Author Topic: Looking for Cohorn plans (Read 1553 times)

CU_Cannon · « **on:** November 20, 2004, 05:48:54 AM »

Is there anyone that has plans of a cohorn mortar or knows where I could find some? Even some dimensions would be great. I am planning on building a golf ball size cohorn that I would like to be scaled from the original. The golf ball mortar just has to look like a cohorn but at some point I would like to build a full size one. That is when I have some extra money.

Cat Whisperer · « **Reply #1 on:** November 20, 2004, 07:58:01 AM »

Do you have a cad package?

Are you looking for an example of something historical, and if so, how complex operations can you do?

The simplest ones are the confederates - simple machining straight walls (OD) and easier that the Yankee ones with more detail in the wall thickness.

Then is the complexity of the base. Simplest is to use wood. Many of the larger ones have cast side plates and several parts inbetween.

CU_Cannon · « **Reply #2 on:** November 20, 2004, 02:55:18 PM »

I like the looks of the more complex cohorns with wood bases. I would like to be as historically accurate as possible.

Machining will not be that much of a problem. I have access to a machine shop with equipment that can handle a golf ball to beer can size mortar. I have a considerable amount of machining experience but not a lot of experience boring. I have enough to get the job done with some learning along the way.

I have AutoCAD 2002.

Double D · « **Reply #3 on:** November 20, 2004, 03:51:31 PM »

I can help you. What part of the country are you in?

Cat Whisperer · « **Reply #4 on:** November 20, 2004, 03:56:23 PM »

The ones in-the-white are golf ball caliber, parkerized one is beer-can caliber. I have plans in AC 2000i. These are CNC made from 4140 (tubes), CRS - trunions, and structural c-channel (base). Same barrel and trunion could easily be afixed to wooden base.

CU_Cannon · « **Reply #5 on:** November 20, 2004, 05:45:25 PM »

I am in upstate New York near Saratoga Springs and Lake George.

This is what I have so far:

It is based on the pictures that I have seen online. The proportions look about right. I would still like it to be closer to scale.

Cat Whisperer · « **Reply #6 on:** November 21, 2004, 02:25:03 AM »

Not bad. But getting it closer to scale at this point will take making measurements from an original. As you know one can draw in full scale and then scale the whole drawing to make an exact scale model of whatever scale.

Two suggestions. Add 9 fillets/rounds to the outside contours - it really dresses it up and makes cleaning easier. (Adding them to the inside of the bore increases strength as well.) Second, consider 1.70" bore diameter vs. 1.75" for golf balls. Take carefull measurements. Golf balls are light and a tighter fit helps a lot.

On mine, I've fastened the trunion to the tube with 2 socket head set screws, heads counter bored into the trunion rather than welding. Welding is OK too, but use 4130 rather than 4140 and ensure that you preheat to the proper temperature (get a certified welder to do it!)

Cat Whisperer · « **Reply #7 on:** November 21, 2004, 02:38:03 AM »

Here's what I've done:

Cat Whisperer · « **Reply #8 on:** November 21, 2004, 02:47:19 AM »

PS. I've gotten some good bullet moulds (brass tri-folders - sugar loaf form) at yard sales in Saratoga Springs!

(We live in Virgina and have relatives in Vermont and other places).

You are fairly close to the Watervliet Arsenal - they have good collection and might be pursuaded to let you take some measurements.

Double D · « **Reply #9 on:** November 21, 2004, 03:07:20 AM »

CW,

Didn't I send my coehorn plans down to to you?

CU-Cannon,

I have set of arsenal drawings for the Coehorn that I would be willing to loan you.

The reason I asked where you were , was if you were close I was going to coerce you into boring out my beercan mortar from 2.75 t0 3.67 for the use of the plans.

Cat Whisperer · « **Reply #10 on:** November 21, 2004, 03:18:03 AM »

If it were a snake - ...

The 1841. I guess I've got to get busy and do some CAD drafting!
Found them right at my FEET next to the easy chair!

Stay tuned. Tube and trunion won't take long to put into a DWG and JPG formats. Base will follow. Have to fix the window in car first this afternoon though. Fact of life: things break.

Thanks for the reminder, DD!

CU_Cannon · « **Reply #11 on:** November 21, 2004, 10:47:04 AM »

I didn't even think of the Watervliet Arsenal. Once I get a chance I'll have to take a trip down there.

CW,
What is the best way to round the inside end of the bore as is shown on your drawing? I can see it being a PITA on a manual lathe. The curved outside will be fun enough.

DD,
I would love to bore your mortar out but I would rather have some more experience bore before I do something for someone else. Screwing up your own stuff is one thing screwing up someone elses is another.

Cat Whisperer · « **Reply #12 on:** November 21, 2004, 11:03:31 AM »

As drawn is tough to do the rounding unless you have one of two things: a) cnc capability or b) a ball end-mill of the right diameter.

