Cannon Design and Construction- Suggestions Welcome

The breech plate installed into the magazine sleeve.

 

Bottom View

 

I included this shot to show the adjustment screws that set/ adjust the height of the round in the breech.

 

The breech plate/ magazine sub assembly is added on top of the manifold. I used a bunch of screws (not all shown) because the breech plate has to seal on top of the magazie without the aid of any media to improve sealing. The two machined surface are simply clamped to one another and a seal that is effective enough for our purposes results. The Delrin breech plate does conform to the magazine to a small degree. The seal is actually very good.

 

Here is the ball valve body all by it's lonesome. This particular one has yet to have the outlet port tapped to accept the rotation bushing. The threads for the MPA pilot and the ones for the accumulator to screw to are present though.

 

Another view:

 

That's it for now. I will contunue with some pics of the ball valve inner components later.

 

Impressive

 

I like that loading tube jig you made. Have you thought about how to make a larger magazine? I've heard some discussion of making a central part that includes the loading tubes and breech, with the bulk of the magazine being a separate piece that can be used to make different sizes of magazine. I saw a design using two large 3/16" or 1/4" pins to hold the accumulator to the valve body. It still used the o-ring to seal the accumulator, but looked a bit simpler to make.

 

I have put a bit of thought into a "stockable" cannon. Due to the massive difficulty in obtaining cannons for big gun, and its negative effects on growing the hobby I had considered (even started designing) a cannon that could be built and stocked in larger quantities. It was close in overall concept (as far as cost effectively having cannons pre made without having to stock tons of different configurations) My thought was to still make the magazine in one piece, but stocked without the feed holes or riser holes. Then when a order was received, using a set of fixtures similar to mine, the holes would be drilled in the needed configuration on a drill press and ship the cannon out. Properly designed, and with some rules "flexability" on barrel spacing, only one set of drilling fixtures would be needed for all magazine diameters, and virtually no skill or special equipment needed to take a magazine from a generic undriled unit to a dual barrel, 7/32 diameter ammo or a triple 1/4 diameter ammo simply by choosing the appropriate fixtures and drilling as the orders came in.

I had also considered a "quick release" accumulator allowing for changing/ servicing/ etc. My first thought was very similar to what you mentioned, but using any type or pin to hold the magazine on would require it to be offset to one side to be out of the path of the rush of air. That would probably also require two pins to even out the load as the pressure rapidly built and dropped to keep from fatiguing the mounting points. It would also need to be close tolerance holes to keep from allowing a hammering effect that would wear out the holes. Due to the difficulty in accurately drilling off center holes to a close tolerance which is a pain at the beginning, middle, and end of a parts run, I opted to just thread the connection. That eliminates the hammering effect, the weakening of the two components due to holes, etc. Also, once set up, the threads would run accurately unattended throughout the run. Off center holes are a pain because inevitably they walk around, more so as the tools dull, so without close watch the opportunity fro scrap goes way up, driving up costs.

Those were my thoughts on a "commercially" available cannon. Not that all I say here is absolute truth, just why I did what I did and didn't do what I didn't do. I still toss around the possibility of providing them myself, so all of what I build is sort of a test been for that, just because I love the big gun rule set and want more people to build for it. I think that smaller big gun clubs and newly started ones are fighting an uphill battle due to lack of cannons.

I am getting ready to unveil my new big gun torpedo I am building on my Uritski build thread, so anyone who hasn't followed that may want to take a look. There is another inclusion to the ball valve which reduces weight and space requirements. I am going to post pics before I have it fully operational due to all the discussion I became involved in on cannons.

 

Wow! Sounds awesome!! I had guessed that anything requiring a good finish (sealing surfaces) would need a touch up after printing. I can't wait to see what you have come up with!!

 

The volume of air would be around 2.3 cu. in., I think the typical recommendation is around 2cu.in. per barrel for 1/4" projectiles?? Considering that is usually for Indiana cannons, you *might* get away with it depending on your design efficiency. A torpedo doesn't have to pressurized a large manifold like an Indiana cannon does, so its volume requirement should be less. Seems to me you might need another 1" of length minimum though. Running at 150 paid should allow you to run an accumulator on the smaller side. I think the thinner walled PVC is the schedule 40 stuff, and is what most people use. The thicker stuff I think is schedule 80, has a bunch more burst strength, but I'm not sure of the availability. I would go for the abs tube from McMaster, and if you print in abs you can solvent weld it just like PVC if you wish. Of course, I'm an overkill kind of person... I hate to blow off a body part.... or worse yet,someone else's..... I wonder if there is a composite type material (like Kevlar) that could be laminated to the outside of PVC to help control failure.

 

Drawing looks great! You work fast!
See uritskis build thread for a view of internals. The pushrod I used was 1/8 brass, threaded about 1/2 inch with a 6-32 die. It does not provide a proper thread because the rod O.D. is too small, but it holds good in the buna ball. I pre drilled the balls, I will have to check on the size. I can track down the part number of the stainless spring I used, it came from McMaster also. So did the buna balls. I know the spring O.D. is a little under 1/4 inch.

 

Oops, You did say ABS.... My bad!! I used the thicker wall stuff and bored a little out of the I.D. to leave it thicker than the wall thickness of 1" schedule 40 pvc, but I don't remember the thickness off hand.... .137" maybe??? If so I probably went with .150 Wall thickness.

 

I think the 1/8" brass rod can be threaded with a 5-40 die to bite into the Buna ball. On the other end, I think a 5-40 nut was too big to fit inside an actuator. I had to drill and tap the center of the rod 2-56 and have a small bolt and washer retain the spring.

One problem with the traditional positive pressure cannon is that the 1/8" rod is right in the middle of the air path. I think it is possible to use a negative pressure cannon to eliminate this inefficiency, and a negative pressure cannon is easier if you can build your own actuator into the valve body instead of using an MPA-7.