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Gun drills (through coolant drill) are straight fluted drills which allow cutting fluid (either compressed air or a suitable liquid) to be injected through the drill's hollow body to the cutting face. They are used for deep hole drilling—a depth-to-diameter ratio of 300:1 or more is possible. Gun barrels are the obvious example; hence the name. Other uses include moldmaking, diemaking, and the manufacture of combustion engine parts such as crankcase, cylinder head, and woodwind musical instruments, such as uilleann pipes, as gun drills can drill long straight holes in metal, wood, and some plastics. The coolant provides lubrication and cooling to the cutting edges and removes the swarf or chips from the hole. Modern gun drills use carbide tips to prolong life and reduce total cost when compared with steel tips. Speed of drilling depends on the material being drilled, rotational speed, and the drill diameter; a high speed drill can cut a hole in P20 steel at 75 centimeter (30 inches) per minute.
Gun drilling can be done on several kinds of machine tools. On lathes, it is generally practical with hole depths of less than 50 centimeters. There are also purpose-built gun drilling machines, where longer aspect ratios can be drilled.