Here is my entry in the Golf Ball mortar contest.
I decided to make a simple basic mortar using the minimum Machine tools. I employed Millwright- gunsmith techniques in building the mortar and not precise Machinist techniques. The only only exact measurement required for this mortar is those to make the bore and breech plug. All other measurements are MIF-Make It Fit. You can adjust the measurements to fit your materials.
The only heavy machine tool operations are the making of the barrel and breech plug. Even those operation are minor. Three simple lathe operations are required; boring, turning and drilling. All other operations are done on the drill press.
When completed this Mortar replicates a Confederate 24 PDR Coehorn.
If you click on the drawings, you can print them out full size.
Use of materialsI used the DOM and solid piece to make the barrel and breech plug.
The wood was used for the base.
I added a piece of steel for the trunnion bar and another piece for the caps. Standard hardware store screws and bolts were used for assembly.
I use a dry erase board in the shop for small single operations. I decided in keeping with the K.I.S.S. theme, I would submit my drawings from the dry erase board. CAD drawings have everything you need and sometimes to much. They often are difficult to read and understand. These drawings keep it simple...
Making the mortar.Breech plug
Turn one end of a 2 inch piece of the solid stock to a slip fit, about 1.490", in the original bore of the DOM to length in the drawing. Leave the other end original dimension. Drill a 1/2" hole in the large end then turn a cone in the end extending full diameter of the breech plug.
Barrel
The bore of the barrel is opened so the original diameter of the breech plug is a slip fit in the barrel. A square shoulder is left for the breech plug to seat against. The unturned section in the barrel should be a few thousandth longer than the turned section of the breech plug.
The breech plug should be a slip fit in the barrel to seat.
Barrel length with the measurements given should not be less than 4 inches overall.
Trunnion bar.
My mortar was made with a 1" x 3.75" piece of round stock. I would not use a smaller diameter and bigger is okay. Length was determined by the width of my base. I attached the trunnion with a 2 #10 socket head cap screws. They are spaced one inch apart and centered. Care must be taken that the powder chamber isn't broached by the holes for the screws. Depending on which screw size you use you may want to space the screws further apart.
The Trunnion holds the breech plug in place.
Vent
With the trunnion in place drill a hole all the way through the barrel and through the breech plug into the powder chamber. Be sure to measure and drill the hole so it comes out in the bottom of the powder chamber. Tap the hole and make a vent liner by drilling hole down the center of a bolt. You can skip this step, and use a vent liner, but you must disassemble the mortar after every firing to clean the breech plug and barrel junction before corrosion starts.
The Base
I used two pieces of oak to make the base. Drill a hole centered on the side of the base for the trunnion. Drill the hole so the edge of the hole is at the top edge of the oak. After drilling the hole, cut away the wood so you have a U shaped hole to allow the trunnion to drop in place.
Drill four cross bolt holes to hold the two sides together. The holes should be in pairs in the lower half of the base. Cross bolts can be made from all thread cut to length.
Inlet or chisel out an area to allow the mortar tube to tilt over at 45 degrees.
Trunnion Caps
Make two trunnion caps and attach them with lag screws.
When completed either paint the base or rub in several coats of boiled linseed oil.
Mortar matchThe powder chamber will hold 20 to 25 grs of black powder. I found 23 grs of Cannon grade would drop the ball right at the 30 yard aiming stake.
Here are my distances.
62' 2"
14' 1 1/2"
19' 1"
20" 9 1/4"
21' 9 3/4"
27' 4 1/4"
52' 2 1/4"
Toss out the two longest, my total is 103' 1 3/4"
