Golfball Bore 10" Seacoast Mortar, Model 1840

The Model 1840 10" Seacoast Mortar was the Army's biggest and heaviest mortar during the period before the Civil War. The barrel alone averaged 5,775 pounds, so these guns weren't meant to be moved around a lot. These mortars were usually placed in forts to protect strategic spots along coasts or rivers by dropping explosive shells on enemy ships. The Federals used them in siege operations during the Civil War, but they were soon made obsolete by the larger Model 1861 13" Seacoast Mortar which threw a heavier shell to a greater range. The Confederates kept the 10" mortar in service well after the Federals due to their chronic shortage of artillery pieces.

Here's a period photograph of a 10" Seacoast Mortar battery.

When I first got my lathe I had grand ideas about making a 13" long golf ball howitzer based off of one of CU_Cannon's plans. I bought some 4" 1018 and started working on the cascabel, but the project was a little over my skill level and possibly my lathe's capacity. This hunk of metal has been sitting under my workbench for 2–3 years and I decided it'd be a good candidate for my 10" seacoast mortar. I'm using GGaskill's plans which are conveniently scaled to golf ball bore.

I cut off the cascabel and muzzle so I had 7" of material. Obviously what was left of the howitzer's pilot hole will start the bore of the new mortar.

Next I faced off the muzzle so I could measure back to where the diameter changes.

Turning the taper over the bore.

Boring bore boring. :D I opened it up to 1.723", so it's right at 1/40th windage.

Drilling a 5/8" powder chamber for a maximum capacity of around 100 grains. I figure that will get me out to 100 yards with steel Fox balls at 45º. I went with 5/8" because from what I've read, I think long narrow powder chambers are a little more reliable and efficient than wider ones.

I don't have a ball turning attachment, so I drew the breech in CAD and then drew a horizontal line every 50 thousandths. Then I could select the next step and see how long the cut should be. Believe it or not, this is the first time I've used the tick marks on my lathe's knobs. I took a "machining" class in college and it was really a lot of fun. I didn't learn much because there were too many students and the teacher was truly incompetent. However, that class was responsible for making me to get my own lathe to play around with. Anyway, I'm starting to figure out easier ways than stopping the lathe every 30 seconds to measure with calipers.

The stepped off hemisphere.

10 minutes with an angle grinder and files.

Done! Or so I thought. I can't believe I forgot the little "lip" at the start of the breech's taper. I think I was so excited about how well the hemisphere came out that I completely ignored it.

I managed to get the tube back on my lathe relatively straight and deepened the breech's taper to make that lip just behind the cylindrical part. I also bought 75 steel balls. Man these suckers are no joke! I tossed one up in the air and it came down with a satisfying "THUNK!"

I usually like trying to make everything myself, but cutting the mortar bed out is a bit much. I'm sure I could do it, but instead I drew up some plans and took them to a local metal fabricator and am having him cut them out with a water jet. The main piece is 3/4", while the thinner outside piece is 3/16". I plan to weld the seam and try to blend everything together with a grinder/dremel to make it look like one cast piece. The lifting lug is 1/2" and the inner trunnion supports are 3/8".

Here's the parts I got back from the metal fabricator. He had to make two lifting lugs because otherwise the piece would fall through the grate on the machine. I'm glad it worked out like that because it'll give me a second chance if I mess one up welding.

The material for the trunnion is 1-3/8" 1018 which I turned down to 1.3 or so. It's a little loose in the sides of the mortar bed, but a few layers of paint should tighten it up. The next problem was how to scoop out a hollow for the hemispherical end of the mortar. I don't have a milling machine, so I did the next logical thing and grabbed an angle grinder and started for China! I would have gotten further but for some reason my grinding wheel seemed to have disappeared.

After buying another grinding wheel I had the trunnion finished. It fits surprisingly well.

I didn't take a picture, but prior to welding I beveled the edges of the trunnion so I could get way down to the centerline. I'm not a great welder, but I think I got a solid weld. At least the trunnion hasn't fallen off yet. :D

I cleaned up the weld a bit and stuck it on the mortar bed. For some reason pictures make the sizes look wrong, at least to me. I think once I put the wooden cross pieces in the bed won't look so flimsy and the proportions will look right.

The oak pieces that go between the metal sides are cut out, and I welded on the lifting lug and rimbases. I was surprised at how close to level the mortar sits when suspended by the lifting lug. I've also been grinding, filing, bondo-ing, and sanding the joint between trunnion and tube. It's getting there but it still needs a bit of work.

Next I made the bolts and drilled 1/4" holes through the side plates for them.

To make sure the holes lined up, I clamped the second side to the first and made some shallow divots.

Then I unclamped them and finished drilling all the way through.

Next I put the bolts through the holes to align the two sides. Then I carefully removed the bolts and shimmed my wooden pieces to the correct height and clamped the sides together. Then I drilled the wooden pieces with a cordless drill. My bit wasn't long enough to make it through the 3/4" side and the 4" piece of wood so I had to drill from both sides. The holes came out a bit crooked so I enlarged the two ends of the hole to 9/32" and about 3/8" deep which allowed the bolt to slide easily through the hole.

