Editing Rodman gun (section)

== Casting ==
[[file:RodmanCasting.jpg|thumb|Engraving showing a gun being cast using Rodman's hollow-casting technique. The engraving shows the gun mold in the casting pit. The outer iron flask, the fire built outside the flask, and the cooling core are also shown.]]

Traditionally, a gun was [[casting (metalworking)|cast]] as a solid body, with the bore then drilled out of the solid metal. In this method, a newly cast gun cooled from the outside in. Castings [[Shrinkage (casting)|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 [[hollow 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.

In Rodman's method, a cooling [[core (manufacturing)|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 {{convert|50,000|USgal|m3}} 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 [[trunnion]]s were turned on a trunnion [[lathe]], and a vent was drilled.

Columbiads were not the only guns cast using Rodman's method. [[Dahlgren gun#Dahlgren shell guns and Rodman casting|Dahlgren XV-inch shell guns]] for the [[United States Navy|US Navy]] were also hollow cast. A 20-inch hollow cast gun, which may not have been identical to the two guns supplied to the [[United States Army|US Army]], was sold to [[Peru]].

Rodman guns were cast at the [[Fort Pitt Foundry]], [[Pittsburgh, Pennsylvania]];<ref>{{ citation | first= Donald B. | last =  Webster Jr. | date =July–August 1962 | title=Rodman's Great Guns | journal = Ordnance: The Journal of the Army Ordnance Association | issue =July–August 1962| url = http://www.clpgh.org/exhibit/neighborhoods/strip/strip_n41.html | access-date = 2008-07-05 | url-status = dead | archive-url = https://web.archive.org/web/20080704200115/http://www.clpgh.org/exhibit/neighborhoods/strip/strip_n41.html | archive-date = 2008-07-04 }}</ref><ref>{{ cite news |title=A Twenty-Inch Gun; Casting at the Fort Pitt Foundry |newspaper=[[The New York Times]] |url=https://timesmachine.nytimes.com/timesmachine/1864/02/21/90528150.pdf |access-date=2008-11-14 | date=1864-02-21}}</ref> the Scott Foundry, [[Reading, Pennsylvania]]; Cyrus Alger & Co., [[Boston, Massachusetts]]; and the [[West Point Foundry]], [[Cold Spring, New York]].