Who has tried to roll your own weapons?

Who has tried to roll your own weapons?

I'm giving it a crack... based largely on Queen's Own designs.

I'm most worried about making sure the air pressure is constant across the mulitple shells. For Big Gun I'll be firing 3x 1/4" shells; & the last thing I want is one shell flying & two dribbling.

The other thing I'm worried about is bending the tube for the & magazine. However, my father is quite skilled at bending pipe for musical instruments. I have "hired" him to make a few for me.

 

Before I offer any advice, you should know my limited experience with gun design. You can judge for yourself whether or not my advice is worthwhile. I have disected and rebuilt the forward cannon of my Scharnhorst (expanded the accumulator so it could fire three balls). I built a fixed triple MJV-2 cannon for the stern of Scharnhorst. I helped with construction (but not design) for the torpedos used in my Cadorna and two Capitani Romani cruisers. I designed and built the torpedos used in my Spahkreuzer. I have studied many cannon designs, including several "classified" experimental designs, to understand the theory of operation and improvements over traditional designs. I bent barrels for every cannon except Scharnhorst's forward cannon. You should know that I work in Neal's garage. He used to be a profesional machinist, and provides machining advice, assistance, and the occasional miracle to ensure a high level of quality in my work.

The Queen's Own seems to like to chain together Clippard MJV-2 valves or custom made valves to make multiple cannons in a single turret. While this works, it is bulkier than the standard big gun cannon, the J.C. White or Indiana cannon, which uses a single valve to power a whole turret with a built in bearing for rotation. If you own or have access to a machine lathe and mill, I would recommend going with the standard Indiana cannon. My opinion is that MJV-2 cannons should be used in 1/144 scale for fixed cannons only.

I have some rules of thumb I picked up from Neal. A lot of them sound like common sense, but its amazing how few people actually follow these rules.

fluid dynamics:
1) Maintain the same cross-sectional area through the cannon, or reduce it as it goes along. Avoid increasing the cross-sectional area at any point, it will reduce the efficiency.
Area of 3x 1/4" barrels = 3*(pi)*(1/2)*(1/8)^2 = 0.0736 in^2
Area of 1/2" ball valve - area of 1/8" actuator rod = (1/2)*(pi)*((1/4)^2-(1/16)^2) = 0.0920 in^2
Area of 1" ID accumulator - area of 3/4" buna ball = (1/2)*(pi)*((1/2)^2-(3/8)^2) = 0.1718 in^2
Cross-sectional area decreases as air moves from accumulator to valve to barrels.
2) Have as few direction changes as possible in the air path. Every time you hit a direction change, the air slows down. Example: canister cannons operate at a lower pressure than indiana cannons because they eliminate one 90 degree turn. MJV-2 cannons eliminate two turns, and Wreno's sliding breach cannon eliminates three turns.
3)The distance the air must fill from the valve to any and all possible exits (loading tube, barrels, leaks, etc) should be within 10% or less of the same distance for the other barrels. This will synchronize the air pulse and ensure that all shells are as close to equal as possible. Compare this to the exhaust manifold in a well tuned car which is timed to stay in sequence with the pistons.

