotation cable is near the top. That is one of the things I was waiting to model until after I found out if the rest was do-able.
So I finally found my piston pictures! Shown are 7/32 ball bearing, closed end spring, and binding post barrel. I would like to add some material around the binding post barrel to help minimize piston wobble. Any suggestions on a good material to use? I also have not drilled out the inside of the piston yet as is evident by the threads still showing.
I've tried to make a flat accumulator similar to your second design. The problem isn't really with material strength, it's with the glue joint. If you take a 3" accumulator and fill it with 150 psi CO2, you have 1060 pounds of force trying to separate the top and bottom. On my try the flat top and bottom bulged out and the glue joints failed. I anticipated this a little and had some internal ribs like you have, but they didn't help enough. Fortunately when the glue failed, the accumulator depressurized gently instead of explosively.
You have to remember that it is not just material thickness but geometry that matters... 1/8" material in a tube that takes X will readily take something very different when made into a flat cap. there is a good reason why there are very few pressure vessels out there that use flat caps. they are materially intensive (absurdly so) compared to spherical, elliptical and the inverted elliptical. here is an example from work, where we effectively design pressure vessels. a cap that needed to be 0.5mm thick in one configuration to keep stresses in the safe range needed to be almost 2mm in the flat configuration. The other problem one has is fatigue of the vessel. this one can be a real killer of joints, particularly in areas where there are welds, adhesives, etc that are not as strong as the parent material.
one thing on the pistons. stephen was making injection molded pistons. if he has those available, I would HIGHLY recommend them over anything you do. You have to remember how the piston selector works. The heavier the piston or the stiffer the spring, the longer it will take for the piston to rise and block off rounds. the cannon is a dynamic system, I made the following mistake once: My piston was too heavy and my spring too stiff. the synamics of the spring mass system coupled with it's dynamic loads set up a situation that was neat, and not what I was going for. The cannon controllably full auto'd. what I mean by that is that the piston cycled once per round fired. the parts effectively were tuned to one another so the piston would cycle up, feed round, block remainder, round goes to O-ring stops, pressure rises and round fires, however the piston moved downward as round fired, chambering another round before newly decreased pressure drove it up. it was a balancing act. too stiff a spring/heavy piston and the piston requires too large of a pressure difference to remain up, so durring the final stage in filling the volume behind the round at the breech, the piston dropped enough to chamber another round. too heavy a piston and too stiff a spring also retards the upward direction motion, which can lead to multiple shots per trigger pull in a spurt type configuration (also done that, reliable 2 or 3 rounds per spurt is doable, but fussy to maintain)
Thanks guys, I'll take the accumulator info under advisement. As to the pistons, I'm sure injection molded ones would work great. What I've got are aluminum ones from McMaster so I figure the weight isn't an issue. Concerning the spring, if I knew more about the dynamics involved I'm sure I could calculate what strength of spring needed. I don't so I figured from the beginning that it was going to be a trial and error sorta thing.
You actually can make them work without spring at all. the spring is there to provide a garantee to overcome static friction (see the old ball bearing selector mechanisms if the info is still on the web). my general rule of thumb was the softest spring that prevents teh piston from getting hung up. I machined my pistons from delrin.
I've got a PDF of it anyhow, plus there is Tuggies CPOE cannon build that while the breach may not make him happy the bearing selector part sure seemed to! Also, I'm hopefully going to be doing a little more Sketchup modeling this week end. Mostly I want to get the solenoid & manifold sorted out on variation 2. I'd also like to get my basic barrel idea in. Also, one thing to know about Sketchup is that even if make sure all points are properly connected and that every thing is correct, it can and will occasionaly throw a massive screwball at you! Don't get me wrong I absolutly love the program. BUT, some times it just makes me want to cry. inch: For instance, on this project each one of those plates seems to have developed a tendency to randomly develop hidden lines and for parts of pre-existing lines to turn hidden....needless to say this adds much more work and further irritation to the process. Some might say it's just that it is the freebie version...nope it's the pro version too. Granted this is by far the worst I've encountered it doing this. Normally it might do this once or twice or not at all on a project. This time it is doing it every time I do anything. Okay, I've vented enough now. Have fun with the program now Clark.
Don't forget, guys, that Big Gun cannons don't need to meet the same rate-of-fire requirements that Fast Gun cannons do. So you may find that gravity will work just as well to return the piston to its resting position. That's how the other set of cannons I saw worked. Greg, you mentioned curved surfaces for pressure vessels. I wonder how easy it would be to 3D print curved endcaps for accumulators, instead of flat ones.
You also mentioned that it had issues with having to be cleaned so that it traveled easily. The spring will help to mitigate that. As far as print 3D end caps, I know you could do it, the question should be how well does the printed plastic hold together as compared to one that was milled. The only place I could have a curved end cap would be on the bottom and that is going to end up having 1/4" or more material; plus, if I use the clippard manifolds, then that is going to add 3/8x1x1" across the bottom. Additionally I've added 5 more bolt holes to help with clamping. If I decide to go with Kip soleniods, then I might have some other changes as well that would increase volume of the accumulator chambers a little bit.
