Gun drills are straight fluted drills which allow cutting fluid

Cutting fluid

Cutting fluid is a type of coolant and lubricant designed specifically for metalworking and machining processes. There are various kinds of cutting fluids, which include oils, oil-water emulsions, pastes, gels, aerosols , and air or other gases. They may be made from petroleum distillates, animal...

 (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 drilling—a depth-to-diameter ratio of 300:1 or more is possible. Gun barrel

Gun barrel

A gun barrel is the tube, usually metal, through which a controlled explosion or rapid expansion of gases are released in order to propel a projectile out of the end at a high velocity....

s are the obvious example; hence the name. Other uses include moldmaking

Moldmaker

Moldmaker is the designation for a profession in the metalworking and ceramics industry. It is a variety of the profession of the toolmaker. The moldmaker produces molds for the injection molding , die casting and ceramics industries....

, diemaking

Tool and die maker

Tool and die makers are workers in the manufacturing industry who make jigs, fixtures, dies, molds, machine tools, cutting tools , gauges, and other tools used in manufacturing processes...

, and the manufacture of woodwind musical instrument

Musical instrument

A musical instrument is a device created or adapted for the purpose of making musical sounds. In principle, any object that produces sound can serve as a musical instrument—it is through purpose that the object becomes a musical instrument. The history of musical instruments dates back to the...

s, such as uilleann pipes

Uilleann pipes

The uilleann pipes or //; ) are the characteristic national bagpipe of Ireland, their current name, earlier known in English as "union pipes", is a part translation of the Irish-language term píobaí uilleann , from their method of inflation.The bag of the uilleann pipes is inflated by means of a...

, 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

Swarf

Swarf, also known as turnings, chips, or filings, are shavings and chippings of metal — the debris or waste resulting from metalworking operations including milling and grinding. It can usually be recycled, and this is the preferred method of disposal due to the environmental concerns regarding...

 or chips from the hole. Modern gun drills use carbide

Tungsten carbide

Tungsten carbide is an inorganic chemical compound containing equal parts of tungsten and carbon atoms. Colloquially, tungsten carbide is often simply called carbide. In its most basic form, it is a fine gray powder, but it can be pressed and formed into shapes for use in industrial machinery,...

 tips

Tool bit

The term tool bit generally refers to a non-rotary cutting tool used in metal lathes, shapers, and planers. Such cutters are also often referred to by the set-phrase name of single-point cutting tool. The cutting edge is ground to suit a particular machining operation and may be resharpened or...

 to prolong life and reduce total cost when compared with metal 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 30 inches per minute.

Requirement

With a standard twist drill, it is difficult to drill a straight and accurately sized hole of a depth more than about 5 times the diameter. This is a problem in many manufacturing processes, especially the firearms industry: the barrel of a gun must be very straight and accurately sized. Gun barrels are far longer than their inside diameter; as an example, the .223 inch (5.56 mm) caliber barrel of the M-16

M16 rifle

The M16 is the United States military designation for the AR-15 rifle adapted for both semi-automatic and full-automatic fire. Colt purchased the rights to the AR-15 from ArmaLite, and currently uses that designation only for semi-automatic versions of the rifle. The M16 fires the 5.56×45mm NATO...

rifle is 20 inches (508 mm) long, nearly 90 times the diameter of the bore. The gun drill was developed to drill such long, straight holes.

Gun drilling is possible over a range of depths and diameters. For diameters between 1-3mm, gun drilling can be performed successfully with special equipment. It is a common process between 3-50mm in diameter. It is also possible for the 50-75mm range, however less efficient than BTA deep hole drilling.

Types

There are two basic types of gun drill: 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 through the hole in the tip, up through the shaft, and out. This type is more common to BTA deep hole drilling.

The external chip removal bit has a notch cut in the outside of the drill tip. Coolant is forced down through the hollow shaft, and forces chips to exit along the notch. This type is more common in most gun drilling applications. These types of bits may also vary in the coolant hole. For maximum coolant flow, the drill will have a larger, kidney-shaped hole, or two round holes. Standard flow drills will simply have one round hole.

Further reading

• [Machinery editors]. (1910) Deep hole drilling, 2nd edition. Machinery's Reference Series Number 25. A small monograph on gun drilling practice as of 1910. Compiled as part of a series by the editors of the journal Machinery by revising and republishing information from the journal. Reprinted, along with 2 other Machinery articles on deep hole drilling from 1926 and 1927, by Lindsay Publications, Inc, ISBN 1-55918-258-X.

External links

• Lilja article on rifle barrel drilling
• Gun drill article on definition and applications of gundrilling