Author Topic: Mortar in the making, (Read 1263 times)

Charcoal Burner · « **on:** March 12, 2006, 10:24:18 AM »

Here's a pic of the start of my 3 1/8 mortar , the barrel liner is 4in OD 3 in ID . the outer liner is 8 3/4 od with 1/4 wall , 3/8 end caps, trunnion is 2 in . ( Do you think that needs to be bigger?)No breech plug yet , Hopefully i'll have this completed by summers end. I'm learning this picture posting thing , i'll post some of my others soon...Mike

Rickk · « **Reply #1 on:** March 14, 2006, 12:11:00 PM »

what are you going to fill the in-between space with ?

GGaskill · « **Reply #2 on:** March 14, 2006, 12:29:16 PM »

Would you care to post the details of your design?

Cat Whisperer · « **Reply #3 on:** March 14, 2006, 12:36:14 PM »

Should it be thicker (the 4"OD 3"ID tubing)?

Three things need to be in concert:

1. Good design principles.
2. Appropriate materials and methods of manufacture.
3. Reasonable size and weights of bullets, size and types of charges and techniques of fire.

For example:

If the bullet is a concrete filled tin can with 1/40 of the diameter for windage using Fg powder in a powder chamber in the breach plug (appropriately designed) and the tubing is seamless then it's probably OK.

I start with commonly available ammunition (cement filled cans, golf balls or cement filled PVC pipe). Others start with cast zinc (about as dense as iron) or aluminum and have a mould made.

Then put the design on paper - publish it (here) for review of a lot of folks that will (gently) point out concerns.

If your design is consistant with experience-proven principles there's a good chance of success.

THEN cut a chip, strike an arc and dig out the brass polish!

Charcoal Burner · « **Reply #4 on:** March 14, 2006, 01:28:15 PM »

OK mabey I can post this time, I'll scratch out the details of my mortar and hopefully get them scaned by the weekend , we had a two tornados go through town one hit the shop where i work I'm a roofer so we are really busy,, anyway the answer to the in-between question is concrete,, But i'm open for suggestions for that along with constructive criticism on any of my design ,, That's why I joined this board is to learn from people in the know ,I don't have anybody else to learn from here,, Thanks Mike

Rickk · « **Reply #5 on:** March 14, 2006, 01:35:50 PM »

I know nothin about this, but concrete is pretty hard and brittle. It tends to fall apart when repeatedly beat on.

There are some really tough 2 part RTV's that would fill the space nicely with little effort, but getting only a gallon or so may be a hastle. There was a time where the company I work for threw out expired material all the time and I could have sent ya a couple gallons, but we are so busy right now we can just barely keep the stuff in stock.

Lead maybe? Not sure how much shrinkage you would get when it solidafied. If it didn't work out it would be as easy to get it out as it is to put it in.

Charcoal Burner · « **Reply #6 on:** March 14, 2006, 01:56:57 PM »

Hey rickk you have hit on somthing ,there's a two part epoxy we use to fill pans for roof penatrations , when it sets up it's like really hard rubber. Would something like that be suitable for this?

Rickk · « **Reply #7 on:** March 14, 2006, 02:19:09 PM »

I do a lot with epoxy for a living ( electronics for military stuff that goes boom). There are all sorts of epoxies, with all sorts of properties. Some are better than others at different things.

It may be wise to weld the two tubes together rigidly with end plates, andattach the trunion to the bottom of the inner tube. Then the outer tube will be mostly for show. If the epoxy experiment did not work out, at least nothing bad would happen.

If the epoxy is soft it would be safer if anything when wrong than if it is hard.

Cat Whisperer · « **Reply #8 on:** March 14, 2006, 02:24:17 PM »

Filler - why? If it's for strength, then look at the forces - the barrel liner is trying to EXPAND. Hoop strength is one term that applies. Look at the tensile strength around in a circle (hoop) resisting it enlarging. Hence, there is NO PLACE for concrete (except to add weight) it adds NO strength in the directions needed.

A two part resin - they're used to bind things together (like fiberglass cloth or fibers and carbon fibers) that provide the tensile strength needed. An example is the super lightweight .22 barrels made of a VERY thin piece of rifled tubing wrapped with carbon fiber bonded with epoxy resins.

Rickk · « **Reply #9 on:** March 14, 2006, 02:40:35 PM »

I've dealt with epoxy manufacturers for years... when they make a new latest and greatest formulation they do some simple tests on it and then give you a sample to try. The manufacturer rarely knows as much about it as you might think they might. If you ask them how much strength an epoxy has when subjected to an impulse force from a cannon blast they will say "huh?".

