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"Cannon shell?" Topic


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280 hits since 19 Nov 2010
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Personal logo Balin Shortstuff Supporting Member of TMP19 Nov 2010 10:29 a.m. PST

I'm making a cannon for a local Ren Fair, and I can get the form ok, but trying to decide what to use for an outer shell.

The cannon will be made around a 3" PVC pipe. The various reenforcing rings will be made out of plywood and slip over the pipe. The bulk of the form will be made out of 1.5" construction foam, but I need to protect the foam. My first thought is fibreglass, but I've heard the resin would desolve the foam. The next thought was a plaster of paris cast, but I'm concerned how brittle the surface would be.

Any other ideas?

Top Gun Ace Inactive Member19 Nov 2010 2:08 p.m. PST

Sheet plastic, perhaps. Styrene, or another material.

I've heard of people using white glue too, which hardens well when dry, and works for smaller projects. Not sure if it will scale up to the size you need.

Instead of making the reinforcing rings out of plywood, why not just slightly larger diameter PVC pipe. I think that might be quicker, and easier to work with, but don't know if that will work with your tools. Fill in any gaps with epoxy, fiberglass, or similar material.

Personal logo Balin Shortstuff Supporting Member of TMP19 Nov 2010 3:22 p.m. PST

The ID of the pipe is 3", the OD is 3.5", and the outer diameter of the gun it's going to be is about 8", tapering from one end to the other. Since it'll be used teaching kids "how to load for fun and profit", I want it as tough as I can reasonably make it. The plywood rings don't really reinforce the pipe, they're to represent the reinforcing rings on the real thing, and since they stand out .75" to 1", I hope they'll add a bit of protection to the shell, what ever I end up with. This is going to be over 5' long.

Personal logo Klebert L Hall Supporting Member of TMP20 Nov 2010 9:17 a.m. PST

Make the whole outside from Bondo, instead of construction foam.

If you know someone with a big lathe, make it out of a log.

Maybe look into coldcast, though I bet that's seriously expensive in this size.
-Kle.

Personal logo Balin Shortstuff Supporting Member of TMP21 Nov 2010 8:06 a.m. PST

Construction foam is just meant to give it the right shape, I'm after a hard shell to protect the shape.

Fibreglass: the resin would desolve the foam(?) and way messy.

Plaster (for casts): might tend to chip on the surface and subject to rain damage.

Bondo: would that be strong enough for a large surface?

I'm afraid I may have to do some experiments.

Personal logo Klebert L Hall Supporting Member of TMP21 Nov 2010 8:58 a.m. PST

Bondo: would that be strong enough for a large surface?

As long as you don't drop it, it should be.
You could layer it with rabbit wire, to act as rebar.

I suppose you could make a mold, and then cast an outer shell from hydraulic cement.
-Kle.

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