Thundermug?

I've been haunted by the idea of a sand cast thundermug, ever since some of you cast up cannons back on the AA board, but I'm a little leary due to my design idea.

I'd like to make it have the appearance of a rook chess piece, cast from aluminum. That way, when not in use, it could be used as a decorative doorstop or bookend.
My problem is that to appear as a castle parapet, with the look of stacked block or stone, I'll have to have a network of false mortar lines around the outside. I'm concerned that this feature may just act like a weakness in the wall, causing it to fail in a catastrophic manner, not unlike a hand grenade of old.

I plan on using not more than about 100-200 grains of black powder, in a bore of about 1.25", with a ping-pong ball as a projectile. Chamber pressures should be minimal, with such a lightweight object. Ignition source would be standard cannon fuse through a drilled touch hole.

I call myself an "eyeball engineer", so the math escapes me to calculate wall thickness to get the strength that will be required to safely contain the explosion of ignition.

My eyeball engineering suggests a wall thickness of 2", so that would give me an O.D. of about 5.25", and a proportional height of 10" or 12 " tall.

Am I insane, and should I abandon this idea?

Thanks for any input you might want to offer.

Roger

 

I wouldn't consider aluminium due to potential porosity, metal fatigue and possibly age hardening over time: some cast aluminium items can shatter when dropped after 20 years or so. It might be better to have a steel liner out of something substantial like 4140 so the aluminium is purely decorative.

 

I considerd a steel liner, but we all know that shrink rates between disimilar metals don't work out so well with a cast-in liner. Maybe I should just start with a steel billet and machine the whole thing. I do, after all, have a lathe and mini mill.

Roger

 

Casting it in could affect the temper of the steel too, if it were me I'd cast the aluminium and bore it for a shrink fit or even a Loctite fit of the steel barrel. Those old time short barreled siege mortars were made from cast bronze so that would be do-able for the home foundry not to mention being a highly machinable material.

 

if you use a ping pong ball as the projectile, it will fail before the bore so there is 0 chance of the barrel failing, of course the ping pong ball might as well just be wadding, because it will be shredded by the explosion, think of the videos of people shooting 50 cal rounds in shotguns only much moreso

V/r HT1

 

You know ping pong balls are made from nitrocellulose....you could probably power a cannon with them as the charge.

 

I thought about a golf ball sized bore, but I don't really want the projectile to travel that far. Just something small, to act as wadding, but still blow out of the barrel. Wasn't sure a ping-pong ball would survive, but if it did, it wouldn't travel far enough to make it out of my yard.

Roger

 

Mark might have itterated the best solution. Go ahead and make my casting, then shrink-fit a steel liner into it.
I'm liking that solution more than wondering if I'm going to set off a large grenade in my yard someday.

Shouldn't be overly difficult. Fabricate my liner and stick it in the freezer for a day. Bore the casting for about a .004" interference fit, stick it in the oven at 400*F for an hour, and force the two together. The liner shouldn't have any movement once temps are equalized. It's not like this is an internal combustion engine running at 4 grand.

This is why I like these boards so much.....brainstorming. Three heads are soooo much better than my one shoulder-mounted vaccuum chamber.

Roger

 

See?.....brainstorming.

Thanks a bunch, Kelly. I think you're right on holding the taper/foot down on my lathe. I'm probably asking too much, and I should shoot for a slip fit. Maybe with some loctite and screws for retention.

Roger

 

I am going to go back to the idea of pouring aluminum around the sleeve. Maintaining straightness of the sleeve and maintaining temper of the sleeve are off the table for the sort of purpose envisioned. The coefficient of thermal expansion of steel is 1/2 that of aluminum. So as the molten metal is poured, the iron will expand about 1/2 the expected shrinkage of the aluminum. Once everything cools, the sleeve will be very tightly gripped (tons) by the aluminum shell. If the shell is "thick" it will not rupture as it would if say a 1/4" thick shell were poured over a 1/4" thick sleeve. The grip of the shell will be so strong that the risk of the liner becoming a projectile will be very small.

To prevent premature freeeze of the pouring metal, preheating the steel to 800-1000 or so degrees is probably wise. A weed burner would do that easily.

Ordinary low-carbon "black pipe" would serve well. No worries about hardening. Machines very easily.

Denis

 

A shrink fit is a risky proposition at the best of times, the only way I'd try it would be with under a thou of interference and by using heat on the aluminium and dry ice/acetone on the steel tube. I think Denis' suggestion eliminates all the critical tolerances and still gives a tightly gripped steel barrel in aluminium. Any heat related tempering or annealing of the steel can be compensated by extra thickness and material selection: 4140 is still twice as strong as mild steel in it's normalized state.

 

I was worried for a bit when you said you were casting a "thundermug". That's how my grandmother referred to a chamber-pot, and I just kept wondering why would anybody want to cast one of those?

I'm glad it's something a little more interesting, but probably just as explosive.

Don

 

As a follow-up to this thread, I've done a whole bunch more investigation, to find that in the black powder cannon circles, it's standard practice to maintain a bore's dimension of wall thickness around the breech, or powder chamber, and that's with steel cannons. Therefore, if I do a 1" bore, then the cannon must be not less than 3" o.d. at the breech end.
I've decided to play it safe, and dig up some solid steel round stock to machine the whole thing out of.

Roger

 

I've made a few mortars for golf balls and 1" lead balls using seamless tubing. With short, 8-12"
length and 50-60 gr of 2f or 3f black powder, preferably 2f, there's little to no chance of failure unless
a very heavy projectile is used. The chamber is machined with small cavity matched to powder charge.
I have a half scale Napoleon cannon, cast iron, 2" bore firing a slightly under size lead ball, using
600gr cannon grade powder. The projectile should be slightly undersized to provide "windage" to control
pressure and insure not binding in the bore when fired. I made "slap hammer" type ignition using # 209 shotgun shell primers.
If just wanting a loud report, what a thunder mug is typically used for, I use wadded up bread or a potato, that way
no mess to clean up down range and feeds the critters too.