Author Topic: 8" id 11" od BB mortar (Read 1862 times)

191701 · « **on:** July 08, 2010, 02:31:16 PM »

I was given a small piece of seamless tube today.

It will be torch cut around 2 1/2' long. I can square it up and bore it on the lathe. I have a design in mind which I will post when i get a drawing made. My plan is to bore it to 8.625" leaving a shoulder maybe 4" in the rear. Make a powder chamber 8.620" od with a 8" od smaller shoulder about 4 1/2" long. drop or press in and weld the 1/2" sticking out. Maybe even weld the inside . What I'm looking for is ideas and suggestions. Bad idea or your an idi@t comments are fine. How big can I safely make the powder chamber? 2 1/2" id would leave 2 3/4 wall plus 1 3/16" wall on the tube. Thanks for the help

RocklockI · « **Reply #1 on:** July 08, 2010, 03:42:33 PM »

welcome to the board 191701 . This is the place for questions .

Cat Whisperer · « **Reply #2 on:** July 08, 2010, 05:15:26 PM »

COOL project!

I would heat shrink the two pieces together - that seals the joint from corrosion from the products of combustion.

I would weld it at the back and NOT at the front. Welding it at the front would cause porositity where it was welded - the very reason seamless is used - and would change the hardness and brittleness adjacent to the weld.

My two cents worth.

GGaskill · « **Reply #3 on:** July 08, 2010, 05:34:21 PM »

Only change I would make is to turn the plug to 8.635" dia. and press the pieces together. They will never come apart. You wouldn't even need to weld them as there is no force that would tend to separate the pieces. The rear part could be .010" over too or not. It wouldn't have to be much over if any.

But you will need a heavy duty press.

6 ounces of Cannon in mine gives about 600 yards. How long is your firing area?

And I would seal the interior joint with some gasket sealer just for the heck of it.

191701 · « **Reply #4 on:** July 08, 2010, 05:53:01 PM »

With heating the tube and freezing the plug I wonder how much expansion and shrinkage I would get?

GGaskill · « **Reply #5 on:** July 08, 2010, 06:14:46 PM »

Thermal expansion of steel is 6 millionths of an inch per inch of metal per degree Fahrenheit. Normal rule of thumb for shrink fit is .001" per inch of diameter plus .001".

Double D · « **Reply #6 on:** July 08, 2010, 06:52:11 PM »

George,

What about thermal contraction-cold is it the same?

GGaskill · « **Reply #7 on:** July 08, 2010, 07:24:38 PM »

Change in size is same up or down for human tolerable temperatures. May change at extreme temps.

Cat Whisperer · « **Reply #8 on:** July 09, 2010, 03:34:31 AM »

I would want to do that with experienced folks and the right setup, as you'll only have one shot at doing it.

191701 · « **Reply #9 on:** July 09, 2010, 05:22:02 AM »

Before I take the plug to size I will freeze it overnight and compare measurements. Add .005-.010" to the bore except where it seats. I have access to a big press that I think will work. Should I put the trunnions in the rear or sides? I am thinking rear. Thanks for the ideas.

GGaskill · « **Reply #10 on:** July 09, 2010, 12:36:50 PM »

You want the breech plug to be larger than the sleeve. Definitely larger on the part that engages the shoulder in the sleeve. OK to be equal or only a little larger in the rest.

A 50-100 ton press would be a good thing to have available. It should be able to press the pieces together even without heating the sleeve. You should chamfer the edge of the muzzle and leading edge of the engaging shoulder on the breech plug so things get started on the square.

191701 · « **Reply #11 on:** July 10, 2010, 03:54:22 AM »

Is 2 feet going to be a good length? I can go a little longer if needed.

dominick · « **Reply #12 on:** July 10, 2010, 05:00:05 AM »

Two calibers long is the typical length for a Coehorn style mortar. 18" Unless you're building a full scale siege Howitzer.

GGaskill · « **Reply #13 on:** July 10, 2010, 09:57:59 AM »

Is 2 feet going to be a good length?

If you would post a dimensioned drawing of your plans, we could give better advice. Doesn't have to be CAD or anything formal, just give all the relevant info.

smokemjoe · « **Reply #14 on:** July 10, 2010, 11:29:00 AM »

Turn the plug to just start and then add for the oversize dem.,This will keep the plug straight when you press it in.

191701 · « **Reply #15 on:** July 10, 2010, 12:16:49 PM »

Quote from: GGaskill on July 10, 2010, 09:57:59 AM

Is 2 feet going to be a good length?

If you would post a dimensioned drawing of your plans, we could give better advice. Doesn't have to be CAD or anything formal, just give all the relevant info.

Did a rough cad drawing just not sure how to post it or convert it to something readable.

Cat Whisperer · « **Reply #16 on:** July 10, 2010, 01:00:20 PM »

when in the CAD program there are two ways you could do it.

Export it using a CAD command to a jpg or bmp file.

or Hit ctrl-printscreen which will copy the image to the clipboard, then open a new jpg file and hit ctrl-V which will paste it into the jpg. Save it. Post it.

191701 · « **Reply #17 on:** July 17, 2010, 05:39:48 AM »

I think I could build this thing easier than converting it to a jpg. I priced a piece of 9" 1018 12" long. $310

thanks for the ideas

GGaskill · « **Reply #18 on:** July 17, 2010, 07:48:19 AM »

Steel is sold by the pound and the pounds go up really fast as you increase diameter (by the square of the diameter, in fact.) Look for a place that sells remnants (also called drops) and they should be cheaper than buying new pieces cut off a long bar.

