This 12pdr Coehorn Mortar was built about 5 years ago. It was my first full scale mortar. I was lucky enough to have access to a large lathe, milling machines and welding equipment at the time.
Total cost for materials was about $300-$350. I would say that it took about 80 hours over the course of a year and a half to complete not including design time. It is a fairly involved project and requires a wide range of skills.
7.00” dia. x 12.63” long hot rolled 1018 steel (1 req.)
(cold rolled would have been better but hot rolled was cheaper)
2.25” dia. x 10.13” long steel (1 req.)
0.25” steel plate (2 req.)
2” x 8”x 8’ long framing lumber (3 req.)
0.50” dia. x 12” long steel bar (6 req.)
1.25” dia. x 1.5” long steel bar (6 req.)
1.00” square x 0.5” steel (6 req.)
0.75” square x 0.4” long steel (4 req.)
3/8” x 6” carriage bolt (4 req.)
0.13" x 2.00" flat bar x 2.0" long (4 req.)
0.25” x 2.00” flat bar x 10.0” long (4 req.)
0.25" x 1.63" flat bar x 4.75" long (2 req.)
Tools / Equipment:
Lathe (as big as you can find)
General hand tools
Wood carving tools
This is my first instructable so please let me know if there is anything I can do to make improvements.
Teachers! Did you use this instructable in your classroom?
Add a Teacher Note to share how you incorporated it into your lesson.
Step 1: Background
I have had an interest in cannons ever since I was a kid. Several years ago I participated in a civil war reenactment and I was hooked. After doing some research I decided that a mortar would give me the most bang for my buck. I came across a small mortar called a Coehorn Mortar. The design was originally developed by the Dutch engineer Barron Von Coehorn in the late 1600’s. Variations of the design were used in the American revolution and Civil war.
My mortar is based on the 24 pdr Federal 1841 model used during the American Civil war. I decided to build mine as a 12 pdr. The size refers to the weight of a solid shot. The shot for a 12 pdr is half the weight of a 24 pdr but about 80% the size.
Technically my mortar is not full scale but at 12pdr I still consider it full size. The confederates did have 12pdr mortar but I liked the Federal design better.
I want to note two things before continuing. First, this mortar is not a toy. It is a very real artillery piece and should be treated as such. Second this mortar 100% legal. It is not considered a “destructive devices” and does not need to be registered in the US. For more information see the National Firearms Act, sections 26 U.S.C. 5845 and 27 CFR 479.11, both can be found on the ATF website.
Step 2: Design
A large number of designs found online are dubious at best and outright dangerous at worst. I based mine on a proven design used during the civil war using modern materials that are of greater strength than the originals.
These drawings were made for my own use so there may be some information missing. If there are any questions about some of the details let me know.
Step 3: The Barrel
The barrel started out as a solid piece of 1018 steel 7" in dia. The original would have been cast out of bronze. Since I didn't win the lottery steel was used.
I started by chucking one end of the bar in the lathe. At about 135 lbs it was about all I was able to handle by myself.
I faced the end of the bar and center drilled it. The bar could then be supported with a center. The breech end of the barrel was roughed out.
After reversing and re-chucking the barrel, most of the outer profile was machined. Then breech end of the barrel was left as is for now.
Step 4: The Barrel Continued
With the outside of the barrel complete as far as possible, it is time to start the bore.
The bore was drilled to 2" (the biggest drill that I had available).
The bore was then increased using a boring bar. This took a long time. As you can see the boring bar I built was kind of small compared to the bore. I didn't feel like building another one. In hind sight I wish I had.
Once I got close to full size I made a temple and a follower to machine the hemispherical bottom of the bore. It worked quite well but I did have some problems with chatter. You can see some ridges in the bottom of the bore. This was why I wish I took the time to make a new bar. I will eventually smooth them out with a die grinder.
The wood block was originally used to support the barrel when it was being chucked up but became a makeshift steady rest when I started to get some chatter. It actually worked quite well and I didn’t need to make any additional tooling.
The two small steps in the bottom of the bore is the start of the powder chamber. It was created similarly using a template. Unfortunately I don't have any pictures of the setup.
Step 5: Finishing the Barrel
With the bore complete the outside of the barrel could then be finished.
The compound rest was set at 6.5 degree and the shallow taper was turned on the breech end of the barrel. The compound rest was then set to 45 degrees so that additional material could be removed in preparation for the round portion.
I made an Excel spreadsheet to calculate the coordinatesof the radius. With the compound rest set at 90 degrees I machined the radius by hand using the coordinates. The result was a stepped radius that was smoothed with a file.
Step 6: Attaching the Trunnion
The trunnion / barrel connection was designed such that the amount of material removed from the barrel was minimized. To accomplish this both the barrel and the trunnion were notched.
A piece of bar stock was cut to length and turned to size to form the trunnion. The only piece of bar that I had was 3.00” dia. That meant a good amount of turning to get the bar down to 2.25”. I always hate to waste so much material but I had the bar on hand and didn’t want to spend any more than I had to on this project.