other rules of thumb:
4) If possible, close the breech so air doesn't go to the magazine. This is not neccesary, but will help improve efficiency. Air that goes into the magazine is air that doesn't propel balls out the barrels. This is easier to do on an MJV-2 cannon than on an Indiana cannon, though not impossible.
5) Longer barrels will reduce operating pressure by allowing the air more time to accelerate to the proper speed, as well as help with Rule of Thumb #3 above.
6) Minimize volume between valve and breech. As the air expands to fill the volume, the pressure decreases. You want to minimize this pressure drop by keeping the volume down. Air in the volume between the valve and breech is air that isn't propelling balls out the barrels.
7) Maintain a good finish, especially on sealing surfaces such as the ball valve. Even a tiny scratch is enough to cause a leak that will render you harmless.
8) Keep the actuation rod (not sure if that's the technical term, I'm referring to the rod that the MPA-7 pushes that is connected to the buna ball) for the ball valve short. If it's too long, it will droop when fired and won't reliably seal back up. If you must make it long, such as in a tandem cannon, try to give it a support that will hold it in place. There are several pocket battleships with cannons that don't always re-seat after firing. An internal support or a shorter valve would remedy this problem.
8.5) Drilling and tapping small holes can be a major pain. Have extra taps. I watched another person working in the garage go through three 2-56 taps and ruin as many components.
9) Prepare for future failures. For example, if you use plastic threads to hold the MPA-7 actuator on the valve, leave enough material to drill it out and put in a threaded metal insert in case the plastic threads strip. Another example is how all our cannons are standardized to use 4-40 screws, so if any strip out they can be drilled out and replaced with 6-32's.
10) standardize components whenever possible. For example, use ALL 4-40 stainless hex head screws, or use ALL 6-32 stainless phillips head screws, so that if you ever misplace a screw you know exactly what size to get.
11) make sure that the cannon conforms to historical dimensions. In other words, make sure that the magazine fits UNDER the deck, the barrel risers and depression servo fit WITHIN the scale barbette, and the barrels fit within the turret cover at scale height. This will work wonders for the appearance of your ship, especially if you actually install the barbettes and turret covers. Look at Ben Lee's ships in the WWCC photo gallery. All his ships comply with this or rule 11.5.
11.5) if you need to raise your barrel height, raise the entire superstructure by the same amount, and keep it to a minimum. The King George V in the WWCC has its entire superstructure raised by 1/2" and nobody can tell the difference.
12) Never build only the number of guns that you need. Once you start working, set up a process and make several extras. You can use them as spares (especially if you make a mistake late in construction), sell them, or keep them for your next project. Example: Neal's policy is that whatever you make in his garage, you also make enough for him. This ranges from hulls to cannons. He will either trade, sell, or use everything at some point, and it helps him support his workshop. Spahkreuzer's torpedos (built at Neal's) have a sister set that was recently sold and installed in the sister hull for Cadorna (also built at Neal's).
13) Give the barrels some slop. If a tight-tolerance barrel is hit, it's out of action. Better to have a slightly higher operating pressure than to lose a barrel in combat. Adunno about anybody else, but when I'm too close to hit my target's hull, I aim for their gun barrels instead.
14) Build the cannon to withstand much higher than operating pressure, and test it. Pressurize it, then seal it off from the CO2, and check the pressure after 24 hours. You'll know if it has any leaks and whether it can easily survive normal operations.

Dimensions and proportions we normally use:
15) 1.5-2.5 cubic inches of accumulator volume per barrel. When in doubt, round up. Neal tested this, using an MJV-2 and different size accumulators. He found that under 1.5 cu in needed a much higher pressure, while anything more than 2.5 gave negligible benefit.
16) for 3x 1/4" balls, you want at least a 1/2" dia. bore for the ball valve (though bigger is better), a 3/4" dia. buna ball, and a 1" ID accumulator. Larger dia. accumulator is recommended to save length. Note that this is in compliance with Rule of Thumb #1.

Philosophy of construction: the AK-47 construction style
At Neal's garage, we build according to what we call the "AK-47" style. This style emphasizes reliability and ease of maintenance through simplicity and durability. Our motto is "Built like a brick, and hits like one, too." This is a very apt description, because like our ships and cannons, a brick is very durable, and nearly impervious to the rigors of combat. When used as a weapon, a brick is very simple to use, and will make quite an impact. Basically, build things simple and tough. And most importantly, if something fails, find out why and remove the reason. There are bunches of details on HOW we make guns and ships simple and tough, but if I were to list them all I'd be typing for the next week. I have heard some people claim that we over-engineer our ships, but its hard to argue with results. I claim that I could have my boat running again half an hour after a sink. I could then load the torpedos with rusty ball bearings (I always feel better when returning ammunition to its rightful owner ) or even small pebbles (haven't tried this, yet...) and still blast holes in my opponents.

 

Carl pretty much hits it on the head. I've been helping Jason build the Indiana cannon since he took over the production from Bill N. We've played with a few basic designs and the Indiana design has alot going for it. Mainly it just works and the design is reliable. If you sink, pour some alcohol in and fire it a few times (clears out the water). And head back out.

Andy

 

I agree, I think Carl should repost what he just wrote in the "tips & tricks" section under pneumatics.

 

Seriously Carl, just copy your tip & paste in into a post on the Tips & Tricks section.