Beer Can Caliber 24 Pounder Coehorn Mortar


Here's a historical photo of the kind of mortar I want to model. The mortar beds had three general shapes: rounded corners, beveled, or roundover. The roundover was what was listed on ordnance drawings, so that's what I'll go with. The others were probably field expedient changes. Basically these mortars were light enough for 4 men to move in the battlefield, but they weren't powerful enough for protracted sieges.



The first order of business was to make a bigger steady rest. The one that came with the lathe only opens up to about 3.5" or so. So I scrounged up scrap materials from work and built it in a few evenings. The tubing is 1.25" square tubing with .120 wall thickness. The clamp piece under the ways is 1/2" plate. The white plastic piece is HDPE I cut on a table saw. The threaded rod and nuts are 5/8-11. The threaded rod have a 3/8" hole drilled 3/4" of an inch in to hold the brass tips.



The round bar is 1018, 4" diameter by 7.5" long. I started off with some smaller drill bits and worked my way up to 1", which is the largest I have. I only drilled down the length of the bore, 4.1"



Next I started boring. A problem with my steady rest is the HDPE block sticks out a little preventing me from moving the carriage in an extra 2 inches which would help keep the boring bar from sticking out so much. Since that boring bar isn't very rigid, I only took off .010" per pass. Increasing a hole from 1" to 2.667" = 84 passes. O_O Whoever decided to name boring "boring" sure picked the right word. :D



Now that the bore was finished I could get in there with my 1" drill bit again and drill a 1.75" powder chamber.



The bore ended up being 3 thousandths off at 2.670", but I'm not complaining. I can't wait to send a few concrete filled cans skyward!



Next I flipped the barrel end for end and worked on the outside profile. The taper is 6.5º and took some concentration to not to get my knuckles knocked on the chuck.



Here's how I make curves when I don't want to free hand them. I use a free program called DraftSight to draw the curve I want to make. Then I use the array tool to make a bunch of lines spaced out however much I want to take off at a time. In this case I spaced them out every .010". Then I set a dial indicator up on the ways to keep track of how long my cut is. Then I select the line in DraftSight to get its length and crank the carriage over until the dial indicator tells me I've gone that far. In this case I'm 3/4 the way done with the curve and my next cut is .598". It's a good idea to delete the line before selecting the next so you don't get confused.



And here's the curve stepped off.



Filed and sanded.



And finally out of the lathe. So far I've got 18.5 hours in it, but boring took forever. If I had a bigger drill bit it would go a lot faster.



The next item was making the trunnion. It's 5.8" long and 1-5/16" diameter. On my 10" seacoast mortar I ground one out by hand with a 4.5" angle grinder, but that was tedious. Eventually I hit on the idea of grabbing the trunnion with the 4 jaw chuck and just doing the reverse of the curve I put on the breech of the tube. Here's my CAD drawing of what's supposed to happen. I started off trying to face off the cuts, but quickly decided maybe drilling a 1" hole and boring it out would be better. Then I realized my boring bar couldn't reach to the bottom of the 1" hole so I only bored to where the taper of the drill bit starts.



And here's my setup. I select a line in CAD, see how long it is, and crank until the dial indicator tells me I've gone that far. Back out, move over .010" and do it another 90 times.



Then I came back and cleaned up the rest of the curve by hand. I'd turn a bit and then check to see how the tube fit in the trunnion. Make a note of where it hit and turn some more. Kind of a roundabout way of doing it, but it worked.



Next I welded the trunnion onto the tube. My welding skills have vastly improved since the spatter covered seacoast mortar. Very little excess weld to grind off.



Then I drilled the vent. A few pieces of scrap wood level out the trunnion and a C clamp holds everything down. I used a #35 drill since I like 1.8mm visco fuse. #35 (0.1065") is perfect for 1.5 x 1.8mm fuse diameter (0.1063")



Next I built the bed out of some maple.



You can never have too many clamps



I left everything ~1/8 big so I could run it through the planer to get everything flat. I trimmed the ends with the table saw.



Laying out the round over. I figure I'll just eyeball this side and do the same to the other end after each cut.



Worked pretty well.



Then I attacked the corners with a 7" sander. It's best to lightly sand completely from one side to the other, rather than try and form the arc as you go along. Just hit the ridge and make 2 ridges. Then sand those 2 ridges into 4 and so on until you get the curve.



Next was a Forstner bit and some chisels to mortise out the slot for the trunnion.



Then I used a candle to put soot on the bottom of the mortar to inlet it into the bed. The soot will rub off onto the high spots, then shave the high spots off with a gouge. It's best to keep your gouge razor sharp so you can cut across the grain. I spent 5 hours inletting the barrel.



A proper inletting job makes it look like the wood grew around the metal and holds the barrel at 45º.



Last are the handles. I've been putting them off because I remember the ones on my little 1" bore Coehorn being an absolute pain to make. Anyway, I made a bending jig out of scrap material to get everything the same. Don't mind that extra hole, sometimes my uh... assistant... has trouble remembering which side of the number to mark when the tape measure is upside down.



Then I made the flat part of the handles. I tacked everything together in one big block and drilled the holes so I knew they were spaced the same. Then I cut the shape with a 4.5" angle grinder and smoothed the corners out on a bench grinder.



Next I made a jig to weld the pieces together. The bent piece slides up against the stop, and the flat pieces slip over the studs in the block of wood. I just held the ground against the handle and tacked everything in place.



Then I could really stick them on good.



Then cut off the excess and smooth it out with a grinder and files.



Then drilling holes and cutting threaded rod to make the bolts. I used a drill press and drilled from both sides to meet in the middle. It's a good idea to back out and get rid of the sawdust frequently. The 3/8" Forstner bit for the two big bolts aligned perfectly, while my 1/4" brad point bit for the handles was a tad off. I wallowed out the hole as best I could and then beat a piece of threaded rod through with a hammer to clean up the rough spot.



Just in case you're looking for paint, here's what I used for the bed. It's supposed to be very close to the color used on Civil War carriages.



And here she is all painted up!



And here's a powder can to show the scale.


Here's a video before I painted it. The first shot was 125 grains Fg, the second was a full chamber of 250 grains Fg. My cans of cement had only been drying for 5 hours, so they blew apart on takeoff. Afterward I went over to the 25 yard range to shoot my 1858 New Army and a few guys wanted to know "what in the tarnation was you shootin' over there?!" :D



Here's another video after I painted it. All shots were 100 grains Goex Fg and the cans were 1/3 full of Rockite expansion cement. They went between 85 and 105 yards. As the fouling built up on the bore, the cans went farther due to the reduction in windage.