Recently, I thought I would try to get my nephew, 9, away from his video games, so I proposed that we build a potato cannon together. Grandpa was not very keen on the idea of us firing it handheld, so in order to placate him I agreed to mount it on a carriage.
I chose the 105mm Howitzer as a model. This is pretty typical as light artillery pieces go, I think.
I scaled various relative dimensions from pictures, and once we got our cannon built, used those relative dimensions to size the other parts. These would only be guidelines, naturally. I used AutoCad to draw and dimension everything.
We built the cannon first. It’s a 36” x 2” barrel and a 14” x 4” combustion chamber. The wheels (from McMaster-Carr) are 20”. Try as I might, I could not find a 20” wheel with a solid rim to simulate the real thing.
This is the beginning of the axle, or the frame. I don’t know the correct names for many of these parts, so I’m making them up. It’s 1/2” baltic birch ply, with lightening holes, top and bottom, and 2x lumber milled square to 3 1/2” wide for the sides.
This shot shows the “trail” in the towing position. We’re not actually going to tow it.
This is the trail in the firing position. There are 2 bolts in each arm, one farther back for the pivot, which will be secured with a nut, and one close to the forward end, which will come out, and drop into one of two holes, for each position. These pieces have been ripped a hair narrower than the frame side pieces so they will pivot easily.
Next are the elevating arcs. You can see these in the picture of the real thing. This is also 1/2” baltic birch ply. The curved slot is a circular arc, centered on the 3/4” hole on the upper right; the trunnion (cannon’s elevation pivot) drops into the hole, and there will be a hanger stud sticking out of the trunnion assembly and through the slot, with a locking knob on it. The arc goes from -10 to +45 degrees, so when the piece is resting on the trail, elevation range will be about 0 to +55 degrees. My lightening holes are unevenly spaced; that’s what you get when you eyeball rather than measure. I did all the cuts with both pieces double-side taped together.
Same shot; you can see the “trunnion bearing cap” in place here. The bearing blocks & caps are 2x lumber, milled square. The trunnion is a piece of 3/4” threaded rod. It will wear these holes out, but it’s not going to be moving very much, and there’s very little weight on it, since the hanger studs and knobs will be right near the cannon’s center of gravity, so I’m not worried. If they get bad, I’ll drill them out and put in a bushing.
Next installment: Frame/elevating arc assembly and trunnion.
-- "A man may conduct himself well in both adversity and good fortune, but if you want to test his character, give him power." - Abraham Lincoln