Gun drilling is the oldest method for deep hole cutting, first used in the manufacture of gun barrels. It was developed specifically for deep hole cutting. Deep hole cutting is defined as a hole with a depth to diameter ratio greater than 5, and gun drilling excels at ratios greater than 20. It is one of three methods of deep hole drilling, alongside BTA/STS and ejector drilling. In spite of being the oldest of the methods, it has advanced considerably in the last 100 years and remains competitive with the newer methods of deep hole drilling.
The gun drill itself is a hollow tube with a V-groove or flute cut into its outer surface. It has a single carbide cutting edge designed to cut its own guide bushing, which can also be modified for cutting forms instead of just holes, and also consists of guide pads to maintain straightness and burnish the inside surface of the hole. Note that this single cutting edge differentiates from the twist drill, as well as from BTA drilling/STS drilling and ejector drilling.
Gun drilling involves forcing high pressure coolant from the spindle and tool system through small holes in the center of the gun drill. As it exits the bottom of the gun drill, it provides lubrication for the cutting process and carries the chips upward, along the flute/groove, and out of the newly cut hole.
Gun drilling can produce very small holes, as opposed to BTA/STS and ejector drilling methods. This is due in large part to how it differs from these methods: in gun drilling the coolant is injected inside the tool and exits along the outside surface of the tool, while both BTA/STS and ejector methods have the coolant exiting from inside the drill tube. Gun drilling also achieves an excellent surface finish and straightness due to its burnishing guide pads, often not requiring any secondary finishing operations such as honing or reaming. This can significantly reduce the cost of production.
Gun drilling can be used to produce holes with diameters between 1 mm and 50 mm. It cannot achieve the penetration rates that ejector drilling and BTA/STS drill can, but requires less power and can cut smaller diameter holes than either method. BTA/STS is considered more efficient than gun drilling for diameters in excess of 50 mm.
Gun drilling is most commonly used for diameter between 3 mm and 25 mm, with its major strength being its ability to cut small diameter deep holes. It is also well adapted to harder metals, working well with just about any type of material. Gun drills can be used on conventional machining equipment, although it may require additional modification for non-enclosed machining systems. The main requirements are high pressure through tool coolant flow, which can be achieved through retrofitting.
In short, gun drilling is a classical method for deep hole drilling that works with a variety of materials, can achieve diameters smaller than BTA/STS and ejector drilling, and can often produce holes with sufficient straightness, roundness, tolerances, and surface finishes that no additional finishing operations are required.