Coehorn construction question.

[Victor3]

I'm getting ready to build a couple of "block" type (solid, fixed bbl angle) bases for my golf ball and 1" Coehorn bbls.

Are the bbls on these normally just set into somewhat rough inletting around the rear of the bbl where they are free to swivel back until the bbl contacts wood, or is something else used that fixes them in place?

I'm sure that tight fitting caps would help keep the bbl from moving freely, but is there anything else that's normally done to prevent the bbl from moving at all?

[Rickk]

On mine, the wood is tightly inletted and it can not move. If it tries to swing back, the back of the barrel will hit the wood and stop it immediately. Inletting was the hardest part of making the base. I drilled a hole to clear the trunnion, squared the sides of the hole off going up to the top with a circular saw, and then the rest was done with a chisel. My barrel weighs over 90#, so simply testing the fit every so often was a major workout.

If you are going to paint it rather than stain it, and find that you cut away a bit too much wood here and there, Bondo Fiberglass Gel will help you out. I masked the base of the barrel with celophane tape first to insure that the stuff did not stick to the barrel.

[jeeper1]

Here is a picture of the base I made for my GB mortar. It's ugly inside but the ugly doesn't show when assembled.

I actually cut the slot too deep because I used the narrowest board for setting up the drill jig. I added spacers under the bars that hold the trunnions in and these also work as clamps so the tube angle doesn't change.

[Double D]

Here is a picture of the base I made for my GB mortar. It's ugly inside but the ugly doesn't show when assembled.

Well if you hadn't spilt your rose' !!!

[Rickk]

jeeper1 ... you shoulda kept us all in awe !   

[GGaskill]

Having made a couple of inletted block Coehorn mounts, I will concur that the inletting takes more time than most of the rest of the project. 

However, since the breech of the Coehorn is symmetrical with respect to the long axis, it would be possible to make a custom cutter that would do the inletting except for the trunnion (which is the easiest part) using a machine.  Basically something that looked like a spade drill with the profile of a Coehorn would be all that is needed.  Then the trunnion slot could be machined with a ball nose end mill and the only chiseling necessary would be making the ends of the trunnion slot cylindrical.

[Victor3]

Thanks for the ideas guys...

GGaskill - Interesting thought about a form cutter. For me in my garage however, making such a large one for a "one-off" would probably be impractical. My GB mortar would require one 3.5" diameter with a step and full radius on the end.

I suppose a flycutter could be set to cut the required radii of the bbl (With the block set at 45 degrees in a vise) in two steps though. Then the full radius at the rear near the trunnion could be rough milled and chisled to fit. That would at least make the front area of the inletting a near perfect fit, and I could cast the rear with Acraglas to match. I think I'll do that.

As far as the trunnion slot goes, I was thinking of milling slots with the side of an end mill in the separate pieces individually. That way, the slot would be cylindrical to fit the trunnion diameter and flat on the ends (No chisel required). The stack could then be glued and bolted.

[Cat Whisperer]

Thanks for the ideas guys...
...
As far as the trunnion slot goes, I was thinking of milling slots with the side of an end mill in the separate pieces individually. That way, the slot would be cylindrical to fit the trunnion diameter and flat on the ends (No chisel required). The stack could then be glued and bolted.

That works.  Leave a little extra to custom fit the trunnion.  Some folks will then glass bed the trunnion as well.

[GGaskill]

The problem with cutting the cavity with standard tooling is that the breech end of a Coehorn is tapered as well as sort of hemispherical.  So a straight in feed will not cut a properly shaped hole.

But you are right about making a special tool for only a one off part.

[Victor3]

Fortunately (At least as far as the inletting work goes), mine are straight-turned, not tapered as in the originals. That makes things a bit less complicated...

[copdoc]

Hi Victor3
I drilled the trunnion slots with an adjustable bit(Irwin Lockhead).   For the barrel I used a wood chisel and for some of the later ones got a chain saw type blade for the angle grinder(Lancelot).  As you can imagine it would eat you up it you are not careful but made it easier. I will post a pic if anyone wants to see it. It was a pain to inlet the full sized 24 pounders and that Bugundian howitzer.  I may work on a carriage/sled/tripod/barrel for a year or 2 at a time.  The Confederate Coehorn barrel weighs over 200 lbs.

Jeeper1 That is a nice piece of wood.  I built one (Kufstein Mortar) out of walnut by luck as a friend just happened to have some and asked if I could use it for a cannon.  I would not even know where to buy that much walnut if I could afford it.

[jeeper1]

Here is a link to my mortar base build.
http://www.gboreloaded.com/forums/index.php/topic,93894.30.html

[copdoc]

Oh purpleheart, I thought it was walnut.  That is going to be nice.