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What Is a Tapered Drill Bit?

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  • Written By: Jay Leone
  • Edited By: M. C. Hughes
  • Last Modified Date: 31 August 2016
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All drill bits are designed to allow users to bore circular holes into wood and other construction substances. Unlike traditional drill bits, which are uniform in diameter, the diameter of a tapered drill bit gets progressively smaller from top to tip. These drill bits are essentially designed for drilling pilot holes in various materials; these holes are made in preparation for the insertion of screws, bolts, and other hardware. Most tapered drill bit manufacturers employ durable, high-quality steel in the construction of their tapered drill bits.

Driving screws into a substance without pilot holes can easily cause damage. While soft woods and other softer materials may be able to accept screws directly without suffering much damage, other materials such as hardwoods are likely to crack from the pressure that results from driving a screw in without a pilot hole. It is essential to drill a pilot hole in most materials with the proper-sized tapered drill bit to decrease the likelihood of splitting, cracking, warping, or otherwise damaging the material.

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A screw's root diameter ā€” also known as its minor diameter ā€” refers to the diameter of the screw at the points between threads. Root diameters on #4 and #6 wood screws, for example, are 0.075 inches (1.879 millimeters) and 0.094 inches (2.387 millimeters), respectively. In general, when drilling pilot holes for a specific wood screw to be used on hardwood, tapered drill bits that can produce pilot holes that match the screw's root diameter should be used. Pilot holes for softwoods should be about 15 percent smaller than the root diameter of the screws that will be used on them.

Many types of drill bits, including tapered drill bits, are constructed out of high-speed steels (HSS). HSSs are types of strong steels that contain elements such as chromium, molybdenum, carbon and tungsten. A tapered drill bit made out of HSS is very hard, and resistant to corrosion, abrasion, and high temperatures. Carbon adds to the strength of steel, molybdenum increases flexibility, and the combination of tungsten and cobalt allow tapered drill bits to work efficiently at higher temperatures.

An HSS tapered drill bit can retain its hardness even while production drilling at extremely high temperatures. The Rockwell scale is used to reference material hardness. Most grades of HSS offer hardnesses in the range of between 64 and 70 on the Rockwell ā€œCā€ scale for hardness. These values are higher than many types of metal, including iron and most grades of steel.

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