Cat Whisperer · « **Reply #19 on:** July 17, 2010, 11:46:53 AM »

Add an insert/sleave around the touch-hole to seal crud from getting between the two pieces.

Be VERY carefull in drilling the touch-hole through the radius - you will tend to break bits there. Might want to move it up a whisker.

Yup - drops and rounds - the odd scrap pieces come at sometimes GOOD prices.

191701 · « **Reply #20 on:** July 17, 2010, 12:13:35 PM »

Quote from: Cat Whisperer on July 17, 2010, 11:46:53 AM

Add an insert/sleave around the touch-hole to seal crud from getting between the two pieces.

Should I go just deep enough to get into the powder chamber wall with the sleeve? Or all the way and weld it up? Wouldn't want it becoming a projectile. What about welding the inner bore corner with a small tig weld to seal it? Thanks again

Double D · « **Reply #21 on:** July 17, 2010, 12:42:01 PM »

Quote from: 191701 on July 17, 2010, 12:13:35 PM

Quote from: Cat Whisperer on July 17, 2010, 11:46:53 AM
Add an insert/sleave around the touch-hole to seal crud from getting between the two pieces.

Should I go just deep enough to get into the powder chamber wall with the sleeve? Or all the way and weld it up? Wouldn't want it becoming a projectile. What about welding the inner bore corner with a small tig weld to seal it? Thanks again

The artists and free spirits here will find my suggestion lacking in imagination and creativity, but I will dare again refer to the traditional.

Here is a sketch of how a vent liner should be. It's threaded so it can be removed and replace when it erodes. Original were copper, stainless will work. Always minimize welding where you can.

191701 · « **Reply #22 on:** July 17, 2010, 03:06:42 PM »

You guys are great. Thanks

191701 · « **Reply #23 on:** July 18, 2010, 11:34:45 AM »

For the powder chamber. Max is 2oz fg per inch if I remember correctly? that is 17oz. I want to be able too send it far but I don't think I need a 17oz powder chamber. or do I? What would be a good size? I have 600+ yards to play with now. When I figure wall thickness should I just go by the inner plug and not consider the outer tube? I would like the option to use the max safe load if I ever find a safe place to do it. Would a 2 5/8" powder chamber deep enough to hold 12 oz's be ok? Would that empty place create any problems when shooting lighter loads? Again I thank everyone for sharing what they know.

Zulu · « **Reply #24 on:** July 18, 2010, 11:48:47 AM »

A 17 oz. load will send it into space!

Zulu

armorer77 · « **Reply #25 on:** July 18, 2010, 12:04:21 PM »

In this case your working dia. is your powder chamber . Mine is 1.5" x 2.5" 450grs. of 1Fg is less than 1/2 full for a nice 100 yards . Armorer77

Double D · « **Reply #26 on:** July 18, 2010, 12:32:04 PM »

Quote from: 191701 on July 18, 2010, 11:34:45 AM

For the powder chamber. Max is 2oz fg per inch if I remember correctly? that is 17oz. I want to be able too send it far but I don't think I need a 17oz powder chamber. or do I? What would be a good size? I have 600+ yards to play with now. When I figure wall thickness should I just go by the inner plug and not consider the outer tube? I would like the option to use the max safe load if I ever find a safe place to do it. Would a 2 5/8" powder chamber deep enough to hold 12 oz's be ok? Would that empty place create any problems when shooting lighter loads? Again I thank everyone for sharing what they know.

The base plug you are talking about is 9 inches, so the max chamber diameter would be 3 inches. 2 oz. per in a 3 inch chamber is 6 oz, not 17. In a mortar the powder chamber is the bore size.

In a bore this large and with a projectile this heavy you want to be sure you use Cannon grade powder. Pressure will be substantially higher with Fg and should not be used for large cannons. With Cannon grade use 3 oz per inch or 9 oz. max load.

There is no advantage using the higher pressure Fg powder in these guns.

In this application there will be no adverse effect from the air space. The base of the projectile is round and not flat and is in the expansion chamber where the pressure hammer is dispersed.

Make yourself a dummy plug plug of wood. I would drill a 3 inch hole through a 2" x 4". Fill the hole level full of Cannon grade BP and then weigh the powder. Divide that weight into 9 oz maximum weight. Multiply that number by the thicknes of the 2 4 and that is how deep your chamber must be to hold that maximum weight.

(George-check my math please...

)

GGaskill · « **Reply #27 on:** July 18, 2010, 02:52:03 PM »

My only complaint with your math is the constant 1.5; I would have said "the thickness of the 2 x 4" instead since they aren't necessarily 1.5" thick.

I would add, however, that a 16 lb bowling ball, even though it sounds heavy, is about one quarter the weight a shell of the same diameter would be. On the other hand, the wall thickness of a 1860 cast iron mortar was a lot greater than an oxygen cylinder.

Cat Whisperer · « **Reply #28 on:** July 18, 2010, 04:00:15 PM »

and on the other hand CAST IRON is much different that steel of an 02 tank.

Double D · « **Reply #29 on:** July 18, 2010, 04:10:10 PM »

Quote from: GGaskill on July 18, 2010, 02:52:03 PM

My only complaint with your math is the constant 1.5; I would have said "the thickness of the 2 x 4" instead since they aren't necessarily 1.5" thick.

Good point George! I can always count on you to keep me honest.