.177 Coehorn Construction Project

[Microboomer]

Hi Everyone,
I've been reading for a few months now, and am finally getting around to posting a project:
I had found a good deal on a Harbor Freight Mini Mill last summer, and by Fall I had it set up pretty well and started getting itchy to work on a project that wasn't a tool part.
I found the "Model_Cannons" group on yahoo and was fascinated by some photos of a "BB mortar".  There weren't any plans, but it was pretty easy to figure out that the diameter was about 5/8".  It occurred to me that because the two outer rings are the same diameter, a Coehorn would be pretty easy to make on a mill by putting stock in a collet and using lathe bits clamped in a milling vise.  I found CAD plans for a larger Coehorn in the same yahoo group's file section, and started scaling it down to 5/8".

The first problem I had was figuring out the bore size.  I don't own a BB gun anymore, but I remembered a pretty loose fit from when I was a kid.  So which is .177" - the BB or the Bore?
The easy solution was to buy a tube of BB's and measure a few.  I miked 20 BBs and found the average was .1729 and the largest was .1739".  I figured that .177 would be a good bore size if I could expect BB's to be less than .174".  (Now that I"ve found the great resources here, I realize that the idea bore size would be just over .178")  The great thing about a .177 bore is that you can do it with standard drills - drill a #17 hole (.173) and make a D-bit reamer out of a #16 drill blank (.177) to clean it up.
My final design ended up being stretched a bit from the original Coehorn dimensions - mine is 1.25" long and .61" diameter.  I bought a 5/8" collet and a 5/8" bar of brass and went to work.
I didn't take any pictures of construction, but I have some for my second mortar, which I'll post later in this thread.
Here's a picture with my 'temporary' sled:

I've got to run - more to come!
andy

[Double D]

Sometimes we have difficulty understanding perspective here on the board.  Some years ago a friend  Blaster suggested that to establish a scale we  include a powder can in the picture.  I really think that in this case, to clearly understand what you built. some thing like that would help...doesn't have to be a powder can, even a pop can will suffice.

[subdjoe]

Sometimes we have difficulty understanding perspective here on the board.  Some years ago a friend  Blaster suggested that to establish a scale we  include a powder can in the picture.  I really think that in this case, to clearly understand what you built. some thing like that would help...doesn't have to be a powder can, even a pop can will suffice.

On this one I have no trouble with the scale.  We know that the bore is roughly 1/5 of an inch.  We also have the fuse to help give scale.  Muzzle face is maybe about the diameter of a penny?

[Mike H.]

For scale laying a quarter next to it would help.  Nice work.  It is often difficult to make something tiny that still looks good.

[jlwilliams]

That's prett cool.  I'd never thought of a BB bore artilery piece.  Why not?

[Microboomer]

Double D: Sorry, I'm using old pictures that predate my first powder can. Here's a perspective shot:


I'd stopped thinking of this as small after going through some old posts here where folks have built much smaller mortars (search "nano mortar" if you don't know what I'm talking about).
I think the most interesting part of this project was to making something on a milling machine that you'd normally make on a lathe.  Most people get a lathe before a milling machine, and you can make or buy milling attachments for lathes.  Before I got the mini-mill, it never occurred to me that it could be used as a lathe w/o any attachments.  I chose a small project because I have a small mill!
As I mentioned, I don't have construction pictures for the mortar shown above, but I took pictures when I built a second one, and I'll use them to talk about the process.
The nice thing about making a cannon barrel is that their are few places where true precision is needed.  The bore is the hardest part, so I decided I'd start there first.  I'd drill/ream the bore, then drill the vent, and I already have a working cannon!  Everything after that is just decoration...

Here I have the bore and vent drilled, and am setting up to drill a hole for the trunnion bar on a Coehorn:

There is a 1/16" piece of drill rod stuck in the vent.  I need to drill a hole for the trunnion bar that is perpendicular to the vent, so I just rotate the barrel in the vise so that the drill rod is horizontal, and tighten up the jaws.  I don't know if you can tell, but I have some aluminum foil in the vise to protect the work.  You can see a 1/8" end mill over a black dot on the barrel. What I did next was to start the mill and make passes perpendicular to the barrel axis while lowering the tool.  When the tool just touches the barrel, I'll have a tiny flat rectangle milled on it, and I mark the center of that for the spot to drill the trunnion hole. After marking, I switch to drill bits to drill the hole.
Once the hole is finished, I'll put my 5/8" collet in the spindle, and put the barrel in the collet so I can start turning.

This is a picture after I've finished the easy part of the turning:

Hey, it's starting to look like a Coehorn!
You can see the rings, the vent, and the trunnion hole.

We're having thunderstorms - I'm gonna post this and finish later!
andy

[Double D]

Most people would have just plunked the cannon down in front of the can and take a picture, I would have. Setting it on top of the can is a cool step....I like it.

Who needs a lathe to make a cannon, innovative, creative...major kudo's to you.