Some of the holes in the wood are mighty close to the surface, but I think they'll be ok.

And here's the bed put together. I also drilled a 1.75" long, 3/32" diameter vent. It intersects right where the 5/8" cylindrical section of the powder chamber turns into the drill bit point.

The next thing to do was to make the half moon shapes in the side so the nuts can freely rotate. I should have drawn them on my CAD drawing and let the water jet cut them out, but I didn't think that far ahead. So instead I chucked an abrasive die grinder bit in the drill press and set the depth so I could grind out the shape. It took a long time.

Next I cut the bolts off to the right length and took the threads off the end for the handspike ones. The reason they aren't installed is because I forgot to take into account the width of the nuts. :o They lined up perfectly with the side of the carriage though!

New handspike bolts made and installed.

Next up was the metal straps around the wooden spacers. I cut the four strips out of 18 gauge sheet metal with a skill saw. Then drilled a 1/16" pilot hole in the wood, 3/32" clearance hole in the metal, and then countersunk with a 5/16" bit. I used a flathead eye glasses screwdriver set to screw in the #2 wood screws.

Here's the piece that supports the center part of the trunnion. My trunnion is 1.3" in diameter, so I started with a piece of DOM tubing 1-3/8" ID and 5/8" wall thickness. I faced it off until it would just slide between the sides of the carriage, then I used a hand held band saw to cut a 90º wedge out.

Then I clamped it in place and welded. I've been holding off shooting the mortar until this piece was installed because I wanted as much support under the trunnion as possible. I'm sure it wouldn't matter with golf balls, but I primarily want to shoot Fox balls in this one.

Up to 34 pounds!

The first test fire! 50 grains of FFg black powder sent the 11 ounce steel ball ~150 yards!

I've got 2 layers of Bondo and 3 layers of glazing putty around the trunnion/rimbase/tube joint, and around the lifting lug. I also put a thin fillet of glazing putty around the muzzle and base ring.

Next I used an airbrush kit to spray on some flat black Rustoleum enamel. I've never used an airbrush before, although my father used it quite a bit in his duck decoy carving hobby. It took a fair amount of experimenting before I realized the air hose was clogged, then I found out the paint nozzle and air nozzle have to be just right. By that point I'd thinned the paint down so much thinking it was too viscous to be sucked up the tube. Eventually I got out the instructions and things went together better. :D

I stamped the muzzle face before painting but forgot to take a picture of it. My initials are where the inspectors would be, it's No. 1, and I took the average of 3 weights to come up with mine. At first I thought about calculating the volume of my mortar and seeing what it would be in cast iron and then scaling to full scale, but my powder chamber is different than an original so there was no point in doing that. I didn't stamp the trunnions because I filled up the hole for my live center with a welder and the weld was too hard to stamp.

And here's a shot of the tube from the side. The paint's a little thin in places so it needs a few more coats.

Another first for me during this project is threading. I've been putting off making the elevating screw and nut but finally I took the plunge and gave it a shot. Sorry for the lack of in progress pictures, but I got so carried away I forgot about taking them.

First I decided on 10 threads per inch, so eventually I figured out that works out to a tooth/groove width/depth of .050". I made my cutter out of of O-1 steel, heated the tip to bright orange and quenched in old motor oil. Then I attacked with a Dremel and tried to make the tooth as square and sharp as possible.

I used 12L14 for the nut and bolt because there's no need to make cutting them any harder for myself. I drilled a 7/16" hole in the nut, and then started making .005" passes with my cutter. I was honestly surprised how well it cut and pretty soon I had the nut made. Then I turned down the outside diameters and drilled 8 3/16" holes around the outside. I assume those were for handspikes or some sort of lever to turn the nut.

Next up was the bolt which gave me more trouble than the nut. I used a parting tool to cut the grooves, but I don't think my nut's teeth and grooves are exactly .050". I tried deepening the grooves of the bolt but finally realized the bolt's outside diameter was just too big. I turned it down some and now the nut threads on easily. Surprisingly my first threading operation turned out pretty good. There's a barely perceptible amount of backlash, but I'm not sure how close the teeth go to the bottom of the groove.

Next I cut a slot down the side of the bolt so a pin will prevent the bolt from turning. I blocked up a parting tool sideways until it was on the centerline of the bolt and engaged the back gear so the spindle wouldn't turn. Then I started scraping it along the side taking off no more than .003" per pass. After about 60 passes I had a pretty nice keyway.

I got the elevating screw and its bracket installed this evening. The threaded rod and square nuts are 8-32.

Drilling the holes took some care because the center 8-32 bolt is pretty close to the crosswise 1/4" bolt hole. However, I just took it slow and measured as many ways as I could.

Here it is all put together. I've left the elevating screw long until I assemble the bed again and see how it works. I think cutting it off flush with the wood while it's down as far as it can go is what I'll end up doing.

And here's the completed mortar!

Next I took it to the gun range and did some tests to see how far the 11 ounce steel ball would go on a certain amount of powder.