So, next best, is to simply do the flat bottom (or the drilled powder chamber) and radius the inside corners. That is done by either rounding the corner of the drill bit done to do the drilling - with a grinder, or by using a rounded cutter on the boring bar.

With wall thickness being what we've both drawn, there is a wealth of material there and strentgh would only become an issue using say - an ounce or so of Unique or Bullseye. But I think it is important to do the easy thing of providing a rounded inside corner everywhere present to greatly reduce the stress points.

If you can, use the BIGGEST bit you can afford to cut the hole. That reduces the number of passes on boring.

I've got pictures of a home-built boring bar (using a milling cutter) that will cut 3/8 of metal in a pass. Takes clearing chips often, but that beats taking .020 at at time.

Cat Whisperer · « **Reply #13 on:** November 21, 2004, 03:16:16 PM »

Here are the dimensions that are published (or derived from published drawings/tables).

Dimensions ARE readible, though not easily. DWG AC2000i on request.
2d drawing so far. Don't have any more drawings to know shape of reinforcements around trunions (but if you're machining the piece that wouldn't matter much.)

Lyle · « **Reply #14 on:** November 23, 2004, 04:19:32 AM »

Peterson's Roundshot and Rammers page 40, 55, 87 & 99.

Lyle

Double D · « **Reply #15 on:** November 23, 2004, 05:51:24 AM »

Round shot and rammers does have scaled drawings and one could surely make a gun from the pictures. Be a little more work measuring and scaling but it could be done.

Round shot and rammers is very handy reference on Muzzle loading artillery and anyone with a interest in Muzzle loading artillery should have a copy in their library.

CU_Cannon · « **Reply #16 on:** November 28, 2004, 02:38:55 PM »

The drawing is great. That is what I was looking for. I only have one question; what is the size and location of the vent?

I wouldn't mind seeing the pictures of the boring bar. If I could use a ball end mill I would have a bore with nice rounded corners.

Cat Whisperer · « **Reply #17 on:** November 28, 2004, 03:08:32 PM »

CU:

In the original the touch hole is near the bottom of the powder chamber at a slight angle towards the rear, in mine it's through the thickest part - straight out through the cylindrical portion.

venting a barrel:
http://www.graybeardoutdoors.com/phpbb2/viewtopic.php?t=38059
I like the vent hole to be large enough so the fuse blows clear, others are more concerned about not loosing too much pressure. Flip a coin.

boring bar:
http://www.graybeardoutdoors.com/phpbb2/viewtopic.php?t=37178
(Take a look at the 3rd picture. Home built 1" boring bar - from a dot-matrix printer. Tooling head was cut from a 1" bolt and a milling cutter bolted into place - did a 3/8" wide boring cut on EACH pass!) A rounded corner cutter insert could be substituted or the inside corner being cut coud be rounded by adjusting the infeed and cross feed such that the inside corner was radiused appropriately.

CU_Cannon · « **Reply #18 on:** November 29, 2004, 09:51:22 AM »

This is what I have so far for the design. I made the powder chamber a little larger. I am planning on rounding the corners of the bore to make the barrel stronger and easer to clean. I found a piece of 4140 that was 3 7/8 x 6 1/2 so the size of the barrel dimensions have changed slightly. If I can find a piece of 4140 for the trunions I still plan on welding it. If not I will probably connect it using socket screws. I'm not sure how strong a weld between dissimilar steels would be.

That is the plan for my first attempt. I opted for a simpler design than trying for a true scale replica. If it turns out well I may look into building the molds and having a 1/2 scale coehorn cast. Cast iron is much nicer to work with than steel.

Cat Whisperer · « **Reply #19 on:** November 29, 2004, 12:44:43 PM »

Lookin' good.

Three suggestions.

1. Extend the length of the large diameter back to go past the touch hole. (As in the original.) The cannon I watched explode was intact from 1/2 the touch hole back - it's a weak point.

2. Radius the inside corners (at the 3-7/8 depth AND the 4-7/8 depth).

3. Weld? 4140? You had better use nothing less than a certified welder that knows what he's doing. The problem with welding 4140 (4130 is considered about as far as you should go) is CRACKING.

The good news is that in golf ball caliber (with golf-balls) the pressure is mild.

CU_Cannon · « **Reply #20 on:** December 04, 2004, 02:32:03 PM »

I think that I will end up attaching the trunions with screws. I think it will be much easer than finding someone to weld it.

Would expanding the powder chamber to 1in present a problem? I found a 1 in ball end mill that would make a great powder chamber but I don't want to end up with a chamber that is too big.

Cat Whisperer · « **Reply #21 on:** December 04, 2004, 02:45:36 PM »

Good decision on the trunion attachment - with 4140.

No problem on the powder chamber - even with air space the key is consistancy.

In golf ball caliber, mine simply have a 1/8" or so fillet in the corner - no powder chamber. Not critical with golf balls and such THICK walls - lots of margin for safety.