Gun drills 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 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 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 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.
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 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.
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.
Eldorados Youtube Channel (several gun drilling videos)
- Production Technology – Chapter 6: Deep Hole Drilling and Trepanning
- [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.
- Lilja article on rifle barrel drilling
- Gun drill article on definition and applications of gundrilling
This information originally retrieved from http://en.wikipedia.org/wiki/Gun_drill
on Thursday 17th November 2011 11:45 am EST
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