[Microboomer]

Thanks for the kind words, Double D!
Now back to where I was before the lights started flickering...

To make the curved base of the mortar, I cut a series of steps giving a jagged approximation of the curve.
Later I go back and file it smooth.  I carefully calculated a complete set of steps, but ended up just making a few cuts to the proper depth, and then eyeballing cuts in between.
From the picture in my last post, you can see that the mortar-to-be is essentially hanging upside down from the mill spindle.  As I made steps closer to the bottom of the mortar (which is higher up in that picture) I'm getting closer and closer to cutting the whole thing loose from the bar stock.  When there was just a bit of metal left holding things together, I stopped the mill and cut the mortar off with a hacksaw and it looked like this:

The idea is that the inside corners of the cuts define the final curve.
It's hard to tell from the picture, but the cut off point is above the center of the trunnion hole, so that the trunnion bar will be held in place fairly well prior to soldering it in place.

The barrel bands are still close to the original 5/8" diameter, so now I flip the barrel over, and put in in the collet (with two layers of Al foil for protection) and start filing.  Filing goes really fast, because there's very little metal to remove (and it's brass).
After filing to shape I finish with 100, 220, and 320 sandpaper.  I cover the mill ways to keep grit out.
Then I put the barrel in the vise and countersink the touch hole.
Finally I silver soldered the trunnion bar in place (using a Harbor Freight $7.99 butane torch - nice little tool!), cleaned up around the solder joint, and it was done!

.30 Cal Mortar finished

OK, my construction pics were of my second mortar, but let me get back to the BB mortar.  Initial smoke and flame was nice, but disappointing in the sound department - "pssssssssssssssssst" was all I got.  Part of the problem was my powder - I went to any number of local gun shops and couldn't find real black powder, so I bought some pyrodex P.  After discovering this forum, I found out that Bass Pro has the real deal, and there's one less than an hour from the house, so I got some Goex.  This gave me a little 'pop'  with the BB mortar, and actually shot a BB pretty well, but it still wasn't that impressive.  Part of the problem might be that the fuse hole cross section area is about 14% of the bore area!  Lots of fire comes out of the touch hole when fired.
I haven't tried smoke and flame pictures yet, but I'll try that this week and post if successful!
  My current goal is to find a way to make a big bang in a small package.  The 30 Cal mortar was a failed attempt at that.
andy

[Co. Batguano]

Bang "loudness" is a function of pressure mostly.  That's why cannons are louder than mortars, and rifles are louder than handguns.  The distance you hear it away from the muzzle is more a function of the amount of air that is displaced, and the length of the wave.  So a bigger caliber gun will have a longer wave (deeper pitched sound) than a smaller caliber one, and you will hear it from further away.

[keith44]

pressure, and velocity.  The projectile that is traveling at "sub-sonic" speeds when it exits the barrel will be quieter than one moving at "super-sonic" velocities.  This is also why sometimes a gun will boom, and other times it will crack, and seem louder.

[Rayfan87]

The easiest thing for future ones is make the vent smaller and use something other than green fuse to light it. I'd try putting a little powder on top of the vent and making a linstock

[Double D]

Check with Cannonfuse.com, http://www.cannonfuse.com/store/pc/viewCategories.asp?idCategory=2 for 1.8mm fuse.  THey might even have something smaller. 

I like  Cannonfuse.com they are in Montana and supported the last Cut  Bank cannon convention.

[Microboomer]

I know my fuse looks huge in the 1st picture, but it's a 1/16"(1.6mm) fuse I got from pyrocreations.com  It measures slightly less than that, so I drilled the vent with a #52(.063) drill.  I have to clean the vent out carefully to get the fuse in!  Lately I've been putting a few grains of 3F in the touch hole and lighting with my butane torch - this seems to be more reliable than the fuse.  I'll probably make a linstock at some point.
My current project is a small handgonne.  I drilled a 2.5" long .178" bore in a rod and drilled/countersinked a 1/16" vent.  Test firing with powder in the vent was amazingly loud - the extra length makes a HUGE difference!
I'm not sure if I want to try less than 1/16" for a vent - it would be tough to clean out.  On the other hand, it would be easy enough to test a tiny vent, and then drill larger if there's any problems...
andy

[Microboomer]

Here's the Smoke and Flame shots:


This is Max displaying good safety technique.


I like the way the fire looks in this one.
andy

[Rayfan87]

Very nice. But when you're getting that much flame from the touch hole compared to the muzzle, its a sign to use a smaller touch hole.

[Microboomer]

Yes, I'll definitely try a smaller touch hole when I make another in the same caliber.  When I built this I thought a fuse was my only option for firing.  Now that I'm past that, it will be interesting to see how small of a hole I can use and still fire reliably.
andy

[Rayfan87]

If you can get your hands on a can of Goex Meal-D, you're pretty much only limited by what size drill you can find.