So, lacking any real safe way to figure out how strong the epoxy is for your application, I would say stay on the way far side of safe and use it only for a cosmetic filler and not for strength.

Double D · « **Reply #10 on:** March 14, 2006, 06:26:02 PM »

Am I the only one who under stands the concept of using concrete as a filler?

He has a smaller 4in OD 3 in ID seamless tube that is the barrel. He is going to build a powder chamber in the bottom of that. That is his "mortar". That is where all the strength is. Everything from this tube out is decoration and only has a minor part to play in strenght. It is almost laike the golf ball mortars we all built last year.

He is going to weld bottom and face plates on the Seamless tube. then weld the face plates to the outer tube. He his going to fill the space between inner "cannon" tube and outer decorative tube with concrete.

The concrete will be sandwiched between two layers of steel and serves only to provide mass. Mass that you wouldn't get from epoxy, at least the ones I am familiar with. Actually when you think about expansion wise you may even get a slightly magnified hoop strenght effect from laminating steel-concrete-steel.

Fill it full of concrete, get the mass and use the money saved from buying concrete instead of epoxy to buy more powder.

GGaskill · « **Reply #11 on:** March 14, 2006, 08:13:29 PM »

trunnion is 2 in. ( Do you think that needs to be bigger?)

Your barrel is about the size of a full size Coehorn mortar. The trunnion for one of those is 2.75" in diameter.

What is going to be used for shot?

Charcoal Burner · « **Reply #12 on:** March 15, 2006, 03:04:27 PM »

DD that's the way I looked at it, after the breech is installed,I planned on welding the trunnion to the bottom of the breech,( That Will have a powder chamber) then notching the outer tube for the trunnion to fit into, welding that in place before putting the bottom plate on( with all things centered) after that i'll manufacture my vent liner ,, install that ,,lube the threads so that when I fill the void I'll be able to remove the liner if needed,fill the void( with a yet to be determined compound),, and finally welding the plate on the muzzel end .
The intent for the projectile is a 4lb zink ball.A lead ball would weigh 7 lbs I have plenty of lead though and I wouldn't count that out of the equation either.thanks Mike

Rickk · « **Reply #13 on:** March 15, 2006, 05:16:01 PM »

One thing to think about when choosing the filler material... when welding the final plate in place things are going to get hot. Epoxy burns when heated, and can give off toxic vapors when it burns. . RTV usually can take heat - they hold jet engine parts in place with it.. Concrete is funny stuff when heated... make sure it is really dry, as it will start to pop apart if trapped water is present.

Depending on what you pick, you may be able to weld it all together, leaving a little hole someplace to inject the filler after the major welding is done.

If there is no structural reason for filler, Maybe nothing is needed at all?

Minimally expanding foam in a can might make an acustical difference without adding significant weight.

GGaskill · « **Reply #14 on:** March 15, 2006, 05:42:24 PM »

If you think you might use a lead 7 lb shot, I would suggest shrinking another piece of half inch tubing over the bottom end of the main tube to get some extra reinforcing around the ball seat.

Double D · « **Reply #15 on:** March 16, 2006, 12:17:58 AM »

Actually he doesn't need to reinforce the the tube any more If he considers just the inner tube as the mortar he can use a powder chamber that is 1.33 inches in diameter. If the outer tube is filled with concrete with it's 8 inch plus OD I would think you could go even larger. In theory you could have a 2.91 diameter powder chamber. I would think that no more than 2 inches would fine. 1 3/4" better.

Just keep in mindn the thickness of the breech walls als applies to the bashe of the breech. So if you make a 2 in chamber, you have to have 2 inches thick wall at the base of the chamber also.

I would also opt for a bigger trunnions. 2.5 or 3 inch

I would pass on shooting lead balls that big. Besides once you use your mold for zinc you won't be able to cst lead in it. The zinc balls will deform less when shot.

Charcoal Burner · « **Reply #16 on:** March 16, 2006, 08:44:53 AM »

Well here's my rough draft ,, scratched out as promised

It's a little hard to view so i'll give ya some numbers... breech legnth is 6 3/4 in powder chamber is 1 in by 3 1/8,, with an approx 1 1/4 wall at the base.I read in these posts somewhere that a longer narrower powder chamber gave greater velocity. Bore legnth is 3 calibers ( just over 9 in) I would like to radius the end of the breech to fit the ball( I'll have to run that by my machinest) so all you folks in the know let me know what I can and should do with this,, after drawing this up it appears that the 2 in trunnion is a bit small Thanks Mike