With the trunnion cut to size it was then clamped in the milling machine. The center was milled down to half its thickness. Steps were formed to match the corresponding notch in the barrel.
The barrel was clamped in the milling machine and milled to accept the trunnion.
Two small pieces of plate were also cut to size and fit. They are not really necessary but recreate details found on the original cast barrels.
Step 7: Welding the Trunnion
With all the pieces fit and assembled the barrel was then welded. I wouldn’t consider myself a welder by any stretch of the imagination. I did succeed in laying down enough metal that there is not worry of it falling apart.
I’ll admit that this is a poorly designed weld joint. The sharp corners would lead to inclusions in the weld that woudl be unacceptable in a critical joint. Since the primary function of the joint is to hold everything together it is okay. The strength of the joint is more than adequate for any loads it will see.
After the welding was complete everything was ground sooth. Any spots that were low were then filled with JB-Weld and sanded. The goal was to make it look like the barrel was one casting rather than separate pieces welded together.
Step 8: Drilling the Vent
The last thing to do was to drill the vent. This is my favorite part because this is the point that the barrel goes from a hunk of steel to a mortar.
The vent is located such that it will meet the powder chamber about a half inch from the bottom.
Since the vent is located on the tapered part of the barrel the hole was started with a center drill. The vent was then drilled with a #7 drill. Original cannons would have a vent 0.2” dia. At 0.201”, a #7 drill is perfect. The size is also just right for 1/8” cannon fuse.
When all was said and done I would estimate that about 30 hours were spent on the barrel.
Step 9: Hardware
None of the hardware used is standard. With an exception of four carriage bolts, everything was built from scratch.
The nuts were made from square bar. Each one was cut to length on the band saw, chucked in the lathe and faced square. A hole was then drilled and tapped in each. Each nut was then de-burred and the tops chamfered. All together it took about 2 hrs to make the 10 nuts required.
The carriage bolts were made in two pieces. The head was turned separately and welded to the shaft. Once welded, the square portion was milled. A small jig was made to hold the bolt while milling the square portion.
Each bolt took about 1.5 hours. I probably could have done it faster but I decided to cut the threads in the lathe. A die would have been much faster but I wanted the practice.
Four washers were made from 1/8” flat bar. They were rough cut on the band saw then turned round and chamfered in the lathe. A small mandrel was made to hold them. It took about an hour for the four washers.
The only hardware that was not custom made were the 4 carriage bolts that hold the barrel in place. The only the very end of the threads are visible. It just wasn’t worth the effort to make them from scratch.
Step 10: Hardware Continued
The handles were made from 0.25” flat bar that I had a lot of.
The profile was cut out on the band saw and ground smooth. The bar was then bent into the shape of the handle. They look good but are to narrow. When carried they tend to cut into your hand. I may make a proper set using round bar at some point. The handles took about 6 hours to complete.
The last part of the hardware is the trunnion caps. They consist of pieces of flat bar cut to size, drilled and beveled. They were the fastest parts to make. Total time for fabrication was about a half hour.
Step 11: The Base
Unfortunately I didn’t get any good pictures of the base as it was being built. The project had gone on long enough that I just wanted to get it done.
The base was laminated from 2x8 framing lumber. The original would have been white oak, ash, hickory or some other hardwood. A block of oak this size would be rather expensive and hard to find not to mention heavy. At this point the project had gone on for the better part of a year, I was just anxious to get it done.
Each piece was cut to length and drilled with two holes for the tie bolts and four for the handles. The end detail was cut using a band saw. I also roughed out the recess for the barrel.
The pieces were glued together using Titebond II to form a solid block. Pieces of threaded rod were used in the holes to clamp the pieces together.
Once everything dried the pieces of rod were removed. This required some significant pounding as they had also become glued in place.
A hand plane was used to flatten the top and bottom of the block.
A recess was carved in the block to accept the barrel. This took a lot of carving, checking the fit and more carving. All together this was one of the most time consuming tasks for the entire build. The base took about 15 hours to complete.
Step 12: Finishing Everything
All the steel was cleaned and given a coat of primer followed by two coats of black Rust-Oleum spray paint. The inside of the bore was left unpainted.
The wood was sanded. There were a few spots that needed some wood filler. The entire block was then painted with green paint. I got the color from the New York State Military museum. I was told it was matched from original artillery.
The total mortar weighs about 140 lbs.
Step 13: Firing
So far I have only shot blanks with the mortar. The powder is contained in foil packets. This makes handling much easier and safer.
For ignition I have used both friction primers and cannon fuse. Cannon fuse is cheaper but there is a delay between lighting the fuse and firing. A lot can happen in that time.
I would like to do some live firing as some point. Potential projectiles include, candle pin bowling balls, 4” pvc pipe filled with concrete and frozen grapefruit. Replica shells are available but they are quite expensive. I’ll try one of the options at some point but I need to find a large open field first.
I have only used true black power. In my experience, the synthetic powders do not work well in cannons. I think they need to build high pressure to burn correctly.
That about sums up everything. If you have stuck with me this long I would like to thank you and I hope that you enjoyed it.