Rodman Gun - Hollow Casting

Hollow Casting

Guns had been traditionally cast solid and the bore was bored out of the solid metal. With this traditional method, the gun cooled from the outside inward. Castings shrink as they cool. As each succeeding layer cooled it contracted, pulling away from the still molten metal in the center, creating voids and tension cracks. Drilling out the bore removed the voids, but the tensions in the metal were still toward the outside. Rodman devised a method of casting where the gun cooled from the inside out, so that as cooling occurred, it created compression rather than tension. This resulted in a much stronger gun.

With Rodman’s method of casting, a cooling core was placed in the mold before casting. This core consisted of a watertight cast iron tube, closed at the lower end. A second, smaller tube, open at the bottom was inserted into the first. As the molten iron was poured into the mold, water was pumped through the smaller tube to the bottom of the larger tube. The water rose through the space between the two tubes and flowed out at the top. The water continued flowing as the metal cooled. To further ensure that the gun cooled from the inside out, a fire was built around the iron flask containing the gun mold, keeping the gun mold nearly red-hot. For an 8-inch Rodman columbiad, the core was removed 25 hours after casting and the flow of water continued through the space left by the core for another 40 hours. Over 50,000 gallons of water was used in the process. For larger guns, the cooling periods were longer and more water was used.

After cooling the gun the machining process began. The bore was bored out to proper size, the exterior was turned smooth, the trunnions were turned on a trunnion lathe, and a vent was drilled.

Columbiads were not the only guns cast using Rodman’s method. Dalgren XV-inch shell guns for the U.S. Navy were also hollow cast. A 20-inch hollow cast gun, which may not have been identical to the two guns supplied to the U.S. Army, was sold to Peru.

Rodman guns were cast at the Fort Pitt Foundry, Pittsburgh, Pennsylvania; the Scott Foundry, Reading, Pennsylvania; Cyrus Alger & Co., Boston, Massachusetts; and the West Point Foundry, Cold Spring, New York.