The Craftmans Shop > Backyard Ballistics
Black Powder Cannon
Lew_Merrick_PE:
Frank (& All),
1) Here in the U.S. the laws vary all over the place. In certain States (California, Massachusetts, & New York I believe) you cannot own such a device personally. In other States, you have to register it with your local police or sheriff's department. In other States, you have no restrictions at all. I am not conversant with the specifics of these regulations.
2) Black Powder is a deflagerant which means that it builds pressure more slowly than other propellants. The issue is how much gas and what pressure & temperature results from igniting it. This is usually published as 100 cmł bomb pressure & temperature data. (Understand that in pyrotechnical terms a bomb holds and prevents the release of energy of combustion. It is a bombast that actually bursts to release such energy.) The absolute pressure & temperature is the energy released by combusting the propellant. The values you get from a 100 cmł bomb will be for a specific (and small) amount of propellant. Be sure to read the entire document to find out how to expand such values to your load!
3) The resistance to energy is the area under the curve of a stress-strain diagram. You will want to have enough material to absorb the propellant's energy with a factor of safety that is no less than 3 to the yield point's energy. Thus, for A36 (common structural) steel, you will typically have an area under the curve of (36,000 lb/inČ X 36,000 lb/inČ / 13,500,000 lb/inČ/in =) 96 in-lb/inł. This is highly simplified as there are geometry effects involved, but this is one aspect you need to account.
4) The maximum pressure pulse is another. For a thick-walled tube (i.e. a cannon barrel), the shear stress = Pressure X (RoČ + RiČ)/(RoČ - RiČ). Where Ro is the Outer Radius & Ri is the Inner Radius. The allowable shear stress for most steels may be approximated as 0.63 X Ultimate Tensile Strength and should have a factor of safety applied that is no less than 7.
5) Also based on the maximum pressure pulse is the hoop stress engendered. This is tensile stress = (Pressure X Ro)/Radial Thickness. For A36 steel, the allowable tensile yield strength is 36,000 lb/inČ. A factor of safety of no less than 3 should be applied to this.
6) And then their is the dissipation of retained heat which will determine your maximum rate of fire. It is fairly easy to raise the temperature of the barrel into the 600°F range which will reduce all your allowable stress & allowable energy values by anywhere from 30% to 70% depending on the material. This was the most common cause of early cannon failure!
All I have done here is to give you a very general overview of the problems you will face in making such a device. My background in this arena has more to do with insertion & removal of rounds, stripping said rounds from magazine presentation, and the mechanics of boring and rifling barrels for such devices. The equations I have cited for energy, shear, & tensile stress are readily available (Machinery's Handbook is one reference for them, just about any engineering stress analysis text will give you more detail on them). Be very careful if you decide to proceed! One failed barrel can ruin a lifetime...
BronxFigs:
Lew Merrick, and others....thank you for your interest and for the suggestions. I will definitely take all comments under consideration.
Years ago, while still in my Salad Days, I used to shoot a .50 caliber Hawkins rifle and loaded it with about 120 grains of Pyrodex. I think the hex. barrel was about 1.00" across the flats. I don't think the cannon that I plan to make will have a bore larger than .750" diameter, and the breech section, from the trunnions back to the cascabel, will be at least 3.00" - 3.50" diameter. I think the barrel will be similar to the old Wiard rifled cannon....basically.
I'm guessing that this heavy breech section will have enough strength to contain the pressure of 120-150 grains of Pyrodex. The OAL of the barrel will be roughly 13". I'll still proof the barrel before firing the occasional salute. I think any commonly found steels will work well for this purpose.
Frank
BronxFigs:
Yep. I decided that my short attention span will tolerate turning a Wiard-style cannon barrel. Just basically a two diameter barrel. Larger diameter section from trunnions back to breech, and a smaller diameter section from trunnions to muzzle. Neat, clean lines, easy job....done.
So, I'll chuck full-diameter bar-stock, cut to length, center-drill, then deep-hole drill, then ream/finish the 0.750" bore.
Unchuck work. Bore two recessed holes for trunnions on milling machine.
Re-chuck work, and finish turning outside profile of barrel. OAL: 13" Chase to breech section; 3.00" dia. Chase to muzzle approx. 2.00" dia.
Turn and mount trunnions (Braze/Silver Solder).
Proof load. Light the fuse. If all is OK, start making some noise.
Thanks for the interest, and all the suggestions.
Frank
Arbalist:
Let's see some pictures when it's done Frank!
I've thought about making a Canon as well but thought about making it from larger stock so that the trunnions are part of the barrel, not additions. I understand in bygone days they used a "trunnion" Lathe to achieve this but I could not find any details of how this was actually done.
BronxFigs:
Arbalist-
Thanks for the interest. I will take some photos of the barrel-making process and capture my screw-ups, failures, and triumphs. But then, I will need help with how to post them. Computers and me don't mix.
Trunnion Lathes, etc.
I don't know how large a diameter barrel that you are going to machine, but (I'm speculating), making a barrel with intergral trunnions will require an incredible amount of machining, and stock removal. Also, how will you machine the band of remaining metal from between the trunnions? You could turn the profile of the barrel from muzzle to breech, except for the area surrounding the trunnions, which will still be at full diameter across the trunnions. Even if you found a way to machine the trunnions (milling machine, w/boring head, etc.), and remove the band of extra metal, you will still need to hand file and then finish the area around the rimbase of the trunnions, to blend them into the curved and tapered, barrel. (I'm envisioning a traditional cannon, i.e. a barrel that is tapered from breech to muzzle).
I suppose you could remove that problematic, band of extra metal from between the trunnions, by doing some kind of a set-up in a milling machine so that the barrel could be slowly rotated under a cutter, and by degrees cutting away the metal to match the taper at that section of the barrel. You might even be able to machine away that band by using the lathe carriage and a cutting tool, as a "shaper", while the barrel is mounted, and rotated while still chucked in the lathe. I'm no machinist, so maybe others will come up with alternative, and more efficient ways to machine away all this extra metal. My guess would be that the whole process is quite a huge p.i.t.a. One bad cut, and oops!
Honestly, I don't think there is any "easy" way to machine integral trunnions. Most modern reproduction cannons made from bar-stock, have trunnions welded onto the tubes....unless of course, the finished cannon barrel started as a casting.
Judging from the old engravings that I've seen, I believe that "trunnion lathes" were mounted at 90 degrees to the length of the barrel and came in with some kind of cutting tools to machine the circumference of the trunnions while the barrel was still mounted in the lathe....but the extra metal must still be machined from between the trunnions. But how? I have no clue.
Go for it... Make a barrel. We only live once.
Frank
EDIT: Go to: "Seacoast Artillery" for photo of a cannon barrel being machined in the area of the trunnions.
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