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While the above is a common use of center drill bits, it is a technically incorrect practice and should not be considered for production use. The correct tool to start a traditionally drilled hole (a hole drilled by a high-speed steel (HSS) twist drill bit) is a spotting drill bit (or a spot drill bit, as they are referenced in the U.S.). The ...
Drill bit sizes are written as irreducible fractions. So, instead of 78/64 inch, or 1 14/64 inch, the size is noted as 1 7/32 inch. Below is a chart providing the decimal-fraction equivalents that are most relevant to fractional-inch drill bit sizes (that is, 0 to 1 by 64ths).
Originally, all tool bits were made of high carbon tool steels with the appropriate hardening and tempering.Since the introductions of high-speed steel (HSS) (early years of the 20th century), sintered carbide (1930s), ceramic and diamond cutters, those materials have gradually replaced the earlier kinds of tool steel in almost all cutting applications.
Roller cone bits are characterized by the use of rolling cones at the head rather than the typical auger-type design of most drill bits. Rock hardness is one of the determining factors taken into account when selecting an appropriate drill bit. The cutting structure of the bits varies according to the rock formation.
Exploration diamond drilling differs from other geological drilling (such as Reverse Circulation (RC) Drilling [3]) in that a solid core is extracted from depth, for examination on the surface. The key technology of the diamond drill is the actual diamond bit itself. [4] It is composed of industrial diamonds set into a soft metallic matrix.
The typical test uses a 10 mm (0.39 in) diameter steel ball as an indenter with a 3,000 kgf (29.42 kN; 6,614 lbf) force. For softer materials, a smaller force is used; for harder materials, a tungsten carbide ball is substituted for the steel ball. The indentation is measured and hardness calculated as: