Gun drill
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Gun drills are straight fluted drills which allow coolant (either compressed air or a suitable liquid) to be directed through the drill's body, directly to the cutting face. They are used for deep drilling of which gun barrels are the obvious example. Other uses include the manufacture of woodwind instruments, such as uilleann pipes, as gun-drills are capable of drilling long straight holes in metal, wood and some plastics. The coolant provides lubrication and cooling to the cutting edges as well as ejecting the swarf or chips back out the drills length. Modern gun drills take advantage of carbide tips to reduce expense and prolong life. The images show the coolant holes in the drills shank and tip, as well as full length (12 mm dia × 635 mm long) gun drill and the carbide tip of a second (25 mm) gun drill.
[edit] Motivation
It is difficult to drill a deep hole (where deep is approximately 3 times the diameter of the hole) that is both straight and accurately sized with a standard twist drill. This is a problem in many manufacturing processes, especially the firearms industry. The barrel of a gun must be very straight and quite accurately sized. Gun barrels are far longer than their inside diameter; as an example, the .223 inch (5.56 mm) barrel of the M-16 rifle is 20 inches (508 mm) long, nearly 90 times the diameter. Thus the gun drill was invented to create these long straight holes.
[edit] Construction
There are two basic types of gun drills: the internal chip removal type and the external chip removal type. The internal type is basically a tube with a cutting bit on the front. Coolant is forced around the outside of the tube, and pours around the front of the drill forcing the chips into the center of the tube, and out of the hole.
The external chip removal type operates similarly except there is a notch cut in the outside of the drill, through which the chips exit, and a passage for the coolant goes through the center of the bar.
[edit] External links
- Drillmasters, a deep hole drilling company providing gun drills
- Lilja, custom firearms barrel maker who also offers non-firearms related deep drilling services
- Lilja article on rifle barrel drilling
 Metalworking:
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